Functional interfaces with conjugated organic materials: energy - PowerPoint PPT Presentation

Functional interfaces with conjugated organic materials: energy level tuning and "soft" metallic contacts Norbert Koch Emmy Noether-Nachwuchsgruppe "Supramolecular Systems" Institut für Physik Humboldt-Universität zu Berlin

Outline: 1. Interfaces in "organic electronics": conjugated molecules (semiconductors) and electrodes (conductors) 2. Optimizing energy levels at organic/metal interfaces with strong electron acceptors/donors - work function increase with a molecular acceptor - work function reduction with a molecular donor 3. "Soft" metallic contacts to individual C 60 molecules Conclusion

"Organic Electronics" Devices 1 S S S Organic Field-Effect Transistors S S S (OFET) Organic Light Emitting Diodes (OLED) Source Drain Organic channel V DS Gate insulator Gate V G Organic Photovoltaic Cells Organic Memory Cells (OPVC) + (-) E NC COM F F NC CN E NC CN F F - (+)

Why are interfaces important: example: Organic Light Emitting Devices 1 OLED - How do electrodes and organics interact? E vac - Physico-chemical properties? U- � �� U - ( � 1 - � 2 ) � � - Energy level alignment at interfaces? � � - Influence on charge injection? CB (LUMO) E F - Morphological/structural aspects of � � interface-formation? SE h � Molecular Electronics": h n Interface-Only Devices! E F Injection-limited current: VB (HOMO) cathode anode organic �� � charge injection barrier material � � � 2 j AT exp � � k T � � B

Estimating charge injection barriers: The Schottky-Mott Limit 1 if Schottky-Mott limit ( vacuum level alignment ) applies: charge injection barriers � can be predicted from materials parameters: • metal work function � � � � � IE h • organic material ionization energy IE � � � � • organic material electron affinity EA EA e E vac E vac � i substrate work function IE ionization energy IE IE � h,i EA hole injection barrier E F Fermi level � 1 � 2 � e E vac vacuum level E F � h,2 = � h,1 – ( � 2 – � 1 ) � h,1 � h,2 E F

Ionization Energy, Work Function & Charge Injection Barriers from photoelectron spectroscopy 1 h � measurements: spectrometer in ultrahigh vacuum (p < 10 -9 mbar) - sample e sample preparation: Secondary electron cutoff (SECO) - molecular layers evaporated (stepwise) in situ s t n HOMO or E F u - polymers spin coated o C ex situ E kin E kin,SECO E kin,HOMO E kin,EF ionization energy = h � – (E kin,HOMO – E kin,SECO ) work function = h � – (E kin,EF – E kin,SECO ) Organic hole injection barrier = E kin,EF – E kin,HOMO Substrate core-levels: type of interaction

Example for physisorptive organic/metal interface: pentacene on Au(111) 1 E vac � vac,PEN =0.95 � Au =5.50 � e = 45° � PEN =4.55 (1) MT(Å) intensity (arb. units) E F 0.60 0 2 (2) 4 8 (1) 16 50 � PEN = 0.60 eV PEN 110 ID = 0.95 eV (change of � ) 3 2 1 0 binding energy (eV) Estimated from � Au (5.40 eV) and IE PEN (5.1 eV): � est = IE PEN - � Au = - 0.3 eV � exp = 0.6 eV Measured: Koch, Vollmer, Duhm, Sakamoto, Suzuki, Adv. Mater . 19 (2007) 112

Invalidity of Schottky-Mott model for organic/metal interfaces: Interface Dipoles 1 � � � � � IE ID h � � � � � EA ID e Interface Dipole (ID or � vac ): Schottky-Mott Limit • charge transfer • bond formation � i substrate work function • metal electron "push-back" IE ionization energy � hole injection barrier E F Fermi level Ishii, Sugiyama, Ito, Seki, Adv. Mater . 11 (1999) 605 E vac vacuum level Koch, ChemPhysChem 8 (2007) 1438

2 2a � Organic/metal interface energy level tuning 2b � Bonding of an acceptor molecule on a metal

Systematic tuning of energy levels 2 metal surface potential � changes as (linear) function of acceptor coverage due to � � metal � adsorbate charge transfer (CT). CT creates localized dipoles hole injection barrier height HIB HIB reduction and � increase small HIB reduction and � increase small max Helmholtz-Equation: � N � � N µ + + + + + + 1 1 � eN � � � � � HIB HIB reduction and � increase large HIB reduction and � increase large 0 min � N � � N µ + + + + + + + + + + + + 2 2 for � ... effective diel. const. 0 ML ca. 1 ML equiv. to Topping-model acceptor coverage mechanism works in general: F4TCNQ TCAQ FAQ tetrafluoro-tetracyano- NC CN F O F quinodimethane organic semiconductor F F F F + + + + + + + + + + + + NC CN F F F O F NC CN predictable tuning of HIB for F F NC CN any subsequent organic layer by up to 1.4 eV Koch, Duhm, Rabe, Vollmer, Johnson, Phys. Rev. Lett. 95 (2005) 237601

Molecular energy levels after charge transfer: simple model of integer charge transfer and molecular ions 2 E =0 vac (LUMO+1) binding energy (LUMO) E F (HOMO) N nP nBP N neutral molecule insulating/semiconducting nP "negative Polaron" (anion) metallic nBP "negative Bipolaron" (dianion) insulating/semiconducting

Energy Levels and � of F4TCNQ on Cu(111): Simple charge transfer? F F 2 NC CN NC CN Comparison UPS and Density Functional Theory (DFT) * F F * Zojer & Brédas groups, TU-Graz/GA-Tech ��� Å) 0 5 10 15 5.8 UPS ���� eV � intensity (arb. units) LUMO of F4TCNQ becomes filled 5.6 DFT 5.4 located below E F : non-metallic 5.2 5.0 4.8 work function � increases: E F Cu(111): 5.0 eV F4TCNQ/Cu: 5.6 eV -10 -8 -6 -4 -2 0 binding energy (eV) Estimation of �� : 2 electrons transferred from Cu to F4TCNQ 2.5 Å F4TCNQ-Cu(111) bonding distance � �� should be + 5 eV ! (experiment: + 0.6 eV !)

Detailed mechanism of metal � -increase: F4TCNQ on Cu(111) 2 X-ray standing waves (XSW) Density functional theory (DFT)* x y F F NC CN 3.6 (3.3) 2.1 (2.7) NC CN 0.0 (0.0) F F z x Bonding distances from Cu: F4TCNQ conformation is changed due to adsorption on Cu: Theory Experiment • quinoid (bulk) to aromatic (adsorbed) � CT F: 3.6 Å F: 3.3 Å • bulk F4TCNQ: planar N: 2.1 Å N: 2.7 Å F4TCNQ on Cu(111): non-planar � non-planarity induces dipole that decreases � ! * Zojer & Brédas groups, TU-Graz/GA-Tech

Bonding mechanism and bi-directional charge transfer 2 Orbital occupation analysis Metal � Molecule charge transfer: LUMO ( � -level) filled with 1.8 e Molecule � Metal charge transfer: H-9 etc. ( � -levels) depleted of e L H-9 net CT: 0.6 e transferred to F4TCNQ � � due to net charge transfer Including all effects: � � due to bent molecular conformation total work function increase from theory : 0.7 eV � experiment: 0.6 eV Romaner, Heimel, Brédas, Gerlach, Schreiber, Zegenhagen, Duhm, Koch, Zojer, Phys. Rev. Lett. 99 (2007) 256801

Gold work function reduction by 2.2 eV with an air-stable molecular donor layer 2 methyl viologen (MV0) 1,1'-dimethyl-1H,1'H-[4,4']bipyridinylidene N N �� 5.50 eV pristine Au a intensity (arb. units) 1 ML MV0/Au b 3.30 eV c 4.20 eV d 3.30 eV 4.10 eV e 8 9 10 11 12 electron injection barriers lowered by: kinetic energy (eV) 0.8 eV for Alq 3 Bröker, Blum, Frisch, Vollmer, Hofmann, Rieger, Müllen, 0.7 eV for C 60 Rabe, Zojer, Koch, Appl. Phys. Lett. 93 (2008) 243303

3 Organic Electronics � Molecular Electronics

How to make "good metallic" contacts to individual molecules ? 3 UPS (density of valence states) challenges in molecular electronics : � lateral separation of individual molecules (reduce lateral cross-talk) � metallic contact changes molecular electronic properties (molecule changes/loses its function) Example: C 60 on Ag(111) scanning tunneling microscopy (STM) close packed C60 monolayer lattice constant � molecular diameter � 1 nm "bulk" C 60 : large energy gap � electronic cross-talk (no DOVS close to E F ) between neighboring molecules monolayer C 60 : gap-state near E F � not a "semiconductor"

Designed molecular acceptor to pre-pattern Ag(111) 3 hexa-azatriphenylene-hexanitrile (HATCN) UPS (density of valence states) NC CN N N NC CN N N N N NC CN STM: monolayer HACTN/Ag(111) honeycomb structure w/ hole lattice constant � 2 nm HATCN / Ag(111) is metallic partially filled LUMO cuts E F and extends into vacuum side calculated electron density distribution @ E F

"Soft metallic" contacts: C 60 on HATCN/Ag(111) 3 UPS (density of valence states) STM: lattice constant � 2 nm C 60 in hexagonal lattice individual C60 molecules (reduced cross-talk) Using "soft molecular metal" as structural template, i.e., HATCN/Ag(111): � metallic contact to individual C 60 molecules � function ("semiconductor") preserved C 60 on HATCN / Ag(111) has � at room temperature bulk electronic structure Glowatzki, Bröker, Blum, Hofmann, Rabe, Müllen, Zojer, Koch, Nano Lett. 8 (2008) 3825