Organic Electronics. MatWi II summer term Priv. Doz. Bert Nickel

Transcription

1 Organic Electronics MatWi II summer term 2018 Introduction: Organic electronics Fabrication and characterization of organic thin films Devices: solar cells, OLEDs, OFETs blackboard part: OFET Priv. Doz. Bert Nickel

2 Organic Electronics

3 AMOLED displays

4 OLED TVs Sony 10 Mio. :1 Kontrastverhältnis 0,1ms Reaktionszeit Freier Betrachtungswinkel 15 Zoll / 38 cm Diagonale 40 Watt 1.6 kg 1400 EUR ( )

5 OLED TV 55 Zoll (140 cm) 15 years R&D Full HD 999

6 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.

7 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. Note: Today we are at 12%. After using more than 150 Mil USD investment capital, Konarka filed bankruptcy. Alan Heeger / University of California - Santa Barbara

8 CIGS: Kupfer-Indium-(Gallium)-Schwefel-Selen Grätzel Wikipedia: Solar cell

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10 Ultrathin and lightweight organic solar cells with high flexibility Martin Kaltenbrunner, Matthew S. White, Eric D. Głowacki, Tsuyoshi Sekitani, Takao Someya, Niyazi Serdar Sariciftci, Siegfried Bauer Nature Communications 3 (2012)

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12 How is this possible, or what do we need for organic electronics?

13 Silicon: microstructering and doping Intel 80486DX2 Strukturierter Si-wafer

14 Organic materials (hydrocarbons) in everydays life

15 are good insulators Polyethylen (PE) all electrons covalently bound in sigma bonds

16 electron density MatWi I

17 electron mobility (Drude Model)

18 sp2 hybridisation (C 2 H 4 ): s s s s s p-orbital

19 conjugated and aromatic molecules: p-electrons

20 molecular orbitals (MO) of aromatic molecules: Benzol

21 Naming orbitals: HOMO and LUMO

22 Nobel Price Chemistry 2000: Alan J. Heeger

23 Doped polymers are conducting: PDOT : PSS poly(styrenesulfonate),

24 Nobel prices for aromatic and conjugated materials Graphene 2010 physics Polyacetylene 2000 chemistry Fullerenes 1996 chemistry

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27 Linear acenes: HOMO LUMO band gap Bsp. Pentacene: Absorption

28 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

29 The vision of organic electronics: mass printing

30 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 ]

31 Organic materials deposition and growth

32 Record material: Pentacene C22H14 Properties: hole mobility larger than electron mobility forms well-ordered layers when evaporated in vacuum at RT gold contact are reasonably well matched

33 Pentacene-deposition by vacuum sublimation, accuracy ca. 0.1 nm (QMC)

34 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)

35 Diffusion limited Aggregation (DLA) diffusion nucleation surface modification = Diffusionsenergie = Diffusionskonstante temperature deposition rate island formation R = D / F

36 Molecular structure: Bragg-scattering APL (2004)

37 Pentacene growth (thick films)

38 Comparison: Coronene films 1.5µm 3µm M. Huth (LMU), diploma thesis (2006)

39 surface energy determines growth mode (Wulf construction)

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41 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

42 Carrier mobility

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44 little overlap - bands are flat mass of the carriers is high mobility is low scattering time depends on sample quality

45 Measurement of mobility: Time-Of-Flight: Kepler & LeBlanc 1960 for Anthracene crystals TOF-Geometry 1. Generation of elektron-eole 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

46 typical mobility for organics m = 1 cm 2 /Vs same as amorphous Si

47 traps reduce mobility (Shockley-Read-Hall) 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) = where m 0 (T) ~ T -n N T m 0 (T) 1 + [exp( ) -1] N 0 E T k B T n depending on material, scattering mechanisms, etc.

48 grain boundaries reduce mobility Horowitz Adv. Mat. (2000)

49 Comparison of organic and inorganic Semiconductors E [ev] 0 E vac narrow bands ( ca. 100meV at 300K ) 2.2 E gap E cb 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

50 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 of the arrangement of the molecular orbitals and symmetry determine the electronic properties of organic crystals organic molecules have large band gaps (typically 2 ev), few or no intrinsic carriers at RT bands are flat, dispersion typically 100 mev crystal quality matters for mobility (grain boundaries, traps)

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

52 Thin film transistors

53 Thin film transistors - design -8 2 Thermal SiO 2 (200nm)( C ox F / cm ) ~ 48 nm Pentacene Gold-structure(bottom-contact) 200 µm 250 µm 750 µm 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

54 Field effect-transistors from pentacene molecules

55 Organic field effect transistors (see blackboard for characteristic curve) Precise saturation behaviour Linear regime I SD * 10-5 [A] 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

56 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 2005

57 Diodes, solar cells

58 Optical excitations in organic crystals Singlet excitons diffuse within the organic crystal until they decay fluorescent or radiationless. Lifetime is very short (fs-ps), therefore diffusion length is only nm.

59 Si the p/n- junction energy band deformation by doping

60 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

61 Organic solar cells e h

62 Summary active elements (diode, transistor) made from organic electronics materials: OLEDs, solar cells, field-effect-transistors (FETs) materials science aspects have huge influence on performance: structural defects, chemical impurities, lifetime and stability need to be optimized the molecular character is usually not completely lost in crystals: strongly bound excitons, low epsilon

63 Organic Electronics summer term 18 Pentacene Linear Acenes:

64 Comparison Pentacene-Gold MOSFET Metal-Oxide-Semiconductor field-effect-transistor here: p-type, normally off similar: MISFET (metal insulator...) IGFET (insulator-gate...)

65 working principle of an OFET plate capacitor gate electrode contact + semicond. film

66 I-V curve of a MOSFET conceptionally, L>>d (channel length >> oxide thickness) Moors law requires thin oxides as main technological challenge for small MosFETS with Si with organic FETs, parasitic problems dominate so far: contact barriers, traps, contaminations,...

67 linear range: amplifier saturation regime: logic

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