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

Nickel oxide (NiO_x ) 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 NiO_x /perovskite interface, which results in a low open-circuit voltage (V_OC ), instability, and phase separation of the NiO_x -based wide-bandgap perovskite (Br > 20%). In order to simultaneously address the abovementioned phase separation problem and redox chemistry at the perovskite/NiO_x interface, the bandgap is widened from 1.64 to 1.67 eV by adding inorganic CsPbCl₃ -clusters (3 mol%) to the Cs₂₂ Br₁₅ perovskite precursor solution. Moreover, adding extra 2 mol% CsCl enriches the NiO_x /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 NiO_x -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 V_OC 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.