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 (E-g > 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, PbBr2) 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%. This 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.