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Organic Electronics – Where Are We, Where Do We Go? Organic Electronics – where are we, where do we go? Karl Leo Institut für Angewandte Photophysik, TU Dresden, 01062 Dresden, Germany, www.iapp.de and Fraunhofer-COMEDD, 01109 Dresden EDS Webinar, September 27, 2012 Outline •Introduction •What are organic semiconductors? •Organic Light Emitting Diodes (OLED) •Organic Solar Cells •A few remarks on manufacturing © Fraunhofer COMEDD For internal use only Organic Semiconductors • Large area & flexible substrates possible • Large variety of materials • Low cost Organic Organic light Photovoltaic Transistors materials emitting diodes cells and memory Future Markets for Organic Devices • IdTechex study: • 330 billion US$ in 2027 • Most important markets: - Logic/Memory - OLED-Display - Photovoltaics Progression of Organic Products 4th wave: Organic electronics 3rd wave: Solar cells 2nd wave: OLED lighting 1st wave: OLED Displays Time Organic Light Emitting Diode (OLED) OLEDs: Basic Principles LUMO Cathode E - A - - Cathode V Emissive layer Transparent anode Anode Glass substrate + + + HOMO Light emission x Device structure Energy diagram Organic materials needed: ≈ 1 g/m2 ! OLED display market forecasts – and the reality.. OLED display market forecasts – and the reality.. The 55‘ OLED TV is coming… Outline •Introduction •What are organic semiconductors? •Organic Light Emitting Diodes (OLED) •Organic Solar Cells •A few remarks on manufacturing © Fraunhofer COMEDD For internal use only The basics of organic semiconductors: Conjugated π-electron systems Sp2-hybridised Carbon: p -orbital p -orbital z π -bond z plane of the σ-bond sp2-orbitals π -bond σ∗ π∗ pz pz π sp2 sp2 σ Molecules with conjugated π-electron system π-electron systems delocalize! • VdW crystals • small π−π-overlap, narrow bands • saturated electron system delocalizeddelokalisierte π −Elektronen π-electrons σ∗ π LUMO ( *) => EC „conduction band“ 6 x pz π HOMO ( ) => EV „valence band“ 18 x sp2 Technology: vacuum evaporation Polymers with conjugated π-electron system • broad bands • transport limited by interchain hops n σ∗ π CB ( *) n x pz 6 x pz VB (π) 18 x sp2 sp2 Technology: solution processing Carbon: the influence of dimensionality 2D covalent broad bands 1D covalent: broad bands 104 Van der Waals- 101 coupling: 2D Narrow bands 1D 10-2 y t i l i b o 0D Source: Castro Neto, Geim et al. m Band dispersion in vdW-coupled organics • Dispersion from angle- resolved photoelectron spectroscopy • Bandwidth 4t ≈ 200 meV: compare to exciton binding of about 500meV • Agrees with mobility of 3.8cm2/Vs in broad band model H. Yamane et al., Phys. Rev. B 68 (2003) 033102 Materials with low symmetry: packing is important • Molecules often have comparably low symmetry • Consequently, crystals have low- symmetry crystal class: Monoclinic or triclinic • Disorder plays a big role Phthalocyanine-crystal Mobility as a function of disorder • Rocking width correlates with mobility Typical OLED today! • Even small disorder reduces µ strongly • Conductivities are accordingly low N. Karl et al. 2001 Growth of ordered layers: a challenge QT (Quaterrylen) Directly on Au(111) on HBC on Au(111) 10 nm reconstructed Au(111)-surface 1 ML HBC on Au(111): d = 3 Å [ courtesy T. Fritz; R. Forker et al., Adv. Mat., 20, 4450 (2008) ] Outline •Introduction •What are organic semiconductors? •Organic Light Emitting Diodes (OLED) •Organic Solar Cells •A few remarks on manufacturing © Fraunhofer COMEDD For internal use only Single layer OLEDs A n o d e O r g a n i c C a t h o d e A n o d e HTL EL ETL C a t h o d e Φ E v a c - - A E s Φ - F - - - e V I s + E F + + + + + V • Here: hole mobility larger than electron mobility => losses by holes reaching the cathode => losses by exciton quenching at the cathode => low outcoupling efficiency Electron and hole blocking layers A n o d e O r g a n i c C a t h o d e A n o d e HTL EL ETL C a t h o d e Φ E v a c - - A E s Φ - F - - - e V I s + E F + + + + + V • self balancing system => γ=1 • no exciton quenching by metal contacts • outcoupling can be optimized Five layer device: every layer optimized for specific specific for optimized layer every device: layer Five function • • Polymer OLED: Balance more difficult to achieve to difficult more Balance OLED: Polymer difficult Small molecule OLED: balance easy for RGB, white more more white RGB, for easy balance OLED: molecule Small ITO Achieve Carrier Balance: Blocking Layers Blocking Balance: Carrier Achieve X HTL Electron BL Emitter Hole BL ETL Al Why doped layers: organic vs. Inorganic LED Undoped Organic LED Inorganic LED (e.g., GaAs/AlGaAs) CB ETL EFe Metal HTL p n i EFh VB • space charge limited currents • Flat-band under operation • low work function metals needed ITO-preparation necessary • Low work-function contacts not needed ! • • Anode Carriers are injected through thin space-charge layers space-charge thin through injected are Carriers Device operates in flat-band condition flat-band in operates Device p p-HTL structure pin-OLED The Electron Blocker Emitter i Hole Blocker n-ETL n Cathode P-doped ZnPc: Conductivity vs. Doping Concentration T= 30°C ] ZnPc series 1 10-2 m ZnPc series 2 c / S [ σ y t i -3 v - i 10 t e c u d n o linear dependence C 10-4 10-3 10-2 10-1 Molar Doping Ratio F -TCNQ : ZnPc 4 Nominally undoped ZnPc: ≈10-10 S/cm ⇒ Doping increases conductivity by orders of magnitude M. Pfeiffer, A. Beyer, T. Fritz, K. Leo, Appl. Phys. Lett. 73, 3202 (1998); K. Walzer, B. Maennig, M. Pfeiffer, K. Leo, Chem. Rev. 107, 1233 (2007) Thickness of space charge layers • MeO-TPD doped with F4-TCNQ • Sparge-charge layer thickness scales with dopant density • Few monolayers for high dopant density S. Olthof et al. J. Appl. Phys. 106, 103711 (2009) The most simple device: pn homo junction Al ZnPc - homo ( d = 30 nm ) 1x10-1 i -2 40 nm n-ZnPc 1x10 Al n - ZnPc ) -3 i-ZnPc 2 1x10 i - ZnPc 30 nm p - ZnPc m -4 c 1x10 15 nm p-ZnPc / ITO A ( -5 y 1x10 24.0 °C t ITO i s -1.8 °C n -6 e 1x10 -26.6 °C D t n -7 -42.6 °C e 1x10 r r -58.7 °C u C 1x10-8 -82.8 °C 1x10-9 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 Voltage ( V ) ZnPc: K. Harada et al., Phys. Rev. Lett. 94, 036601 (2005); Pentacene: K. Harada et al., Phys. Rev. B 77, 195212 (2008) Where ist the voltage limit: Results for thermodynamically ideal device 1E8 1E8 1E7 1E7 1000000 1000000 100000 100000 2 10000 10000 • At 100Cd/m : ) 2 1000 1000 only about 20% red: 1,56V m / 100 100 excess voltage over d green: 1,95V C ( 10 10 theoretical limit e blue: 2,28V c n 1 1 a n i 0,1 0,1 m u L 0,01 0,01 • Voltages at high 1E-3 1E-3 brightness can still be 1E-4 1E-4 improved! 1E-5 1E-5 0,0 0,5 1,0 1,5 2,0 2,5 Voltage (Volt) Best value for red: R. Meerheim et al., Proc. SPIE 6192, 61920P/1 (2006) 1.89V (Novaled) The all-organic device: Red pin OLED at 2.4V Best devices: 1.89V ≈ thermodynamic limit + 20% What determines OLED efficiency? hν η = b × ×η ×η external I eU recomb optical Recombination efficiency √ • bI: Electron and hole current balance: 1 can be reached • eU: Operating voltage should be ≈ photon energy √ ∀η recomb: 75% Triplet, 25% Singlet excitons: 0.25 for fluorescent emitter: Use phosphorescent emitters (Forrest/Thompson), optimize recombination zone √ ∀η optical: about 20% in flat structure: 80% lost to wave guide modes Spin Statistics: Phosphorescent Emitters are needed (Thompson & Forrest) hole electron exciton Ir(ppy)3 + Triplet + Triplet Ir N + Triplet 3 + Singlet Phosphorescent Emitter: Ir(ppy)3 • e-h-recombination: 75% triplet- and 25% singlet-excitons • Phosphorescent emitters: triplets are used as well due to spin-orbit coupling by heavy metals (Ir, Pt, Cu…) • ≈ 100% internal quantum efficiency reached What determines OLED efficiency? hν η = b × ×η ×η external I eU recomb optical Outcoupling efficiency • √ bI: Electron and hole current balance: 1 can be reached • eU: Operating voltage should be ≈ photon energy √ ∀η recomb: 75% Triplet, 25% Singlet excitons: 0.25 for fluorescent emitter: Use phosphorescent emitters (Forrest/Thompson), optimize recombination zone √ ∀η optical: about 20% in flat structure: 80% lost to wave guide modes Distribution of Power in Modes 3 • Outcoupled modes • Substrate modes (1) • Organic modes (2) • Plasmonic losses (3) Substrate Modes: Outcoupling easily achieved 3 Source: Temicon Optical Simulation: The Model • Emitter is treated as electrical dipole • Power spectrum K is calculated by a transfer matrix approach • Isotropic orientation of molecules is Mauro Furno assumed • Total emitted power by the dipole is Ag given by n ∞ ∞ 2 F(λ) =∫ K(λ,u)du = 2∫ uK(λ,u)du p 0 0 ITO Low-n (LI) glass [1] W.L. Barnes, J. Mod. Opt. 45, 661 (1998)) [2] M. Furno et al., Phys. Rev. B 85, 115205 (2012) Waveguide Modes Glass ITO Organics M. Furno et al. Cathode Emitting Center Surface Plasmon Modes Glass ITO Organics M. Furno et al. Cathode Emitting Center High Losses due to Coupling to Metal! Distribution of power into different modes • Calculations by Mauro Furno (M. Furno et al. Proc. SPIE 7617, 761716 (2010); Phys. Rev. B 85, 115205 (2012)) • Model includes Purcell effect • Model can be tested by variation of electron transport layer thickness R.
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