Revealing key factors of efficient narrow-bandgap mixed lead-tin perovskite solar cells via numerical simulations and experiments

Mixed lead and tin (Pb-Sn) halide perovskites promise high-performance solar cells because of their narrow bandgaps and other outstanding essential properties. However, there is still a big gap between the performance of the practical devices and theoretical limitations. Here we investigate key factors for realizing high-performance narrow-bandgap Pb-Sn perovskite solar cells (PSCs) via numerical simulations. We first study the effects of extrinsic factors on device performance, which predicts that a p-i-n structure along with appropriate charge transport layers is superior to an n-i-p structure, benefiting from a better energy band alignment. We further investigate key intrinsic factors and demonstrate that surface defect density, body defect density, and film thickness of perovskite absorbers play a pivotal role in determining device performance. The simulation results imply that narrow-bandgap Pb-Sn PSCs with fully-powered optimization should be able to realize a power conversion efficiency of 28.56%. Motivated by the simulations, we also successfully fabricate efficient narrow-bandgap Pb-Sn single-junction and 4-terminal all-perovskite tandem PSCs, which deliver high efficiencies of 21.01% and 26.01% measured under reverse voltage scans, respectively. This work points the way toward the improvement of narrow-bandgap Pb-Sn PSCs in the future.