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NEWS FEATURE
NEWS FEATURE
The solar cell of the future
If the latest photovoltaic technologies can team up, they promise to capture the sun’s energy
far more effectively than ever before.
Stephen Battersby, Science Writer
In principle, the deluge of energy pouring down on us convert about 15 to 19% of the energy in sunlight into
from the sun could meet the world’s power needs electricity (2). That efficiency is the result of decades of
many times over. Already, in the United States, the research and development. Further improvements are
total power capacity of installed solar photovoltaic increasingly hard to come by.
(PV) panels is around 60 gigawatts, an amount Material shortages, as well as the size and speed of
expected to double in the next 5 years, and China the requisite investment, could also stymie efforts to
increased its PV capacity by nearly 60 gigawatts in scale up production of existing technologies (3). “If we
2017 alone (1). Meanwhile, improvements in PV panel
are serious about the Paris climate agreement, and we
technology have driven down the price of solar elec-
want to have 30% [of the world’s electricity supplied
tricity, making it cost competitive with other power
by] solar PV in 20 years, then we would need to grow
sources in many parts of the world.
That’s not a bad start. But to take full advantage of the capacity of silicon manufacturing by a factor of
that energy deluge and make a real impact on global 50 to build all those panels,” says Albert Polman,
carbon emissions, solar PV needs to move into tera- leader of the photonic materials group at the AMOLF
watt territory—and conventional panels might strug- research institute in Amsterdam. “It may happen, but
gle to get us there. Most PV panels rely on cells made in parallel we should think about ways to make solar
from semiconducting silicon crystals, which typically cells that take less capital.”
Silicon solar panels have become cheaper and more efficient, but a slew of exotic materials and optical tricks promises to
increase solar power’s potential far more in the coming years. Image credit: Shutterstock/Smallcreative.
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Published under the PNAS license.
www.pnas.org/cgi/doi/10.1073/pnas.1820406116 PNAS | January 2, 2019 | vol. 116 | no. 1 | 7–10
and there’s still room for improvement. For example,
the interface between a CdTe layer and the metal
conductor beneath it has defects that can help holes
and electrons recombine, and so prevent them
from contributing to the cell’s current. There is an
opportunity to reduce this source of inefficiency, says
Markus Gloeckler, chief scientist at First Solar Inc. in
Tempe, AZ, which makes most of the world’s CdTe
panels. But CdTe and CIGS both depend on rare
elements—tellurium and indium—and it may be
impossible to deploy these on terawatt scales (3).
So researchers are investigating a wealth of other
materials. Organic molecules such as polymers and
dyes, synthesized in bulk from simple ingredients, can
form the light-absorbing layer in a PV cell. “The mate-
rials we use are, in principle, extremely inexpensive,”
says Stephen Forrest, who leads an optoelectronics
Among the most promising of PV materials being
explored, perovskites all share the same crystal research group at the University of Michigan in Ann
structure, shown here. Image credit: ScienceSource/ Arbor, MI. However, although organics are potentially
ELLA MARU STUDIO. cheap, the cost of silicon continues to fall as well.
Forrest suggests that, rather than becoming direct
competitors with silicon, organics will fill a different
A slew of new technologies is aiming to tackle the niche. “They can do things that silicon can’t,” he says.
terawatt challenge. Some could be cheaply mass Unlike silicon, organic cells are flexible. So they can
produced, perhaps printed, or even painted onto easily be rolled out on rooftops or stuck onto other
surfaces. Others might be virtually invisible, integrated surfaces, without requiring heavy glass plates. Organic
neatly into walls or windows. And a combination of cells can also be designed to absorb mainly infrared
new materials and optical wizardry could give us light and remain fairly transparent to visible light,
remarkably efficient sun-traps. In different ways, all which means they can be integrated into windows.
of these technologies promise to harvest much more Forrest’s group, for example, has demonstrated or-
ganic PV cells with 7% efficiency that allow 43% of
solar energy, giving us a better chance of transforming
visible light to pass through (4). That might sound like
the world’s energy supply in the next 2 decades.
a dim and dingy window, but it’s comparable to
standard office windows with an antireflection coating.
Material Benefits
Transparent organics could also get an efficiency
Most PV cells work in basically the same way. A layer
boost from electrodes made of graphene—a thin,
of semiconductor material absorbs photons of light,
conducting, and transparent sheet of carbon atoms. In
generating electrons and positive charge carriers
2016, researchers at the Massachusetts Institute of
known as holes (vacancies where an electron would
Technology in Cambridge, MA, managed to glue a
normally be). The electrons are siphoned off to flow
graphene electrode onto experimental cells (5).
around a circuit and do useful work, before recom-
The most efficient organic PV cells have proved
bining with the holes at the other side of the cell.
susceptible to oxidation, giving them a relatively short
lifetime. But placing them inside a sealed double-
glazed window panel would protect them from dam-
“Organics have a real opportunity in aging oxygen and water. “Organics have a real oppor-
building-integrated solar cells.” tunity in building-integrated solar cells,” says Forrest.
—Stephen Forrest
Efficiency Drive
Organic solar cells may be cheap, but the price of a
A silicon layer needs to be about 200 micrometers cell is only one part of the economic equation. The
thick to absorb a good proportion of the light that hits real bottom line is called the levelized cost of elec-
it. But other materials absorb more strongly and form tricity (LCOE): its cost per kilowatt-hour, across the
effective light-collecting layers that are only a few whole lifetime of an installation. That cost includes
equipment such as inverters, which turn a panel’s low-
micrometers thick. That makes cells based on these
voltage direct current into higher-voltage alternating
materials potentially cheaper and less energy inten-
current. Other costs include installing and eventually
sive to manufacture. recycling the panels. Although super-cheap panels
Some of these thin-film technologies are well offer one route to low LCOE (Box 1), researchers are
established. Cadmium telluride (CdTe) and copper also working to improve two other crucial economic
indium gallium selenide (CIGS) share about 5% of inputs: the lifetime of a panel and its power efficiency.
today’s global PV market (2). Commercial CdTe panels Perovskites are among the most promising of the
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have recently matched silicon’s efficiency and cost, new PV materials. They all share the same crystal
8 | www.pnas.org/cgi/doi/10.1073/pnas.1820406116 Battersbystructure as a calcium titanium oxide mineral, the carriers, internal resistance, reflection from the face of
original perovskite that gives this family of materials the cell, and other effects.
its name. Different types of ion or molecule can oc- But existing materials can do much better by com-
cupy each of the three sites in this structure, meaning bining forces. In tandem cells there are two semi-
that perovskite chemistry can produce a panoply of conductor layers: an upper layer with a wide bandgap
different materials. Some of these, such as methyl- can make the most of visible light, whereas most of the
ammonium lead halides, form effective thin-film cells infrared shines through so that it can be mopped up by
with efficiencies recorded up to about 23% (6). a second layer with a narrower bandgap. Tandem cells
Perovskite cells have reached this impressive out- are perfect for materials with bandgaps that are rela-
put after barely a decade of research. “They are tively easy to tune. Tinkering with chemistry makes this
growing rapidly in efficiency in a way that no one possible in organics and perovskites. So in a perov-
expected,” says Francisco Garcı́a de Arquer at the
skite–silicon tandem, the perovskite can be engineered
University of Toronto in Ontario, Canada. One reason
to have a bandgap of 1.7 electron volts, which provides
for their high efficiency is that perovskites tend to have
the best light-absorbing complement to silicon’s 1.1 elec-
a low density of defects in their crystal structure, en-
tron volts. The theoretical efficiency limit for these
suring that relatively few electrons and holes are lost
two bandgaps combined is 43%.
to premature recombination. A recent study implies
As ever, the real-world performance is not up to
that the relatively flexible lattice is ineffective at re-
that ideal. But in June 2018, spin-out company Oxford
moving heat energy from charge-carrying electrons,
which could help explain perovskite’s high efficiencies Photovoltaics set a record efficiency of 27.3% for
and promise further improvements (7). What’s more, perovskite–silicon tandem cells (10). The company
all the materials in perovskites are abundant, and the says it is relatively simple to take existing silicon wafers
solution-based methods used to make them are po- and add the perovskite layer by using an electrically
tentially cheaper than the high-temperature process- conductive adhesive to stick them together. “We have
ing needed for silicon cells. an almost commercially ready product,” says the
But perovskites do have an Achilles’ heel or two. company’s chief technology officer Chris Case. They
They usually include lead, a toxic element that might expect early versions of the product to have around
hinder their commercialization, so several teams are 25 to 26% efficiency, improving to better than 30% in
looking at nontoxic alternatives, such as tin (8). Pe- the coming years. The company is also embarking on
rovskites are also prone to degrade, especially in the a project to build all-perovskite cells with two or more
presence of moisture, giving them short lifetimes and layers, targeting an eventual efficiency of 37%.
therefore poor LCOE. Encapsulating them in plastic Three layers would be better than two, and re-
helps but adds cost. At the Swiss Federal Institute of searchers are increasingly looking to nanostructured
Technology in Lausanne, Switzerland, a team led by materials to complete such a trio. Quantum dots, for
Giulia Grancini has found another way around the example, are tiny semiconductor particles that turn
problem, which involves adding an extra surface layer out to be particularly good at capturing photons, and
of perovskite to the cell. This material uses the same changing their size offers a straightforward way to tune
ingredients as the PV perovskite below but has a dif- their bandgap (See Core Concept: Quantum dots,
ferent structure that is more resistant to moisture. This www.pnas.org/content/113/11/2796).
seals and protects the cell, which shows no loss in per-
formance over 10,000 hours of operation, and should be
a cheaper option than plastic encapsulation (9).
Box 1
The Power of Print
Band Together
For solar power to make a substantial contribution to the global power
Despite the rising efficiencies of the perovskites and
supply will require tens of thousands of square kilometers of solar panels.
other new PV materials, they all face a fundamental
Printing could enable makers to churn them out rapidly, without the need
limit on their performance. This is set by their char-
for enormous capital investment.
acteristic bandgap—the energy needed to set free a
At the University of Newcastle in Callaghan, Australia, Paul Dastoor’s
bound electron so it becomes a charge carrier. In sil-
team has developed printable PV that’s on the verge of commercial de-
icon, this gap is 1.1 electron volts. Photons with less
ployment. Their organic light absorber, a thiophene polymer, is prepared
than that energy cannot generate a charge carrier, so
in ink form and deposited by commercial printing presses, as is one of the
they are wasted. Photons with more than that energy
electrodes, by using silver-based ink.
can generate carriers, but any energy above
Last year, Dastoor’s team tested the system in a 100-square-meter
1.1 electron volts is lost as heat. Given the spectrum of
installation and reached an efficiency of around 1%, with a projected
sunlight arriving at the surface of the Earth, it’s possi-
lifetime of 1 to 2 years. That may sound poor, but because their cells are so
ble to calculate what proportion of solar energy can
cheap to manufacture and install, just 2% and 3 years would make them
possibly be captured by a material, known as its
cost competitive with other forms of PV, according to Dastoor’s economic
Shockley–Queisser efficiency limit. For a bandgap of
model (14). The panels can literally be rolled out and fixed down by Velcro.
1.1 electron volts, the limit is about 32%. The ideal
They would have to be replaced quite frequently, however, which makes
bandgap of 1.34 electron volts does only a little
recycling vital. “Early indications are that it is straightforward to separate
better, with a limit of 33.7%. In practice, cell effi-
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the components,” says Dastoor.
ciency drops because of the recombination of charge
Battersby PNAS | January 2, 2019 | vol. 116 | no. 1 | 9A triple cell might have a perovskite layer tuned to some of it passes through without being captured.
blue and green light, a silicon layer for red and near The nanocylinders have the right spacing to reflect this
infrared, and a quantum dot layer for the longest unabsorbed light back into the perovskite layer,
wavelengths. “This could add up to 6% power con- allowing it a second chance to be absorbed.
version efficiency with little addition in cost,” says In contrast, the longer-wavelength light can pass
Garcı́a de Arquer, part of a team developing quantum straight through the nanocylinder layer without being
dot PV systems (11). reflected so that it can reach the silicon beneath.
Similar methods could improve light trapping in many
Tricks of the Light forms of solar cell, bouncing the light back and forth
Novel optics could conjure even more power from until it is absorbed.
sunlight. Nanostructured materials could provide better Spectrally selective reflectors such as these could
antireflection coatings, which allow more sunlight to also enable better tandem cells. Sticking one layer on
enter a solar cell. They could also be used to restrict the top of another creates several problems, including
wasteful emission of radiation when electrons and holes having to match the currents generated by each layer.
recombine. And electrodes made from a grid of nano- This is difficult enough for a two-layer tandem, never
wires can be almost perfectly transparent. mind three or more. “If light levels change, one of the
In Amsterdam, Polman’s research group has found cells can generate less current, which draws down the
that nanocylinders can supercharge solar cell perfor- entire stack,” says Polman. So he is working with Harry
mance in several ways. Although superficially similar Atwater and his group at the California Institute of
to quantum dot arrays, nanocylinders are made from Technology in Pasadena, CA, to build a device that
an insulating material instead of a semiconductor. uses reflector layers to channel light into six cells, each
Rather than absorbing light, they simply have a dif- tuned to a different waveband and stacked side by
ferent refractive index than the surrounding material. side (12). The aim is to produce a device with an
As a result, certain wavelengths of light bounce off the overall efficiency of 50%—and other optical enhance-
array, whereas others are transmitted. ments could take this higher still (13).
Polman is working on a reflector based on nano- It’s not yet clear which of these technologies will
cylinders of titanium oxide to boost the performance come together to form the super-cells of the future, but
of perovskite–silicon tandem cells. These nanocylinders the momentum seems to be unstoppable. “PV is less
form a separate layer between the perovskite and expensive than fossil fuel almost everywhere in the
silicon. As light enters the cell, the perovskite layer US,” says Forrest. And it’s only going to get cheaper.
absorbs most of the short-wavelength light—but “Things,” he says, “are moving fast.”
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