Stacking perovskites, a crystalline material, onto a conventional silicon solar cell may dramatically improve the overall efficiency of the cell, scientists from Stanford University concluded in a new study.
“Right now, silicon solar cells dominate the world market, but the power conversion efficiency of silicon photovoltaics has been stuck at 25% for 15 years,” explained Professor Michael McGehee, who co-authored the study recently published in the journal Energy & Environmental Science. One cost-effective way to improve efficiency is to build a tandem device made of silicon and another inexpensive photovoltaic material, he said, but “until recently, we didn’t have a good material for the top cell. Then perovskites came along.”
Perovskites are synthetic crystalline materials with the same crystal structure as the mineral perovskite. They are inexpensive and easy to produce in the lab. In 2009, scientists showed that perovskites made of lead, iodide, and methylammonium could convert sunlight into electricity with an efficiency of 3.8%. Since then, researchers have achieved perovskite efficiencies above 20%, rivaling commercially available silicon solar cells. The advancements have piqued the interest of silicon manufacturers worldwide.
The tandem solar cells work by converting photons of sunlight into an electric current that moves between two electrodes. Silicon solar cells generate electricity by absorbing photons of visible and infrared light, while perovskite cells harvest only the visible part of the solar spectrum, where the photons have more energy.
“Absorbing the high-energy part of the spectrum allows perovskite solar cells to generate more power per photon of visible light than silicon cells,” said Stanford graduate student Colin Bailie, co-lead author of the study.
One hurdle to building an efficient perovskite-silicon tandem has been a lack of transparency. Also, perovskites are easily damaged by heat and readily dissolve in water. So the team used a sheet of plastic with silver nanowires on it and built a tool that uses gentle pressure to transfer the nanowires onto the perovskite cell (Figure 4). When the team stacked a perovskite solar cell with an efficiency of 12.7% on top of a low-quality silicon cell with an efficiency of just 11.4%, the results were impressive. “We improved the 11.4% silicon cell to 17% as a tandem, a remarkable relative efficiency increase of nearly 50%,” McGehee said.
In another experiment, the research team replaced the silicon solar cell with a cell made of copper indium gallium diselenide (CIGS). The researchers stacked a 12.7% efficiency perovskite cell onto a CIGS cell with 17% efficiency. The resulting tandem achieved an overall conversion efficiency of 18.6%.
The team is now working on resolving the “unanswered question” of how perovskites fare in terms of long-term stability. “You can heat [silicon] to about 600 degrees Fahrenheit, shine light on it for 25 years, and nothing will happen. But if you expose perovskite to water or light, it likely will degrade. We have a ways to go to show that perovskite solar cells are stable enough to last 25 years,” McGehee said.
—Sonal Patel, associate editor