Unraveling the ammonia sensing behavior and degradation pathways of a novel lead-free MA2CuBr4 based ammonia sensor†

We report a lead-free copper-based halide perovskite gas sensor to detect ammonia gas at ambient temperature. The sensor uses methylammonium copper bromide as the active material and can trace ammonia through both a visual color change method and electrical readout. The maximum calibrated sensitivity based on the optical response of the sensor is ∼95% upon exposure to 10 ppm ammonia gas, which is the best among the colorimetric sensors using halide perovskites. The sensor can be operated at 0.5 V bias with an output current of ∼12 μA at 2 ppm ammonia gas exposure, making our device compatible with low-power gas sensors. Furthermore, we studied the degradation mechanism by subjecting the MA₂CuBr₄ film to ammonia-exposure cycles. We found that there were two factors responsible for the degradation of the sensor: (i) loss of methylamine gas due to formation of NH₄Br, and (ii) reduction of Cu²⁺ to Cu⁺. Increasing the proportion of MABr in the system increased the material's tolerance to ammonia exposure by solving the methylamine gas escape problem. Further, we showed that the stability of the device could be enhanced by depositing porous polymethylmethacrylate over the copper perovskite.