Cesium Manganese Bromide Nanocrystal Sensitizers for Broadband Vis-to-NIR Downshifting

Simultaneously achieving both broad absorption and sharp emission in the near-infrared (NIR) is challenging. Coupling of an efficient absorber such as lead halide perovskites to lanthanide emissive species is a promising way to meet the demands for visible to NIR spectral conversion. However, lead based perovskite sensitizers suffer from relatively narrow absorption in the visible range, poor stability, and toxicity. Herein, we introduce a downshifting configuration based on lead-free cesium manganese bromide nanocrystals acting as broad visible absorbers coupled to sharp emission in the NIR-I and NIR-II spectral regions. To achieve this, we synthesized CsMnBr₃ and Cs₃MnBr₅ nanocrystals and attempted to dope them with a series of lanthanides, achieving success only with CsMnBr₃. The correlation of the lanthanide emission to the CsMnBr₃ visible absorption was confirmed with steady-state excitation spectra and time-resolved photoluminescence measurements, whereas the mechanism of downconversion from the CsMnBr₃ matrix to the lanthanides was understood by density functional theory (DFT) calculations. This study shows that lead-free metal halides with an appropriate phase are an effective sensitizer for lanthanides and a route to efficient downshifting applications.