Halogen Engineering for Operationally Stable Perovskite Solar Cells via Sequential Deposition

The performance of perovskite solar cells (PSCs) relies on the synthesis method and chemical composition of the perovskite materials. So far, PSCs that have adopted two-step sequential deposited perovskite with the state-of-art composition (FAPbI(3))(1-)(x)(MAPbBr(3))(x) (x < 0.05) have achieved record power conversion efficiency (PCE), while their one-step antisolvent dripping counterparts with typical composition Cs(0.05)FA(0.81)MA(0.14)Pb(I0.85Br0.15)(3) with more bromine have exhibited much better long-term operational stability. Thus, halogen engineering that aims to elevate bromine content in sequential deposited perovskite film would push operational stability of PSCs toward that of antisolvent dripping deposited perovskite materials. Here, a Br-rich seeding growth method is devised and perovskite seed solution with high bromine content is introduced into a PbI2 precursor, leading to bromine incorporation in the resulting perovskite film. Photovoltaic devices fabricated by Br-rich seeding growth method exhibit a PCE of 21.5%, similar to 21.6% for PSCs having lower bromine content. Whereas, the operational stability of PSCs with higher bromine content is significantly enhanced, with over 80% of initial PCE retained after 500 h tracking at maximum power point under 1-sun illumination. This work highlights the vital importance of halogen composition for the operational stability of PSCs, and introduces an effective way to incorporate bromine into mixed-cation-halide perovskite film via sequential deposition method.