Bimolecular Additives Improve Wide-Band-Gap Perovskites for Efficient Tandem Solar Cells with CIGS

Tandem solar cells coupling narrow- and wide-band-gap thin-film polycrystalline absorbers are attractive for achieving ultrahigh efficiency with low manufacturing cost. For established narrow-band-gap CIGS thin-film bottom cells, a challenge is to develop highly efficient polycrystalline wide-band-gap top cells. Here, we demonstrate a 1.68-eV (FA0.65MA0.20Cs0.15)Pb(I0.8Br0.2)3 wide-band-gap perovskite solar cell with an efficiency of ∼20% enabled by using PEAI and Pb(SCN)2 complementary additives in the perovskite precursor. The coupling of PEA+ and SCN− provides a synergistic effect that overcomes growth challenges with either additive individually and improves perovskite film quality with enhanced crystallinity, reduced formation of excess PbI2 (in comparison to using Pb(SCN)2 additive alone), lower defect density and energetic disorder, and an improved carrier mobility (∼47 cm2 V−1s−1) and lifetime (∼2.9 μs). When coupling a semi-transparent 1.68-eV perovskite top cell fabricated by this approach with a 1.12-eV CIGS bottom cell, we achieve 25.9%-efficient polycrystalline perovskite/CIGS 4-terminal thin-film tandem solar cells.