Role of CsMnCl₃Nanocrystal Structure on Its Luminescence Properties

Cesium manganese chloride (CsMnCl₃) nanocrystals (NCs) have recently been recognized as potential lead-free perovskite candidates for red emission. To ascertain how the luminescence properties depend on the NC structures formed under different synthesis conditions, we synthesized CsMnCl₃NCs in two polymorphic structures, namely, cubic (c-CsMnCl₃) and rhombohedral (r-CsMnCl₃), by tuning the reaction temperature of a hot injection route. c-CsMnCl₃NCs are found to be nonemissive, whereas r-CsMnCl₃NCs exhibit red emission at 670 nm with a photoluminescence quantum yield of 40%. Density functional theory calculations reveal an indirect band gap for c-CsMnCl₃-the electronic transitions between valence and conduction band edges are prohibited by orbital symmetry and spin. Conversely, r-CsMnCl₃NCs possess a direct band gap. Further, transient absorption measurements suggest self-trapped exciton formation in r-CsMnCl₃NCs, which contributes to their emission characteristics. Our proof-of-concept demonstration of photocurrent generated from the emitting r-CsMnCl₃NCs indicates their suitability for luminescent solar concentrator applications. The findings of this work highlight the importance of understanding structure-luminescence relationship of emerging lead-free perovskites providing design criteria for red-emitting materials.