Professor Mike McGehee and graduate student Colin Bailie use a microscope to examine silver nanowires on a solar cell. Credit:L.A. Cicero
Scientists at Stanford University have discovered that when perovskite, a crystalline material, is stacked on top of a conventional silicon solar cell, it provides a big boost to the efficiency of the solar cell.
We've been looking for ways to make solar panels that are more efficient and lower cost," said study co-author Michael McGehee, a professor of materials science and engineering at Stanford. "Right now, silicon solar cells dominate the world market, but the power conversion efficiency of silicon photovoltaics has been stuck at 25 percent for 15 years.
Co-author Michael McGehee, a professor of materials science and engineering at Stanford.
McGehee added that one method to improve the efficiency of silicon photovoltaics was to make a tandem device that includes silicon and another low-cost photovoltaic material.
Making low-cost tandems is very desirable. You simply put one solar cell on top of the other, and you get more efficiency than either could do by itself. From a commercial standpoint, it makes a lot of sense to use silicon for the bottom cell. Until recently, we didn't have a good material for the top cell, then perovskites came along.
Co-author Michael McGehee, a professor of materials science and engineering at Stanford.
Perovskite is an inexpensive, crystalline material that can be easily manufactured in laboratories. Scientists had, back in 2009, demonstrated that perovskites had the capability to generate electricity from sunlight with 3.8% efficiency. These perovskites were made using iodide, methylammonium and lead. Researchers have made progress over time and have presently achieved more than 20% efficiency. This is comparable to the efficiencies of commercial silicon solar cells, and hence has generated significant interest among producers of silicon.
Our goal is to leverage the silicon factories that already exist around the world," said Stanford graduate student Colin Bailie, co-lead author of the study. "With tandem solar cells, you don't need a billion-dollar capital expenditure to build a new factory. Instead, you can start with a silicon module and add a layer of perovskite at relatively low cost.
Stanford graduate student Colin Bailie, co-lead author of the study.
In solar cells, photons of sunlight are converted into an electric current, and the current moves to and fro between two electrodes. The photons of infrared and visible light are absorbed by silicon solar cells for generation of electricity. Photons in the solar spectrum’s visible part possess more energy, and only this visible part is harvested by perovskite cells.
"Absorbing the high-energy part of the spectrum allows perovskite solar cells to generate more power per photon of visible light than silicon cells," Bailie said.
However, the lack of transparency has been an important deterrent in making a perovskite-silicon tandem.
"Colin had to figure out how to put a transparent electrode on the top so that some photons could penetrate the perovskite layer and be absorbed by the silicon at the bottom," McGehee said. "No one had ever made a perovskite solar cell with two transparent electrodes."
Perovskites easily dissolve in water and can also get damaged by heat easily. This makes Perovskites very unstable and hence electrodes cannot be applied onto perovskite solar cells using conventional techniques. Hence, Bailie did this process manually.
"We used a sheet of plastic with silver nanowires on it," he said. "Then we built a tool that uses pressure to transfer the nanowires onto the perovskite cell, kind of like a temporary tattoo. You just need to rub it to transfer the film."
As part of this study, the research team used a perovskite solar cell with 12.7% efficiency and a low-quality silicon cell that had 11.4% efficiency. The perovskite solar cell was stacked over the silicon cell.
"By combining two cells with approximately the same efficiency, you can get a very large efficiency boost," Bailie said.
The researchers obtained impressive results.
"We improved the 11.4 percent silicon cell to 17 percent as a tandem, a remarkable relative efficiency increase of nearly 50 percent," McGehee said. "Such a drastic improvement in efficiency has the potential to redefine the commercial viability of low-quality silicon."
The research team conducted another experiment, where they used a copper indium gallium diselenide (CIGS) cell instead of the silicon solar cell. The CIGS cell had an efficiency of 17% and a perovskite cell with 12.7% efficiency was stacked onto the CIGS cell. The resulting overall efficiency of conversion achieved was 18.6%.
"Since most, if not all, of the layers in a perovskite cell can be deposited from solution, it might be possible to upgrade conventional solar cells into higher-performing tandems with little increase in cost," the authors wrote.
However, McGehee stated that the stability of perovskites in the long-term was still not known.
"Silicon is a rock," he said. "You can heat it 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. My vision is that some day we'll be able to get low-cost tandems that are 25 percent efficient. That's what companies are excited about. In five to 10 years, we could even reach 30 percent efficiency."
M. Greyson Christoforo from Stanford co-led the study. The team that wrote the study included Christoforo, Eva Unger, Alberto Salleo, Andrea Bowring, and William Nguyen from Stanford; Norman Pellet, Michael Grätzel and Julian Burschka, from the Swiss École Polytechnique Fédérale de Lausanne; Jungwoo Lee, Tonio Buonassisi and Jonathan Mailoa, from the Massachusetts Institute of Technology; and Rommel Noufi, who was with the U.S. National Renewable Energy Laboratory earlier.
The U.S. Department of Energy has provided the primary support for this study through Stanford’s Bay Area Photovoltaic Consortium along with Stanford's Global Climate and Energy Project.
This study has been published in the journal, Energy & Environmental Science.
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