POWERING THE WORLD WITH SUNLIGHT
A White Paper Describing the Discussions and Outcomes of the
1st Annual Chemical Sciences and Society Symposium (CS3)
Kloster Seeon, Germany, July 23-25, 2009 1

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This white paper was prepared by science writer Leslie A. Pray, PhD, in consultation with the Gesellschaft
Deutscher Chemiker e. V. and American Chemical Society, and reviewed by members of the CS3 ScientificCommittee.

TABLE OF CONTENTS

Executive Summary
Research Recommendations
About this White Paper

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I. Converting Solar Energy into Chemical Fuel: If a Leaf Can Do It, We Can Do It
Developing Catalysts for Hydrogen Production
Developing Catalysts for Oxygen Production
Self-Healing: A Desirable but Elusive Quality
Building an “Artificial Leaf”
Catalysts for CO2 Reduction
CO2 Reduction: Not for Fuel Alone

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II. Accessing Solar Energy Stored as Biomass
Are Biofuels a Solution?
Ways to Improve Biofuel Production
Energy is a Systems-Level Issue
Use of Biofuels

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III. Converting Solar Energy into Electricity: Next-Generation Photovoltaic (PV) Technologies
Overview of PV Technology: Chemists Look for Ways to Cut Cost
Existing and Emerging PV Technologies
Crystalline Silicon (c-Si) PV Technology
Thin-Film Solar Cells (TFSC)
New-Concept PV Technologies
The Search for Earth-Abundant Materials
The Future of PV Technologies

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IV. Storing Solar EnergyWays to Store Solar Energy
The Storage of Distributed Energy
What Kind of Chemical Fuel?
Large-Scale, Grid-Connected Solar Energy Storage

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Appendix A: Program

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Appendix B: List of Participants

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EXECUTIVE SUMMARY

The sun provides more energy to the earth in an hour than the world consumes in a year.
Compare that single hour to the one million years required for the earth to accumulate the sameamount of energy in the form of fossil fuels. As the global population continues to expand andwith life expectancies increasing worldwide, scientists estimate that total energy consumption of
the world will more than double by 2050 if society continues to use energy at its current rate.
Fossil fuels are not a sustainable resource, and we must break our dependence on them. Solar
power is among the most promising alternatives. The challenge is: How can solar power beharnessed on the scale that is needed?
To initiate international cooperation and innovative thinking on solar solutions to theglobal energy challenge, thirty leading scientists from five national chemical societies 2 , as well
as representatives from the national science funding agencies of each country, convened in
Kloster Seeon, Germany, in July 2009. The meeting was the first Chemical Sciences and Society(CS3) Symposium, a new annual series of symposia designed to bring top scientists together todiscuss how the science of chemistry can provide solutions to some the world’s most pressing
challenges. The presentations, discussions, and outcomes of this first CS3 symposium revolved
around the questions: How can solar energy be captured, converted, and stored on the scaleneeded? What are the most urgent scientific problems that must solved before solar energy canbe used to meet the world’s energy needs?
The international and interdisciplinary nature of the CS3 symposium set a new paradigm
for the cross-fertilization of ideas that will be necessary for harnessing the power of the sun. The
nearly unanimous agreement on many of the issues among an international group of experts
specializing in different areas of the chemical sciences, many of whom had never met before,
made a powerful statement about the readiness of the international chemistry community to seeka solar solution to the global energy needs of the 21st century.
Equally important, it became clear that national and international energy initiatives must
make a stronger commitment to the central role of chemistry in realizing the potential of solar
energy. This is because the conversion of energy from one form to another almost always
involves a chemical reaction, and understanding the molecular nature of those chemical reactions
is vital to developing new materials and processes that can be used to drive those reactions inaffordable, sustainable ways.
While several overarching themes emerged over the course of the two-and-a-half day
dialogue, CS3 participants identified the three key take-home messages:•

The science of today is the technology of tomorrow: There is no single best solution to
the energy problem. Science and society must seek more affordable, sustainable solutions
to the global energy challenge by examining and considering all next-generation options.
It is difficult to predict how scientific breakthroughs and emerging technologies being
explored today will advance in the future.

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The Chinese Chemical Society; Gesellschaft Deutscher Chemiker e. V.; Chemical Society of Japan; Royal Societyof Chemistry; and American Chemical Society. The meeting was supported by the national science fundingorganizations of the five countries: National Science Foundation of China; German Research Foundation; Japan
Society for the Promotion of Science; British Engineering and Physical Sciences Research Council; and U.S.
National Science Foundation.

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•Investing in chemistry is investing in the future: Having a strong basic research programin chemistry is indispensable to realizing the potential of solar energy. The discovery of
new materials and the design of new processes will be vital to making solar energy anaffordable option for large-scale use.

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The chemistry students of today are the energy scientists of tomorrow: Society needs a
new generation of energy scientists -- young minds eager to explore innovative ways tocapture, convert, and store solar energy. Developing and optimizing solar energy
technologies for large-scale use will be a long-term effort. Efforts should be made toencourage young investigators to address the wide range of scientific challenges thatmust be met before the tremendous potential of solar energy can be realized.

Research Recommendations

While silicon-based photovoltaic (PV) cells are the most common way of using solar energy,
the high cost of purified silicon limits its widespread use. No matter how efficient an energy
conversion technology is, much of the world will not be able to use it unless it is also affordable.
During the final wrap-up session of the symposium, CS3 participants discussed the major
scientific challenges and priority research goals in four areas of solar energy science. Almost allof the goals revolve around the need to develop not just efficient but also affordable solar energy
technologies:(1) Converting solar energy into chemical fuel: Artificial photosynthesis is any process thatconverts solar energy into chemical energy, mimicking what plants do during natural
photosynthesis. Before artificial photosynthesis can become an affordable, sustainable
solution for widespread use, chemists must:•

develop chemical catalysts for the two major processes of artificial photosynthesis -water splitting and CO2 reduction -- that can be applied commercially and are madeof affordable, earth-abundant materials; and•create an “artificial leaf” by coupling water splitting and CO2 reduction in a waythat eliminates the need for an external, sacrificial electron donor.

(2) Accessing solar energy that already exists in nature: Biofuels derived from non-crop biomass
allow access to the enormous reservoir of solar energy that has already been naturally
converted into plant mass. Before biofuel production can become an affordable, sustainablesource of energy for widespread use, chemists must:•

develop biochemical methods that can be used to create more biomass; and•develop catalytic processes that improve the efficiency of biomass conversion.

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(3) Converting solar energy into electricity: The widespread use of silicon-based photovoltaic(PV) cells, which convert solar energy directly into electricity, is limited by their high cost.
Before PV cells can become an affordable, sustainable mode of energy, chemists must:•

develop low-cost, non-toxic, earth-abundant PV materials for use in nextgeneration PV cells.

(4) Storing newly harnessed and converted solar energy: We must develop systems that not only
convert solar fuel into other forms of energy but also store that converted energy for futureuse. This is an especially critical issue for areas of the world without access to centralizedenergy delivery systems. Before low-cost, sustainable solar storage systems can be built,
chemists must:•

develop new catalysts and materials from low-cost, earth-abundant elements thatcan be used to build affordable, sustainable solar energy transformation-andstorage systems.

About this White Paper

This white paper summarizes the main issues and outcomes of this year’s CS3, with anemphasis on the four areas of solar energy science listed above: Chapter I focuses on theconversion of solar energy into chemical fuel (i.e, water-splitting and CO2 fixation), chapter II
the conversion of biomass into biofuel, chapter III the conversion of solar energy into electricity(i.e., PV, or solar cells), and chapter IV the storage of solar energy. The CS3 program is attached
as Appendix A and the list of participants as Appendix B. This paper should not be viewed as acomprehensive report of the many technical details addressed during the course of the 2-1/2 day
workshop, nor does it cover all comments made by CS3 participants.

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I

CONVERTING SOLAR ENERGY INTO CHEMICAL FUEL:
IF A LEAF CAN DO IT, WE CAN DO IT 3

Plants use solar energy all the time -- they capture and convert light into chemical fuelthrough photosynthesis. In chemical parlance, photosynthesis is the conversion of water (H2O)
and carbon dioxide (CO2) into carbohydrates (CH2O) and oxygen (O2). The actual conversion
comprises several separate reactions, the two main ones being the splitting of water intohydrogen (H2) and oxygen (“water splitting”) and the reduction of CO2 using electrons released
during the water splitting (“CO2 reduction” 4 ). The question is, can chemists replicate what plants
do naturally?
Chemists have in fact mimicked most of what is required to make these separate reactions
proceed. In essence, they have replicated photosynthesis. But it hasn’t been easy, and chemists
have yet to replicate the separate reactions in an integrated fashion and in a way that can becommercially applied on a wide scale. For example, as described below, scientists have only
recently developed an experimental O2 production reaction that is potentially affordable enoughfor widespread use; most current commercial O2 production methods rely on the use of
expensive platinum catalysts.
Most of the CS3 discussion on the conversion of solar into chemical energy revolved
around current research efforts aimed at developing affordable catalysts for driving the various
photosynthetic reactions forward. This chapter summarizes that discussion.

Developing Catalysts for Hydrogen Production

Dr. Kazunari Domen of the University of Tokyo, Japan, opened the session by describing
current research efforts focused on developing more efficient and more affordable H2 production
catalysts using a new type of water-splitting technology: “photocatalytic overall water splitting.”
The technology makes use of nanostructured photocatalytic particles dissolved in solution,
whereby the catalytic reactions and production of H2 and O2 occur in contact with these particles.
This is different from conventional approaches, which use separate catalytic-coated electrodesfor H2 and O2 production. The key advantage of photocatalytic overall water splitting is its
reliance on less expensive materials than PEC cells require. Therefore, it can be more affordably
scaled up for widespread use.
But first, Domen described the two conventional water-splitting technologies. The first
involves the combined use of a photovoltatic (PV) cell (often called a “solar cell” or simply“PV”; see chapter III) and an electrolytic device, whereby the PV absorbs and converts the lightenergy into electricity and the electrolytic device uses that electricity to split the water. He
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This section summarizes the presentations and discussions that took place during the first session of CS3,
“Artificial Photosynthesis, Photocatalytic Water Splitting, CO2 Fixation,” which was organized by Dr. Haruo Inoue,
Tokyo Metropolitan University, Tokyo, Japan.
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A reduction reaction requires an electron donor. In natural photosynthesis, water serves as the electron donor.

6mentioned that Honda has been using the first approach for several years in an effort to develop
an experimental “solar-powered water-electrolyzing hydrogen station” where hydrogen fueloperated cars could re-energize. The second approach involves the use of a photoelectrochemical(PEC) cell, a device that contains both the PV cell and electrolytes in tandem. By combining thelight-harvesting and water-splitting processes in one device and eliminating the need for aseparate electrolyzer, PEC cells provide a more direct, less expensive means of using solar
energy to drive the water-splitting reactions. Domen pointed to several examples of the second
approach.
Both conventional approaches are further advanced than photocatalytic overall water
splitting, which Domen said is still very much in the research phase and not yet ready to be
developed for any sort of commercial application. Chemists have built PEC cells, for example,
with about 5% solar energy conversion efficiency. Photocatalytic overall water splitting systems,on the other hand, are only about 1 to 3% efficient at best. Domen predicts that it will takeanother 10 years of research to increase their average solar energy conversion efficiency to 5%.
One of the key challenges is that most of the photocatalytic materials work only when activatedby ultraviolet (UV) light, not visible light. 5 He described current efforts by his and other researchgroups to develop new materials that work efficiently under visible light. For example, after
experimenting with a range of metal oxide materials, he and his collaborators have demonstrated
good photocatalytic activity using a mix of gallium nitride (GaN) and zinc oxide (ZnO).
Following Domen’s talk, there were several additional comments on some of thetechnical details of H2 production. Dr. Haruo Inoue of Tokyo Metropolitan University, Tokyo,
Japan, discussed the molecular details of how the electrons used in H2 production are actuallyobtained from water during the water-splitting process. He argued that a two-electron oxidation
approach to photochemical water splitting may be more promising than either a single-electron
or multiple-electron approach to developing an artificial photosynthetic system that uses water as
the electron donor (i.e., instead of an external, sacrificial electron donor). Dr. Chen-Ho Tung and
Dr. Can Li, both of the Chinese Academy of Sciences, Beijing, China, discussed some of thedetailed chemistry of the H2 production component of water splitting.

Developing Catalysts for Oxygen Production

In the following session on biomass (see Chapter II) but of relevance to water splitting,
Dr. James Barber of Imperial College London, UK, briefly reviewed some of the history ofchemical research on water splitting, beginning with work in the late 1960s by Bessel Kok.
Understanding the basics of water splitting – for example, precisely where and how the O2
molecules are formed – has been the subject of intense study for decades. He then described his
team’s recent discovery of the first complete structure of one of the key protein complexes
involved in water splitting and the site of O2 formation, photosystem II (PSII). They identified
both the shape and manganese (Mn) and calcium (Ca) content of PSII, finally giving scientists a
glimpse at catalytic machinery that has been operating on the planet for the past 2.5 billion years.
As both Barber and, later during the symposium (see Chapter IV), Dr. Daniel Nocera
discussed, Barber’s discovery of the structure of PSII also guided Nocera and his research groupat the Massachusetts Institute of Technology (MIT), Cambridge, Mass., U.S.A., in their
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While some sunlight is within the UV range (<400 nm), most solar light is visible (400-800 nm).

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development of an artificial water-splitting system based on a novel cobalt-based O2 production
catalyst. Nocera’s group demonstrated that by placing cobalt (Co) ions and potassium phosphatein solution with an indium tin oxide electrode and applying an electrical current, they were able
to not only produce O2 but to do so continuously. When a current is run through the solution, thecobalt, potassium and phosphate accumulate on the electrode and form an amorphous catalyst
that drives the oxygen production component of the water-splitting reaction, forming oxygen gas(O2). Hydrogen (H2) is produced at a separate, platinum-coated electrode 6 .
Nocera noted the “striking” similarities in both catalytic function and self-assembly
between the cobalt catalyst and the naturally existing Mn-based PSII protein complex. The twocomplexes share the same basic structure except that the synthetic catalyst contains cobalt
instead of manganese. Importantly, the synthetic catalyst can self-assemble (and re-assemble),
eliminating the need to add additional raw material as the water splitting proceeds and the needto dispose of used reagent at the end of the reaction. So the catalyst continuously breaks down,
but it also continuously builds back up, just like PSII; PSII is not very stable, with plants making
repairs about every 30 minutes.
While the water splitting demonstrated by Nocera’s group is not a new achievement inand of itself, the use of an inexpensive (i.e., non-platinum) catalyst for at least, in this case, theO2 half-reaction, is remarkable. Nocera stated that if chemists could develop an affordable,
efficient means of using solar energy to split water and produce hydrogen fuel, society would be
able to power the world simply by splitting, every second, one-third the amount of water inMIT’s swimming pool. That is a compelling image.
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Self-Healing: A Desirable but Elusive Quality
Of relevance to Nocera’s emphasis on the importance of self-assembly, Dr. Kazuhito
Hashimoto, University of Tokyo, Japan, commented during this session on two desirable aspectsof any next-generation PV technology, indeed any type of solar energy technology: (1) theability to self-assemble or self-organize and (2) the ability to self-heal. Noting that natural
photosynthesis is typically only 4 ½-5% efficient at best (see Chapter II), Hashimoto argued thatmost artificial systems are already “better” than natural photosynthesis with respect to solar
energy conversion efficiency. Clearly, however, artificial systems are lacking something thatnature has – specifically, the ability to self-assemble and the ability to self-heal. Hashimoto
described a paddy-field power generation system that uses “microbial fuel cells” (bacteria thatcan convert biomass into electricity) to convert stored carbohydrates into electricity. Under theright conditions, the solar-to-electricity paddy-field power generation system can survive on its
own, thereby satisfying the self-assembly criterion of a natural system. But he and his colleagues
have yet to figure out how to build a self-healing system. The solar energy conversion efficiencyof the system is only about 0.3%, although Hashimoto said that it could probably be improved
ten-fold by improving some of the technical components.

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When asked about the hydrogen catalyst component of his new water-splitting system, Nocera mentioned that heand his team have developed a nickel alloy-based electrode to replace the prohibitively expensive platinum-based
electrode typically used, but they have yet to determine whether it performs as well or better.

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Building an “Artificial Leaf”:
Coupling Water Splitting and CO2 Reduction Artificial Photosynthesis
Domen stated that the ultimate goal of artificial photosynthesis is to combine the water
splitting and CO2 reduction reactions, using the electrons released during the water splitting todrive the CO2 reduction and produce a liquid fuel, such as methanol. Achieving this would, inessence, fulfill the vision of creating an “artificial leaf” – a device that not only splits water but
also uses the products of the water-splitting reaction to create a more usable organic fuel,mimicking what real leaves do.
As Dr. Haruo Inoue, Tokyo Metropolitan University, Tokyo, Japan, explained, coupling
CO2 reduction with water splitting would be an ideal system from both an energy conversion and“circulation-of-elements” perspective. An artificial leaf would neither require energy (i.e, otherthan what it gathers from the sun) nor emit CO2. CO2 reduction does not have to be coupled withwater splitting in order to be used for fuel production. Unfortunately, however, the major sourceof H2 used in most CO2 reduction processes that are currently in operation is the steam reformingof methane, which not only requires energy (i.e., currently in the form of fossil fuels) but alsoemits CO2.
Dr. Etsuko Fujita of Brookhaven National Laboratory, Upton, New York, agreed that atleast one overall goal of research efforts in the field of artificial photosynthesis should be to
combine CO2 reduction with water splitting. She laid out a vision for the future where CO2 iscaptured at fossil fuel burning plants before being released into the atmosphere and then reduced
using electrons (and protons) released during water splitting. There would be no need for asacrificial electron donor. But the challenge is formidable. As both Fujita and Dr. Anthony
Harriman, Newcastle University, United Kingdom, discussed, coupling the two halves of whatmakes a leaf a leaf would require joining together several complicated multi-electron reactions.
Meanwhile, until H2 production can be coupled with CO2 reduction, chemists aredeveloping systems that use H2 as an end product. For example, Nocera described how his
cobalt-based O2 catalyst could be integrated into a larger PV-and-fuel-cell system for hydrogenfuel production and storage: the H2 produced from the water splitting would be used as a fuel(i.e., in a fuel cell, where it is would be combined with O2 to release energy), and CO2 reduction
would not be part of the process. As a first step toward realizing this vision, Nocera iscollaborating with Sharp Electronics to see if the system can be integrated with Sharp’s PV
technology.
Catalysts for CO2 Reduction
As Dr. Koji Tanaka, Institute for Molecular Science, Okazaki, Japan, and others
discussed, building an artificial leaf will require developing better catalysts not just for H2 and O2
production but also CO2 reduction. In fact, Dr. Can Li of the Chinese Academy of Sciences,
Beijing, China, suggested that “solving CO2 reduction” is more urgent than “solving water
splitting.” Fujita agreed that, just as with H2 and O2 production, chemists have yet to optimize
catalysis of the CO2 reduction reaction. She commented on how both Dr. Osamu Ishitani’s
research group at the Tokyo Institute of Technology, Japan, and her research group have beenworking on the reduction of CO2 for the past 20 years. Ishitani’s team recently developed a novel
ruthenium (Ru)-rhenium (Re) photocatalyst, but challenges remain. For example, the CO2

9reduction takes a lengthy ten hours, owing to the extreme stability of CO2 molecules and theinstability of the catalyst. Another problem, Li said, is that often the final hydrocarbon product of
CO2 reduction is very quickly re-oxidized, making the net hydrocarbon yield quite low. He
emphasized the need to study natural systems in an effort to better understand how this can be
avoided.
CO2 Reduction: Not for Fuel Alone
Whether coupled to water splitting or not, CO2 reduction has other uses besides fuelproduction. In fact, CO2 is often used as a raw material for the production of a wide range of
non-fuel products, including urea (an important component of nitrogen fertilizers), salicylic acid(a pharmaceutical chemical), cyclic carbonates, ethylene and propylene polycarbonates, andpolyurethanes. It has been proposed in the past that the chemical industry, through its use of CO2
as a chemical feedstock, might be able to contribute to lowering the level of CO2 in theatmosphere by using more CO2 during manufacturing. However, as both Fujita and Dr. Walter
Leitner of the Institute for Technical and Macromolecular Chemistry, Aachen, Germany,
discussed, the scale of CO2 consumption is small compared to the total annual CO2 emissions.
The chemical industry uses the equivalent of less than 0.5% of total anthropogenic CO2
emissions. Moreover, many polymeric products that could potentially be manufactured using
CO2 as a starting material would need to be made more durable before their production could bescaled up. Clearly, Fujita argued, we cannot rely on CO2 utilization by the chemical industry tosignificantly mitigate CO2 emissions from fossil fuel burning.
Leitner suggested that, because CO2 utilization by the chemical industry cannot
significantly mitigate the CO2 emissions from fossil fuel burning, the question is not “How much
CO2 are we going to fix?” Rather, the question is, “What is the difference between using CO2 as
opposed to some other starting material to make a product?” Efforts should be re-directed toward
thinking about the value added when CO2 is used as a chemical feedstock and away fromthinking about how using CO2 as a raw material can mitigate CO2 emissions.

Converting Solar Energy into Chemical Fuel
Artificial photosynthesis is any process that converts solar energy into chemical energy,
mimicking what plants do during natural photosynthesis. What must be achieved before artificial
photosynthesis can become an affordable, sustainable solution for widespread use? During thefinal wrap-up session of CS3, a breakout group identified the following as the most pressing
scientific challenges:
Develop chemical catalysts for the two major processes of artificial photosynthesis -- water
splitting and CO2 reduction -- that can be applied commercially and are made of affordable,
earth-abundant materials.
Create an “artificial leaf” by coupling water splitting and CO2 reduction in a way thateliminates the need for an external, sacrificial electron donor.

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II
ACCESSING SOLAR ENERGY STORED AS BIOMASS 7

In his opening presentation of the second session of CS3, Dr. James Barber of Imperial
College London, UK, described the planet as a “macroscopic reaction vessel” driven by aphotosynthetic process that has been going on for 2.5 billion years. None of the plant mass onearth, nor the fossil fuels stored beneath the surface of the earth, would exist as they do today
without photosynthesis -- that is, the conversion of solar energy into carbohydrates. While many
countries are already tapping into this vast resource of already existing and naturally converted
solar energy, there is considerable uncertainty and debate around the extent to which fuels
derived from plant biomass (“biofuels”) can meet the world’s energy demands. Much of the CS3
discussion on biofuels revolved around this uncertainty, as well as on ways to improve biofuel
production. This chapter summarizes that discussion.

Are Biofuels a Solution?

CS3 participants agreed that biomass is not a complete solution to the global energy
challenge. As Barber explained, its use is limited by two related factors. First, natural
photosynthesis is a relatively inefficient process, with most plants unable to convert more than, at
best, about 4.5% of absorbed solar energy into carbohydrate fuel. (Following Barber’s talk, there
was some discussion about the optimism of 4.5%, with a couple of CS3 participants suggestingthat most plants’ solar energy conversion efficiencies are much lower. Not only is much of thelight that hits a plant reflected [and therefore not absorbed], but also the photosynthetic process
becomes saturated at very high light intensities. Barber explained that the 4.5% figure is a
maximum and that solar energy conversion efficiencies vary over time and among plants. For
example, the conversion of light energy into chemical bonds is much higher during rapid growth,with plants accumulating biomass more quickly at certain times. Additionally, some plants
accumulate more biomass than others under a given amount of light simply because of their
genetic makeup.) Solar energy conversion efficiencies of biomass crops would need to be
increased, for example through genetic or biochemical engineering, before biofuels can become amore widespread practical solution.
Second, fuel biomass requires land. The rate at which light energy is converted into
chemical bonds in terrestrial biomass amounts to the equivalent of about 100 terawatt-hours(TWh 8 ) per year. Barber remarked that this is not very much, considering that the world uses
about 14 TWh per year, a rate that is expected to more than double by 2050. He said that it isunrealistic to expect that, by 2050, one-third of the total land biomass on earth could be
harvested for biofuel production. Again, this points to the need to improve the solar energy
conversion efficiency of fuel biomass.
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This section summarizes the highlights of the presentations and discussions that occurred during the second sessionof CS3, “Solar Radiation to Energy via Biomass,” which was organized by Dr. Laurie Peter, University of Bath, UK.
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A terawatt (TW) is the same as one trillion, or 1012, watts. A terawatt-hour (TWh) is a measure of total energy usedover time.

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While biofuel production may not be a practical global solution, CS3 participants agreed
that biofuel production serves as a good local or regional solution in some circumstances. For
example, Brazil has been successfully converting sugarcane (Saccharum sp.) into bioethanol forthe past several decades. Today, sugarcane-derived biofuel provides about one-third of Brazil’s
total energy, and about half of all cars sold in Brazil operate with a bioethanol-gasoline blend.
The United States, on the other hand, relies on biofuel, most of which is corn (Zea mays)-based,for only about 3% of the country’s energy needs. While the U.S. Department of Energy has
predicted that this figure could increase to 30% in the future, Barber explained that there areseveral caveats to this prediction. For example, it is not clear whether there would be enoughavailable (or dedicated) landmass to achieve that level of biofuel production.
Whether biofuel production is a practical global solution or not, bioethanol production isincreasing worldwide. For example, Barber commented on a northern Italian plastics company
that has demonstrated the capacity to grow 5 to 10 tons of dry matter per hectare (by growing
one crop in winter and a second crop in summer), which according to the company’s predictions
would yield about one ton of bioethanol per hectare. Barber emphasized that the bottom line for
a company is not efficiency but cost and whether production will yield a profit. In this case, thecompany has decided that the amount of biomass that they can grow and the rate at which they
can convert it into biofuel is sufficient to earn a profit. (Following Barber’s presentation, there
was a question about whether the one-ton figure might be overly optimistic and whether factors
such as the amount of fertilizer and water and the energy required for harvesting had been
included in the calculations. Barber stated that the company had taken these factors into
consideration. Hopefully, he remarked, biofuel crops of the future will have fewer energy
demands than those being used today.)
Following Barber’s presentation, there were a couple of comments about biofuels not
being a global solution but nonetheless playing unique role in the energy field. Dr. Mark Davisof the U.S. National Renewable Energy Laboratory (NREL), Golden, Colorado, commented on
their suitability as a liquid fuel for transportation applications. He said that other solar
technologies, like photovoltaic (PV) cells, will probably figure more prominently in electrical
production, but it is difficult to conceive that PV cells and some of the other technologies being
developed are going to be useful for producing liquid fuels. Dr. Ferdi Schüth of the Max Planck
Institute für Kohlenforschung, Mülheim, Germany, stated that while there are many exaggerated
claims about the extent to which biofuels can help meet the energy demands of the world andwhile, to a large extent, much of our future energy system will probably be electricity-based,some of it will be material-based; biomass is one of the most readily available material-based
fuels.

Ways to Improve Biofuel Production

Barber also discussed how the field must move forward in order to optimize biofuel
production and produce biofuels on a scale that is competitive with fossil fuels. Most importantly,
because fuel biomass often competes with food crops for land, scientists have proposed using thecellulose bioproducts of agriculture instead of the food itself for biofuel production. But in orderfor biofuel production to become cellulose-based, instead of food-based, scientists need todevelop “energy crops” (i.e., non-food plants) and new agronomic practices for growing and12harvesting those crops. For example, many chemists are studying the potential to use Miscanthus,
a high-yielding grass, instead of corn (or another food crop) as a biofuel source. Additionally,
chemists need to develop better organic and synthetic catalysts for converting cellulose into
simple sugars and for converting those simple sugars into fuel products. As Barber explained,
cellulose is a very complicated structure and is difficult to break apart. Some chemists areapproaching the problem by investigating natural enzymes in termites and other pests and testingwhether those same enzymes can be used in an energy conversion system.
In the discussion following Barber’s talk, Davis agreed that better catalysts are needed forboth the front end (breaking polysaccharides into smaller molecules) and the back end (building
those smaller molecules back up into fuel products) of biomass conversion; better catalysts
would reduce the amount of energy being used breaking and building chemical bonds. Davis alsocommented on the need to change some of the chemical processing steps of biofuel production inorder to lower the cost of production and, therefore, the cost of biofuel. In sum, using different
types of crops (e.g., Miscanthus instead of sugar or corn), developing better harvesting
techniques, and changing some of the chemical processing steps would all contribute to bringing
the cost down and making biofuel production more competitive with fossil fuels.
As an example of the type of work being done to build better catalysts for biofuel
conversion, Dr. Michikazu Hara of the Tokyo Institute of Technology, Japan, commented on
work that his research group has been doing with a particulate- and carbon-based solid acidcatalyst for the conversion of cellulose into sugars. Unlike other acid catalysts that cannot be reused and must be separated out and treated as waste when the reaction has been completed, his
substance can be re-used in subsequent reactions.
While most of the CS3 discussion on biofuels focused on land biomass, Dr. Fang Huangof the Chinese Academy of Sciences’s Institute of Botany, Beijing, China, commented on her
research group’s experiments with the use of microalgae as a biomass source for fuel.

Energy is a Systems-Level Issue

Davis’ discussion of the cost of biofuel production led into a discussion about theimportance of examining the full life cycle of biomass-to-biofuel conversion. That is, how muchenergy is required to produce biofuel? Which steps are the most costly in terms of energy loss?
Where can improvements be made? Even in Brazil, despite decades of widespread use ofbioethanol, the full life cycle of bioethanol is still unclear. CS3 participants agreed that the needto consider the full life cycle of an energy conversion process is true of all solar energy
technologies, not just biofuels. Even with artificial photosynthesis, for example, when thetechnology matures it will be important to consider not just energy output but also the amount ofenergy required to build the devices and carry out the processes. Davis emphasized, however,that caution be taken when evaluating any lifecycle or systems-level analysis of solar energy
conversion technologies, as the results are highly depending on which factors are considered andthe assumptions of the analysis. Many life system analyses of biofuels are based on assumptions
derived from corn, which involves fairly high-intensity agricultural practices. If those same
assumptions are used when evaluating biofuels derived from a non-corn crop that is not asenergy-intensive, the results may not be as favorable as they should be.

13

Use of Biofuels

Dr. Walter Leitner, Institut für Technische und Makromolekulare Chemie, Aachen,
Germany, initiated some discussion on the use of biomass not just as a source of fuel but also as
a source of added value. That is, products can be made from biomass than cannot be made using
any other material. Biomass is already regularly used to produce high-value carbon-based
chemicals, like high electrolyte oils used as lubricants or in the medical industry. Leitnerreiterated that “first-generation” biofuels, namely ethanol (i.e., from sugar cane and corn) andbiodiesel (e.g., from sunflowers and soy beans), are clearly not a sustainable solution except incertain regions. “Second-generation” biofuels -- that is, those that use liquid cellulose as afeedstock, as Barber and Davis discussed – are a better option because of their higher energy
content and other factors. Still, Leitner said, they are not a global solution. Thus, he urged
consideration of a “third-generation” biomass conversion approach, one that scientists at his
research institution have adopted: the conversion of biomass into value-added products. These
products can include fuel, but they also include chemicals, lubricants, pharmaceuticals, and otherproducts – things that cannot be made using other technologies.

Accessing Solar Energy That Already Exists in Nature
Biofuels derived from non-crop biomass allow access to the enormous reservoir of solar energythat has already been naturally converted into plant mass. What must be achieved before biofuel
production can become an affordable, sustainable source of energy for widespread use? During
the final wrap-up session of CS3, a breakout group identified the following as one of the mostpressing scientific challenges:
Develop biochemical methods that can be used to create more biomass 9 .
Develop catalytic processes that improve the efficiency of biomass conversion 10 .

9The enzyme (known as “RuBisCO9”) that takes CO2 out of the atmosphere and incorporates it into plant biomass isvery sluggish and does not discriminate well between CO2 and O2. Often, it grabs hold of O2 instead of CO2. Not
only does a plant waste energy when RuBisCO makes this mistake, it wastes even more energy fixing the mistake.
By improving RuBisCO’s performance through protein engineering, chemists estimate that they could increaseavailable plant biomass by at least 50%.
10
The most abundant forms of biomass contain a high level of oxygen, and too much oxygen reduces the amount ofenergy available (per unit of biomass). Developing resource- and energy-efficient ways to remove oxygen from thecomplex molecules involved in biomass conversion would increase available biomass energy.

14

III

CONVERTING SOLAR ENERGY INTO ELECTRICITY: NEXT-GENERATIONPHOTOVOLTAIC (PV) TECHNOLOGIES 11

Photovoltaic (PV) cells (“solar cells”) are devices that convert sunlight into electricalenergy. 12 While photovoltaic PV technology is arguably the most popular type of solar
technology in use today, with many people associating the notion of solar power with an imageof solar panels on rooftops, it constitutes only a fraction of global energy production. According
to Dr. Alvin Compaan, University of Toledo, Ohio, about 20 terawatts (TW) of electricity were
produced worldwide in 2008. Only 7 gigawatts (GW) of that 20 TW (0.035%) were in the formof PV modules. Widespread use of PV technology is limited by the high cost of silicon. Most
PVs currently being manufactured and used worldwide are made of solar-grade silicon (Si) 13 .
Repeated calls were made throughout the course of the two-and-a-half-day CS3 for more
affordable PV materials and manufacturing methods. In fact, scientists continue to develop andexperiment with different semiconductor materials and solar cell structures. But they face someserious challenges. For example, while innovative organic molecule-based PV technologies arepotentially much less expensive than silicon-based solar cells, the stability and longevity of theorganic materials is a major concern. In this third session of the symposium, CS3 participants
discussed the challenges and opportunities of several “next-generation” PV technologies. This
chapter summarizes that discussion.

Overview of PV Technology: Chemists Look for Ways to Cut Cost

Dr. Junhao Chu of the Shanghai Institute of Technical Physics, Shanghai, China, opened
the session with an overview of PV technology. PVs were among the first solar power
technology developed, by Bell Lab scientists in the 1950s, and worldwide production has been
increasing practically exponentially over the past decade. According to Chu, total worldwide
production of PVs was about 125.8 megawatts (MW) in 1997, compared to 3733.4 MW in 2007.
Chu predicted that the annual installation of PV power will continue to increase at a rapid rate in
the near future, with a conservative estimate of more than 13,000 MV MW of solar power being
generated in 2012.
The majority of PV production occurs in China, mostly for export. In 2008, about onethird (33%) of all PV production occurred in China, followed by Germany (21%), Japan (16%),
Taiwan (7%), elsewhere in Europe (7%), elsewhere in Asia (7%), U.S.A. (6%), and India (1%).
By 2011, China and Germany are expected to remain the leading two producers of PV
11This section summarizes the highlights of the presentations and discussions that occurred during the third sessionof CS3, “Photovoltaics, Including Organic Solar Cels, Inorganic Solar Cells and Dye-Sensitized Solar Cells,” which
was organized by Dr. Chen-Ho Tun of the Chinese Academy of Sciences, Beijing, China.
12
Illumination of a semiconductor material inside the PV device generates an electric current.
13
Solar-grade silicon is different than pure silicon; among other differences, it is treated with chemicals that increase
its electrical conductivity.

15

technology, accounting for 19% and 13% of all PV manufacturing, respectively, followed by
Malaysia (11%). Even though China is the greatest producer of commercial PV technology, it
uses very little of what is produced; its 2008 share of the global market was only 0.17%.
Chu emphasized that while there has been considerable rapid progress in PV production
over the past several years, PV science and technology remains a very active area of research andinnovation. Continued research and innovation will be essential to the development of more
affordable materials. This is true even though the cost of solar-grade silicon has dropped, from$350-400/kg in December 2007 to $100/kg in December 2008. Silicon is still the most costly
component of PV production, accounting for nearly 60% of the cost of producing a single solar
cell module, or panel. 14

Existing and Emerging PV Technologies

Chu described three different types of PV technologies:(1) Crystalline silicon (c-Si) PV technology
Currently, c-Si technology dominates the marketplace, with about 87% of all solar arrayinstallations worldwide comprised of c-Si cells. However, there is ample room for improvement.
Most importantly, while c-SI PV technology is the most efficient of all PV technologies (and,arguably, all solar energy technologies), with an average 15-20% efficiency of converting
absorbed sunlight into electricity, the current cost of c-Si technology is prohibitively expensive
for widespread use. Chu emphasized the need to continue to reduce the thickness of the siliconwafer that makes up such a large part of each first-generation c-Si PV cell. Although the average
thickness of wafers has decreased in size more than 50% over just the past few years, from about300-350 microns in 2003 to about 150 microns by 2010, it needs to be decreased even further inorder to put the cost of the technology into a more affordable range.
While decreasing the wafer thickness makes good economic sense, it creates a new set ofchallenges. Specifically, it increases the chance that not all photons will be absorbed, with somebeing reflected back into the atmosphere, thereby decreasing the cell’s solar conversion
efficiency. Chu mentioned two ways that cells can be modified in an effort to avoid this problem:(1) “light trapping” and (2) improved surface passivation. Light trapping increases the amount ofabsorbed light that gets “trapped,” for example by creating an inverted pyramid silicon surface oradding mirror-like metal layers to the back of the device. Passivation renders the surface of thesemiconductor chemically and electrically “passive,” thereby reducing its chemical reactivity
and improving the likelihood that the charge carriers (i.e., the electrons and holes) will betransported into the external circuit. Chu showed a few examples of how these techniques havebeen used to make c-Si cells more efficient. For example, by adding passivating layers of SiO2 toboth the front and rear surfaces, Fraunhofer Institute for Solar Energy Systems (Fraunhofer ISE),
Freiburg, Germany, developed a 21.6% efficient LFC-PERC (laser-fired contact -- passivatedemitter, rear cell) solar cell. SunPower Corporation, San Jose, California, and Sanyo ElectricCompany, Moriguchi, Japan, have both used passivation techniques to reduce the cost and14

A solar module, or panel, is an assembly of interconnected solar cells. It is the functional unit of most solar PV
systems.

16

improve the efficiency of their respective proprietary c-Si PV technologies. SunPower hasreported 21% efficiency, and Sanyo has reported 22.3% efficiency.
Chu also discussed some recent research on other materials besides SiO2 that can be used
for passivation. For example, researchers at RWTH Aachen University, Germany, have been
studying the use of silicon nitride (SiNx) as a passivating material; scientists at Applied Materials,
Inc., Santa Clara, Calif., have been investigating passivation using a combination of SiO2 andSiNx; and researchers from Eindhoven University of Technology, Netherlands, the Institute for
Solar Energy Research at Leibniz University, Hannover, Germany, and the Tokyo Institute ofTechnology, Japan, have been studying the use of aluminum oxide (Al2O3) films as a passivating
material. To date, the “world record” 15 using such passivating materials with c-Si cells is
Sanyo’s reported 22.3% efficiency.
(2) Thin Film Solar Cells (TFSC)
Chu spent most of the remainder of his time discussing “second-generation” thin-film PV(TFPV) technologies, which include amorphous silicon (aSi)-based TFSCs as well as TFSCs
comprised of other chemical compounds, such as polycrystalline copper indium (di)selenide(CIS), copper indium gallium (di)selenide (CIGS), cadmium telluride (CdTe), or epitaxial 16
layers of indium gallium phosphide (InGaP) and gallium arsenide (GaAs).
Most TFPV cells have relatively low efficiencies compared to c-Si cells, with typicalefficiencies in the 10-13% efficiency range. Most TFPV production modules are about 5-10%
efficient. However, as Chu discussed, there is wide variation in TFPV efficiency, which among
factors depends on whether a cell is “multi-junction” or not. Multi-junction PV cells containmultiple layers of different semiconductor materials, with each type of material absorbing a
different wavelength of light. Increasing the range of light wavelengths that can be absorbed
increases the amount of solar energy that can be converted into electrical energy. Some multiple
epitaxial layer cells (e.g., “triple junction” cells) have efficiencies greater than 40%. As just one
example of new materials being explored for their use in multi-junction TFPV devices, Chu
pointed to the use of gallium-indium-nitride-arsenide (GaInNAs) and boron-gallium-indiumarsenide-phosphide (BGaInAs) layers by scientists at the U.S. National Renewable Energy
Laboratory, Golden, Colorado.
In addition to their lower efficiency (compared to silicon wafer PV cells), stability poses
another key challenge for TFPV. Many thin-film solar cells are inherently unstable because theirmaterials degenerate over time when exposed to light. As Chu explained, lack of stability can bematerials-related (e.g., while CIGS is relatively stable, a-Si is susceptible to light-induced
degradation), adhesion-related (i.e., peeling can occur between layers or from the substrate), or
capsulation-related (i.e, moisture can penetrate the encapsulated module through laminatededges).
Despite their efficiency and stability drawbacks, thin-film solar cells require fewer raw
materials and are less expensive to build than c-Si cells. They also have a shorter energy“payback” period. That is, all solar cell modules require energy to produce, but TFPV modules“pay back” that energy more quickly in terms of generated electricity compared to silicon wafer
technology. It takes about four years for a typical wafer silicon cell to “pay back” all the energythat was required to make the cell in the first place, compared to only about one year for a thin15
16“World record” efficiencies are for research laboratory settings; efficiencies in practical settings are usually lower.
An epitaxial layer is a type of semiconductor layer with a particular crystalline structure.

17

film silicon-based solar cell. Added to their lower cost and shorter energy payback period, theirflexibility and light weight make them suitable for applications not possible with the moreconventional silicon wafer technology (e.g., for space, military, building-integrated installations).
While TFPVs are not as popular as c-Si PVs, a growing percentage of PV installations
are thin-film based. For example, Chu mentioned a planned 1.3 MW (megawatt) power station
being built in Germany using CdTe-based solar cells (the Dimbach Solar Park); and an
amorphous silicon-based thin-film system on the roof of the Beijing New Capital Museum,
Beijing, China, which provides 300kW of power using flexible panels developed by United Solar
Ovonic, a subsidiary of Energy Conversion Devices, Rochester Hills, MN. By 2012, Chu
predicts that 75% of all PV installations will be c-Si-based (down from the current figure ofabout 87%), the remaining 25% thin film-based.
More on Thin-Film Solar Cells
Following Chu’s presentation, Dr. Alvin Compaan of the University of Toledo, Ohio,echoed Chu’s comments about TFPV becoming a more popular choice in the future. In fact,
Compaan stated that, in the United States, most silicon-based PV production is with thin-film,
not wafer, technology (i.e., by Uni-Solar). Compaan mentioned three inorganic thin-film PV
technologies that have reached large-scale manufacturing in the past four years: Ohio-based First
Solar’s cadmium telluride (CdTe) TFPV cells, Michigan-based Uni-Solar’s amorphous-silicon
TFPV cells, and Arizona-based Global Solar’s copper indium gallium diselenide (CIGS) TFPV
cells. At least one of these companies (First Solar) has experienced major cost breakthroughs as
well.
However, many chemical challenges remain before some types of TFPV technologies canbe advanced and other types can be scaled up for widespread use. Compaan mentioned two: (1)
TFPVs made of polycrystalline materials pose the same type of passivation problem that somematerials used in c-Si technology create, as Chu discussed. (2) While some TFPVs demonstrate
relatively high efficiencies in research lab settings, their actual efficiencies following large-scale
manufacturing are much lower. CIGS, for example, is a difficult material to manufacture and so,while CIGS TFPVs may be 20% efficient in the lab, actual modules manufactured for sale areless than 10% efficient.
Compaan then discussed in more detail his collaboration with First Solar and the future of
CdTe TFPV technology. As with indium (which was mentioned following Chu’s comments onorganic molecule PV technologies – see below), there is concern about Te availability. In 2008,
CdTe module production used a total of about 50 tons of Te, all of which came from copper (Cu)mine tailings. 17 Currently, Te is extracted from Cu tailings at only about 33% efficiency,
yielding about 500 tons a year. If that extraction efficiency could be increased to 80%, Compaan
explained, it would support about 25 GW of CdTe-based PV production a year.
Compaan also addressed concern about Cd toxicity, stating that most Cd emissions areindirect emissions produced during the coal-fired electricity generation needed for the module
production process. Since the production of CdTe modules requires less electricity than the
production of c-Si wafer modules, Cd emissions are actually quite low in comparison. When
asked about recovery costs at the end of the lifecyle of a CdTe cell, Compaan stated that First
Solar has set up an escrow account for every panel sold and which will be used for recycling all
17

There are some Te-enriched reserves in areas of China that are just beginning to be explored, but to date all Te isextracted from other mining operations.

18

old panels to make sure that the Cd is properly sequestered. Still, Dr. Dan Nocera of MIT
commented on the “huge hidden costs” with CdTe technology.
(3) “New-concept” PV technologies, including dye-sensitized cells (DSC) and organic cells.
Chu mentioned three types of “third-generation” new concept solar cell technologies:
1. Dye-sensitized solar cells (DSSC). First engineered by Michael Grätzel in 1991,
DSSC (also known as “Grätzel” cells) use dye molecules to absorb incoming light;
the photon-excited dye generates an electric current in a separate, non-silicon-based
semiconductor material (e.g., titanium dioxide, TiO2). Chu explainedt that DSSC areless expensive than c-Si cells, but they also have a lower efficiency than both c-Si and
TFPV cells (with a maximum reported efficiency of just over 11%) and suffer fromsome of the same stability problems that other 18 TFPV cells have.
Following Chu’s talk, Dr. Qingbo Meng of the Chinese Academy of Sciences,
Beijing, China, noted that not only are DSSCs less expensive than c-Si cells, they arealso easier to produce. In China, researchers are investigating whether lithium iodide(LiI), the traditional electrolyte material used in DSSCs, can be replaced by the lessexpensive aluminum iodide (AlI3) to bring their cost down even further.
Also following Chu’s talk, Dr. Laurie Peter, University of Bath, UK, briefly
described the history of DSSC technology, including Grätzel’s discovery and thenMassachusetts-based Konarka Technologies’ commercialization of DSSC. Today,
UK-based G24 Innovations (G24i) is selling another, proprietary DSSC technology.
Meanwhile, scientists at the Fraunhofer Institute for Solar Energy Systems ISE,
Germany, and elsewhere are continuing to investigate new ways to increase theefficiency of DSSC technology. As Peter explained, while the greatest reported solar
energy conversion efficiency of a DSSC is 11.4%, that record is for a single cell in a
research laboratory setting. Efficiencies in practical settings are lower, on the order of
8-9%.
2. Conducting polymer solar cells. Chu explained that, like DSSC, conducting polymer
solar cells are potentially cheaper to manufacture than other types of PV technologies
because they use semiconducting polymer materials instead of silicon, but they are
not as efficient. They are also not very stable, with typical longevity for conducting
polymer cells being only 3 to 5 years. Following Chu’s presentation, Dr. Peter
Bäuerle of Universität Ulm, Ulm, Germany, remarked that Konarka recently reported
6.4% efficiency, which is the highest performance to date for a conducting polymer
solar cell in a research laboratory setting. However, the company’s reported rooftop
efficiency is only 3.3%. Dr. Zhigang Schuai, Tsinghua University, Beijing, China,
commented on some of the research approaches being taken in an effort to increase
the efficiency of polymer solar cells. These include using different types of polymers
and nanostructured materials.
3. Molecular (or “small molecule”) organic cells. As with the other new-concept
technologies, molecular organic cells are potentially cheaper than silicon wafer and18

While identified by Chu as a “new concept” technology, DSSC are also considered a type of TFPV.

19

thin-film technologies, both because of the materials used and because of their
manufacturing processes. However, their solar energy conversion efficiency is only
about 4%. As Bäuerle pointed out following Chu’s presentation, Heliatek GmbH,
Dresden, Germany, reported an efficiency of about 6%, which is the record for small
molecule organic solar cells at the moment. Their limited efficiency is due in part tothe fact that most currently available molecular semiconductor materials absorb only
a narrow wavelength of light and therefore are not capable of harvesting much light.
Also following Chu’s presentation, there was a question about indium tin oxide(ITO), one of the materials commonly used as an electrode in organic solar cells19
and the movement to steer away from ITO because of sustainability issues (i.e., there
isn’t enough indium in the earth to provide for large-scale production). However, notall participants agreed that sustainability is an issue. In China, for example, the supplyof ITO is not considered to be in short supply. Sustainability aside, there was stillsome concern expressed about the use of ITO because of performance and stability
problems stemming from indium’s chemical reactivity with organic materials in thecells.

The Search for Earth-Abundant Materials

Discussion around the resource implications of CdTe-based TFPV’s potential expansion
in the future led Dr. Laurie Peter, University of Bath, UK, to speculate on the availability of PV
technology resources in general and whether the earth actually provides enough materials to
realize the large-scale production of solar-generated electricity. He noted that both Cd and Te,
for example, are quite rare in comparison to other elements in the earth’s crust, like zinc (Zn),copper (Cu) and tin (Sn). Zn, Cu and Sn are also relatively cheap compared to some of theelements being considered for next-generation PV technologies, like indium (In). He asked: Is
there a way to replace some of the more expensive, less abundant and potentiallyenvironmentally harmful materials that are currently being used with less expensive, moreabundant, non-toxic materials? He argued that part of the reason that Te and many of the othermaterials being used today is historical and that chemists need to re-visit the periodic table ofelements, as well as historic work, and experiment with new combinations of materials thatwould have the same properties as, for example, CIGS, but which would be more suitable forlong-term, scaled-up use. To his knowledge, there are only a handful of research groups
investigating the performance of alternative materials in PV cells.

19

The organic material is typically sandwiched between a layer of ITO and a layer of another electrode material.

20

The Future of PV Technologies

Chu emphasized four important next steps for advancing solar cell technologies:(1) Improve existing processing technologies for first-generation c-Si solar cells. Even if
the cost of silicon were to significantly drop, it would affect only 60% of the cost of ac-Si solar cell module. The processing and manufacturing of the materials and cells
still need to be improved as well in order to bring the remaining 40% of the cost of cSi technology down.
(2) Develop new materials for both thin-film and new-concept PV technologies, not justin order to bring the cost down but also for performance reasons.
(3) Continue to develop new structures, processes and concepts for both thin-film andnew-concept PV technologies in order to improve the performance of thesepotentially less expensive technologies.
(4) Combine chemists’ efforts with those of scientists in other disciplines (e.g., physics,
materials science and engineering).
Chu’s overview prompted several questions about which, if any, of the multitude of PV
technologies currently being explored and developed is likely to be the most successful in termsof very large-scale delivery of electricity worldwide. For example, which of these technologies
are more (or less) resource- or cost-limited and therefore less (or more) likely to be widelyadopted? Chu responded that silicon is still the “best” material in terms of efficiency and that,within the next ten years or so, Si-based cells will become more affordable because they will bebuilt with less Si than they are today. However, some of the other thin-film materials, like CiGS,
will become more popular over time. He also predicts that molecular organic solar cells inparticular will become very important in the future because of their low cost, although theirefficiency and particularly their stability will need to be improved.
The fact that the efficiency and stability of molecular organic solar cells will need to be
improved before they can become a viable option for large-scale use led into further discussion
about the many basic scientific challenges that still remain. As Peter put it, “There is still a lot ofchemistry that needs to be figured out.”
Converting Solar Energy into Electricity
The widespread use of silicon-based photovoltaic (PV) cells, which convert solar energy directly
into electricity, is limited by their high cost. What must be achieved before PV cells can become
an affordable, sustainable mode of energy? During the final wrap-up session of CS3, a breakout
group identified the following as one of the most pressing scientific challenges
Develop low-cost, non-toxic, earth-abundant PV materials for use in next-generation PV cells.

21

IV
STORING SOLAR ENERGY 20

While finding ways to reduce the cost of water-splitting catalysts, increase the efficiencyof biomass conversion, and build better-performing next-generation PV technologies will be vitalto making solar energy-derived fuels competitive with fossil fuels, a couple of CS3 participants
remarked that it is just as important that chemists figure out how to store all this energy once it
has been converted. After all, as Dr. Daniel Nocera of the Massachusetts Institute of Technology,
Cambridge, Mass., stated, “you can’t run a society only if and when the sun shines.” He argued
that solar energy amounts to less than 0.1% of the energy market today not just because of costand other material and manufacturing issues but also because of a lack of long-term stationary
storage options. 21 Until solar energy can be stored for future (long-term) use, solar energy
conversion technologies will have a difficult time penetrating the global energy market. The
focus of the fourth session of CS3 was on solar energy storage technologies, with much of the
discussion revolving around differences between personalized and centralized storage as well as
on types of chemical fuels that should and could be stored. This chapter summarizes that
discussion.

Ways to Store Solar Energy

Nocera opened the session with a brief overview of the various ways that solar energy
can be stored:(1) Mechanically, through compressed air energy storage. Solar energy can be used tocompress air in underground reservoirs (e.g., aquifers, salt caverns) such that when the air
is de-compressed, about 0.5 megajoules (MJ)/kg of energy are released.
(2) Hydroelectric pumping systems. Solar energy can be used to pump and store water in
uphill reservoirs such that when the water flows back downhill, it releases about 0.001
MJ/kg energy. Nocera remarked that the energy density using this method is so low thathydroelectric pumping can’t really be used for practical purposes.
(3) Batteries. Energy yield of batteries ranges from 0.54-0.72 MJ/kg for lithium (Li)batteries to 0.14-0.22 MJ/kg for nickel-cadmium (NiCd) batteries to 0.14-0.17 MJ/kg forlead (Pb) batteries. That’s about as energy dense as batteries can be without making them
20

This section summarizes the highlights of the presentations and discussion that occurred during the fourth sessionof CS3, “Storage, Transportation, and Distribution of Discontinuously Supplied Energy,” which was organized by
Dr. Alvin Compaan of the University of Toledo, Toledo, Ohio.
21
Storage is also a very important issue with respect to using converted solar energy for transportation purposes, anarea of energy use that was mentioned but not elaborated on during CS3. Most of the discussion focused on
stationary energy use.

22

any heavier than they already are. Nocera opined that, while batteries are good fortransportation because their stored energy can be so quickly extracted, they are not a very
good option for long-term, non-grid storage.
(4) Capacitors. While capacitors can store energy, their yield is only about 0.01 MJ/kg(supercapacitors) to 0.0206 MJ/kg (ultracapacitors).
(5) Chemical bonds: Chemical bonds have the highest energy density of any solar energy
system. Compressed hydrogen gas (70 MPa H2), for example, yields 143 MJ/kg, andother liquid fuels yield on the order of 44 MJ/kg. As Nocera explained, “nature ‘chose’chemical fuels for storage” because of this high energy density and, moreover, “nature‘chose’ to fix hydrogen with CO2.” He argued that perhaps we should take our cues fromnature and not only use chemical bonds as our primary mode of long-term solar energy
storage but also fix the hydrogen (i.e., the H2 derived from water splitting) with CO2 andmake liquid fuels. However, as discussed in Chapter I, chemists have yet to develop apractical solar-powered way to reduce CO2. Until that goal is reached, Nocera’s vision ofa personalized energy system relies on the use of H2 as a fuel.

The Storage of Distributed Energy

Nocera remarked that, in addition to storage technologies being a key driving force ofmarket penetration, personalized storage technologies in particular will also be key to making
solar energy an accessible option for the non-legacy world – that is, the three billion people alive
today that are considered low-energy users (e.g., people who use off-grid energy) plus anadditional three billion low-energy users who will be inhabiting the planet over the next century.
Most of the increased energy consumption worldwide over the next several decades will be
among these six billion people. While the legacy world would arguably be better served bycentralized grids, for example with converted solar energy being stored in batteries (seediscussion below), the non-legacy world be better served by a decentralized system, with eachhousing complex or building relying on its own individual storage unit.
However, in order to make solar energy an accessible option for people who use off-gridenergy, new types of solar energy storage systems need to be developed. The challenge is greaterthan simply scaling down currently existing systems. Nocera compared it to the challenge ofdesigning personal computers: “You can’t simply make mainframes smaller. You have to startfrom scratch and design new, personal computers. In much the same way, you can’t simply scale
down currently existing systems to meet the demands of all the new energy users. You need tocreate new personalized systems.” As is the case with so many next-generation solar energy
technologies, the question is, how?
Nocera described his vision of a personalized energy system, which is based partly on
work that his laboratory group has done with a new water-splitting catalyst (see Chapter I for amore detailed discussion of the chemistry of the catalyst): A rooftop PV array would convert
solar energy into electricity for use during the day, with surplus electrical energy being diverted
into a water-splitting device for the production of H2 and O2. The H2 and O2 would be stored in a23

local tank and, at night, recombined in a fuel cell 22 to generate whatever electricity is needed to
power the home when the PV array isn’t operating. In theory, powering a single home or clusterof dwellings would require only a very small amount of water as raw material. As mentioned in
Chapter I, Nocera and has entered into an agreement with Sharp Electronics to integrate thewater splitting part of the system with Sharp’s PV technology. Nocera emphasized that his work
is not necessarily “the answer,” rather it is the beginning of a new area of research anddevelopment in decentralized solar energy storage.

What Kind of Chemical Fuel?

Following Nocera’s presentation and re-visiting some of the discussion that took place
during the first session (as summarized in Chapter I), Dr. Robert Schlögl of Fritz-Haber-Instituteder Max-Planck-Gesellschaft, Berlin, Germany, stated that while, in the past, hydrogen fuel was
viewed as the only option for the chemical storage of solar energy, scientists have since realizedthat in fact there are other options and other types of fuels. He remarked that different types offuels could be produced depending on the situation. Dr. Etsuko Fujita, Brookhaven National
Laboratory, Upton, New York, commented on the desirability of methanol, because it can beused either as a stationary or transportation fuel and because there are many options available forits production. Either way -- whether H2 (e.g., H2 produced from water splitting) is used as a fuelby itself or converted into another fuel -- H2 will likely become a very important component of
the future energy systems of many countries. Therefore, Schlögl said, we need to start thinking
about how to store it. He re-emphasized, however, that although chemists have successfully
demonstrated that it is possible to use solar energy to power the various chemical reactions thatcomprise water splitting, none of these processes have been advanced to a point where they canbe applied. Chemists have yet to develop artificial photosynthesis catalysts that not only work
efficiently enough to apply in power station or other energy conversion systems but also aremade of abundant, affordable materials
Nocera then asked whether it would be possible to scale down the CO2 reduction process
so that it could be used in personalized energy storage systems. That is, are there catalysts that
could be used to reduce CO2 and form other fuels besides hydrogen for use in small energy
systems? Schlögl responded that, yes, it is possible, but it would require the development of newreactors and simpler processes. In other words, while the chemistry of catalysis is obviously atremendously important issue for many areas of solar energy science and technology, in this casethere are other issues of concern. Dr. Sossina Haile, California Institute of Technology, Pasadena,
California, noted that scientists at Pacific Northwest National Laboratory are developing
miniaturized energy-converting technologies that might be applicable to this sort of small-scale
CO2 reduction scenario.

22

A fuel cell is a device that converts chemical energy into electrical energy. The simplest fuel cells use H2 as theirenergy source, reacting the H2 with O2 to produce water and, in so doing, release energy. Other fuel cells use varioushydrocarbons, like methane, as their chemical source, converting the methane to hydrogen first and then reacting thehydrogen with oxygen to release energy.

24Large-Scale, Grid-Connected Solar Energy Storage

During the discussion following Nocera’s presentation, Dr. Ferdi Schüth, Max-PlanckInstitut für Kohlenforschung Kaiser-Wilhelm-Platz, Mülheim, Germany, reiterated the need toconsider different types of storage systems for different locations, depending on local
infrastructure and need. While it might make sense to store solar energy as a gas or liquid
chemical fuel (e.g., hydrogen, methanol or methane) in areas without grid infrastructures, it
makes better sense to “stay electric” as long as possible in grid-connected areas. Building acentralized energy system around solar-powered electricity, however, will still require buffering(i.e., storage capacity). Germany, for example, is required to maintain a 90-day supply of oil
reserves, which corresponds to about 20 million tons. Moving away from oil would require
finding an alternative way to store 90-days worth of fuel.
Dr. Jürgen Janek, Justus Liebig University Giessen, Giessen, Germany, commented on
the feasibility of using batteries as one of those alternatives. He remarked that unlike most of theother technologies discussed during the course of the symposium, the challenge for batteries as astorage mode is coordination, not chemistry. A grid-connected battery storage system of that sizeneed only be designed and organized correctly. Other than these brief comments, the scientificand technological challenges of advancing battery storage were not addressed in any detail.
Finally, Haile described a centralized solar energy storage energy vision for the futurethat involves an approach analogous to what Nocera laid out for personal use: In addition to
producing electricity for daytime use, centralized plants would also use some of the solar energy
captured during the day to produce fuel via a ceria-based thermochemical process and the fuelused to deliver electricity at night. She and her research team have demonstrated that ceria can beused as a thermochemical reaction medium for the production of hydrogen, syngas 23 or methane
upon exposure to water and CO2. Based on the amount of ceria currently available (about 40
megatons worldwide, which is relatively abundant compared to other elements), one could
conceivably build 50,000 such 100-megawatt power plants at the equivalent of about US$140k
each (in raw ceria materials cost).

Storing Newly Harnessed and Converted Solar Energy
Chemists must develop systems that not only convert solar fuel into other forms of energy but
also store that converted energy for future use. This is an especially critical issue for areas of theworld without access to centralized energy delivery systems. What must be achieved in order tobuild low-cost, sustainable solar storage systems? During the final wrap-up session of CS3, a
breakout group identified the following as the most pressing scientific challenge:
Develop new catalysts and materials from low-cost, earth-abundant elements that can be used
to build affordable, sustainable solar energy transformation-and-storage systems.

23

Syngas is a combustible gas mixture that contains carbon monoxide and hydrogen – it can be used as a fuel or as
an intermediate in other types of fuel production systems.

25

APPENDIX A
ProgramFirst Chemical Sciences and Society Symposium (CS3)
Kloster Seeon, Germany, July 23-25, 2009
Powering the World with Sunlight
Wednesday, July 22, 2009
Afternoon

Arrival and registration

6:00 PM

Welcome and informal dinnerThursday, July 23, 20097:30 AM

Breakfast

8:45-9:00 AM

Welcome and Opening by Klaus Müllen, GDCh President

Session 1

Artificial Photosynthesis, Photocatalytic Water Splitting, and CO2 Fixation,organized by Dr. Hauro Inoue, Tokyo Metropolitan University, Tokyo, Japan

9:00-9:40 AM

Introductory Lecture by Dr. Kazunari Domen, University of Tokyo, Japan

9:40-10:30 AM

Statements and Discussion

10:30-11:00 AM

Break

11:00-12:00 PM

Statements and Discussion, cont.

12:30-2:00 PM

Lunch

Session 2

Solar Radiation to Energy via Biomass, organized by Dr. Laurie Peter,
University of Bath, UK

2:00-2:40 PM

Introductory Lecture by Dr. James Barber, Imperial College London, UK

2:40-3:30 PM

Statements and Discussion

3:30-4:00 PM

Break

4:00-5:00 PM

Statements and Discussion, cont.

26

Friday, July 24, 20097:30 AM

Breakfast

Session 3

Photovoltaics: Organic Solar Cells, Inorganic Solar Cells, and Dye-SensitizedSolar Cells, organized by Dr. Chen-Ho Tung, Chinese Academy of Sciences,
Beijing, P.R. China

9:00-9:40 AM

Introductory Lecture by Dr. Junhao Chu, Shanghai Institute of Technical Physics,
Shanghai, P.R. China

9:40-10:30 AM

Statements and Discussion

10:30-11:00 AM

Break

11:00-12:00 PM

Statements and Discussion, cont.

12:30-2 PM

Lunch

Session 4

Storage, Transportation, and the Distribution of Discontinuously Supplied
Energy, organized by Dr. Alvin Compaan, University of Toledo, Toledo, Ohio,
U.S.A.

2:00-2:40 PM

Introductory Lecture by Dr. Daniel G. Nocera, Massachusetts Institute ofTechnology, Cambridge, Mass.

2:40-3:30 PM

Statements and Discussion

3:30-4:00 PM

Break

4:00-5:00 PM

Statements and Discussion, cont.

6:00-7:30 PM

DinnerSaturday, July 25, 20097:30 AM

Breakfast

9:00-10:30 AM

Breakout Sessions, organized by Dr. Ferdi Schüth, Max-Planck-Institut fürKohlenforschung Kaiser-Wilhelm-Platz, Mülheim, Germany.

10:30-11:00 AM

Break

11:00-12:00 PM

Wrap-Up and Recommendations

27

APPENDIX B
List of Participants
China
Prof. Dr. Chen-Ho Tung, Technical Institute of Physics and Chemistry, Chinese
Academy of Sciences, Beijing: CS3 Scientific Committee member
Prof. Dr. Hongzheng Chen, Department of Polymer Science & Engineering, Zhejiang
University, Hangzhou
Prof. Dr. Junhao Chu, Shanghai Institute of Technical Physics, Shanghai
Prof. Dr. Fang Huang, Research Center for Photosynthesis, Institute of Botany, Chinese
Academy of Sciences, Beijing
Prof. Dr. Can Li, Dalian Institute of Chemical Physics, Chinese Academy of Sciences,
DalianProf. Dr. Qingbo Meng, Institute of Physics, Chinese Academy of Sciences, Beijing
Prof. Dr. Zhigang Shuai, Department of Chemistry, Tsinghua University, Beijing: Chinese
Chemical Society [CCS] and National Science Foundation of China [NSFC]
liaisonGermany
Prof. Dr. Ferdi Schüth, Max-Planck-Institut für Kohlenforschung Kaiser-Wilhelm-Platz,
Mülheim: CS3 Scientific Committee member
Prof. Dr. Peter Bäuerle, Universität Ulm, Abteilung Organische Chemie II, Ulm
Prof. Dr. Jürgen Janek, Physikalisch-Chemisches Institut, Justus-Liebig-Universität Gießen,
GießenProf. Dr. Walter Leitner, Institut für Technische und Makromolekulare Chemie, Aachen
Prof. Dr. Dr. h. c. Hartmut Michel, Max-Planck-Institute für Biophysik, Abteilung Molekulare
Membranbiologie, Frankfurt am Main
Prof. Dr. Klaus Müllen, Max-Planck-Institut für Polymerforschung, Mainz
Prof. Dr. Robert Schlögl, Fritz-Haber-Institute der Max-Planck-Gesellschaft, Berlin
Dr. Kurt Begitt, Gesellschaft Deutscher Chemiker e.V. (GDCh), Frankfurt am Main:
GDCh liaison
Dr. Markus Behnke, Deutsche Forschungsgemeinschaft (DFG), Bonn: DFG liaison
Japan
Prof. Dr. Haruo Inoue, Department of Applied Chemistry, Graduate School of Urban
Environmental Sciences, Tokyo Metropolitan University, Tokyo: CS3 ScientificCommittee member
Prof. Dr. Kazunari Domen, Department of Chemical System Engineering, School ofEngineering, University of Tokyo, Tokyo
Prof. Dr. Michikazu Hara, Materials and Structures Laboratory, Tokyo Institute ofTechnology, Yokohama
Prof. Dr. Kazuhito Hashimoto, Department Applied Chemistry, School of Engineering,
The University of Tokyo, Tokyo
Prof. Dr. Osamu Ishitani, Department of Chemistry, Graduate School of Science andEngineering, Tokyo Institute of Technology, Tokyo

28

Prof. Dr. Koji Tanaka, Institute for Molecular Science, Okazaki
Dr. Teruto Ohta, The Chemical Society of Japan (CSJ), Executive Director, Secretary
General, Tokyo: CSJ liaison
Prof. Dr. Yoshio Okahata, Senior Program Officer, Chemistry Group, Research Groupfor Science Systems, Japan Society for the Promotion of Science (JSPS) and Professor, Graduate
School of Bioscience and Biotechnology, Tokyo Institute of Technology: JSPS liaison
United Kingdom
Prof. Dr. Laurie Peter, Department of Chemistry, University of Bath, Bath: CS3
Scientific Committee member
Prof. Dr. James Barber, Imperial College London, London
Prof. Dr. Anthony Harriman, Professor of Physical Chemistry and Co-Director of the
Molecular Photonics Laboratory, Newcastle University, Newcastle upon Tyne
Prof. Dr. Timothy Jones, Department of Chemistry, University of Warwick, Coventry
Prof. Dr. Andrew Mills, Department of Pure & Applied Chemistry, University ofStrathclyde, Glasgow
Prof. Dr. Chris Pickett, School of Chemical Sciences & Pharmacy, University of East
Anglia, Norwich
Dr. Richard Pike, Chief Executive, Royal Society of Chemistry (RSC), London: RSC
liaisonKatie Daniel, Senior Portfolio Manager, Engineering & Physical Sciences ResearchCouncil, Swindon: EPSRC liaison
United States
Prof. Dr. Alvin Compaan, Department of Physics and Astronomy, University of Toledo,
Toledo, Ohio: CS3 Scientific Committee member
Dr. Mark Davis, Principal Scientist & Group Manager of Chemical and Catalyst
Sciences, National Renewable Energy Laboratory, Golden, Colorado
Dr. Etsuko Fujita, Senior Chemist, Chemistry Department, Brookhaven National
Laboratory, Upton, New YorkProf. Dr. Sossina M. Haile, Materials Science and Chemical Engineering, California
Institute of Technology, Pasadena, California
Prof. Dr. Stephen Maldonado, Department of Chemistry, University of Michigan, Ann
Arbor, Michigan
Prof. Dr. Daniel G. Nocera, Department of Chemistry, Massachusetts Institute ofTechnology, Cambridge, Massachusetts
Prof. Dr. Mary Jane Shultz, Department of Chemistry, Tufts University, Medford,
Massachusetts
Dr. Julie Callahan, Global Network Content Development Manager, American Chemical
Society (ACS), Washington, D.C.: ACS liaison
Dr. Carol A. Bessel, Program Director, Inorganic, Bioinorganic, and Organometallic
Chemistry Program, National Science Foundation (NSF), Arlington, Virginia: NFS liaison
Dr. Luis Echegoyen, Director, Division of Chemistry, National Science Foundation,
Arlington, Virginia: NFS liaison
Other Participants
Dr. Leslie A. Pray, Science Writer, Holyoke, Massachusetts, United States
Sarah Everts, European Correspondent, Chemical & Engineering News, Berlin, Germany

29