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 CsMnCl3 NCs in two polymorphic structures, namely, cubic (c-CsMnCl3) and rhombohedral (r-CsMnCl3), by tuning the reaction temperature of a hot injection route. c-CsMnCl3 NCs are found to be nonemissive, whereas r-CsMnCl3 NCs 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-CsMnCl3 NCs possess a direct band gap. Further, transient absorption measurements suggest self-trapped exciton formation in r-CsMnCl3 NCs, which contributes to their emission characteristics. Our proof-of-concept demonstration of photocurrent generated from the emitting r-CsMnCl3 NCs 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.