Electronic Processes in Organic Semiconductors: An Introduction

422 Pages·4.683 MB·English

by  Köhle A., Bässler H.

AnnaKöhlerand
HeinzBässler
ElectronicProcessesinOrganic
Semiconductors
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AnnaKöhlerand
HeinzBässler
Electronic Processes in Organic
Semiconductors
AnIntroduction
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V
Contents
Preface XI
TableofBoxes XIII
1 TheElectronicStructureofOrganicSemiconductors 1
1.1 Introduction 1
1.1.1 WhatAre“OrganicSemiconductors”? 1
1.1.2 HistoricalContext 3
1.2 DifferentOrganicSemiconductorMaterials 5
1.2.1 MolecularCrystals 5
1.2.2 AmorphousMolecularFilms 7
1.2.3 PolymerFilms 9
1.2.4 FurtherRelatedCompounds 14
1.2.5 ACommentonSyntheticApproaches 15
1.3 ElectronicStatesofaMolecule 17
1.3.1 AtomicOrbitalsinCarbon 17
1.3.2 FromAtomicOrbitalstoMolecularOrbitals 19
1.3.3 FromOrbitalstoStates 25
1.3.4 SingletandTripletStates 28
1.4 TransitionsbetweenMolecularStates 31
1.4.1 ThePotentialEnergyCurve 31
1.4.2 RadiativeTransitions:AbsorptionandEmission 37
1.4.2.1 TheElectronicFactor 38
1.4.2.2 TheVibrationalFactor 41
1.4.2.3 TheSpinFactor 45
1.4.3 AClassicalPictureofLightAbsorption 48
1.4.3.1 TheLorentzOscillatorModelandtheComplexRefractiveIndex 48
1.4.3.2 RelatingExperimentalandQuantumMechanicalQuantities:TheEinsteinCoefficients,
theStrickler–BergExpression,andtheOscillatorStrength 52
1.4.4 Non-RadiativeTransitions:InternalConversionandIntersystemCrossing 56
1.4.4.1 TheFranck–CondonFactorFandtheEnergyGapLaw 57
1.4.4.2 TheElectronicCouplingJ 58
1.4.4.3 AcceptingModes,PromotingModes,andtheIsotopeRule 59
1.4.4.4 ImplicationsoftheEnergyGapLaw 60
1.4.4.5 TheStrongCouplingLimit 61
1.4.5 BasicPhotophysicalParameters:LifetimesandQuantumYields 62
1.5 SpectroscopicMethods 64
1.5.1 PhotoluminescenceSpectra,Lifetimes,andQuantumYields 67
1.5.1.1 SteadyStateSpectraandQuantumYields 68
VI Contents
1.5.1.2 SpectraandLifetimesintheNanosecondtoSecondRange 72
1.5.1.3 SpectraandLifetimesinthePicosecondtoNanosecondRange 73
1.5.1.4 SpectraandTimeScalesbelowthePicosecondRange 74
1.5.2 ExcitedStateAbsorptionSpectra 75
1.5.2.1 SteadyStateSpectra(PhotoinducedAbsorption) 75
1.5.2.2 SpectraintheNanosecondRange(FlashPhotolysis) 77
1.5.2.3 SpectraintheFemtosecondRange(fsPump–ProbeMeasurements) 78
1.5.3 FluorescenceExcitationSpectroscopy 79
1.6 FurtherReading 80
References 81
2 ChargesandExcitedStatesinOrganicSemiconductors 87
2.1 ExcitedMoleculesfromtheGasPhasetotheAmorphousFilm 87
2.1.1 EffectsduetoPolarization 87
2.1.2 EffectsduetoStatisticalAveraging 91
2.1.3 EffectsduetoEnvironmentalDynamics 94
2.1.4 EffectsduetoElectronicCouplingbetweenIdenticalMolecules–Dimersand
Excimers 99
2.1.4.1 ElectronicInteractionintheGroundState 99
2.1.4.2 ElectronicInteractionintheExcitedState 99
2.1.4.3 OscillatorStrengthofDimerandExcimerTransitions 105
2.1.4.4 SingletandTripletDimers/Excimers 107
2.1.5 EffectsduetoElectronicCouplingbetweenDissimilarMolecules–Complexesand
Exciplexes 111
2.1.6 ElectromersandElectroplexes 113
2.2 ExcitedMoleculesinCrystallinePhases–TheFrenkelExciton 114
2.2.1 TheFrenkelExcitonConceptforOneMoleculeperUnitCell 114
2.2.2 TheFrenkelExcitonConceptforTwoMoleculesperUnitCell 117
2.2.3 CoherentandIncoherentMotionofFrenkelExcitons 118
2.2.4 FörsterandDexterTypeEnergyTransfer 119
2.2.5 ExperimentalExamplesforFrenkelExcitonsinOrderedMolecularArrays 123
2.2.5.1 MolecularCrystals:AnthraceneandTetracene 123
2.2.5.2 CyclicArraysofChromophores:Light-HarvestingProteins 124
2.2.5.3 MolecularJandHAggregates:CyanineDyesandCarotenes 126
2.2.5.4 WeaklyInteractingHandJAggregateswithVibronicCoupling 127
2.3 ExcitedStatesinπ-ConjugatedPolymers 133
2.3.1 CrystallinePolymers:Poly(diacetylene)s(PDAs) 133
2.3.2 ConceptsforNoncrystallinePolymers 136
2.3.2.1 TheBasicIdea 136
2.3.2.2 QuantitativeApproaches:ExcitonModels 141
2.3.2.3 ComparisonAgainstExperimentalData 143
2.3.3 BriefOverviewOverDifferentClassesofConjugatedPolymers 144
2.3.3.1 Poly(ene)s/Poly(acetylene)s 144
2.3.3.2 Poly(p-phenylenevinylenes) 147
2.3.3.3 Poly(p-phenylene)s 150
2.3.3.4 Poly(thiophene)s 152
2.3.3.5 Poly(silane)s/Poly(silylene)s 153
2.3.3.6 Low-GapDonor–AcceptorPolymers 154
2.4 ChargedMolecules 155
2.4.1 TheCreationofChargedMoleculesbyInjection,AbsorptionandDoping 157
2.4.1.1 ByInjection 157
Contents VII
2.4.1.2 ByAbsorption 158
2.4.1.3 ByDoping 159
2.4.2 ChargedMoleculesinDisorderedFilms 161
2.4.2.1 TheEnergyofChargedMolecules 161
2.4.2.2 TheAbsorptionofChargedMolecules 162
2.4.3 ChargedMoleculesinCrystals 164
2.4.4 DeterminingtheEnergyLevelsofChargedMoleculesbyCyclovoltammetryand
PhotoemissionSpectroscopy 167
2.4.4.1 Cyclovoltammetry 167
2.4.4.2 PhotoemissionSpectroscopy(UPS,XPS,IPES) 169
2.4.4.3 TheDifferencebetweenElectricalGapandOpticalGap 170
2.5 AComparisonbetweenInorganicandOrganicSemiconductors 171
2.5.1 Crystals 171
2.5.2 AmorphousSolids 174
2.5.3 TheSu–Schrieffer–Heeger(SSH)ModelforConjugatedPolymers 175
2.6 FurtherReading 181
References 182
3 ElectronicandOpticalProcessesofOrganicSemiconductors 193
3.1 BasicAspectsofElectricalCurrentinaDevice 194
3.1.1 InjectionLimitedCurrents 195
3.1.2 UnipolarSpaceChargeLimited(SCL)Current 196
3.1.3 BipolarSpaceChargeLimitedCurrent 200
3.2 ChargeInjectionMechanisms 201
3.2.1 Fowler–NordheimTunnelingInjection 202
3.2.2 Richardson–SchottkyThermionicInjection 203
3.2.3 ThermallyActivatedInjectionintoaDisorderedOrganicSemiconductor 204
3.3 ChargeCarrierTransport 208
3.3.1 ExperimentalTechniquestoMeasureChargeCarrierMobility 208
3.3.1.1 TheTimeofFlightTechnique 208
3.3.1.2 Space-Charge-LimitedCurrents(SCLC) 209
3.3.1.3 CarrierExtractionbyLinearlyIncreasingVoltage(CELIV) 210
3.3.1.4 Field-EffectTransistors(FETs) 211
3.3.1.5 Time-ResolvedMicrowaveConductivity(TRMC) 212
3.3.1.6 TerahertzSpectroscopy 212
3.3.1.7 OpticalProbing 213
3.3.2 CarrierTransportintheBandRegimeandintheHoppingRegime 213
3.3.2.1 BandTransport 215
3.3.2.2 HoppingTransport 217
3.3.2.3 PolaronicTransport 217
3.3.2.4 Disorder-ControlledTransport 223
3.3.2.5 SuperpositionofPolaronandDisorderEffects 233
3.3.3 TrappingEffects 235
3.3.4 TransportatHigherChargeCarrierDensities 237
3.3.5 TheImpactofMorphologyonTransport 239
3.3.5.1 TheInfluenceofExcimersandTraps 239
3.3.5.2 TheRoleofAggregatesandCrystallites 240
3.3.5.3 Self-OrderinginDiscoticLiquidCrystals 241
3.3.5.4 PolycrystallineFilms 243
3.3.6 ChargeTransportonShortLengthsScalesandTimeScales 244
3.4 Non-GeminateChargeCarrierRecombination 246
VIII Contents
3.4.1 RecombinationwithoutTraps(Langevin-TypeRecombination) 246
3.4.2 RecombinationwithTraps(Shockley–Read–Hall-LikeRecombination) 247
3.5 GenerationofExcitations 249
3.5.1 OpticalGeneration 249
3.5.2 ElectricalGeneration 251
3.5.3 SecondaryProcesses 252
3.6 DissociationofExcitations 254
3.6.1 GeminatePairCreation 254
3.6.1.1 TheTimescaleofChargeTransfer 254
3.6.1.2 PropertiesofGeminatePairsinSingle-CompoundMaterials 256
3.6.1.3 GeminatePairsinMaterialsatanInterfaceorContainingTraps 259
3.6.1.4 GeminatePairsinDonor–AcceptorSystems 260
3.6.2 TheDissociationoftheGeminatePair 263
3.6.2.1 TheOnsager(1938)Model 263
3.6.2.2 TheOnsager–BraunModel 265
3.6.2.3 HongandNoolandi’sTime-DependentFormalism 266
3.6.2.4 Pump–Push–ProbeExperimentstoMonitortheGeminatePairPopulation 267
3.6.2.5 ContemporaryModelsConsideringtheEffectsofConjugationLengths 269
3.6.2.6 TheInfluenceofDisorderonGeminatePairDissociation 272
3.7 DiffusionofExcitations 274
3.7.1 ExcitonDiffusioninaMolecularCrystal 274
3.7.2 DiffusionofExcitationsinAmorphousCondensedPhases 276
3.7.3 ExperimentalTechniquestoMeasureExcitonDiffusion 276
3.7.3.1 MeasuringDiffusionbyLuminescenceQuenching 276
3.7.3.2 MonitoringSpectralDiffusion 279
3.8 DecayofExcitations 283
3.8.1 MonomolecularDecay 283
3.8.1.1 Fluorescence 283
3.8.1.2 Phosphorescence 286
3.8.2 BimolecularProcesses 287
3.8.2.1 Singlet-Singlet-Annihilation 288
3.8.2.2 Triplet–Triplet-Annihilation 288
3.8.2.3 Triplet-Charge-Annihilation 290
3.8.2.4 Singlet–Triplet-Annihilation 291
3.9 FurtherReading 292
References 292
4 FundamentalsofOrganicSemiconductorDevices 307
4.1 BasicSolarCellsandLight-EmittingDiodeStructures 311
4.1.1 BasicFabricationSteps 311
4.1.2 ElectrodeGeometries 315
4.1.3 TheBasicOperationofaSingle-LayerOLED 317
4.1.4 Multi-LayerOLEDArchitectures 322
4.1.5 TheCurrent–Voltage–LuminanceCharacteristicsofanOLED 324
4.1.6 TheBasicOperationofanOSC 326
4.1.7 TheCurrent–VoltageCharacteristicsofanOSC 327
4.2 SolarCellPerformance 331
4.2.1 DeterminingSolarCellEfficiencies 331
4.2.2 StrategiestoIncreasethePhotocurrent 334
4.2.2.1 TheBilayerDevice 337
4.2.2.2 TheBulkHeterojunctionDevice 339