NEWS FEATURE

The solar cell of the futureIf 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 usfrom the sun could meet the world’s power needsmany times over. Already, in the United States, thetotal power capacity of installed solar photovoltaic(PV) panels is around 60 gigawatts, an amountexpected to double in the next 5 years, and Chinaincreased its PV capacity by nearly 60 gigawatts in2017 alone (1). Meanwhile, improvements in PV paneltechnology have driven down the price of solar elec-tricity, making it cost competitive with other powersources in many parts of the world.

That’s not a bad start. But to take full advantage ofthat energy deluge and make a real impact on globalcarbon emissions, solar PV needs to move into tera-watt territory—and conventional panels might strug-gle to get us there. Most PV panels rely on cells madefrom semiconducting silicon crystals, which typically

convert about 15 to 19% of the energy in sunlight intoelectricity (2). That efficiency is the result of decades ofresearch and development. Further improvements areincreasingly hard to come by.

Material shortages, as well as the size and speed ofthe requisite investment, could also stymie efforts toscale up production of existing technologies (3). “If weare serious about the Paris climate agreement, and wewant to have 30% [of the world’s electricity suppliedby] solar PV in 20 years, then we would need to growthe capacity of silicon manufacturing by a factor of50 to build all those panels,” says Albert Polman,leader of the photonic materials group at the AMOLFresearch institute in Amsterdam. “It may happen, butin parallel we should think about ways to make solarcells that take less capital.”

Silicon solar panels have become cheaper andmore efficient, but a slew of exotic materials and optical tricks promises toincrease solar power’s potential far more in the coming years. Image credit: Shutterstock/Smallcreative.

Published under the PNAS license.

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NEW

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A slew of new technologies is aiming to tackle theterawatt challenge. Some could be cheaply massproduced, perhaps printed, or even painted ontosurfaces. Others might be virtually invisible, integratedneatly into walls or windows. And a combination ofnew materials and optical wizardry could give usremarkably efficient sun-traps. In different ways, allof these technologies promise to harvest much moresolar energy, giving us a better chance of transformingthe world’s energy supply in the next 2 decades.

Material BenefitsMost PV cells work in basically the same way. A layerof semiconductor material absorbs photons of light,generating electrons and positive charge carriersknown as holes (vacancies where an electron wouldnormally be). The electrons are siphoned off to flowaround a circuit and do useful work, before recom-bining with the holes at the other side of the cell.

A silicon layer needs to be about 200 micrometersthick to absorb a good proportion of the light that hitsit. But other materials absorb more strongly and formeffective light-collecting layers that are only a fewmicrometers thick. That makes cells based on thesematerials potentially cheaper and less energy inten-sive to manufacture.

Some of these thin-film technologies are wellestablished. Cadmium telluride (CdTe) and copperindium gallium selenide (CIGS) share about 5% oftoday’s global PV market (2). Commercial CdTe panelshave recently matched silicon’s efficiency and cost,

and there’s still room for improvement. For example,the interface between a CdTe layer and the metalconductor beneath it has defects that can help holesand electrons recombine, and so prevent themfrom contributing to the cell’s current. There is anopportunity to reduce this source of inefficiency, saysMarkus Gloeckler, chief scientist at First Solar Inc. inTempe, AZ, which makes most of the world’s CdTepanels. But CdTe and CIGS both depend on rareelements—tellurium and indium—and it may beimpossible to deploy these on terawatt scales (3).

So researchers are investigating a wealth of othermaterials. Organic molecules such as polymers anddyes, synthesized in bulk from simple ingredients, canform the light-absorbing layer in a PV cell. “The mate-rials we use are, in principle, extremely inexpensive,”says Stephen Forrest, who leads an optoelectronicsresearch group at the University of Michigan in AnnArbor, MI. However, although organics are potentiallycheap, the cost of silicon continues to fall as well.Forrest suggests that, rather than becoming directcompetitors with silicon, organics will fill a differentniche. “They can do things that silicon can’t,” he says.

Unlike silicon, organic cells are flexible. So they caneasily be rolled out on rooftops or stuck onto othersurfaces, without requiring heavy glass plates. Organiccells can also be designed to absorb mainly infraredlight and remain fairly transparent to visible light,which means they can be integrated into windows.Forrest’s group, for example, has demonstrated or-ganic PV cells with 7% efficiency that allow 43% ofvisible light to pass through (4). That might sound likea dim and dingy window, but it’s comparable tostandard office windows with an antireflection coating.Transparent organics could also get an efficiencyboost from electrodes made of graphene—a thin,conducting, and transparent sheet of carbon atoms. In2016, researchers at the Massachusetts Institute ofTechnology in Cambridge, MA, managed to glue agraphene electrode onto experimental cells (5).

The most efficient organic PV cells have provedsusceptible to oxidation, giving them a relatively shortlifetime. But placing them inside a sealed double-glazed window panel would protect them from dam-aging oxygen and water. “Organics have a real oppor-tunity in building-integrated solar cells,” says Forrest.

Efficiency DriveOrganic solar cells may be cheap, but the price of acell is only one part of the economic equation. Thereal bottom line is called the levelized cost of elec-tricity (LCOE): its cost per kilowatt-hour, across thewhole lifetime of an installation. That cost includesequipment such as inverters, which turn a panel’s low-voltage direct current into higher-voltage alternatingcurrent. Other costs include installing and eventuallyrecycling the panels. Although super-cheap panelsoffer one route to low LCOE (Box 1), researchers arealso working to improve two other crucial economicinputs: the lifetime of a panel and its power efficiency.

Perovskites are among the most promising of thenew PV materials. They all share the same crystal

Among the most promising of PV materials beingexplored, perovskites all share the same crystalstructure, shown here. Image credit: ScienceSource/ELLA MARU STUDIO.

“Organics have a real opportunity inbuilding-integrated solar cells.”

—Stephen Forrest

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structure as a calcium titanium oxide mineral, theoriginal perovskite that gives this family of materialsits name. Different types of ion or molecule can oc-cupy each of the three sites in this structure, meaningthat perovskite chemistry can produce a panoply ofdifferent materials. Some of these, such as methyl-ammonium lead halides, form effective thin-film cellswith efficiencies recorded up to about 23% (6).

Perovskite cells have reached this impressive out-put after barely a decade of research. “They aregrowing rapidly in efficiency in a way that no oneexpected,” says Francisco Garcı́a de Arquer at theUniversity of Toronto in Ontario, Canada. One reasonfor their high efficiency is that perovskites tend to havea low density of defects in their crystal structure, en-suring that relatively few electrons and holes are lostto premature recombination. A recent study impliesthat the relatively flexible lattice is ineffective at re-moving heat energy from charge-carrying electrons,which could help explain perovskite’s high efficienciesand promise further improvements (7). What’s more,all the materials in perovskites are abundant, and thesolution-based methods used to make them are po-tentially cheaper than the high-temperature process-ing needed for silicon cells.

But perovskites do have an Achilles’ heel or two.They usually include lead, a toxic element that mighthinder their commercialization, so several teams arelooking at nontoxic alternatives, such as tin (8). Pe-rovskites are also prone to degrade, especially in thepresence of moisture, giving them short lifetimes andtherefore poor LCOE. Encapsulating them in plastichelps but adds cost. At the Swiss Federal Institute ofTechnology in Lausanne, Switzerland, a team led byGiulia Grancini has found another way around theproblem, which involves adding an extra surface layerof perovskite to the cell. This material uses the sameingredients as the PV perovskite below but has a dif-ferent structure that is more resistant to moisture. Thisseals and protects the cell, which shows no loss in per-formance over 10,000 hours of operation, and should bea cheaper option than plastic encapsulation (9).

Band TogetherDespite the rising efficiencies of the perovskites andother new PV materials, they all face a fundamentallimit on their performance. This is set by their char-acteristic bandgap—the energy needed to set free abound electron so it becomes a charge carrier. In sil-icon, this gap is 1.1 electron volts. Photons with lessthan that energy cannot generate a charge carrier, sothey are wasted. Photons with more than that energycan generate carriers, but any energy above1.1 electron volts is lost as heat. Given the spectrum ofsunlight arriving at the surface of the Earth, it’s possi-ble to calculate what proportion of solar energy canpossibly be captured by a material, known as itsShockley–Queisser efficiency limit. For a bandgap of1.1 electron volts, the limit is about 32%. The idealbandgap of 1.34 electron volts does only a littlebetter, with a limit of 33.7%. In practice, cell effi-ciency drops because of the recombination of charge

carriers, internal resistance, reflection from the face ofthe cell, and other effects.

But existing materials can do much better by com-bining forces. In tandem cells there are two semi-conductor layers: an upper layer with a wide bandgapcan make the most of visible light, whereas most of theinfrared shines through so that it can be mopped up bya second layer with a narrower bandgap. Tandem cellsare perfect for materials with bandgaps that are rela-tively easy to tune. Tinkering with chemistry makes thispossible in organics and perovskites. So in a perov-skite–silicon tandem, the perovskite can be engineeredto have a bandgap of 1.7 electron volts, which providesthe best light-absorbing complement to silicon’s 1.1 elec-tron volts. The theoretical efficiency limit for thesetwo bandgaps combined is 43%.

As ever, the real-world performance is not up tothat ideal. But in June 2018, spin-out company OxfordPhotovoltaics set a record efficiency of 27.3% forperovskite–silicon tandem cells (10). The companysays it is relatively simple to take existing silicon wafersand add the perovskite layer by using an electricallyconductive adhesive to stick them together. “We havean almost commercially ready product,” says thecompany’s chief technology officer Chris Case. Theyexpect early versions of the product to have around25 to 26% efficiency, improving to better than 30% inthe coming years. The company is also embarking ona project to build all-perovskite cells with two or morelayers, targeting an eventual efficiency of 37%.

Three layers would be better than two, and re-searchers are increasingly looking to nanostructuredmaterials to complete such a trio. Quantum dots, forexample, are tiny semiconductor particles that turnout to be particularly good at capturing photons, andchanging their size offers a straightforward way to tunetheir bandgap (See Core Concept: Quantum dots,www.pnas.org/content/113/11/2796).

Box 1The Power of PrintFor solar power to make a substantial contribution to the global powersupply will require tens of thousands of square kilometers of solar panels.Printing could enable makers to churn them out rapidly, without the needfor enormous capital investment.

At the University of Newcastle in Callaghan, Australia, Paul Dastoor’steam has developed printable PV that’s on the verge of commercial de-ployment. Their organic light absorber, a thiophene polymer, is preparedin ink form and deposited by commercial printing presses, as is one of theelectrodes, by using silver-based ink.

Last year, Dastoor’s team tested the system in a 100-square-meterinstallation and reached an efficiency of around 1%, with a projectedlifetime of 1 to 2 years. That may sound poor, but because their cells are socheap to manufacture and install, just 2% and 3 years would make themcost competitive with other forms of PV, according to Dastoor’s economicmodel (14). The panels can literally be rolled out and fixed down by Velcro.They would have to be replaced quite frequently, however, which makesrecycling vital. “Early indications are that it is straightforward to separatethe components,” says Dastoor.

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A triple cell might have a perovskite layer tuned toblue and green light, a silicon layer for red and nearinfrared, and a quantum dot layer for the longestwavelengths. “This could add up to 6% power con-version efficiency with little addition in cost,” saysGarcı́a de Arquer, part of a team developing quantumdot PV systems (11).

Tricks of the LightNovel optics could conjure even more power fromsunlight. Nanostructuredmaterials could provide betterantireflection coatings, which allow more sunlight toenter a solar cell. They could also be used to restrict thewasteful emission of radiation when electrons and holesrecombine. And electrodes made from a grid of nano-wires can be almost perfectly transparent.

In Amsterdam, Polman’s research group has foundthat nanocylinders can supercharge solar cell perfor-mance in several ways. Although superficially similarto quantum dot arrays, nanocylinders are made froman insulating material instead of a semiconductor.Rather than absorbing light, they simply have a dif-ferent refractive index than the surrounding material.As a result, certain wavelengths of light bounce off thearray, whereas others are transmitted.

Polman is working on a reflector based on nano-cylinders of titanium oxide to boost the performanceof perovskite–silicon tandem cells. These nanocylindersform a separate layer between the perovskite andsilicon. As light enters the cell, the perovskite layerabsorbs most of the short-wavelength light—but

some of it passes through without being captured.The nanocylinders have the right spacing to reflect thisunabsorbed light back into the perovskite layer,allowing it a second chance to be absorbed.

In contrast, the longer-wavelength light can passstraight through the nanocylinder layer without beingreflected so that it can reach the silicon beneath.Similar methods could improve light trapping in manyforms of solar cell, bouncing the light back and forthuntil it is absorbed.

Spectrally selective reflectors such as these couldalso enable better tandem cells. Sticking one layer ontop of another creates several problems, includinghaving to match the currents generated by each layer.This is difficult enough for a two-layer tandem, nevermind three or more. “If light levels change, one of thecells can generate less current, which draws down theentire stack,” says Polman. So he is working with HarryAtwater and his group at the California Institute ofTechnology in Pasadena, CA, to build a device thatuses reflector layers to channel light into six cells, eachtuned to a different waveband and stacked side byside (12). The aim is to produce a device with anoverall efficiency of 50%—and other optical enhance-ments could take this higher still (13).

It’s not yet clear which of these technologies willcome together to form the super-cells of the future, butthe momentum seems to be unstoppable. “PV is lessexpensive than fossil fuel almost everywhere in theUS,” says Forrest. And it’s only going to get cheaper.“Things,” he says, “are moving fast.”

1 Solar Energy Industries Association (2018) Solar market insight report: 2018 Q3. Available at https://www.seia.org/research-resources/solar-market-insight-report-2018-q3. Accessed December 1, 2018.

2 Philipps S, Warmuth W (2018) Fraunhofer ISE photovoltaics report. Available at https://www.ise.fraunhofer.de/content/dam/ise/de/documents/publications/studies/Photovoltaics-Report.pdf. Accessed December 1, 2018.

3 Feltrin A, Freundlich A (2008) Material considerations for terawatt level deployment of photovoltaics. Renew Energy 33:180–185.4 Li Y, et al. (2017) High efficiency near-infrared and semitransparent non-fullerene acceptor organic photovoltaic cells. J Am Chem Soc139:17114–17119.

5 Song Y, Chang S, Gradecak S, Kong J (2016) Visibly-transparent organic solar cells on flexible substrates with all-graphene electrodes.Adv Energy Mater 6:1600847.

6 National Renewable Energy Laboratory (2018) Best research-cell efficiencies. Available at https://www.nrel.gov/pv/assets/images/efficiency-chart.png. Accessed December 1, 2018.

7 Gold-Parker A, et al. (2018) Acoustic phonon lifetimes limit thermal transport in methylammonium lead iodide. ProcNatl Acad Sci USA115:11905–11910.

8 Pantaler M, et al. (2018) Hysteresis-free lead-free double-perovskite solar cells by interface engineering. ACS Energy Lett3:1781–1786.

9 Grancini G, et al. (2017) One-year stable perovskite solar cells by 2D/3D interface engineering. Nat Commun 8:15684.10 Osborne M (2018) Oxford PV takes record perovskite tandem solar cell to 27.3% conversion efficiency. PVTECH. Available at https://

www.pv-tech.org/news/oxford-pv-takes-record-perovskite-tandem-solar-cell-to-27.3-conversion-effi. Accessed December 1, 2018.11 Jo JW, et al. (2018) Acid-assisted ligand exchange enhances coupling in colloidal quantum dot solids. Nano Lett 18:4417–4423.12 Atwater Research Group (2018) Photovoltaic materials and devices. Available at https://daedalus.caltech.edu/research/photovoltaic-

materials-and-devices/. Accessed December 1, 2018.13 Polman A, Atwater HA (2012) Photonic design principles for ultrahigh-efficiency photovoltaics. Nat Mater 11:174–177.14 Mulligan CJ, et al. (2015) Levelised cost of electricity for organic photovoltaics. Sol Energy Mater Sol Cells 133:26–31.

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