Physics of Organic Semiconductors

Transcription

1 Physics of Organic Semiconductors Edited by Wolfgang Brutting WILEY- VCH WILEY-VCH Verlag GmbH & Co. KGaA

2 Contents List of Contributors XV Introduction to the Physics of Organic Semiconductors 1 W. Brutting 1 History 1 2 Materials 2 3 Basic Properties of Organic Semiconductors Optical Properties Charge Carrier Transport Device Structures and Properties 9 4 Outline of this Book 12 Part I Growth and Interfaces 25 1 Organic Molecular Beam Deposition: Growth Studies beyond the First Monolayer 17 F. Schreiber 1.1 Introduction Organic molecular beam deposition General concepts of thin film growth Issues specific to organic thin film growth Overview of popular OMBD systems Films on oxidized silicon PTCDA DIP Phthalocyanines Pentacene Films on aluminium oxide PTCDA DIP Phthalocyanines Pentacene 29 Physics of Organic Semiconductors. Edited by W. Brutting Copyright 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN O550-X

3 VI I Contents 1.5 Films on metals PTCDA Structure and epitaxy of PTCDA/Ag(lll) Comparison with other substrates Dewetting and thermal properties Real-time growth DIP Phthalocyanines Pentacene Films on other substrates More complex heterostructures and technical interfaces Summary and Conclusions 37 Y? Electronic Properties of Interfaces between Model Organic Semiconductors and Metals 41 M. Knupfer and H. Peisert 2.1 Introduction Experimental methods Photoemission spectroscopy Polarization dependent XAS Adsorption geometry and substrate roughness Organic/metal interfaces: general properties and trends Reactive surfaces: indium-tin oxide and PEDOT:PSS Concluding remarks 64 3 Kelvin Probe Study of Band Bending at Organic Semiconductor/Metal Interfaces: Examination of Fermi Level Alignment 69 H. Ishii, N. Hayashi, E. Ho, Y. Washizu, K. Sugi, Y. Kimura, M. Niwano, O. Ouchi, and K. Seki 3.1 Introduction Energy level alignment at organic/metal interfaces Basic aspects of band bending Band bending in thermal equilibrium Band bending in nonequilibrium KP measurements for insulating surface Experimental Results ans Discussion C 60 /metal interface; case of band bending TPD/metal interface: case for no band bending Potential gradient built in Alq 3 film Concluding Remarks 90

4 Contents VII 4 Thermal and Structural Properties of the Organic Semiconductor Alq 3 and Characterization of its Excited Electronic Triplet State 95 M. Colle and W. Brutting 4.1 Introduction Crystalline Phases of Alq Thermal Properties of Alq The Molecular Structure of fi>alq High resolution powder diffraction using synchrotron radiation Vibrational Analysis Population and properties of the electronic excited triplet state Population of the triplet states Phosphorescence of Alq Summary 225 Part II Photophysics Ultrafast Photophysics in Conjugated Polymers 232 C. Lanzani, G. Cerullo, D. Polli, A. Gambetta, M. Zavelani-Rossi, C. Gadermaier 5.1 Introduction Femtosecond laser systems The pump-probe experiment Coherent vibrational spectroscopy Vibrational relaxation Electronic relaxation Triplet and soliton pairs Charge state dynamics A brief overview about charge photogeneration Field-assisted pump-probe spectroscopy The role of higher-lying states Conclusion The Origin of the Green Emission Band in Polyfluorene Type Polymers Gamerith, C. Gadermaier, U. Scherf, E.J. W. List 6.1 Introduction Photo-physical Properties of Polyfiuorene-type Polymers Optical properties of pristine polyfluorenes Optical properties of polyfluorenes in the 6-phase Fluorescent on chain defects in polyfluorene-type polymers Formation of keto-type defects in PF-type polymers The formation of keto defects during the synthesis Formation of keto defects during thermal degradation Formation of keto defects due to photo-oxidative degradation The origin of the green emission band at ev 266

5 VIM Contents Degradation in polyfiuorene-based electroluminescent devices Conclusion Exciton Energy Relaxation and Dissociation in Pristine and Doped Conjugated Polymers 283 V. /. Arkbipov and H. Bdssler 7.1 Introduction-, Field-induced exciton dissociation probed by luminescence quenching Steady state fluorescence quenching Time resolved fluorescence quenching A model of field-assisted dissociation of optical excitations Exciton dissociation in doped conjugated polymers Experiments on photoluminescence quenching in doped polymers Exciton energy relaxation and quenching by Forster energy transfer Exciton quenching at charge transfer centers Photoconductivity in pristine and weakly doped polymers 208 7A.I Intrinsic photogeneration Dopant assisted photogeneration A model of dopant-assisted charge photogeneration Photoconductivity in polymer donor/acceptor blends Enhanced exciton dissociation at high concentration of electron acceptors A model of efficient exciton dissociation at a donor/acceptor interface Conclusions Polarons in 6-conjugated Semiconductors: Absorption Spectroscopy and Spin-dependent Recombination 235 M. Wohlgenannt 8.1 Introduction Polarons in o-conjugated polymers and oligomers Organic electroluminescence Spin dependent exciton formation cross-sections Polaron absorption spectra Experimental Experimental results Polaron absorption spectra in doped oligomers; non-degenerate ground state systems Polaron absorption spectra in doped oligomers; degenerate ground state systems Polaron absorption spectra in photodoped polymer films Is polaron recombination (exciton formation) spin-dependent? Experimental 245

6 Contents IX Experimental Results Conclusions Phosphorescence as a Probe of Exciton Formation and Energy Transfer in Organic Light Emitting Diodes 257 M. Baldo and M. Segal 9.1 Introduction Phosphorescence, the Spin Dependence of Exciton Formation and Triplet Energy Transfer Phosphorescent Studies of Exciton Formation Energy Transfer in Blue Phosphorescent OLEDs Conclusions 267 Part III Transport and Devices Electronic Traps in Organic Transport Layers 273 R. Schmechel and H. von Seggern 10.1 Introduction Origin of trap states Trap detection techniques Traps in non-doped and doped small molecule semiconductors Pristine traps and their electronic structure Effect of doping on electronic traps Doping Induced Electrical and Optical properties Structure related traps and their influence on device performance Traps in polymeric semiconductors Conclusions Charge Carrier Density Dependence of the Hole Mobility in poly(p-phenylene vinylene) 305 C. Tanase, P. W. M. Blom, D. M. de Leeuw, E.J. Meijer 11.1 Introduction Experimental results and Discussion Charge carrier mobility in hole-only diodes Local mobility versus field-effect mobility in FETs Comparison of the hole mobility in LEDs and FETs Conclusions Analysis and Modeling of Organic Devices 329 Y. Roichman, Y. Preezant, N. Rappaport, and N. Tessler 12.1 Introduction Motivation Charge transport 322

7 X Contents The Gaussian DOS The Equilibrium Charge Density The diffusion coefficient The mobility coefficient The operation regime of organic devices Contact workfunction Organic Light-Emitting Diodes Space charge regime Contact limited regime I-V Analysis of LEDs Field Effect Transistors Photo-Ceils Fabrication and Analysis of Polymer Field-effect Transistors 343 S. Scheinert and G. Paascb 13.1 Introduction Models The Shockley current characteristics Material models Drift-diffusion-model and numerical simulation Analytical estimates Basic dependencies Interrelation between material and device properties Cut-off frequency Experimental A.I Preparation of transistors and capacitors Measuring techniques Device characterization and analyzes Basic procedure: P3OTdevices as an example Layer characterization MOS capacitor Thin film transistors Devices made from MEH-PPV Further transistor designs Finger structure Short channel design Subthreshold currents and trap recharging Hysteresis Selected simulation studies Formation of inversion layers Trap distributions in top and bottom contact transistors with different source/drain work functions Short-channel effects Conclusions 385

8 Contents XI A Non-degenerate approximation for the Gaussian distribution 387 B DDM with traps and trap distributions Organic Single-Crystal Field-Effect Transistors 393 R. W. I. de Boer, M. E. Gershenson, A. F. Morpurgo, and V. Podzorov 14.1 Introduction Fabrication of single-crystal organic FETs Single-crystal growth Crystal characterization Polarized-light microscopy The time-of-flight experiments Space charge limited current spectroscopy Fabrication of the field-effect structures Electrostatic bonding technique "Direct" FET fabrication on the crystal surface Characteristics of single-crystals OFETs Unipolar operation Field-effect threshold Sub-threshold slope Double-gated rubrene FETs Mobility Mobility anisotropy on the surface of organic crystals Preliminary results for the OFETs with high-k dielectrics Conclusion Acknowledgments Charge Carrier Photogeneration and Transport in Polymer-fullerene Bulk-heterojunction Solar Cells 433 I. Riedel, M. Pientka, and V. Dyakonov 15.1 Introduction Experimental Photoinduced charge transfer in polymer: fullerene composites Photovoltaic characteristics and charge carrier photogeneration efficiency of polymer-fullerene bulk heterojunction solar cells Analysis of the electrical transport properties Illumination intensity dependence Temperature dependence Bulk-heterojunction solar cells with increased absorber thickness Dependence of IPCE on reverse bias Conclusions 448

9 XII Contents 16 Modification of PEDOT:PSS as Hole Injection Layer in Polymer LEDs 452 M. M. de Kok, M. Buechel, S. I. E. Vulto, P. van de Weijer, E. A. Meulenkamp, S. H. P. M. de Winter, A.J. G. Mank, H.J. M. Vorstenbosch, C. H. L Weijtens, and V. van Elsbergen 16.1 Introduction Experimental Results A Chemistry' and bulk properties Surface properties Device performance Discussion Effect of NaOH on PEDOT:PSS properties Effect of NaOH addition on PLED device performance Conclusions Insights into OLED Functioning Through Coordinated Experimental Measurements and Numerical Model Simulations 475 D. Berner, H. Houili, W. Leo, and L. Zuppiroli 17.1 Introduction Device Fabrication The Model Coulomb effects Equations of motion and injection Transport in the organic material Recombination Numerical procedure A Case Studies A.I Electrode effects Time of flight simulations Internal electric field Experimental analysis Discussion of Simulations Charge and current distribution inside the device Electric field density distribution Dye doping Experimental results: Analysis Through Simulation Mobility variation inside the doped part Recombination rate distribution Energy level shift inside the doped region General recombination picture Transport characteristics Summary 507

10 Contents XIII 18 Optimizing OLED Structures for a-si Display Applications via Combinatorial Methods and Enhanced Outcoupling 522 W. Riefi, T. A. Beierlein, and H. Riel 18.1 Introduction Experimental Results and Discussion Combinatorial Device Optimization Layer-Thickness Variations: Charge-Carrier Balance Layer-Thickness Variation: Optical Effects Optical Outcoupling inch a-si AMOLED Display Conclusions 525 Index 529