AMOLED displays. OLEDs for lightening. Organic Electronics

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1 Organic Electronics AMOLED displays MatWi II summer term 2015 Priv. Doz. Bert Nickel Introduction: Organic electronics Fabrication and characterization of organic thin films Devices: solar cells, OLEDs, OFETs blackboard part: OFET Organic Electronics 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. 1

2 OLED TV 55 Zoll (140 cm) after 15 years R&D ca 2000 EUR (LG & Samsung) Copper-Indium-Gallium-Diselenid (CIGS) Wikipedia 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. How is this possible, or what do we need for organic electronics? Note: Today we are at 10%. After using more than 150 Mil USD investment capital, Konarka filed bankruptcy. Alan Heeger / University of California - Santa Barbara 2

3 Silicon: microstructering and doping are good insulators Polyethylen (PE) Intel 80486DX2 Strukturierter Si-wafer all electrons covalently bound Organic materials (hydrocarbons) in everydays life MatWi I electron density 3

4 conjugated and aromatic molecules: p-electrons electron mobility (Drude Model) sp2 hybridisation (C 2 H 4 ): molecular orbitals (MO) of aromatic molecules: Benzol s s s s s p-orbital 4

5 Naming orbitals: HOMO and LUMO Doped polymers are conducting: PDOT : PSS poly(styrenesulfonate), Nobel Price Chemistry 2000: Alan J. Heeger Nobel prices for aromatic and conjugated materials Graphene 2010 physics Polyacetylene 2000 chemistry Fullerenes 1996 chemistry 5

6 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 6

7 The vision of organic electronics: mass printing Organic materials deposition and growth Status Quo Record material: Pentacene C 22 H % 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 ] Properties: hole mobility larger than electron mobility forms well-ordered layers when evaporated in vacuum at RT gold contact are reasonably well matched 7

8 Pentacene-deposition by vacuum sublimation, accuracy ca. 0.1 nm (QMC) Diffusion limited Aggregation (DLA) diffusion nucleation surface modification = Diffusionsenergie = Diffusionskonstante temperature deposition rate island formation R = D / F Pentacene film growth Molecular structure: Bragg-scattering H H H H H H Si OH OH OH OH OH OH SiO 2 Si mm 10 mm Phys. Rev. B (2003) APL (2004) 8

9 1.5µm Pentacene growth (thick films) surface energy determines growth mode (Wulf Konstruktion) Comparison: Coronene films 3µm M. Huth (LMU), diploma thesis (2006) 9

10 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 little overlap - bands are flat mass of the carriers is high mobility is low Carrier mobility Güte des Kristalls 10

11 Measurement of mobility: Time-Of-Flight: Kepler & LeBlanc 1960 for Anthracene crystals TOF-Geometry traps reduce mobility (Shockley-Read-Hall) µ 0 (T): intrinsic mobility E T : trap energy N T /N 0 : trapping vs. conduction states 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 injection free method electron and hole current separately Experiment mobility, in the presence of shallow traps: m(t) = where m 0 (T) ~ T -n n N T m 0 (T) 1 + [exp( ) -1] N 0 E T k B T depending on material, scattering mechanisms, etc. grain boundaries reduce mobility typical mobility for organics m = 1 cm 2 /Vs same as amorphous Si Horowitz Adv. Mat. (2000) 11

12 Comparison of organic and inorganic Semiconductors E [ev] E gap E vac E cb narrow bands ( ca. 100meV at 300K ) high mass Thin-film devices 5.8 Compared to tetracene: E F E vb x bandgap E gap 3.6eV (diamond E gap = 5.5eV) k B T = 26meV at 300K no free carriers in thermal equilibrium transistors p-n contacts (diodes, solar cells, oleds) 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 of the arrangement of the molecular orbitals and symmetry determine the electronic properties of organic crystals Thin film transistors 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) 12

13 Thin film transistors - design Organic field effect transistors (see blackboard for characteristic curve) -8 2 Thermal SiO 2 (200nm)( C ox F / cm ) Gold-Structure(bottom-contact) Precise saturation behaviour ~ 48 nm Pentacene 200 µm 250 µm 750 µm Linear regime V G = 0 V V G = -10 V V G = -20 V V G = -30 V Contacts (60 nm Au + 3nm Ti) w = 10 µm l = 20 µm Channel I SD * 10-5 [A] Transistor channel Pentacene on SiO µm 2.5 µm 25 µm 5 µm Ohmic contacts V SD [V] p-type pentacene bottom-contact OTFTs Fieldeffect-Transistors from pentacene molecules 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

14 Optical excitations in organic crystals Diodes, solar cells 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. Si the p/n- junction energy band deformation by doping 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 14

15 Organic solar cells e h 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 15

16 Organic Electronics summer term 15 Comparison Pentacene-Gold Pentacene Linear Acenes: MOSFET Metal-Oxide-Semiconductor field-effect-transistor here: p-type, normally off similar: MISFET (metal insulator...) IGFET (insulator-gate...)

17 working principle of an OFET I-V curve of a MOSFET plate capacitor gate electrode contact + semicond. film 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,...

18 linear range: amplifier saturation regime: logic