Synergistic Effect of Pseudo-Halide Thiocyanate Anion and Cesium Cation on Realizing High-Performance Pinhole-Free MA-Based Wide-Band Gap Perovskites

The performance of wide-band gap perovskite solar cells has a profound impact on the multijunction tandem device efficiency. However, once bromide (Br^–) has been adopted to substitute the iodide (I^–) in the MAPbI₃ framework, it becomes very challenging to achieve uniform and high crystalline perovskite films. Here, a synergistic effect of pseudo-halide anion thiocyanate (SCN^–) and inorganic cation cesium (Cs⁺) on the crystallization and film formation of MA-based wide-band gap perovskite is reported. It is found that the intrinsic ability of SCN^– for increasing the perovskite crystal size can make the crystallization process more tolerable to the different affinity of the initial inhomogeneous small particles. However, the introduction of SCN^– usually comes along with undesired large PbI₂ aggregates. By further incorporating Cs⁺ in the precursor solution to improve the solubility of the halide/pseudo-halide coordination to Pb²⁺, the formation of the aggregated PbI₂particles is successfully inhibited. As a result, uniform pinhole-free MA_0.9Cs_0.1PbI₂Br(SCN)_0.08 perovskites with a wide band gap of 1.77 eV can be achieved. The corresponding photovoltaic device exhibits a record-high fill-factor over 80% and a promising power conversion efficiency of 16.3%.