January 21, 2015 | Leave a Comment
Study co-author Michael McGehee, a professor of materials science and engineering at Stanford said, “We’ve been looking for ways to make solar panels that are more efficient and lower cost. 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.”
One cost-effective way to improve efficiency is to build a tandem device made of silicon and another inexpensive photovoltaic material, he said. The research team described their novel perovskite-silicon solar cell in this week’s edition of the journal Energy & Environmental Science.
McGehee explains, “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 pervoskites came along.”
Perovskite is a crystalline material that is inexpensive and easy to make in the lab. Researchers showed on 2009 that perovskites made of lead, iodide and methylammonium could convert sunlight into electricity with an efficiency of 3.8%. Today the efficiency is above 20% and attracting commercial attention.
Stanford graduate student Colin Bailie, co-lead author of the study said, “Our goal is to leverage the silicon factories that already exist around the world. 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.”
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,” Bailie explained.
But there is a kind of roadblock to building an efficient perovskite-silicon tandem, its a lack of light transparency. “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 are easily damaged by heat and readily dissolve in water. This inherent instability ruled out virtually all of the conventional techniques for applying electrodes onto the perovoskite solar cell, so Bailie did it 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.”
For the experiment, the Stanford team stacked a perovskite solar cell with an efficiency of a 12.7% on top of a low-quality silicon cell with an efficiency of just 11.4%.
“By combining two cells with approximately the same efficiency, you can get a very large efficiency boost,” Bailie said.
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. “Such a drastic improvement in efficiency has the potential to redefine the commercial viability of low-quality silicon.”
In another experiment with a better grade of cell, 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 a 17% efficiency. The resulting tandem achieved an overall conversion efficiency of 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.
A big unanswered question is the long-term stability of perovskites, McGehee added.
“Silicon is a rock,” he said. “You can heat it to about 600º F 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% efficient. That’s what companies are excited about. In five to 10 years, we could even reach 30% efficiency.”
For now the tandem stack looks like a huge potential boon to the low end silicon solar cell market, enabling closing the gap to the better ones an perhaps very little cost. Just how to apply the perovskites to better cells and get big gains is a questions a lot of folks are surely asking today.