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 PbI2 particles is successfully inhibited. As a result, uniform pinhole-free MA0.9Cs0.1PbI2Br(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%.