Anomalous NH₃-Induced Resistance Enhancement in Halide Perovskite MAPbI₃ Film and Gas Sensing Performance

Despite the growing interest in halide perovskite-based NH₃ sensors, the NH₃ sensing mechanism is still not well understood. Here, we report an anomalous behavior of resistance enhancement in CH₃NH₃PbI₃(MAPbI₃) perovskite films upon exposure to NH₃ gas, which is contrary to a resistance drop trend in previously reported perovskites. We propose a NH₃ sensing mechanism in which the anomalous resistance enhancement is dominated by grain boundaries of perovskites. It is demonstrated that NH₃ molecules can substitute MA⁺ cations of MAPbI₃ to form the insulating NH₄PbI₃·MA intermediate layers onto the surface of crystal grains, thereby resulting in an increase of resistance. Additionally, we construct the MAPbI₃-based sensor, and achieve a gas response of 472% toward 30 ppm of NH₃. This study suggests the potential of the perovskite-based NH₃ sensors, and also provides guidance for developing high-performance sensing perovskite materials.