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 C60 molecules

Conclusion

Organic Light Emitting Diodes (OLED) Organic Field-Effect Transistors (OFET) Organic Photovoltaic Cells (OPVC) Organic Memory Cells

+ (-)

• (+)

NC COM E E Source Drain Gate Gate insulator

Organic channel

VDS VG

1

"Organic Electronics" Devices

CN CN F F F F NC NC

S S S S S S

• T

k AT j

B

barrier injection charge exp

2

cathode anode

• rganic

material

EF

hn

EF Evac U-

• VB (HOMO)

CB (LUMO)

SE

U - (1 - 2)

Why are interfaces important: example: Organic Light Emitting Devices

OLED

• How do electrodes and organics interact?
• Physico-chemical properties?
• Energy level alignment at interfaces?
• Influence on charge injection?
• Morphological/structural aspects of

interface-formation? Molecular Electronics": Interface-Only Devices!

1 h

Injection-limited current:

Estimating charge injection barriers: The Schottky-Mott Limit

1 IE h,1 EF 2 IE h,2 EF Evac Evac

h,2 = h,1 – (2 – 1)

if Schottky-Mott limit (vacuum level alignment) applies: charge injection barriers can be predicted from materials parameters:

• metal work function
• organic material ionization energy IE
• organic material electron affinity EA

i substrate work function IE ionization energy h,i hole injection barrier EF Fermi level Evac vacuum level

1

EA IE

e h

• EA

e

Ionization Energy, Work Function & Charge Injection Barriers from photoelectron spectroscopy

ionization energy = h – (Ekin,HOMO – Ekin,SECO) work function = h – (Ekin,EF – Ekin,SECO) hole injection barrier = Ekin,EF – Ekin,HOMO core-levels: type of interaction

Secondary electron cutoff (SECO) HOMO or EF Ekin C

• u

n t s E E

kin,HOMO kin,EF

Ekin,SECO 1 measurements:

in ultrahigh vacuum (p < 10-9 mbar)

sample preparation:

• molecular layers evaporated

(stepwise) in situ

• polymers spin coated

ex situ

Substrate

Organic

sample spectrometer

e

• h

Example for physisorptive organic/metal interface:

pentacene on Au(111)

3 2 1

(2) (1)

MT(Å) 110 50 16 8 4 2

e= 45°

intensity (arb. units) binding energy (eV)

Estimated from Au (5.40 eV) and IEPEN (5.1 eV): est = IEPEN- Au = - 0.3 eV Measured: exp = 0.6 eV

PEN

PEN= 0.60 eV ID = 0.95 eV (change of )

Au=5.50 vac,PEN=0.95

0.60

PEN=4.55

Evac EF

(1)

Koch, Vollmer, Duhm, Sakamoto, Suzuki, Adv. Mater. 19 (2007) 112

1

Invalidity of Schottky-Mott model for organic/metal interfaces: Interface Dipoles

Schottky-Mott Limit

i substrate work function IE ionization energy

• hole injection barrier

EF Fermi level Evac vacuum level Ishii, Sugiyama, Ito, Seki, Adv. Mater. 11 (1999) 605 Koch, ChemPhysChem 8 (2007) 1438

1 Interface Dipole (ID or vac):

• charge transfer
• bond formation
• metal electron "push-back"

ID EA ID IE

e h

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

Systematic tuning of energy levels

metal surface potential changes as (linear) function of acceptor coverage due to metaladsorbate charge transfer (CT). CT creates localized dipoles

• Helmholtz-Equation:
• eN
• mechanism works in general:

predictable tuning of HIB for any subsequent organic layer by up to 1.4 eV

Koch, Duhm, Rabe, Vollmer, Johnson, Phys. Rev. Lett. 95 (2005) 237601

+ + +

N

1

HIB reduction and increase small

+ + + + + +

µ N

2

HIB reduction and increase large

+ + +

µ N

• 1

HIB reduction and increase small

+ + + + + +

N

• 2

HIB reduction and increase large

hole injection barrier height HIB

max

• ca. 1 ML

acceptor coverage 0 ML HIB

min

O O F F F F F F F F

CN NC NC CN

CN CN F F F F NC NC

F4TCNQ tetrafluoro-tetracyano- quinodimethane TCAQ FAQ

+ + + + + + + + + + + +

• rganic semiconductor

2

for ... effective diel. const.

• equiv. to Topping-model

Molecular energy levels after charge transfer: simple model of integer charge transfer and molecular ions

binding energy E =0

vac

N nP nBP

EF

(HOMO) (LUMO)

N neutral molecule insulating/semiconducting nP "negative Polaron" (anion) metallic nBP "negative Bipolaron" (dianion) insulating/semiconducting

(LUMO+1)

2

Energy Levels and of F4TCNQ on Cu(111): Simple charge transfer?

• 10
• 8
• 6
• 4
• 2

5 10 15 4.8 5.0 5.2 5.4 5.6 5.8 UPS DFT

EF intensity (arb. units) binding energy (eV)

eV Å)

Comparison UPS and Density Functional Theory (DFT) *

LUMO of F4TCNQ becomes filled located below EF: non-metallic

work function increases: Cu(111): 5.0 eV F4TCNQ/Cu: 5.6 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 !) 2

CN CN F F F F NC NC

* Zojer & Brédas groups, TU-Graz/GA-Tech

Detailed mechanism of metal -increase: F4TCNQ on Cu(111)

CN CN F F F F NC NC

x z y x

3.6 (3.3) 2.1 (2.7) 0.0 (0.0)

X-ray standing waves (XSW) Density functional theory (DFT)* Bonding distances from Cu: Theory Experiment F: 3.6 Å F: 3.3 Å N: 2.1 Å N: 2.7 Å

F4TCNQ conformation is changed due to adsorption on Cu:

• quinoid (bulk) to aromatic (adsorbed) CT
• bulk F4TCNQ: planar

F4TCNQ on Cu(111): non-planar non-planarity induces dipole that decreases !

* Zojer & Brédas groups, TU-Graz/GA-Tech

2

Bonding mechanism and bi-directional charge transfer

H-9 L

Metal Molecule charge transfer: LUMO (-level) filled with 1.8 e Molecule Metal charge transfer: H-9 etc. (-levels) depleted of e

net CT: 0.6 e transferred to F4TCNQ

Including all effects: due to net charge transfer 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

Orbital occupation analysis

2

Gold work function reduction by 2.2 eV with an air-stable molecular donor layer

8 9 10 11 12

e d c b a

• 4.10 eV

3.30 eV 4.20 eV 3.30 eV 5.50 eV

intensity (arb. units)

kinetic energy (eV)

N N methyl viologen (MV0) 1,1'-dimethyl-1H,1'H-[4,4']bipyridinylidene

pristine Au 1 ML MV0/Au electron injection barriers lowered by: 0.8 eV for Alq3 0.7 eV for C60

2

Bröker, Blum, Frisch, Vollmer, Hofmann, Rieger, Müllen, Rabe, Zojer, Koch, Appl. Phys. Lett. 93 (2008) 243303

3 Organic Electronics Molecular Electronics

How to make "good metallic" contacts to individual molecules ?

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: C60 on Ag(111)

scanning tunneling microscopy (STM) UPS (density of valence states) close packed C60 monolayer lattice constant molecular diameter 1 nm electronic cross-talk between neighboring molecules "bulk" C60: large energy gap (no DOVS close to EF) monolayer C60: gap-state near EF not a "semiconductor"

3

Designed molecular acceptor to pre-pattern Ag(111)

N N N N N N CN CN CN NC NC NC

hexa-azatriphenylene-hexanitrile (HATCN)

STM: monolayer HACTN/Ag(111) honeycomb structure w/ hole lattice constant 2 nm UPS (density of valence states) HATCN / Ag(111) is metallic partially filled LUMO cuts EF and extends into vacuum side

3 calculated electron density distribution @ EF

"Soft metallic" contacts: C60 on HATCN/Ag(111)

STM: lattice constant 2 nm C60 in hexagonal lattice individual C60 molecules (reduced cross-talk) UPS (density of valence states)

Using "soft molecular metal" as structural template, i.e., HATCN/Ag(111): metallic contact to individual C60 molecules function ("semiconductor") preserved at room temperature

C60 on HATCN / Ag(111) has bulk electronic structure

Glowatzki, Bröker, Blum, Hofmann, Rabe, Müllen, Zojer, Koch, Nano Lett. 8 (2008) 3825

3

Conclusions

• rganic/metal electrodes:

rather complex multiple mechanisms; simple models do not apply.

• metal electron "push-back"
• charge transfer
• bond formation
• model with reliable predictive character still missing

(for adsorption on "clean" metals) + injection barrier tuning with acceptors/donors: concept transfer from UHV to even air feasible Using "soft molecular metal" as structural template: metallic contact to individual C60 molecule function ("semiconductor") preserved at room temperature

Acknowledgements

HU-Berlin Georg Heimel Jürgen P. Rabe

Financial Support:

• Sfb 448 (DFG)
• Emmy Noether Program (DFG)
• SPP 1355 (DFG)
• H. C. Starck GmbH
• EC (STREP "ICONTROL")

BESSY Antje Vollmer Supramolecular Systems Ralf-Peter Blum Benjamin Bröker Steffen Duhm (now Chiba U) Johannes Frisch Fatemeh Ghani Hendrik Glowatzki Sven Käbisch Ingo Salzmann Raphael Schlesinger Rasmus Talviste Jörn-Oliver Vogel Shuwen Yu Jian Zhang TU-Graz Lorenz Romaner Egbert Zojer Georgia-Tech Jean-Luc Brédas Hasylab Robert L. Johnson

• H. C. Starck

Andreas Elschner U Tübingen Alexander Gerlach Frank Schreiber ESRF Jörg Zegenhagen MPI-Polymer Res. Ralph Rieger Klaus Müllen