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.

In principle, the deluge of energy pouring down on us from the sun could meet the world’s power needs many times over. Already, in the United States, the total power capacity of installed solar photovoltaic (PV) panels is around 60 gigawatts, an amount expected to double in the next 5 years, and China increased its PV capacity by nearly 60 gigawatts in 2017 alone (1). Meanwhile, improvements in PV panel technology have driven down the price of solar electricity, making it cost competitive with other power sources in many parts of the world.

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.

That’s not a bad start. But to take full advantage of that energy deluge and make a real impact on global carbon emissions, solar PV needs to move into terawatt territory—and conventional panels might struggle to get us there. Most PV panels rely on cells made from semiconducting silicon crystals, which typically convert about 15 to 19% of the energy in sunlight into electricity (2). That efficiency is the result of decades of research and development. Further improvements are increasingly hard to come by.

Material shortages, as well as the size and speed of the requisite investment, could also stymie efforts to scale up production of existing technologies (3). “If we are serious about the Paris climate agreement, and we want to have 30% [of the world’s electricity supplied by] solar PV in 20 years, then we would need to grow the capacity of silicon manufacturing by a factor of 50 to build all those panels,” says Albert Polman, leader of the photonic materials group at the AMOLF research institute in Amsterdam. “It may happen, but in parallel we should think about ways to make solar cells that take less capital.”

A slew of new technologies is aiming to tackle the terawatt challenge. Some could be cheaply mass produced, perhaps printed, or even painted onto surfaces. Others might be virtually invisible, integrated neatly into walls or windows. And a combination of new materials and optical wizardry could give us remarkably efficient sun-traps. In different ways, all of these technologies promise to harvest much more solar energy, giving us a better chance of transforming the world’s energy supply in the next 2 decades.

Material Benefits

Most PV cells work in basically the same way. A layer of semiconductor material absorbs photons of light, generating electrons and positive charge carriers known as holes (vacancies where an electron would normally be). The electrons are siphoned off to flow around a circuit and do useful work, before recombining with the holes at the other side of the cell.

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

—Stephen Forrest

A silicon layer needs to be about 200 micrometers thick to absorb a good proportion of the light that hits it. But other materials absorb more strongly and form effective light-collecting layers that are only a few micrometers thick. That makes cells based on these materials potentially cheaper and less energy intensive to manufacture.

Some of these thin-film technologies are well established. Cadmium telluride (CdTe) and copper indium gallium selenide (CIGS) share about 5% of today’s global PV market (2). Commercial CdTe panels have recently matched silicon’s efficiency and cost, 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 materials we use are, in principle, extremely inexpensive,” says Stephen Forrest, who leads an optoelectronics research group at the University of Michigan in Ann Arbor, MI. However, although organics are potentially 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 niche. “They can do things that silicon can’t,” he says.

Unlike silicon, organic cells are flexible. So they can easily be rolled out on rooftops or stuck onto other surfaces, without requiring heavy glass plates. Organic cells can also be designed to absorb mainly infrared light and remain fairly transparent to visible light, which means they can be integrated into windows. Forrest’s group, for example, has demonstrated organic PV cells with 7% efficiency that allow 43% of visible light to pass through (4). That might sound like a dim and dingy window, but it’s comparable to standard office windows with an antireflection coating. Transparent organics could also get an efficiency boost from electrodes made of graphene—a thin, conducting, and transparent sheet of carbon atoms. In 2016, researchers at the Massachusetts Institute of Technology in Cambridge, MA, managed to glue a graphene electrode onto experimental cells (5).

The most efficient organic PV cells have proved susceptible to oxidation, giving them a relatively short lifetime. But placing them inside a sealed double-glazed window panel would protect them from damaging oxygen and water. “Organics have a real opportunity in building-integrated solar cells,” says Forrest.

Efficiency Drive

Organic solar cells may be cheap, but the price of a cell is only one part of the economic equation. The real bottom line is called the levelized cost of electricity (LCOE): its cost per kilowatt-hour, across the whole lifetime of an installation. That cost includes equipment such as inverters, which turn a panel’s low-voltage direct current into higher-voltage alternating current. Other costs include installing and eventually recycling the panels. Although super-cheap panels offer one route to low LCOE (Box 1), researchers are also working to improve two other crucial economic inputs: the lifetime of a panel and its power efficiency.

Perovskites are among the most promising of the new PV materials. They all share the same crystal structure as a calcium titanium oxide mineral, the original perovskite that gives this family of materials its name. Different types of ion or molecule can occupy each of the three sites in this structure, meaning that perovskite chemistry can produce a panoply of different materials. Some of these, such as methylammonium lead halides, form effective thin-film cells with efficiencies recorded up to about 23% (6).

Perovskite cells have reached this impressive output after barely a decade of research. “They are growing rapidly in efficiency in a way that no one expected,” says Francisco García de Arquer at the University of Toronto in Ontario, Canada. One reason for their high efficiency is that perovskites tend to have a low density of defects in their crystal structure, ensuring that relatively few electrons and holes are lost to premature recombination. A recent study implies that the relatively flexible lattice is ineffective at removing heat energy from charge-carrying electrons, which could help explain perovskite's high efficiencies and promise further improvements (7). What’s more, all the materials in perovskites are abundant, and the solution-based methods used to make them are potentially cheaper than the high-temperature processing needed for silicon cells.

But perovskites do have an Achilles’ heel or two. They usually include lead, a toxic element that might hinder their commercialization, so several teams are looking at nontoxic alternatives, such as tin (8). Perovskites are also prone to degrade, especially in the presence of moisture, giving them short lifetimes and therefore poor LCOE. Encapsulating them in plastic helps but adds cost. At the Swiss Federal Institute of Technology in Lausanne, Switzerland, a team led by Giulia Grancini has found another way around the problem, which involves adding an extra surface layer of perovskite to the cell. This material uses the same ingredients as the PV perovskite below but has a different structure that is more resistant to moisture. This seals and protects the cell, which shows no loss in performance over 10,000 hours of operation, and should be a cheaper option than plastic encapsulation (9).

Among the most promising of PV materials being explored, perovskites all share the same crystal structure, shown here. Image credit: ScienceSource/ELLA MARU STUDIO.

Band Together

Despite the rising efficiencies of the perovskites and other new PV materials, they all face a fundamental limit on their performance. This is set by their characteristic bandgap—the energy needed to set free a bound electron so it becomes a charge carrier. In silicon, this gap is 1.1 electron volts. Photons with less than that energy cannot generate a charge carrier, so they are wasted. Photons with more than that energy can generate carriers, but any energy above 1.1 electron volts is lost as heat. Given the spectrum of sunlight arriving at the surface of the Earth, it’s possible to calculate what proportion of solar energy can possibly be captured by a material, known as its Shockley–Queisser efficiency limit. For a bandgap of 1.1 electron volts, the limit is about 32%. The ideal bandgap of 1.34 electron volts does only a little better, with a limit of 33.7%. In practice, cell efficiency drops because of the recombination of charge carriers, internal resistance, reflection from the face of the cell, and other effects.

But existing materials can do much better by combining forces. In tandem cells there are two semiconductor layers: an upper layer with a wide bandgap can make the most of visible light, whereas most of the infrared shines through so that it can be mopped up by a second layer with a narrower bandgap. Tandem cells are perfect for materials with bandgaps that are relatively easy to tune. Tinkering with chemistry makes this possible in organics and perovskites. So in a perovskite–silicon tandem, the perovskite can be engineered to have a bandgap of 1.7 electron volts, which provides the best light-absorbing complement to silicon’s 1.1 electron volts. The theoretical efficiency limit for these two bandgaps combined is 43%.

As ever, the real-world performance is not up to that ideal. But in June 2018, spin-out company Oxford Photovoltaics set a record efficiency of 27.3% for perovskite–silicon tandem cells (10). The company says it is relatively simple to take existing silicon wafers and add the perovskite layer by using an electrically conductive adhesive to stick them together. “We have an almost commercially ready product,” says the company’s chief technology officer Chris Case. They expect early versions of the product to have around 25 to 26% efficiency, improving to better than 30% in the coming years. The company is also embarking on a project to build all-perovskite cells with two or more layers, targeting an eventual efficiency of 37%.

Three layers would be better than two, and researchers are increasingly looking to nanostructured materials to complete such a trio. Quantum dots, for example, are tiny semiconductor particles that turn out to be particularly good at capturing photons, and changing their size offers a straightforward way to tune their bandgap (See Core Concept: Quantum dots, www.pnas.org/content/113/11/2796).

A triple cell might have a perovskite layer tuned to blue and green light, a silicon layer for red and near infrared, and a quantum dot layer for the longest wavelengths. “This could add up to 6% power conversion efficiency with little addition in cost,” says García de Arquer, part of a team developing quantum dot PV systems (11).

Tricks of the Light

Novel optics could conjure even more power from sunlight. Nanostructured materials could provide better antireflection coatings, which allow more sunlight to enter a solar cell. They could also be used to restrict the wasteful emission of radiation when electrons and holes recombine. And electrodes made from a grid of nanowires can be almost perfectly transparent.

In Amsterdam, Polman’s research group has found that nanocylinders can supercharge solar cell performance in several ways. Although superficially similar to quantum dot arrays, nanocylinders are made from an insulating material instead of a semiconductor. Rather than absorbing light, they simply have a different refractive index than the surrounding material. As a result, certain wavelengths of light bounce off the array, whereas others are transmitted.

Polman is working on a reflector based on nanocylinders of titanium oxide to boost the performance of perovskite–silicon tandem cells. These nanocylinders form a separate layer between the perovskite and silicon. As light enters the cell, the perovskite layer absorbs most of the short-wavelength light—but some of it passes through without being captured. The nanocylinders have the right spacing to reflect this unabsorbed light back into the perovskite layer, allowing it a second chance to be absorbed.

In contrast, the longer-wavelength light can pass straight through the nanocylinder layer without being reflected so that it can reach the silicon beneath. Similar methods could improve light trapping in many forms of solar cell, bouncing the light back and forth until it is absorbed.

Spectrally selective reflectors such as these could also enable better tandem cells. Sticking one layer on top of another creates several problems, including having to match the currents generated by each layer. This is difficult enough for a two-layer tandem, never mind three or more. “If light levels change, one of the cells can generate less current, which draws down the entire stack,” says Polman. So he is working with Harry Atwater and his group at the California Institute of Technology in Pasadena, CA, to build a device that uses reflector layers to channel light into six cells, each tuned to a different waveband and stacked side by side (12). The aim is to produce a device with an overall efficiency of 50%—and other optical enhancements could take this higher still (13).

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

Box 1

The Power of Print

For solar power to make a substantial contribution to the global power supply will require tens of thousands of square kilometers of solar panels. Printing could enable makers to churn them out rapidly, without the need for enormous capital investment.

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

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