WhitePaper
from the 7th Chemical Sciences and Society Summit (CS3)

Solar energy and photonics
for a sustainable future

Dalian, China • September 2017

CHALLENGES AND SOLUTIONS IN A CHANGING WORLD

04

Introduction

05

Executive Summary

12

Chapter 1: Photovoltaics

18

Chapter 2: Photonic materials

23

Chapter 3: Artificial photosynthesis and CO2 reduction

29

Chapter 4: Methods and standards

33

References

35

CS3 Participants

3

CHALLENGES AND SOLUTIONS IN A CHANGING WORLD

About the Chemical Sciences andSociety Summit (CS3)
The CS3 brings together leading researchers to discuss how the chemical sciences
can help to tackle some of the most daunting challenges that our world faces. Previous
summits have tackled topics as diverse as water resources, human health, andsustainability.

This White Paper, Solar energy and photonics for

Chemical Society (GDCh), the Chemical Society

a sustainable future, summarizes the outcomesof Japan (CSJ), the Royal Society of Chemistryof the 7th

CS3, held in Dalian, China, on 5–8(RSC), and the American Chemical Society (ACS).

September 2017. More than 30 scientists from the

The symposia are supported by the National

five participating countries gathered to discuss four

Science Foundation of China (NSFC), the German

major themes in solar energy: photovoltaics; photonic

Research Foundation (DFG), the Japan Science andmaterials; artificial photosynthesis and CO2 reduction;

Technology Agency (JST), the UK Engineering andand methods and standards. Their goals were to

Physical Sciences Research Council (EPSRC), andassess the current state of solar energy technologies,

the US National Science Foundation (NSF). CS3

identify the most pressing research challenges insummits are held every two years, and rotate among

these areas, and make recommendations about how

participating nations.

the field should progress.
This White Paper was written in consultation with the
Chinese Chemical Society and CS3 participants.

The CS3 initiative is a collaboration between the
Chinese Chemical Society (CCS), the German

4CHALLENGES AND SOLUTIONS IN A CHANGING WORLD

Executive summary
Solar energy can help to tackle climate change by drastically reducing carbon emissions
and providing sustainable energy for the world. To radically increase our solar power
capacity, we need a wide-ranging discovery programme to develop new light-harvestingmaterials, catalysts, and structures that help to manage light in solar energy devices.
Computational screening, ultrafast analytical methods and other cutting-edge techniques
will play a vital role in this effort.

Introduction

as a transportation fuel. Hydrogen is also a valuable

Climate change is one of the greatest challenges everreagent for making other chemicals — including liquid

faced by humankind. The world’s energy consumption

fuels and plastics — through reactions with CO2,

continues to rise, and the majority of our energy

a strategy that could ultimately help to reduce thesources — specifically coal, oil and gas — release

concentration of CO2 in the atmosphere.

the greenhouse gas carbon dioxide (CO2). To tackle
global warming, we need to cut CO2 emissions by

The chemical sciences play a crucial role inmaking a dramatic shift to renewable energy sources.

developing these technologies. Researchers are
discovering materials, based on inexpensive andSolar energy can make a huge contribution to thisabundant elements, which can capture light or speed

effort. Photovoltaic (PV) modules already provide

up photochemical reactions. They are also using aalmost 2% of global electricity, and collectively offer

wide range of techniques to measure, understandmore than 300 gigawatts (GW) of power capacity. But

and control how these processes work, in order toto decarbonize our energy systems, some projections

make them more reliable and efficient.

suggest that PV capacity must grow rapidly in the

In September 2017, the 7th Chemical Science andcoming decades, perhaps supplying 40% of ourSociety (CS3) summit in Dalian, China, broughtelectricity by 2050. To meet that target, we urgently

together leading researchers from around the world

need new technologies to harvest and store solar

to discuss the current state of these solar energy

energy more effectively.

technologies, and to identify the key researchchallenges they face. Their recommendations,

Although PV electricity can be stockpiled in batteries,

documented in this White Paper, underline thea promising alternative involves sequestering solar

significance, urgency and growing importance of thisenergy in chemical bonds, just as nature does

research.

during photosynthesis. For example, ‘artificial
photosynthesis’ systems can harvest sunlight to

It is clear that breaking out of the existing framework

drive the production of hydrogen, potentially usefulin the field of energy could bring potential clues to

5

CHALLENGES AND SOLUTIONS IN A CHANGING WORLDsolving problems common to all humankind. Global-

further improvements, researchers need a better

scale approaches and initiatives can accelerate

understanding of fundamental processes inside theresearch drastically in this area.

cell, such as how light separates electrical charges,
how those charges travel through the perovskite, andPhotovoltaics

how the interfaces between different materials affectcharge transport.

Over 90% of the world’s PV modules rely on thesemiconductor material silicon to harvest sunlight,

Organic PV (OPV) materials also show great promise.

and in recent years their cost has plummeted,

They are inexpensive, flexible, readily processed into

thanks to lower materials costs and economies oflarge-area films, and cell efficiencies have improved

scale in factories. To spur the deployment of PV

steadily to more than 13%. Researchers are exploring

capacity, researchers need to develop solar cells thata wide variety of ways to improve their performance

generate electricity more efficiently, and that costfurther: OPVs are also being used with quantum dots,even less money and energy to manufacture. It isfor example, and inverted bulk heterojunction OPVs

equally important to reduce the overall system costs,

containing polymers and small molecules may also

since non-module costs dominate the cost of solar

become an important technology.

electricity, and this can be assisted by higher cell
efficiency and variations in module product form.

In all of these burgeoning PV materials, including
chalcopyrites and kesterites, there is a need for more

There are various ways to lower the cost of silicon

fundamental research. That is also true of novel andcells. Expensive components can be replaced withso far mostly unexplored multinary materials withcheaper alternatives: swapping silver electrodes

optoelectronic properties equivalent to perovskites.

for earth-abundant metals such as copper or nickel,

Besides important research on these emerging

for example. Researchers are working on newmaterials, scientists should search for new wide-

methods to prepare thinner silicon films, which do not

gap semiconductor materials that are needed forcompromise the electronic quality of the material. To

devices harnessing the full solar spectrum, by taking

boost their efficiency, silicon cells can also be teamed

advantage of novel synthesis and computer-aided

with other PV cells that absorb a different part of thematerials theory.

spectrum of light, forming so-called tandem or triple(multi-junction) cells.

One of the principal arguments for developing
alternatives to silicon PV cells is that their fabrication

Perovskite and wide-gap chalcopyrite cells areis very energy-intensive. In contrast, thin-film cells

examples of promising tandem partners. Perovskite

use less material, and are far less resource intensive.

cells have made rapid efficiency gains over the

Yet the vast investment in silicon PV manufacturing

past few years and should be relatively easy to

makes it difficult for new technologies to compete in

manufacture, although there are concerns about

the market. Smart specialisation in currently niche

their long-term stability and the lead content

markets may open doors for new technologies.

of the most promising perovskites. To makeFor example, many of the technologies discussed

6

CHALLENGES AND SOLUTIONS IN A CHANGING WORLD

needed.

in this report could be used in building-integrated
photovoltaics (BIPVs), an application that is expected

1. Target higher efficiency

to gain a huge market in the long term.

• Multi-junction devices — understanding, measuring
Researchers developing new PV semiconductors

and controlling interfaces is key

typically spend a great deal of time making and testing• Light management — taking into account real

the materials. Using computational models to predict

properties of sunlightthe properties of those materials could accelerate this

2. Material development

process considerably, by winnowing down the shortlist• Generates fundamental understanding with widerof materials to pursue in the lab. This computational

scope

approach must be underpinned by solid theoretical• Targeting application specific properties (BIPV etc)

foundations, along with internationally-agreed

3. New materials discovery

standards that rate the effectiveness of solar energy• Move away from empirical approaches

materials. Indeed, this strategy — building robust• Fusion of computational selection with experimentation

datasets and reliable theories to enable computational

including high throughput approaches

screening — should be applied to all of the solar• Development of screening tools / methods andenergy materials discussed at the CS3 summit. An

metrics (other than devices)extended ‘materials genome’ approach, based on

4. Sustainability

an better understanding of fundamental cause-effect• Energy efficiency/energy balance

relations and an improved control of materials design• Abundance

and manufacturing across the different length scales,• Manufacturability

could become a key tool for further materials and• Toxicity

device optimization.

Photonic materials
Researchers also need to consider the sustainabilityof the elements in their cells, and the CO2 emitted

Another way to improve the efficiency of solar energy

during the manufacture of a module. Rigorous life-

devices is simply to make better use of the lightcycle analyses can help to answer both of thesethat falls on them. This ‘photonic’ approach has thequestions. Ultimately, next-generation PV cells

potential to significantly improve PV cell efficiencies

should rely on readily available elements, have ausing low-cost technologies.

minimal environmental footprint, and boast a lifespanof 20 years or more. Crucially, they will only achieve

Changing the design of the solar cell, by strategicallycommercial success if they offer high efficiencies that

adding reflecting layers, for example, can increase

are comparable to current technologies.

efficiencies by a percentage point or more. Tiny
structures such as towers, spheres or pyramid

Summary: New PV materials with abundantelements

shapes on the surfaces of cells can scatter more lightinside. Other structures, such as nanopillars andnanowires, can help the semiconductor to absorb

New materials and fundamental understanding is stilllight more efficiently.

7

CHALLENGES AND SOLUTIONS IN A CHANGING WORLD

Meanwhile, devices called luminescent solar

Research on photonic materials could offer significant

concentrators can harness diffuse sunlight overbenefits to solar PV devices, but it may have an evena large area and funnel it down to a much smaller

greater impact on photochemical synthesis (seecell. This offers a very large light-collecting area

below).

for a relatively small cell size, potentially reducing
the materials intensity of the cell itself. One of theSummary: Photonic materials and photon
upconversion

more promising near-term applications for LSCs
involves using them in BIPVs, either in windows or as
decorative features.

1. Key metric: Radiative efficiency• Need to measure luminescence, identify pathways

Another approach aims to overcome an important

for relaxation

limitation of semiconductors: they can only absorb• Opportunity for characterization of electronicpart of the solar spectrum, so a lot of the sunlight that

relaxation and structural relaxation: vibronic relaxation

hits them is wasted. Materials known as upconverters(couple electronic relaxation measurements to

can help to avoid this waste by combining two low-

ultrafast structural dynamics using coherent free-energy photons to make a high-energy photon, whichelectron laser light sources)

can then be absorbed by the semiconductor.

• Need predictive theory for excited state lifetime anddecay

In all these cases, researchers are trying to develop
a deeper fundamental understanding of the energy-

2. Opportunity: Data-driven Discovery

transfer processes involved in order to create• Large theoretical and experimental databases canphotonic materials that are more efficient, long-lived

be interrogated with machine learningand inexpensive. Data-driven discovery programmes• Need databases of key materials properties (mobilities

are needed to accelerate the development of new

and excited state lifetimes)

photonic materials, and this work would be helped• Data driven discovery can also be applied toby well-maintained and accessible databases

photonic design and device designof materials properties, to enable computational
screening of the composition, shape and structure of

3. Direction: Focus on schemes for ‘Beyond-SQ

materials for optimum performance.

Limit’ Solar Energy• Photonic design for light harvesting: reduce

Manufacturers need cheap and scalable methods

materials cost of absorbers by light trapping (plasmonics,

to make photonic structures for light capture anddielectric light trapping)conversion. Various laboratory methods have been

• Upconversion, downconversion, downshifting: candeveloped, but one of the more promising techniques

be integrated with existing silicon solar PV; biggest

is 3D printing. Chemists and materials scientists canneed is new materials

help to develop the materials and technologies that

• Multijunctions face a choice: join Si (Si-based

could turn 3D printing into a practical manufacturing

tandem) or compete (requires new materials)

technology for photonic structures.

• IB band, MEG

8

CHALLENGES AND SOLUTIONS IN A CHANGING WORLD• Hot carrier collection for PV and PEC: key issue is tosignificant challenge, not least because it involves

collect carriers prior to relaxation

breaking the strong carbon-oxygen bonds in CO2 and• Thermophotonics (LSCs, solar heat engines)

form new carbon-carbon or carbon-hydrogen bonds.
All of these processes are multi-electron reactions,

Artificial photosynthesis and CO2 reduction

and are energy demanding. For direct air conversion,

Generating electricity is not the only way to exploit

pre-concentrate the CO2 to a local concentration forsolar energy. Sunlight can also be used to drive

catalysis that is significantly higher than 400 ppm.

it may also be necessary to develop materials toreactions that ultimately store energy within chemicalbonds, so that energy from the Sun can be used afterSome approaches use inorganic catalysts to speed

it sets. Researchers are developing devices such as

up these reactions; others use materials such as

photoelectrochemical cells (PECs) that can achievecarbon nitride or conjugated organic polymers; yetthis by splitting water to make hydrogen, for example.

others unite semiconductors with enzymes, or evenliving organisms, to create hybrid catalysts.

Yet current systems are too expensive and inefficient

Photoelectrochemical systems will have a betterfor commercial deployment. They need new light-

chance of widespread use if they are tailored to make,absorbing materials — and selective, effective

if needed, more complex higher-value molecules,

electrocatalysts — that are cheap, safe, efficient,

as well as simple fuels. Indeed, researchers should

abundant, selective and robust. Researchers needconsider the practicalities of these systems fromto understand, at a molecular level, how the catalysts

an early stage, assessing whether they have theinteract with their surrounding; and how chargepotential to be cost-effective and viable at large

and energy flow from a light-absorbing material toscales.

the catalyst. Research in these areas should help
to integrate materials and catalysts with atomic

To make truly significant progress in this area —precision.

for example, by photochemically converting CO2
to higher alkanes — there is a need for continued

Photoelectrochemical devices also need more

fundamental research that explores new concepts,

efficient light-harvesting materials that can standunfettered initially by overly narrow technoeconomic

up to the rigors of the water-splitting reactions. In

analyses, since it is unlikely that small changes tomany of these challenges, researchers can takeexisting systems will attain this goal.

inspiration from natural photosynthesis itself, whichuses a remarkable self-repairing catalyst made from

Photocatalytic systems are at an early stage ofabundant elements to oxidize water.

development, so there is still a need for basic
To minimize the impact of climate change, many

research on wide range of approaches. While it is too

researchers argue that we also need to develop

early for any detailed technoeconomic analysis to betechnologies that remove CO2 from the atmosphere

useful in this area, an evaluation of the net energy

on an industrial scale. But using photochemicalcosts of the conversion process will be useful inprocesses to turn CO2 into useful products poses a

determining their viability.

9

CHALLENGES AND SOLUTIONS IN A CHANGING WORLD

Summary: Artificial photosynthesis and CO2
reductionthese techniques, by sharing expertise, facilities, and

1. Water oxidation, CO2 reduction

Researchers also need to agree on the best ways to• Target molecules, larger C-containing molecules

measure the performance of solar energy materials,• H2 and thermal catalysis

in order to make fair comparisons between devices.

• CO2 reduction and N2 fixation

PV cells can be tested and certified at independent

best practice.

centres, but these services are not available for
2. Light harvesting, charge separation

photoelectrochemical cells, the catalysts they depend• band gaps, Fermi level and catalysts

upon, or upconversion/downconversion materials —nor have the most appropriate tests been developed.

3. Catalysis

In order to drive research along the most promising• New catalyst concepts, inspired by natural

paths, researchers need to agree on the best metrics

photosynthesisthat rate the performance of their devices.

4. Integration of multifunctional materials andOne option worth considering is for journals tovarious components with atomic precision

require researchers to provide standard performance• New aspects, including light manipulation andmeasurements for their solar energy conversion

modeling

systems, including solar cell efficiency and solar-tofuel performance. To make this practicable, it would

Methods and standards

be necessary to establish validated test centres,
as for photovoltaics, to verify these measurements

Solar energy research is underpinned by the methods

independently, particularly when reported results

used to prepare and analyze materials. Analyticalbreak existing records or introduce a fundamentally

methods provide the data needed to develop

new type of chemical reactivity.

new theories, and to identify the most promisingcomputational screening.

Summary: Photofunctional materials andstructures for light manipulation

CS3 delegates discussed a range of analytical

1. Development of experimental techniques for atechniques that could help researchers in this area,

better understanding of fundamental photophysicalparticularly when studying processes that take place

and photochemical processes at interfaces toon incredibly short timescales and atomic spatial

understand what determines efficiency in working

scales. Some of these techniques rely on large

devicesmaterials by using data-driven approaches such as

facilities such as free-electron lasers that produce
very short, intense pulses of X-rays or ultraviolet light.

2. Develop theoretical and computational approaches

All aspects of solar energy research can benefit fromto understand the complexity of photofunctional

international and interdisciplinary collaboration on

systems, from the molecular level of light-driven

10

CHALLENGES AND SOLUTIONS IN A CHANGING WORLDcharge transfer processes between solid andenergy clearly needs a broad base of scientificelectrolytes up to the device levelresearch. Some systems have value because of
their technological potential; others because of

3. Development of cheap and scalable methods

what we can learn about fundamental mechanisms.

for the creation of 2D and 3D structures at the sub-Nevertheless, researchers should keep an eye on

micrometer length scale to improve the efficiency ofthe potential commercial viability of their devices,light conversion systems

considering practicalities such as whether systems
will be cost effective, scalable and sustainable.

4. Explore opportunities to couple light energy into
And one size does not fit all. Each country has its

biological systems

own unique set of natural resources, energy systems
5. Develop standards, best practices, and databasesand economic circumstances. Using PV electricity tomake hydrogen by electrolysis may make sense inConclusions

one location; in other places, photoelectrochemicalcells will be more viable. The key message is thatCS3 delegates agreed that the path to a much greater

the world needs a range of solar energy options, anddeployment of solar energy technologies must include

funding should reflect that by supporting a variety ofa wide-ranging materials discovery programme, often

fundamental research and emerging technologies.

branded in the United States as a ‘materials genome’approach. Researchers need to consider, where

Multidisciplinary collaboration is increasingly

possible, potentially using computational selection

important in solar energy research. Discoveries

to help guide their experiments. This will help toabout PV semiconductors can often be applied todevelop a genuine synergy between theory andPECs, and vice versa; photonic strategies can help

experiment. High-throughput robotic discovery may

both. Consequently, the field needs to educate

also have an important role to play, particularly whenexperienced researchers and bring in early-careercoupled with computational selection (for example, a

researchers who are able to take a systems view

‘funnel’ approach). Researchers should use a rangeof the field, offering a broader perspective on solarof analytical methods to probe the mechanisms ofenergy problems that draws on many areas, fromsolar energy processes, rather than focusing on

physics, chemistry and biology to engineering andperformance metrics alone, thus building both large

other areas. The trend towards training students indata sets and fundamental understanding to driveinterdisciplinary centres gives cause for genuine

research forwards. Fundamental studies are more

optimism about the future of collaboration in this area.

vital than ever, to advance knowledge and to develop
more reliable predictive theories about key properties,
such as charge transfer across the interfaces and theIndeed, young scientists should flock to the field —actual detailed mechanisms of photocatalytic solar

after all, developing through robust fundamental

fuel production.

research solar energy technologies to tackle climatechange is one of the most important and inspiring
challenges that chemistry has ever faced.

This report is not about picking winners — solar

11

CHALLENGES AND SOLUTIONS IN A CHANGING WORLD

Chapter 1: Photovoltaics
The cost of photovoltaic electricity has fallen dramatically over the past decade, driving a
rapid increase in the world’s capacity to harvest solar power. This success story has reliedon silicon solar cells, but the technology may face limitations that will hinder the massive
capacity expansion needed to curb greenhouse gas emissions. Chemists, materials
scientists and physicists have a vital role to play in improving the performance of siliconcells, and developing the next generation of photovoltaic materials.

Introduction

in renewable energy, for example, and a gradual

Solar power is booming. The manufacture ofthose investments. More than 90% of PV cells use thephotovoltaic (PV) cells, which harvest light andsemiconductor material silicon, mostly in crystalline

convert its energy into electrical current, is now theform, and today’s commercial modules can convert

world’s largest optoelectronics industry. PV devices

about 20% of the light that falls on them into electricity

provide about 2% of global electricity, and total more

(in the lab, silicon cell efficiencies can top 26%).

increase in cell efficiency has improved the return on

than 300 gigawatts (GW) of power capacity (see Fig. 1.1),
which corresponds to about 75 GW of conventional

Arguably the most significant factor, though, has

power stations running around the clock. PV

been the dramatic fall in the price of PV modules:

manufacturing capacity has grown rapidly, especiallytypically, one watt-peak of solar power capacity now

in China, to around 70 GW per year.

costs less than US$0.5. This is largely due to lower
materials costs and economies of scale in Chinese

Solar power is booming. The manufacture ofmanufacturing plants — indeed, CS3 delegates heard

photovoltaic (PV) cells, which harvest light andthat a Chinese solar company recently released aconvert its energy into electrical current, is now the

21.5% efficiency PV module at a cost of less than $0.2

world’s largest optoelectronics industry. PV devices

per watt-peak, comparable to coal power.

provide about 2% of global electricity, and offered
more than 300 gigawatts (GW) of power capacity in

The blossoming of solar power is an astonishing and2017 (see Fig. 1.1), which corresponds to about 75

somewhat unexpected success story. A recent meta-

GW of conventional power stations running around

analysis found that previous predictions from thethe clock. PV manufacturing capacity has grown

International Energy Agency, the Intergovernmental

rapidly, especially in China, to around 70 GW per

Panel on Climate Change and other groups had

year.

consistently underestimated future PV growth.1 It now
seems possible that the world could get 40% of its

This growth has been propelled by a range of factors.

electricity from PV by 2050, potentially making a huge

Economic incentives have helped to drive investment

contribution to reducing humans’ CO2 emissions.

12

CHALLENGES AND SOLUTIONS IN A CHANGING WORLDYet that target presents enormous challenges. It

the economics of solar PV. Meanwhile, finding ways

would demand an additional 1000 GW of PV capacity

to use less silicon, in cheaper forms, could improve

to be installed every year from about 2030. It also

the industry’s sustainability while reducing module

demands major innovations in electricity storage,

costs, leading to wider use of solar PV.

such as using large batteries to smooth out the peaks
and troughs of solar power — a technology reliant

The longer-term challenge is to identify silicon’son chemistry and materials science, yet beyond theextension and/or successor technologies. This next

scope of this report as well as developments in power

generation of PV cells must be more efficient, lower

distribution systems and demand management.. A

cost, suitable for mass manufacture, and operate forpotentially more efficient solution is to convert the

decades — all with a minimal environmental footprint.

electricity into storable fuels, an approach that is

Improving silicon

Installed Capacity (GW)

discussed in Chapter 3.
300

Silicon itself makes up about 15% of the cost of a250

PV module. So using less silicon without hampering

200

performance, or using a cheaper semiconductor
altogether, offer routes to lower costs. Today’s PV

150

cells typically use a 120-150 micrometre-thick layer of100

crystalline silicon — any thinner, and the silicon tends

50
0

to be fragile. Thus creative solutions are needed torealize high-quality crystalline silicon films on novel

20072008 20092010 2011 2012 2013 20142005 2006
Year

substrates, or to develop cost-effective methods for

Figure 1.1: Global cumulative PV capacity
The International Renewable Energy Agency(IRENA), Renewable Energy Statistics 2017
http://bit.ly/2rH2b2M

epitaxial growth and subsequent lift-off techniques.
A second strategy is to replace other expensive

Despite the remarkable rise of silicon PV cells, morecomponents within the module. Silver electrodes candevelopments in cell technology are likely to beaccount for up to 20% of a module’s cost, so replacing

needed in order to reach that 2050 target. Achieving

silver with earth-abundant metals such as copper andhigher module efficiencies and lower-cost production

nickel could reduce prices.

will require new materials, new module designs, andThe cost-effectiveness of a solar PV module also

a better understanding of the fundamental processes

depends on how much electricity it can deliver.

involved in exploiting solar energy.

Broadly, that process is well understood: whena photon of light of sufficient energy hits the

In the near term, one of the key challenges forsemiconductor material in a PV cell, it excites achemists, materials scientists and physicists is tonegatively-charged electron across the bandgap into

improve the performance of silicon cells as muchthe conduction band, and leaves behind a positively-

as possible: even adding two percentage points tocharged ‘hole’ in the valence band. Electrons andsilicon PV efficiency could make a huge difference toholes then migrate to opposing electrodes, generating

13

CHALLENGES AND SOLUTIONS IN A CHANGING WORLDsuch as those based on perovskites, have only beena current and a voltage.

investigated in labs for a few years. Up till now, there
The maximum theoretical efficiency of this process

has been a lack of effort to develop high-energy-

is known as the Shockley-Queisser (SQ) limit. It

bandgap absorbers.

depends on a combination of factors, such as theDeveloping thinner films

cell’s ability to absorb only certain wavelengths oflight above the bandgap. Cells may be held back fromthe SQ limit by, for example, the loss of absorbed

Roughly 5% of installed PV capacity uses CdTe cells,

energy when electrons and holes recombine (measured

with typical efficiencies of about 22% in the lab andas the cell’s ‘external radiative efficiency’). If global

16% in commercial modules. The semiconductor

PV capacity is to grow by an order of magnitude,

layer itself is cheaper than that of a typical silicon

researchers may need to push solar cells closer

cell, because it employs a very thin film, rather than atowards (and perhaps even past) the SQ limit.

relatively thick layer of silicon.

One way to improve efficiencies relies on controlling

One measure of a PV cell’s environmental footprint

light. Certain coatings can change the wavelength

involves tallying all the material involved in the mining,of some of the incoming light, for example, so thatextraction, processing and assembly that is involved

the semiconductor can absorb more of the photons

in its manufacture — its ‘materials intensity’. CdTethat fall on it. Alternatively, materials that concentratecells have a materials intensity of 74 tonnes per GW;3diffuse light onto a PV cell can improve module

by contrast, silicon cells consume about 1000 tonnes

efficiency. These ‘photonic’ approaches are discussedof materials per GW.4 This is a powerful illustration of

in more detail in Chapter 2.

why PV cells can and must improve on silicon.

Another approach for boosting the efficiency of silicon

Yet CdTe cells must also contend with a drawback:PV cells is to add a second or third cell on top of it.

cadmium is toxic. In contrast, chalcopyrite (CIGSe

In these multi-junction cells, the top cell absorbs one

or CIGSSe) modules are not hampered by concerns

part of the light spectrum, and the unused light passes

about toxicity, and more than 5 GW per year ofthrough it before being absorbed by the lower cell.

production capacity is currently being installed in

Tandem cells and triple cells are already commerciallyChina.

available, but there is plenty of scope for improving
their cost and performance.2

One of the more recent arrivals to the family of thin-

Many of the potential tandem or triple partners forfilm cells utilizes a class of materials known assilicon use alternative semiconductor materials. The

perovskites. Since 2009, the efficiency of perovskite

semiconductor chalcopyrite CIGSSe (copper indium

cells has skyrocketed from about 4% to more thangallium sulfo-selenide) is already mass-produced in

22%.5 The cells can be created by cheap, low-

Japan and China, and its higher-bandgap cousin,

temperature solution processing, and the structure of

based on the sulfide only, is a suitable tandem

the perovskite compounds are relatively easy to fine-partner for silicon. Other potential tandem partners,

tune, enabling researchers to modify their optical and

14

CHALLENGES AND SOLUTIONS IN A CHANGING WORLD

electronic properties.

these materials, such as the relation between theirstructure, defects and performance, is necessary to

But the best-performing cells in this class all rely

boost efficiencies in the near future.

on lead-based perovskites, raising health andenvironmental concerns. There are also lingering

It will be hard for alternative PV materials to competequestions about the long-term stability of perovskites,

directly against commercial silicon modules.

which may hamper their commercial prospects.

Industry has invested around $800 billion in silicon

Nevertheless, there is much optimism that thesemanufacturing infrastructure, and that is not about toproblems can be overcome, and companies are

be usurped. But tandem cells offer next-generation

already developing production systems to makesemiconductors an opportunity to piggyback onperovskite cells in tandem with silicon. Indeed, thesilicon’s success, and establish a commercial footholdtunability of perovskites may eventually enable

that will enable further cost savings as manufacturing

perovskite-perovskite tandems, with each layer

processes mature. To improve the performance ofabsorbing different wavelengths.

these thin-film multi-junction devices, though, the cells
will need better interfaces between the materials, with

Most of the progress in perovskites, and other thincontrolled adjustments. That must be supported byfilm materials like chalcopyrites, has been based on

basic research to understand, measure and control

empirical work. CS3 attendees agreed that morethese interfaces.

fundamental research is needed to understandGoing organic

exactly how the perovskite cell works in order toboost performance further. In addition, an improved
understanding on the precise control of relevant

Another promising form of PV cell relies on carbon-

manufacturing processes is still missing, as it is forbased (organic) molecules such as polymers. Organic

many other novel thin-film absorber materials. All thisPV (OPV) materials have several advantages: they

research should run in parallel with efforts to improveare cheap, flexible, straightforward to make fromdevice integration and develop technologies that willreadily-available materials, and easy to process into

aid mass production.

large-area films. The molecules’ structures can beeasily tweaked to adjust their properties, and this has

Another promising, but less studied, class of cells

delivered steady improvements in cell efficiency, touses materials called kesterites, with efficiencies

over 13%.7that have risen from less than 1% in 1997 to 13.8%
in 2016.6 In addition, there are very many multinary

OPVs come in an array of colours, and can be used

elemental combinations that form semiconductors

as a tinted coating for glass that also generates

with the right absorption properties (bandgaps).

electricity. This offers the cells access to a niche

However, the increasing complexity and missing

market that silicon cannot fill. These include someknowledge in controlling their physical properties

aspects of building-integrated photovoltaics (BIPVs),

have so far hindered a systematic evaluation of theira market that is forecast to grow rapidly. However,PV promise. Addressing fundamental questions about

the performance of OPVs can decline by 20% over15

CHALLENGES AND SOLUTIONS IN A CHANGING WORLD

their lifetime, caused by poor long-term stability to

the excited electrons and holes, and even how theoxygen, ultraviolet light, moisture or temperature —orientation of polymer molecules in the film can be

all problems that currently prevent their use in BIPVs,

altered to improve performance (see Chapter 4).

but that basic research can help to solve. (BIPVs
need not be transparent: CIGS modules, for example,

Conclusions

are homogeneously black or coloured, and are very
To judge the true potential of new PV materials

appealing to architects.)

and devices, researchers must be able to compare
A further application for organic and inorganic

results between different systems. Consequently,

molecular systems are their integration with inorganic

reports of spectacular cell efficiencies must be

absorber materials in hybrid systems, as alreadybacked up by independent confirmation at national

shown to be valuable for contact formation in manylaboratories such as the US National Renewable

perovskite cells. Researchers are exploring a wide

Energy Laboratory (NREL). In addition, some journals

variety of ways to improve the performance of OPVs.

have a checklist of performance metrics that must be

They are also being used with quantum dots, forreported for PV cells, and this is a useful protocol thatexample, and inverted bulk heterojunction OPVs

should be adopted more widely to ensure that results

containing polymers and small molecules may also

can be assessed fairly.

become an important technology.
Efficiency is a critical way to judge the success of a PV
Mass manufacture is currently a challenge for OPVs,

cell, of course, but there are other factors to consider.

as for any other new PV material. Processes used

If the semiconductor is particularly cheap and easy to

to make high-quality films in the lab need to beprocess, or more reliable over a longer time span, it

translated to higher volume fabrication processes

could be preferable to slightly more efficient devices.

for commercial production, and this can reduce

Researchers also need to consider the sustainability

their efficiency. So researchers are investigatingof the elements in their cells, including the abundancealternative manufacturing processes that could beof the elements, how they are mined and purified,

used commercially, along with PV polymers that are

their toxicity, and the energy used in manufacturing abetter suited to mass manufacture. Small molecules

PV module.

and quantum dot materials also provide opportunities
It is also vital to consider the balance between thefor low-cost photovoltaics.

CO2 savings of deploying a particular PV technology,
Fundamental research questions lie at the heart ofand the emissions caused by manufacturing thethese stability, efficiency and processing issues.

cell. Different PV materials will pay back this ‘carbon

Understanding exactly how these problems arise isdebt’ in varying amounts of time. The field is moving

the first step to solving them. That includes figuring

so quickly that it would be helpful to have more up-out how the molecular structure of the polymer

to-date life-cycle analyses of the various competing

affects charge separation, how the distribution oftechnologies to understand their overall impact on the

charge within the polymer affects the movement ofenvironment.

16

CHALLENGES AND SOLUTIONS IN A CHANGING WORLD

Research into new semiconductor materials often

Recommendations

relies on time-consuming empirical research —essentially, building each new cell in the lab and thentesting it. Materials discovery could be speeded up by

1. Improve the efficiency of silicon-based cells

using computer modelling to predict the behaviour of(including multi-junctions).

new materials, calculating properties such as band

Researchers need to understand, measure andgap, optical absorption, or defect tolerance. This

control the interfaces between materials in multi-

sort of computational selection could help to directjunction devices; and develop materials andwhich materials are pursued in the lab, especiallystructures that can maximize light harvesting and

if researchers also develop new high-throughput

energy capture.

experimental screening methods.
2. Develop existing alternatives.
Computational models must be underpinned by

Further efforts to understand the fundamental

improved theories about the mechanisms involved insources of energy loss within cells will lead to higher

light harvesting and charge separation, particularly inefficiencies. Exploiting niche markets that could

materials such as perovskites and polymers. These

become transformative in the long term, such as

theories could also be highly relevant to research onbuilding-integrated photovoltaics, will help production

batteries or photoelectrochemical cells (see Chapter 3).

methods to mature, potentially lowering costs.

Even if alternative PV materials cannot beat

3. Discover new PV materials.

commercial silicon cells today, they are still worth

Researchers must move away from trial-and-error

pursuing. Perovskite cells, for example, would notresearch. Computational modelling of materials’

have experienced such a stellar rise if they had notproperties, high-throughput screening, along withbuilt on two decades of work on dye-sensitized andnew analytical methods and metrics, will all help toorganic solar cells.

accelerate materials discovery programmes.

The commercial success of silicon PV modules offers

4. Consider sustainability at all stages.

the first glimpse of a revolution in energy supply that

Even at the basic research level, researchers

could take place over the coming decades. But thatneed to think about the abundance, toxicity,

will only happen if researchers develop alternative PV

materials intensity, energy investment and potential

materials alongside silicon, while gaining a deeper

manufacturability of new PV materials.

understanding of their operation.

17

CHALLENGES AND SOLUTIONS IN A CHANGING WORLD

Chapter 2: Photonic materials
The efficiency of photovoltaic cells and other solar energy devices could be significantly
improved by manipulating the light that falls on them. Various materials can be used tofunnel more sunlight into cells, for example, or alter the spectrum of the light to improveenergy conversion. Data-driven discovery programmes will help to develop cheap andefficient photonic materials for these tasks.

Introduction

to develop new photonic materials that are efficient,
long-lived and inexpensive.

Much of the research effort to improve photovoltaic (PV)

Redesigning the cell

devices has focused on semiconductor materials
that capture light to generate an electrical current.

One of the most important ways to move PV cells

Increasingly, though, there is a growing focus on

towards their SQ limit is to improve their externalmanipulating light itself, as it falls on the PV cell andradiative efficiency. After incoming photons generate

travels through it.8

electrons and holes within a semiconductor material,
those charge carriers often recombine before theyThis ‘photonic’ approach has the potential to

can reach the cell’s electrodes. Recombination may

significantly improve PV cell efficiencies using low-release a photon that can generate more electron-

cost technologies, and in some cases may push cells

hole pairs elsewhere in the semiconductor, apast the Shockley-Queisser (SQ) efficiency limit (seeprocess called photon recycling; or it may lead to

Chapter 1). It should also benefit other solar energy

non-radiative processes, which often simply heattechnologies, such as photoelectrochemical cells (PECs

the semiconductor material. The external radiative— see Chapter 3).

efficiency of a cell essentially measures how often
recombination leads to beneficial photon recycling.9

In some cases, redesigning the internal structureof a PV cell can make better use of the light within

Meanwhile, a new design of silicon PV cell has

its semiconductor layer. Miniature structures on the

become increasingly popular in recent years (seesurfaces of the cell can help to channel more light

Fig. 2.1). This so-called Passivated Emitter Rear Cell

into its active layer. And luminescent materials can(PERC) includes a ‘passivation’ layer at the bottom

concentrate the Sun’s rays to increase the cell’sof the cell, and a smaller contact area between the

power output, and also alter the wavelength ofrear metallic coating and the silicon wafer. Along withincoming light so that the semiconductor is able toother tweaks to the cell’s architecture, this helps it toabsorb more photons.

absorb more incoming light; reflects light back into
Although researchers have seen promising results inthe semiconductor material that would otherwise

all of these areas, the main challenge for the future isescape; and reduces energy-wasting recombination

18

CHALLENGES AND SOLUTIONS IN A CHANGING WORLD

Total Si Capacity, %100%

to capture more light. Small towers, spheres or

80%

Si-Tandem

pyramid shapes can act as anti-reflective coatings

60%

Rear J

that scatter more light into the cell, for example,

40%

HJT

which improves the device’s efficiency (see Fig. 2.2).

20%

PERC

Other structures, such as nanopillars and nanowires,can absorb incoming light and generate an electric

Al-BSF
25

22

19

17

15

field that increases photon absorption within the

20

20

20

20

20

20

14

0%

semiconductor.

Figure 2.1: Passivated Emitter Rear Cells (PERCs)
are becoming increasingly popularInternational Technology Roadmap for Photovoltaic(ITRPV) 2016 Results http://bit.ly/2DJCmVp

But one of the key challenges in this area is tounderstand which materials and shapes offer

pathways. This design typically adds one percentage

the best performance, and how best to arrange

point to the efficiency of a cell, and more than half ofthem on the surfaces of the cell. Building on that

all PV cells manufactured in 2020 are expected to beknowledge, researchers also need to develop

PERC cells.

technologies for cheap, large-scale production ofthese nanostructures. Various laboratory methods

Texture the surface

have been developed: colloidal particles on the

Researchers and PV manufacturers are able towhile plastic molds and 3D printing are also used to

texture the surfaces of their cells with tiny structures

produce nanostructures.

surface of the cell can act as a template, for example,

Figure 2.2: Surface structures can help to draw more light into PV cells
J. Zhu et al, Nano. Lett., 2009, 9, 279.

3D printing, which is now widely used inchemicals at precise points in space. Another process

manufacturing, builds complex shapes using

called interference lithography can offer similar

a computer-controlled dispenser that deposits

results by using laser light to etch patterns into small

19

CHALLENGES AND SOLUTIONS IN A CHANGING WORLD

structures. These processes enable gratings or holes

because their surface area catches more light — but

to be patterned into tiny pyramids or cones, which

they also use more materials, which makes them

could make them more effective at scattering light into

more expensive and resource-intensive. One way tothe cell.

circumvent this problem is to use a luminescent solar
concentrator (LSC), a large panel that gathers lightand funnels it into a much smaller solar cell.

Although 3D printing is unlikely to be commercially
viable as a way to create these structures on alarge scale within the next 5 years, the technology

Plastic LSC panels contain chemical compounds

is progressing very quickly. Chemists and materials

called luminophores, which absorb incoming lightscientists can accelerate that progress by developing

and then re-emit it. This re-emitted light travels along

materials that are suitable for printing, and studying

the LSC towards a solar cell that is typically mounted

how their shape and structure affects light absorption.

along the edge of the panel. This offers a very large

Researchers also need to develop a fundamental

light-collecting area for a relatively small cell size,

understanding of the processes involved in energy

potentially reducing the materials intensity of the celllosses at the interface between the photonic structure

itself (see Chapter 1). LSCs can also exploit diffuse

and the semiconductor. Based on this knowledge,

light, such as indoor lighting or the weaker sunshine

it is already possible to computationally screenof overcast northern skies. Crucially, the LSC should

the shapes of photonic structures, as well as theuse cheaper elements than a PV cell, and be easiermaterials themselves, to optimize their performance,

to manufacture.

and this is a key area for future advances.
The LSC concept has been around for a long time,
but these devices need to concentrate light more
efficiently in order to be commercially viable. To
achieve that, researchers are already developing
materials, and new LSC designs, that are able tointernally reflect re-emitted light at more obliqueangles, to improve energy transport to the PV cell.11
Chemists are also developing a wide range ofluminophores that can absorb and emit light
more efficiently and with low reabsorption losses.
These include quantum dots that emit across a

Figure 2.3: A luminescent solar concentrator (LSC) is a large
panel that gathers light and funnels it into a much smaller solar
cell (black band, right)
V. Ferrybroad spectral range, as well as dye molecules.12
Luminophores must be non-toxic, and remain stable
over the lifetime of the plastic panel, potentially 20

Catching the lightyears or more. Since the plastic panel itself may

Larger solar panels can produce more electricity

polymers would also help to commercialize LSCs.

degrade over that length of time, more durable

20

CHALLENGES AND SOLUTIONS IN A CHANGING WORLDOne of the more promising near-term applications

and photocatalysis.

for LSCs involves using them as building-integrated

Some upconverters are based on nanocrystals

panels, either in windows or as decorative features.

containing lanthanide elements, such as erbium andWindows are often tinted by adding a plastic layer

ytterbium. Ytterbium absorbs infrared light, and thenbetween two sheets of glass, and if that could betransfers the energy to erbium, which emits green

replaced by a LSC that channels light to a PV celland red light. But these systems generally need to beat the edge of the window, it could provide useful

illuminated with bright light to upconvert efficiently,

power. CS3 delegates heard that glassmakers

and it can be difficult to fine-tune the materials’

are increasingly interested in this approach, andcomposition so that they emit a particular wavelength

the efficiency of the system does not have to beof light.

particularly high for the window to eventually ‘payfor itself’ with the electricity it generates, as long as it

Certain organic molecules can also act as

does not adversely affect the optical properties of theupconverters. Light is absorbed by ‘sensitizer’window.

molecules, which ultimately transition into a longerlived form known as a triplet state. This energy thentransfers to ‘emitter’ molecules, which can comeChange the spectrum

together to release their energy as a single photon in
Semiconductors can only absorb light within a certaina process called triplet-triplet annihilation (see Fig.

range of wavelengths — the range depends on the2.3). Researchers working on these upconverters

material — and this wastes the energy carried byaim to develop materials that absorb as much light

light of other wavelengths. But some materials canas possible, upconvert it with no energy loss, andtake two low-energy photons that are not absorbed

then emit as much energy as possible at a shorterby a PV semiconductor, and convert them into awavelength. Promising strategies include extending

single higher-energy photon that can be absorbed.

the lifetime of excited triplet states, speeding up

This process, called upconversion, allows a PV cell

energy migration, and avoiding processes thatto exploit a broader swath of the spectrum of light.

deactivate the triplets. Another hot topic in this area

Upconversion of infra-red to visible light is particularlyis ‘singlet fission’, in which a singlet excited state

important, because it could open up access to a large

generates two triplet states.

chunk of the spectrum that is generally not used by
One of the key challenges is to ensure that two

PV cells.

energized emitters can actually meet. Some systems
Upconversion can theoretically increase the SQ

have relied on emitter molecules that diffuse throughlimit of a silicon cell from about 34% to 44%. Today’sa polymer until they collide with each other, but

upconversion technology could be improved bythis diffusion process tends to be slow, and many

developing better upconverter materials. Moreemitters lose their energy before they meet aefforts are required to improve the efficiency ofpartner. A potential solution to this problem would(near) infrared-to-visible as well as visible-to-UV

be the control of energy diffusion by controlling theupconversion, which is more directly relevant to PV

structural arrangement of the materials. For example,

21

CHALLENGES AND SOLUTIONS IN A CHANGING WORLD

it is possible to use platinum complexes to capture

theoretical efficiency limits. Researchers should

activated emitters and hold them in position, until

develop the materials, screening techniques andenough emitters accumulate around the particle totheoretical understanding needed to commercialize

undergo triplet-triplet annihilation.13 An alternative

photonic approaches.

approach does not require emitter molecules to movearound at all — instead, energy passes along a relay

2. Use a range of photonic tactics.

of emitters until two neighbours are energized, andDevelop materials that retain more light within theable to join forces to emit a photon.14

semiconductor layer and improve their externalradiative efficiency. Channel more light into the cell,

In principle, upconversion technologies might add atusing surface nanostructures and luminescent solar

least one percentage point to the efficiency of siliconconcentrators. Upconvert or downconvert unused

cells. They could have an even greater impact on

wavelengths of light to help semiconductors absorb

photochemical synthesis, which uses light to drive

more photons.

chemical reactions that make fuels and other usefulproducts (see Chapter 3). For example, it may be

3. Discover new photonic materials.

possible to upconvert visible light into ultraviolet

Develop cheaper, more efficient photonic materials.

light, which could provide a titanium dioxide photo-

Create large, well-maintained and accessible

electrode with sufficiently energetic photons todatabases of materials’ properties to drive discovery

oxidize water and produce hydrogen gas in a

programmes, with the help of machine learning andphotoelectrochemical cell.

high-throughput screening. Screen materials for key
properties such as luminescence. Improve theoretical

In other situations, the reverse process —understanding of photonic processes such as

downconversion of an ultraviolet photon into two

upconversion/downconversion.

visible-light photons — may also be useful for some
4. Find cheap and scalable methods to makeforms of solar energy harvesting.

photonic structures.

Recommendations

Researchers should develop the materials andmethods needed to mass-produce photonic

1. Manipulate light to boost the efficiency of PV

nanostructures, including techniques such as 3D

cells and other solar devices.

printing, in order to commercialize light-management

Photonic materials can help PV cells to exceed theirstrategies.

22

CHALLENGES AND SOLUTIONS IN A CHANGING WORLD

Chapter 3: Artificial photosynthesis and CO2 reduction
Sunlight can be used to drive chemical reactions, such as splitting water to make hydrogengas. This could offer a much more efficient way to store solar energy than feeding batteries
with photovoltaic electricity. To make these ‘artificial photosynthesis’ systems commercially
viable, though, researchers must develop new light-absorbing materials and catalysts, andunderstand precisely how they work.

Introduction

our food — without photosynthesis, humans could not

Despite the huge improvements in photovoltaic

exist on Earth.

cells over the past few decades (see Chapter 1),

Photosynthesis is already used to store energy instoring the electricity they produce remains a major

liquid transportation fuels. The ethanol that powers

challenge. Even state-of-the-art batteries are ancars in Brazil, for example, is produced by fermenting

inefficient, expensive and resource-intensive way

sugars from sugar cane. But storing solar energy inthis way uses vast areas of land, competes with foodto stockpile energy — although the costs of lithium

crops for resources, and is most effective in tropicalion batteries, for example, are falling sharply. On

climates.

average, a battery weighing one kilogram can store
up to 0.5 megajoules (MJ) of energy. That stands in

It is possible to store the Sun’s energy in hydrogenstark contrast to compressed hydrogen, which canby using photovoltaic cells to generate electricity,

store up to 140 MJ per kg. Hydrogen is also versatile:which can then split water by electrolysis. But thisit can be used as a fuel, or reacted with othertwo-step process may introduce inefficiencies, andcompounds to make methane, ammonia, liquid fuels

electrolysers are relatively expensive. So researchers

and a host of other essential chemicals.

are developing artificial photosynthesis systems thatmimic aspects of the natural process, while potentially

Storing energy in chemical bonds is hardly a newbeing much more efficient.

idea — after all, that is how nature hoards solar
energy during photosynthesis, when plants use

Artificial photosynthesis harvests sunlight to separate

sunlight to react carbon dioxide with water to makeelectrical charges in a material, and then uses thosecarbohydrates and oxygen. On a sunny day, one

charges to drive catalytic processes that store energy

mature tree can process 10,000 litres of CO2,

in chemical compounds. Some systems simply split

producing 12 kg of carbohydrates and enough oxygen

water to produce hydrogen and oxygen, while others

to meet the needs of five humans. Globally, about 150

go on to react that hydrogen with CO2 to produce

billion tonnes of carbon is captured by photosynthesis

more complex compounds. The challenge forevery year, according to estimates presented by CS3

chemistry is to develop light-absorbing materials thatdelegate Wolfgang Lubitz. It is the primary source of

can drive these processes, and catalysts that make23

CHALLENGES AND SOLUTIONS IN A CHANGING WORLD

them proceed at a practically useful rate.

to make hydrogen gas (H2). Both electrodes havecatalysts on their surfaces to accelerate these

Splitting water

reactions, and these materials are typically formed
into a thin layer that allows light through to the

One of the most promising approaches to artificial

semiconductor beneath. The catalysts need to be

photosynthesis is the photoelectrochemical cell

cheap, safe, efficient, abundant, selective and robust.

(PEC). The cell contains two electrodes, immersed
in water that contains electrolytes, and separated by

The semiconductor in the HER electrode is often

a semi-permeable membrane (see Fig. 3.1). One —silicon, although cadmium selenide and copper

sometimes both — of these electrodes is made from aoxides are also used. Texturing the surface of thesemiconductor material that harvests light to separate

semiconductor can help to increase the surface area

electrons and holes, just like a photovoltaic device (seefor the HER catalyst and capture more light, improving

Chapter 1).

the overall efficiency of the system.15 Meanwhile, theOER electrode may use light-absorbing materials

The holes drive an oxygen evolution reaction (OER)

such as titanium dioxide, iron oxide, bismuth

at the cell’s anode, which splits water into oxygen

vanadate,16 or tantalum nitride,17 amongst others. In

gas (O2) and protons (H+). Meanwhile, electrons

both cases, a range of HER and OER catalysts are

drive a hydrogen evolution reaction (HER) at the

being investigated, often using materials based oncell’s cathode, with protons and electrons combining

cobalt, molybdenum, and nickel.18

Figure 3.1: Artificial photosynthesis systems can be ‘wireless’, composed of from asandwich of materials (left), or ‘wired’ to create a photoelectrochemical cell (right).
I. Roger et al, Nat. Rev. Chem. 2017, 1, 0003.

In theory, the minimum voltage needed to split water

overpotential, is needed to make the reaction run

under standard conditions at room temperature

at a practically useful rate. Semiconductors that are

and one atmosphere of pressure is 1.23V (perable to supply this additional voltage tend to absorb

electron). But an additional voltage, known as theshorter wavelength light, which limits the range of the24

CHALLENGES AND SOLUTIONS IN A CHANGING WORLDsolar spectrum they can absorb and reduces their

methods to measure, understand and control charge

efficiency. This trade off is one of the key challenges

separation processes in order to improve molecularfor PECs, and there are several potential solutions.

catalysts, materials, and assemblies thereof (seeChapter 4).

One approach is to use more efficient catalysts.

Photocatalysis

By speeding up the reaction, they reduce theoverpotential that is needed to drive the watersplitting processes at an acceptable rate. For

Many researchers developing photocatalysts areexample, a tantalum oxynitride photoanode canlooking to nature for inspiration.22 Plants contain aperform photoelectrochemical water splitting underprotein called photosystem II, which uses the energy

visible light. It relies on cobalt oxide nanoparticles

from light to convert water into dioxygen and protons.

to scavenge photogenerated holes and suppress

The protons are then used in a variety of otherdeactivation on the surface of the tantalum oxynitride.

chemical processes in the cell, including one that

Connected to a platinum counter-electrode, theultimately turns CO2 into carbohydrates. At the heart

photoanode can generate dioxygen with a relativelyof photosystem II is a catalytic structure containing

low applied voltage.19

four manganese ions that bind and cleave twomolecules of water, as well as a calcium ion to help
guide the water into place.23

The best PECs using photoanodes and/or
photocathodes have a solar-to-hydrogen conversion
efficiency of about 2.5%.16, 17 Meanwhile, using high-Photosystem II offers many important lessons

performance triple-junction solar cells to generate

for chemists trying to replicate photosynthesis.

electricity, and then feeding that power into a water

Manganese and calcium are cheap, abundant

electrolysis system, can achieve a solar-to-hydrogen

elements. The catalyst avoids high-energy

conversion efficiency of more than 20%.20

intermediates, and big energy jumps, as it breaks
water apart. It binds water molecules in a way

However, higher efficiencies are expected for better-that produces oxygen more efficiently. Its catalytic

adjusted multi-absorber structures. Energy can often

structure is embedded in a protein that manages the

be lost as charge travels between the materials,

flow of inputs and outputs, and protects the catalyst

and solving that problem will require a basic

from degradation. The catalytic structure also self-

understanding of the transport processes going on at

assembles, and self-heals when it breaks down.

the interfaces.

That is an impressive resumé for any catalyst, andchemists must learn all they can about every step inTiny variations in the structure of photoactive

photosystem II’s catalytic cycle if they are to replicate

materials can have a big impact on their ability to

it successfully in an artificial system.

separate electrons and holes. Seeding materials
such as titanium dioxide with dopants can help, as

Researchers are also investigating catalytic organic

can optimizing the shape and crystal structure ofmaterials such as polymers.24 One advantage ofcatalyst particles.21 But researchers still need betterusing an organic catalyst is that researchers can25

CHALLENGES AND SOLUTIONS IN A CHANGING WORLDeasily tweak its structure to modify its properties, forremain a barrier to commercialization.

example. Whereas organic catalysts are generally
tailored to perform either the HER or OER, very

The carbon challenge

recently Chinese researchers reported that a
To minimize the impact of climate change, it ispolymer called PTEB could catalyze both HER andHowever, there are significant challenges in

not enough to merely curb our greenhouse gas

understanding the catalytic reaction mechanism inemissions: many researchers argue that we need tothese systems, as well as improving their long-term

develop chemical technologies that remove CO2 fromstability under operating conditions.

the atmosphere on an industrial scale. In general, that

25

OER.

involves ‘reducing’ CO2 molecules, by substituting
Artificial photosynthesis systems can have a ‘wireless’

their oxygen atoms for hydrogen or carbon atoms, forconfiguration, combining HER catalyst, photovoltaic

example. These chemical reactions can produce fuels

materials and OER catalyst into a single sandwich

or other useful chemicals. For example, ruthenium-(see Fig. 3.1). These can be used as a photocatalytic

based catalysts can turn CO2 into syngas, a mixture

sheet, often described as an ‘artificial leaf’, or free-of carbon monoxide and hydrogen that is used infloating particles.27

major industrial processes to make ammonia, fuels26

and other products.30
One way to harvest a wide range of visible light
involves using two different photocatalysts connected

Using materials that combine different catalytic

by a ‘shuttle redox couple’ — a chemical reaction

properties can also help. A highly stable zinc oxide–that can transfer electrons from one photocatalyst

zirconium oxide catalyst can convert hydrogen andto the other. For example, an iodate-iodide redox

CO2 into methanol, for example, because each metal

couple can be used with niobate and tungsten trioxide

plays a different catalytic role in the whole reaction

photocatalysts to generate hydrogen and oxygen

sequence.31

gases under visible light.28
It should be noted that because CO2 is very stable,
A major challenge for PEC water splitting is toconverting CO2 into fuels costs energy and cannot

efficiently separate charge within semiconductor-

contribute usefully to carbon emissions mitigation

based photocatalysts. That could be achieved inwhen fuel synthesis costs more than the energy thatcertain crystals, such as bismuth vanadate, if theis finally supplied by burning the fuel.

oxidation and reduction reactions involved in water
splitting occur on different facets of the crystal. BY

One of the remarkable aspects of natural

selectively depositing different cocatalysts on eachphotosynthesis is that it draws CO2 directly fromface of the crystal, photogenrated electrons and holes

the air, even though the concentration of the gas
in the atmosphere is 400 parts per million. The

29

can be separated to different faces of the material.

prospects of developing a synthetic catalyst that
The efficiencies of these systems are gradually

can process CO2 directly from air are still remote,improving, but the cost of the materials involved may

not least because many of these catalysts cannot

26

CHALLENGES AND SOLUTIONS IN A CHANGING WORLD

operate in the presence of oxygen. But roughly 10%

into methane; and by introducing indium phosphideof anthropogenic CO2 emissions come from heavy

photocathodes and titanium dioxide photoanodes,

industries such as steel or cement works, where thethe system can be fully solar-powered.35

CO2 output is much more concentrated, and thus

Conclusions

a more promising feedstock for catalytic systems.
Indeed, porous materials called covalent organic
frameworks can already help to concentrate CO2 for

PEC systems that split water to produce solar

catalytic reduction.32

hydrogen have made good progress over the past
decade. Researchers must now make these systems

Another interesting option is to create artificial

more durable, using cheaper and more abundantphotosynthesis systems by uniting semiconductors

materials, and more suitable for scale up. PECs alsowithorganisms.

need to be more efficient, and this could be achieved

Semiconductors are good at capturing light andnot only by developing new materials — following

separating charge, while the enzymes in biologicalsimilar strategies as for advanced PV systems — butsystems are highly selective and efficient catalysts.

also by understanding how energy losses can occur at

Combining the two into a semiconductor-enzyme or

the interfaces between those materials (see Chapter

semiconductor-organism hybrid could, in principle,1). Photonic techniques, such as upconversion, could

offer the best of both worlds.

also play a key role here (see Chapter 2), as could aenzymesor

evenlivingcombination of computational and high-throughput
For example, a light-absorbing ruthenium complex

screening techniques. This work must be underpinned

can deliver electrons to a hydrogenase enzyme,

by basic research to understand the mechanisms ofwhich unites the electrons with protons to produce

light harvesting, charge separation, charge transfer,More recently, researchers haveand catalytic processes in molecules and materials.

used nanocrystals of semiconductors to drive

Natural photosynthesis should continue to serve as a

enzymes such as hydrogenases and nitrogenases.34

source of inspiration for new catalyst concepts.

hydrogen gas.

33

This general approach could be applied to a wide
range of chemical reactions, potentially opening up

At translational stages researchers must also

solar-powered routes to high-value chemicals.

consider the practicalities of these systems froman early stage, assessing whether they have theOutside of their native cellular environments, however,potential to be viable in terms of energy investment

enzymes are often fragile. Moreover, extraction ofas well as cost, and whether they are achievable

pure enzymes may not be scalable or economical. So

at large scales. Technoeconomic and energy cost

semiconductor-organism hybrids have recently been

analyses can help to assess the costs and benefits

explored as an alternative approach. For example,of various solar technologies, to understand how

the bacterium Methanosarcina barkeri has been

photoelectrochemical approaches compare with, forteamed with platinum or nickel sulfide, which are

example, photovoltaics.

biocompatible hydrogen evolution reaction (HER)
Chemically reducing CO2 remains a huge challenge,

electrocatalysts. Together, they can convert CO2

27

CHALLENGES AND SOLUTIONS IN A CHANGING WORLDbut it is a key frontier for science because it has thefor water-splitting and CO2 reduction that are cheap,

potential to be such a transformative technology,

safe, efficient, abundant, selective and robust.

allowing this greenhouse gas to be removed from

2. Study these materials at the molecular level

the atmosphere. Chemical systems that reduce

Understand their chemical, structural, and electronic

CO2 will have a greater commercial viability if they

properties; probe water oxidation and CO2 reduction

produce higher-value molecules. The energy costs

mechanisms in more detail using all the advancedof CO2 reduction may, however, limit their potential toexperimental tools available, from synchrotrons tocontribute to CO2 emissions mitigation at scale whenultrafast lasers, to materials analysis with atomic

compared with alternative mitigation routes. Similarly,

precision. A directed effort to achieve synergy

finding ways to use the oxygen produced from water-

between theory and experiment is essential here.

splitting, as well as hydrogen, could add value to the

3. Improve the integration of photoactive andprocess and effectively lower the cost of the hydrogencatalytic components.

it produces.

Understand and control the interfaces between
these materials to minimize the loss of voltage and

It is also important to bear in mind that ‘one size does

current in integrated systems. Develop cheap andnot fit all’. Countries have significantly different naturalcommercially viable deposition techniques to control

resources and energy systems, and there is certainly

the structures of materials and interfaces with atomic

not a single, global solution to harvesting solar power.

precision.

Research funding should reflect that by supporting a

4. Compare the costs and benefits ofbroad range of fundamental research and emerging

photoelectrochemical systems.

technologies.

Life-cycle analyses and analysis of energy balanceand potential scale will help to identify most promising

Recommendations

technologies to pursue, especially when coupling
different components to existing devices such as

1. Develop new photoelectrochemical materials.

photovoltaic cells. For systems that convert CO2

Researchers should continue to seek light-absorbing

into other products, focus on making more complex,

molecules, assemblies and materials and catalysts

higher-value molecules as well as C1 products.

28

CHALLENGES AND SOLUTIONS IN A CHANGING WORLD

Chapter 4: Methods and standards
Analytical techniques provide the vital data needed for solar energy research to makeprogress. Researchers can use these data to accelerate research, by exploiting
approaches such as the computational screening of materials. They must also collaborate
to agree on common standards for measuring the performance of solar energy devices, sothat the most promising avenues of research are easier to identify.

research could proceed much more quickly byIntroduction

exploiting computational screening — running ‘virtual
Delegates at the CS3 summit were clear that newexperiments’ on novel materials to find promisingmaterials, along with better theories to explain andcandidates before actually synthesizing them in thepredict their behaviours, are absolutely critical tolaboratory. Meanwhile, novel preparative methods

achieving a major increase in the use of solar energy

can help researchers to fine-tune the properties oftechnologies.

these materials, or develop solar energy devices thatare more amenable to mass manufacture.

These two factors — experiment and theory — are
intimately connected, driving each other forwards inAnalytical and preparative techniques are often

a virtuous circle. Experimental results help to shape

applicable across a wide range of scientific fields,

new theories, and these theories offer guidance andand interdisciplinary collaboration can provide an

fresh challenges for experimentalists. However, thisimportant way to improve and then apply thesesynergy depends on another crucial component:

techniques to new problems. Researchers also

experimental techniques. Preparative techniques

need to share expertise and best-practice in theseenable researchers to make new materials andmethods, along with common standards so thatmodify their properties, while analytical techniques

experimental results can be compared.

allow scientists to measure those properties.
Innovations in both help research to leap forwards.

Measuring up

Solar energy research demands a diverse array of

The fundamental processes of solar energy devices,

analytical methods, capable of taking measurements

such as light absorption, charge separation andat many different timescales and spatial scales.

charge transfer, depend on many different properties

The data provided by these techniques can helpof the materials involved, and different techniques areto uncover the basic mechanisms that determine

used to measure each one.

the performance of solar energy devices. They
can also help to make theories more predictive,

Experimentalists can use time-resolved microwave

enabling researchers to develop materials in a

conductivity to measure the mobility of charge

more strategic way. For example, solar energy

carriers, for example, and to assess how that29

CHALLENGES AND SOLUTIONS IN A CHANGING WORLDis affected by the orientation of molecules in amaterial.

36

Electron diffraction offers another way to determineThis can help researchers to design new

how structures change when they absorb light.

materials with better charge mobilities for photovoltaic

Recently, researchers used a powerful ‘electron

applications, and study how charges move at thecamera’ at the SLAC National Accelerator Laboratoryinterfaces between materials.

in California to show that when light hits a perovskite
material (see Chapter 1), it deforms the lattice of leadIn artificial photosynthesis systems, researchers

and iodine atoms in the crystal, potentially helping

need to be able to track how tiny variations in theelectrical charges to move through the material.39

structure or composition of a catalyst affect its

This could explain why certain perovskite cells are so

performance. Take titanium dioxide, which is a widely

efficient, and point scientists in the right direction toused photocatalyst that absorbs ultraviolet light (seeimprove that performance.

Chapter 3). Seeding crystals of the material with otherelements — a process called doping — can shift the

Another technique, electronic spectroscopy, canspectrum of light that it absorbs. But to control thisreveal how the structure of a molecule, such as aeffect, researchers need to work out which dopants

protein, changes as it absorbs light,40 and it has been

to use, where to place them in the crystal, and in what

used to study how the structure of molecules involved

concentration and chemical state. Techniques suchin natural photosynthesis affects their function.

as electron microscopy can help to answer thesequestions with atomic precision. Meanwhile, spatially-

Advanced surface science techniques allowing in-resolved surface photovoltage spectroscopy cansitu or even in operando experimental studies of

reveal how electrons and holes separate to different

intermediates and their electronic properties at solid/

parts of a photocatalyst.

37electrolyte interfaces are under development at many
synchrotron sources. These techniques, together with

Photochemical processes often happen at ultrafast

theoretical electronic structure calculations, will allow

timescales, taking just nanoseconds (10-9 s, or

researchers to identify which electronic structures andbillionths of a second) or even femtoseconds (10-15

electronic states are essential for efficient electrons, or billionths of one billionth of a second). Working

transfer processes.41

at these timescales, sensitive instruments canfreeze-frame the action to capture the vibrations of a

Finally, two-dimensional optical spectroscopy canmolecule, or the trajectory of an electron. Take X-ray

measure how the electronic as well as the vibrational

crystallography, for example, which has been usedstates of a molecule change as it absorbs a photon.

for a century to determine the structure of crystals,

The technique first excites the molecule with one laser

revealing precisely where its atoms sit in space. Morepulse, known as the ‘pump’; and then follows it anrecently, time-resolved X-ray crystallography has

instant later with another pulse, known as the ‘probe’,

been able to produce a series of snapshots showing

to measure how the molecule has changed. Recent

how a molecule’s structure changes after it absorbs aadvances include ultrafast multipulse experiments thatphoton, revealing how a catalyst’s structure affects its

allow researchers to follow energy propagation along

behaviour in a photocatalytic reaction.

38

the molecules in real time.42

30

CHALLENGES AND SOLUTIONS IN A CHANGING WORLD

Researchers have used these types of multipulse

with the right absorption properties (bandgaps).

techniques to study, for example, how solar energy

However, the increasing complexity and lack ofis converted to electricity in PV devices that include

knowledge in controlling their physical properties

a light-absorbing dye molecule,43 and the techniquehave so far hindered a comprehensive evaluation of

can be applied to a wide range of other systems.

their PV potential, following a systematic materials

This brief survey offers a mere glimpse of the

genome project of advanced solar converter systems.

techniques that can be used to build up large datasetsof materials’ properties. These datasets can help toThis sort of computational design is much harderhone theories and improve computational screening,for polymer-based materials, however, because thewhich promises to be an extremely powerful way tomechanisms of charge separation and transport inaccelerate materials discovery programmes. It may

these systems are not fully understood. In situations

also be possible to use machine learning to tease out

like this, where researchers cannot make reliablepredictive patterns in these data sets.

predictions about a material’s properties, they can
speed up materials discovery using an approach

Computational screening relies on models thatcalled combinatorial chemistry. This method is

can accurately predict properties such as bandalready widely-used in drug development, and uses

gap, optical absorption, defect tolerance, charge

robots to analyze large libraries of different materials

mobility and more, based on the composition of ain high-throughput screening tests. Such high-material. Unfortunately, there are still many instances

throughput screening methods could also be applied

where these models fail. For example, some of theto the photonic materials discussed in Chapter 3.

processes that influence charge transfer acrossConclusions

material interfaces are still shrouded in mystery, andfundamental studies are needed to understand thisso that researchers can reduce energy losses in

Many of the analytical techniques outlined in thisdevices.

chapter are routinely used in university laboratories
and research institutions. But some depend on large

Nevertheless, computational screening is alreadyfacilities such as free-electron lasers or synchrotrons

being used to study classes of materials withthat can produce incredibly brief, high-energy laser

relatively well-understood behaviours. For example,

pulses. These include the Dalian Coherent Light

researchers have applied computational screening to

Source in China, the European XFEL in Germany,

predict which composition of lead bismuth sulfide has

the UK’s Diamond Light Source, the SLAC National

the optimum light-absorption and mobility properties

Accelerator Laboratory in the United States, andfor PV cells.44 Meanwhile, a recent computational

SACLA in Japan. Indeed, SACLA recently helped

screen of more than 11,000 possible perovskites

to reveal how the catalytic complex at the heart ofidentified a group of materials that avoided using

photosystem II (see Chapter 3) forms the double bondtoxic lead compounds, while retaining good

between two oxygen atoms.46 This underlines theperformance.45 There are very many more multinary

importance of such national facilities and international

elemental combinations that form semiconductors

cooperation in in solar energy research.

31

CHALLENGES AND SOLUTIONS IN A CHANGING WORLDAs they explore how best to apply all these methods,

centres in this field could be authorized to verify these

solar energy researchers should collaborate widely

measurements independently, particularly whento share their expertise. Techniques that provereported results break existing records.

successful in one area — PV cells, for example —Recommendations

could prove invaluable in studying some puzzling
aspect of artificial photosynthesis, for example.
Indeed, progress in solar energy research may

1. Develop and apply analytical techniques acrossdepend on developments in completely different

all relevant time-, energy-, and spatial-scales tofields, such as electronic engineering. Programmes

study solar energy processes.

that bring together expertise from a very wide

Use these techniques to probe fundamental

range of disciplines to address problems inphotophysical and photochemical processes insolar energy research can help to push the field

molecules, materials, and at interfaces, the wholeforwards, particularly in overcoming obstacles to

way through to understanding what determines

commercialization. CS3 delegates noted successful

efficiency in working devices.

examples such as the Joint Center for Artificial
Photosynthesis in the US, the Korean Center for2.

Develop

and

apply

theoreticalandArtificial Photosynthesis, and the Solar Energy Action

computational approaches to understand solar

Program of the Chinese Academy of Sciences.

energy processes.
Tackle the problem at every scale, from the molecular

Researchers must also collaborate to determine thelevel of light-driven charge transfer processes up to

best ways to assess the performance of their solar

the device level. Continue integration of experimentenergy devices. The community of photovoltaic

and theory.

researchers has benefitted enormously fromlaboratories that will conduct independent tests of

3. Develop cheap and scalable methods to createa photovoltaic cell’s efficiency, such as the service

new photofunctional materials.

provided by the US National Renewable Energy

Control the composition and structure of materials

Laboratory. This has helped to identify the mostat many scales, from atomistic to macroscopic; use

promising PV systems to pursue.

high-throughput systems to make and screen novel
materials for key properties.

For the photoelectrochemical cells that have beendeveloped so far, research groups often measure

4. Develop standards, databases and share best

the performance of their systems in different ways,

practice.

making it difficult to make fair comparisons. In such

Agree on key metrics and reference standards tocases, it would be useful for researchers to agree on

measuring the performance of photofunctionalreference standards that researchers can compare

materials; foster multidisciplinary and international

their own systems against; and for journals to ensurecollaborations to make best use of analytical andthat researchers report their results in that context.

preparative techniques.

In addition, renowned laboratories or research

32

CHALLENGES AND SOLUTIONS IN A CHANGING WORLD

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34

CHALLENGES AND SOLUTIONS IN A CHANGING WORLD

CS3 Partici pants

China
Can LiJianhui Hou
Lizhu Wu
Gang LiuFei Huang
Yongsheng Zhao
Xi Zhang
Yongjun Chen
Zhigang Shuai
Fengtao Fan
Suping Zheng
Huajun Ju
Lidong Han
Min Zheng

Dalian Institute of Chemical Physics, Chinese Academy of Sciences
Institute of Chemistry, Chinese Academy of Sciences
Technical Institute of Physics and Chemistry, Chinese Academy of Sciences
Institute of Metal Research, Chinese Academy of Sciences
South China University of Technology
Institute of Chemistry, Chinese Academy of Sciences
Tsinghua University
National Nature Science Foundation of China
Tsinghua University
Dalian Institute of Chemical Physics, Chinese Academy of Sciences
Chinese Chemical Society
Chinese Chemical Society
Chinese Chemical Society
Dalian Institute of Chemical Physics, Chinese Academy of Sciences

35

CHALLENGES AND SOLUTIONS IN A CHANGING WORLD

Germany
Ferdi Schueth
Wolfgang Lubitz
Wolfram Jaegermann
Susanne Siebentritt
Bernd Rech
Markus Behnke
Hans-Georg Weinig

Max-Planck-Institut für Kohlenforschung
Max-Planck-Institut für Chemische Energiekonversion
Technische Universität Darmstadt
University of Luxembourg
Helmholtz-Zentrum Berlin für Materialien und Energie, Institute Silicon
Photovoltaics
German Research Foundation (DFG)
German Chemical Society (GDCh)

Japan
Kazunari Domen
Ryu AbeHiroko Yamada
Nobuhiro Yanai
Shu Seki
Yoshinori Nishikitani
Tetsuya Ito
Mitsuo Sawamoto

The University of Tokyo
Graduate School of Engineering, Kyoto University
Nara Institute of Science and Technology
Kyushu University
Kyoto University
Waseda University
Japan Science and Technology Agency (JST)The Chemical Society of Japan/Chubu University

United Kingdom
Martin Heeney
Andy Cooper
Jenny Nelson
Hugo Bronstein
Julia Weinstein
Sarah Thomas

Imperial College London
University of Liverpool
Imperial College London
University College London
University of Sheffield
Royal Society of Chemistry

United States
Suljo Linic
Jerry Meyer
Vivian Ferry
Harry Atwater
Gordana Dukovic
Christopher LaPrade
Lin HeUniversity of Michigan
University of North Carolina
University of Minnesota
California Institute of Technology
University of Colorado Boulder
American Chemical Society
National Science Foundation

36