nanostructured and photoelectrochemical systems for solar photon conversion, 2008, p.781

NANOSTRUCTURED AND
PHOTOELECTROCHEMICAL
SYSTEMS FOR
SOLAR PHOTON CONVERSION
SERIES ON PHOTOCONVERSION OF SOLAR ENERGY
Series Editor: Mary D. Archer (Cambridge, UK)
Vol. 1: Clean Electricity from Photovoltaics
eds. Mary D. Archer & Robert Hill
Vol. 2: Molecular to Global Photosynthesis
eds. Mary D. Archer & Jim Barber
Vol. 3: Nanostructured and Photoelectrochemical Systems
for Solar Photon Conversion
eds. Mary D. Archer & Arthur J. Nozik
Forthcoming
From Solar Photons to Electrons and Molecules
by Mary D. Archer
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Imperial College Press
ICP
Series on Photoconversion of Solar Energy — Vol. 3
Editors
Mary D. Archer
Imperial College, UK
Arthur J Nozik
National Renewable Energy Laboratory, USA
NANOSTRUCTURED AND
PHOTOELECTROCHEMICAL
SYSTEMS FOR
SOLAR PHOTON CONVERSION

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Copyright © 2008 by Imperial College Press
Series on Photoconversion of Solar Energy — Vol. 3
NANOSTRUCTURED AND PHOTOELECTROCHEMICAL SYSTEMS FOR
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This volume is dedicated


to


Olga I. Mićić

21 December 21 1934 – 24 May 2006


a fine scientist and pioneer in the field of quantum dots


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vii

CONTENTS
About the authors xi

Preface xix



1 Overview

M. D. Archer

1.1 Themes 1

1.2 Historical perspective 6

1.3 Extremely thin absorber (ETA) cells 10

1.4 Organic solar cells 12

1.5 Dye-sensitised solar cells (Grätzel cells) 16

1.6 Regenerative solar cells 18

1.7 Future prospects 23

App.
1A
The vacuum scale of electrode potential and the concept of the
solution Fermi level
24

1A.1 SHE and SCE scales of electrode potential 25

1A.2 Absolute electrode potentials 25

1A.3 Absolute electrode potential of the SHE 27

1A.4 The solution Fermi level 28

1A.5 Vacuum scale of electrode potential 29



2 Fundamentals in photoelectrochemistry

R. J. D. Miller and R. Memming

2.1 Introduction 39

2.2 Photophysics of semiconductors and semiconductor particles 41

2.3 Carrier relaxation 55

2.4 Charge transfer at the semiconductor–electrolyte interface 84

2.5 Conversion of solar energy 120

2.6 Photocatalysis 130

2.7 Summary 132

3 Fundamentals and applications of quantum-confined structures

A. J. Nozik

3.1 Introduction 147

3.2 Quantisation effects in semiconductor nanostructures 151

Contents

viii

3.3 Optical spectroscopy of quantum wells, superlattices and quantum dots

163

3.4 Hot electron and hole cooling dynamics in quantum-confined
semiconductors
167

3.5
High conversion efficiency via multiple exciton generation in quantum
dots
176

3.6 Quantum dot solar cell configurations 190

3.7 Summary and conclusions 194

4 Fundamentals and applications in electron-transfer reactions

M. D. Archer

4.1 Introduction 209

4.2 Historical perspective 213

4.3 Thermodynamics of ET and PET reactions 218

4.4 Classical Marcus theory 223

4.5 Semiclassical theories of nonadiabatic electron transfer 232

4.6 Electron transfer in donor–bridge–acceptor supermolecules 238

4.7 Electrochemical electron transfer 247

4.8 Rate control by reorganisation dynamics 261

4.9 Optimisation of photoinduced electron transfer in photoconversion 263

App.
4A
Solution density-of-states functions 265

App.
4B
Derivation of high-temperature limit Marcus rate equation for

homogeneous electron transfer using density-of-states approach
266



5 Fundamentals in metal-oxide heterogeneous photocatalysis

N. Serpone and A. V. Emeline

5.1 Introduction 275

5.2 The complex science underlying metal-oxide photocatalysis 277

5.3 Metal-oxide photochemistry, photophysics and modelling 310

5.4 Challenges in heterogeneous photocatalysis 329

5.5 Theoretical description of quantum yields 345

5.6 Evidence for a gas/solid surface reaction being photocatalytic 374

5.7 Concluding remarks 381



6 Inorganic extended-junction devices

R. Könenkamp

6.1 Introduction 393

6.2 Concepts for extremely thin absorber cells 398

Contents ix

6.3 Preparation of substrates, absorber and transporting layers 403

6.4 Electronic and optical aspects 419

6.5 Devices 434

6.6 Advanced photovoltaic concepts and new routes to other electronic
devices
441

6.7 Summary 443



7 Organic donor–acceptor heterojunction solar cells

J. J. Benson-Smith and J. Nelson

7.1 Introduction 453

7.2 Basic principles of photovoltaic conversion in organic materials 457

7.3 Donor–acceptor bilayer devices 461

7.4 Donor–acceptor bulk heterojunction devices 465

7.5
Relationship between material and device parameters and photovoltaic
performance
473

7.6 Challenges 478

7.7 Summary 489



8 Dye-sensitised mesoscopic solar cells

M. Grätzel and J. R. Durrant

8.1 Introduction 503

8.2 Historical background 504

8.3 Mode of function of dye-sensitised solar cells 505

8.4 DSSC research and development 515

8.5 Solid-state dye-sensitised cells 526

8.6 Pilot production of modules, outdoor field tests and commercial DS
SC
development
527

8.7 Outlook 530



9 Semiconductor/liquid junction photoelectrochemical solar cells

S. Maldonado, A. G. Fitch and N. S. Lewis

9.1 Introduction 537

9.2 Variation of the solution redox couple 539

9.3 Non-aqueous solvents 548

9.4 Chemical modification of semiconductor surfaces 551

9.5 Future directions 569



Contents

x

10 Photoelectrochemical storage cells

S. Licht and G. Hodes

10.1 Introduction 591

10.2 Comparative solar energy storage processes 594

10.3 Modes of photoelectrochemical storage 600

10.4 Optimisation of photoelectrochemical storage 603

10.5 Examples of photoelectrochemical storage cells 611

10.6 High-efficiency multiple-bandgap cells with storage 622

10.7 Conclusions 625



11 Measuring ultrafast photoinduced electron-transfer dynamics

X. Ai and T. Lian

11.1 Introduction 633

11.2 Techniques for measuring ultrafast electron transfer 635

11.3 Current understanding of ultrafast electron transfer 645

11.4 Summary 659



12 Experimental techniques in photoelectrochemistry

L. M. Peter and H. Tributsch

12.1 Introduction 675

12.2 Electrical methods 676

12.3 Photocurrent, photovoltage and microwave reflectance methods 683

12.4 In-situ spectroscopic methods 697

12.5 Time-resolved optical and spectroscopic techniques 702

12.6 Modulation spectroscopies 706

12.7 Frequency-resolved light modulation methods 712

12.8 Imaging techniques 720

12.9 In-situ X-ray analysis and EXAFS 723

12.10 Differential mass spectrometry (DEMS) 723

12.11 Combination of electrochemistry with vacuum spectroscopy 725



Appendices

I Fundamental Constants 737

II Useful Quantities and Conversion Factors 738

III List of Symbols 739

IV Acronyms and Abbreviations 743



Index 747


xi

ABOUT THE AUTHORS
Xin Ai received her BS degree from Jilin University, Changchun, China. In 2004, she
obtained her PhD in chemistry from Emory University, Atlanta, Georgia, where she
worked on the investigation of photoinduced interfacial electron-transfer dynamics on
dye molecule and inorganic semiconductor nanocomposite films using femtosecond
infrared spectroscopy. She then joined National Renewable Energy Laboratory,
Golden, Colorado, as a postdoctoral associate. Her primary research interest is in the
photochemical and photoelectrochemical properties of novel molecular materials,
including conjugated polymers, carbon nanotubes and quantum dots, which have been
used to fabricate a new generation of solar cells. She currently focuses on fundamental
understanding of the photoinduced interfacial charge-transfer processes occurring in
these materials, using femtosecond transient spectroscopy, terahertz spectroscopy and
time-resolved and steady-state photoluminescence spectroscopy. The goal of her
work is to understand the factors affecting the efficiency of photovoltaic cells and,
through this understanding, to provide insight into improving the performance of the
working devices.

Mary Archer read chemistry at Oxford University and took her PhD from Imperial
College, London, in 1968. From 1968 to 1972, she did post-doctoral work in electro-
chemistry with Dr John Albery at Oxford, and she then spent four years at The Royal
Institution in London, working with Lord Porter (then Sir George Porter) on photo-
electrochemical methods of solar energy conversion. She taught chemistry at
Cambridge University from 1976 to 1986. From 1991 to 1999, she was a Visiting
Professor in the Department of Biochemistry at Imperial College, London, and from
1999 to 2002, she held a Visiting Professorship at ICCEPT (Imperial College Centre
for Energy Policy and Technology). She is President of the UK Solar Energy Society
and the National Energy Foundation and a Companion of the Energy Institute. She
was awarded the Melchett Medal of the Energy Institute in 2002 and the Eva Philbin
Award of the Institute of Chemistry of Ireland in 2007.

Jessica Benson-Smith was awarded the British Marshall Scholarship in 2004. As a
recipient of this fellowship, she became a postgraduate student in the Experimental
Solid State Physics Department at Imperial College, London, from which she received
her PhD in 2007. She specialises in the spectroscopy of organic bulk heterojunction
films for organic solar cell applications.

About the Authors

xii

James Durrant is Professor of Photochemistry in the Department of Chemistry at
Imperial College, London. After completing his undergraduate studies in physics at
the University of Cambridge, he obtained a PhD in biochemistry at Imperial College,
London, in 1991, studying the primary reactions of plant photosynthesis. After
postdoctoral positions and a BBSRC Advanced Fellowship, he joined the Chemistry
Department at Imperial College in 1999. His interests are in photochemical

approaches to solar energy conversion, electron-transfer dynamics and excitonic solar
cells.

Alexei Emeline obtained his MSc in Physical Chemistry from Tomsk State University
in 1990 and his PhD in Molecular Physics from St Petersburg State University in
1995. He started his academic research career at St Petersburg State University as a
researcher at the V. A. Fock Institute of Physics in 1995, and later in the same year as
an assistant professor of the Faculty of Physics. In 1996 he was awarded a NATO
Science Fellowship to take up a post-doctoral fellowship at Concordia University in
Canada under the supervision of Professor Nick Serpone. From 1998 to 2004, he
remained in the same laboratory at Concordia University as an associate researcher. In
2005 he was awarded a JSPS Fellowship and spent one year in the group of Professor
Akira Fujishima at Kanagawa Academy of Science and Technology in Japan. He is
currently a senior researcher, working for his DSc in the V. A. Fock Institute of
Physics of St Petersburg State University. His research interests focus on fundamental
studies of interfacial photophysical and photochemical processes in heterogeneous
systems, particularly on the role of photoexcitation conditions on the direction and
efficiency of different photoprocesses.

Anthony Fitch received his undergraduate degree at the University of Nebraska at
Kearney and is currently pursuing his PhD at Caltech under the advisement of Nathan
S. Lewis.

Michael Grätzel is a professor at the École Polytechnique de Lausanne, where he
directs the Laboratory of Photonics and Interfaces. He discovered a new type of solar
cell based on dye-sensitised mesoscopic oxide particles and pioneered the use of
nanocrystalline materials in electroluminescent and electrochromic displays, as well as
lithium ion batteries and bioelectronic sensors. Author of over 500 publications, two
books and inventor of more than 50 patents, his work has received over 40,000
citations so far, ranking him amongst the most highly-cited scientists worldwide. He

has received several prestigious awards, including the Faraday Medal of the (British)
Royal Society of Chemistry, the Dutch Havinga award, the Italgas prize, the European
About the Authors


xiii

Millennium award for Innovation, the 2006 World Technology Award in Materials
and the Gerischer award. In 2006, he was selected by Scientific American as one of
the fifty top researchers in the world. He received his doctor’s degree in Natural
Science from the Technical University, Berlin, and holds honorary doctorates from the
Universities of Delft, Uppsala and Turin. He is a member of the Swiss Chemical
Society and the European Academy of Science and an elected honorary member of the
Société Vaudoise de Sciences Naturelles.

Gary Hodes received his BSc and PhD from Queen’s University of Belfast in 1968
and 1971 respectively, and has been at the Weizmann Institute of Science, Rehovot,
Israel since 1972. His research has focused on semiconductor film deposition from
solutions (initially electrochemical and later chemical bath deposition) and on various
types of solar cells (liquid junction, thin film, polycrystalline and nanoporous) and
quantum dots using these films. Throughout his career, he has also studied various
aspects of semiconductor surface treatments. More recently, he is continuing work on
various aspects of chemical bath deposition mechanisms and also increasingly
concentrating on nanocrystalline, semiconductor-sensitised solar cells.

Rolf Könenkamp is the Gertrude-Rempfer Professor of Physics at Portland State
University in Portland, Oregon. His present research interests lie in the field of nano-
science. He has worked extensively on semiconductor devices, such as nanostructured
solar cells and nanowire light-emitting diodes and transistors, and he holds several
patents in this area. He has led the design and construction of a new high-resolution

photoelectron microscope since 2002. This will be one of the first aberration-corrected
microscopes of this type and it will be used to explore transport and confinement
effects on the nanoscale. He has worked at NREL, HMI Berlin, Hitachi Tokyo,
Princeton University and at the IST in Lisbon, and he is a member of the national
R&D team for thin-film photovoltaics in the US.

Nathan Lewis is George L. Argyros Professor of Chemistry at the California Institute
of Technology, where he has been on the faculty since 1988. He has also served as the
Principal Investigator of the Beckman Institute Molecular Materials Resource Center
at Caltech since 1992. From 1981 to 1986, he was on the faculty at Stanford, as
assistant professor from 1981 to 1985 and associate professor from 1986 to 1988. He
received his PhD in Chemistry from the Massachusetts Institute of Technology. He
has been an Alfred P. Sloan Fellow, a Camille and Henry Dreyfus Teacher–Scholar
and a Presidential Young Investigator. He received the Fresenius Award in 1990, the
ACS Award in Pure Chemistry in 1991, the Orton Memorial Lecture award in 2003
About the Authors

xiv

and the Princeton Environmental Award in 2003. He has published over 200 papers
and supervised about 50 graduate students and postdoctoral associates. His research
interests include light-induced electron transfer reactions, both at surfaces and in
transition metal complexes, surface chemistry and photochemistry of semi-
conductor/liquid interfaces, novel uses of conducting organic polymers and
polymer/conductor composites and development of sensor arrays that use pattern
recognition algorithms to identify odorants, mimicking the mammalian olfaction
process.

Tianquan Lian received his BS degree from Xiamen University in 1985, his MS
degree from the Chinese Academy of Sciences in 1988 and his PhD from the

University of Pennsylvania in 1993. After postdoctoral training in the University of
California at Berkeley, he joined the faculty of chemistry department at Emory
University in 1996. He was promoted to associate professor in 2002 and full professor
in 2005. He has been a recipient of the NSF CAREER award and the Sloan
fellowship. His research interest is focused on the ultrafast dynamics of nanomaterials
and interfaces. He is particularly interested in fundamental physical chemistry
problems related to nanomaterials-based solar energy conversion concepts and
devices. These problems include the dynamics of electron transfer, energy transfer,
vibrational energy relaxation and solvation at interfaces and in nanomaterials.

Stuart Licht has over 250 publications in renewable energy chemistry, physical
chemistry and analytical chemistry, and was the recipient of the 2006 Electrochemical
Energy Research Award. He has developed theory and experiment for the highly
efficient solar generation of hydrogen fuel, introduced the contemporary use of
caesium to enhance solar cell voltage and established the chemistry of an efficient
solar cell that functions day and night. He has originated the field of Fe(VI) redox
chemistry for charge storage (the ‘Super-Iron Battery’), as well as novel sulphur
batteries and a variety of new aluminium electrochemical storage cells. En route to
new pathways to utilise renewable energy, the Licht group continues to explore a
range of fundamental physicochemical processes ranging from quantum mechanics to
thermodynamics of water, hydrogen, halide, chalcogenide and transition metal
chemistry, and to introduce new analytical methodologies, in dilute, concentrated or
molten media, as needed to facilitate the research. Licht has chaired a regional section
of the national American Chemical Society and also founded, and chaired, the New
England, and the Israel, Sections of the Electrochemical Society.


About the Authors

xv

Stephen Maldonado was a Beckman Scholar in 2000–2001 for his work on proton
exchange membrane fuel cell system testing. After receiving a BS in Chemistry from
the University of Iowa in 2001, he was awarded an NSF Fellowship and a Huntington
Fellowship for graduate studies at the University of Texas at Austin. His thesis work
centred on designing electrocatalytically active graphitic carbon nanotubes. In 2006,
he obtained his PhD in Chemistry and joined the research laboratory of Professor
Nathan S. Lewis as a postdoctoral research scientist at the California Institute of
Technology. His current research focuses on the electrical and electrochemical
properties of metal–silicon contacts using chemically modified silicon.

Rüdiger Memming obtained his PhD degree in Physical Chemistry from the
University of Stuttgart, Germany, in 1958, working with Professor Förster, and then
did post-doctoral work at the Chemistry Department of the University of Minnesota,
Minneapolis, working with Professor R. S. Livingston for two years. In 1960, he
started to work in the Philips Research Laboratory in Hamburg, Germany, where he
continued until 1987. In addition, he had a research group at the Chemistry
Department of the University of Hamburg from 1981 to 1987. After this he started a
new government Institute for Solar Energy Research in Hanover, from which he
retired in 1994. In 1991, he went to Japan for four months as a JSPS-Fellow.

R. J. Dwayne Miller obtained a BSc Honours degree in 1978 from the University of
Manitoba and his PhD degree in Chemistry from Stanford University in 1983, and
then did post-doctoral work as a NATO Science Fellow at the Université de Joseph
Fourier, Grenoble. He started his academic research career at the University of
Rochester in 1984, where he was a faculty member in Chemistry and the Institute of
Optics. He relocated his research group to the University of Toronto in 1995, where
he is currently the Director of the Institute for Optical Sciences and full professor in
the Departments of Chemistry and Physics. He is a Fellow of the Royal Society of

Canada and the holder of the Canada Research Chair in femtoscience. His early
research interests focused on the primary events controlling electron transfer at
surfaces. This work demonstrated how truly fast electron-transfer processes can be at
conducting surfaces, and led to his current research, which focuses on femtosecond
electron pulse generation to give atomic-level views of transition state processes. He
welcomed the opportunity to return to his roots to write this review on the
photophysical processes at semiconductor surfaces with the hope that this overview
will help researchers solve the last hurdles to economically viable solar power.


About the Authors

xvi

Jenny Nelson is a Professor of Physics at Imperial College, London, where she has
researched novel types of solar cell since 1989. Her current research focuses on
photovoltaic energy conversion using molecular materials, characterisation of the
charge transport, charge separation and morphological properties of molecular
semiconductors, the theory of charge transport in organic semiconductors and
modelling of photovoltaic device behaviour. She has published over 100 papers on
photovoltaic materials and devices and a book on the physics of solar cells.

Arthur Nozik graduated from Cornell University in Chemical Engineering in 1959.
After a brief spell in the aerospace industry, he entered Yale University to work for a
PhD in physical chemistry. The birth of his daughter caused him to intermit these
studies and join the American Cyanamid Company, but he returned to Yale and
finished his PhD in 1967. He then returned to Cyanamid for seven years, introducing
Mössbauer spectroscopy to the company. In 1974, he joined Allied Chemical
Corporation to work on semiconductor photoelectrochemistry as applied to solar
photoconversion. At Allied, he became the first to demonstrate the ‘zero bias’

photoelectrolysis of water, using an n-TiO
2
photoanode and a p-GaP photocathode,
and also the photoreduction of dinitrogen on p-GaP. He also developed the
‘photochemical diode’, the forerunner of today’s particulate semiconductor
suspensions. In 1978, he moved to the new Solar Energy Research Institute (now
NREL) at Golden, Colorado, where he was Branch Chief of the Photoconversion
Branch, 1980–1984, and has been a Senior Research Fellow since 1984. He was Team
Leader of the NREL Chemical Sciences Team from 1985 to 2006 and he has been
Professor Adjoint at the University of Colorado at Boulder since 1998. At NREL, his
research has centred on the behaviour of hot carriers in quantum wells, superlattices
and quantum dots. In 2005, thirty years of work in solar photon conversion were
rewarded when he and his research group demonstrated efficient multiple exciton
generation in lead chalcogenide quantum dots. He was awarded the 2008 Eni Award
for Science and Technology.

Laurie Peter gained his PhD in Southampton in 1969, before being awarded a CIBA
Postdoctoral Fellowship to work in the Group of Heinz Gerischer, who was then at the
Technische Hochschule in Munich. Subsequently, he moved with Gerischer’s group
to the Fritz Haber Institute in Berlin, where he remained as a member of staff until
1975, when he returned to the UK to take up a lectureship in Southampton. He
remained in Southampton for the next 17 years and was promoted to professor before
moving to Bath in 1992 to become Professor of Physical Chemistry and subsequently
Head of Department. Laurie Peter was an editor of the Journal of Electroanalytical
About the Authors


xvii

Chemistry from 1999 to 2005, and has been awarded the Electrochemistry Prize of the

Royal Society of Chemistry and the Pergamon Medal of the International Society of
Electrochemistry. He currently leads the UK SUPERGEN Excitonic Solar Cell
Consortium (Bath, Cambridge, Edinburgh and Imperial College), which is studying
non-classical solar cells.

Nick Serpone obtained a BSc Honours Chemistry in 1964 from Sir George Williams
University in Montreal and his PhD degree in Physical-Inorganic Chemistry from
Cornell University in 1968, following which he joined the chemistry faculty of
Concordia University as Assistant Professor. His early research involved NMR studies
of Group IV coordination complexes. After sabbatical leaves at the University of
Bologna, Italy, in 1975 and at the École Polytechnique Fédérale de Lausanne,
Switzerland, as an invited professor in 1983, his research interests focused on the
photochemistry of coordination complexes and on fundamental and applied studies in
heterogeneous photocatalysis in which, together with others, he has been instrumental
in developing the technology to degrade environmental organic pollutants and to
dispose of toxic metals. In 1981, he co-founded the Canadian Centre for Picosecond
Laser Spectroscopy at Concordia University and was its director until 2002. His other
principal research interests have involved studies of ultra-fast photophysical and
photochemical events in metal chalcogenide and silver halide semiconductors.
Following his appointment as a University Research Professor and Professor Emeritus
in 1998, he joined the Chemistry Division of the National Science Foundation in
Washington DC as an IBO Program Director from 1998 to 2001. He was a Visiting
Professor in Italy’s programme ‘Rientro dei Cervelli’ at the University of Pavia from
2002 to 2005, where he carried out research into the photochemistry of sunscreen
active agents.

Helmut Tributsch obtained his PhD degree in physical chemistry at the Technical
University, Munich, in 1968, working with Heinz Gerischer, and subsequently
continued research with Melvin Calvin at the University of Berkeley. For the next ten
years he worked in different institutions including Stanford University, the CNRS in

Paris and the Fritz-Haber Institute in Berlin. Since 1982, he has been Professor of
Physical Chemistry at the Free University in Berlin and head of the department at the
Hahn-Meitner Institute specialising in research on sustainable energy systems.

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xix

PREFACE



Thus daily were my sympathies enlarged,
And thus the common range of visible things
Grew dear to me: already I began
To love the sun, a Boy I lov’d the sun,
Not as I since have lov’d him, as a pledge
And surety of our earthly life, a light
Which while we view we feel we are alive;
But, for this cause, that I had seen him lay
His beauty on the morning hills, had seen
The western mountain touch his setting orb,
In many a thoughtless hour, when, from excess
Of happiness, my blood appear’d to flow
With its own pleasure, and I breath’d with joy.

William Wordsworth, The Prelude: Book 2: School-Time, 1805.


More solar energy falls on the Earth’s surface every day than the total amount of

energy the world’s population would consume in 16 years at present rates of
utilisation. To harness this potential to provide reliable and economic carbon-free
sources of electricity and fuels remains a challenge, even in current times of high
energy prices and action to mitigate climate change. However, there are encouraging
signs. The annual global market for photovoltaic (PV) modules was valued at
US$12.9bn in 2007 and is predicted to grow by 15% compound per annum. Although
crystalline silicon p–n junction cells still dominate this market, a new generation of
photovoltaic and photoelectrochemical devices is emerging to challenge them, many
based on the unique properties of matter at the nanoscale.
It is this new generation of solar photon conversion devices that are covered in this
book. They are less highly developed than those described in Volumes 1 and 2 of this
series, but their promise is at least as great. That promise is two-fold: on the one hand
highly efficient devices with sophisticated architectures in which the Shockley–
Queisser limit on efficiency is finally overcome, and on the other very low-cost
plastic or organic-based devices that are cheap enough to be disposable.
The leitmotifs of these devices include bespoke dye sensitisers, space-quantised
nanoscale structures that enable hot carrier or multiple exciton generation, molecular
and solid-state junction architectures that lead to efficient exciton dissociation and
charge separation, and charge collection by percolation through porous or mesoscale
phases. Another common theme underlying the devices discussed in this book is the
Preface

xx

orthogonalisation of the pathways for photon absorption and carrier collection.
Contrast the classical silicon solar cell, in which the two pathways are parallel with an
ETA or bulk heterojunction cell, in which they are orthogonal. In the silicon cell, the
base layer has to be sufficiently thick to absorb incoming photons, so minority carrier
diffusion lengths have to be (and are) as long as 200–500 µm, placing great demands
on materials quality. In an ETA or bulk heterojunction cell, the junction architecture

allows efficiencies of over 5% to be achieved with exciton or charge carrier diffusion
lengths that are as much as one million times shorter, and materials of much lower
electronic quality suffice.
Photocatalysis is closely related to photoelectrochemistry, and the fundamentals of
both disciplines are covered in this volume. Their applications to photoelectrolysis
and other solar fuel-forming or waste-destroying photochemical and photoelectro-
chemical processes will form the main subject matter of the fourth and final volume in
this book series.
To satisfy the global need for carbon-free energy, the fields of photovoltaics and
photoelectrochemistry must continue to develop. The key to progress lies in the
quality of the fundamental research being conducted in this area. It is worrying that
global funding streams for research to develop advanced solar photon conversion
technologies remain fragile despite the concerted and powerful case for a ‘Manhattan
project’ effort to do so made by the international scientific community during a
special conference in 2005 on basic research needs for solar energy utilisation
promoted by the US Department of Energy’s Office of Science, Basic Energy
Sciences Division. However, commercialisation of some of these devices is
beginning, and a January 2008 report from BCC Research predicts that the market for
nanostructured thin films and silicon and dye-sensitised solar cells is set to grow at
more than 50% per annum through to 2013 as the technology matures.
Our warmest appreciation goes to our fifteen authors, who between them have
provided so rich a picture of the scientific frontiers they are exploring. We also thank
Alexandra Anghel, Carol Burling, Barrie Clark and Stuart Honan for their editorial
assistance, David Ginley, John Kelly and Reshef Tenne for providing information,
James Bolton for his early input into some of the material in Chapters 1 and 4, the
staff of World Scientific Press who expertly drew many of the diagrams, and Lenore
Betts, Lizzie Bennett and Katie Lydon of IC Press for guiding us along the winding
road to publication.

Mary D. Archer

Arthur J. Nozik

March 2008


1

CHAPTER 1
OVERVIEW
MARY D. ARCHER
The Old Vicarage, Grantchester, Cambridge CB3 9ND, UK

Where from citadels on high
Her imperial standards fly,
Let the hot sun
Shine on, shine on.
W. H. Auden, Twelve Songs, 1935–1938.


1.1 Themes

The major themes of this book are announced by its title: nanostructured and photo-
electrochemical systems for solar energy conversion. It deals mainly with the direct, i.e.
non-thermal, conversion of solar photonic energy into electrical power by photo-
electrochemical or advanced photovoltaic means in extended-junction, mesoporous,
nanocomposite or space-quantised structures and devices. Other themes are the
fundamentals of electron transfer and photoinduced electron transfer in supramolecular
assemblies, photocatalytic reactions at semiconductor dispersions, and experimental
techniques for the characterisation of semiconductor photoelectrochemical systems.

Semiconductors have been the electrode materials of choice for solar photon
conversion for nearly thirty years, on account of their favourable optoelectronic
properties and chemical versatility. Semiconductor bandgap energies E
g
commonly fall in
the range 1–3 eV, which overlaps well with the spectrum of terrestrial sunlight, as shown
in Fig. 9.1, and also with the decomposition potentials of such important reactions as
water splitting, as shown in Fig. 2.17. Absorption by a semiconductor of photons of
energy greater than the bandgap energy leads to the creation of ‘free’ holes and electron
(in broadband inorganic semiconductors) and excitons (in organic semiconductors). At
the junction of a photovoltaic device, these free carriers or excitons are separated into a
flow of electrons in one direction and a flow of holes in the other at a potential difference
determined by the light intensity and the junction characteristics, leading to the
generation of electric power on illumination.

M. D. Archer

2

Photoelectrochemical cells for solar photon conversion are usually designed to
produce either electric power or solar fuels; this book focuses on the latter. Power-
producing solar cells are designed to be operated at their maximum-power point to
produce electric power at the energy conversion efficiency η
mp


mp mp
mp
S
o

i V
E
η
=
(1.1)
where
S
o
E
is the incident solar irradiance, i
mp
is the maximum-power photocurrent
density and V
mp
is the maximum-power voltage. The ratio between the maximum power
generated and the product of the short-circuit photocurrent density i
sc
and the open-circuit
voltage V
oc
is known as the fill factor, η
fill
. The higher the value of η
fill
, the better the
quality of the device.

mp mp
fill
sc oc

i V
i V
η
=
(1.2)
‘Classical’ silicon photovoltaic cells are capable of excellent performance,
approaching the detailed balance limit for a single-bandgap device: non-concentrator
single-crystal cells have reached an energy conversion efficiency
1
of 24.7%, and
concentrator cells 27.6%. They are, however, minority-carrier devices, meaning that the
photocurrent must be carried to the junction by electrons through p-type material, and by
holes through n-type material. Minority carriers are highly susceptible to bulk recomb-
ination, as well as to trapping and interfacial recombination. A high level of materials
quality and fastidious attention to cell design and fabrication are therefore needed to
endow minority carriers in a silicon cell with sufficient lifetime to reach and flow across
the junction without loss by hopping or recombination. The minimum thickness of a
photovoltaic cell is determined by the width of the absorber layer needed to absorb
incident light efficiently. Since crystalline silicon is an indirect-gap material, it is not
intensely absorbing, and so a comparatively thick wafer of it is required to absorb
incident sunlight efficiently, even with such refinements as surface texturisation, internal
light scattering and back-surface reflection to increase the optical path length of light in
the cell. Thus the excellent performance of the classical silicon photovoltaic cell is in
some ways a triumph of materials and device optimisation over basically unfavourable
materials characteristics. Few other inorganic semiconductors, and no organic semi-
conductors, are capable of being developed to deliver similarly good performance in a
photovoltaic cell of classical, planar-junction architecture. Moreover, in a classical p–n

1
Conversion efficiencies are, or should be, quoted for standard test conditions, which are 1000 W m

–2
of
AM1.5 global insolation and a cell temperature of 25 C.
Overview

3
junction cell the same material is required both to absorb light and to permit charge
transport along the same dominant parallel pathway, which is perpendicular to the planar
junction, as shown in Fig. 1.1a.
Extended-junction and nanostructured photoconversion devices can escape from
these constraints by orthogonalising the pathways for light absorption and charge
collection, as illustrated in Fig. 1.1b. The pathways for charge collection are much
shorter, allowing the use of inexpensive low-quality materials, and also of organic
semiconductors in which light absorption generates not free charge carriers but short-
lived excitons that must reach an interface in order to separate at it and generate
photocurrent. Additional and important advantages of nanosized semiconductor
structures and particles are the increased carrier lifetimes arising from space quantisation,
the enhanced redox potentials of photogenerated holes and electrons arising from the
increased effective bandgap and the possibility of multiple exciton generation by one
absorbed photon in a quantum dot.
In this chapter, I give an account of the historical development of semiconductor
photoelectrochemistry and nanostructured photovoltaic devices in Section 1.2, and then
Sections 1.3–1.6 provide a brief introduction to the major cell types discussed in the
remainder of the book: the ETA (extremely thin absorber) cell, organic and hybrid cells,
dye-sensitised solar cells (Grätzel cells) and regenerative solar cells.
In Chapter 2, Miller and Memming present an advanced treatment of the solid-state
physics and photoelectrochemistry of semiconductors. In Chapter 3, my co-editor Art
Nozik covers the fundamentals and applications of quantum-confined structures and
explains how the unique ability of quantum dots to generate multiple pairs of charge
carriers with a single high-energy photon could lead to a new generation of photovoltaic

cells. In Chapter 4, I turn to electron-transfer theory, and how its understanding through

Figure 1.1
(a) Classical planar n-on-p photovoltaic cell junctio
n, showing the dominant parallel direction of
the light and charge separation pathways; (b) Extended, structured junction with interposed absorber layer
(shaded in grey), showing the dominant non-parallel direction of the light and charge separation pathways.

+

+

n p n absorber p
(a) (b)
–

–

hν

hν


M. D. Archer

4

the powerful prism of Marcus theory has led to the design and synthesis of molecular
dyads, triads and polyads with optimised hole–electron lifetimes and energies, which
might in future be linked into energy-funnelling antennae or nanoscopic current-

collecting systems to create molecular power-producing cells. Nick Serpone and Alexei
Emeline provide a comprehensive account of the fundamentals of metal-oxide
heterogeneous photocatalysis, with particular emphasis on dispersed titanium dioxide
systems, in Chapter 5.
In Chapters 6–10, we turn to important cell types: inorganic extended-junction
devices are described by Rolf Könenkamp, who has pioneered their development, in
Chapter 6, and polymer and polymer-composite cells by Jenny Nelson and Jessica
Benson-Smith in Chapter 7. In Chapter 8, dye-sensitised solar cells are discussed by
James Durrant and their inventor, Michael Grätzel. Another authority in the field of solar
photoelectrochemistry, Nate Lewis, and two colleagues, Stephen Maldonado and
Anthony Fitch, provide an overview of non-dye-sensitised semiconductor/liquid junction
solar cells in Chapter 9. In Chapter 10, Stuart Licht and Gary Hodes describe their own
and others’ development of photoelectrochemical storage (PECS) cells, which have the
conceptual advantage over the other types of power-producing cell described in this
volume of being able to produce continuous rather than intermittent power. Figure 1.2
shows how the performance of all these cell types has improved over time. Finally, Xin
Ai and Tianquan Lian deal with the measurement of electron-transfer dynamics at the
molecule/semiconductor interface in Chapter 11, and Laurie Peter and Helmut Tributsch
cover techniques for the characterisation of photoelectrochemical systems in Chapter 12.
One type of photoelectrochemical device not covered in this book is the photo-
galvanic cell. By this term is meant power-producing or storage cells in which the
products of an endoergonic photoredox reaction that occurs in solution are harvested at
metal (or at any rate photoinactive) electrodes. Although Albery and Archer (1977) took
a sanguine view of the maximum power conversion efficiency (5–9%) that might be
obtained from such a cell, subsequent experimental studies have shown that the
combination of long optical lengths, low diffusivities of short-lived redox products and
imperfect electrode selectivity in practice restrict conversion efficiencies to well below
1%, rendering the photogalvanic cell impractical as a power-producing device (Archer
and Ferreira, 1980).
Each chapter is comprehensively referenced, and the reader may also find some of the

following recent reviews and books helpful: Fujishima and Zhang (2005), Soga (2006),
Durrant et al. (2006), Hodes (2007), Kamat (2007) and Licht (2007). The Festschrift
issue of the Journal of Physical Chemistry (Vol. 100, No. 50, 21 December 2006) in
honour of my co-editor’s seventieth birthday also contains many papers of relevance.