Controlling crystal growth by chloride-assisted synthesis: towards optimized charge transport in hybrid halide perovskites

Understanding the charge carrier dynamics and charge transport in metal halide perovskites and their correlation with the synthesis procedure is crucial for the fabrication of competitive thin film photovoltaic devices and their further improvement. In this work we investigate two CH₃NH₃PbI₃ (MAPI) films both deposited by a two-step protocol and only differing in the optional addition of a chloride containing salt to the precursor solution. Although being highly akin from a chemical and structural point of view, these films show substantial differences in their optoelectronic characteristics. For chloride-treated perovskite films we find an enhanced average power conversion efficiency of 10.3% and charge carrier mobility of μ=2.16 cm²/Vs in comparison to 5.3% and μ=1.62 cm²/Vs for untreated MAPI films, respectively. Moreover, we observe different light-soaking behavior and increased photoluminescence lifetimes of 70 ns in the case of chloride-treated MAPI and 100 ns for pure MAPI. From in-situ photoluminescence lifetime measurements during the crystallization process we conclude that the chloride addition during film deposition results in slower crystal growth, leading to fewer defects and higher crystalline order.