Electronic Structure of Organic Semiconductor Interfaces: Looking Back to the Future

Transcription

1 E TM Electronic Structure of Organic Semiconductor Interfaces: Looking Back to the Future O O Al O Antoine Kahn S S S S Department of Electrical Engineering Princeton University, Princeton, J o S S c c o c c Tel Aviv University October 10, 2004

2 E TM Outline Interest in organic materials and thin films Outstanding metal-organic interface issues Re-visiting key concepts Surface photovoltage Interface formation o S-parameters o Defects o Induced density of interface states Doping as a means of enhancing charge injection

3 E TM π-conjugated molecules Alq 3 (tris(8-hydroxy-quinoline)aluminum) (ET material; EL in green) α-pd (, -diphenyl-, -bis(l-naphthyl)-l,l biphenyl- 4,4 diamine ( HT material) IE = 5.8 e E opt = 2.7 e O Al O O IE = 5.5 e E opt = 3.1 e MPc M-phthalocyanine (M=Cu, Zn) IE = 5.1 e E opt = 1.6 e Zn 2 ML of PTCDA on Au(111) [01] PTCDA (3,4,9,10 perylenetetracarboxylic dianhydride) IE = 6.7 e E opt = 2.2 e o o o C C F 4 -TCQ (tetrafluoro-tetracyano-quinodimethane); p-dopant IE = 8.34 e C C C F F C C o o o F F C C C I. Chizhov et al. J. Cryst. Growth 208, 449 (2000) 2nd layer [10] = -1.58, I = 0.10 na S ~ 2 A 1st layer 20 nm x 20 nm

4 E TM Ultra-flat, ultra high work function, patterned surface

5 E TM Organic Light-Emitting Device (OLED) 1000 Å C.W. Tang and S.A. anslyke, Appl. Phys. Lett. 51, 913 (1987) Mg:Ag (40:1) Alq 3 α-pd ITO/ glass Anode: Indium-Tin oxide (ITO) transparent + acuum Level Cathode: low work function - Red, green and blue OLEDs High quantum efficiency Brightness > 10 5 cd/m 2 demonstrated (~ 100 cd/m 2 for displays 10~50 µa/mm 2 ) low operating voltages 100cd/m 2 ) stability: depends critically on how hard OLED is driven;

6 E TM Organic Light-Emitting Devices (OLED) Sony/Kodak prototype high resolution Display (2001) Kodak Digital Camera (2003) 15 ; 1280x720 white OLED 2.2 ; 512x218 AMOLED OLED display introduced in CLIE 'PEG-Z90' handheld SOY BEGIS MASS PRODUCTIO OF FULL-COLOR ORGAIC LIGHT EMITTIG DIODE (OLED) DISPLAYS ew Thin Screens for Mobile Devices Realize CRT-Quality Picture Clarity and Color Gamut TOKYO, Japan, Sept. 14, This month, Sony Corporation will commence mass production of a full-color Organic Light Emitting Diode (hereafter OLED) display.

7 E TM Organic Field Effect Transistor (OFET) Pentacene OFET Small molecule organic SC; Low-T deposition Gate dielectric treated with octadecyltrichlorosilane Thin film µ > 3cm 2 /-s, ~1cm 2 /-s typical (single transistor) I on /I off =10 8 Statistics on 1cm OFETs array Average µ=0.81cm 2 /-s; competitive with a-si Average I on /I off = T.. Jackson, ACS ProsPective, Jan. 04

8 E TM Organic Photovoltaics 3.5 e 3.5 e 4.8 e ITO 5.3 e PEDOT CuPc 5.2 e 4.5 e C 60 BCP Al 4.2 e ITO/PEDOT/200Å CuPc/400Å C 60 /150Å BCP/800Å Al 6.2 e 7.0 e After Peumans et al., APL 79, 126 (2001)

9 E TM Advantages of π-conjugated molecular films Quasi-infinite number of molecules and derivatives, with wide span of electronic properties Electronic and optical properties of the films determined in first approximation by the molecular moiety System more tolerant of defects (not electronically active); no dangling bonds! great flexibility on choice of substrates ITO; metal; conducting polymer; insulator Ability to modify materials with optical or electrical dopants for wave length shift or enhancing conductivity Excellent optical emission properties Control of molecular deposition down to the fraction of molecular plane Unmatched freedom for device architecture

10 E TM Device engineering: the hole-blocker E AC 0.25 e 0.6 e E AC Recombination zone e e ITO α-pd PtOEP in Alq 3 BCP Alq 3 Mg:Ag Hole injection Blocking barrier Electron injection

11 E TM Molecular level alignment Do separately determined material parameters, e.g., φ M, IE, EA, define the real energetics of metal-organic interfaces? = 0 EA dipole barrier (O) φ M φ M IE? φ Bn Metal Metal Metal Semiconductor Schottky-Mott model Ishii et al, IEEE Trans. Electron Devices, 44, 1295 (1997) Hill et al., Appl. Phys. Lett. 73, 662 (1998)

12 E TM Key questions concerning metal-organic interfaces Origin(s) of interface dipoles Mechanism(s) of molecular level alignment Engineering of metal-organic interfaces

13 E TM Experimental approach Growth of molecular layers and metal deposition in UH (10-10 Torr) Incremental build-up of interface and (mostly amorphous) films ultraviolet photoemission spectroscopy (UPS) and inverse photoemission spectroscopy (IPES) for valence and empty state X-ray core level spectroscopy (XPS) for interface chemistry Kelvin probe for contact potential difference (CPD) and surface photovoltage In-situ current-voltage measurements (I-) (M) φ M Dipole barrier (O) EA CPD (with KP) IPES Metal (20nm) UPS φ Be φ Bh IE Substrate (Au/Si) XPS (interface chemistry)

14 E TM UPS/IPES picture of electronic structure hν Intensity (arb. units) UPS occupied states Alq 3 5.4e IPES empty states UPS: h + transport level UPS: vacuum level IPES: e - transport level EA E t IE Binding energy (e) Onset of photoemission I.G. Hill et al., Chem. Phys. Lett. 317, 444 (2000)

15 E TM UPS on metal/organic interface metal-organic interface ZnPc on Au hv=21.2e ZnPc on Au hv=21.2e e 0.76e e 128Å ZnPc 64Å ZnPc 32Å ZnPc 16Å ZnPc 8Å ZnPc 4Å ZnPc clean Au 0.76e Binding energy to (e) 128Å ZnPc 64Å ZnPc 32Å ZnPc 16Å ZnPc 8Å ZnPc 4Å ZnPc clean Au Binding Energy to (e) Au 0.90e ZnPc narrower gap at metal interface due to polarization effect Is the interface dipole real? Are the molecular levels flat away from the interface? Do interface barriers correlate with current injection?

16 E TM Re-visiting old demons! GaAs (110) E AC LT Stiles and Kahn PRL 60, 440 (1988) E C E + adsorbates Transition between single-metal-atom induced states and metal-induced states? RT To some extend, yes; but not entirely! SP Alonso, Cimino et al. Surface Photovoltage! Mao et al., JST B9, 2083 (1991) Could SP be affecting photoemission results on wide gap organic semiconductors?

17 E TM Re-visiting old friends!

18 E TM Interface dipole: UPS vs. CPD α-pd on Au Excellent agreement between results of two techniques involving radically different measurement concepts α-pd on Au hν=82 e UPS α-pd on Au hν= 82 e 5.2 e Au - + α-pd 3.1 e 1.4 e Interface dipole IE=5.4 e Hole injection barrier Intensity (arb. units) 20 Å α-pd/ Au 10 Å α-pd/ Au 4 Å α-pd/ Au 20Å α-pd on Au 10Å α-pd on Au 4Å α-pd on Au φ relative to substrate (e) Kelvin probe CPD pure a-pd on Au KP UPS clean Au clean Au Kinetic energy (e) Kinetic energy (ev) Film Thickness (Å)

19 E TM Correlation between dipole, barrier and current Sputtered clean Au 5.4 e α-pd 1.3 e 5.3 e 400Å Au α-pd 400Å Au Au/Cr/Si Au Contaminated 4.8 e α-pd 0.3 e 5.3 e 0.8 e 1.2 e

20 E TM Mechanisms of dipole formation (I) EA > φ M M φ M EA Charge transfer from metal to empty molecular states; raises organic molecular levels to stop electron transfer Ex: F 16 CuPc on Mg and Al; PTCBI on Mg and Ag; PTCDA on Mg, In and Sn M e - PTCBI on Ag He I, -3 bias PTCBI on Ag He I Charge transfer 0.2 e 64Å 32Å 16Å 8Å 4Å Ag PTCBI 64Å 32Å 16Å 8Å 4Å Ag Polaron + bipolaron localized at the interface Energy Relative to (e) Energy Relative to (e) I. Hill et al., Organic Electronics, 1, 5 (2000)

21 E TM (II) EA < φ M < IE and reactive interface Mechanisms of dipole formation Chemical reaction induces filled and empty states which pin O, leading to charge exchange to align O and M M M e - O Ex: Alq 3 and Mg or Al Mg on Alq 3 He I (21.22e) Mg- (Al-) + Alq3 organo-metallic complex Intensity (a.u.) x3 128A 64A 32A 16A 8A Chemistry-induced gap states 0.6 e x3 4A 2A 0A Binding Energy (e) defects C. Shen et al, J. Appl. Phys. 89, 449 (2001) S. Meloni et al., J. Am. Chem. Soc. 125, 7808 (2003)

22 E TM (III) EA < φ M < IE and non-reactive interface Mechanisms of dipole formation M Compression of metal surface charge density; lowers the metal work function M Ex: α-pd on Au; Alq 3 on Ag or Au; CBP on Ag or Au + r - + r - organic molecules surface surface Large work function metals have strong surface dipole components Ishii et al., Advanced Materials, 11, 605 (1999) X. Crispin, et al., J. Am. Chem. Soc. 124, 8131 (2002)

23 E TM General approach for organic/metal interfaces Abrupt interface Interfaces formed by gentle deposition of large organic molecules on clean metal surfaces Ideal to test models based on intrinsic aspects of interfaces, in particular the modification of the semiconductor interface electronic structure by the continuum of the metal.

24 E TM S - interface parameter for metal-organics φ M dipole barrier φ Bn (O) Interface position (e) Mg φ Bn Al Ag Au Alq 3 IE = 5.8 e S=0.8 S Mg dφbn = dφ m S = 0: Fermi level pinned S = 1: Schottky-Mott limit ZnPc IE = 5.2 e S~0.25 Au Interface dipole Metal Metal work function (e) Ishii et al, IEEE Trans. Electron Devices, 44, 1295 (1997) Hill et al., Appl. Phys. Lett. 73, 662 (1998) Organic F 16 -CuPc PTCBI PTCDA ZnPc Pentacene α-pd CBP Alq 3 E t (e) S ~ 0 ~ 0 ~

25 E TM Induced Density of Interface States (IDIS) model Proposed for Inorganic Semiconductor/metal interfaces.. Heine, Phys. Rev. 138, A1689 (1965) S.G. Louie and M.L. Cohen, Phys. Rev. B 13, 2461 (1976) C. Tejedor, F. Flores and E. Louis, J. Phys. C: Solid St. Phys. 10, 2163 (1977) J. Tersoff, Phys. Rev. Lett. 52, 465 (1984) Charge eutrality Level (CL) S Interface Slope Parameter: S = de dφ F M = 1 + 4πe 2 1 D(E F )δ A Induced interface states pin the Fermi level

26 E TM Back of the envelope application to MO interfaces Slope parameter S = dφ dφ Bn m 2 2e D gs δit = 1 + ε iε 0 1 D gs D gs = density of interface states per unit energy at E CL molecular area density 4x energy gap S.G. Louie and M.L.. Cohen, Phys. Rev. B 13, 2461 (1976) δ it average molecule-metal distance α-pd CBP BCP Alq 3 ZnPc pentacene PTCDA PTCBI D gs (cm -2 e -1 ) E g (e) S cal S exp ~0 ~0

27 E TM IDIS and CL for unreactive MO interfaces 1. Electronic structure of isolated molecule. Ab-initio DFT calculation + correlation. 2. Interaction with metal broadens molecular levels. T i,υ δ(e E Φ M IDIS 2 2 = 2π c T ρ (E ) 3. CL: The induced density of states up to the CL integrates to the number of electrons in the isolated molecule. Γ i = 2π υ j,α T iυ i,j 2 υ j,α e - in isolated molecule i ) α,α = i H. ázquez et al., Europhys. Lett (2004)

28 E TM Induced DOS, CL and interface The CL tends to align with the metal Fermi level PTCDA π states σ states Φ M (Au) CL S (Φ M CL) H. ázquez et al., Europhys. Lett (2004)

29 E TM S - Interface slope parameter at MO interfaces S = de dφ F M = π e 2 1 D(E F ) δ A PTCBI/Au PTCDA/Au CBP/Au CuPc/Au Pent/Au S (theory) (d=3.0å) S (exp) (ZnPc) 0.37 Interface position(e) Mg Schottky- Mott limit S=1 Ag S~0 PTCBI IE= 6.2 e Au Mg In S=0 Sn PTCDA IE= 6.8 e Au Mg Ag S=0.6 CBP IE = 6.2 e Au Mg ZnPc IE = 5.2 e S~0.25 Au Sm S~0.37 Pentacene IE = 5.0 e Au Metal work function (e)

30 E TM Modifying interface barriers Interface dipole approach: changing the work function of the substrate I. Campbell et al., Appl. Phys. Lett. 71, 3528 (1997) Stepping-stone approach: ultra-thin interlayer I.G. Hill et al. J. Appl. Phys. 86, 2116 (1999) CuPc α-pd Interface doping

31 E TM Control of injection via electrical doping p-doped region tunneling Create a narrow depletion region for carrier tunneling

32 E TM Electronic structure of ZnPc and F 4 -TCQ ZnPc F 4 -TCQ Intensity (a.u.) UPS HeI 2.98e 1.94e IPES Intensity(a.u.) UPS HeI 4.35e 3.10e IPES Energy Relative to (e) Energy Relative to (e) Zn 3. 34e 5.28e 5.24e e- 8.34e C C C F F F C F C C ZnPc F 4 -TCQ W. Gao and A. Kahn, Appl.Phys.Lett., 79, 4040 (2001)

33 E TM Depletion region at the doped ZnPc/Au interface 0.76e 0.55e 0.48e 5.14e 5.28e 5.14e 5.24e 5.14e 5.24e Au Au < 128Å Au < 32Å 0.90e 0.80e 0.42e 0.38e 0.74e 0.18e 0.56e ZnPc on Au ZnPc:0.3% F 4 TCQ on Au ZnPc:3% F 4 TCQ on Au W. Gao and A. Kahn, Appl.Phys.Lett., 79, 4040 (2001) and Organic Electronics 3, 53 (2002)

34 E TM SP at doped interface? α-pd 5.14e Au 0.86e 5.52e 1.24e φ relative to substrate (e) Undoped α-pd on Au pure a-pd on Au KP UPS Film Thickness (Å) α-pd:4% F 4 -TCQ 5.14e Au 0.68e 1.10e 30-40Å 5.56e 0.62e 0.48e Film Thickness (Å) PES and CPD in perfect agreement no SP; fast recombination and low carrier mobility likely to prevent charge separation at the interface φ relative to substrate (e) a-pd:0.5%f4-tcq on Au 0.5% doped α-pd on Au KP UPS C. Chan et al, J. ac. Sci. Technol. A 22, 1488 (2004)

35 E TM Doping enhancement of hole injection in α-pd Curren Density (ma/cm 2 ) Au α-pd Au substrate Applied oltage () Au/ 170nm α-pd:0.5% F 4 -TCQ /Au α-pd Au/ 8nm α-pd:0.5% F 4 -TCQ + 162nm pure α-pd /Au α-pd Au/ 170nm pure α-pd/au α-pd Hole injection enhancement of 4-7 orders of magnitude via tunneling through the depletion region W. Gao and A. Kahn, J. Appl. Phys., 94, 359 (2003)

36 E TM Summary As for inorganic semiconductors, surface/interface physics has been highly instrumental in advancing basic understanding of organic interface electronic structure Organic molecular semiconductors depart in major ways from inorganic counterparts, but key interface concepts developed two or three decades ago are, to the least, extremely useful as guiding principles for understanding metal-organic interfaces

37 E TM Ultra-flat, ultra high work function, patterned surface Extremely high work function surface (meaning? hard work done on this surface or work function so high that nothing can escape) Essentially organic Perfectly clean; no apparent contamination, down to the submolecular level In spite of quasi-regular patterns, extremely small surface corrugation, nearly impossible to measure with scanning probes (Yossi can confirm!) Absolutely not able to reproduce in our labs and offices!

38 E TM Ideal ultra-flat high work function organic surface Expanding the field of view does reveal some inhomogeneities חמות וברכות טוב מזל