Michael McGehee (right), a materials scientist at Stanford University in Palo Alto, California, examines a tandem perovskite cell. 

Linda A. Cicero / Stanford News

Low-cost solar cells poised for commercial breakthrough

Cheap materials called perovskites are insinuating themselves into silicon solar cells—a first step toward ultimately usurping the reigning cell material. Last week, at a meeting here of the Materials Research Society (MRS), researchers announced that “tandem” cells, in which perovskites are layered on top of silicon and other photovoltaic materials, have achieved record-setting efficiencies at turning sunlight into electricity. Now, researchers are moving fast to surmount the lack of durability and other problems that have hindered the commercialization of perovskites.

“I think perovskites are going to make it to market,” says Aslihan Babayigit, a perovskite researcher at Hasselt University in Diepenbeek, Belgium. The progress has been “amazing,” adds David Cahen, a materials scientist at the Weizmann Institute of Science in Rehovot, Israel. “Even if all the problems are not solved, most look solvable.”

Known since the 1830s, perovskites are a class of crystals with a common 3D structure. It wasn’t until 2009 that researchers in Japan first realized their potential as a photovoltaic material. The first perovskite devices converted only 3.8% of light energy into electricity, far less than crystalline silicon, today’s dominant commercial technology, which tops out at 25.3% efficiency for the best research cells. (Commercial cells usually vary between 16%–20%.) But researchers tinkered with their perovskite recipes, and the efficiencies of the cells quickly skyrocketed. The record now stands at 22.1%, demonstrated earlier this year by researchers in South Korea.

Tandems, which combine cells optimized to capture different parts of the solar spectrum, can do even better. Silicon, for instance, preferentially absorbs reddish light, whereas perovskites tend to soak up blue and green photons. Slapping a perovskite cell on top of silicon need not cost much because the ingredients are dirt cheap, and the crystals can be grown easily at low temperatures. Tandems also allow perovskites to piggyback on the entrenched silicon industry.

Stronger together

By stacking perovskite solar cells in tandem with others, researchers are nearing the record efficiency of single crystal silicon, the industry’s commercial standard. Two-terminal (2T) devices layer the materials into a single cell; four-terminal (4T) devices stack together two electrically independent cells.

C. Aycock/Science; Michael McGehee, Stanford Univ.

At the MRS meeting, Michael McGehee, a materials scientist at Stanford University in Palo Alto, California, reported that by growing a perovskite on silicon, he and his colleagues had created a tandem cell with an efficiency of 23.6%, better than the efficiencies of either component. Another group led by Christophe Ballif of the Federal Polytechnic School of Lausanne in Neuchâtel, Switzerland, reported in July that a silicon-perovskite tandem with a more complex architecture had reached an efficiency of 25.2%.

Tandems are likely to continue improving for years. Researchers have yet to build in all the finer tricks of the trade, such as optimizing the electricity-carrying layers in the cells and adding coatings that minimize surface reflections. Even with current perovskite materials, over the next couple years silicon-perovskite tandems could reach efficiencies of 30%, McGehee predicts. At that threshold, says Henry Snaith, a physicist at the University of Oxford in the United Kingdom, solar companies will start to add perovskites into their commercial panels, driving further improvements in the materials that could ultimately help them supplant silicon altogether.

At the MRS meeting, some researchers foreshadowed that day. Giles Eperon, a materials scientist at the University of Washington in Seattle, explained that when getting his Ph.D. at Oxford, he made a perovskite that strongly absorbs reddish light—the wavelengths that silicon has specialized in. Partnering with McGehee’s group, Eperon layered his red absorber on top of a more standard blue absorber, achieving an efficiency of 20.3% in a pure perovskite tandem. Although not yet as good as perovskite-silicon tandems, the perovskite components in the cells are still rapidly improving, whereas silicon has flatlined.

For all their gains in efficiency, perovskites have faced lingering problems. Water vapor, high temperatures, or even prolonged sun exposure can dissolve or degrade perovskites within hours. But at the MRS meeting, McGehee reported exceptional stability for new perovskite recipes that replace an organic component called methylammonium with formamidinium or the element cesium. When encapsulated to protect them from moisture, these cells showed no sign of degradation for 6 weeks, even when exposed to temperatures of 85°C and a relative humidity of 85%, a standard test of durability. “Panels that pass it usually will not fail due to heat and humidity over 25 years outside,” McGehee says.

Others are reporting improvements in manufacturing commercial-sized cells rather than the small, bespoke crystals used for setting records. Christopher Case, the chief technology officer for Oxford Photovoltaics (Oxford PV) in the United Kingdom, a perovskite solar cell company launched by Snaith, says the company has scaled up the postage stamp–sized research cells to ones that are 10 centimeters square and that have passed industry durability standards. Last month, the company acquired a former photovoltaic pilot facility in Germany. It is now gearing up to produce perovskite cells atop full-sized commercial silicon wafers, 15 centimeters on a side, Case says. Oxford PV also recently announced that they raised an additional £26 million ($33 million) over the last 18 months from investors, and Case says the company has inked partnerships with several of the top 10 silicon solar cell producers to investigate adding perovskites to their cells. If all goes well, he says, the first pilot products could appear in 2018.

That leaves safety as the major outstanding roadblock to commercialization. The most efficient perovskites contain a highly soluble form of lead, a dangerous neurotoxin that could leach into homes, soil, or groundwater if the cells degrade. Babayigit says there are potential solutions, such as encapsulating the perovskite in protective shells or adding sulfides around the cell, which would bind and quarantine any lead that managed to escape. For now, she says, “it’s a heavily underresearched field that needs attention.” Given how quickly perovskites are moving to market, it’s a safe bet that someone will soon take on the project.

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