Spin-dependent processes in organic devices [Elektronische Ressource] / Sebastian Schaefer

Sujets

• Physik

Informations

• Publié par: freie_universitat_berlin
• Publié le: 01 janvier 2010
• Nombre de lectures: 15
• Langue: English
• Poids de l'ouvrage: 7 Mo

Extrait

Spin-Dependent Processes in Organic
Devices




Dissertation to obtain the academic degree

Dr. rer. nat.

submitted to the Departement of Physics of Freie Universität Berlin by:

Sebastian Schaefer

Berlin, 2010

























































Betreuer: Dr. Wolfgang Harneit

1. Gutachter: Prof. Dr. R. Bittl

2. Gutachterin: Prof. Dr. M. Ch. Lux-Steiner

Tag der Disputation: 2. Juni 2010




1
Abstract
By bringing together a systematic IVharacterization and EDMR experiments, transport and
degradation processes were studied in organic devices. In a first step, two Zinc phthalo
cyanine (ZnPc) single layer devices with different electrodes were investigated, a coplanar
Au/ZnPc/Au sample and a sandwich type ITO/ZnPc/Al device. They served as a testbed for
the correlation of IV- and EDMR measurements. The insights gained in this study were then
appliedtomorecomplexbilayerZnPc/C eterojunctionsolarcells.60
A transport study at low voltages shows that bulk transport with Ohmic IV characteristics is
dominantinthecoplanarZnPc,whereasthetransportinthesandwichdeviceiscontroledbya
Schottkybarrieratthealuminumcontact. BothsamplesshowSCLurrentswithexponential
trap distribution in the high voltage limit, characteristic for ZnPc. The degradation analysis
indicate that the ITO/ZnPc/Al - device suffers from oxidation of the aluminum electrode, ex
hibiting a pronounced Schottky emission IVehavior. This degradation could be prevented
by an effective encapsulation, using a glass cover and UVlue. The results of the solar cells
alsoindicateanoxygennduceddegradation. Thisdegradationisrelatedtoanincreaseofthe
resistivityintheC layer,duetooxygenimpurities.60
TheEDMRmeasurementsindicatethatpolaronrecombinationisthedominantprocessinthe
organic devices investigated in this work. However the recombination process shows dis
tinct impact on the electric transport in the individual devices. Whereas the EDMR signal is
photocurrent quenching in the coplanar sample it reverses sign in the sandwich device. The
resultsofthetransportmeasurementsindicateachargeaccumulationattheoxidizedZnPc/Al
contact. As a consequence a model was proposed in which recombination involving these
accumulated carriers can lead to a current enhancement. This model was verified by voltage
dependent EDMR measurements, where it consistently explains a sign reversal when chang
ingfromnegativetopositivebias.
In degraded solar cells a similar charge accumulation as in the ZnPcayer is suspected. This
chargeaccumulationmanifestsitselfinanEDMRsignalwithidenticalpropertiestotheonein
ZnPc and is assumed to happen at the ZnPc/C - interface, during degradation. Furthermore,60
EDMR studies indicate that spinependent recombination happens during the exciton dis
−+sociation process at the ZnPc/C - interface, in the charged transfer complex (ZnPc , C ).60 60
Thisprocessisobservedtoquenchthephotocurrentinthesolarcells.
InfurtherspinstudiesRabibeatoscillationsunderspinockingconditionswereobservedfor
the first time in the EDMR of ZnPc and solar cells. This phenomenon exhibits a signal os
cillation at twice the Rabirequency that appears only when two pair spins are excited at the
same time. The impact of this beat oscillation on EDMR lineshapes as well as its microwave
power dependence were studied in detail. The effect of exchange coupling in the spinair
wasanalyzedinthecontextofthebeatoscillationsandalockinphaseanalysis.2Contents
1 TheoreticalBackground 11
1.1 ChargeTransportinOrganicMaterials . . . . . . . . . . . . . . . . . . . . . 11
1.1.1 SpaceChargeLimitedCurrents(SCLC) . . . . . . . . . . . . . . . . 12
1.1.2 SCLCinthePresenceofDefectStates . . . . . . . . . . . . . . . . . 13
1.2 MetalemiconductorContacts . . . . . . . . . . . . . . . . . . . . . . . . . 14
1.2.1 SchottkyContact . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
1.2.2 FermievelPinning . . . . . . . . . . . . . . . . . . . . . . . . . . 16
1.3 ElectronSpinResonance(ESR) . . . . . . . . . . . . . . . . . . . . . . . . 17
1.4 ElectricallyDetectedMagneticResonance(EDMR) . . . . . . . . . . . . . . 18
1.4.1 EDMRinOrganicSemiconductorDevices . . . . . . . . . . . . . . 19
1.4.2 ExchangeCouplinginEDMR . . . . . . . . . . . . . . . . . . . . . 20
1.5 PulsedElectricallyDetectedMagneticResonance . . . . . . . . . . . . . . . 22
1.5.1 TransientEDMRSignal . . . . . . . . . . . . . . . . . . . . . . . . 22
1.5.2 RabiOscillation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
2 ExperimentalBackground 33
2.1 OrganicMaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
2.1.1 FullereneC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3360
2.1.2 Phthalocyanine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
2.2 EPRinZnPc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
2.2.1 ZnPcRadicalCationgaluedeterminedbyEPR . . . . . . . . . . . 39
2.3 DeviceFabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
2.3.1 CoplanarAu/ZnPc/AuSamples . . . . . . . . . . . . . . . . . . . . 42
2.3.2 SandwichDevicesforEDMR . . . . . . . . . . . . . . . . . . . . . 43
2.3.3DevicesforIVharacteristics . . . . . . . . . . . . . . . 44
2.4 ExperimentalSetup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
2.4.1 IVloveboxetup . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
2.4.2 EDMRSetup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
34 CONTENTS
3 ChargeTransportinPhthalocyanineDevices 47
3.1 BulkTransportinAu/ZnPc/AuCoplanarDevices . . . . . . . . . . . . . . . 47
3.1.1 IV-TemperatureDependency . . . . . . . . . . . . . . . . . . . . . . 48
3.2 ITO/ZnPc/ALSchottkyolarCells . . . . . . . . . . . . . . . . . . . . . . . 50
3.2.1 IVharacteristics(injectionvsbulkproperties) . . . . . . . . . . . . 50
3.2.2 DegradationoftheAlContactInterface . . . . . . . . . . . . . . . . 56
3.2.3 EncapsulationRevisited . . . . . . . . . . . . . . . . . . . . . . . . 60
3.2.4 DiscussionandConclusions . . . . . . . . . . . . . . . . . . . . . . 62
4 SpinDependentTransportinZnPc 65
4.1 BulkTransportvs. InjectionLimitedTransport . . . . . . . . . . . . . . . . 65
4.1.1 EDMRinCoplanarAu/ZnPc/AuDevices . . . . . . . . . . . . . . . 65
4.1.2 EDMRinITO/ZnPc/AlSandwichDevices . . . . . . . . . . . . . . 67
4.2 EDMRSignalSaturation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
4.3 EDMRatDifferentVoltages . . . . . . . . . . . . . . . . . . . . . . . . . . 70
4.4 TheRoleofExchangeCouplinginZnPc . . . . . . . . . . . . . . . . . . . . 72
4.4.1 DiscussionofTheLineshapes . . . . . . . . . . . . . . . . . . . . . 73
4.4.2 LocknPhaseAnalysis . . . . . . . . . . . . . . . . . . . . . . . . 75
4.5 MagnetoresistanceinZnPc . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
5 SpinDynamicsinZnPc 83
5.1 PulsedEDMRinZnPcLayers . . . . . . . . . . . . . . . . . . . . . . . . . 83
5.2 ThePhenomenonofSpinocking . . . . . . . . . . . . . . . . . . . . . . . 86
5.2.1 LineShapesUnderSpinockingConditions . . . . . . . . . . . . . 89
5.2.2 BeatOscillationsatdifferentMWowers . . . . . . . . . . . . . . . 89
5.3 PulsedEDMRinAu/ZnPc/AuCoplanarDevices . . . . . . . . . . . . . . . 90
5.3.1 DecoherenceinAu/ZnPc/Audevices . . . . . . . . . . . . . . . . . 92
5.4 DiscussionandConclusions . . . . . . . . . . . . . . . . . . . . . . . . . . 94
6 Spin-DependentTransportinZnPc/C SolarCells 9560
6.1 IVharacteristicsofSolar1new . . . . . . . . . . . . . . . . . . . . . . . . 95
6.2 SpinDependentProcessesinBilayerCells . . . . . . . . . . . . . . . . . . . 98
6.2.1 ContinuousWaveEDMR . . . . . . . . . . . . . . . . . . . . . . . . 98
6.2.2 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
6.2.3 PulsedEDMRinZnPc/C olarCells . . . . . . . . . . . . . . . . 10060
6.2.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
6.3 SignalDecompositionbyLightIntensityControl . . . . . . . . . . . . . . . 102
6.3.1 IVharacteristicofSolar2new . . . . . . . . . . . . . . . . . . . . . 102
6.3.2 PulsedEDMRResultsforSolar2new . . . . . . . . . . . . . . . . . 103CONTENTS 5
6.3.3 EDMRLightIntensityDependence . . . . . . . . . . . . . . . . . . 106
6.4 RabiBeatOscillations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108
6.5 RabiBeatoftheQuenchingSignal . . . . . . . . . . . . . . . . 112
7 SummaryandOutlook 1156 CONTENTSCONTENTS 7
Preface
In past years the potential of organic materials for optoelectronic devices and spintronic ap
plications has gained more and more attention. The extremely high absorption coefficients
of some organic molecules facilitate the fabrication of low cost thin film photoetectors and
solar cells, whereas organic light emitting diodes (OLEDs) are already used in commercial
displays and illuminants. Preparation methods of organic materials like sp