A Highly Red‐Emissive Lead‐Free Indium‐Based Perovskite Single Crystal for Sensitive Water Detection

Abstract Low‐dimensional luminescent lead halide perovskites have attracted tremendous attention for their fascinating optoelectronic properties, while the toxicity of lead is still considered a drawback. Herein, we report a novel lead‐free zero‐dimensional (0D) indium‐based perovskite (Cs 2 InBr 5 ⋅H 2 O) single crystal that is red‐luminescent with a high photoluminescence quantum yield (PLQY) of 33 %. Experimental and computational studies reveal that the strong PL emission might originate from self‐trapping excitons (STEs) that result from an excited‐state structural deformation. More importantly, the in situ transformation between hydrated Cs 2 InBr 5 ⋅H 2 O and the dehydrated form is accompanied with a switchable dual emission, which enables it to act as a PL water‐sensor in humidity detection or the detection of traces of water in organic solvents.