Table Of ContentTHIRD GENERATION
PHOTOVOLTAICS
Edited by Vasilis Fthenakis
Third Generation Photovoltaics
Edited by Vasilis Fthenakis
Published by InTech
Janeza Trdine 9, 51000 Rijeka, Croatia
Copyright © 2012 InTech
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Statements and opinions expressed in the chapters are these of the individual contributors
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First published March, 2012
Printed in Croatia
A free online edition of this book is available at www.intechopen.com
Additional hard copies can be obtained from [email protected]
Third Generation Photovoltaics, Edited by Vasilis Fthenakis
p. cm.
ISBN 978-953-51-0304-2
Contents
Preface VII
Chapter 1 Solar Spectrum Conversion for
Photovoltaics Using Nanoparticles 1
W.G.J.H.M. van Sark, A. Meijerink and R.E.I. Schropp
Chapter 2 Multi-Scale Modeling of Bulk Heterojunctions
for Organic Photovoltaic Applications 29
Varuni Dantanarayana, David M. Huang, Jennifer A. Staton,
Adam J. Moulé and Roland Faller
Chapter 3 Organic Photovoltaics: Technologies and Manufacturing 61
Yulia Galagan and Ronn Andriessen
Chapter 4 Life Cycle Assessment of Organic Photovoltaics 91
Annick Anctil and Vasilis Fthenakis
Chapter 5 Investigating New Materials and
Architectures for Grätzel Cells 111
Alex Polizzotti, Jacob Schual-Berke, Erika Falsgraf and Malkiat Johal
Chapter 6 Nonisovalent Alloys for Photovoltaics Applications:
Modelling IV-Doped III-V Alloys 141
Giacomo Giorgi, Hiroki Kawai and Koichi Yamashita
Chapter 7 Concentrated Photovoltaics 167
Robert McConnell and Vasilis Fthenakis
Chapter 8 Durable Polymeric Films for Increasing the
Performance of Concentrators 183
T. J. Hebrink
Chapter 9 Performance of Photovoltaics Under
Actual Operating Conditions 201
George Makrides, Bastian Zinsser, Matthew Norton
and George E. Georghiou
Preface
The quest for alternative sustainable energy sources has led us to the third generation
of photovoltaics. The feasibility for PV to become a major contributor in the electricity
mixture of the U.S. and other major energy consuming countries has been proven. The
production and sales of photovoltaics have increased by an average of 45% per year
over the last fifteen years. As with all energy-generation technologies, PV deployment
is subsidized but the expectation is that, within a decade, the cost of PV electricity
generation will be in parity with electricity from the grid, making additional subsidies
unnecessary.
For PV grid cost parity we need high efficiencies and low production costs. So far
we have made strides on either high performance as exemplified by Sunpower
producing modules with 20% efficiencies at a production cost of about $1.75 /Wp, or
on low cost as mastered by First Solar producing modules at a cost of 75 ¢/Wp and
efficiencies of 12.5%. Third generation technologies promise further advances in
each pathway and, more importantly, bridging the two with technologies of both
high efficiency and low cost. In this book we present notable advances of 3rd
generation technologies, namely organic cells and nanostructures, dye-sensitized
cells and multijunction III/V cells. The underlining promise of most 3rd generation
technologies is that the thermodynamic Shockley-Queisser limit of single junction
solar cells will be surpassed. In 1st generation crystalline silicon and 2nd generation a-
Si, CdTe and CIGS solar cells, only photons within a narrow wavelength,
corresponding to the semiconductor band gap, are effectively absorbed. Photons
with energy lower than the semiconductor band gap are not absorbed and their
energy is not used for carrier generation, whereas photons with energy larger than
the band gap are absorbed but excess energy is lost to heat, negatively affecting the
voltage generation of the devices. Primarily for this reason, the thermodynamic
Shockley-Queisser of single junction cells is only about 31%, with spectral losses
being as large as 50%. Several approaches have been proposed to reduce or
eliminate spectral losses, e.g., multiple stacked cells, intermediate band gaps,
multiple exciton generation, quantum dot concentrators, down- and up-converters,
and down-shifters. The first approach is already commercialized, with III/V solar
cells of efficiencies as high as 42% being produced in the lab and 38% efficient cells
being deployed in terrestrial concentrator systems. The others involve the
VIII Preface
generation of more than one electron from a highly energetic photon and the
upshifting of low energy photons so that their energy is also utilized in electron
production. Nanotechnology is essential in realizing these concepts. Nano-
technology enabled organic photovoltaics (OPV), in addition to enabling high
photon conversion efficiencies, have the potential to further lower the production
cost of photovoltaics by using inexpensive materials. Dye-sensitized solar cells seem
especially suited for low irradiation regions and their efficiencies can also be
augmented with nanostructures; III/V solar cells are commercially produced and
started being deployed, but challenges remain in decreasing their system costs.
The book begins with an overview of approaches based on nanotechnology for
surpassing the Shockley-Queisser limit. The principles and advances of tuning
chemical and physical properties in nanosized materials and inducing
downconversion, downshifting and upconversion of photon energies, are explained
in terms understood by general physics and engineering readership. The first
chapter also introduces the reader into modelling of spectral conversion, a field
where further advances are necessary to improve the understanding of the plethora
of combinations with interlinked influences on a solar cell system. The second
chapter presents specific cases of multi-scale modeling of quantum chemical
interactions at the molecular level of organic solar cells. Case-specific calculations
are shown to offer a universal view of such systems, from local interactions and
structure, up to morphology and phase behavior. This chapter shows that a realistic
description of OPV requires examination at multiple scales and that defining
appropriate time and length scales is a prerequisite to selecting or developing a
model. The discussion on organic solar cells continues in the third chapter with a
tutorial on manufacturing organic solar cells, which will be especially interesting to
people studying the transition from lab-scale to commercialization. This transition
requires adjusting and optimizing of process and system parameters, such as solvent
types, deposition techniques, drying conditions, and dimensions of individual cells
and modules. The chapter establishes a knowledge base concerning the influence of
different parameters and process conditions on the performance, cost and lifetime of
polymer solar cells.
It is imperative that new energy technologies are safe and friendly to the environment.
While solar cells, as an alternative to fossil fuel energy production, prevent the
pollution and climate change impacts of the later, proactive evaluations are needed to
ensure that new environmental issues are not created. For this reason, life cycle
assessments (LCA) are performed of emerging organic technologies to evaluate a
priori the environmental impact of their production and to identify sustainable energy
production pathways. In chapter 4 a comprehensive LCA methodology, and its
application on emerging organic photovoltaics, are presented. A major difficulty for
applying LCA to new technologies is the availability of data representing actual
commercial production. Based on extrapolation of laboratory data, this chapter shows
that fullerenes used as exciton acceptors are the most energy-intensive components of
Preface IX
organic photovoltaics and that a trend towards using higher quantities of larger
fullerenes in OPV will increase the cumulative energy demand associated with their
life cycle. Nevertheless, the LCAs of OPV inherently carry large uncertainties and the
need for further studies on the environnmental impacts and recyclability of these
technologies is pointed out.
Chapter 5 gives an overview of the state of knowledge about dye-sensitized solar cells
and probes into the type of developments needed to produce durable and reliable PV
modules from these cells. As advances are made in more effective semiconductor
morphologies, lower-cost, and higher-efficiency dyes, and more effective and long-
lasting hole conduction materials, these devices could become commercially viable.
The chapter describes the functions and challenges for each of the components of a
Grätzel cell, and outlines research needed for innovative ways of separating and
transporting charges within these devices.
In chapter 6, an overview of III/V solar cells used in 3rd generation high concentration
photovoltaics (HCPV) is given, explaining how these cells enable efficiencies well
above the Shockley-Queisser limits and illustrating associated developments. The
focus is on the importance of theoretical models for the prediction of the structural and
electronic properties of IV-doped III-V alloys used in tandem structures, showing that
efficiencies of 60% could be obtained. Concentrator systems are commercialized and
Amonix is the industry leader with well tested HCPV based on Fresnel lenses and
III/V solar cells. These systems are described in chapter 7 and performance data are
presented along with the life-cycle environmental profile of the Amonix systems. One
way to bring down the cost of generating electricity from photovoltaics is to replace
relatively expensive reflecting materials in concentrating systems with low cost
polymeric wavelength selective mirror films. Chapter 8 describes such films made by
3M Corporation that have the potential to significantly reduce the cost of electricity
derived from PV.
Reliability of photovoltaics deployed in the field is an essential element of sustainable
growth of these technologies. Accelerated aging tests, prescribed by qualification
standards, can minimize risks to reliability. Proving the reliability of 3rd generation
solar cells is a demanding challenge as some use electrolyte solutions, other use
polymeric materials that have less resistance to light and temperature cycles than
silicon or glass, and light concentrating systems that function at higher voltages and
generate higher currents than the previous generations’ solar cells. Reliability is best
confirmed by decades of field operation. Chapter 9 describes how PV systems perform
in the sunny and dusty south European/Middle-eastern climate of Cyprus. This
chapter presents useful information of the seasonal performance and the effects of
temperature, soiling and power rating of contemporary PV systems that would also
guide the field evaluation of the forthcoming new technologies.
I am grateful to the authors of the chapters for sharing with us the technical
knowledge for which they are renowned experts. I have worked together with
X Preface
several of the contributors and fully appreciate that in their busy schedules they
found the time and energy to be part of this project. I also want to extend special
thanks to Anja Filipovic, InTech Publishing, for keeping all of us on track.
Prof. Dr. Vasilis M. Fthenakis
Brookhaven National Laboratory and Columbia University
USA
