Unraveling the irreversible transformation by nucleophilic substitution: A hint for fully transparent perovskite

Methylamine molecular treatment (MMT) either in precursor ink or final film of perovskite has been widely proposed to trigger either high‐efficiency, room‐temperature, green‐solvent, fast crystallization, and high‐throughput in processing perovskite solar cells but exclusively limited in methylammonium (MA)‐based perovskites, which is due to the reversible exchange between external methylamine (MA0) molecule and the same molecule's cation form (MA+) in the crystallographic A‐site of MA‐based perovskites. Executing MMT into formamidinium (FA)‐based perovskite (e.g., FAPbI3) that has optimal electronic band structure may bring new insights not only in high‐quality FAPbI3 towards higher efficiency but also in manufacturing merits such as fast and low‐temperature processability. By this motivation, we introduced MMT into the processing of FAPbI3 but obtained a new stabilized bleached crystal with complete transparency in the visible range. This can be ascribed to the irreversible reaction between the external MA0 molecule and the larger‐size FA+ cation in the lattice. We systematically investigated the chemical interaction in the MMT‐FAPbI3 system and proposed a nucleophilic substitution mechanism to understand this irreversible transformation. Surprisingly, this highly stable transparent crystal exhibits high transparency with average visible transmission (AVT) of 77.07% and color rendering index (CRI) of 95.81, and shows promising photodetector performance with response time less than 2 ms and on/off ratio of 1.18 × 103 under ultraviolet (UV) radiation, providing a novel and stable transparent candidate for UV photodetector application. The interaction between methylamine (MA0) and formamidinium (FA) based perovskite causes an irreversible transformation into a transparent bleach state film. Nucleophile attack and substitution mechanism is proposed to understand the involved chemical reaction. Benefiting from this novel reaction, a highly transparent layer is synthesized which shows high transparency with average visible transmission (AVT) of 77.07% and color rendering index (CRI) of 95.81, providing high potential in the application of transparent photodetectors.