Indirect-to-direct bandgap transition in layered metal halide perovskite – CsPb2Br5

Abstract

All-inorganic layered halide perovskite CsPb₂Br₅ has many potential applications due to its interesting optical properties and long-term stability. However, its photoluminescence mechanism remains controversial due to the contradiction between its indirect bandgap nature and experimental observation of efficient green emission. The optical properties of CsPb₂Br₅ are highly dependent on the sample quality and preparation method, which is partially responsible for the controversy. Here, we prepared high-quality millimeter sized CsPb₂Br₅ single crystals using a saturated solvent evaporation crystallization method. The non-emissive CsPb₂Br₅ single crystals were found to convert into highly efficient green emitters with emission enhancement of up to four orders of magnitude by a simple thermal annealing process or irradiation with UV light or femtosecond laser pulses via multi-photon absorption. Through comprehensive characterization studies and theoretical calculations, a mechanism of the thermally induced indirect-to-direct bandgap transition associated with defect formation was proposed to explain the dramatic change in the optical properties of CsPb₂Br₅.