Anti-Stokes Photoluminescence from CsPbBr3 Nanostructures Embedded in a Cs$_4$PbBr$_6$ Crystal

Lead halide perovskites possess high photoluminescence (PL) efficiency and strong electron-phonon interactions, and therefore the optical cooling using up-conversion PL has been expected. We investigate anti-Stokes PL from green-luminescent Cs$_4$PbBr$_6$, whose origin is attributable to CsPbBr3 nanostructures embedded in a Cs$_4$PbBr$_6$ crystal. Because of the high transparency, low refractive index, and high stability of Cs$_4$PbBr$_6$, the green PL displays high external quantum efficiency without photo-degradation. Time-resolved PL spectroscopy reveals the excitonic behaviors in recombination process. The shape of the PL spectrum is almost independent of excitation photon energy, which means that the spectral width is determined by homogeneous broadening. We demonstrate that the phonon-assisted process dominates the Urbach tail of optical absorption and anti-Stokes PL at room temperature. Anti-Stokes PL is observed down to 70 K. We determine the temperature dependence of the Urbach energy and estimate the strength of the electron-phonon coupling. Our spectroscopic data show that CsPbBr3 nanostructures have potentially useful features for optical cooling.