Efficient up-conversion photoluminescence in all-inorganic lead halide perovskite nanocrystals

Up-conversion photoluminescence (UCPL) refers to the elementary process where low-energy photons are converted into high-energy ones via consecutive interactions inside a medium. When additional energy is provided by internal thermal energy in the form of lattice vibrations (phonons), the process is called phonon-assisted UCPL. Here, we report the exceptionally large phonon-assisted energy gain of up to ~ 8k_BT (k_B is Boltzmann constant, T is temperature) on all-inorganic lead halide perovskite semiconductor colloidal nanocrystals that goes beyond the maximum capability of only harvesting optical phonon modes. By systematic optical study in combination with a statistical probability model, we explained the nontrivial phonon-assisted UCPL process in perovskites nanocrystals, where in addition to the strong electron-phonon (light-matter) coupling, other nonlinear processes such as phonon-phonon (matter-matter) interaction also effectively boost the up-conversion efficiency.