Organische Elektronik

Transcription

1 Organic Electronics MatWi II (summer term 2012) Introduction: Organic electronics Fabrication and characterization of organic thin films Devices: solar cells, OLEDs, OFETs blackboard part: OFET Priv. Doz. Bert Nickel Organische Elektronik 1

2 Papers / year Trends in organic electronics 300 organic light emitting diode OR OLED organic field effect transistor OR OFET organic solar cell organic field effect transistor sensor 40 inch Samsung OLED display OLED FETs Philips Solar year Georgia Institute of Technology (Pentacene+C60 solar cell, 2.7 % efficiency) AMOLED displays 2

3 OLEDs for lightening AP photo Anil Duggal, who heads up GE Global Research's Organic Electronics Project, says sheets of organic light-emitting diodes, such as the one above, might be the future of lighting. OLED TV announced for 2nd half of Zoll (140 cm) 3

4 O-Solar cells Science 2007 "The result is six and a half percent efficiency," said Heeger. "This is the highest level achieved for solar cells made from organic materials. I am confident that we can make additional improvements that will yield efficiencies sufficiently high for commercial products." He expects this technology to be on the market in about three years. Alan Heeger / University of California - Santa Barbara How is this possible, or what do we need for organic electronics? 4

5 Silicon: microstructering and doping Intel 80486DX2 Strukturierter Si-wafer Organic materials (hydrocarbons) in everydays life 5

6 are good insulators Polyethylen (PE) all electrons covalently bound MatWi I electron density 6

7 electron mobility (Drude Model) sp2 hybridisation (C 2 H 4 ): s s s s s p-orbital 7

8 conjugated and aromatic molecules: p-electrons Molekulare Orbitale (MO) von aromatischen Molekülen: Bsp. Benzol 8

9 Bezeichnung der Orbitale: HOMO und LUMO Nobel Preis Chemie 2000: Alan J. Heeger 9

10 Doped polymers are conducting: PDOT : PSS poly(styrenesulfonate), Nobelprices for aromatic and conjugated materials Graphene 2010 physics Polyacetylene 2000 chemistry Fullerenes 1996 chemistry 10

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12 Linear acenes: HOMO LUMO band gap Bsp. Pentacene: Absorption Translation valence band, conduction band, electron-hole pair, doping, traps for charge carriers, phonons, energy bands, Drude model, electroluminescence, surface states LUMO, HOMO, excitonic states, chemical impurities, vibrations, hopping, fluorescence and phosphorescence, singulets, triplets, anhilation, oxidation, reduction 12

13 The vision of organic electronics: mass printing Status Quo... 90% of OLEDs, by far the most important application of organic electronics at the moment, are produced by vapor deposition of small molecules [Source: ICB 10 July 2008 ] 13

14 Organic materials deposition and growth Record material: Pentacene C 22 H 14 Properties: hole mobility larger than electron mobility forms well-ordered layers when evaporated in vacuum at RT gold contact are reasonably well matched 14

15 Pentacene-deposition by vacuum sublimation, accuracy ca. 0.1 nm (QMC) Pentacene film growth H H H H H H Si OH OH OH OH OH OH SiO 2 Si mm 10 mm Phys. Rev. B (2003) 15

16 Diffusion limited Aggregation (DLA) diffusion nucleation surface modification = Diffusionsenergie = Diffusionskonstante temperature deposition rate island formation R = D / F Molecular structure: Bragg-scattering APL (2004) 16

17 1.5µm Pentacene growth (thick films) Comparison: Coronene films 3µm M. Huth (LMU), diploma thesis (2006) 17

18 surface energy determines growth mode (Wulf Konstruktion) 18

19 Summary: Deposition and structure Growth mode of small molecules largely determined by molecular shape and surface energy molecular arrangement can be determined by x-ray experiments Carrier mobility 19

20 little overlap - bands are flat mass of the carriers is high mobility is low Güte des Kristalls 20

21 Measurement of mobility: Time-Of-Flight: Kepler & LeBlanc 1960 for Anthracene crystals TOF-Geometry 1. Generation of Elektron-Hole pairs by hard light pulse ( pulse 0.76ns) Charge carriers are generated at the surface due to adsorption 2. measurement of displacement current Experiment injection free method electron and hole current separately typical mobility for organics m = 1 cm 2 /Vs same as amorphous Si 21

22 traps reduce mobility µ 0 (T): intrinsic mobility E T : trap energy N T /N 0 : trapping vs. conduction states mobility, in the presence of shallow traps: m(t) = N T N 0 m 0 (T) E T 1 + [exp( ) -1] k B T where m 0 (T) ~ T -n n depending on material, scattering mechanisms, etc. grain boundaries reduce mobility Horowitz Adv. Mat. (2000) 22

23 Comparison of organic and inorganic Semiconductors E [ev] E gap E vac E cb narrow bands ( ca. 100meV at 300K ) high mass E F 5.8 E vb x bandgap E gap 3.6eV (diamond E gap = 5.5eV) k B T = 26meV at 300K Compared to tetracene: no free carriers in thermal equilibrium strong influence of traps Summary electronic properties conjugated and aromatic molecules have delocalized electrons (p electrons) small organic molecules form highly ordered crystals, while polymer films are only partially ordered Details der Anordnung der MO und Symmetrie bestimmen die elektronischen Eigenschaften organic molecules have large band gaps (typically 2 ev), few or no intrinsic carriers at RT bands are flat, dispersion typically 100 mev Quality matters for mobility (grain boundarys, traps) 23

24 Thin-film devices transistors p-n contacts (diodes, solar cells, oleds) Thin film transistors 24

25 Thin film transistors - design -8 2 Thermal SiO 2 (200nm)( C ox F / cm ) Gold-Structure(bottom-contact) ~ 48 nm Pentacene 200 µ m 750 µ m 250 µ m Contacts (60 nm Au + 3nm Ti) w = 10 µm l = 20 µm Channel Transistor channel Pentacene on SiO µm 2.5 µm 25 µm 5 µm Fieldeffect-Transistors from pentacene molecules 25

26 I SD * 10-5 [A] Organic field effect transistors (see blackboard for characteristic curve) Precise saturation behaviour Linear regime V G = 0 V V G = -10 V V G = -20 V V G = -30 V Ohmic contacts V SD [V] p-type pentacene bottom-contact OTFTs Pentacene TFTs : Trap density 1st measurement last measurement Threshold shift V T deep traps Energy level far from HOMO level Fixed interface charges Hysteresis V shallow traps Energy level near HOMO level Can be released thermally ID 0099: ID 0099: M. Fiebig (LMU, LS Kotthaus), Diploma thesis

27 Diodes, solar cells Si the p/n- junction enegy band deformation by doping 27

28 Optical excitations in organic crystals Singulet excitons diffuse within the organic crystal until they decay fluorescent or radiationless. Lifetime is very short (fs-ps), therefore diffusion length is only nm. Exciton splitting at hetero-junctions Appl. Phys. Lett. 48, 183 (1986) Two layer organic photovoltaic cell C. W. Tang Modellsystem: Pentacene/C60 (C60 erst ab 1985 bekannt) p n 28

29 Organic solar cells e h Wikipedia: Solar cell 29

30 Summary all important devices can be made by organic electronics: oleds Field-Effect-Transistors (FETs) Solar cells Materials science aspects have huge influence on performance: structural defects, chemical impurities, lifetime and stability 30