Charge transport in disordered organic field-effect transistors - - PDF document

Charge transport in disordered

• rganic field-effect transistors

Eduard Meijer

Charge transport in disordered

• rganic field-effect transistors

PROEFSCHRIFT

ter verkrijging van de graad van doctor aan de Technische Universiteit Delft,

• p gezag van de Rector Magnificus prof.dr.ir. J.T. Fokkema,

voorzitter van het College voor Promoties, in het openbaar te verdedigen op vrijdag 20 juni 2003 om 16.00 uur door

Eduard Johannes MEIJER

natuurkundig ingenieur geboren te Drachten.

Dit proefschrift is goedgekeurd door de promotor: Prof.dr.ir. T.M. Klapwijk Samenstelling promotiecommissie: Rector Magnificus, voorzitter Prof.dr.ir. T.M. Klapwijk, Technische Universiteit Delft, promotor Prof.dr. P.W.M. Blom, Rijksuniversiteit Groningen Prof.dr. L.D.A. Siebbeles, Technische Universiteit Delft Prof.dr. G.G. Malliaras, Cornell University, Ithaca, New York, USA Prof.dr. H. B¨ assler, Philipps Universit¨ at, Marburg, Deutschland Prof.dr. D. Emin, University of New Mexico, Albuquerque, USA

• Dr. D.M. de Leeuw,

Philips Research Laboratories, Eindhoven Prof.dr. S.W. de Leeuw, Technische Universiteit Delft, reservelid

This work is part of the research programme of the Stichting voor Fundamenteel Onderzoek der Materie (FOM), which is financially supported by the Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO). The work described in this thesis has primarily been carried out at the Philips Research Laboratories, Eindhoven, The Netherlands, as part of the Philips Research Programme. Cover design by Henny Herps.

Meijer, Eduard Johannes Charge transport in disordered organic field-effect transistors / Eduard Johannes Meijer Ph.D. thesis, Delft University of Technology. - With ref. - With summary in Dutch. ISBN 90-6734-306-4 Keywords: Organic semiconductors / polymers / field-effect transistors / charge transport / doping

As we seek to improve our surrounding world, we should never forget how miraculous nature already is.

Contents

1 General Introduction 1 1.1 Historical perspective . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.2 Charge transport in polymeric semiconductors . . . . . . . . . . . . . . . 3 1.2.1 Hopping transport . . . . . . . . . . . . . . . . . . . . . . . . . 6 1.2.2 Multiple trapping and release model . . . . . . . . . . . . . . . . 7 1.2.3 Polarons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 1.3 Scope of this work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 1.3.1 The field-effect transistor . . . . . . . . . . . . . . . . . . . . . . 9 1.4 Outline of this thesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 2 The switch-on voltage and the field-effect mobility in disordered organic field- effect transistors 15 2.1 The switch-on voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 2.1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 2.1.2 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 2.1.3 The classical threshold voltage . . . . . . . . . . . . . . . . . . . 17 2.1.4 Definition of the switch-on voltage . . . . . . . . . . . . . . . . . 18 2.1.5 Modeling and discussion . . . . . . . . . . . . . . . . . . . . . . 18 2.1.6 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 2.2 The field-effect mobility . . . . . . . . . . . . . . . . . . . . . . . . . . 22 2.2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 2.2.2 Calculation of the charge distribution . . . . . . . . . . . . . . . 23 2.2.3 Modeling the mobility variation . . . . . . . . . . . . . . . . . . 24 2.2.4 Discussion of the field-effect mobility . . . . . . . . . . . . . . . 26 2.2.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 2.3 Unifying the charge transport in polymeric FETs with PLEDs . . . . . . 26 2.3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 2.3.2 Experimental results . . . . . . . . . . . . . . . . . . . . . . . . 27 2.3.3 The mobility - charge density relation . . . . . . . . . . . . . . . 28 2.3.4 Unification of the LED and FET models . . . . . . . . . . . . . . 29 2.3.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

viii Contents 3 The Meyer-Neldel rule in organic field-effect transistors 33 3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 3.2 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 3.3 Demonstration of the MNR . . . . . . . . . . . . . . . . . . . . . . . . . 35 3.4 Implications for the charge transport . . . . . . . . . . . . . . . . . . . . 37 3.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 4 The isokinetic temperature in disordered organic semiconductors 41 4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 4.2 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 4.3 Discussion of the isokinetic temperature . . . . . . . . . . . . . . . . . . 46 4.3.1 The field-dependent mobility . . . . . . . . . . . . . . . . . . . . 46 4.3.2 The Meyer-Neldel rule . . . . . . . . . . . . . . . . . . . . . . . 47 4.3.3 Comparison of the MNR with the field-dependent mobility . . . . 47 4.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 5 Scaling behavior and parasitic series resistance in disordered organic field- effect transistors 51 5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 5.2 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 5.3 Scaling behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 5.4 Parasitic series resistance determination . . . . . . . . . . . . . . . . . . 53 5.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 6 Frequency behavior and the Mott-Schottky analysis in poly(3-hexyl thiophene) metal-insulator-semiconductor diodes 61 6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 6.2 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 6.3 The Mott-Schottky analysis . . . . . . . . . . . . . . . . . . . . . . . . . 63 6.4 Equivalent circuit modelling . . . . . . . . . . . . . . . . . . . . . . . . 63 6.5 The relaxation time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 6.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 7 Photoimpedance spectroscopy of poly(3-hexyl thiophene) metal-insulator-semi- conductor diodes 71 7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72 7.2 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72 7.3 Flat-band shift under oxygen exposure . . . . . . . . . . . . . . . . . . . 73 7.4 Photoimpedance spectroscopy . . . . . . . . . . . . . . . . . . . . . . . 74 7.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81

Contents ix 8 Dopant density determination in disordered organic field-effect transistors 83 8.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84 8.2 Motivation and Realization . . . . . . . . . . . . . . . . . . . . . . . . . 84 8.3 Interpretation and Analysis . . . . . . . . . . . . . . . . . . . . . . . . . 86 8.3.1 Determination of the dopant density . . . . . . . . . . . . . . . . 86 8.3.2 Determination of the bulk mobility . . . . . . . . . . . . . . . . . 89 8.4 Results for PTV and P3HT . . . . . . . . . . . . . . . . . . . . . . . . . 89 8.5 Shift of the switch-on voltage . . . . . . . . . . . . . . . . . . . . . . . . 91 8.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95 9 Solution-processed ambipolar organic field-effect transistors 97 9.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98 9.2 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101 9.3 Ambipolar transistor operation . . . . . . . . . . . . . . . . . . . . . . . 101 9.4 CMOS-like inverter operation . . . . . . . . . . . . . . . . . . . . . . . 105 9.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107 10 Ambipolar field-effect transistors based on a single organic semiconductor 109 10.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110 10.2 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111 10.3 The PIF ambipolar transistor and inverter . . . . . . . . . . . . . . . . . 111 10.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117 Summary 119 Samenvatting 123 List of Publications 127 Patent Application 128 Curriculum Vitae 129 Dankwoord 131

Chapter 1 General Introduction

1.1 Historical perspective

Solid-state electronics was founded by the invention of the bipolar transistor by Bardeen, Brattain and Shockley1 in 1947. Germanium, the original semiconducting material to fabricate diodes and transistors was soon replaced by single-crystalline silicon. The next major development in the field of solid-state electronics was the invention of the first inte- grated circuit2 in 1960, which catapulted us into the information age. Today, the number

• f devices manufactured on a chip has grown to over several hundreds of million and the

feature sizes have shrunk to submicron resolution. Our information, condensed into bits and bytes, travels across the globe, processed by grains of sand, as silicon technology has dominated integrated device manufacturing and will do so for the foreseeable future. Plastics, or organic materials, are strong, lightweight, adaptable, and they can be produced at low temperatures, giving them an economic and technological edge over competing materials, such as wood or ceramics, for the fabrication of packaging materials, furniture, domestic appliances, etc. In the 1960s the development of photo-conductive

• rganic materials for xerographic applications [4] emerged. However it was the discovery
• f the first highly conducting polymer, chemically doped polyacetylene3 in 1977, that

demonstrated that polymers could be used as electrically active materials as well. This discovery resulted in a huge research effort on conjugated organic materials. Combining the interesting properties of polymers with opto-electronic functional- ity, has demonstrated that organic materials, in their own right, can be the ”core” of a wide range of new opto-electronic devices, such as polymeric light-emitting diodes, poly- meric solar cells, and organic integrated circuits. The polymers used in these applications are soluble in organic solvents and can therefore be processed from solution, using tech- niques such as spin coating, film casting or even inkjet printing. These techniques for film deposition allow large areas to be coated, which is important for the realization of large-

1The invention of the transistor won them the Nobel prize in physics in 1956 [1,2] 2which was awarded the Nobel prize in physics in 2000 to Alferov, Kroemer and Kilby [3] 3This discovery won McDiarmid, Shirakawa and Heeger the Nobel prize in chemistry in 2000 [5–7]

2 General Introduction

Figure 1.1: The top left-hand photo shows a 64 by 64 polymer-dispersed

liquid-crystal display driven by 4096 polymer TFTs, with solution- processed PTV as the semiconductor. An image containing 256 grey levels is shown, while the display is refreshed at 50 Hz. The top right-hand photo shows a fully processed 150-mm wafer foil con- taining all-polymer transistors and integrated circuits. The bottom photo shows a monochrome polymer LED segment display, which is used as battery charge state indicator on a Philips shaver. This is the first Polymer LED product that was launched on the consumer market in June 2002. Photos: Philips Research

1.2 Charge transport in polymeric semiconductors 3 area displays as well as for high-volume production of integrated circuits [11]. Also, the mechanical toughness of polymers and the flexibility of polymeric thin films allow their use in flexible displays and flexible electronics (see Fig.1.1). Furthermore, polymers are usually associated with low-processing costs, allowing them to be used in disposable products. The properties of polymers can be tuned by adding functionalizing sidegroups, or by building in elements such as sulfur or nitrogen. A lot of effort is being put in the synthesis of new materials, with improved performance and novel properties. These ad- vantages make them interesting candidates for low-cost, flexible industrial applications. Understanding the optical and electronic properties of these materials on a microscopic level has turned out to be an intriguing challenge that merges the fields of physics and chemistry. For the operation and performance of all these devices the charge transport through the polymer layer is the dominant factor. It is therefore crucial to gain insight into their charge transport mechanisms. In the following section the most widely used descriptions

• f charge transport in disordered organic semiconductors are presented. Emphasis is put
• n the presence of disorder. In the context of charge transport, the scope of this thesis

is in the physical description of field-effect transistors based on disordered organic semi- conductors.

1.2 Charge transport in polymeric semiconductors

Conjugated polymers are intrinsically semiconducting materials. They lack intrinsic mo- bile charge, but are able to transport charge generated by light, injected by electrodes, or provided by chemical dopants. The main constituent of conjugated polymers is the carbon

• atom. It is the nature of the bonds between the carbon atoms that gives the conjugated

polymer its interesting physical and chemical properties. To understand the basics of these molecular bonds it is instructive to understand the shape of the electronic orbitals4

• f the atoms participating in the molecular bond [8].

Carbon, in the ground state, has four electrons in the outer electronic level. The or- bitals of these electrons may mix, under creation of four chemical bonds, to form four equivalent degenerate orbitals referred to as sp3 hybrid orbitals in a tetrahedral orienta- tion around the carbon atom, If only three chemical bonds are formed, they have three coplanar sp2 hybridized orbitals which are at an angle of 120o with each other. These bonds are called σ-bonds, and are associated with a highly localized electron density in the plane of the molecule. The one remaining free electron per carbon atom resides in the pz orbital, perpendicular to the plane of the sp2 hybridization. The pz orbitals on neigh- boring atoms overlap to form so-called π-bonds [9, 10]. A schematic representation of this hybridization is given for the simplest conjugated polymer, polyacetylene, in Fig.1.2. Molecules with σ and π-bonds are schematically represented by single and double alter- nating chemical bonds between the carbon atoms, and are called conjugated molecules. The π-bonds establish a delocalized electron density above and below the plane of the

4An atomic orbital is derived using the mathematical tools of quantum mechanics, and is a representation of

the three-dimensional volume (i.e. the region in space) in which the probability of finding an electron is highest.

4 General Introduction

• σ
• π
• σ
• π
• σ
• π
• Figure 1.2: (a) The moleculare structure of polyacetylene, for clarity, hydrogen

atoms are not shown. The alternating double and single bonds in- dicate that the polymer is conjugated. (b) Schematic representation

• f the electronic bonds in polyacetylene. The pz-orbitals overlap to

form π-bonds.

• molecule. These delocalized π-electrons are largely responsible for the opto-electronic

behavior of conjugated polymers. There are large differences between the three-dimensional crystal lattice of most in-

• rganic semiconductors and the amorphous structure of conjugated polymers. Inorganic

semiconductor crystalline lattices, such as silicon and germanium, are characterized by long range order and strongly coupled atoms. For silicon and germanium this results in the formation of long-range delocalized energy bands separated by a forbidden energy gap [12]. Charge carriers added to the semiconductor can move in these energy bands with a relatively large mean free path. The limiting factor for this band transport is scat- tering of the carriers at thermal lattice vibrations, i.e. phonons [10, 12]. This is depicted schematically in Fig.1.3a. As the number of lattice vibrations decreases with decreasing temperature, the mobility of the charge carriers increases with decreasing temperature. In conjugated polymers the polymer chains are weakly bound by van der Waals

• forces5. These polymers typically have narrow energy bands, the highest occupied molec-

ular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), which can easily be disrupted by disorder. Although electric charge is delocalized along the π- conjugated segments of the polymer backbone, the length of such perfectly conjugated segments is typically limited to length scales of around 5 nm, separated by chemical de- fects, such as a nonconjugated sp3-hybridized carbon atom on the polymer backbone, or by structural defects, such as chain kinks or twists out of coplanarity. Due to the disorder the semiconductor can not be regarded simply as having two delocalized energy bands separated by an energy gap. Instead, the charge transporting sites, which are the seg- ments of the main chain polymer, are subject to a Gaussian distribution of energies (see Fig.1.4b), implying that all states are localized [13]. The shape of the density of states (DOS) is suggested to be Gaussian, because of the observed Gaussian shape of the optical spectra [13].

5We note that the carbon atoms in a polymer chain are strongly coupled.

1.2 Charge transport in polymeric semiconductors 5

• Figure 1.3: Charge transport mechanisms in solids. (a) Band transport. In a

perfect crystal, depicted as the straight line, a free carrier is delo-

• calized. There are always lattice vibrations that disrupt the crystal
• symmetry. Carriers are scattered at these phonons, which limit the

charge carrier mobility. The mobility for band transport increases with decreasing temperature. (b) Hopping transport. If the carrier is localized due to defects, disorder or selflocalization, e.g. in the case of polarons, the lattice vibrations are essential for a carrier to move from one site to another. For hopping transport the mobility increases with increasing temperature. The figure is adapted from

• ref. [10]

6 General Introduction

• Figure 1.4: (a) Schematic view of polymer chains broken up in conjugated seg-

ments, which are represented as charge transport sites, between which the charge carriers hop. (b) A representation is given of the smeared out density of states, which is often approximated by a Gaussian distribution for the HOMO and LUMO levels. The specific shape of the DOS is rarely investigated, as it is difficult to determine

• experimentally. Via chemical doping the DOS itself is often altered by the presence of

the dopant counter ions, and via the field-effect no unique DOS can be determined. The shape of the DOS, which is a manifestation of the disorder of the system, is however im- portant for the description of charge transport. Charge carriers move from localized site to localized site, on chain as well as between chains in order to percolate through a thin-film device (see Fig.1.4a) [10,13]. It can be concluded that the charge transport and semicon- ducting properties of polymeric semiconductors are sensitive to the morphology of the polymer chains and the local structural order within the film. Structural and energetic disorder in conjugated polymer systems are therefore of importance in the description of charge transport.

1.2.1 Hopping transport

Due to the disorder and the localization of charge, the motion of the charge carriers in

• rganic semiconductors is typically described by hopping transport, which is a phonon-

assisted tunneling mechanism from site to site [14, 15]. This hopping transport takes place around the Fermi level6. Many of the hopping models are based on the single- phonon jump rate description as proposed by Miller and Abrahams [16]. In this model

6The Fermi level is defined as the highest energy level occupied by charge at a temperature of 0 K. At finite

temperatures, some levels above the Fermi level are filled and some levels below are empty. The distribution of filled energy levels around the Fermi level is given by the Fermi-Dirac distribution function [12]. The position

• f the Fermi level is determined by the charge neutrality condition of the system.

1.2 Charge transport in polymeric semiconductors 7 the hopping rate between an occupied site i and an adjacent unoccupied site j, which are separated in energy by Ei − Ej and in distance by Rij, is described by: νij = ν0 exp

• −2γ Rij
• exp
• − Ei−E j

kB T

• Ei > E j,

1 Ei < E j, (1.1) where γ −1 quantifies the wavefunction overlap between the sites, ν0 is a prefactor, and kB is Boltzmanns constant. The Miller-Abrahams model addresses hopping rates at low tem- peratures between shallow three-dimensional impurity states, assuming that the electron- lattice coupling is weak. When the Miller-Abrahams model is applied to polymeric semi- conductors, it is assumed that the conjugated segments of the polymers play the role of nearly isolated impurity states, and that Eq.1.1 is still valid at high temperatures [16]. Depending on the structural and energetic disorder of the system it can be energet- ically favorable to hop over a longer distance with a low activation energy (energy dif- ference between sites), than over a shorter distance with a higher activation energy. This extension to the Miller-Abrahams model is called variable range hopping [17]. Monroe developed a model describing hopping transport around the Fermi level in an exponential density of states [18]. He found that this hopping description is analytically very simi- lar to a model in which charge carriers are thermally activated to a transport level. For the description of the temperature and gate voltage dependencies of organic field-effect transistors Vissenberg and Matters [19] developed a percolation model based on variable range hopping in an exponential density of states, which will be used throughout this thesis.

1.2.2 Multiple trapping and release model

For polycrystalline organic semiconductor layers the temperature dependent transport data is often interpreted in terms of a multiple trapping and release model [20]. In this model the organic semiconductor film consists of crystallites which are separated from each other by amorphous grain boundaries. In the crystallites the charge carriers can move in delocalized bands, whereas in the grain boundaries they become trapped in lo- calized states. The trapping and release of carriers at these localized states results in a thermally activated behavior of the field-effect mobility, which depends on the gate volt-

• age. The description of trapped, i.e. localized, charges which can be thermally activated

to a transport level, in this case the band, is very similar to hopping in an exponential density of states [18,19] as stated in the previous section. As the grain boundaries in a polycrystalline system determine the DC charge transport and typically an exponential trap distribution is used to model the experimental data, no clear distinction can be made between hopping transport and multiple trapping and release, on the basis of the temper- ature dependence of the mobility. One should be able to separate a trap-limited mobility from a hopping mobility if the Hall mobility in a Hall effect experiment could be deter- mined, since in a magnetic field, there will be no Lorentz force on trapped charges [21]. Unfortunately, due to the low charge carrier mobilities, the Hall effect is very difficult to measure experimentally.

8 General Introduction

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Figure 1.5: Molecular

structures

• f

the conjugated materials stud- ied in this thesis: (a) poly(2,5-thienylene vinylene) (PTV), (b) poly(3-hexyl thiophene) (P3HT), (c) poly([2-methoxy- 5-(3’,7’-dimethylocyloxy)]-p-phenylene vinylene) (OC1C10- PPV), (d) poly([2,5-(3’,7’-dimethylocyloxy)]-p-phenylene vinylene) (OC10C10-PPV), (e) pentacene, (f) poly(3,9-di-tert- butylindeno[1,2-b] fluorene) (PIF), (g) [6,6]-phenyl C61-butyric acid methyl ester (PCBM).

1.3 Scope of this work 9

1.2.3 Polarons

Next to the disorder-induced localization of charge, the strong electron-phonon coupling in organic materials results in localization of charge. An excess charge carrier on a conju- gated polymer chain can minimize its energy by a local lattice deformation. This quasipar- ticle consisting of charge and a lattice deformation, or phonon cloud, is called a polaron. As polarons represent a local distortion of the lattice, the associated energy levels must split off from the HOMO level and the LUMO level. These energy levels, which reside in the energy gap, have often been observed in optical absorption experiments on charged conjugated polymer films [22–24]. In polaronic charge transport, not only the charge moves under an applied electric field but the lattice deformation moves with it. Typically, for hopping transport of po- larons, a description in terms of Miller-Abrahams hopping [16] is insufficient, as multi- phonon hopping rates need to be considered [25,26].

1.3 Scope of this work

In this thesis we study charge transport in organic semiconductors. We do this by focusing

• n the physical characterization of disordered organic field-effect transistors. It will be

made clear that the disorder in the polymer films is crucial for the interpretation of the

• data. The field-effect transistor geometry allows variation of the charge carrier density in

the semiconductor, without the presence of counter ions. Therefore, the transistor allows a rather clean study of the charge transport in organic semiconductors as a function of the charge carrier density and temperature. In the following section the operation of the silicon metal-oxide-semiconductor field-effect transistor (MOSFET) is described. In the experiments we find that the organic transistors are in several respects not comparable to silicon MOSFETs. Therefore, in this thesis we redefine and reevaluate basic transistor parameters, such as the threshold voltage, the field-effect mobility, the contact resistance and the dopant density. Subsequently, we study the charge transport as a function of charge density and temperature, giving insight into the charge transport mechanisms. And finally, we investigate the stability of the polymer layer and discuss why typically only unipolar transistor behavior is observed experimentally. The conjugated organic materials used throughout this thesis are given in Fig.1.5.

1.3.1 The field-effect transistor

The MOSFET can basically be considered as a parallel plate capacitor, where one con- ducting electrode, the gate electrode, is electrically insulated, via an insulating oxide layer, from the semiconductor layer (see Fig.1.6). Two electrodes, the source and the drain, are contacted to the semiconductor layer. By applying a gate voltage, Vg, with respect to the source electrode, charge carriers can electrostatically be accumulated or depleted in the semiconductor at the semiconductor-insulator interface. Due to this field-effect the charge carrier density in the semiconductor can be varied. Therefore, the resistivity of the semiconductor, and hence the current through the semiconductor (upon application of a source-drain field), can be varied over orders of magnitude [27]. Since the MOSFET can

10 General Introduction

• Figure 1.6: Schematic of a thin-film field-effect transistor.
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level band diagram

• f

an ideal metal-insulator- semiconductor diode structure with a p-type semiconductor: (a) flat-band condition, (b) accumulation, (c) depletion. be switched between a conducting and a non-conducting state, it is widely used as the basic building block of binary logic. The band-bending diagrams of a p-type transistor in the different operating regimes are schematically given in Fig.1.7. In equilibrium the Fermi levels of the materials align, by charge carriers which move to or from the semiconductor-insulator interface. When a bias is applied which is equal to the difference between the Fermi levels of the gate metal and the semiconductor, no band bending will occur in the semiconductor at the semiconductor-insulator interface, i.e. the energy bands in the semiconductor will be flat (see Fig.1.7a). This biasing condition is defined as the flat-band voltage. If the Fermi level of the metal and semiconductor are similar this flat-band voltage will be 0. For a p-type semiconductor, the application of a negative gate voltage will induce charges at the semiconductor-insulator interface (these charges are supplied by the source and drain contacts). In effect the Fermi level of the gate metal is varied with a value of qVg, where q is the elementary charge, causing band bending in the semiconductor layer as is schematically represented in Fig.1.7b. For a positive applied Vg the energy bands in the p-type semiconductor are bent downwards, and the mobile positive charge carriers are depleted from the semiconductor-insulator interface. In this case the transistor is biased

1.4 Outline of this thesis 11 in the depletion mode (see Fig.1.7c).

1.4 Outline of this thesis

In Chapter 2 the switch-on voltage and the field-effect mobility in disordered organic field-effect transistors are introduced. It is argued that the use of classical MOSFET theory for the description of these transistors is not a priori justified. In fact a scruti- nizing look at the data of FETs based on three different organic semiconductors reveals that MOSFET theory neglects basic properties of these materials, particularly disorder, and should therefore not be used. Alternative characterization parameters are introduced which will be used throughout the remainder of this thesis. In the third section of chapter 2 the hole transport in light-emitting diodes and field- effect transistors is compared. The experimental hole mobilities extracted from both types

• f devices, based on a single polymeric semiconductor, can easily differ by three orders
• f magnitude. This apparent discrepancy in the charge transport description is resolved

by demonstrating that the hole mobility depends strongly on the charge carrier density in disordered organic semiconducting polymers. In Chapter 3 the temperature dependence of the field-effect mobility is investigated in two solution-processed disordered organic field-effect transistors (PTV and pentacene). We find thermally activated behavior, with an activation energy that depends on the in- duced charge density in the transistor. Upon extrapolation of the data to infinite temper- ature we find an empirical relation, termed the Meyer-Neldel rule, which states that the mobility prefactor increases exponentially with the activation energy. From this analy- sis a characteristic temperature is extracted that does not vary much between different

• materials. The possible implications of this observation in terms of charge transport are

given. As a follow-up to the previous chapter the field dependence of the in-plane conduc- tivity is investigated in PTV and P3HT in Chapter 4. Using an empirical relation for this field-dependence we find an isokinetic temperature that is comparable to the values

• btained from the Meyer-Neldel experiments of Chapter 3. Implications of these findings

are addressed. In Chapter 5, we investigate the scaling behavior of FETs. It is demonstrated that downsizing of the transistor channel does not automatically result in an improve- ment of integrated circuit performance. This is due to parasitic resistances at the metal- semiconductor contact. We find an empirical relation between the charge-carrier mobility in the polymeric semiconductor and the parasitic resistance. In Chapter 6 we investigate the dopant density in polymeric semiconductors in the classical way by means of impedance spectroscopy of metal-insulator-semiconductor

• diodes. The diodes, based on poly(3-hexyl thiophene), are measured and analyzed as a

12 General Introduction function of bias voltage, measurement frequency and temperature. We find that simple Mott-Schottky analysis can only be applied here if the relaxation time of the semicon- ducting polymer is taken into account. The long relaxation times are a direct result of the disorder in the semiconducting polymer layer. In Chapter 7 we investigate the instability of P3HT under oxygen exposure and

• illumination. P3HT forms a charge-transfer complex with molecular oxygen, which re-

sults in an acceptor density increase with time. This acceptor density change with time, is investigated by means of photoimpedance measurements as a function of wavelength. The measurements show that the acceptor creation efficiency peaks upon excitation of the molecular oxygen-polythiophene contact charge transfer complex. An alternative method for the determination and monitoring of the dopant density is given in Chapter 8, where the dopant density is extracted directly from the transfer characteristics of transistors based on PTV and P3HT. It is demonstrated that, due to the fact that in disordered semiconductors the charge carrier mobility depends on the charge density, the bulk current can be separated from the field-effect current in the total current

• density. It is found that the morphology of the polymer film is crucial for the stability of

transistors that are prone to oxygen doping effects. In the first 5 chapters we have looked at unipolar p-type organic field-effect tran-

• sistors. In Chapter 9 we use a blend of hole-conducting OC1C10-PPV and electron-

conducting PCBM to construct a solution-processed ambipolar organic field-effect tran-

• sistor. The characteristics of this kind of transistor are measured and analyzed and CMOS-

like inverter operation is demonstrated, paving the way for solution-processable CMOS- like logic circuitry. As was demonstrated in the previous chapter a suitable choice of electrode and semi- conductor can result in ambipolar FETs. In Chapter 10 we show that even a single solution-processed organic semiconductor can be used in ambipolar transistors, demon- strating that in principle an intrinsic organic semiconductor can transport both polarities

• f charge. Important parameters for experimental observation of this ambipolar behavior

are extrinsic effects such as workfunction mismatch and material purity.

References

[1] http://www.nobel.se/physics/laureates/1956/press.html [2] W. Shockley, Bell Syst. Techn. J. 28, 435 (1949). [3] http://www.nobel.se/physics/laureates/2000/press.html [4] D.M. Pai and B.E. Spingett, Rev. Mod. Phys. 65, 163 (1993). [5] http://www.nobel.se/chemistry/laureates/2000/press.html [6] C.K. Chiang, C.R. Fincher, Y.W. Park, A.J. Heeger, H. Shirakawa, E.J. Louis, S.C. Gan, A.G. MacDiarmid, Phys. Rev. Lett. 39, 1098 (1977). [7] C.K. Chiang, M.A. Druy, S.C. Gau, A.J. Heeger, E.J. Louis, A.G. McDiarmid, Y.W. Park and H. Shirakawa, J. Am. Chem. Soc. 100, 1013 (1977). [8] P.W. Atkins, Physical Chemistry, Oxford University Press (1986). [9] R.E. Peierls, Quantum theory of solids, Oxford University Press, London (1955). [10] M. Pope and C.E. Swenberg, Electronic Processes in Organic Crystals and Poly- mers, Oxford University Press (1999). [11] A.R. Brown, C.P. Jarrett, D.M. de Leeuw and M. Matters, Synth. Met. 88, 37 (1997). [12] C. Kittel, Introduction to solid state physics, 6th edition (John Wiley & Sons, Inc 1986). [13] H. B¨ assler, Phys. Stat. Sol. B 175, 15 (1993). [14] E.M. Conwell, Phys. Rev. 103, 51 (1956). [15] N.F. Mott, Canadian J. Phys. 34, 1356 (1956). [16] A. Miller and E. Abrahams, Phys. Rev. 120, 745 (1960). [17] N.F. Mott and E.A. Davies, Electronic processes in non-crystalline materials, 2nd Edition, Oxford University Press, London (1979). [18] D. Monroe, Phys. Rev. Lett. 54, 146 (1985).

14 References [19] M.C.J.M. Vissenberg and M. Matters, Phys. Rev. B 57, 12964 (1998). [20] G. Horowitz, R. Hajlaoui and P. Delannoy, J. Phys. III 5, 355 (1995). [21] D. Emin, private communication, D. Emin, Phys. Today 35, 34 (1982). [22] K.E. Ziemelis, A.T. Hussain, D.D.C. Bradley and R.H. Friend, Phys. Rev. Lett. 66, 2231 (1991). [23] P.A. Lane, X. Wei and Z.V. Vardeny, Phys. Rev. Lett. 77, 1544 (1996). [24] R. ¨ Osterbacka, C.P. An, X.M. Jiang and Z.V. Vardeny, Science 287, 839 (2000). [25] T. Holstein, Ann. Phys. 8, 325 (1959). [26] D. Emin, Adv. Phys. 24, 305 (1975). [27] S.M. Sze, Physics of semiconductor devices, 2nd edition, John Wiley & Sons, (New York, 1981).

Chapter 2 The switch-on voltage and the field-effect mobility in disordered

• rganic field-effect transistors

Abstract

In this chapter we critically evaluate two characterization parameters of disordered or- ganic field-effect transistors.

• The switch-on voltage is defined as the flat-band voltage, and is used as character-

ization parameter. The transfer characteristics of the solution processed organic semiconductors pentacene, poly(2,5-thienylene vinylene) and poly(3-hexyl thio- phene) are modeled as a function of temperature and gate voltage with a hopping model in an exponential density of states. The data can be described with reason- able values for the switch-on voltage, which is independent of temperature. This result also demonstrates that the large threshold voltage shifts as a function of tem- perature reported in the literature constitute a fit-parameter without a clear physical basis.

• The experimentally determined field-effect mobility is compared to the local charge

carrier mobility, which takes into account the charge density distribution in the tran-

• sistor. It is demonstrated that the experimentally determined field-effect mobility is

a reasonable estimate for the local mobility of the charge carriers at the interface. In the last section of this chapter the transistor description is compared to the models used for polymeric light-emitting diodes and it is demonstrated that the two device descriptions can be unified, when the large differences in charge densities in the two device geometries are taken into account.

16 The switch-on voltage and the field-effect mobility

2.1 The switch-on voltage

2.1.1 Introduction

The charge transport in organic field-effect transistors (FETs) has been a subject of re- search for several years. It has become clear that disorder severely influences the charge transport in these transistors [1, 2]. Studies on the effect of molecular order ultimately resulted in the observation of band transport in high quality organic single crystals [3]. The electrical transport in these crystals is well described by monocrystalline inorganic semiconductor physics [3,4]. However, devices envisaged for low-cost integrated circuit technology are typically deposited from solution [5, 6], resulting in amorphous or poly- crystalline films. In these solution-processed organic transistors the disorder in the films dominates the charge transport, due to the localization of the charge carriers. The disor- der is observed experimentally through the thermally activated field-effect mobility and its gate voltage dependency [7,8]. These observations have thus far been modeled using multiple trapping and release [9], variable range hopping [8] and grain boundary charg- ing [10]. A further common feature of disordered organic field-effect transistors is the temperature dependence of the threshold voltage, Vth [10]. In this paragraph, the temperature dependence of Vth in disordered organic field- effect transistors is addressed. It is argued that Vth, as used in literature for the description

• f organic transistor operation, is a fit parameter with no clear physical basis. Instead, a

switch-on voltage, Vso, is defined for the transistor at flat-band. We model the experimen- tal data obtained on solution-processed pentacene, poly(2,5-thienylene vinylene) (PTV) and poly(3-hexyl thiophene) (P3HT), with a disorder model of variable-range hopping (VRH) in an exponential density of states [8]. The modeling shows that good agreement with experiment can be obtained with reasonable values for the switch-on voltage, which is independent of temperature. Furthermore, it is found that the shift of the Fermi-level with temperature has no influence on Vso.

2.1.2 Experimental

In the experiments we used heavily doped Si wafers as the gate electrode, with a 200 nm thick layer of thermally oxidized SiO2 as the gate-insulating layer. Using conventional lithography, gold source and drain contacts are defined with an interdigitated geometry. The SiO2 layer is treated with the primer hexamethyldisilazane (HMDS) to make the surface hydrophobic. The samples are measured under high vacuum (10−7 mbar) in an Oxford optistat CF-V flow cryostat, using a Hewlett-Packard 4156A semiconductor pa- rameter analyzer. The films of PTV are truly amorphous whereas the pentacene films are

• polycrystalline. The P3HT films can be considered nanocrystalline, as ordered regions
• f this regioregular polymer alternate with disordered regions [1]. We do not observe

any hysteresis in the current-voltage characteristics and the curves are stable with time (in vacuum). The field-effect mobilities in the devices have been estimated from the transconductance [7] at a gate voltage, Vg = −19 V at room temperature and are given in Table 2.1. For the P3HT transistor described here the processing conditions were not

• ptimized to give the high mobilities reported in literature [1].

2.1 The switch-on voltage 17

Table 2.1: Values obtained by using Eq.2.6 to model the transfer characteris-

tics of solution-processed pentacene, PTV and P3HT. TDOS repre- sents the width of the exponential density of states, σ0 is the conduc- tivity prefactor, α−1 the effective overlap parameter, Vso the switch-

• n voltage, and µRT the field-effect mobility at Vg = −19 V and

room temperature. TDOS[K] σ0[106S/m] α−1[ ˚ A] Vso[V] µRT [cm2/Vs] PTV 382 5.6 1.5 1 2 × 10−3 pentacene 385 3.5 3.1 1 2 × 10−2 P3HT 425 1.6 1.6 2.5 6 × 10−4

2.1.3 The classical threshold voltage

The difficulty of defining a threshold voltage in disordered organic transistors was already pointed out by Horowitz et al. [11]. The threshold voltage in inorganic field-effect tran- sistors is defined as the onset of strong inversion [4]. However, most organic transistors

• nly operate in accumulation and no channel current in the inversion regime is observed.

Nevertheless, classical metal-oxide-semiconductor field-effect (MOSFET) theory is often used to extract a Vth from the transfer characteristics of organic transistors in accumula-

• tion. The square root of the saturation current is then plotted against the gate voltage, Vg.

This curve is fitted linearly and the intercept on the Vg-axis is defined as the Vth of the

• transistor. For disordered transistors this method neglects the experimentally observed de-

pendence of the field-effect mobility on the gate voltage [7,12]. In an attempt to take this into account in the parameter extraction several groups have used an empirical relation to fit the field-effect mobility [10,13], µ = K

• Vg − Vth

γ , (2.1) where K, γ and Vth are fit parameters. Fitting of current-voltage characteristics of the transistors, using either this empirical relation or the square root technique, has resulted in a temperature dependent Vth [10, 14]. The shift of Vth with temperature is as large as 15 V in the temperature range of 300 K to 50 K [10]. However, for transistors based

• n the same materials in the crystalline phase, for which MOSFET theory is valid, the

shift of Vth with temperature is at most ≃0.5 V [4]. This observation raises the question: why, for a disordered system, the shift of Vth with temperature is so much larger than in its crystalline counterpart. To answer this question, we first have to realize that, in both types of analysis mentioned, the extracted Vth is a fit parameter. This fit parameter has no direct relation with the original definition of the threshold voltage in MOSFET

• theory. Also, depending on the range of Vg over which the data is fitted in disordered

transistors, the value of the extracted Vth will be different. Therefore, we argue that Vth as defined in MOSFET theory has no physical relevance in the description of the operation

• f disordered organic transistors. Despite these issues, some suggestions have been given

18 The switch-on voltage and the field-effect mobility in literature to explain the large temperature dependence of the apparent Vth, such as a widening of the bandgap [14], and displacement [10] of the Fermi level with decreasing temperature.

2.1.4 Definition of the switch-on voltage

Instead of Vth as characterization parameter, we will use the gate voltage at which there is no band-bending in the semiconductor, i.e. the flat-band condition (see Fig.1.7a). We call this the switch-on voltage, Vso, of the transistor. Below Vso the variation of the channel current with the gate voltage is zero, while the channel current increases with Vg above

• Vso. For an unintentionally doped semiconductor layer, Vso is then only determined by

fixed charges in the insulator layer or at the semiconductor/insulator interface. In that case Vg becomes Vg − Vso. Without these fixed charges Vso should in principle be zero [4].

2.1.5 Modeling and discussion

Here we will model the experimental DC transfer characteristics obtained on three differ- ent disordered organic field-effect transistors to estimate the temperature dependence of

• Vso. Because we are looking at disordered systems, we use the variable range hopping

model proposed by Vissenberg and Matters [8]. The charge transport in this model is gov- erned by hopping, i.e. thermally activated tunneling of carriers between localized states around the Fermi level, EF. The carrier may either hop over a small distance with a high activation energy or over a long distance with a low activation energy. In the disordered semiconducting polymer the density of states (DOS) is described by a Gaussian distribu- tion [16]. For a system with both a negligible doping level compared to the gate-induced charge and at low gate-induced carrier densities the Fermi level is in the tail states of the Gaussian, which is approximated by an exponential DOS [8](see also section 2.3): g(E) = Nt kBTDOS exp

• E

kBTDOS

• (2.2)

where Nt is the number of states per unit volume, kB is Boltzmann’s constant, and TDOS is a parameter that indicates the width of the exponential distribution. The energy dis- tribution of the charge carriers is given by the Fermi-Dirac distribution. If a fraction, δ ∈ [0, 1], of the localized states is occupied by charge carriers, such that the density of carriers is δNt, then the position of the Fermi-level is fixed by the condition [8]: δ = exp

• EF

kBTDOS

• πT

TDOS sin

• π

T TDOS

. (2.3) Using a percolation model of variable range hopping, an expression for the conductivity as a function of the charge carrier occupation δ and the temperature T is derived [8]: σ (δ, T ) = σ0   δNt (TDOS/T )4 sin

• π

T TDOS

• (2α)3Bc

 

TDOS T

(2.4)

2.1 The switch-on voltage 19 where σ0 is a prefactor of the conductivity, Bc is a critical number for the onset of percola- tion, which is ≃2.8 for three-dimensional amorphous systems [17], and α−1 is an effective

• verlap parameter between localized states. To calculate the field-effect current we have

to take into account that in a field-effect transistor the charge density is not uniform. Using the gradual channel approximation (|Vg| ≫ |Vds|, where Vds is the source-drain voltage), we neglect the potential drop from source to drain electrode. To take into account that the charge-density decreases from the semiconductor-insulator interface to the bulk, we integrate over the accumulation channel: Ids = WVds L

• t

dxσ [δ(x), T] , (2.5) where L, W, and t are the length, width and thickness of the channel, respectively. From

• Eqs. 2.4 and 2.5 we obtain the following expression for the field-effect current:

Ids = WVdsǫsemiǫ0σ0 Lq

• T

2TDOS − T 2kBTDOS ǫsemiǫ0 ×   

• TDOS

T

4 sin

• π

T TDOS

• (2α)3 Bc

  

TDOS T

× ǫsemiǫ0 2kBTDOS

• Ci
• Vg − Vso
• ǫsemiǫ0

2TDOS

T

−1

(2.6) where q is the elementary charge, ǫ0 is the permittivity of vacuum, ǫsemi the relative di- electric constant of the semiconductor, and Ci is the insulator capacitance per unit area. Eq.2.6 is used to model the transfer characteristics of solution processed PTV, pentacene, and P3HT as a function of Vg and T . The four parameters σ0, α−1, TDOS, and Vso were used to model all the curves, with a value of Bc=2.8. After this initial fit, each curve was individually modeled with only Vso as variable parameter, with the other parameters fixed. From this modeling, no temperature dependence of Vso was observed. The results of the modeling are shown in Figs.2.1, 2.2, and 2.3. The fit parameters are given in Table 2.1. Good agreement is obtained for all three semiconductors. The single constant Vso for all temperatures accounts for any fixed charge in the oxide and/or at the semiconductor- insulator interface. Also, the low values obtained for Vso are realistic numbers. Because the measurement resolution in the low current regime is limited to 1-10 pA, the onset of the experimental curves in Figs.2.1, 2.2, and 2.3, seems to be shifting to more negative gate voltages with decreasing temperature. This is due to current decrease as a function

• f temperature, as a result of the thermally activated field-effect mobility (see Chapter

3) [18]. This effect does not translate in a temperature dependence of Vso. Analysis of the data with the square root technique yields an apparent threshold voltage shift with temperature of 15 V for the PTV. Eq. 2.1 gives similar results. The values obtained for the prefactor of the conductivity σ0 seem to be too high with respect to theoretical con- siderations [15]. The conductivity prefactor is discussed further in Chapters 3 and 4.

20 The switch-on voltage and the field-effect mobility

• Figure 2.1: Ids versus Vg of a PTV thin-film field-effect transistor for several
• temperatures. The solid lines are modeled using Eq.2.6. W=2 cm,

L=10 µm, Vds = −2 V . The inset shows the structure formula of PTV.

• Figure 2.2: Ids versus Vg of a pentacene thin-film field-effect transistor for

several temperatures. The solid lines are modeled using Eq.2.6. W=2 cm, L=10 µm, Vds = −2 V. The inset shows the structure formula of pentacene.

2.1 The switch-on voltage 21

• Figure 2.3: Ids versus Vg of a P3HT thin-film field-effect transistor for several
• temperatures. The solid lines are modeled using Eq.2.6. W=2.5

mm, L=10 µm, Vds = −2 V . The inset shows the structure formula

• f P3HT.
• )
• )
• .
• Figure 2.4: Fermi-level displacement as a function of temperature, calculated

using Eq.2.3 and the parameters of Table 2.1

22 The switch-on voltage and the field-effect mobility We note that, the Fermi level shifting with decreasing temperature [10] has no effect

• n Vso. The Fermi level shift, which results from the Fermi-Dirac distribution of the

charge carriers in the exponential density of states, is calculated from Eq.2.3 and is found to be ≃0.04 eV over a temperature range of 200 K (see Fig.2.4). This displacement does not result in a shift of Vso with temperature.

2.1.6 Conclusion

It was argued that the threshold voltage extracted from the transfer characteristics of dis-

• rdered organic transistors, using MOSFET theory or Eq.2.1, is only a fit parameter if

the strong inversion regime is not observed in the transfer characteristics. The use of this parameter in the physical description of organic field-effect transistors is therefore incor-

• rect. Instead, we have defined a switch-on voltage for unintentionally doped disordered
• rganic field-effect transistors as the gate voltage that has to be applied to reach the flat-

band condition. Using a disorder model of hopping in an exponential density of states, the experimental data of solution-processed PTV, pentacene and P3HT could be described with reasonable values for the switch-on voltage, which is temperature independent. The use of Vso as characterization parameter of disordered organic field-effect transistors is not limited to the model described here, but is generally applicable.

2.2 The field-effect mobility

2.2.1 Introduction

As mentioned in the previous section the transport properties in disordered organic semi- conductors are dominated by localized states. For polymeric light-emitting diodes an important consideration for device modelling is the non-uniform charge distribution in the device. As stated above, the charge distribution in a FET is also non-uniform, it de- creases from the semiconductor/insulator (S/I) interface to the bulk, and it depends on the applied Vg. Therefore, it is not trivial to assign one field-effect mobility to all carri- ers in the accumulation channel of a disordered organic field-effect transistor. Here, we calculate the variation of the charge carrier mobility through the thickness of the accumu- lation channel. It is found that only a small error is made by assuming that all carriers are moving at the semiconductor-insulator interface with the same mobility, at a given Vg. In polymeric field-effect transistors (FETs) charge carriers are induced by a gate electrode across an insulating layer. By applying a negative voltage at the gate electrode the top of the valence band bends upward closer to the Fermi level (see Fig.1.7b). This band bending gives rise to a positive accumulation layer into the semiconductor next to the interface. Applying a voltage Vds between the source and the drain contact gives rise to a current in the channel. In literature, typically this current in the linear regime of the FET is described by: Ids = WCi VgµFE Vds L , (2.7) with Ci Vg the total amount of accumulated charge carriers, Vds/L the electric field in the channel, and µFE the field-effect mobility. It is important to note that the use of the total

2.2 The field-effect mobility 23 amount of induced charge CiVg in Eq.2.7 is only valid when all charge carriers have the same mobility. In that case, the field-effect mobility is calculated from [7]: µFE = L WCi Vds ∂ Ids ∂Vg

• Vds→0

. (2.8) To check the validity of this equation for disordered organic FETs, we calculate the charge distribution in the device from Poisson’s equation. Then we use the hopping model described in the previous section to calculate a local mobility, and compare the results with the values obtained from Eq.2.8 for both P3HT and PTV field-effect transistors.

2.2.2 Calculation of the charge distribution

Again, an unintentionally doped system is considered. In the gradual channel approxima- tion the distribution of charge carriers has to be described only in the direction perpendic- ular to the S/I interface (x, see Fig.1.6). The distribution of the electric field, Fx, in the accumulation layer perpendicular to the S/I interface is given by [12]: Fx =

• 2

ǫ0ǫsemi

• V

q

• δNt
• V ′

dV ′

• 1/2

, (2.9) where V ′ is the local potential, which varies from zero far away in the semiconductor bulk to V in the accumulation channel. The potential distribution as a function of the distance from the S/I interface, x, follows from the relation: x = V0

V

dV ′ Fx (V ′) (2.10) where V0 is the surface potential of the S/I interface. From the variation of the gate- induced potential V (x) as a function of distance x the density of holes δNt(x) can be

• calculated. The induced charge per unit area Qind is related to the gate voltage as follows:

Vg = Qind Ci + Vso = ǫ0ǫsemi Fx(0) Ci + Vso, (2.11) where Fx (0) = Fx (V = V0) is the electric field at the S/I interface. By increasing the gate voltage the surface potential increases resulting in an increase of charge carrier den-

• sity. Assuming Vso=0, δNt(x) is calculated for an undoped semiconductor with Ci=15.5

nF/cm2 and ǫsemi=3. In Fig.2.5 the concentration of charge carriers as a function of dis- tance x is shown for gate voltages of Vg = −19 V and Vg = −10 V. At Vg = −19 V the charge carrier density decreases from 3.5·1019 cm−3 at the S/I interface (x = 0) to 3·1016 cm−3 at a distance of 10 nm from the S/I interface. For Vg = −10 V the total in- duced charge is about half that of Vg = −19 V . This calculation is performed using clas- sical electrostatics, and we have neglected quantum effects close to the semiconductor- insulator interface. It should be noted that the calculated charge distribution is not specific for organic semiconductors but is generally applicable to field-effect devices of undoped semiconductors in the linear operating regime, since it only depends on Ci and ǫsemi.

24 The switch-on voltage and the field-effect mobility

• δ

W

• J

J

Figure 2.5: Numerical calculated distribution of charge carriers in the ac-

cumulation channel perpendicular to the S/I interface for Vg = −19 V and Vg = −10 V, calculated for ǫsemi = 3 and Ci=15.5 nF/cm2.

2.2.3 Modeling the mobility variation

In order to take into account the charge carrier dependent mobility we use again the hop- ping model described in the previous section. From Eq.2.4 an expression for the local mobility is derived: µl = σ (δ, T ) qδNt = σ0 q   (TDOS/T)4 sin

• π

T TDOS

• (2α)3Bc

 

TDOS T

(δNt)TDOS/T −1 (2.12) Using the parameters from Table 2.1 the local mobility is calculated as a function

• f distance from the S/I interface and is plotted in Fig.2.6 for Vg = −19 V at room

temperature for PTV and P3HT. For PTV the modeled local mobility varies from 2.1·10−3 cm2/Vs at the S/I interface to 8.4·10−4 cm2/Vs at a distance of 5·10−10 m from the S/I

• interface. For P3HT the local mobility varies from 6.6·10−4 cm2/Vs at the interface to

1.2·10−4 cm2/Vs at a distance of 5·10−10 m from the interface. The calculations show that in a polymeric FET even for moderate gate voltages, variations in the local mobility are considerable. Thus, due to the inhomogeneous charge carrier density in a disordered FET the local mobility demonstrates a strong variation in the active channel.

2.2 The field-effect mobility 25

• O
• O
• Figure 2.6: The local charge carrier mobility as function of position in the

accumulation layer for (a) a PTV FET and (b) a P3HT FET at Vg = −19 V . The extracted field-effect mobility as determined from Eq.2.8 is given as the solid symbol in both figures.

26 The switch-on voltage and the field-effect mobility

2.2.4 Discussion of the field-effect mobility

For the interpretation of the charge transport in disordered FETs it is crucial to understand how such a mobility distribution compares to the experimentally extracted field-effect mobility, using Eq.2.8. In Fig.2.6 the local mobility (Eq.2.12) and the field-effect mobility (Eq.2.8) are compared. With this modeling we find that the local mobility of the charge carriers at the S/I interface at Vg = −19 V is 15% and 9% larger then the extracted field- effect mobility for PTV (Fig.2.6a) and P3HT (Fig.2.6b), respectively. The reason for this relatively small difference is that (as shown in Figs.2.5 and 2.6) not only a major part

• f the charge carriers is located close to the interface, but also that these charge carriers

have the highest mobility. As a result the field-effect current is mainly determined by the charge carriers at the interface. Consequently, the error due to the approximation used in Eq.2.8, namely that all charge carriers have the same mobility, is relatively small and amounts typically to 10-15%.

2.2.5 Conclusion

In disordered organic field-effect transistors the dependence of the mobility with gate voltage is determined by the charge carrier dependence of the local mobility. Taking into account the distribution of the charge carrier density in the active channel perpendicular to the insulator the local mobility has been calculated as a function of position in the accumulation layer. It is demonstrated that for disordered organic FETs, in spite of the strong variations in the local mobility in the active channel, the experimentally determined field-effect mobility (Eq.2.8) is a reasonable estimate for the local mobility of the charge carriers at the semiconductor-insulator interface.

2.3 Unifying the charge transport in polymeric FETs with PLEDs

2.3.1 Introduction

The experimental hole mobilities extracted from FETs can differ by three orders of mag- nitude from the hole mobilities extracted from polymeric light-emitting diodes (PLED), based on the same polymeric semiconductor. We resolve this apparent discrepancy by considering that the hole mobility depends strongly on the charge carrier density in dis-

• rdered semiconducting polymers. This is demonstrated in this section by a system-

atic study of the hole mobility as a function of temperature and applied bias in PLEDs and FETs based on poly(2-methoxy-5-(3’,7’-dimethyloctyloxy)-p-phenylene vinylene) (OC1C10-PPV) and in PLEDs and FETs based on amorphous poly(3-hexyl thiophene) (P3HT). In contrast to the variable range hopping description in an exponential density of states for FETs described in the previous section, the transport in polymeric light-emitting diodes (PLED) is typically described by hopping in a Gaussian density of states with long- range electronic correlations [16,19–21]. We unify the charge transport description in the two device geometries for both materials by demonstrating that in the energy range of

2.3 Unifying the charge transport in polymeric FETs with PLEDs 27

• Figure 2.7: Temperature dependent current density versus voltage character-

istics of a P3HT hole-only diode, with a thickness of 95 nm and active area 10 mm2. The solid lines represents the description of the space-charge limited current model, incorporating the field de- pendence of the mobility (Eq.2.13). The inset shows the molecular structure of P3HT. interest the exponential density of states, which consistently describes the FET measure- ments at high carrier densities, is a good approximation for the Gaussian density of states, which consistently describes the PLED measurements at low carrier densities.

2.3.2 Experimental results

Field-effect transistors were fabricated using P3HT and OC1C10-PPV as the semicon- ductor (For experimental details see section 2.1.2) and were modeled with Eq.2.6. The modeled parameters are given in Table 2.2. For P3HT and OC1C10-PPV we find a field- effect mobility at -19 V of respectively 6×10−4 cm2/Vs and 5×10−4 cm2/Vs. Also PLEDs of P3HT and OC1C10-PPV were fabricated and the current density, J, versus applied bias, V , is measured as a function of temperature. The J−V characteristics

• f OC1C10-PPV PLEDs can accurately be described by space-charge limited currents,

with a field- and temperature dependent mobility [19–21]. This mobility is well described by a transport model based on hopping in a correlated Gaussian disordered system [16, 22]: µh = µ∞ exp

• −

3σDOS 5kBT 2 + 0.78 σDOS kBT 3

2

− 2 qaF σDOS

• ,

(2.13) with µ∞ the mobility in the limit T → ∞, σDOS the width of the Gaussian DOS, a the intersite spacing, and F the applied electric field. The J − V characteristics of a P3HT

28 The switch-on voltage and the field-effect mobility

Table 2.2: Values obtained by modeling the transfer characteristics of

OC1C10-PPV and P3HT FETs using Eq.2.6. The P3HT data is taken from Table 2.1. TDOS represents the width of the exponential DOS, σ0 is the prefactor of the conductivity, α−1 is the effective

• verlap parameter between localized states, and Vso is the switch-
• n voltage as defined in section 2.1. Also given are the values ob-

tained using Eq.2.13 to model the J −V characteristics of the hole-

• nly diodes. The OC1C10-PPV diode data are taken from [19].

σDOS is the width of the Gaussian DOS and a is an average trans- port site separation. TDOS[K] σ0[106S/m] α−1[ ˚ A] Vso[V ] σDOS[eV] a[nm] OC1C10PPV 540 31 1.4 0.5 0.112 1.4 P3HT 425 1.6 1.6 2.5 0.098 1.7 PLED can also accurately be modeled with space-charge limited currents in combination with Eq.2.13, as is demonstrated in Fig.2.7. The modeled parameters from Eq.2.13 are given in Table 2.2. We find at low electric fields hole mobility values for P3HT and OC1C10-PPV of respectively 3×10−5 cm2/Vs and 5×10−7 cm2/Vs. These values are upto three orders of magnitude lower as compared to the mobility values obtained from the FETs.

2.3.3 The mobility - charge density relation

Experimentally, we find for both semiconductors large differences in mobility values when measured in different device geometries. Because we study amorphous polymer films, we argue that anisotropy in the transport parallel or perpendicular to the substrate surface can not be the origin of this mobility difference. Instead, we propose to investi- gate the mobility versus volume charge density relation for both type of devices, as it is well known that the charge carrier mobility in disordered organic semiconductors depends strongly on the charge carrier density. For the diode, the mobility at low electric fields and at room temperature can directly be obtained from the space-charge limited currents [19, 23]. The lowest charge carrier density, pl, in a diode, where the current is limited by space-charge, is found at the non- injecting contact and is given by [23]: pl = 3 4 ǫ0ǫsemi V ql2

• ,

(2.14) where q is the elementary charge, and l is the semiconductor layer thickness. For the FET, the experimental field-effect mobility is determined as a function of gate bias using Eq.2.8 and we calculate the volume charge carrier density, p = δNt, at the semiconduc- tor/insulator interface as a function of gate voltage, as outlined in section 2.2.

2.3 Unifying the charge transport in polymeric FETs with PLEDs 29

• µ

K

• '26
• '26
• Figure 2.8: Charge carrier mobility as a function of the hole density for

P3HT and OC1C10-PPV determined in a hole-only diode (p < 1017 cm−3) and in a field-effect transistor (p > 1017 cm−3). The dashed lines are a guide to the eye. The hole mobility versus the volume charge density for P3HT and OC1C10-PPV are given in Fig.2.8, where the values at low charge density (p < 1017 cm−3) are derived from the PLED data and the values at high charge density (p > 1017 cm−3) are derived from FET data. From Fig.2.8 we see that the mobility starts to increase rapidly with charge carrier density when the charge density is larger than a certain minimum value. Fig.2.8 shows that when measured at the same high values of volume charge carrier den- sity the field-effect mobility of OC1C10-PPV is nearly equal to the field-effect mobility of

• P3HT. Furthermore, the dependence of the field-effect mobility on charge carrier density

is stronger for OC1C10-PPV, which is likely due to the presence of stronger energetic dis-

• rder in the OC1C10-PPV as compared to P3HT, as reflected by the larger value of TDOS

for OC1C10-PPV. The strong energetic disorder explains the low mobility values reported for OC1C10-PPV based light-emitting diodes [19], which operate at relatively low carrier densities as compared to field-effect transistors. The large differences in mobility values

• btained from diodes and FETs, based on a single semiconducting polymer, are direct

results of the large difference in charge densities in these devices.

2.3.4 Unification of the LED and FET models

To further emphasize this point we compare the modeled parameters obtained from the PLED and the FET current voltage characteristics. In Fig.2.9 the obtained Gaussian den- sity of states for P3HT is plotted as a function of energy, in a semilogarithmic plot. For the total number of states per unit volume, Nt, we have used a value of 3·1020 cm−3, which

30 The switch-on voltage and the field-effect mobility

• '26
• Figure 2.9: The Gaussian DOS (dashed line), as obtained from the hole-only

diode analysis and the exponential DOS (solid line), as obtained from the field-effect transistors as a function of energy for P3HT. The exponential DOS is found to be a good approximation of the Gaussian DOS. roughly corresponds to 1/a3 (a=1.5 nm). Additionally, the exponential DOS of P3HT as

• btained from the FET characteristics is shown, which is described by the characteristic

temperature TDOS. For the charge carrier density range in which the P3HT FET operates, we find that the exponential distribution with TDOS=425 K is a good approximation of the Gaussian DOS with σDOS=0.098 eV. This same analysis holds for the obtained OC1C10- PPV data [24]. This demonstrates that the two theoretical descriptions are consistent. The mobility description at high carrier densities, which employs an exponential DOS, is an accurate representation of the mobility description at low carrier densities, which uses a Gaussian DOS.

2.3.5 Conclusion

In conclusion, the large mobility differences reported for conjugated polymers used in PLEDs and FETs have been shown to originate from the strong dependence of the mo- bility on the charge carrier density. The exponential density of states, which consistently describes the field-effect measurements, is shown to be a good approximation of the tail states of the Gaussian density of states used in the description of PLEDs.

References

[1] H. Sirringhaus, P.J. Brown, R.H. Friend, M.M. Nielsen, K. Bechgaard, B.M.W. Langeveld-Voss, A.J.H. Spiering, R.A.J. Janssen, E.W. Meijer, P.T. Herwig and D.M. de Leeuw, Nature (London) 401, 685 (1999). [2] S.F. Nelson, Y.-Y. Lin, D.J. Gundlach and T.N. Jackson, Appl. Phys. Lett. 72, 1854 (1998). [3] M. Pope and C.E. Swenberg, Electronic Processes in Organic Crystals and Polymers, Oxford University Press (1999). [4] S.M. Sze, Physics of Semiconductor Devices (Wiley, New York, 1981) [5] C.J. Drury, C.M.J. Mutsaers, C.M. Hart, M. Matters and D.M. de Leeuw, Appl. Phys.

• Lett. 73, 108, (1998).

[6] G.H. Gelinck, T.C.T. Geuns and D.M. de Leeuw, Appl. Phys. Lett. 77, 1487 (2000). [7] A.R. Brown, C.P. Jarrett, D.M. de Leeuw and M. Matters, Synth. Met. 88, 37 (1997). [8] M.C.J.M. Vissenberg and M. Matters, Phys. Rev. B, 57, 12964 (1998). [9] G. Horowitz, R. Hajlaoui and P. Delannoy, J. Phys. III, 5, 355 (1995). [10] G. Horowitz, M.E. Hajlaoui and R. Hajlaoui, J. Appl. Phys. 87, 4456 (2000). [11] G. Horowitz, R. Hajlaoui, H. Bouchriha, R. Bourguiga and M. Hajlaoui, Adv. Mater. 10, 923 (1998). [12] M. Shur, M. Hack and J.G. Shaw, J. Appl. Phys. 66, 3371 (1989). [13] N. Lustig, J. Kanicki, R. Wisnieff and J. Griffith, MRS Symp. Proc. 118, 267 (1988). [14] B-S. Bae, D-H. Cho, J-H. Lee and C. Lee, MRS Symp. Proc. 149, 271 (1989). [15] D. Emin, private communication. [16] H. B¨ assler, Phys. Stat. Sol. B 175, 15 (1993). [17] G.E. Pike and C.H. Seager, Phys. Rev. B 10, 1421 (1974).

32 References [18] E.J. Meijer, M. Matters, P.T. Herwig, D.M. de Leeuw and T.M. Klapwijk, Appl.

• Phys. Lett. 76, 3433 (2000).

[19] P.W.M. Blom, M.J.M. de Jong and J.J.M. Vleggaar, Appl. Phys. Lett. 68, 3308 (1996). [20] P.W.M. Blom, M.J.M. de Jong and M.G. van Munster, Phys. Rev. B 55, R656 (1997). [21] H. C. F. Martens, P. W. M. Blom and H. F. M. Schoo, Phys. Rev. B 61, 7489 (2000). [22] S.V. Novikov, D.H. Dunlap, V.M. Kenkre, P.E. Parris and A.V. Vannikov, Phys. Rev.

• Lett. 81, 4472 (1998).

[23] M.A. Lampert and P. Mark, Current Injection in Solids (Academic, New York, 1970). [24] C. Tanase, E.J. Meijer, P.W.M. Blom and D.M. de Leeuw, submitted.

Chapter 3 The Meyer-Neldel Rule in organic field-effect transistors

Abstract

We have measured and analyzed the temperature and gate voltage dependencies of the field-effect mobility in organic field-effect transistors. We find that the mobility prefactor increases exponentially with the activation energy in agreement with the Meyer-Neldel

• rule. This behavior is demonstrated in the mobility data of solution-processed pentacene

and poly(2,5-thienylenevinylene) and in mobility data reported in literature. Surprisingly, the characteristic Meyer-Neldel energy for all analyzed materials is close to 40 meV. Possible implications for the charge transport mechanism in these materials are discussed.

34 The Meyer-Neldel rule

3.1 Introduction

The temperature and gate voltage dependence of the charge carrier mobility, µFE, of

• rganic-based field-effect transistors (FET) have been the subject of research for some

years now [1–7]. However, the charge transport mechanisms in these organic devices are still not fully understood. Reports vary from thermally activated behavior [2, 3, 5] to temperature independent transport [4]. Moreover, large variations in the experimental data on even nominally the same samples make it difficult to obtain an accurate picture of the transport mechanism [4, 6]. Band-like transport in extended states has been reported in the past for high-purity single crystals [1]. It should be noted that in contrast to this highly orderded system the carrier transport in disordered or partially ordered systems is governed by localized states, which results in a different transport mechanism. The temperature and gate voltage dependencies of µFE in organic FETs have been described in terms of multiple trapping [3], hopping [7] and Coulomb blockade [5]. A common factor in all these models is the gate voltage dependence of the activation energy, Ea. Whenever a property, say X, has a thermally activated behavior, X = X0 exp −Ea kBT

• ,

(3.1) and Ea is a variable, it is empirically found [8] that the prefactor, X0, increases exponen- tially with the activation energy: X0 = X00 exp Ea EMN

• .

(3.2) This relation between the prefactor X0 and Ea is known as the Meyer-Neldel rule (MNR) [8]. Here kB is the Boltzman constant, T the absolute temperature, X00 is a constant prefactor and EMN is the so-called Meyer-Neldel energy. A combination of Eqs. 3.1 and 3.2 gives the general form: X = X00 exp

• −Ea

1 kBT − 1 EMN

• ,

(3.3) which implies a single crossing point for different activation energies at an isokinetic temperature determined by the Meyer-Neldel energy: T0 = EMN/kB. The MNR has been

• bserved in a wide variety of physical, chemical and biological processes [9]. However,

the microscopic origin of the MNR and therefore, the physical meaning of EMN, are still a topic of discussion in literature [9]. In this work we demonstrate that the MNR applies to the field-effect mobility data

• f solution-processed pentacene and poly(2,5-thienylene vinylene) (PTV) FETs, as well

as to mobility data reported in literature. We discuss the relation between the transport mechanism in organic FETs and the possible origin of the MNR.

3.2 Experimental

In the experiments we used heavily doped Si wafers as the gate electrode, with a 200 nm thick layer of thermally oxidized SiO2 as the gate-insulating layer. Using conven- tional lithography, gold source and drain contacts were defined with a channel width

3.3 Demonstration of the MNR 35

• G

V

• J

Figure 3.1: Ids vs. Vg for pentacene (Vds=-2 V (open squares) and -20 V (filled

squares)) and PTV (Vds=-2 V (open circles) and -40 V (filled cir- cles)) at 290 K in vacuum (10−7 mbar). Vg was swept from +2 to

• 30 V and back to +2 V.

W=2 cm and length L=10 µm. The SiO2 layer was treated with the primer hexamethyl- disilazane (HMDS) to make the surface hydrophobic. The films of both pentacene and PTV were deposited using a precursor-route process [2,10,11]. The obtained PTV films are truly amorphous [12]. The pentacene films are polycrystalline with a planar spacing

• f 14.3 ± 0.1 ˚

A [12], which corresponds to the bulk triclinic phase of pentacene [13,14]. The samples were measured under high vacuum (10−7 mbar) in an Oxford optistat CF-V flow cryostat, using a Hewlett-Packard 4156A semiconductor parameter analyzer. The source-drain current, Ids, is plotted as a function of gate voltage, Vg, in Fig. 3.1 for both the pentacene and PTV transistors at 290 K. We do not observe any hysteresis in the measurements and the curves are stable in time (in vacuum).

3.3 Demonstration of the MNR

The temperature dependence of µFE is evaluated in the linear regime [2] of the Ids − Vds characteristics (at a low source-drain voltage Vds = −2 V). The applied gate field in this regime is much larger than the in-plane drift field, which results in an approximately uniform density of charge carriers in the active channel. We calculate µFE [2,3] from µFE = L WCi Vds ∂ Ids ∂Vg , (3.4) where Ci is the capacitance of the insulator per unit area. In Fig. 3.2 we plot µFE vs. T −1 [15] for pentacene.

36 The Meyer-Neldel rule

• J

J J J J

• )

(

• D
• J

Figure 3.2: Temperature dependence of the field-effect mobility of pentacene.

The inset shows the dependence of the activation energy on the gate voltage.

• J

J J J J J

• )

(

• D
• J

Figure 3.3: Temperature dependence of the field-effect mobility of PTV. The in-

set shows the dependence of the activation energy on the gate volt- age.

3.4 Implications for the charge transport 37

• D

Figure 3.4: extrapolated prefactor, µ0, as a function of Ea for pentacene(filled

squares) and PTV(open circles). We can fit the data [22] in Figs. 3.2 and 3.3 with µFE = µ0 exp −Ea kBT

• (3.5)

and upon extrapolation to high T we clearly find a common crossing point of the curves. Plotting the prefactor, µ0, logarithmically as a function of Ea for both pentacene and PTV (see Fig. 3.4), results in a straight line. This demonstrates that the experimental results are in agreement with the MNR. From Fig. 3.4 we find EMN ≈ 38 meV and EMN ≈ 42 meV for pentacene and PTV respectively. When this analysis is also used

• n the gate voltage dependent mobility data reported in literature, we find that the MNR

also holds for the mobility data on dihexyl-sexithiophene measured by Horowitz et al. [3] (EMN ≈ 43 meV) and for the previous studies of pentacene (EMN ≈ 34 meV) and PTV (EMN ≈ 35 meV) [2]. Furthermore, for C60 FETs it was found that EMN ≈ 36 meV [23,24]. Surprisingly, for all these materials the value of EMN is close to 40 meV.

3.4 Implications for the charge transport

Now we will discuss the relation between the transport mechanism and the origin of the

• MNR. In inorganic amorphous semiconductors the MNR has been attributed to a dis-

placement of the Fermi level in an exponential density of states (DOS) [16, 17, 25, 26]. A consequence of this interpretation is, that in order to calculate a physically reasonable value for the prefactor (of conductivity or mobility) one has to assume physically unrea- sonable values for the attempt frequency for hopping, which can vary between 103 and

38 The Meyer-Neldel rule 1028 s−1 [27–31]. Furthermore, Yelon et al. [29, 30] have argued that the MNR can not be solely due to an exponential DOS, as the MNR is much more generally applicable. They attribute the difficulty of interpreting the prefactor values in the DOS model to the assumption that the excitation process involves only one phonon [32,33]. If the activation energy is large compared to the typical phonon energies available, multiple excitations are required for a hopping event to occur. They show that a multiphonon process can explain the MNR and that the large spread in values of the prefactor can be accounted for with reasonable values for the attempt frequencies [29,30]. In organic FETs, the interpretation of the MNR in terms of a Fermi level shift would be consistent with the hopping [7] and multiple trapping models [3]. In that case EMN is equal to the width of the DOS [27] (the TDOS parameter from Chapter 2) and no physical meaning is attributed to the prefactor µ0. We argue that the ubiquitous value of EMN is more likely due to a characteristic transport mechanism in organic materials rather than to one general DOS. Whether the interpretation given by Yelon et al. [29, 30, 34] is applicable to our results on organic FETs hinges on the question whether it is justified to describe the hopping of a charge carrier from one conjugated segment (or molecule) to the next, as a multiphonon process. We note, that if the charge carrier is a localized polaron [35,36], Emin argued that the transport should be a multiphonon process, even if the phonon energies are comparable to Ea [37,38]. This interpretation would imply that the observed MNR is a direct consequence of the polaronic nature of the charge carriers.

3.5 Conclusions

In summary, we have shown the validity of the MNR in organic-based FETs. The ubiqui- tous value of EMN ≈ 40 meV is an indication of a common origin of the MNR in organic

• FETs. We have argued that the MNR is directly linked to the charge transport mechanism

and possibly even to polaronic carriers.

References

[1] M. Pope and C. E. Swenberg, “Electronic processes in organic crystals”, Oxford Uni- versity Press, New York, (1982). [2] A. R. Brown, C. P. Jarrett, D. M. de Leeuw and M. Matters, Synth. Metals 88, 37 (1997). [3] G. Horowitz, R. Hajlaoui and P. Delannoy, J. Phys III France 5, 355 (1995). [4] S. F. Nelson, Y.-Y. Lin, D. J. Gundlach and T. N. Jackson, Appl. Phys. Lett. 72, 1854 (1998). [5] W. A. Schoonveld, J. Wildeman, D. Fichou, P. A. Bobbert, B. J. van Wees and T. M. Klapwijk, to be published in Nature. [6] L. Torsi, A. Dodabalapur, L. J. Rothberg, A. W. P. Fung and H. E. Katz, Phys. Rev. B 57, 2271 (1998). [7] M. C. J. M. Vissenberg and M. Matters, Phys. Rev. B 57, 12964 (1998). [8] W. Meyer and H. Neldel, Z. Tech. Phys. 18, 588 (1937). [9] For examples see references in [29,30] [10] A. R. Brown, A. Pomp, D. M. de Leeuw, D. B. M. Klaassen, E. E. Havinga, P. T. Herwig, K. M¨ ullen, J. Appl. Phys. 79, 2136 (1996). [11] P. T. Herwig and K. M¨ ullen, Adv. Mater. 11, 480 (1999). [12] A. R. Schlatmann, E. J. Meijer and D. M. de Leeuw, unpublished X-ray diffraction results. [13] R. B. Campbell and J. M. Robertson, Acta Cryst. 14, 705 (1961). [14] I. P. M. Bouchoms, W. A. Schoonveld, J. Vrijmoeth and T. M. Klapwijk, Synth. Met. 104, 177 (1999). [15] We note that the MNR in inorganic FETs is sometimes evaluated by analyzing the sheet conductance of the channel [16,17]. However, the extraction of EMN, using the sheet conductance, may be inaccurate due to a decrease of the effective accumulation channel thickness with increasing Vg [18, 19]. We therefore look at µFE instead, as suggested by Fortunato and co-workers [20,21].

40 References [16] R. Schumacher, P. Thomas, K. Weber and W. Fuhs, Sol. State. Comm. 62, 15 (1987). [17] R. Schumacher, P. Thomas, K. Weber, W. Fuhs, F. Djamdji, P. G. Le Comber and R.

• E. I. Schropp, Phil. Mag. B 58, 389 (1988).

[18] M. Yamaguchi and H. Fritsche, J. Appl. Phys. 56, 2303 (1984). [19] A. P. Gn¨ adinger and H. E. Talley, Proc. IEEE, 916 (1970). [20] G. Fortunato, L. Mariucci and C. Reita in “Amorphous and Microcrystalline Semi- conductor Devices” Volume 2, 355, Editor: J. Kanicki, Artech House, Norwood (1992). [21] G. Fortunato, D. B. Meakin, P. Migliorato and P. G. Le Comber, Phil. Mag. B, 57, 573 (1988). [22] We note that the mobility curves of the PTV show a deviation from simple Arrhenius behavior at low temperatures. This behavior has thus far been modelled with a specific distribution of traps [3] and hopping in an exponential density of states [7]. [23] J. Paloheimo and H. Isotalo, Synth. Met. 55, 3185 (1993). [24] J. C. Wang and Y. F. Chen, Appl. Phys. Let. 73, 948 (1998). [25] H. Overhof, J. Non-Cryst. Sol. 97/98, 539 (1987). [26] M. Ortuno and M. Pollak, Phil. Mag. B 47, L93 (1983). [27] P. Irsigler, D. Wagner and D.J. Dunstan, J. Phys. C 16, 6605 (1983). [28] B. Movaghar, J. Phys. Coll. C4, 73 (1981). [29] A. Yelon and B. Movaghar, Phys. Rev. Lett. 65, 618 (1990). [30] A. Yelon, B. Movaghar and H. M. Branz, Phys. Rev. B 46, 12244 (1992). [31] A. M. Szpilka and P. Vis˘ cor, Phil. Mag. B 45, 485 (1982). [32] A. Miller and E. Abrahams, Phys. Rev. 120, 745 (1960). [33] N. F. Mott and E. A. Davis “Electronic Processes in Non-Crystalline Materials”, second edition, Clarendon Press Oxford (1979). [34] H. M. Branz, A. Yelon and B. Movaghar, MRS Symp. Proc. 336, 159 (1994). [35] T. Holstein, Ann. Phys. 8, 325 (1959). [36] A. J. Heeger, S. Kivelson, J. R. Schrieffer and W.-P. Su, Rev. Mod. Phys. 60, 781 (1988). [37] D. Emin, Phys. Rev. Lett. 32, 303 (1974). [38] D. Emin, Adv. Phys. 24, 305 (1975).

Chapter 4 The isokinetic temperature in disordered organic semiconductors

Abstract

We have investigated the field dependence of the in-plane conductivity in poly(2,5-thieny- lene vinylene) and poly(3-hexyl thiophene) thin films. The conductivity is found to have a square root dependence on the lateral electric field. The values for the characteristic tem- perature, obtained from the empirical field-dependent mobility relation are very similar to the values found for the isokinetic temperature in Meyer-Neldel experiments on poly(2,5- thienylene vinylene) and poly(3-hexyl thiophene) field-effect transistors. The possible relation between the field-dependent mobility and the Meyer-Neldel rule is discussed in the context of charge transport in disordered organic semiconductors.

42 The isokinetic temperature

4.1 Introduction

Due to possible industrial applications, opto-electronic devices based on disordered or- ganic semiconducting layers are receiving much attention [1–4]. The disorder in the or- ganic films dominates the charge transport. Typically, low mobilities with a thermally activated behavior are observed. Transport is mostly described by hopping. Several inter- esting physical features are associated with transport through disordered materials, such as the Meyer-Neldel rule (MNR) [5], which we demonstrated in the previous chapter. This rule states that the prefactor of the thermally activated mobility increases exponen- tially with the activation energy. For a number of materials we found the characteristic isokinetic temperature associated with the MNR to be in the range of 440-510 K [6]. At high electric fields in disordered systems the mobility becomes field dependent, which can be described by the empirical relation [7,8]: µ = µ0 exp − kBT + γ √ F

• (4.1)

with, γ = B 1 kBT − 1 kBT0

• ,

(4.2) where kB is Boltzmann’s constant, T the absolute temperature, F the applied electric field and the low field activation energy. The field dependence as given in Eq.4.1 is

• bserved in a wide range of disordered materials, with typical values for the parameters
• f =0.5 eV, T0 ∼500-600 K and B=3·10−5eV(m/V)1/2 [9–11]. It has been suggested

that the Meyer-Neldel rule and the field dependent mobility are related effects [11,12]. To investigate this hypothesis we have studied both effects in two organic semiconductors, poly(2,5-thienylene vinylene) (PTV), and poly(3-hexyl thiophene) (P3HT).

4.2 Results

The in-plane conductivity, σ, was measured using interdigitated gold contacts on glass and the MNR using regular MISFET structures, as described in Chapter 2. X-ray ex- periments on the PTV films did not yield reflections [6], indicating that the PTV films are amorphous. The P3HT films are nanocrystalline. In Figs.4.1 and 4.2 σ is plotted as a function of F1/2 for various temperatures. We do not observe space-charge limited currents in the range of electric fields shown in Figs.4.1 and 4.2. At low fields ohmic behavior is observed in Fig.4.2. Figs.4.3 and 4.2 show the temperature dependence of σ for different electric fields. The data are well described by Eq.4.1 [13] and the values of , B, and T0 fitted for both PTV and P3HT are given in Table.4.1. The obtained values are in agreement with data published for other materials [9–11]. Fig.3.3 shows the field-effect mobility for a PTV transistor as a function of T −1 for different gate voltages. The same is plotted for P3HT in Fig.4.5. The curves are fitted to Arrhenius behavior with a gate voltage dependent prefactor and activation energy, as was outlined in the previous chapter.

4.2 Results 43

• 6

Q

• Figure 4.1: In-plane conductivity of PTV as a function of F1/2 for various tem-
• peratures. L=2 µm, W=2 cm. The lines are fits to Eq.4.1. The

inset shows the structure formula of PTV.

• 6

&+ Q

• Figure 4.2: Measured in-plane conductivity of P3HT as a function of F1/2 for

various temperatures. L=5 µm, W= 50cm. The lines are fits to Eq.4.1. The inset shows the structure formula of P3HT.

44 The isokinetic temperature

• Figure 4.3: In-plane conductivity of PTV as a function of T −1 for various elec-

tric fields. The lines are Arrhenius fits. The intersection of the fitted curves at high temperatures demonstrates the MNR [5,6] with a characteristic isokinetic temperature of 4.9·102 K for PTV and 4.6·102 K for

• P3HT. Interestingly, the characteristic temperature of the MNR in PTV and P3HT is close

to these values. This indicates that the origin of the T0 in the field dependence (Figs.4.3 and Fig.4.4) could be the same as the isokinetic temperature observed in the Meyer-Neldel rule (Fig.3.3 and Fig.4.5) [11,12].

Table 4.1: Values obtained by fitting the field dependence of the conductivity

σ with Eq.4.1 for PTV and P3HT. is the low field activation en- ergy, B is the field dependent coefficient as given in Eq. 4.2, T0,σ is the isokinetic temperature determined from the field-dependence data of Figs.4.3 and 4.4, and T0,FE is the isokinetic temperature determined from the field-effect experiments of Figs.3.3 and 4.5. [eV] B[eV(m/V)1/2] T0,σ[K] T0,FE[K] PTV 0.46 2.3 · 10−5 5.2 · 102 4.9 · 102 P3HT 0.40 4.7 · 10−5 5.3 · 102 4.6. · 102

4.2 Results 45

• Figure 4.4: In-plane conductivity of P3HT as a function of T −1 for various

electric fields. The lines are Arrhenius fits.

• )

(

• (

D

• >

H 9 @ 9J>9@

Figure 4.5: Field effect mobility of a P3HT transistor as a function of T −1 for

different gate voltages. The lines are Arrhenius fits.The inset shows the activation energy as a function of the gate voltage.

46 The isokinetic temperature

4.3 Discussion of the isokinetic temperature

An investigation into a possible relation between the isokinetic temperature observed in the Meyer-Neldel experiments (see the previous chapter) and in the field-dependence of the mobility, requires a careful examination of the empirical relations from which they

• riginate, Eq.3.3 and Eq.4.1, and the underlying physical mechanisms.

4.3.1 The field-dependent mobility

Although Eq.4.1 gives a good description of the mobility as a function of temperature and electric field, it still lacks theoretical justification. Other approaches to understand the ln(µ) ∼ √ F relation have been proposed. B¨ assler argued that the observed field dependence of the mobility is related to the intrinsic charge transport in disordered materials [14]. The disorder model developed by B¨ assler and coworkers [14, 15] is based on a Gaussian distribution of localized states, and considers that hopping between sites is subject to both energetic and spatial disorder. Monte Carlo simulations of hopping transport in a Gaussian distribution of localized states has suggested that the field dependence of the mobility is a natural consequence of the presence of disorder [14]. There is general agreement that the field-dependency observed at fields as low as 1 MV/m is related to the presence of correlations in site-energies in the disordered material, and can be described by [16]: µ = µ∞ exp

• −

3σDOS 5kBT 2 + 0.78 σDOS kBT 3

2

− 2 qaF σDOS

• ,

(4.3) with µ∞ the mobility in the limit T → ∞, σDOS the width of the Gaussian DOS, and a the intersite spacing. Various physical mechanisms for the origin of these correlations in the Gaussian model have been suggested. Yu et al. considered molecular geometry fluctuations such as phenylene-ring torsion out of the plane of the molecule, which will influence the steric energy of neighbouring molecules, and results in long-range energy correlations between molecular sites [17]. Rakhmanova and Conwell considered struc- tural disorder, where the morphology of the polymer is crucial, and the long-range cor- relations between sites arise due to local variations in order. Sites in ordered regions are suggested to have lower site energies than those in amorphous regions [18], which again will result in long-range energy correlations. Similar modeling based on the argument

• f structural disorder was given by Vissenberg [19]. In principle any model of Gaussian

disorder that incorporates long-range energy correlations between sites will give rise to a √ F dependence of ln(µ). In addition to the different physical interpretations of the field-dependent mobility, the Gaussian disorder model and the empirical relation of Eq.4.1 also predict that the temperature dependence of the mobility should be distinct from each other. The zero field mobility is expected to vary as µ = µ0 exp(−/kBT) from Eq.4.1, whereas it should follow a much stronger variation as µ = µ0 exp(−[2σDOS/(3kBT )]2) from Eq.4.3. An evaluation of the µF=0 as a function of temperature should therefore allow the applica- bility of the two models to be tested. Unfortunately, in polymer systems typically only a

4.4 Conclusions 47 small temperature range can be scanned experimentally, and therefore a clear distinction between ln(µ) ∼ T −1 and ln(µ) ∼ T −2 cannot be made.

4.3.2 The Meyer-Neldel rule

As stated in the previous chapter, the Meyer-Neldel rule is also an empirical relation, the physical justification of which is still heavily under debate. The discussion of the origin

• f the MNR has focussed on multi-phonon hopping rates. Entropy change associated with

these hopping rates would lead to the MNR [20,21](see the previous chapter).

4.3.3 Comparison of the MNR with the field-dependent mobility

It is instructive to compare the two physical pictures that emerge from the discussion around the isokinetic temperature. In the first picture, it is the environment in which the charge carriers move that is important: a manifold of localized states in a Gaussian energy distribution with long- range correlations [16]. The nature of the charge carriers themselves, or the specifics

• f the charge transfer from one localized state to the next, is taken as simple as possi-
• ble. Mostly Miller-Abrahams hopping rates [22] are considered, a single-phonon assisted

transfer process, for holes and electrons. In the second picture, the nature of the carriers and the charge transfer description is

• f importance and the long-range environment is not: polaronic carriers, quasi particles

that consist of charge accompanied by a lattice deformation, that require multiple phonons from their surroundings to move from a localized site to the next [21,23]. This requires a much more complex mathematical treatment of the site to site hopping process.

4.4 Conclusions

The fact that polarons are the charge carrying species in these polymeric systems and that disorder is tantamount for the description of charge transport, would suggest that a combination of the two interpretations arising from the discussion of the isokinetic temperature would give a more complete description of charge transport. This then results in a charge transport picture of polaronic charge carriers, that move with a multi-phonon transfer rate between localized states, which have a Gaussian energy distribution and long- range energy correlations.

References

[1] C.J. Drury, C.M.J. Mutsaers, C.M. Hart, M. Matters and D.M. de Leeuw, Appl. Phys.

• Lett. 73, 108 (1998).

[2] A.R. Brown, C.P. Jarrett, D.M. de Leeuw and M. Matters, Synth. Met. 88, 37 (1998). [3] H. Sirringhaus, P.J. Brown, R.H. Friend, M. Nielsen, K. Bechgaard, B. Langeveld- Voss, A. Spiering, R.A.J. Janssen, E.W. Meijer, P.T. Herwig and D.M. de Leeuw, Nature 401, 685 (1999). [4] Z. Bao, A. Dodabalapur and A.J. Lovinger, Appl. Phys. Lett. 69, 4108 (1998). [5] W. Meyer and H. Neldel, Z. Techn. Phys. (Leipzig), 18, 588 (1937). [6] E.J. Meijer, M. Matters, P.T. Herwig, D.M. de Leeuw and T.M. Klapwijk, Appl. Phys.

• Lett. 76, 3433 (2000).

[7] D.M. Pai, J. Chem. Phys. 52, 2285 (1970). [8] W.D. Gill, J. Appl. Phys. 43, 5033 (1972). [9] P.W.M. Blom and M.C.J.M. Vissenberg, Mater. Scie and Engin. R27, 53 (2000) and references therein. [10] P.W.M. Blom, M.J.M. de Jong and M.G. van Munster, Phys. Rev. B 55, R656 (1997). [11] D.B.A. Rep, B.-H. Huisman, E.J. Meijer, P. Prins and T.M. Klapwijk, Mat. Res. Soc.

• Symp. Proc. 660 (2001).

[12] A. Peled and L. Schein, Phys. Scripta 44, 304 (1991). [13] We assume that the field and temperature dependence of σ are due to the mobility and not the charge density. [14] H. B¨ assler, Phys. Stat. Sol. B 175, 15 (1993). [15] M. Abkowitz, H. B¨ assler and M. Stolka, Phil. Mag. B 63, 201 (1991). [16] S.V. Novikov, D.H. Dunlap, V. Kenkre, P. Parris and A. Vannikov, Phys. Rev. Lett. 81, 4472 (1998).

50 References [17] Z.G. Yu, D.L. Smith, A. Saxena, R.L. Martin and A.R. Bishop, Phys. Rev. Lett. 84, 721 (2000). [18] S. V. Rakhmanova and E.M. Conwell, Appl. Phys. Lett. 76, 3822 (2000). [19] P.W.M. Blom and M.C.J.M. Vissenberg, Mater. Scie. Eng. R27, 53 (2000). [20] A. Yelon and B. Movaghar, Phys. Rev. Lett. 65, 618 (1990). [21] D. Emin, Phys. Rev. B 61, 14543 (2000). [22] A. Miller and E. Abrahams, Phys. Rev. 120, 745 (1960). [23] T. Holstein, Ann. Phys. 8, 325 (1959).

Chapter 5 Scaling behavior and parasitic series resistance in disordered organic field-effect transistors

Abstract

The scaling behavior of the transfer characteristics of solution-processed disordered or- ganic field-effect transistors with channel length is investigated. This is done for a variety

• f organic semiconductors in combination with gold injecting electrodes. From the chan-

nel length dependence of the transistor resistance in the conducting ON-state we deter- mine the field-effect mobility and the parasitic series resistance. The extracted parasitic resistance, typically in the M-range, depends on the applied gate voltage, and we find experimentally that the parasitic resistance decreases with increasing field-effect mobility.

52 Scaling behavior and parasitic series resistance

5.1 Introduction

The interest in organic field-effect transistors has grown rapidly due to envisaged ap- plications such as integrated circuits [1] and active-matrix displays [2]. Research has mainly been focused on improving the field-effect mobility, µFE [3–7], which is known to depend on material purity and processing conditions. For transistors based on solution- processed organic semiconductors µFE typically ranges between 10−4 and 10−1 cm2/Vs. The switching speed of organic integrated circuits can be estimated from the performance

• f the individual transistors and is roughly proportional to ∼ µFE/L2 [8], where L is the

channel length of the transistor. To reach higher switching speeds, the search for higher mobility materials is therefore important, but it is also of great interest to downsize the transistor geometries. In this work the scaling behavior of the transfer characteristics with transistor channel length is investigated for a variety of solution-processable organic field-effect transistors.

5.2 Experimental

In the experiments we use heavily doped Si wafers as the gate electrode, with a 200-nm- thick layer of thermally oxidized SiO2 as the gate-insulating layer. Using conventional lithography, gold source and drain contacts of 100 nm thick are defined with channel widths ranging from 1 mm to 1 cm and channel lengths between 0.75 and 40 µm. The structures typically have an underetch of 0.5 µm, which we neglect in the following anal-

• ysis. A 10 nm layer of titanium acts as an adhesion layer for the gold on the SiO2.

The SiO2 layer is treated with the primer hexamethyldisilazane to make the surface hy-

• drophobic. No special care is taken to clean the gold surface prior to deposition of the
• semiconductor. Poly(2,5-thienylene vinylene) (PTV) films as semiconductor layer are

deposited using a precursor-route process [8]. We systematically varied the processing conditions for the conversion from precursor to PTV and we determined the average de- gree of conversion using the method described by Fuchigami et al. [9]. This enabled us to systematically study PTV transistors at various degrees of conversion ranging from 60% to 100%, and consequently over a range of field-effect mobilities, between 10−4 and 10−3 cm2/Vs. Poly(3-hexyl thiophene) (P3HT) is spincoated from a 1 wt% chlo- roform solution [7]. Films of poly([2-methoxy-5-(3’,7’-dimethyloctyloxy)]-p-phenylene vinylene) (OC1C10-PPV) and poly([2,5-di-(3’,7’-dimethyloctyloxy)]-p-phenylene viny- lene) (OC10C10-PPV), are spun from a 0.5 wt% toluene solution. Pentacene thin films are deposited using a precursor-route process [4, 8]. The measurements are performed

• n freshly prepared samples in order to minimize external doping and degradation ef-

fects [10]. The PTV, OC1C10-PPV, OC10C10-PPV and pentacene samples are measured in air, whereas the P3HT samples are measured in vacuum and dark after a thermal dedope procedure [11]. The electrical characteristics are recorded using an HP4155B semicon- ductor parameter analyzer.

5.3 Scaling behavior 53

5.3 Scaling behavior

Typical source drain current, Ids, versus gate voltage, Vg, characteristics for solu-tion- processed PTV and P3HT are shown in Fig.5.1, for different channel lengths, where the channel width, W, is kept constant. The position of the switch-on voltage, Vso (see Chap- ter 2) [12], which determines the onset of the field-effect and is defined as the flat-band condition of the transistor, does not vary much between the transistors with different chan- nel length. At low source-drain voltage, Vds = −2 V , where the in plane electric field is much smaller than the applied gate field (gradual channel approximation) [8], the field- effect mobility is evaluated using: µFE

• Vg
• =

L WCi Vds ∂ Ids ∂Vg , (5.1) where Ci is the capacitance of the insulating layer per unit area. The field-effect mobilities for both PTV and P3HT are found to depend on the chan- nel length of the transistor, which can be seen from the insets of Fig.5.1. This means that the extracted µFE is a device parameter rather than a material parameter of the organic

• semiconductor. By comparing the output characteristics multiplied by the channel length,

i.e. at constant source-drain field, for short- and long channel transistors we clearly see an effective current decrease for shorter channel lengths, which is demonstrated for P3HT in Fig.5.2. Furthermore, at low drain voltages the output characteristics of the long tran- sistor show ohmic behavior [13], whereas for the short channel transistors, at low drain voltages, superlinear output characteristics are observed. Because µFE is decreasing with reduced L, the reduction of the channel length will not result in the expected increase of the switching speed in circuits. From amorphous silicon thin-film transistors it is well known that the presence of source and drain parasitic resistances, Rs and Rd respectively, can give rise to an apparent µFE that decreases with decreasing channel length [14,15]. This is due to the fact that in shorter channels, a relatively larger fraction of the applied source-drain voltage drops

• ver the parasitic resistance, as compared with the long channel transistors. To be able

to evaluate the performance of the organic semiconductor, a correction for the parasitic resistance, Rp = Rs + Rd, is required [16,17]. A theoretical approach to this end was presented by Horowitz et al. [18]. Experimentally, it has been demonstrated that the influence of Rp can be reduced by modifying the interface between the current injecting contacts and the organic semiconductor [19, 20]. Kelvin probe force microscopy has been employed for experimental evaluation of Rp [13]. Here, we investigate the scaling behavior of the transistor current [15,21,22] to estimate Rp.

5.4 Parasitic series resistance determination

We plot the total device-resistance, RON = Vds/Ids, as a function of the nominal channel length, L, for different gate voltages in Fig.5.3. In the linear operating regime of the transistor the channel resistance varies linearly with the channel length. The parasitic resistance, Rp = Rs + Rd, at the source and drain contacts is assumed to be independent

54 Scaling behavior and parasitic series resistance

• G

V

• J

/ µP µP µP µP

µ)(FRUUHFWHGIRU5S

µP µP µP µP

• µ

) (

• J
• G

V

• J

/ µP µP µP µP µP

µ)(FRUUHFWHGIRU5S

• µ

) (

• J

µP µP µP µP µP

Figure 5.1: Ids vs Vg at Vds = −2 V , for different channel lengths for (a) PTV,

converted at 80oC under 150 mbar of HCl partial pressure [8]. The characteristics are measured in air at room temperature, W=1 mm. (b) P3HT, W=1 mm, in vacuum at room temperature after a thermal dedoping treatment [11]. The insets show the corresponding µFE- values derived from the gatesweeps by using Eq.5.1.

5.4 Parasitic series resistance determination 55

• J

J J J J

• G

V

• GV

Figure 5.2: The normalized output characteristics for two P3HT transistors

with L=0.75 µm (closed circles) and L=40 µm (open squares). Clearly the current in the short transistor is more dominated by the parasitic series resistance as compared to the long transistor.

• f L. The RON can then be expressed as [15]:

RON (L) = ∂Vds ∂ Ids

• Vds→0,Vg

= Rch (L) + Rp. (5.2) The experimental data are, in first order, well described by this equation, with RON de- pending linearly on L (see Fig.5.3). From the slopes of the plots in Fig.5.3 we find the channel resistance, Rch, the inverse

• f which, [RON/L]−1, is the channel conductivity. From the derivative of the channel

conductivity, the field-effect mobility, corrected for Rp can be obtained: ∂

• RON

L

−1 ∂Vg = µFE

• Vg
• WCi

(5.3) The resulting corrected mobilities are plotted in the insets of figure 5.1. The corrected curve yields a higher overall µFE

• Vg
• . From the insets of Fig.5.1b it is clear that for the

40 µm-channel, the influence of Rp is small, as µFE obtained from Eq.5.1 is close to the corrected mobility. We note that any non-linearity of µFE with Vg in our samples cannot a priori be attributed to the presence of an Rp [18], but is more probably the result of a specific density of states in the semiconductor at the semiconductor/insulator interface (see Chapter 2). From the analysis with Eq.5.2 we find Rp in Fig.5.3 as the intercept

56 Scaling behavior and parasitic series resistance

• 2

1

Ω µ

9J 9 9J 9 9J 9 9J 9 9J 9

• 2

1

Ω µ

9J 9 9J 9 9J 9 9J 9 9J 9

Figure 5.3: Total device resistance RON, calculated with Eq.5.2 from the data

in Fig.5.1, as a function of the mask channel length for various gate voltages, for (a) PTV, (b) P3HT.

5.4 Parasitic series resistance determination 57

• 379PEDU+&O

FRQYHUVLRQGHJUHHYDULHG EHWZHHQDQG 379PEDU+&O FRQYHUVLRQGHJUHHYDULHG EHWZHHQDQG 379PEDU+&O FRQYHUVLRQGHJUHHYDULHG EHWZHHQDQG 2&&339 2&&339 3+7 3+7IURPUHI>@ SUHFXUVRUSHQWDFHQH

• S
• )(
• Figure 5.4: The parasitic resistance times the channel width as a function of

the effective field-effect mobility for a number of polymeric semi- conductors and pentacene.

• f RON at L = 0. This Rp, typically on the order of M for our devices, is found to

decrease with increasing gate voltage, i.e. with increasing carrier density. From the analysis with Eq.5.2 we can find Rp in Fig.5.3 as the intercept of RON at L = 0. This intercept depends on the applied gate voltage, which implies that Rp decreases with increasing gate voltage, i.e. with increasing carrier density. We find experimentally that both the field-effect mobility and the parasitic resistance depend on Vg. In Fig.5.4 we plot the experimentally determined Rp, multiplied with the channel width, and effective µFE obtained from the scaling analysis of several organic

• semiconductors. Also data from a P3HT study of Sirringhaus et al. [16] is included.

An empirical relation is observed between the charge carrier mobility in the polymeric semiconductor and the parasitic resistance for the polymeric semiconductor in contact with the gold/titanium stack. A possible reason for this empirical observation is that the density of localized states in the polymer is of importance for the charge injection

• efficiency. The dependence of Rp on µFE is probed experimentally by accumulating

charge in the semiconductor, by means of the field-effect, where we change the position

• f the Fermi level in the density of states. Why this results in a very similar dependence
• f µFE on Rp for different polymeric semiconductors is unclear at present. In literature

it has been demonstrated that injection-limited current into a disordered polymer can be described by thermally assisted hopping from the electrode into the localized states of the polymer, which are broadened due to disorder [23]. As a representation of this effect, the Rp for injection into a semiconducting polymer is found to depend on the charge carrier mobility in the polymer [24]. For the molecular semiconductor pentacene the data is more scattered and does not follow the trend observed for the polymeric transistors (see Fig.5.4). We attribute this to the polycrystalline nature of the pentacene films, which

58 Scaling behavior and parasitic series resistance depends on the processing conditions. The origin of the observed parasitic series resistance, or injection-limited current, can be due to a combination of effects. In general, geometrical or morphological contact problems between the semiconductor film and the gold contacts can be of importance, which is indicated by the scattered pentacene data. However, the data for the polymers are much more consistent and suggest that the injection barrier is related to material parame- ters of electrode and semiconductor layer rather than processing variations. A mismatch between the workfunction of the gold, at 5.1 eV, and the highest occupied molecular or- bital (HOMO) level of the semiconductors (for the materials used here: around 5.2 eV) would lead to an injection barrier for holes. The width of this injection barrier can be nar- rowed by accumulating charge in the semiconductor, through the field-effect, by applying a Vg [25]. For a small barrier height, in the order of the thermal energy kBT, this will result in an ohmic parasitic resistance, whereas for higher barrier heights a non-ohmic parasitic resistance will be present at the electrode/semiconductor interface.

5.5 Conclusions

In conclusion, we have used channel length dependent measurements to experimentally determine the effective field-effect mobility, corrected for parasitic series resistance, in a variety of spin-coated organic field-effect transistors. The understanding and reduc- tion of parasitic series resistances is important for downsizing of the organic transistor geometries, to be able to reach higher switching speeds for integrated circuits. For the investigated transistors we extract a parasitic series resistance which depends on Vg. This parasitic resistance is attributed to an injection barrier with a height in the order of a few times kBT , which results in an ohmic parasitic series resistance. Experimentally, we find that the parasitic resistance decreases with increasing charge carrier mobility for the investigated polymeric field-effect transistors.

References

[1] G.H. Gelinck, T.C.T. Geuns, D.M. de Leeuw, Appl. Phys. Lett. 77, 1487 (2000). [2] H.E.A. Huitema, G.H. Gelinck, J.B.P.H. van der Putten, K.E. Kuijk, C.M. Hart, E. Cantatore, P.T. Herwig, A.J.J.M. van Breemen, and D.M. de Leeuw, Nature (London) 414, 599 (2001) [3] D.J. Gundlach, Y.Y. Lin, T.N. Jackson, S.F. Nelson, and D.G. Schlom, IEEE Elec.

• Dev. Lett. 18, 87 (1997).

[4] P. T. Herwig and K. M¨ ullen, Adv. Mater. 11, 480 (1999). [5] H. Sirringhaus, R.J. Wilson, R.H. Friend, M. Inbasekaran, W.Wu, E.P. Woo, M. Grell, and D.D.C. Bradley, Appl. Phys. Lett. 77, 406 (2000). [6] Z. N. Bao, Y. Feng, A. Dodabalapur, V. R. Raju, and A. J. Lovinger, Chem. Mat. 9, 1299 (1997). [7] H. Sirringhaus, P.J. Brown, R.H. Friend, M.M. Nielsen, K. Bechgaard, B.M.W. Langeveld-Voss, A.J.H. Spiering, R.A.J. Janssen, E.W. Meijer, P.T. Herwig, and D.M. de Leeuw, Nature (London) 401, 685 (1999). [8] A. R. Brown, C. P. Jarrett, D. M. de Leeuw, and M. Matters, Synth. Metals 88, 37 (1997). [9] H. Fuchigami, A. Tsumura, and H. Koezuka, Appl. Phys. Lett. 63, 1372 (1993). [10] M. Matters, D. M. de Leeuw, P. T. Herwig, and A. R. Brown, Synth. Met. 102, 998 (1999). [11] D.B.A. Rep, B.-H. Huisman, E.J. Meijer, P. Prins, and T.M. Klapwijk, Mat. Res.

• Soc. Symp. Proc. 660 JJ7.9.

[12] E.J. Meijer, C. Tanase, P.W.M. Blom, E. van Veenendaal, B.-H. Huisman, D.M. de Leeuw, and T.M. Klapwijk, Appl. Phys. Lett. 80, 3838 (2002). [13] L. B¨ urgi, H. Sirringhaus, and R.H. Friend, Appl. Phys. Lett 80, 2913 (2002). [14] M. Shur and M. Hack, J. Appl. Phys. 55, 3831 (1984).

60 References [15] S. Luan and W. Neudeck, J. Appl. Phys. 72, 766 (1992). [16] H. Sirringhaus, N. Tessler, D.S. Thomas, P.J. Brown, and R.H. Friend, Festk¨

• rper-

probleme 39, 101 (1999). [17] L. Torsi, A. Dodabalapur, and H.E. Katz, J. Appl. Phys. 78, 1088 (1995). [18] G. Horowitz, R. Hajlaoui, D. Fichou and A. El Kassmi, J. Appl. Phys. 6, 3202 (1999). [19] J. Wang, D.J. Gundlach, C.C. Kuo, and T.N. Jackson, 41st Electr. Mater. Conf. Di- gest, pg 16 (1999). [20] Y.Y. Lin, D.J. Gundlach, and T.N. Jackson, Mat. Res. Soc. Symp. Proc. 413, 413 (1996). [21] K. Terada and H. Muta, Jap. J. Appl. Phys. 18, 953 (1979). [22] J.G.J. Chern, P. Chang, R.F. Motta and N. Godinho, IEEE Elect. Dev. Lett. 1, 170 (1980). [23] T. van Woudenbergh, P.W.M. Blom, M.C.J.M. Vissenberg and J.N. Huiberts, Appl.

• Phys. Lett. 79, 1697 (2001).

[24] Y. Shen, M.W. Klein, D.B. Jacobs, J.C. Scott and G.G. Malliaras, Phys. Rev. Lett. 86, 3867 (2001). [25] S.M. Sze, Physics of Semiconductor Devices (Wiley, New York, 1981).

Chapter 6 Frequency behavior and the Mott-Schottky analysis in poly(3-hexyl thiophene) metal-insulator-semiconductor diodes

Abstract

Metal-insulator-semiconductordiodes with poly(3-hexyl thiophene) as the semiconductor were characterized with impedance spectroscopy as a function of bias, frequency, and

• temperature. We show that the standard Mott-Schottky analysis gives unrealistic values

for the dopant density in the semiconductor. From modeling of the data, we find that this is caused by the relaxation time of the semiconductor, which increases rapidly with decreasing temperature due to the thermally activated conductivity of the poly(3-hexyl thiophene).

62 Frequency behavior and the Mott-Schottky analysis

6.1 Introduction

Low-cost organic integrated circuits are being more and more recognised as a poten- tially interesting industrial application. This has increased the efforts to develop high- performance devices. The development of solution-processable high-mobility polymers [1,2] and of technology for all-polymer integrated circuits [3,4] is promising. However, from the application point of view the lifetime of the devices is an important issue. The limited lifetime of current devices is mainly determined by the increase in conductivity, σ, of the semiconductor upon doping in air and light [4,5]. To study the doping effects in high-mobility polymeric semiconductors we used metal-insulator-semiconductor (MIS) diodes with poly(3-hexyl thiophene) (P3HT) as the

• semiconductor. We measured the temperature and modulation frequency dependence in

these devices. The standard Mott-Schottky analysis to extract the dopant density, NA, yields erroneous results for large frequency and temperature ranges. Analysis of the data will show that this is due to the temperature dependence of the relaxation time of the

• P3HT. We model the data with a simple equivalent circuit and argue that the temperature

dependence of the semiconductor relaxation time is due to thermally activated conductiv- ity of P3HT.

6.2 Experimental

The MIS diodes were fabricated on glass, using patterned Indium-Tin-Oxide (ITO) con- tacts as gate electrode. A 300 nm insulating layer of novolak R photoresist was spin- coated on top of the gate. Over the insulator a 200 nm thick P3HT film was spun from a 1 weight % chloroform solution. Finally, a 10 nm gold layer was evaporated through a shadowmask to form an ohmic contact with the P3HT layer. A cross-section of the de- vice is given in the inset of Fig.6.1. The capacitance, C, of the diode can be changed by depleting or accumulating charge in the semiconductor at the interface with the insulator. The thickness of the insulator layer, dins, determines the maximum value of C: Cins = ǫinsǫ0A dins , (6.1) where ǫins is the relative dielectric constant of the insulator, ǫ0 the permittivity of vacuum and A the area of the device. The minimum value of C is determined by the relative di- electric constants of the insulator, ǫins, and the semiconductor, ǫsemi, and the total distance between the conductive layers. When the semiconductor layer is partially depleted, the depletion layer acts as a capacitance in series with the insulator capacitance. We calculate the total diode capacitance from the modulus of the impedance, Z, and its phase angle, , using: C = − sin /(ω|Z|), with ω=2π fmod, where fmod is the modulation frequency. The capacitances of the MIS diodes scaled with the area of the devices, ranging from 9 to 36 mm2, in the entire biasing regime. The results presented in this work represent typical data measured on more than 10 MIS diodes. All impedance measurements were done with a Schlumberger 1260 Impedance Gain-Phase Analyzer in vacuum (<10−5 mbar) in an Oxford CV-flowcryostat.

6.3 The Mott-Schottky analysis 63

• 3+7

5HVLVW *OD VV *R OG ,72

• J

Figure 6.1: Capacitance of the P3HT MIS diode at 137 Hz as a function of Vg

for different temperatures. The area of this device was 36 mm2.

6.3 The Mott-Schottky analysis

Typical capacitance vs gate bias, Vg, (C − Vg) curves at different temperatures are given in Fig. 6.1 for fmod=137 Hz. The gate voltage is applied on the ITO contact, with the gold electrode at 0 V. The capacitance is clearly a function of Vg, and the semiconductor shows p-type behavior. When the temperature is lowered, we observe that the value of the accumulation capacitance becomes lower, whereas the depletion capacitance remains constant. Using the standard Mott-Schottky analysis [6,7], we extract the dopant density from these curves: ∂ ∂Vg

• C−2

= 2 qǫsemiǫ0NA A2 , (6.2) where q is the elementary charge. The derived dopant densities are plotted as a function

• f temperature in Fig.6.2 for several values of fmod. At low temperatures the extracted

NA drastically rises and seemingly depends on the measurement frequency. We consider this behavior of NA to be physically unrealistic, and it is taken to indicate that the use of Eq.6.2 is not justified for the entire temperature and frequency range. We will show below why this is the case and what the criteria are for extraction of NA.

6.4 Equivalent circuit modelling

In Fig. 6.3a we plot the accumulation capacitance, CA, at Vg = −20 V as a function of temperature for several modulation frequencies. Fig. 6.3b gives the corresponding phase,

64 Frequency behavior and the Mott-Schottky analysis

• $
• Figure 6.2: NA as a function of temperature for different frequencies, as de-

rived from the curves in Fig.6.1 using Eq.6.2. A, at Vg = −20 V . Clearly, both CA and A have a frequency dependence. The maximum slope of CA with temperature coincides with the maximum in A. We find that the maximum of A is thermally activated with an activation energy of 0.36 eV, as shown in the inset of Fig.6.3a. We reason that the observed thermally activated behav- ior is due to the conductivity of the semiconductor layer. A similar argumentation was used by Stallinga et al. in a study of pn junctions based on a poly(phenylene vinylene) derivative [8]. Furthermore, this value compares well with EA = 0.36 eV obtained from bulk conductivity experiments on P3HT [9]. The activation energy is usually interpreted as the distance of the Fermi level in the bulk of the semiconductor to a certain transport level higher up in the density of states [9–11]. The assumption of the thermally activated conductivity allows us to model the data in accumulation for different temperatures with a simple equivalent circuit which is given in the inset of Fig.6.4. We describe the semi- conductor layer by its geometric capacitance, Cs, in parallel with the layer resistance, Rs. The geometric insulator capacitance is Cins and we add a contact resistance, Rc, for the ITO and the gold top contact. This simple model yields a good fit to the experimental data (see Fig.6.4). The fitted values are given in Table 6.1. The obtained values for Cins and Cs correspond to their geometrical values, and the conductivity derived from Rs varies from 1.8·10−10 S/cm at 250 K to 4.3·10−9 S/cm at 330 K, which are values comparable to results of bulk conductivity experiments [9]. The expression for the phase angle as a function of frequency is derived as: A = arctan

• − 1 + ω2R2

s Cs (Cins + Cs)

• ωCins
• ω2Rc R2

s C2 s + (Rc + Rs)

• .

(6.3) From the equivalent circuit analysis we find that Rc is small and can be neglected. The

6.4 Equivalent circuit modelling 65

• +]

+] +] +]

• $
• $
• +]

+] +] +]

• I P

D [

• >

+ ] @ 7>.

@

Figure 6.3: (a) Capacitance of the P3HT MIS diode at Vg = −20 V as a func-

tion of temperature for different frequencies. The inset shows the frequency at which the phase angle, A, is at its maximum vs re- ciprocal temperature (b) A of the MIS diode at Vg = −20 V for different frequencies.

66 Frequency behavior and the Mott-Schottky analysis

• $
• PRG
• Figure 6.4: A (at Vg = −20 V ) vs frequency for different temperatures. The

lines are fits to the data, using the equivalent circuit from the inset. The fit results are given in table 6.1.

Table 6.1: Values obtained by fitting the frequency dependence of the

impedance and its phase angle (see Fig. 6.4) at Vg = −20 V for different temperatures. The equivalent circuit is given in the inset

• f Fig. 6.4. Rc is the contact resistance, Cins the insulator capaci-

tance and Rs and Cs the semiconductor resistance and capacitance respectively. T [K] Rc[] Cins[nF] Rs[k] Cs[nF] 250 72 ± 2 3.99 ± 0.03 320 ± 10 3.54 ± 0.04 264 67 ± 2 3.91 ± 0.03 120 ± 4 3.62 ± 0.04 294 76 ± 3 3.93 ± 0.03 24 ± 1 3.59 ± 0.06 330 71 ± 3 3.98 ± 0.03 12.8 ± 0.6 3.53 ± 0.07

6.5 The relaxation time 67 frequency at which A reaches its maximum is derived as [8]: ωmax = 2π fmax = 1 Rs 1 √Cs (Cins + Cs), (6.4) with a maximum value of A: max = arctan −2√Cs (Cins + Cs) Cins

• .

(6.5) Thus max, in accumulation, is purely dominated by the geometric capacitance of the

• device. Using this model, we find max ∼ −70, which is close to the experimental value
• f -72 degrees. For devices with different geometries, max is well described by Eq.6.5.

At low frequencies the fits in Fig.6.4 start to deviate from the experimental data. This is due to leakage currents through the MIS diode and can be accurately modeled with an added leakage resistance parallel to Cins. The leakage resistance is ∼100 M, which means that for fmod >25 Hz we can neglect its influence on the determination of NA. We note that the inclusion of the thermally activated conductivity in the semiconductor resistance Rs combined with a very simple equivalent circuit is sufficient to model both the temperature and frequency behavior of the MIS diode.

6.5 The relaxation time

The erroneous results obtained with the Mott-Schottky analysis are in fact due to the relaxation time or RC-time, τ = RsCs, of the P3HT. The relaxation time causes the increase of NA, shown in Fig.6.2. When the condition 1/ω ≫ τ. (6.6) is no longer satisfied, the charge carriers can not follow the AC voltage anymore. As a result, one measures a smaller capacitance of the MIS diode with increasing frequency, as shown in Fig.6.3a. As Rs increases with decreasing temperature, this restricts the frequency region over which Eq.6.6 is fulfilled even further for lower temperatures. This is why we observe an apparent temperature dependent capacitance as shown in Fig.6.1. We use our understanding of the temperature and frequency behavior to apply Eq.6.2 in a frequency range where these effects are negligible. In this case the lower limit was taken 25 Hz, and we took the upper limit in frequency one decade lower than the inverse

• f τ, for each temperature. For the present device, we extract a dopant density of 5.4·1015

cm−3, comparable to the value of 1·1016 obtained on P3HT by Brown et al [12].

6.6 Conclusions

In summary, we have fabricated and analyzed MIS diodes based on P3HT. We find that the Mott-Schottky analysis may not be used for extraction of the dopant density, NA, over the entire range of temperature and frequency. This is due to the relaxation time of the

68 Frequency behavior and the Mott-Schottky analysis P3HT, ranging from τ = 1 · 10−3 s at 250 K to τ = 5 · 10−5 s at 330 K. The observed thermally activated behavior related to the relaxation time is attributed to the conductivity

• f the P3HT.

References

[1] Z. Bao, A. Dodabalapur, A.J. Lovinger, Appl. Phys. Lett. 69, 4108 (1996). [2] H. Sirringhaus, P.J. Brown, R.H. Friend, M.M. Nielsen, K. Bechgaard, B.M.W. Langeveld-Voss, A.J.H. Spiering, R.A.J. Janssen, E.W. Meijer, P.T. Herwig and D.M. de Leeuw, Nature 401, 685 (1999). [3] C.J. Drury, C.M.J. Mutsaers, C.M. Hart, M. Matters and D.M. de Leeuw, Appl. Phys.

• Lett. 73, 108 (1998).

[4] G.H. Gelinck, T.C.T. Geuns and D.M. de Leeuw, Appl. Phys. Lett. 77, 1489 (2000). [5] M.S.A. Abdou, F. P. Orfino, Y. Son, S. Holdcroft, J. Am. Chem. Soc. 119, 4518 (1997). [6] E.H. Nicollian and J.R. Brews, “MOS (Metal Oxide Semiconductor) Physics and Technology”, Wiley, New York (1982). [7] S.M. Sze, “Physics of semiconductor devices”, Wiley, New York (1981). [8] P. Stallinga, H.L. Gomes, H. Rost, A.B. Holmes, M.G. Harrison and R.H. Friend, J.

• Appl. Phys. 89, 1713 (2001).

[9] D.B.A. Rep, B.-H. Huisman, E.J. Meijer, P. Prins and T.M. Klapwijk, Mat. Res. Soc

• Symp. Proc., 660 (2001).

[10] A.V. Gelatos and J. Kanicki, Appl. Phys. Lett. 56, 940 (1990). [11] M.C.J.M. Vissenberg and M. Matters, Phys. Rev. B 57, 12964 (1998). [12] P.J.Brown, Charge Modulation Spectroscopy of Poly(3-alkylthiophene), PhD thesis, Cambridge (2000).

Chapter 7 Photoimpedance spectroscopy of poly(3-hexyl thiophene) metal-insulator-semiconductor diodes

Abstract

Capacitance-voltage characteristics of metal-insulator-semiconductordiodes with poly(3- hexylthiophene) (P3HT) as p-type semiconductor were investigated as function of time, ambient, and illumination. P3HT is rapidly doped upon exposure to both oxygen and

• light. Changes of the acceptor density profiles with time were determined by using Mott-

Schottky analysis of the capacitance-voltage characteristics. The profiles were determined to be constant over the P3HT film thickness. Wavelength dependent photoimpedance measurements show that the acceptor creation efficiency peaks upon excitation of the molecular oxygen-polythiophene contact charge transfer complex at (1.9±0.1) eV.

72 Photoimpedance spectroscopy of P3HT MIS diodes

7.1 Introduction

Thin-film field-effect transistors based on p-type organic thiophene-based semiconductors usually are unstable under ambient conditions. The conductivity of the organic semicon- ductor often increases under exposure of oxygen, light, or a combination thereof [1,2]. As a result, the current modulation, or on-off current ratio, of discrete transistors decreases with time, the gain of logic gates gets less than unity and, consequently, logic circuits stop functioning. As is described in Chapter 8, the acceptor density can be estimated from the pinch-off voltage of discrete ring-type transistors, where the pinch-off voltage is the applied gate voltage at which the depletion region in the semiconductor becomes equal to the thickness of the semiconductor layer [3]. This reported analysis implicitly assumes that the acceptor density is constant over the semiconductor layer thickness. In this work we determine the acceptor profile in the polymeric semiconductor layer, us- ing the Mott-Schottky analysis on measured capacitance-voltage (C − Vg) characteristics

• f polymer-based metal-insulator-semiconductor (MIS) diodes. Because we used semi-

transparent MIS diodes, changes in the impedance could be investigated as a function of time upon exposure to oxygen and/or light. As a typical example we investigated poly(3- hexyl thiophene) (P3HT) MIS diodes. In order to elucidate the doping mechanism, we compared the wavelength dependent changes in the photoimpedance measurements with the absorption spectra of molecular oxygen-polythiophene contact charge transfer com- plexes as reported by Abdou et al. [2].

7.2 Experimental

The MIS diodes are fabricated on glass, using patterned transparent indium-tin-oxide gate

• electrodes. A 300 nm layer of novolak R

photoresist is spincoated on top of the contacts and subsequently cross-linked upon baking at 150 oC. On top of this insulator a 200 nm thick film of P3HT is spun from a 1 wt% chloroform solution. Finally, a semi-transparant 10 nm gold electrode is evaporated through a shadow mask. The area of the diodes ranges from 9 to 36 mm2. A schematic cross-section of the MIS diode is given in the inset of Fig.7.1. After processing, the sample is inserted in an Oxford CF1204 optical flowcryo- stat, with a vacuum better than 10−5 mbar. Impedance measurements are done using a Schlumberger 1260 impedance gain-phase analyzer, with a modulation frequency, fmod

• f 137 Hz. This frequency is low enough to prevent artifacts in the impedance data due to

the low bulk charge carrier mobility of P3HT, as described in the previous chapter [4]. The diode capacitance, C, then follows from the modulus of the impedance, Z, and the phase angle, , by C = −sin/(2π fmod|Z|). For the wavelength dependent photoimpedance measurements the light of an Oriel 66058 tungsten-filament lamp is fed into a Jobin Yvon H25 PLE monochromator. The resulting light beam is focused on the sample in the cryo-

• stat. The wavelength of the light is varied between 400 nm and 1100 nm. The power

profile of the light incident on the MIS diode is measured. The number of incident pho- tons is calculated from the calibrated power profile of the lamp in combination with the

• monochromator. The absorption spectrum of a thin film of P3HT is measured and taken

equal to the absorption of the P3HT layer in the diode. In the analysis we have assumed

7.3 Flat-band shift under oxygen exposure 73

• L

Q V

J

LQGDUN

PEDU2 PEDU2 PEDU2 PEDU2

LQOLJKWλ QPYDFXXP

• ∆

) %

• Figure 7.1: Normalized C − Vg curves of a P3HT MIS diode in vacuum and af-

ter 82 minutes exposure to a 230 mbar dry oxygen atmosphere. The arrow indicates the direction of the shift of the curve upon oxygen

• exposure. The inset on the left shows a schematic cross section of

the MIS diode. The inset on the right shows the flat-band voltage shift as a function of time, for different oxygen pressures, as well as a measurement in vacuum where the MIS diode is exposed to light (wavelength of 700 nm). that any dispersion in the absorption of the semi-transparant gold electrode can be disre-

• garded. After exposure to oxygen and/or light in the cryostat, the P3HT MIS diodes are

dedoped by annealing the diodes for several hours in vacuum at 150 oC. This process can be repeated, without apparent degradation of the P3HT [5]. Due to this dedoping proce- dure there are some variations in the dopant density at the beginning of each experiment.

7.3 Flat-band shift under oxygen exposure

The capacitance as a function of gate voltage for an undoped P3HT MIS diode in vacuum and dark is presented in Fig.7.1. Accumulating or depleting charge in the p-type P3HT semiconductor changes the capacitance. The maximum capacitance is obtained in accu- mulation at high negative gate bias and is determined by the thickness of the insulator, dins, as: Cins = ǫinsǫ0A dins , (7.1) where ǫins is the relative dielectric constant of the insulator, ǫ0 is the permittivity of vac- uum, and A is the area of the device. At positive gate bias the semiconductor layer is

74 Photoimpedance spectroscopy of P3HT MIS diodes partially depleted. The depletion layer then acts as a capacitance in series with the insu- lator capacitance. The minimum capacitance is obtained when the whole film is depleted and is determined by the dielectric constants of the insulator and the semiconductor, ǫsemi, and by the total distance between the electrodes. The steepness of the C − Vg character- istics of an MIS diode, when biased in the gate voltage range where the semiconductor layer is partially depleted, is the result of the ease with which the semiconductor can be depleted and is related to the acceptor density and its profile as a function of the depth inside the film. The C − Vg characteristics of Fig.7.1 are measured in the dark and in

• vacuum. The measurements do not change with time in vacuum and dark. The acceptor

density and depth profile therefore do not change; P3HT is stable in vacuum and dark. Subsequently, the diodes are measured in dark upon exposure to dry oxygen. The partial oxygen pressure is varied between 0.8 and 230 mbar. The capacitance-voltage characteristics are recorded every four minutes. The final measurement is included in Fig.7.1. These two measurements can be shifted over the voltage axis on top of each other. Apparently, the dominant effect is a shift of the flat-band voltage, VFB, the applied voltage at which there is no band bending in the semiconductor at the semiconductor/insulator

• interface. The flat-band voltage shift is indicated in the insert of Fig.7.1. The increase
• f VFB with time is dependent on partial oxygen pressure. Upon exposure to light in

vacuum the MIS diode C − Vg characteristics also show a flat-band shift (see the inset of Fig.7.1.), which slowly decreases again when the light is turned off. The flat-band voltage shift is likely due to electrostatic charging of the semiconductor/insulator interface. This interpretation agrees with reported shifts of the transfer characteristics of P3HT field- effect transistors (see Chapter 8) [3].

7.4 Photoimpedance spectroscopy

The constant shape of the measurements shown in Fig.7.1 indicate that on the time scale

• f the measurements (80 minutes) P3HT is stable, with only a flat-band voltage shift,

in vacuum and light, and in oxygen in the dark. P3HT is unstable however upon expo- sure to both oxygen and light. The shape of the capacitance-voltage measurements then gradually changes. Normalized capacitance-voltage curves in 230 mbar of oxygen during illumination at a wavelength of light, λ of 700 nm are presented in Fig.7.2, which also shows that the flat-band voltage shifts. The flat-band shift is difficult to quantify because the slopes of the C − Vg curves change as well. Experimentally we find that the changes in the slopes depend on the wavelength of the illumination (not shown). The slope of the C − Vg characteristics depends on the density and the depth profile of the acceptors. For a quantitative interpretation we use the Mott-Schottky analysis [6,7]. The capacitance at a certain gate bias corresponds to the depletion depth, ddepl, in the semiconductor layer by: ddepl

• Vg
• 1

C Vg − 1 Cins

• ǫ0ǫsemi A

(7.2) Eq.7.2 holds when the contribution to the capacitance of interface states and minority carriers can be disregarded [6]. The acceptor density, NA, at a certain depletion depth can

7.4 Photoimpedance spectroscopy 75

• λ
• L

Q V

J

Figure 7.2: Normalized C−Vg curves as a function of time in 230 mbar oxygen

under illumination of light with a wavelength of 700 nm. now be determined using the Mott-Schottky relation: ∂ ∂Vg

• C−2

= 2 qǫsemiǫ0NA A2 , (7.3) where q is the elementary charge. Combination of Eqs.7.2 and 7.3 yields the acceptor den- sity as a function of depletion depth in the semiconductor layer. We note that the acceptor density profile follows from the shape of the C − Vg characteristics only. It does not de- pend on the value of Vg. Hence, in this analysis flat-band voltage shifts have no influence

• n the profile. This profiling technique probes the holes associated with the acceptors,

rather than the acceptors themselves [8]. The evaluated profile of the hole concentration follows the acceptor profile unless the acceptor profile varies spatially over distances less than the Debije length. This is the distance where the electric field emanating from an electric charge falls off by a factor of 1/e: L D =

• ǫsemiǫ0kBT

q2NA , (7.4) where kB is Boltzmann’s constant and T is the absolute temperature. Under flat-band conditions the number of ionized acceptors is equal to the number of mobile holes. The application of a gate bias results in an additional charge in the semiconductor layer. This causes a rearrangement of the mobile holes, which shield the bulk of the semiconduc- tor from the induced charge. The shielding distance, or band-bending region is on the

• rder of the Debije length. Experimentally, this means that the Debije length limitation

prevents accurate profiling closer than 3L D from the interface of the semiconductor with

76 Photoimpedance spectroscopy of P3HT MIS diodes

• $
• GHSO
• λ

Figure 7.3: Acceptor density profile extracted from the C − Vg data of Fig.7.2,

using Eqs.7.2 and 7.3, for an undoped P3HT MIS diode in vacuum and dark (filled squares) and after 60 minutes of exposure to 230 mbar dry oxygen and illumination with λ=700 nm (filled circles). the insulator [6, 8]. A typical acceptor density of about 1016 cm−3 results in a Debije length of about 20 nm at room temperature. The acceptor profile information can then reliably be determined starting from a depletion depth of about 60 nm from the semicon- ductor/insulator interface. Acceptor densities in the dark and after illumination at a wavelength of 700 nm in 230 mbar O2 are presented in Fig.7.3 as a function of depletion depth. In the dark NA is constant over the P3HT film thickness and amounts to about 3·1015 cm−3. Upon illumination NA increases, but in first order is constant over the layer thickness. There is

• nly a slight increase in acceptor density at the top contact. Similar profiles were obtained

for illuminations at other wavelengths. The profiles with time can now be calculated from the temporal C−Vg characteristics

• f Fig.7.2. We take the values of NA at a depletion depth of 100 nm and plot them in

Fig.7.4 as a function of time on a double logarithmic scale. The acceptor density roughly follows a power law dependence under light exposure of 700 nm and O2 exposure of 230 mbar with an exponent of about 0.3. We note that typically, the conductivity of a variety of oligo- and polythiophenes increases with time as tα with α between 0.2 and 0.5, under ambient conditions [9]. This suggests that the increase of the conductivity on the timescale of the measurements is dominated by an increase in acceptor density. A clue for the doping mechanism can be obtained from the wavelength dependent photoimpedance measurements, which are given in Fig.7.5. In these photoimpedance experiments the MIS diode is exposed for two minutes to light of a certain wavelength in 230 mbar O2. Then the C − Vg characteristics were recorded in the dark, with the oxygen

7.4 Photoimpedance spectroscopy 77

• $
• GHSO
• Figure 7.4: Acceptor density at 100 nm depletion depth in the semiconductor

layer as a function of oxygen exposure and/or illumination

• L

Q V

J

λ

• λ

λ λ λ λ

Figure 7.5: Normalized C − Vg curves as a function of the wavelength incident

• n the sample in 230 mbar oxygen. The curves were recorded in

the dark after 2 minutes of light exposure at each wavelength.

78 Photoimpedance spectroscopy of P3HT MIS diodes

• λ

Figure 7.6: Relative acceptor density creation efficiency as a function of the in-

cident illumination, for P3HT in 230 mbar O2 at room temperature. The absorption spectrum of P3HT is given for comparison. pressure constant at 230 mbar O2. This procedure was repeated at several wavelengths

• f light going from λ=1000 nm to λ=600 nm, in a consecutive measurement. Due to the

low absorption in the P3HT layer from λ=1000 nm up to λ=700 nm, we assume that the light is uniformly absorbed in the film. For shorter wavelengths the absorption profile

• f the P3HT film will not be homogeneous anymore, but will result in a depth profile of
• absorption. We neglect this absorption profile in the measurements up to 600 nm.

The acceptor density is determined from Fig.7.5 at a depletion depth of 100 nm as a function of the photon energy. The derivative yields the change in acceptor density with photon energy, where we have assumed that absolute value of NA at the beginning

• f each measurement has no influence on the increase of NA. This number is corrected

for the power profile of the light, by dividing with the incident photon flux. The relative acceptor creation efficiency as a function of photon energy is then obtained by dividing with the absorbance, and is plotted in Fig.7.6. This efficiency, or cross section, indicates the ease with which acceptors are formed in P3HT under illumination in an oxygen at-

• mosphere. The efficiency peaks at (1.9 ± 0.1) eV. This value corresponds roughly to the

reported absorption of the contact charge transfer complex between molecular oxygen and polythiophene at 1.97 eV [2].

7.5 Conclusions

In summary, we have investigated the instability of P3HT using semitransparent MIS

• diodes. We have measured and analyzed the capacitance-voltage characteristics of these

7.5 Conclusions 79 MIS diodes as function of time, ambient and illumination. On the time scale of the mea- surements in vacuum and light, and in oxygen in the dark, the P3HT MIS diodes only show a flat-band voltage shift. However, upon exposure of the P3HT to both oxygen and light, the capacitance-voltage data show a clear increase of the acceptor density, as is demonstrated using Mott-Schottky analysis. The acceptor density profile is in first ap- proximation constant over the semiconductor film thickness. The wavelength dependent photoimpedance measurements show that the acceptor creation efficiency peaks upon ex- citation of the molecular oxygen-polythiophene contact charge transfer complex.

References

[1] G.H. Gelinck, T.C.T. Geuns and D.M. de Leeuw, Appl. Phys. Lett. 77, 1489 (2000). [2] M.S.A. Abdou, F. P. Orfino, Y. Son and S. Holdcroft, J. Am. Chem. Soc. 119, 4518 (1997). [3] E.J. Meijer, C. Detcheverry, P.J. Baesjou, E. van Veenendaal, D.M. de Leeuw and T.M. Klapwijk, J. Appl. Phys. 93, 4831 (2003). [4] E.J. Meijer, A.V.G. Mangnus, C.M. Hart, D.M. de Leeuw, T.M. Klapwijk, Appl. Phys.

• Lett. 78, 3902 (2001).

[5] D.B.A. Rep, B.-H. Huisman, E.J. Meijer, P. Prins and T.M. Klapwijk, Mat. Res. Soc.

• Symp. Proc., 660, JJ7.9.1 (2001).

[6] E.H. Nicollian and J.R. Brews, “MOS (Metal Oxide Semiconductor) Physics and Technology”, Wiley, New York (1982). [7] S.M. Sze, “Physics of semiconductor devices”, Wiley, New York (1981). [8] D.K. Schroder, “Semiconductor Material and Device Characterization”, Wiley (1990). [9] L. Luer, H.-J. Egelhaaf and D. Oelkrug, Optical Materials 9, 454 (1998).

Chapter 8 Dopant density determination in disordered organic field-effect transistors

Abstract

We demonstrate that, by using a concentric device geometry, the dopant density and the bulk charge carrier mobility can simultaneously be estimated from the transfer charac- teristics of a single disordered organic transistor. The technique has been applied to de- termine the relation between the mobility and the charge density in solution-processed poly(2,5-thienylene vinylene) and poly(3-hexyl thiophene) thin-film field-effect transis-

• tors. The observation that doping due to air exposure takes place already in the dark,

demonstrates that photo induced oxygen doping is not the complete picture.

84 Dopant density determination in organic transistors

8.1 Introduction

For the development of polymeric integrated circuits the stability of the characteristics of the organic semiconductor layer is an important issue. One of the limiting mechanisms is an increase in p-type doping due to a charge-transfer reaction with ambient molecular

• xygen (see chapter 7) [1]. An increase of doping leads to an increase of the conductivity
• f the bulk semiconductor, which reduces the current modulation, or on-off ratio, of the

transistor [2,3]. Here we will demonstrate that, by using a ring-type transistor geometry, we can directly estimate the dopant density and the bulk mobility from the transfer char- acteristics of a single disordered organic field-effect transistor (FET). This allows moni- toring of the dopant density change in time. The disentanglement of the dopant density and bulk mobility in the bulk conductivity will be crucial to understand and counteract the instabilities observed in polymeric transistors. As an example we discuss the dopant density increase under the influence of oxygen exposure in poly(2,5-thienylene vinylene) (PTV) and poly(3-hexyl thiophene) (P3HT) field-effect transistors.

8.2 Motivation and Realization

The field-effect behavior of disordered organic FETs has been described in terms of hop- ping of charge carriers in an exponential density of localized states by Vissenberg et

• al. [4]. The relative position of the Fermi level, EF, in the density of states (DOS),

which determines the charge carrier mobility [4,5], is then dominated by the gate induced charge carriers, CiVg, where Ci is the insulator capacitance per unit area. Similar con- cepts have been used to understand the superlinear increase of the bulk conductivity with doping [6,7], where the relative position of EF is dominated by the dopant density, NA. Disordered organic FETs typically operate in accumulation mode. This means that there is no depletion layer, which is present in inversion layer devices [8], that isolates the conducting channel from the semiconductor bulk. As a consequence, a low conductivity in the bulk layer is required for a large on-off current ratio [5], defined as the ratio of currents at Vg = 0 V and -20 V. When the bulk conductivity is not negligible, we ex- pect a clearly observable crossover from field-effect dominated current to bulk dominated current in the transfer characteristics of a doped accumulation mode disordered organic transistor [9]. Conventional devices with an unshielded drain electrode suffer from parasitic cur- rents outside the transistor area [10, 11], which may obscure the crossover from field- effect dominated current to bulk dominated current. These parasitic currents arise due to the unpatterned semiconductor layer. This can be circumvented by using a ring-type transistor geometry, where the source electrode forms a closed ring around the transistor channel, with the drain electrode, at which the current is monitored, in the center (see inset of Fig.8.1). In the experiments we used heavily doped Si wafers as common gate electrode, with a 200-nm-thick layer of thermally oxidized SiO2 as the gate-insulating layer. The SiO2 was treated with hexamethyldisilazane to make the surface hydrophobic. With conven- tional lithography gold source and drain electrodes were defined, with a channel length,

8.2 Motivation and Realization 85

• G

V

• J

YDFXXP VHFLQPEDUDLU PLQLQPEDUDLU PLQLQPEDUDLU PLQLQPEDUDLU

Figure 8.1: PTV FET transfer characteristics as a function of time in 10 mbar

air, in dark, L = 10 µm, W = 2.5 mm. Clearly visible is the crossover from a bulk depletion transistor to an accumulation tran-

• sistor. The inset shows a topview of the source-drain geometry of

the ring-type transistors. L, varying between 10 and 20 µm, and channel widths, W, of 1 mm and 2.5 mm. As a last step a 200 nm thick semiconductor layer is spincoated over the contacts. The semi- conductors used, are poly(2,5-thienylene vinylene), which is applied as precursor from a 0.5 wt% chloroform solution and subsequently formed in-situ by conversion at 150oC in vacuum [12], and poly(3-hexyl thiophene) which was spun from a 1 wt% chloroform

• solution. All transfer characteristics in this study were measured in the linear operating

regime of the transistor, at source-drain voltage, Vds = −2 V , at room temperature, in the dark. Fig.8.1 shows the transfer characteristics of a PTV thin-film field-effect transistor, with a ring-geometry. The initial curve is measured in vacuum after a thermal dedoping procedure [13]. After this measurement 10 mbar of air is let into the chamber. The time at which the valve is opened to admit the air is denoted as t = 0. Subsequently, the evolution

• f the transfer characteristics in air is monitored as a function of time.

The initial curve shows a characteristic p-type semiconducting behavior. At nega- tive gate voltages, Vg, holes are accumulated in the semiconductor at the semiconductor- insulator interface. These accumulated charges move under the influence of the lateral source to drain field, resulting in the source-drain current, Ids. At positive Vg the holes are depleted from the semiconductor layer and no mobile charges are left to carry the current. Upon prolonged exposure to air, in dark, we observe two changes in the transfer char-

• acteristics. Firstly, in the depletion regime of the transistor (at positive gate voltages) an

additional current appears, which increases with oxygen-exposure and time. This feature

86 Dopant density determination in organic transistors can also be found in transfer characteristics of conventional device geometries, published by other groups [14–16]. Secondly, the onset of the field-effect, i.e. the switch-on voltage, Vso, which was defined in Chapter 2 as the gate voltage at which the transistor is at the flat-band condition [17], is shifted slightly to more positive gate voltages with respect to the initial curve.

8.3 Interpretation and Analysis

The features reported in Fig.8.1 are readily understood when the transistors are considered as accumulation mode FETs. When the conductivity of the bulk layer increases, an addi- tional current will flow in the bulk that is not modulated by the field-effect (schematically depicted in Fig.8.2a). The magnitude of this bulk current compared to the channel current determines whether the gate electrode can still switch the FET between the on and the off state, at Vg = 0 V and -20 V. An increase of the dopant density in the semiconductor, results in an effective bulk depletion transistor (similar to a junction field-effect transis- tor) in parallel to the accumulation field-effect transistor. If this kind of FET is driven far enough into depletion, eventually the entire film will be depleted of mobile charge and no more current will flow. The voltage at which this happens is called the pinch-off voltage, Vpinch (see Fig.8.2c). We argue that the occurrence of the clearly observable crossover from field-effect behavior to bulk behavior in Fig.8.1 is due to the fact that in disordered semiconducting polymers the mobility depends on the charge density. In a system where the mobility is constant and the same for both field-effect and the bulk, this crossover is not observed [18].

8.3.1 Determination of the dopant density

From Vpinch, we can directly determine the dopant density, NA, provided that we correct for the experimentally observed shift of Vso, as described in Section 8.5 below. The initial curve, obtained after the thermal dedoping procedure and measured in vacuum, is taken as reference measurement, and its Vso is estimated at a current level of 1 pA. We can now determine Vpinch, which we also extract at a current level of 1 pA, with respect to Vso from the initial transfer characteristics. In the following analysis we assume that the dopant density in the initial curves is negligible. The error introduced in the analysis by determining Vpinch at 1 pA is small as the current drop typically is steep at that current level. For a small source-drain field we assume that the depletion of the semiconductor takes place uniformly over the entire channel length (see Fig.8.2b) and that the dopants are uniformly distributed throughout the semiconductor layer1. The depletion layer width in a doped semiconductor is given by [8]: Wdepl = ǫ0ǫsemi Ci  

• 1 + 2C2

i

• Vg − Vso
• q NAǫ0ǫsemi

− 1   , (8.1)

1Dopant uniformity was investigated in chapter 7

8.3 Interpretation and Analysis 87

• D
• E
• F

LQVXODWRU JDWH VHPLFRQGXFWRU

Figure 8.2: Schematic of an accumulation-modeFET, showing a p-doped semi-

conductor: + indicates a positive charge in the semiconductor ⊖ indicates a negatively charged counterion. (a) The transistor in ac- cumulation, the current is composed of the field-effect current and the bulk current, resulting from the dopant density. (b) Develop- ment of a depletion region when a positive gate bias is applied. The extent of the depletion region is indicated by the dotted line. Here the current only flows in the undepleted bulk. (c) The film is fully depleted, no more current flows beyond pinch-off.

88 Dopant density determination in organic transistors

• $
• Figure 8.3: dopant density, extracted using Eq.8.4, vs time for several air pres-

sures, in dark. where ǫ0 is the permittivity of vacuum, ǫsemi the relative dielectric constant of the semi- conductor, q the elementary charge and A the active transistor area (length times the width). By using the insulator capacitance per unit area, Ci = ǫ0ǫins dins , (8.2) and the semiconductor layer capacitance, Csemi = ǫ0ǫsemi A dsemi , (8.3) we can recalculate Eq.8.1 for the pinchoff condition, Vg − Vso = Vpinch, at which the depletion layer width is equal to the semiconductor layer thickness, dsemi, to: NA = 2Vpinchǫ0 q

• d2

semi

ǫsemi + 2dsemidins ǫins

, (8.4) where dins is the film thickness of the insulator layer, and ǫins is the relative dielectric constant of the insulator. The dopant density, derived using Eq.8.4, as a function of time at different air pres- sures, is plotted in Fig.8.3. We observe that the initial increase of NA upon exposure to

• xygen is the biggest difference between the measurements at different pressures. In PTV

this initial increase occurs within two minutes after exposure, in the dark. For the P3HT this is a much slower process in the dark, as can be seen from Fig.8.3. In most models

8.4 Results for PTV and P3HT 89 the charge-transfer reaction with molecular oxygen, requires exposure to light [1]. The measurements in the dark presented here, demonstrate that further analysis into the dop- ing mechanisms is required, and that the mechanisms differ quantitatively for different materials.

8.3.2 Determination of the bulk mobility

At Vg = Vso there is no band bending at the semiconductor-insulator interface, no ac- cumulation and no depletion (see Fig.1.7a). The measured current at Vso must then be flowing in the bulk, through the entire film-thickness. We find that the current values at Vso in the doped films typically vary linearly with the applied source-drain voltage, in- dicative of bulk ohmic behavior. From the current flowing at Vso we can calculate the bulk charge carrier mobility: µbulk = LIds NAqWdsemiVds

• Vg=Vso

(8.5) Knowing NA from Eq.8.4, we can determine µbulk using Eq.8.5. We remark that the shape of the curves in depletion can be described by modeling the curves with Ids = WVds NAqµbulk L dsemi − Wdepl , (8.6) using the values obtained from Eq.8.4 and Eq.8.5.

8.4 Results for PTV and P3HT

As a typical result, the bulk mobilities of PTV and P3HT as a function of the dopant density are given in Fig.8.4. For both materials the dependence of µbulk on the dopant density is found to be roughly µ ∼ N2.3

A . For comparison the field-effect mobilities,

extracted from the linear operating regime of the transistors, using µFE = L WCi Vds ∂ Ids ∂Vg , (8.7) are also given. The charge accumulated at the semiconductor-insulator interface is cal- culated using Poisson’s equation, and the charge distribution in the semiconductor is ne- glected [19]. We find that the dependence of mobility on charge density is not the same for the bulk and the field-effect [20]. The µbulk − NA relation obtained here was also found in studies at dopant densities of 1019-1020 cm−3 [2]. These observations suggest that chemical doping of the film not only influences the relative position of the Fermi level in the DOS. In fact, the presence of the dopant counterions will alter the DOS itself, a phenomenon well known from studies on amorphous silicon [21]. The bulk mobility of P3HT is found to be much higher than the bulk mobility of PTV (see Fig.8.4). We argue that this is due to a more ordered film in the case of P3HT [22],

90 Dopant density determination in organic transistors

• E

X O N

• )

(

• $LQGXFHG
• EXON

EXON )( EXON EXON EXON EXON )(

Figure 8.4: The bulk charge carrier mobility in PTV and P3HT estimated using

Eq.8.5 vs the dopant density for several air pressures, in dark. For comparison also the field-effect mobility as a function of induced charge at the semiconductor-insulator interface of a transistor in vacuum, is given for both materials. which can result in a higher bulk mobility. From studies of PTV deposited using a dif- ferent precursor-route [2], we initially did not find the bulk features in the dark and air as described above. Only after additional thermal treatment, which did not change the field-effect behavior, the bulk feature appeared. As the polymer did not degenerate due to this thermal treatment, we conclude that a morphology change in the bulk of the polymer results in an increased bulk mobility, and therefore in an added bulk contribution to the

• current. Morphological differences between the semiconductor bulk and the semiconduc-

tor/insulator interface will result in different relative contributions of the bulk current and the field-effect current to the total source-drain current. The bulk current increases with time due to the doping with oxygen. For a system with a low mobility in the bulk in combination with a high field-effect mobility, the influence of this bulk current increase

• n the total transistor current can be neglected for a longer period of time than in a system

with a high bulk mobility. Study of the bulk morphological aspects of disordered organic semiconductors, and their influence on the stability of devices is therefore of importance. We remark that reducing the thickness of the semiconductor layer will also significantly reduce the bulk current, which we observed experimentally by preparing semiconductor layers with a thickness in the order of 10 nm. The results in Fig.8.4 demonstrate the power

• f our experimental technique, which allows the disentanglement of the dopant density

and the bulk mobility in the bulk conductivity.

8.5 Shift of the switch-on voltage 91

• G

V

• JVR
• G

V

• JVR
• G

V

• J

Figure 8.5: The transfer characteristics of Fig.8.1 are shifted over the voltage

axis, such that the field-effect behavior of the curves coincide. The insets shows (a) the data of Fig.8.1 on a linear scale and (b) the same data shifted over the voltage axis on a linear scale.

8.5 Shift of the switch-on voltage

The data of Fig.8.1 can be replotted, by shifting the transfer characteristics along the Vg

• axis. In this way the field-effect part of the characteristics can be perfectly aligned on

top of each other, as demonstrated in Fig.8.5 and Fig.8.6. This suggests that the shifting

• f the curves can be described as a pure shift of the switch-on voltage. Assuming that

this is a correct representation of the data, we can proceed to estimate the dopant density from Vpinch as outlined above. The various measured shifts as a function of exposed air pressure are plotted in Fig.8.7, from which we conclude that Vso depends on the polymer used. We find that the shift of Vso is not directly related to the increase in dopant density. This is illustrated in Fig.8.8, where a P3HT transistor after a substantial time in air (5.8 days) shows both the bulk current in the depletion regime and a Vso. Upon evacuation

• f the sample the current in the depletion regime is not discernable anymore after half an

hour at 10−5 mbar, whereas the shift in Vso is still present. The shift can only be removed with longer pumping times (see the inset of Fig.8.8). This demonstrates that the dopant density changes and Vso are two separate processes in the dark, and that the suggested relation between dopant density and threshold voltage reported in the literature [5, 18] is not valid for our devices. However, the P3HT data in Figs.8.3 and 8.7 suggest that the increase of Vso and NA are time-scale related. Based on the results of Fig.8.8 this is expected to be an indirect relation. The shift of Vso can not be explained by charge stored in the insulator, as we use high-

92 Dopant density determination in organic transistors

• G

V

• JVR
• G

V

• J
• , G

V

• >

m $ @ 9JD9VR>9@

• , G

V

• >

m $ @ 9J>9@

Figure 8.6: (a) P3HT FET transfer characteristics in 1 bar air, after some light

exposures from a lamp, L = 20 µm, W = 1 mm. The inset shows the same data on a linear scale (b) The data are shifted over the voltage axis in such a way that the field-effect part of the curves

• coincide. The inset shows the same data on a linear scale.

8.5 Shift of the switch-on voltage 93

• V

R

• Figure 8.7: Switch-on voltage shift vs time for several air pressures, in dark.

For P3HT this clearly is a large effect.

• DIWHUGD\VLQEDUDLUDQGGDUN

DIWHUPLQXWHVLQYDFXXP DIWHUGD\VLQYDFXXP DIWHUGD\VLQYDFXXP

• G

V

• J
• V

R

• 3+7YDFXXP

Figure 8.8: Evacuation of a P3HT FET as a function of time. L = 10 µm, W =

2.5 mm. The pinchoff voltage, observed in the initial curve (closed squares), is not present anymore after half an hour of pumping at 10−5 mbar (closed circles), whereas the Vso shift remains. After several days of pumping Vso has shifted towards zero. The inset shows the reduction of Vso with time under vacuum and dark.

94 Dopant density determination in organic transistors quality thermally grown SiO2 in the FETs, which is unlikely to incorporate charge upon

• xygen exposure at room temperature [10,18,23]. A dopant profile in the semiconductor

layer can not explain the Vso shift in these organic devices in contrast to standard silicon devices [8], because here we are not dealing with the formation of an inversion layer [17]. We suggest that the observed shifts of Vso are due to interfacial charging at the semiconductor-insulator interface [10]. This is supported by the fact that we observed these shifts with the same magnitude also in the characteristics of transistors with very thin semiconductor layers. The origin and nature of these charges are unclear at present.

8.6 Conclusions

In conclusion, we have shown that the dopant density as well as the bulk charge carrier mobility in disordered organic semiconductors can be simultaneously determined from the transfer characteristics of a single transistor. This is possible due to a concentric device geometry, which excludes parasitic currents outside the transistor area and a mobility dependence on the charge density in the polymer. For two organic semiconductors, P3HT and PTV, it was demonstrated that field-effect and bulk influences could be separated because of a clear crossover from an accumulation transistor to a bulk depletion mode

• transistor. These polymers already exhibit a dopant density increase upon air exposure in

the dark, which requires a re-evaluation of the doping mechanism in terms of a charge- transfer reaction with oxygen under light exposure. We have argued that the morphology

• f the polymeric semiconductor is of importance for the stability of the transistors. The

ability to analyse dopant influences directly from transistor measurements is crucial to study instabilities and lifetime issues of polymeric transistors.

References

[1] M.S.A. Abdou, F. P. Orfino, Y. Son, and S. Holdcroft, J. Am. Chem. Soc. 119, 4518 (1997). [2] A.R. Brown, C.P. Jarrett, D.M. de Leeuw, and M. Matters, Synth. Met. 88, 37 (1997). [3] G.H. Gelinck, T.C.T. Geuns, and D.M. de Leeuw, Appl. Phys. Lett. 77, 1489 (2000). [4] M. C. J. M. Vissenberg and M. Matters, Phys. Rev. B 57, 12964 (1998). [5] G. Horowitz, R. Hajlaoui, H. Bouchriha, R. Bourguiga, and M. Hajlaoui, Adv. Mater. 10, 923 (1998). [6] B. Maennig, M. Pfeiffer, A. Nollau, X. Zhou, K. Leo, and P. Simon Phys. Rev. B 64, 195208 (2001). [7] H.C.F. Martens, PhD thesis, Charge transport in conjugated polymers and polymer devices, Leiden University (2000). [8] S.M. Sze, Physics of Semiconductor Devices Wiley, New York, (1981). [9] For band transport, where the mobility does not depend on the charge density, this crossover is not expected to occur in the transfer characteristics [18], in which case the analysis given here can not be applied. [10] Y.-Y. Lin, D. J. Gundlach, S.F. Nelson, and T.N. Jackson, IEEE Trans. Elec. Dev. 44, 1325 (1997). [11] We expect that patterning of the semiconductor layer will give similar results as the ring-geometry. [12] A.J.J.M. van Breemen, J.J.A.M. Bastiaansen, B.M.W. Langeveld, J. Sweelssen, J.A.E.H. van Haare, P.T. Herwig, K.T. Hoekerd, and H.F.M. Schoo, Int. Display Re- search Conf. Palm Beach, U.S.A. SID 20, 327 (2000). [13] D.B.A. Rep, B.-H. Huisman, E.J. Meijer, P. Prins, and T.M. Klapwijk, Mat. Res.

• Soc. Symp. Proc., 660, JJ7.9.1 (2001).

[14] G. Horowitz, F. Garnier, A. Yassar, R. Hajlaoui, and F. Kouki, Adv. Mater. 8, 52 (1996).

96 References [15] G.C.R. Lloyd, N. Sedghi, M. Raja, R. Di Lucrezia, S. Higgins. and W. Eccleston,

• Mat. Res. Soc. Symp. Proc. 708, BB10.57 (2001).

[16] T.N. Jackson, C.D. Sheraw, J.A. Nichols, J.-R. Huang, D.J. Gundlach, H. Klauk, and M.G. Kane, Int. Display Research Conf. Palm Beach, U.S.A. SID 20, 411 (2000). [17] E.J. Meijer, C. Tanase, P.W.M. Blom, E. van Veenendaal, B.-H. Huisman, D.M. de Leeuw, and T.M. Klapwijk, Appl. Phys. Lett. 80, 3838 (2002). [18] S. Scheinert, G. Paasch, M Schr¨

• dner, H.-K. Roth, S. Sensfuß, and Th. Doll, J. Appl.
• Phys. 92, 330 (2002).

[19] C. Tanase, E.J. Meijer, P.W.M. Blom, and D.M. de Leeuw, submitted. [20] H. Sirringhaus, N. Tessler, D.S. Thomas, P.J. Brown, and R.H. Friend, Adv. Solid

• State. Phys. 39, 101 (1999).

[21] R.A. Street, Hydrogenated amorphous silicon, Cambridge University Press (1991). [22] H. Sirringhaus, P.J. Brown, R.H. Friend, M.M. Nielsen, K. Bechgaard, B.M.W. Langeveld-Voss, A.J.H. Spiering, R.A.J. Janssen, E.W. Meijer, P.T. Herwig, and D.M. de Leeuw, Nature (London) 401, 685 (1999). [23] S.J. Zilker, C. Detcheverry, E. Cantatore, and D.M. de Leeuw, Appl. Phys. Lett. 79, 1124 (2001).

Chapter 9 Solution-processed ambipolar organic field-effect transistors

Abstract

Progress towards electronics based on organic semiconductors is strongly dependent on the successful interplay between functional chemical units and the development of well- performing electronic components. There is ample evidence that organic field-effect tran- sistors analogous to standard metal-oxide-semiconductor (MOS) transistors have reached a stage that they can be industrialized [1–3]. Currently organic semiconductors are di- vided into two classes, electron-transporters and hole-transporters, or n-type and p-type

• materials. This distinction is important for the design of light-emitting diodes [4] and

solar cells [5] where both type of carriers are needed. Monocrystalline Si technology is largely based on complementary MOS (CMOS) structures which use both n-type and p- type transistor channels. This complementary technology has enabled the construction of digital circuits which operate with high robustness, a low power dissipation and a good noise margin. For organic integrated circuits, there is an urgent need to find ways to- wards organic semiconductors which can be ambipolar, i.e. are capable of transporting both types of carriers, while maintaining the attractiveness of easy solution-processing. We report on the fabrication of solution-processed ambipolar field-effect transistors and inverters based on a blend of two suitable organic semiconductors.

98 Ambipolar organic field-effect transistors

9.1 Introduction

Charge transport through amorphous, or poly-crystalline organic semiconductors is not fully understood, but a number of aspects are clear. It is possible to blend an active com- pound with a non-active medium without spoiling the conductive properties, indicative of the role of percolating conducting paths [6]. Secondly, depending on the electrode work- functions and the nature of the organic semiconductors, the contacts play a crucial role. The electrodes need to have a workfunction that allows injection of holes into the highest

• ccupied molecular orbital (HOMO) of the semiconductor and/or injection of electrons

in the lowest unoccupied molecular orbital (LUMO). Therefore usually other contact ma- terials are chosen for electron-transporters and hole-transporters [7]. Concomitantly, one electrode material can be used in combination with two organic semiconductors, when

• ne has its HOMO level and the other has its LUMO level aligned with the metal work-
• function. In the past, this concept was used to construct heterostructure devices by evap-
• rating an n-type semiconductor layer on top of a p-type semiconductor layer, which

resulted in both n-type and p-type transistor operation [8, 9]. In this stacked geometry, the gate field needs to deplete the lower layer first to achieve accumulation in the top

• layer. Also separate n-type and p-type transistors have been evaporated, which could be

combined to CMOS integrated circuits [10]. Instead of evaporating the organic semicon- ductors sequentially, one would like to deposit both semiconductors in one easy process- ing step, as was demonstrated by Tada et al. [11]. The fact that two active compounds can be mixed together in solution, opens up the possibility to deposit simultaneously two strongly interpenetrating networks of percolating conducting paths, with both hole- and electron-transporting capabilities. The interpenetrating networks used in this work, are composed of hole- transport- ing poly[2-methoxy-5-(3’,7’- dimethyloctyloxy)]-p-phenylene vinylene (OC1C10-PPV) and electron-transporting [6,6]-phenyl C61-butyric acid methyl ester (PCBM). This mix- ture is typically used for organic photovoltaic cells research [5]. The molecular struc- tures of these materials are given in Fig.9.1. Gold electrodes were used as injecting

• contacts. The choice of the semiconducting materials in combination with the injecting

electrodes is crucial. The energy levels that come into play are schematically represented in Fig.9.2. For simplicity, the energy levels are drawn as straight lines, but band bending

• ccurs at the semiconductor-insulator interface upon an applied gate bias. This shifts the

Fermi level in the semiconductor, which in turn can result in band bending at the elec- trode/semiconductor interface. The HOMO level of OC1C10-PPV, at 5.0 eV, is aligned with the workfunction of gold, at 5.1 eV, which will result in an ohmic contact for hole injection from gold into the OC1C10-PPV-network. Due to the large bandgap of OC1C10- PPV gold is a blocking contact for electrons into OC1C10-PPV [7]. The alignment of the gold workfunction with the LUMO level of the electron-transporter PCBM is not as good, and the mismatch in energy levels results in an injection barrier, φB, of 1.4 eV for electron injection into the PCBM network. However, this injection barrier is significantly reduced to 0.76 eV, due to the formation of a strong interface dipole layer at the Au/PCBM in- terface [12]. Similar behavior has been observed for the Au/C60 interface by ultraviolet photoemission spectroscopy [13]. In general, the width of an injection barrier can be narrowed application of a large

9.1 Introduction 99

Figure 9.1: Schematic cross-section of the field-effect transistor geometry used

in this study, the molecular structures of PCBM and OC1C10-PPV, and an artist impression of the interpenetrating networks of the two semiconductors.

100 Ambipolar organic field-effect transistors

• Figure 9.2: Device band diagram of interpenetrating networks of OC1C10-PPV

and PCBM in contact with Au electrodes, when no biases are ap- plied to the transistor. For simplicity the energy levels are drawn as straight lines. It should be noted that band bending due to an applied gate voltage can reduce the barrier for electron injection into the PCBM network.

9.2 Experimental 101 source-drain field, or by accumulation of high charge densities in the semiconductor film, for instance through doping or by the field-effect [14]. A narrow injection barrier will allow tunnelling of charge carriers from the electrode to the semiconductor.

9.2 Experimental

In the experiments we used heavily doped Si wafers as the gate electrode, with a 200 nm SiO2, grown via thermal oxidation, as the gate-insulating layer. Using conventional lithography, gold source and drain contacts were defined with a channel width W of 1 mm and length L of 40 µm. The SiO2 layer was treated with the primer hexamethyldisilazane, which makes the surface hydrophobic. The transistors were completed by spinning a solution of PCBM and OC1C10-PPV (4:1 by weight), with a 0.5% weight content in

• chlorobenzene. Prior to spincoating, the solution was stirred for one hour at 80oC. The

completed devices were annealed in vacuum for 15 hours at 90oC. The films were inves- tigated with atomic force microscopy, showing the same surface morphology as reported by Shaheen et al. [15] for the same mixture. This indicates that the constituents are uni- formly mixed. The electrical measurements were performed at room temperature in a vacuum of 10−5 mbar. A schematic cross-section of the transistors is given in Fig.9.1. The semiconductor layer of transistors with PCBM only was spincoated from a 1 wt% PCBM solution in chlorobenzene, while the semiconductor layer of OC1C10-PPV was spun from a 0.4 wt% OC1C10-PPV solution in toluene or chlorobenzene. We note that the wetting behavior of the PCBM and the OC1C10-PPV on the substrates was very differ- ent, with the OC1C10-PPV solutions forming uniform films, whereas the PCBM solutions where difficult to deposit in a uniform film. The transfer characteristics of a transistor with gold electrodes and PCBM as the semiconductor show good electrical performance, and a field-effect mobility was determined of 10−2 cm2/Vs at a gate voltage, Vg = 20 V , demonstrating that the energy level mismatch between gold and PCBM can be overcome with the field-effect.

9.3 Ambipolar transistor operation

Typical output characteristics of a field-effect transistor based on the OC1C10-PPV:PCBM blend (see Fig.9.3) demonstrate operation both in the hole-enhance-ment and electron- enhancement mode. For high negative gate voltages, Vg, the transistor is in the hole- enhancement mode and its performance is identical to a unipolar transistor based on OC1C10-PPV, with a field-effect mobility of 7·10−4 cm2/Vs at Vg = −20 V, which was extracted from the transfer characteristics of Fig.9.4. For low gate voltages and high drain voltages, Vds, the current shows a pronounced increase with Vds (see Fig.9.3a), which is typical of an ambipolar transistor and not present in the unipolar transistor based on OC1C10-PPV. At positive Vg, the transistor operates in the electron-enhancement mode (Fig.9.3b), with a field-effect mobility of 3·10−5 cm2/Vs at Vg = 30 V , two orders of magnitude lower than the electron mobility in the PCBM-only transistor. At low drain voltages we observe a non-linear current increase, indicating that there is a barrier for

102 Ambipolar organic field-effect transistors

• 9

9 9 9 9 9J 9

• G

V

• GV
• 9

9 9 9 9 9 9J 9

• G

V

• GV

Figure 9.3: Output characteristics of the OC1C10-PPV:PCBM ambipolar tran-

sistor operating in (a) hole-enhancement, and in (b) electron- enhancement mode. electron injection from gold into PCBM, which is much more pronounced than in a unipo- lar PCBM transistor, where the output characteristics look qualitatively similar to those of Fig.9.3a1. At low gate voltages and high drain voltages, we again observe a pronounced increase in current, typical of an ambipolar transistor, and which is not observed in the unipolar PCBM transistor. The current increase can readily be understood, when considering that under certain biasing conditions both holes and electrons are accumulated in the transistor channel, forming a pn-junction [16].

1The super-linear output characteristics at low Vds observed in the blend transistor, indicate an injection

problem from the Au into the PCBM, which is probably due to the selective wetting of PCBM and OC1C10- PPV on gold, which can also account for the low electron mobility in the blend transistor as compared to the PCBM-only transistor.

9.3 Ambipolar transistor operation 103

• 9

9 9 9 9 9GV 9

• G

V

• J
• 9

9 9 9GV 9

• G

V

• J

Figure 9.4: The transfer characteristics of the OC1C10-PPV:PCBM ambipolar

• transistor. (a) For Vg <

0 V only the hole contribution is ob- served in the current, whereas for Vg > 0 V the electron current is seen.(b) For Vg > 20 V only the electron contribution to the current is observed. Depending on the value of Vds however, the hole current-contribution is already observed for Vg < 20 V . The assymetry in electron and hole contributions to the total current in (a) and (b) is due to the larger field-effect mobility for holes as compared to the electrons.

104 Ambipolar organic field-effect transistors

• K

H

• Figure 9.5: Schematic representation of the operation of the ambipolar transis-
• tor. a) Hole accumulation at the semiconductor insulator interface

for a large negative Vg and small negative Vd. b) When the condi- tion Vd ≤ Vg−Vso is reached, an electron accumulation region will grow at the drain electrode, while at the same time, holes are still accumulated around the source electrode. The two accumulation layers will form a pn-junction in the channel. This is schematically represented in Fig.9.5. For a large negative Vg and a small neg- ative Vds the transistor behaves as a unipolar hole accumulation transistor (Fig.9.5a)). If in a unipolar device Vds is increased beyond Vds = Vg − Vso, where Vso is the switch-on voltage of the field-effect2, a depletion region around the drain will develop and the drain current saturates. However for an ambipolar transistor, electrons will start to accumulate at the drain electrode. This electron accumulation region forms a pn-junction in the chan- nel with the hole accumulation region at the source electrode (Fig.9.5b)). This electron accumulation region is responsible for the observed current increase at high Vds. When the transistor is biased in the hole-accumulation mode, the current can be described by a model based on hopping of charge carriers in an exponential density of states (DOS) as was described in chapter 2 [17,18]: Ids = AhWCi L

• −Vg + Vso

2

TDOS,h T

−

• −Vg + Vso + Vds

2

TDOS,h T

• ,

(9.1) where Ah is a prefactor for the hole current [17,18], Ci the insulator capacitance per unit area, TDOS,h is the width of the exponential DOS for holes [18], and [[x]] = 1

2x + 1 2|x|.

For a complete description of the electron accumulation mode, we should consider the injection-limited current. This has recently been modelled by thermally assisted hop- ping from the electrode into the localized states of the organic semiconductor, which are broadened due to disorder [20]. For the sake of simplicity, however, we will neglect the

2We defined the switch-on voltage as the gate voltage that needs to be applied to reach the flat-band condition

(see Chapter 2) [17]

9.4 CMOS-like inverter operation 105

• bserved injection barrier for electrons here. Then, in the electron accumulation mode,

the current can be described analogous to Eq.9.1: Ids = −AeWCi L Vg − Vso 2

TDOS,e T

− Vg − Vso − Vds 2

TDOS,e T

• ,

(9.2) where Ae is a prefactor for the electron current, and TDOS,e is the width of the exponential DOS for electrons. Under bias conditions where both holes and electrons accumulate in the channel, the transistor can be described by two transistors in series, one of length Lh where only holes accumulate and one of length Le where only electrons accumulate. The source-drain current can readily be calculated from the condition of current continuity across the pn-junction and the relation Le + Lh = L. From these considerations it follows that the total current is simply the sum of the source-drain currents in Eq.9.1 and Eq.9.2. The ambipolar transistor can be represented as a p-type and n-type transistor, both with length L, connected in parallel. Here it is implicitly assumed that charge transfer across the pn-junction is not a limiting factor in the device performance.

9.4 CMOS-like inverter operation

Due to the fact that both polarities of charge can be induced in the transistor, comple- mentary logic is possible. An inverter based on two identical ambipolar transistors was constructed, with a common gate as the input voltage, VI N . This device demonstrates CMOS-like inverter operation (see Fig.9.6). A high gain of 10 is easily achieved (the steepness of the inverter characteristic), in combination with a good noise margin (the position of the voltage switch in the inverter characteristic). For unipolar logic this com- bination is very difficult to achieve. Depending on the polarity of the supply voltage, VDD, the inverter works in the first or the third quadrant of Fig.9.6, which is a particular feature

• f the ambipolar transistor-based inverter, as unipolar logic works only in one quadrant.

Furthermore, we see a small dependence of the output voltage, VOUT [21], at low and high values of the input voltage, VI N, which is normally not observed in CMOS-based

• inverters. This change of VOUT is a direct result of the fact that both transistors that make

up the inverter can be considered as a parallel circuit of an n-type and a p-type transistor. Using the sum of Eq.9.1 and Eq.9.2 for the ambipolar current, the inverter charac- teristics can already qualitatively be modelled, by using the various biasing conditions in the inverter, and the ratio of the extracted field-effect mobilities for electrons and holes. From the modelling of the p-channel behavior with Eq.9.1 on the data of Fig.9.3, we find TDOS,h = 530 K, we assume TDOS,e = TDOS,h, and we neglect the injection-limited current observed for the n-channel. The inverter characteristics calculated in this way are plotted as solid lines in Fig.9.6. We note that, although we have taken a very simple model for the ambipolar transistor behavior, we already get a very reasonable description for the inverter characteristics, including the change in VOUT .

106 Ambipolar organic field-effect transistors

• 9,1

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'' ''

• 2

8 7

• ,1

Figure 9.6: Schematic

representation and transfer characteristics

• f

a CMOS-like inverter based on two identical ambipolar OC1C10- PPV:PCBM field-effect transistors. Depending on the polarity of VDD, the inverter works in the first or the third quadrant. The full drawn lines are modeled on the basis of Eq.9.1 and Eq.9.2.

9.5 Conclusions

In conclusion, we have demonstrated ambipolar thin-film field-effect transistors, based

• n interpenetrating networks of the solution-processed organic semiconductors OC1C10-

PPV and PCBM. Complementary logic based on these ambipolar transistors has the po- tential to significantly simplify the design and improve the operation of organic integrated circuits, while keeping the technology as simple as for unipolar logic.

References

[1] H.E.A. Huitema, G.H. Gelinck, J.B.P.H. van der Putten, K.E. Kuijk, C.M. Hart, E. Cantatore, P.T. Herwig, A.J.J.M van Breemen and D.M. de Leeuw, Nature (London) 414, 599 (2001). [2] H. Sirringhaus, N. Tessler, R.H. Friend, Science 280, 1741 (1998). [3] G.H. Gelinck, T.C.T. Geuns and D.M. de Leeuw, Appl. Phys. Lett. 77, 1487 (2000). [4] J.H. Burroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K. Mackay, R.H. Friend, P.L Burn and A.B. Holmes, Nature 347, 539 (1990). [5] G. Yu, J. Gao, J.C. Hummelen, F. Wudl and A.J. Heeger, Science 270, 1789 (1995). [6] C.Y. Yang, Y. Cao, P. Smith and A.J. Heeger, Synth. Met. 53, 293 (1993). [7] P.W.M. Blom, M.J.M. de Jong and M.G. van Munster, Phys. Rev. B. 55, R656 (1997). [8] A. Dodabalapur, H.E. Katz, L. Torsi and R.C. Haddon, Science 296, 1560 (1995). [9] A.Dodabalapur, H.E. Katz, L. Torsi and R.C. Haddon, Appl. Phys. Lett. 68, 1108 (1996). [10] B. Crone, A. Dodabalapur, Y.-Y. Lin, R.W. Filas, Z. Bao, A. LaDuca, R. Sarpeshkar,

• H. E. Katz and W. Li, Nature 403, 521 (2000).

[11] K. Tada, H. Harada and K. Yoshino, Jpn. J. Appl. Phys. 35, L944 (1996). [12] J.K.J. van Duren, V.D. Mihailetchi, P.W.M. Blom, T. van Woudenbergh, J.C. Hum- melen, M.T. Rispens, R.A.J. Janssen and M.M. Wienk, submitted. [13] S.C. Veenstra, A. Heeres, G. Hadziioannou, G.A. Sawatzky and H.T. Jonkman,

• Appl. Phys. A, 75, 661 (2002).

[14] S.M. Sze, ”Physics of Semiconductor devices” (Wiley, New York, 1981). [15] S.E. Shaheen, C.J. Brabec, N.S. Sariciftci, F. Padinger, T. Fromherz and J.C. Hum- melen, Appl. Phys. Lett. 78, 841 (2001). [16] G.W. Neudeck, H.F. Bare and K.Y. Chung, IEEE Trans. Electr. Dev. 34, 344 (1987).

108 References [17] E.J. Meijer, C. Tanase, P.W.M. Blom, E. van Veenendaal, B.-H. Huisman, D.M. de Leeuw and T.M. Klapwijk, Appl. Phys. Lett. 80, 3838 (2002). [18] M.C.J.M. Vissenberg and M. Matters, Phys. Rev. B 57, 12964 (1998). [19] C. Detcheverry and M. Matters, Proc. ESSDERC, 328 (2000). [20] T. van Woudenbergh, P.W.M. Blom, M.C.J.M. Vissenberg and J.N. Huiberts, Appl.

• Phys. Lett. 79, 1697 (2001).

[21] K.Y. Chung, G.W. Neudeck and H.F. Bare, IEEE J. Solid-State Circ. 23, 566 (1988).

Chapter 10 Ambipolar field-effect transistors based

• n a single organic semiconductor

Abstract

With the coming of age of organic semiconductors, envisioned applications in the area

• f light-emitting diodes, plastic solar cells and integrated circuits are becoming a reality.

The promise of low-cost flexible integrated circuits for high volume applications such as electronic paper, demands a flexible semiconductor layer, preferably a polymer, which for ease of processing should be deposited from solution, via for instance spincoating

• r inkjet technology. For organic integrated circuits an important improvement in circuit

performance is the step from unipolar logic, which uses either p-type or n-type transistors, to complementary metal-oxide-semiconductor(CMOS) logic, which uses both n-type and p-type transistor channels. Here we expand the work on blends described in the previous

• chapter. We report on the fabrication of ambipolar field-effect transistors and inverters

based on the solution-processed organic semiconductor poly(3,9-di-tert-butylindeno[1,2- b] fluorene) (PIF).

110 Ambipolar transistors based on a single organic semiconductor

10.1 Introduction

Polymer electronics up to now is typically based on unipolar logic, mainly because of the choice of the electrodes in combination with a large band gap semiconductor. For the

• peration of light-emitting diodes and solar cells based on organic semiconductors it is

essential that both electrons and holes can flow through the semiconductor layer under an applied electric field, and that they can be collected in or emitted from the electrodes. This poses requirements on the choice of semiconductor layer in combination with the electrode materials used for the anode and the cathode. In organic solar cells, typically a blend of an electron-transporting and a hole-trans- porting semiconductor is used. Upon illumination, excitons are formed in the blend, which can be separated in electrons and holes by means of an applied electric field. The electron transporter and the hole transporter provide a conduction path for the electrons and holes to the electrodes, thus converting light into electrical current. In polymeric light-emitting diodes, a high workfunction metal is chosen as the anode for hole injection based on the alignment of its workfunction with the highest occupied molecular orbital (HOMO) of the organic semiconductor, and a low workfunction metal is chosen as the cathode for electron injection based on its workfunction alignment with the lowest un-

• ccupied molecular orbital (LUMO) of the organic semiconductor. Due to the typically

large bandgap (>2 eV) of the polymers used in light-emitting diodes, which is required for light emission in the visible range of the spectrum, the anode will be a blocking con- tact with an injection barrier for electrons, and the cathode a blocking contact for injection

• f holes. Upon application of an electric field across the polymeric semiconductor, elec-

trons and holes flow towards each other and form excitons, which can recombine under emission of light. To fabricate ambipolar organic transistors, which operate as either n-channel or p- channel transistors, the concepts of the light-emitting diode and the solar cell are useful, and in this context we already demonstrated that a blend of suitable chosen n-type and a p- type semiconductors, deposited from solution, in combination with Au electrodes results in ambipolar transistor operation (see previous chapter) [2]. In that case two materials were used where one has its HOMO level aligned with the Au workfunction and the

• ther its LUMO level. The main difficulty in achieving ambipolar transistor operation

in a single organic semiconductor is to inject both holes and electrons from the same

• electrode. A good contact for one polarity of charge typically results in an injection barrier

for the other polarity of charge. However, this injection barrier can be reduced by using a material with a smaller energy gap. Furthermore, the width of an injection barrier can be narrowed by applying a large source to drain field, or by accumulation of high charge carrier densities in the semiconductor film by means of the field-effect [3]. For sufficiently high amounts of accumulated charge, the injection barrier becomes small enough to allow tunneling from the electrode into the semiconductor. Next to the requirements of a small band gap also the semiconductor purity is of importance in order to minimize trapping effects.

10.2 Experimental 111

• Figure 10.1: Schematic cross-section of the FET geometry used in this study.

The molecular structure of PIF is given.

10.2 Experimental

As low band gap organic semiconductor in the ambipolar transistors we have used poly(3,9- di-tert-butylindeno[1,2-b]fluorene) (PIF) [4] which has a band gap of 1.55 eV. Gold, with a workfunction of 5.1 eV, is used for the source and drain injecting contacts. In the exper- iments we use heavily doped Si wafers as the gate electrode, with a 200-nm-thick-layer of thermally oxidized SiO2 as the gate-insulating layer. Using conventional lithography, the gold source and drain interdigitated contacts are defined with a channel width W of 2 cm and length L of 10 µm. The SiO2 layer is treated with the primer hexamethyldisilazane. The transistors are completed by spinning a solution of 1wt% PIF in chloroform onto the

• substrate. The molecular structure of PIF and a schematic cross-section the transistor ge-
• metry are given in Fig.10.1. The measurements are performed in a vacuum of 10−4 mbar

at room temperature, after annealing the sample for an hour at 90 oC.

10.3 The PIF ambipolar transistor and inverter

Typical output characteristics of the PIF field-effect transistor (see Fig.10.2) demonstrate that the transistor operates both in the hole-enhancementand electron-enhancementmode. For high negative gate voltages, Vg, the transistor is in the hole accumulation mode (Fig.10.2a), with a small injection barrier for holes, which is visible via the non-linear

• utput characteristics at low Vds. At high positive Vg electrons are accumulated in the

semiconductor at the semiconductor-insulator interface (Fig.10.2b). Also for the elec- trons we observe a small injection barrier from gold to PIF. From the transfer characteris-

112 Ambipolar transistors based on a single organic semiconductor

• J
• G

V

µ GV

• J
• G

V

µ GV

Figure 10.2: The output characteristics of the PIF ambipolar transistor, oper-

ating in (a) hole-enhancement, and in (b) electron-enhancement mode.

10.3 The PIF ambipolar transistor and inverter 113

• G

V

• J

GV GV

• G

V

• J

GV GV Figure 10.3: The transfer characteristics of the PIF ambipolar transistor. (a)

For Vg < −15 V only the hole contribution is observed in the current, whereas for Vg > −15 V the electron current is seen. (b) For Vg > 10 V only the electron contribution to the current is ob-

• served. The hole current-contribution is observed for Vg < 10 V .

114 Ambipolar transistors based on a single organic semiconductor

• 9,1

9'' 9287 $PELSRODU )(7 $PELSRODU )(7

Q

'' ''

• 2

8 7

• ,1

Figure 10.4: Transfer characteristics of a CMOS-like inverter (see inset) based

• n two identical ambipolar PIF-endcapped (see inset) field-effect
• transistors. Depending on the polarity of the supply voltage, VDD,

the inverter works in the first or the third quadrant. tics in the linear operating regimes of the transistor (see Fig.3) we calculate the field-effect mobility using: µFE

• Vg
• =

L WCi Vds ∂ Ids ∂Vg (10.1) For the holes (at Vg = −30 V ) and electrons (at Vg = 30 V) we find a µFE of 4·10−5 cm2/Vs and 5·10−5 cm2/Vs respectively. We find that in the present transistor the

• nset of the field-effect for the electron accumulation was at Vg = 10 V, and for the hole

accumulation at Vg = −15 V (see Fig.10.3). The combination of a single material for the source and drain electrodes and a single material for the semiconductor layer, with evenly matched field-effect mobilities for the holes and electrons allows the most simple fabrication of a logic voltage inverter. Inverter

• peration is observed for two identical PIF ambipolar transistors connected in accordance

with the schematic diagram given in the inset of Fig.10.4. The ambipolar inverter operates in the first and the third quadrant, depending on the applied supply voltage, VDD. This behavior was also observed for ambipolar inverters based on a blend of n-type and p-type semiconductors (see previous Chapter) [2]. To demonstrate that the ambipolar behavior observed in FETs based on PIF, is not particular to the semiconductor used here, but is more widely applicable to other semicon- ductors, we also performed measurements on a solution-processed pentacene film. The sample was measured in vacuum after an anneal of 10 hours at 90oC, and showed electron enhancement at high applied positive Vg, as is shown in Fig.10.5. The electron current is higher than the observed gate currents in the transistor. Also, the electron current is only

• bserved after thermal anneal in vacuum and is not observed anymore if the sample is

10.4 Conclusions 115

• G

V

• J

GV GV

Figure 10.5: Transfer characteristics of a pentacene FET in vacuum, demon-

strating electron enhancement currents. exposed to air. This demonstrates that there are organic semiconductors that can achieve ambipolar transistor operation.

10.4 Conclusions

In conclusion, we have demonstrated ambipolar field-effect transistors based on the solution- processed organic semiconductor poly(3,9-di-tert-butylindeno[1,2-b] fluorene) and also for solution-processed pentacene. Both charge carrying moieties are injected into the

• rganic semiconductor from gold electrodes, and the semiconductor is processed from

solution, which is an important prerequisite for low-cost polymer electronics. Functional CMOS-like inverters have been demonstrated. This result paves the way for CMOS tech- nology based on a single solution-processable organic semiconductor.

References

[1] H. Reisch, U. Wiesler, U. Scherf, N. Tuytuylkov, Macromolecules, 29, 8204 (1996). [2] E.J. Meijer, D.M. de Leeuw, S. Setayesh, E. van Veenendaal, P.W.M. Blom, J.C. Hummelen, T.M. Klapwijk, submitted [3] S.M. Sze ”Physics of Semiconductor devices”(Wiley, New York, 1981). [4] H. Reisch, W. Wiesler, U. Scherf and N. Tuytuylkov, Macromolecules 29, 8204 (1996).

Summary

Charge transport in disordered organic field-effect transistors

Nowadays, plastics are used in a large variety of applications, ranging from packag- ing materials to toys, and from furniture to plastic bags. In recent times a lot of effort is put also in plastics with electronic and optical functionality. This paves the way for “plas- tic electronics”, such as flexible, flat displays, or cheap electronic barcodes. The plastics,

• r organic polymers, that are being used for such applications need to be able to conduct

current, i.e. facilitate charge carrier motion. In this thesis we address the question of how charge carriers move in an organic semiconducting polymer film. The experimental study is performed on field-effect transistor structures, where the semiconducting polymer lies between two electrodes, and on top of an insulating layer. By applying a current between the two electrodes, current flows through the organic semiconductor. This current can be modulated by a third electrode, which lies under the insulating layer. this third electrode, termed “gate”, can modulate the current by changing the density of charge carriers in the semiconducting polymer. Therefore, the charge transport can be studied as a function of the charge carrier density in the field-effect transistor. Of importance for the charge transport is the disorder in the semiconducting polymer

• layer. This disorder is described in theoretical models by a spatial and energetical distri-

bution of energy states. Furthermore, the charge carriers themselves give rise to local lattice deformations, through which they localize in the polymer film. The charge carriers can move in the polymer film under the influence of an applied electric field, by hopping between two neighbouring energy sites. The energy required to hop is obtained from the lattice vibrations, i.e. phonons, of the polymer. In chapter 2 two parameters, that are used for the description of the field-effect tran- sistor, are critically evaluated. It is argued that the characterization parameters used in standard silicon technology, are not useable for disordered organic transistors. Therefore, we define a new physical parameter, the “switch-on” voltage of the transistor, as the volt- age that needs to be applied to the gate electrode to have exactly no charge accumulation in the semiconductor. To test the suitability of this parameter, we measured the current- voltage characteristics of transistors, based on a variety of organic semiconductors, as a function of temperature. These characteristics are theoretically modeled using a descrip- tion based on hopping between sites in the polymer. From the modelling we conclude that the “switch-on” voltage is a useful physical parameter for the description of organic tran-

• sistors. In the second paragraph, the mobility of the charge carriers, which is a measure
• f how easy the carriers can move in the polymer film, is analyzed and determined as a

function of the charge density distribution in the transistor. We conclude that the average

120 Summary mobility of charges directly at the interface with the insulator layer is a useful parameter in the description of the transistors. In the last paragraph of chapter 2 we link the mobility

• btained from the transistors to the mobility as it is determined in light-emitting diodes.

We conclude that the large differences in mobility obtained for a single polymeric semi- conductor in these two devices is mainly due to a large difference in charge carrier density in the two devices. In chapter 3 the mobility of the charge carriers is measured and analyzed as a func- tion of temperature. From these data we observe an empirical rule, termed the Meyer- Neldel rule. Associated with this rule is a characteristic energy, which has a very similar value for all the studied materials, which implies a common origin of this energy in the charge transport mechanism. It is argued that a possible origin of the characteristic energy is due to the fact that a charge carrier can require multiple phonons to hop in the polymer film, rather than just one phonon. In chapter 4 this empirical rule is also related to the experimentally observed electric field dependence of the mobility. For practical applications, it is not only of importance to understand the charge trans- port through the polymer film, but also to gain insight in how the charge carriers are in- jected into the polymer film. The electrodes that we used in this study consist of gold. The charges do not go from the gold to the polymer layer without resistance. To get an idea

• f the resistance involved in this injection process, in chapter 5 we looked at series of

transistors, where the length of the transistor is systematically varied. In this way we can separate the charge transport process in the polymer film from the charge injection process at the electrodes. We find a correlation between the injection resistance and the mobility in the polymer itself and interpret this by considering injection from the electrode into an energetical distribution of states in the polymer. Certain polymers, with promising properties for applications, also have their draw- backs in terms of air and light stability. In chapters 6 and 7 we look at the polymer poly(3-hexyl thiophene), which has a conductivity that increases with time under the in- fluence of light and oxygen exposure. This is the result of a doping process where an increasing amount of charge carriers is present in the polymer layer, that contribute to the conductivity. In chapter 6 we look first at impedance analysis techniques applied to metal-insulator-semiconductor diodes (MIS diodes), to be able to determine the amount

• f dopants in the polymer. We find that the measurement frequency used in the impedance

technique can not be too high, because otherwise the low mobility of the charge carriers in the polymer will dominate the measurement results instead of the dopant density. In chapter 7 this technique is applied to determine how efficient the dopant charges form un- der the influence of light and oxygen. In chapter 8 we demonstrate that the dopant density changes can also directly be determined from field-effect transistor measurements. Fur- thermore we are able to relate the mobility of charge carriers in the polymer bulk to the dopant density. In the previous chapters we have mainly looked at unipolar p-type transistors, which means: transistors in which predominantly positively charged carriers move. In chapter 9 we show that a suitable combination of the electrode material with a mixture of a p-type and an n-type semiconductor can be used to construct ambipolar field-effect transistors. These transistors enable a simplification of the circuit design of integrated circuits. At the same time the circuit performance may improve. Logic gates (such as a voltage inverter,

Summary 121 as is demonstrated in chapter 9) switch in a better defined region of the voltage axis and a have a steeper crossover in voltage upon going from high to low voltage as compared to unipolar logic gates. In chapter 10 we demonstrate that ambipolar transistor operation can also be realized on the basis of a single semiconducting polymer. These findings contribute to develop organic electronics more towards complementary logic, by using the properties of the semiconductors as optimally as possible for electronic circuitry.

Samenvatting

Ladingstransport in wanordelijke organische veldeffecttransistoren

Tegenwoordig worden plastics veelvuldig gebruikt in toepassingen, vari¨ erend van verpakkingsmateriaal tot speelgoed en van meubels tot plastic tassen. Er wordt de laatste tijd zelfs gewerkt aan plastics die elektronische en/of optische functies kunnen vervullen. Dit opent de deur naar de “plastic elektronica”, zoals bijvoorbeeld flexibele, dunne dis- plays, of goedkope elektronische streepjescodes. De plastics, of organische polymeren, die gebruikt kunnen worden in deze toepassingen dienen stroom te kunnen geleiden, en dus moeten ze ladingsdragers kunnen transporteren. In dit proefschrift wordt ingegaan op de vraag hoe deze ladingsdragers bewegen in een organische halfgeleidende polymeer-

• laag. De experimentele studie is gedaan aan de hand van de veldeffecttransistor, waar

het halfgeleidende polymeer zich tussen twee elektrodes bevindt op een isolerende on-

• dergrond. Door een elektrische spanning aan te leggen tussen de twee elektrodes, loopt

er een stroom door de organische halfgeleider. Deze stroom kan gereguleerd worden met een derde elektrode, die zich onder de isolerende laag bevindt. Deze derde elektrode (die men “gate” noemt) reguleert de elektrische stroom door de hoeveelheid ladingsdragers in het polymeer te veranderen. Zo kan het ladingstransport in organische halfgeleiders bekeken worden als functie van de ladingsdichtheid. Van belang voor dit ladingstransport is de wanorde in de polymeer laag. Deze wanorde wordt in theoretische modellen vaak beschreven met een ruimtelijke en ener- getische verdeling van transporttoestanden. Ladingsdragers in zo’n wanordelijke poly- meerlaag zorgen zelf voor een lokale roostervervorming, waardoor zij zich lokaliseren op een plek in de polymere film. Deze ladingen kunnen bewegen in de polymeerfilm on- der invloed van een aangelegd elektrisch veld, door te springen tussen twee nabijgelegen

• transporttoestanden. De energie om zo’n sprong te kunnen maken halen ze uit roostervi-

braties (phononen) van het polymeer. In hoofdstuk 2 worden allereerst twee parameters, die gebruikt worden om de werk- ing van de veldeffecttransistoren te beschrijven, kritisch belicht. We beargumenteren dat de parameters die gebruikt worden in de standaard siliciumtechnologie, niet bruikbaar zijn voor wanordelijke organische transistoren. Daarom defini¨ eren we de startspanning (switch-on voltage) van de transistor als de spanning die op de gate-elektrode aangelegd moet worden zodat er net geen ladingen in de halfgeleiderlaag meer zitten. Om de bruik- baarheid van deze parameter te toetsen hebben we de stroom-spannings karakteristieken van transistoren, die gebaseerd zijn op allerlei verschillende polymeren, doorgemeten als functie van de temperatuur. Deze karakteristieken worden theoretisch beschreven met een model gebaseerd op het springen tussen de energetische toestanden in het polymeer.

124 Samenvatting Uit het modelleren concluderen we dat de startspanning een bruikbare fysische param- eter is voor de beschrijving van organische transistoren. Voorts wordt de mobiliteit van de ladingsdragers, welke aangeeft hoe gemakkelijk ladingen bewegen door de polymeer laag heen, experimenteel bepaald als functie van de ladingsdichtheid in de transistor. We concluderen dat de gemiddelde mobiliteit van ladingen direct aan het oppervlak met de isolator een goede parameter is voor de beschrijving van de transistoren. In de laatste paragraaf van hoofdstuk 2 wordt deze ladingsdichtheidsafhankelijke mobiliteit in veld- effecttransistoren gelinkt met de mobiliteit zoals die bepaald wordt uit lichtemitterende

• diodes. We concluderen dat de grote verschillen in mobiliteit, die waargenomen wor-

den voor een enkel polymeer in de twee verschillende structuren, voor een groot deel het gevolg zijn van een aanzienlijk verschil in ladingsdichtheid tussen de transistor en de lichtemitterende diode. In hoofdstuk 3 wordt de mobiliteit van de ladingsdragers bekeken als functie van de temperatuur. Aan de hand van deze data, nemen we een empirische regel, getiteld de Meyer-Neldel regel, waar. Aan deze regel is een karakteristieke energie verbonden die voor de bestudeerde materialen steeds eenzelfde waarde oplevert. Dit feit duidt mogelijk

• p een gemeenschappelijke oorsprong van deze energie in het ladingstransport in de ver-

schillende materialen. Er wordt beargumenteerd dat een mogelijke grondslag voor deze karakteristieke energie gelegen is in het feit dat een lading voor een sprong tussen twee plaatsen in het polymeernetwerk meerdere phononen per sprong nodig heeft, in plaats van slechts een enkel phonon. In hoofdstuk 4 wordt deze empirische regel tevens gekoppeld aan de experimenteel waargenomen veldafhankelijkheid van de mobiliteit. Voor praktische toepassingen is niet alleen het begrip van het ladingstransport belan- grijk, maar ook hoe ladingen ge¨ ınjecteerd worden in de polymeerlaag. De elektrodes, die we hiervoor gebruikt hebben, bestaan uit goud. De ladingen gaan niet zonder enige weer- stand de polymeerlaag in vanuit het goud. Om een idee te krijgen van de weerstanden die hierbij belangrijk zijn, hebben we in hoofdstuk 5 gekeken naar series van transistoren waarbij de lengte van de transistor systematisch veranderd wordt. Zo kunnen we het lad- ingstransport in de polymeerlaag loskoppelen van de ladingsinjectie van het goud naar de polymeerlaag. We vinden een correlatie tussen de injectieweerstand en de mobiliteit in het polymeer zelf en interpreteren dit in de context van de verdeling van energetische toestanden in het polymeer. Bepaalde polymeren met veelbelovende eigenschappen hebben ook nadelen ten aan- zien van hun lucht- en lichtgevoeligheid. In hoofdstukken 6 en 7 kijken we naar het polymeer poly(3-hexyl thiofeen), waarvan de geleiding in de tijd toeneemt onder invloed van zuurstof en licht uit de omgeving. Dit is het gevolg van een doteringsproces waar- bij er steeds meer ladingsdragers in het polymeer aanwezig zijn, die bijdragen tot de

• geleiding. In hoofdstuk 6 wordt allereerst gekeken naar de impedantieanalysetechniek

toegepast op metaal-isolator-halfgeleider diodes (MIS diodes), voor het bepalen van de hoeveelheid dotering in het polymeer. We vinden dat de meetfrequentie waarbij de bepal- ing wordt gedaan niet te hoog mag zijn omdat anders de mobiliteit van de ladingsdragers in het polymeer de meting domineert in plaats van de hoeveelheid dotering. In hoofd- stuk 7 wordt deze meettechniek toegepast om te bepalen hoe effici¨ ent doteringsladingen gevormd worden onder invloed van zuurstof en licht. In hoofdstuk 8 laten we zien dat de doteringsveranderingenook direct aan de hand van veldeffecttransistoren bepaald kun-

Samenvatting 125 nen worden. Vervolgens kunnen we tevens de bulk mobiliteit van de doteringsladingen relateren aan de hoeveelheid dotering. In de voorgaande hoofdstukken hebben we vooral gekeken naar unipolaire p-type transistoren, dat wil zeggen transistoren waarin slechts positieve ladingen bewegen. In hoofdstuk 9 laten we zien dat een geschikte combinatie van elektrodemateriaal en een mengsel van een p-type halgeleider met een n-type halfgeleider gebruikt kan worden om ambipolaire veldeffecttransistoren te maken. Deze transistoren maken het mogelijk het

• ntwerp van ge¨

ıntegreerde circuits te vereenvoudigen. Tegelijkertijd verbetert de werk- ing van de circuits. De logische poortjes (zoals een spanningsomkeerder), die gebaseerd zijn op ambipolaire transistoren, schakelen meer symmetrisch en hebben een steilere om- schakeling van hoge spanning naar lage spanning, dan de unipolaire logische poortjes. In hoofdstuk 10 tonen we aan dat ambipolaire transistoroperatie ook gerealiseerd kan wor- den op basis van een enkel halfgeleidend polymeer. Deze vindingen dragen ertoe bij de

• rganische elektronica verder te laten ontwikkelen naar complementaire logica.

List of Publications

• 1. The Meyer-Neldel rule in organic thin-film transistors,

E.J. Meijer, M. Matters, P.T. Herwig, D.M. de Leeuw and T.M. Klapwijk, Appl.

• Phys. Lett 76, 3433 (2000). [Chapter 3]
• 2. The isokinetic temperature in disordered organic semiconductors,

E.J. Meijer, D.B.A. Rep, D.M. de Leeuw, M. Matters, P.T. Herwig and T.M. Klap- wijk, Synth. Met. 121, 1351 (2001). [Chapter 4]

• 3. Charge-transport in partially-ordered regioregular poly(3-hexylthiophene) studied

as a function of the charge density, D.B.A. Rep, B.-H. Huisman, E.J. Meijer, P. Prins and T.M. Klapwijk, Mat. Res.

• Soc. Symp. Proc. 660, JJ7.9.1 (2001). [Chapter 4]
• 4. Frequency behavior and the Mott-Schottky analysis in poly(3-hexyl thiophene) metal-

insulator-semiconductor diodes, E.J. Meijer, A.V.G. Mangnus, C.M. Hart, D.M. de Leeuw and T.M. Klapwijk, Appl.

• Phys. Lett. 78, 3902 (2001). [Chapter 6]
• 5. Switch-on voltage in disordered organic field-effect transistors,

E.J. Meijer, C. Tanase, P.W.M. Blom, E. van Veenendaal, B.-H. Huisman, D.M. de Leeuw and T.M. Klapwijk, Appl. Phys. Lett. 80, 3838 (2002). [Chapter 2]

• 6. Charge transport in disordered organic field-effect transistors,
• C. Tanase, P.W.M. Blom, E.J. Meijer and D.M. de Leeuw, Mat. Res. Soc. Symp.
• Proc. 725, P10.9.1 (2002). [Chapter 2]
• 7. Dopant density determination in disordered organic field-effect transistors,

E.J. Meijer, C. Detcheverry, P.J. Baesjou, E. van Veenendaal, D.M. de Leeuw and T.M. Klapwijk, J. Appl. Phys. 93, 4831 (2003). [Chapter 8]

• 8. Scaling behavior and parasitic series resistance in disordered organic field-effect

transistors, E.J. Meijer, G.H. Gelinck, E. van Veenendaal, B.-H. Huisman, D.M. de Leeuw and T.M. Klapwijk, accepted for publication in Appl. Phys. Lett. [Chapter 5]

• 9. Local charge carrier mobility in disordered organic field-effect transistors,
• C. Tanase, E.J. Meijer, P.W.M. Blom and D.M. de Leeuw, submitted. [Chapter 2]
• 10. Unification of the hole transport theories in light-emitting polymeric diodes and

field-effect transistors,

• C. Tanase, E.J. Meijer, P.W.M. Blom and D.M. de Leeuw, submitted. [Chapter 2]

128 List of Publications

• 11. Photoimpedance spectroscopy of poly(3-hexyl thiophene) metal-insulator-semicon-

ductor diodes, E.J. Meijer, A.V.G. Mangnus, B.-H. Huisman, G.W. ’t Hooft, D.M. de Leeuw and T.M. Klapwijk, submitted. [Chapter 7]

• 12. Solution-processed ambipolar organic field-effect transistors and inverters,

E.J. Meijer, D.M. de Leeuw, S. Setayesh, E. van Veenendaal, B.-H. Huisman, P.W.M. Blom, J.C. Hummelen, U. Scherf, and T.M. Klapwijk, submitted. [Chapters 9 and 10]

Patent Application

Solution-processed ambipolar organic field-effect transistors D.M. de Leeuw, E.J. Meijer, S. Setayesh European Patent Application EP 03100177.9

Curriculum Vitae

Eduard Johannes Meijer

April 5th, 1975 Born in Drachten, The Netherlands. 1987 - 1993 Gymnasium at the Ichthus College in Drachten. 1993 - 1998 M.Sc. Applied Physics at the University of Groningen. Graduate research in the Thin-Film Physics group of prof.dr.ir. T.M. Klap-

• wijk. Subject: Charge trapping instabilities in quaterthiophene

thin-film transistors: experimental observations. 1998 Visiting student at Xerox Palo Alto Research Center, Palo Alto, USA, in collaboration with dr. R.B. Apte. Subject: Morphology study of anodized porous aluminum oxide. 1998 - 2003 Ph.D research at Delft University of Technology in the nanophysics group of prof.dr.ir. T.M. Klapwijk. This work was performed at the Philips Research Laboratories in collaboration with dr. D.M. de Leeuw. Subject: Charge transport in disor- dered organic field-effect transistors. april 2003 Research scientist at Philips Research Laboratories, Eindhoven.

Dankwoord

Hoe mooi is het om de culminatie van mijn promotieonderzoek hier gebundeld te hebben en terug te kunnen lezen. Voor de lezer zal dit boek overkomen als een verza- meling van droge feiten, een cijfermatig geheel waarin claims worden gelegd en physica wordt bedreven. Voor mij is het dat al lang niet meer. Het is een klein geschiedenis- boek waarin ik de verschillende perioden en fasen, pieken en dalen, van de afgelopen vier en een half jaar nog weer eens de revue kan laten passeren. Het geheel is meer dan de som der delen en hoewel dit dankwoord maar twee pagina’s beslaat, zijn de mensen die me op allerlei fronten terzijde hebben gestaan ten aanzien van mijzelf en mijn werk, de voedingsbodem geweest om het werk dat beschreven staat in de voorgaande pagina’s tot uitvoer te kunnen brengen. Teun Klapwijk wil ik hartelijk danken voor de kans die hij me gegeven heeft en de gok die hij met me nam door me aan te nemen op het moment dat de verhuizing naar Delft voor de deur stond. Teun, ik heb veel geleerd van je kritische en heldere manier van wetenschappelijk discussi¨

• eren. Ik heb je input en steun altijd zeer gewaardeerd.

Bij het Natuurkundig Laboratorium van Philips, waar ik vier en een half jaar vertoefd heb wil ik graag als eerste Dago de Leeuw bedanken voor de stimulerende en enthousiaste

• samenwerking. Dago, je vermogen de oppervlakte van de wetenschap af te schuimen op

zoek naar de interessante en relevante onderwerpen heeft op mij zeer stimulerend gewerkt. En om dan vervolgens ook nog de diepte in te kunnen gaan door met “boerenverstand” de juiste vragen op te werpen, is iets wat ik als een zeer vruchtbare werkwijze heb ervaren. Voorts, wil ik Bart-Hendrik Huisman bedanken voor de innemende sfeer die hij met zich meebracht naar de laatste kamer van WB 6. Ik heb met veel plezier de kamer met je

• gedeeld. Estrella Mena-Benito, voor haar voortdurende steun, luisterend oor en haar niet

aflatende spraakwaterval, van achter de stepper in de fitnessruimte. Eugenio Cantatore, voor zijn enthousiasme en onze aangename gesprekken over relaties en het leven. Marco Matters voor zijn begeleiding tijdens de beginfase van mijn promotie. En tevens de rest van het polymere elektronica cluster voor de plezierige samenwerking: Patrick Baesjou, Monique Beenhakkers, Celine Detcheverry, Gerwin Gelinck, Tom Geuns, Ben Giesbers, Kees Hart, Edzer Huitema, Karel Kuijk, Albert Mangnus, Alwin Marsman, Andre Mon- tree, Bas van der Putten, Laurens Schrijnemakers, Sepas Setayesh, Fred Touwslager, Erik van Veenendaal en Stephan Zilker. Jan Verhoeven voor het maken van zovele standaard-

• substraten. Verder wil ik Hans Hofstraat en Gerjan van de Walle en tevens de mensen

van de groepen “Polymers and Organic Chemistry” en “Integrated Device Technologies”, in het bijzonder Elize Harmelink en Isabella Geuens, bedanken voor hun gastvrijheid en plezierige werksfeer in de afgelopen jaren. Henny Herps voor het design van de kaft en de gesprekken over creativiteit. Aan de Technische Universiteit Delft was ik altijd meer gast dan groepslid van de groep “Nanophysics”. Niettemin heb ik het altijd prima naar mijn zin gehad aldaar, tijdens

132 Dankwoord mijn tweewekelijkse bezoeken, waarvoor hartelijk dank aan de groepsleden. Diederik Rep, die op velerlei vlak een parallel pad heeft gevolgd aan het mijne, wil ik graag danken voor de mooie tijd die we samen in maar ook buiten polymerenland hebben doorgebracht. Diederik, ik spreek nogmaals de wens uit, nu gezien vanuit het perspectief van mijn pro- motie, dat we ondanks divergerende wegen binnen elkaars cirkel zullen blijven. Verder de andere mensen verbonden aan het organische halfgeleider werk: Ruth de Boer, Jos Koonen, Hon Tin Man en Alberto Morpurgo. Van het secretariaat Monique Vernhout, Maria Roodenburg-van Dijk en Margaret van Fessem voor alle administratieve hulp die ze me geboden hebben de afgelopen jaren. Aan de Rijksuniversiteit Groningen wil ik graag de plezierige en vruchtbare samen- werking met Paul Blom en Cristina Tanase vermelden. Jan Anton, ik wens je veel succes met jouw promotie. Verder heb ik veel baat gehad bij de technische hulp van Minte Mul- der en Bernard Wolfs in de beginfase van mijn promotie. Alex Schoonveld bedankt voor zijn hulp bij het begin van mijn promotie, die eerste maand in Groningen. Mijn vrienden met wie ik door heel Nederland mooie tijden heb beleefd en nog zal beleven. Mijn ouders en mijn broers voor hun steun tijdens het promotiewerk. Arnoud en Richard, ik vind het heel bijzonder dat jullie mijn paranimfen willen zijn. Pimm, bedankt voor je warmte, liefde en steun in alles wat ik doe. Eduard