Table Of ContentSpringerSeriesin
materials science
Editors: R.Hull R.M.Osgood,Jr. J.Parisi
The Springer Series in Materials Science covers the complete spectrum of materials physics,
includingfundamentalprinciples,physicalproperties,materialstheoryanddesign.Recognizing
theincreasingimportanceofmaterialsscienceinfuturedevicetechnologies,thebooktitlesinthis
seriesreflectthestate-of-the-artinunderstandingandcontrollingthestructureandproperties
ofallimportantclassesofmaterials.
51 PointDefects 56 SiO2inSiMicrodevices
inSemiconductors ByM.Itsumi
andInsulators
57 RadiationEffects
DeterminationofAtomic
inAdvancedSemiconductorMaterials
andElectronicStructure
andDevices
fromParamagneticHyperfine
ByC.ClaeysandE.Simoen
Interactions
ByJ.-M.SpaethandH.Overhof 58 FunctionalThinFilms
andFunctionalMaterials
52 PolymerFilms
NewConceptsandTechnologies
withEmbeddedMetalNanoparticles
Editor:D.Shi
ByA.Heilmann
59 DielectricPropertiesofPorousMedia
53 NanocrystallineCeramics
ByS.O.Gladkov
SynthesisandStructure
ByM.Winterer 60 OrganicPhotovoltaics
ConceptsandRealization
54 ElectronicStructureandMagnetism
Editors:C.Brabec,V.Dyakonov,J.Parisi
ofComplexMaterials
andN.Sariciftci
Editors:D.J.Singhand
D.A.Papaconstantopoulos
55 Quasicrystals
AnIntroductiontoStructure,
PhysicalPropertiesandApplications
Editors:J.-B.Suck,M.Schreiber,
andP.Ha¨ussler
Serieshomepage–http://www.springer.de/phys/books/ssms/
Volumes1–50arelistedattheendofthebook.
C.J. Brabec V. Dyakonov
J. Parisi N.S. Sariciftci (Eds.)
Organic Photovoltaics
Concepts and Realization
With148Figures
1 3
Dr.ChristophJ.Brabec ProfessorJu¨rgenParisi
SiemensAG FacultyofPhysics
CTMM1InnovativeElectronics Dept.ofEnergyandSemiconductorResearch
Paul-Gossen-Strasse100 UniversityofOldenburg
91052Erlangen,Germany 26111Oldenburg,Germany
Dr.VladimirDyakonov ProfessorNiyaziS.Sariciftci
FacultyofPhysics InstituteofPhysicsalChemistry
Dept.ofEnergy andLinzInstituteofOrganicSolarCells
andSemiconductorResearch UniversityofLinz
UniversityofOldenburg AltenbergerStrasse69
26111Oldenburg,Germany 4040Linz,Austria
SeriesEditors:
ProfessorR.M.Osgood,Jr.
MicroelectronicsScienceLaboratory,DepartmentofElectricalEngineering
ColumbiaUniversity,SeeleyW.MuddBuilding,NewYork,NY10027,USA
ProfessorRobertHull
UniversityofVirginia,Dept.ofMaterialsScienceandEngineering,ThorntonHall
Charlottesville,VA22903-2442,USA
ProfessorJürgenParisi
Universita¨tOldenburg,FachbereichPhysik,Abt.Energie-undHalbleiterforschung
Carl-von-Ossietzky-Strasse9-11,26129Oldenburg,Germany
ISSN0933-033x
ISBN 978-3-642-05580-5
LibraryofCongressCataloging-in-PublicationData
Organicphotovoltaics:conceptsandrealization/C.J.Brabec...[etal.].
p.cm.–(Springerseriesinmaterialsscience;60)
Includesbibliographicalreferencesandindex.
ISBN 978-3-642-05580-5 ISBN 978-3-662-05187-0 (eBook)
DOI 10.1007/978-3-662-05187-0
1.Photovoltaiccells.2.Organicsemiconductors.I.Brabec,C.J.(ChristophJ.),1966-II.Series.
TK8322.0752003 621.3815’42–dc21 2002044659
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Preface
During and immediately after the oil crisis in the early 1970s, a tremendous
effort was devoted to the development of photovoltaic solar cells, in order
to utilize the cleanest of all possible renewable energy sources, i.e., sunshine.
The well-established silicon device technology totally dominates the market
today. Organic semiconducting materials represent a promising alternative
and a rather new approach, even though the basics of photovoltaic applica-
tions were described some time ago. Compounds such as merocyanines1 and
phthalocyanines2 that are readily deposited as thin films by vacuum evap-
oration gave solar-to-electric power conversion efficiencies of about 1% for
small-sized photovoltaic elements.
Achieving efficient solar energy conversion on a large scale and at low
cost is one of the most important technological challenges for the near fu-
ture.Itthereforeappearshighlyintriguingtothinkofextensiveorganicsolar
cells based on transparent, flexible, cheap, and easy-to-process thin plastic
films, cut from rolls and deployed on permanent structures and surfaces.
Whereas conjugated polymers have found successful technological applica-
tions as light-emitting diodes, the first attempt to prepare photovoltaic el-
ements with these materials alone (e.g., polyacetylene) was rather discour-
aging.3 Indeed, the photovoltaic effect involves the generation of electrons
and holes in the semiconductor device under illumination and their sub-
sequent collection at opposite electrodes. In organic semiconductors, both
charge separation and charge mobility are limited to some extent. Following
the encouraging breakthrough observation of reversible, metastable, and ul-
trafastphoto-inducedelectrontransferfromdonor-typeconjugatedpolymers
to acceptor-type fullerene molecules, a novel photovoltaic device concept –
in a sense similar to the first steps of natural photosynthesis – based on the
structural ordering of an interpenetrating network (by mixing the donor–
acceptor composite to a bulk heterojunction blend with increased effective
1 G.A. Chamberlain: Solar Cells 8, 47 (1983).
2 J. Simon, J.-J. Andre: Molecular Semiconductors (Springer, Berlin, 1985).
3 J. Kanicki: in Handbook of Conducting Polymers, Vol. 1, ed. by T.A. Skotheim
(Marcel Dekker, New York, 1985) p. 543.
VI Preface
interface area) gives rise to power conversion efficiencies up to 2.5% and a
quantum yield approaching unity.4,5,6
Besidestheefficiencycriterion,long-termstabilityisanothercrucialprob-
lemcommontoallpossibleapplicationsofsemiconductingpolymers.Todate,
light-emitting diodes based on conjugated polymer materials have generally
been provided with industrial encapsulation techniques that allow for a shelf
lifetimeofseveralyearsaswellasanoperationallifetimeofsometensofthou-
sands of hours. In the case of a photovoltaic device or even a single device
component, the trade-off between operational lifetime and efficiency versus
cost will determine the market penetration capability of these types of solar
cells.Thepurposeofthepresentvolumeistoprovideasnapshotofthestate
of the art in fundamental organic/plastic solar cell research with particular
emphasis on educational aspects.
The editors would like to express their gratitude to all authors for the
largeamountoftimeandeffortthatwentintopreparingthebroadspectrum
ofcontributionsprintedhereafter.SpecialthanksareduetoClausAscheron,
Angela Lahee, Petra Treiber, and Stephen Lyle from Springer-Verlag, Hei-
delberg, for continuous commitment, efficient support, and skillful technical
assistance.
Erlangen, Oldenburg, Linz, Christoph J. Brabec
January 2003 Vladimir Dyakonov
Ju¨rgen Parisi
Niyazi Serdar Sariciftci
4 N.S.Sariciftci,L.Smilowitz,A.J.Heeger,andF.Wudl,Science258,1474(1992).
5 G.Yu,J.Gao,J.C.Hummelen,F.Wudl,A.J.Heeger:Science270,1789(1995).
6 N.S. Sariciftci, A.J. Heeger: in Handbook of Organic Conducting Molecules and
Polymers,Vol.1,ed.byH.S.Nalwa(Wiley,NewYork,1996)p.414;S.E.Shaheen,
C.J. Brabec, N.S. Sariciftci, F. Padinger, T. Fromherz, J.C. Hummelen: Appl.
Phys. Lett. 78, 841 (2001); C.J. Brabec, N.S. Sariciftci, J.C. Hummelen: Adv.
Funct. Mater. 11, 15 (2001).
Contents
1 Photoinduced Charge Transfer
in Bulk Heterojunction Composites
Christoph J. Brabec, Vladimir Dyakonov .......................... 1
1.1 Conjugated Polymer–Fullerene Blend:
A Highly Efficient System
for Photoinduced Charge Generation........................... 2
1.2 Molecular Semiconductor Picture of Conjugated Polymers ........ 4
1.2.1Molecular Structure..................................... 4
1.2.2Optical and Electronic Properties......................... 6
1.2.3Characterisation of Excited States ........................ 7
1.2.4Photoinduced Electron Transfer
in Pure Conjugated Polymers and Molecules ............... 10
1.2.5Photoinduced Electron Transfer in Donor–Acceptor Systems . 15
1.3 Detection of Charges in Conjugated Polymers................... 16
1.3.1Pump–Probe Spectroscopy............................... 16
1.3.2Application of Pump–Probe Spectroscopy in the ms Range
to Conjugated Polymer/Fullerene Blends .................. 19
1.4 Kinetics of Photoinduced Charge Generation
in Conjugated Polymer/Fullerene Blends ....................... 21
1.5 Light-Induced Electron-Spin Resonance Detection
of the Charge Transfer Process................................ 26
1.6 Oligo-Phenylene Vinylene: A Model System
for Donor–Acceptor Interactions............................... 29
1.7 Conclusion ................................................. 51
References ..................................................... 51
2 Optical and Spectroscopic Properties
of Conjugated Polymers
Davide Comoretto, Guglielmo Lanzani............................. 57
2.1 Material and Optical Properties............................... 58
2.1.1Optical Constants and Electronic Structure ................ 61
2.1.2Determination of n˜ by Spectroscopic Methods .............. 62
2.1.3Anisotropy............................................. 65
2.1.4Examples.............................................. 67
VIII Contents
2.2 Spectroscopic Properties of Excited States...................... 71
2.2.1Basic Notions of Pump–Probe Spectroscopy................ 72
2.2.2Interpretation of Pump–Probe Experiments ................ 73
2.2.3Isolated Molecules ...................................... 75
2.2.4Condensed Phase ....................................... 77
2.3 Conclusion ................................................. 80
2.4 Appendix: Derivation of (2.17) ................................ 80
2.5 Appendix: Overview of Decay Kinetics ......................... 82
References ..................................................... 84
3 Transport Properties of Conjugated Polymers
Reghu Menon .................................................. 91
3.1 Disorder and Localization .................................... 92
3.2 Conduction in Conjugated Systems ............................ 95
3.3 Metal–Insulator Transition ................................... 99
3.4 Hopping Transport .......................................... 105
3.5 Magnetoresistance........................................... 108
3.6 Thermopower............................................... 113
3.7 Conclusion ................................................. 115
References ..................................................... 116
4 Quantum Solar Energy Conversion
and Application to Organic Solar Cells
Gottfried H. Bauer, Peter Wu¨rfel ................................. 118
4.1 Solar Radiation ............................................. 118
4.2 Solar Cells and General Quantum Converters ................... 120
4.2.1Two-Level Systems ..................................... 121
4.2.2Fermi Distribution ...................................... 122
4.2.3Quasi-Fermi Distribution ................................ 122
4.2.4Transition Rates and Optical Properties ................... 123
4.2.5Current–Voltage Characteristic ........................... 125
4.3 Semiconductor Solar Cells as Two-Band Systems ................ 127
4.3.1Fermi Distribution ...................................... 128
4.3.2Doping ................................................ 129
4.3.3Quasi-Fermi Distributions ............................... 130
4.3.4Interaction of Light with Matter.......................... 131
4.3.5Generation of Electron–Hole Pairs ........................ 135
4.3.6Recombination ......................................... 136
4.4 Radiative Limit for Solar Cell Efficiencies....................... 138
4.4.1Current–Voltage Characteristic ........................... 139
4.5 Charge Separation in Solar Cells .............................. 141
4.5.1Charge Transport....................................... 143
4.5.2Transport Equations for Semiconductor Solar Cells.......... 147
4.5.3Charge Transport in Low Mobility Materials ............... 148
Contents IX
4.5.4Carrier Mobilities in Organic Semiconductors............... 150
4.5.5Equivalent Circuits for Solar Cells ........................ 151
4.6 Conclusions for Solar Cell Requirements........................ 153
4.6.1Special Geometrical Design .............................. 153
4.6.2Particular Optical Design/Multispectral Conversion ......... 155
References ..................................................... 156
5 Semiconductor Aspects
of Organic Bulk Heterojunction Solar Cells
Christoph J. Brabec............................................. 159
5.1 Device Architectures......................................... 160
5.1.1Single-Layer Diodes ..................................... 160
5.1.2Heterojunction Diodes................................... 162
5.1.3Bulk Heterojunction Solar Cells .......................... 163
5.2 Device Aspects and Transport Properties....................... 169
5.2.1Transport Properties of Diodes ........................... 169
5.2.2Metal/Conjugated Polymer Contacts...................... 177
5.2.3Simulation and Modelling................................ 183
5.3 Performance Analysis of Bulk Heterojunction Solar Cells ......... 185
5.3.1Precise Calibration of Solar Cells ......................... 186
5.3.2Production: Device Geometry ............................ 188
5.3.3Short-Circuit Current ................................... 190
5.3.4Open-Circuit Voltage V ................................ 205
oc
5.3.5Fill Factor FF.......................................... 214
5.3.6Spectral Response ...................................... 221
5.3.7Temperature Behavior of Bulk Heterojunction Solar Cells.... 229
5.3.8Stability of Polymeric Semiconductors and Devices:
A Molecular View ...................................... 236
5.3.9Processing of Polymeric Semiconductors:
Blending with Conventional Polymers ..................... 240
5.4 Conclusion ................................................. 242
References ..................................................... 243
6 Organic Photodiodes: From Diodes to Blends
Olle Ingana¨s, Lucimara Stolz Roman .............................. 249
6.1 Thin Film Organic Photodiodes............................... 250
6.2 Optical Mode Structure in Thin Film Organic Structures.
Optimization of Bilayer Geometries............................ 251
6.3 Internal and External Quantum Efficiencies
of Organic Photodiodes ...................................... 263
6.4 Electrical Transport in Photodiodes ........................... 264
6.5 Nanostructure in Polymer/Molecule and Polymer/Polymer Blends. 265
6.6 Conclusion ................................................. 270
References ..................................................... 271
X Contents
7 Dye-Sensitized Solar Cells
Jan Kroon, Andreas Hinsch ...................................... 273
7.1 Operating Principles and Cell Structure of the nc-DSC........... 275
7.2 Manufacture of a Standard Glass/Glass nc-DSC................. 277
7.3 Module Designs ............................................. 278
7.3.1Series Connection of Glass/Glass Devices:
Z- and W-Type Interconnection .......................... 279
7.3.2Series Connection: Three-Layer or Monolithic Module ....... 280
7.4 Sealing Aspects ............................................. 280
7.5 Technological Development and the State of the Art ............. 281
7.6 Large Scale Batch Processing of Mini-Modules .................. 283
7.7 Long Term Stability ......................................... 285
7.7.1Stability Tests on Indoor Dye PV Modules................. 285
7.7.2Long Term Stability Tests on High Power nc-DSC .......... 286
7.8 Outlook.................................................... 288
References ..................................................... 289
Index......................................................... 291
