Functional interfaces with conjugated organic materials:energy level tuning and "soft" metallic contacts

Norbert Koch

Emmy Noether-Nachwuchsgruppe"Supramolecular Systems"

Institut für PhysikHumboldt-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

FF

NC

NC

S

S

S

S

S

S

���

����

��

TkATj

B

barrierinjectionchargeexp2

cathodeanode organic

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,2EF

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 functionIE ionization energy�h,i hole injection barrierEF Fermi levelEvac vacuum level

1

EAIE

e

h

������

EA

�e

Ionization Energy, Work Function & Charge Injection Barriersfrom 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

Cou

nts

EE

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 coatedex situ

Substrate

Organic

sample

spectrometer

e-

h�

Example for physisorptive organic/metal interface:pentacene on Au(111)

3 2 1 0

(2)

(1)

MT(Å)

0

110

50

16

84

2

�e= 45°

inte

nsity

(arb

. uni

ts)

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 eVID = 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 barrierEF Fermi levelEvac 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"

IDEAIDIE

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 metal�adsorbate charge transfer (CT). CT creates localized dipoles ��

Helmholtz-Equation:

���0

eN��

mechanism works in general:

predictable tuning of HIB for any subsequent organic layer

by up to 1.4 eVKoch, Duhm, Rabe, Vollmer, Johnson, Phys. Rev. Lett. 95 (2005) 237601

+ + +N �1

HIB reduction and increase small

+ + + + + +µN �2HIB reduction and increase large

+ + +µN ��1

HIB reduction and increase small

+ + + + + +N ��2

HIB reduction and increase large

holeinjection

barrierheight

HIBmax

ca. 1 ML

acceptor coverage

0 ML

HIBmin

O

O

F

F

F

F F

F

F

FCNNC

NC CN

CN

CN

F F

FF

NC

NC

F4TCNQtetrafluoro-tetracyano-quinodimethane

TCAQ FAQ

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

organic 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

bind

ing

e ner

gy

E =0vac

N nP nBP

EF

(HOMO)

(LUMO)

N neutral molecule insulating/semiconductingnP "negative Polaron" (anion) metallicnBP "negative Bipolaron" (dianion) insulating/semiconducting

(LUMO+1)

2

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

-10 -8 -6 -4 -2 0

0 5 10 15

4.85.05.25.45.65.8UPS

DFT

EF

inte

nsity

(arb

. uni

ts)

binding energy (eV)����e

V�

���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 eVF4TCNQ/Cu: 5.6 eV

Estimation of �: 2 electrons transferred from Cu to F4TCNQ2.5 Å F4TCNQ-Cu(111) bonding distance

� � should be + 5 eV ! (experiment: + 0.6 eV !)

2 CN

CN

F F

FF

NC

NC

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

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

CN

CN

F F

FF

NC

NC

xz

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 ÅN: 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 eV3.30 eV

4.20 eV

3.30 eV

5.50 eV

inte

nsity

(arb

. uni

ts)

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 Alq30.7 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 neighboringmolecules

"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

NCN

CN

CNNC

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 EFand 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 temperatureC60 on HATCN / Ag(111) has bulk electronic structure

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

3

Conclusions

� organic/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-BerlinGeorg HeimelJürgen P. Rabe

Financial Support:- Sfb 448 (DFG) - Emmy Noether Program (DFG)- SPP 1355 (DFG)- H. C. Starck GmbH- EC (STREP "ICONTROL")

BESSYAntje Vollmer

Supramolecular SystemsRalf-Peter BlumBenjamin BrökerSteffen Duhm (now Chiba U)Johannes FrischFatemeh GhaniHendrik GlowatzkiSven KäbischIngo SalzmannRaphael SchlesingerRasmus TalvisteJörn-Oliver VogelShuwen YuJian Zhang

TU-Graz

Lorenz RomanerEgbert Zojer

Georgia-TechJean-Luc Brédas

HasylabRobert L. Johnson

H. C. StarckAndreas Elschner

U TübingenAlexander GerlachFrank Schreiber

ESRFJörg Zegenhagen

MPI-Polymer Res.Ralph RiegerKlaus Müllen