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

Transcription

1 unctional 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

2 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

3 1 "Organic Electronics" Devices Organic ield-effect Transistors (OET) Source Organic channel Gate insulator Gate Drain V DS S S S S Organic Light Emitting Diodes (OLED) S S Organic Memory Cells + (-) V G Organic Photovoltaic Cells (OPVC) E COM E - (+)

4 1 Why are interfaces important: example: Organic Light Emitting Devices OLED - How do electrodes and organics interact? U- U - ( 1-2 ) E vac - Physico-chemical properties? - Energy level alignment at interfaces? CB (LUMO) - Influence on charge injection? SE E - Morphological/structural aspects of interface-formation? h hn Molecular Electronics": Interface-Only Devices! E anode VB (HOMO) organic material cathode j AT 2 Injection-limited current: exp charge injection barrier kbt

5 1 Estimating charge injection barriers: The Schottky-Mott Limit 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 h e IE EA E vac 1 EA e IE E vac 2 IE i substrate work function IE ionization energy h,i hole injection barrier E ermi level vacuum level E vac E h,1 E h,2 h,2 = h,1 ( 2 1 )

6 Counts 1 Ionization Energy, Work unction & Charge Injection Barriers from photoelectron spectroscopy sample h spectrometer e - measurements: in ultrahigh vacuum (p < 10-9 mbar) Secondary electron cutoff (SECO) HOMO or E sample preparation: - molecular layers evaporated (stepwise) in situ - polymers spin coated ex situ E kin,seco ionization energy = h (E kin,homo E kin,seco ) work function = h (E kin,e E kin,seco ) hole injection barrier = E kin,e E kin,homo core-levels: type of interaction E E kin,homo kin,e E kin Substrate Organic

7 1 Example for physisorptive organic/metal interface: pentacene on Au(111) E vac vac,pen =0.95 e = 45 (1) Au =5.50 PEN =4.55 intensity (arb. units) (2) MT(Å) E 0.60 (1) binding energy (ev) PEN PEN = 0.60 ev ID = 0.95 ev (change of ) Estimated from Au (5.40 ev) and IE PEN (5.1 ev): est = IE PEN - Au = ev Measured: exp = 0.6 ev Koch, Vollmer, Duhm, Sakamoto, Suzuki, Adv. Mater. 19 (2007) 112

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

9 2 2a Organic/metal interface energy level tuning 2b Bonding of an acceptor molecule on a metal

10 2 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 N µ 1 N µ 2 HIB reduction and increase small HIB reduction and increase large HIB max HIB min 0 ML ca. 1 ML acceptor coverage hole injection barrier height Helmholtz-Equation: en for... effective diel. const. equiv. to Topping-model 0 4TQ tetrafluoro-tetracyanoquinodimethane TCAQ AQ O mechanism works in general: organic semiconductor O Koch, Duhm, Rabe, Vollmer, Johnson, Phys. Rev. Lett. 95 (2005) predictable tuning of HIB for any subsequent organic layer by up to 1.4 ev

11 2 Molecular energy levels after charge transfer: simple model of integer charge transfer and molecular ions E =0 vac binding energy (LUMO+1) E (LUMO) (HOMO) N np nbp N neutral molecule insulating/semiconducting np "negative Polaron" (anion) metallic nbp "negative Bipolaron" (dianion) insulating/semiconducting

12 Energy Levels and of 4TQ on Cu(111): Simple charge transfer? 2 intensity (arb. units) Comparison UPS and Density unctional Theory (DT) * UPS DT Å) E binding energy (ev) ev * Zojer & Brédas groups, TU-Graz/GA-Tech LUMO of 4TQ becomes filled located below E : non-metallic work function increases: Cu(111): 5.0 ev 4TQ/Cu: 5.6 ev Estimation of : 2 electrons transferred from Cu to 4TQ 2.5 Å 4TQ-Cu(111) bonding distance should be + 5 ev! (experiment: ev!)

13 2 Detailed mechanism of metal -increase: 4TQ on Cu(111) X-ray standing waves (XSW) Density functional theory (DT)* x y 3.6 (3.3) 2.1 (2.7) 0.0 (0.0) z x Bonding distances from Cu: Theory Experiment : 3.6 Å : 3.3 Å N: 2.1 Å N: 2.7 Å * Zojer & Brédas groups, TU-Graz/GA-Tech 4TQ conformation is changed due to adsorption on Cu: quinoid (bulk) to aromatic (adsorbed) CT bulk 4TQ: planar 4TQ on Cu(111): non-planar non-planarity induces dipole that decreases!

14 2 Bonding mechanism and bi-directional charge transfer Orbital occupation analysis Metal Molecule charge transfer: LUMO (-level) filled with 1.8 e Molecule Metal charge transfer: H-9 L H-9 etc. (-levels) depleted of e net CT: 0.6 e transferred to 4TQ 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)

15 2 Gold work function reduction by 2.2 ev with an air-stable molecular donor layer methyl viologen (MV0) 1,1'-dimethyl-1H,1'H-[4,4']bipyridinylidene N intensity (arb. units) a b c d e N kinetic energy (ev) 5.50 ev pristine Au 1 ML MV0/Au 3.30 ev 4.20 ev 3.30 ev 4.10 ev Bröker, Blum, risch, Vollmer, Hofmann, Rieger, Müllen, Rabe, Zojer, Koch, Appl. Phys. Lett. 93 (2008) electron injection barriers lowered by: 0.8 ev for Alq ev for C 60

16 3 Organic Electronics Molecular Electronics

17 3 How to make "good metallic" contacts to individual molecules? challenges in molecular electronics: UPS (density of valence states) 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 electronic cross-talk between neighboring molecules "bulk" C 60 : large energy gap (no DOVS close to E ) monolayer C 60 : gap-state near E not a "semiconductor"

18 3 Designed molecular acceptor to pre-pattern Ag(111) hexa-azatriphenylene-hexanitrile (HAT) UPS (density of valence states) N N N N N N STM: monolayer HACTN/Ag(111) honeycomb structure w/ hole lattice constant 2 nm HAT / Ag(111) is metallic calculated electron density E partially filled LUMO cuts E and extends into vacuum side

19 3 "Soft metallic" contacts: C 60 on HAT/Ag(111) 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., HAT/Ag(111): metallic contact to individual C 60 molecules C 60 on HAT / Ag(111) has bulk electronic structure function ("semiconductor") preserved at room temperature Glowatzki, Bröker, Blum, Hofmann, Rabe, Müllen, Zojer, Koch, Nano Lett. 8 (2008) 3825

20 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 C 60 molecule function ("semiconductor") preserved at room temperature

21 Acknowledgements Supramolecular Systems Ralf-Peter Blum Benjamin Bröker Steffen Duhm (now Chiba U) Johannes risch atemeh Ghani Hendrik Glowatzki Sven Käbisch Ingo Salzmann Raphael Schlesinger Rasmus Talviste Jörn-Oliver Vogel Shuwen Yu Jian Zhang inancial Support: - Sfb 448 (DG) - Emmy Noether Program (DG) - SPP 1355 (DG) - H. C. Starck GmbH - EC (STREP "ICONTROL") HU-Berlin Georg Heimel Jürgen P. Rabe TU-Graz Lorenz Romaner Egbert Zojer Georgia-Tech Jean-Luc Brédas H. C. Starck Andreas Elschner MPI-Polymer Res. Ralph Rieger Klaus Müllen Hasylab Robert L. Johnson BESSY Antje Vollmer ESR Jörg Zegenhagen U Tübingen Alexander Gerlach rank Schreiber