CsPbCl3‐Cluster‐Widened Bandgap and Inhibited Phase Segregation in a Wide‐Bandgap Perovskite and its Application to NiOx‐Based Perovskite/Silicon Tandem Solar Cells

Nickel oxide (NiOx) is an attractive hole‐transport material for efficient and stable p–i–n metal‐halide perovskite solar cells (PSCs). However, an undesirable redox reaction occurs at the NiOx/perovskite interface, which results in a low open‐circuit voltage (VOC), instability, and phase separation of the NiOx‐based wide‐bandgap perovskite (Br > 20%). In order to simultaneously address the abovementioned phase separation problem and redox chemistry at the perovskite/NiOx interface, the bandgap is widened from 1.64 to 1.67 eV by adding inorganic CsPbCl3‐clusters (3 mol%) to the Cs22Br15 perovskite precursor solution. Moreover, adding extra 2 mol% CsCl enriches the NiOx/perovskite interface with Cl, thereby preventing the redox reaction at the interface, while controlling the Br content to within 15% improves the photostability of the wide‐bandgap perovskite. Consequently, the power conversion efficiency (PCE) of a single‐junction p–i–n PSC increases from 17.82% to 19.76%, which leads to the fabrication of highly efficient monolithic p–i–n‐type NiOx‐based perovskite/silicon tandem solar cells with PCEs of up to 27.26% (certified PCE: 27.15%). The perovskite to an n–i–p‐type perovskite/silicon tandem solar cell is also applied to deliver a VOC of 1.93 V and a final efficiency of 25.5%. These findings provide critical insight into the fabrication of highly efficient and stable wide‐bandgap perovskites. The bandgap of Cs0.22FA0.78Pb (I0.85Br0.15)3 is widened from 1.64 to 1.67 eV by adding 3 mol% CsPbCl3‐clusters. Adding extra 2 mol% CsCl can enrich Cl at the NiOx/perovskite interface, thereby preventing the redox reaction at the interface and improving the photostability of the wide‐bandgap perovskite. This enables efficient p–i–n‐type NiOx‐based perovskite/silicon tandem solar cells with PCE up to 27.26%.