Structural phase transitions and photoluminescence mechanism in a layer of 3D hybrid perovskite nanocrystals

Although the structural phase transitions in the single-crystal hybrid methyl-ammonium (MA) lead halide perovskites (MAPbX3, X = Cl, Br, I) are common phenomena, they have never been observed in the corresponding nanocrystals. Furthermore, although these materials are promising for the future electronics, optoelectronics, and solar energy harvesting applications, the nature of quasiparticles governing their unique photoluminescence (PL) and transport properties is still unclear. Here using two-photon excited PL spectroscopy, we provide evidence that the structural phase transitions in a layer of 3D MAPbBr3 nanocrystals may occur at about the same temperatures as those in the corresponding single crystals. We also show that room-temperature PL originates from the radiative recombination of the optical-phonon vibrationally excited polaronic quasiparticles with energies might exceed the ground-state Fröhlich polaron and Rashba energies due to optical-phonon bottleneck. Because of small masses and large radii of these polaronic quasiparticles, their high mobility and long-range diffusion become possible.