Single Crystals of Mixed‐Cation Copper‐Based Perovskite with Trimodal Bandgap Behavior.

Two‐dimensional (2D) hybrid perovskites with novel functionalities and structural diversity are a perfect platform for emerging optoelectronic devices such as photodetectors, light‐emitting diodes, and solar cells. Here, we demonstrate that excess concentration of Cesium bromide (CsBr) is key to the formation of easily exfoliated 2D Cs2Cu(Cl/Br)4 perovskite crystal. Furthermore, by employing this trick to 2D perovskite MA2Cu(Cl/Br)4 (MA=methylammonium), we achieve a phase‐pure easily exfoliated 2D mixed‐cation (MA/Cs)2Cu(Cl/Br)4 perovskite crystal, which exhibits reduced bandgap (1.53 eV) with ferromagnetic behavior and photovoltaic property. The resultant mixed‐cation structured device reveals enhanced efficiency compared to all MA and all Cs counterparts. These findings demonstrate the importance of cation‐engineering in developing innovative materials with novel properties. [ABSTRACT FROM AUTHOR]