Bandgap Engineering of Two‐Step Processed Perovskite Top Cells for Perovskite‐Based Tandem Photovoltaics

For high-performance application of perovskite solar cells (PSCs) in monolithic perovskite/silicon tandem configuration, an optimal bandgap and process method of the perovskite top cell is required. While the two-step method leads to regular perovskite film crystallization, engineering wider bandgaps (Eg > 1.65 eV) for the solution-based two-step method remains a challenge. This work introduces an effective and facile strategy to increase the bandgap of two-step solution-processed perovskite films by incorporating bromide in both deposition steps, the inorganic precursor deposition (step 1, PbBr$_2$) and the organic precursor deposition (step 2, FABr). This strategy yields improved charge carrier extraction and quasi-Fermi level splitting with power conversion efficiencies (PCEs) of up to 15.9%. Further improvements are achieved by introducing CsI in the bulk and utilizing LiF as surface passivation, resulting in a stable power output exceeding 18.5% for $E_g$ = 1.68 eV. This additional performance boost arises from enhanced perovskite film crystallization, leading to improved charge carrier extraction. Laboratory scale monolithic perovskite/silicon solar cells (TSCs) (1 cm$^2$ active area) achieve PCEs up to 23.7%. ... mehrThis work marks a significant advancement for wide bandgap two-step solution-processed perovskite films, enabling their effective use in high-performance and reproducible PSCs and perovskite/silicon TSCs.