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

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.

Automated summary

The all-inorganic layered halide perovskite CsPb2Br5 shows potential applications due to its optical properties and stability, but the mechanism of its photoluminescence remains controversial. The optical properties of CsPb2Br5 depend on sample quality and preparation method. High-quality single crystals of CsPb2Br5 were prepared using a crystallization method, and they showed a conversion into efficient green emitters with a significant enhancement in emission intensity through thermal annealing or irradiation. A mechanism involving thermally induced indirect-to-direct bandgap transition associated with defect formation was proposed based on comprehensive characterization studies and theoretical calculations.