The Origin of Lower Hole Carrier Concentration in Methylammonium Tin Halide Films Grown by a Vapor-Assisted Solution Process

A low hole carrier concentration in methylammonium tin halide (MASnX3) perovskite semiconductors is a prerequisite for a nonshorting solar cell device. In-depth film characterizations were performed on MASnI3–xBrx films, fabricated by both a low-temperature vapor-assisted solution process (LT-VASP) and conventional one-step methods, to reveal the origin of the lower hole carrier concentration from films of the former approach. We found that the vaporization of CH3NH3I solid at 150 °C, the temperature at which the LT-VASP occurs, does not supply iodine to the SnX2 (X = Br, I) films. As a result, secondary phases form aside from the desired MASnX3 perovskite; the secondary phases are suggested to be SnO and Sn(OH)2 via a proposed reaction pathway and are further supported by X-ray photoemission spectroscopy (XPS). These nonperovskite Sn2+ phases are beneficial because they assist in achieving the lower hole-doping levels in LT-VASP films. Remarkably, LT-VASP devices demonstrate improved air stability. Overall, our findings suggest that not only the commonly used SnF2 but also other divalent Sn compounds could serve as Sn vacancy suppressors. Further work on modulating the perovskite film compositions could realize more efficient and stable tin-based perovskite solar cells.