Properties and improvements of chlorine-doped methylamine-based perovskites

Abstract

Metal halide perovskite (MHP) has been widely used in optoelectronic devices such as solar cells in recent years due to their high absorption coefficients, long-range charge carrier diffusion lengths, and adjustable band gap, which is expected to achieve commercial application. Methylammonium lead iodide (MAPbI₃) has been fully investigated as a standard perovskite component, however, due to the low formation energy of polycrystalline films fabricated by wet chemical method, crystal defects (including interface and grain boundary defects) are generally inevitable, which is a principal factor leading to phase transition. Therefore, reducing the defect density of perovskite is a prominent approach to improve the stability of perovskite. Although defect passivation is one of the most commonly used methods to fabricate efficient perovskite solar cells (PSCs), the relatively weak secondary bond between molecular passivation group and perovskite crystal may bring difficulties to the application of practical devices, particularly when operating under harsh environments, such as high temperature, humidity, and ultraviolet light. Therefore, improving the intrinsic structure stability of the perovskite via changing its composition can be an effective way. Although perovskites containing chlorine precursors have been empolyed in solar cells device, how chloride ions affect the structural and electronic properties of these films was not understood yet. In this work, two-phase perovskite (MAPbI₂Cl) was fabricated by one-step spin coating with methylamine chloride (MACl) and lead iodide (PbI₂) as precursors. As a result, chloride (Cl) doping can superiorly induce perovskite crystallization and thus stabilize the MAPbI₃ lattice. The Cl doped perovskite layer shows lower defect density, and compared with the original MAPbI₃ film, the carrier lifetime of MAPbI₂Cl is increased by 7 times. Simultaneously, both of PCE and operational stability have been largely improved with PCE increased from 11.41% to 13.68%. There is no obvious degradation in the maximum power point output for nearly 8000 seconds in ambient conditions.