UCLA created a double layer solar cell by spraying a thin layer of perovskite onto a commercially available solar cell. The solar cell that forms the bottom layer of the device is made of a compound of copper, indium, gallium and selenide, or CIGS.
The new cell converts 22.4% of the incoming energy from the sun, a record in power conversion efficiency for a perovskite–CIGS tandem solar cell. The performance was confirmed in independent tests at the U.S. Department of Energy’s National Renewable Energy Laboratory.
The cell’s CIGS base layer, which is about 2 microns (or two-thousandths of a millimeter) thick, absorbs sunlight and generates energy at a rate of 18.7 percent efficiency on its own, but adding the 1 micron-thick perovskite layer improves its efficiency — much like how adding a turbocharger to a car engine can improve its performance. The two layers are joined by a nanoscale interface that the UCLA researchers designed; the interface helps give the device higher voltage, which increases the amount of power it can export.
And the entire assembly sits on a glass substrate that’s about 2 millimeters thick.
“Our technology boosted the existing CIGS solar cell performance by nearly 20 percent from its original performance,” Yang said. “That means a 20 percent reduction in energy costs.”
He added that devices using the two-layer design could eventually approach 30 percent power conversion efficiency. That will be the research group’s next goal.
Perovskite/CIGS tandem cells
Tandem solar cells can boost efficiency by using more of the available solar spectrum. Han et al. fabricated a two-terminal tandem cell with an inorganic-organic hybrid perovskite top layer and a Cu(In,Ga)Se2 (CIGS) bottom layer. Control of the roughness of the CIGS surface and the use of a heavily doped organic hole transport layer were crucial to achieve a 22.4% power conversion efficiency. The unencapsulated tandem cells maintained almost 90% of their efficiency after 500 hours of operation under ambient conditions.
The combination of hybrid perovskite and Cu(In,Ga)Se2 (CIGS) has the potential for realizing high-efficiency thin-film tandem solar cells because of the complementary tunable bandgaps and excellent photovoltaic properties of these materials. In tandem solar device architectures, the interconnecting layer plays a critical role in determining the overall cell performance, requiring both an effective electrical connection and high optical transparency. We used nanoscale interface engineering of the CIGS surface and a heavily doped poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) hole transport layer between the subcells that preserves open-circuit voltage and enhances both the fill factor and short-circuit current. A monolithic perovskite/CIGS tandem solar cell achieved a 22.43% efficiency, and unencapsulated devices under ambient conditions maintained 88% of their initial efficiency after 500 hours of aging under continuous 1-sun illumination.