Abstract
Wide-bandgap (WBG) mixed-halide perovskites as the front cell absorber are accomplishing perovskite-based tandem solar cells with over 29% power conversion efficiency. However, their large voltage deficits limit their ultimate performance. Only a handful of studies probe the fundamental mechanisms underlying the voltage deficits, which remain an unsolved challenge in the field. In this study, we investigate the formation dynamics and defect physics of WBG mixed-halide perovskites in contrast with their corresponding triiodide-based perovskites. Our results show that the inclusion of bromide introduced a halide homogenization process that occurs during the perovskite growth stage from an initial bromide-rich phase toward the final target stoichiometry. We further elucidated a physical model that correlates the role of bromide with the formation dynamics, defect physics, and eventual optoelectronic properties of the film. This work provides a fundamental and unique perspective toward understanding the performance-limiting factors affecting WBG mixed-halide perovskites.
Highlights
Metal halide perovskites with their X-sites of iodine partially substituted with bromine have demonstrated great potential for commercialization of halide perovskite technology for use in tandem photovoltaics (PVs) integrated with conventional PV products such as Si and CuInGaSe2 (CIGS) [1]
We focused on the triple cation mixed-halide WBG perovskite FAMACsPb(I0.8Br0.2)3, as it is one of the most widely reported compositions with a good resistance against light-induced halide segregation and high PV performance
The triiodide reference with the same triple cations, FAMACsPbI3, will serve as the reference to investigate the effects of Br inclusion on the perovskite formation dynamics
Summary
Metal halide perovskites with their X-sites of iodine partially substituted with bromine have demonstrated great potential for commercialization of halide perovskite technology for use in tandem photovoltaics (PVs) integrated with conventional PV products such as Si and CuInGaSe2 (CIGS) [1]. By controlling the I/Br ratio, the optical bandgaps of mixed-halide perovskites can be tuned to be between 1.64 and 1.70 eV, usually referred to as wide-bandgap (WBG) perovskites in the field, ideal for front-cell applications in two-junction tandem PVs [2]. With such WBG mixed-halide perovskites, perovskite-Si and perovskite-CIGS tandem cells have reached remarkable power conversion efficiencies (PCEs) of 29.5 and 24.2%, respectively [3]. The voltage deficits remain the largest bottleneck toward further improving the performance of perovskite-based tandem PVs
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