Mechanistic Investigation of Upconversion Photoluminescence in All-Inorganic Perovskite CsPbBrI2 Nanocrystals

Perovskite quantum dots emit upconversion photoluminescence (UCPL) excited by long wavelength photons. However, UCPL with different wavelengths and widths are irradiated in perovskite colloidal excited by different energy photons. This study investigated efficient UCPL in colloidal CsPbBrI2 nanocrystals and proposed a simple energy level model to illustrate the UC mechanism. Red-shifted UCPL relative to normal PL was attributed to recombination of carriers from the surface to the valence band and involved a single-photon process, whereas unshifted UCPL was attributed to recombination of carriers from the conduction band to the valence band and involved a two-photon process. Trap states within the valence band played an important role in the UC process. Single photon UC emission occurred when the absorbed photons possessed sufficient energy to excite electrons in deep trap states in CsPbBrI2 nanocrystals. However, when the photons could only excite electrons in shallow trap states, some excited photons were absorbed by shallow trap state, producing single photon UCPL, while the remaining photons were absorbed by the valence band, leading electrons transfer from the valence band to the conduction band. Therefore, the UC process was gradually dominated by the two-photon process as the incident photons energy was decreased.