山田 泰裕

Highly efficient anti-Stokes (AS) photoluminescence (PL) is observed from halide perovskite quantum dots (QDs) due to their strong electron-phonon interactions. The AS PL is particularly intriguing as it suggests the potential for semiconductor optical cooling if the external quantum efficiency approaches 100%. However, the PL quantum efficiency in QDs is primarily dominated by multiparticle nonradiative Auger recombination processes under intense photoexcitation, which impose limits on the optical cooling gain. Here, we investigate the Auger recombination of dot-in-crystal perovskites. We quantitatively estimate the maximum optical cooling gain and the corresponding excitation intensity. We further conducted optical cooling experiments and demonstrate a maximum photo-cooling of approximately 9 K from room temperature. Additionally, we confirmed that increasing the excitation intensity leads to a transition from photo-cooling to photo-heating. These observations are consistent with our time-resolved measurements, offering insights into the potential and limitations of optical cooling in semiconductor QDs.

Abstract Strong electron-phonon interactions are frequently considered the origin of the unique electrical and optical properties of lead halide perovskites. Electron-phonon interactions induce the formation of a polaron, which is a charge carrier dressed with a phonon cloud. The details of polaron formation are crucial for carrier transport since polaron formation leads to a larger effective mass of a carrier. Several mechanisms have been proposed regarding the physics of polaron formation in halide perovskites, but the details are still under active debate. While the Fröhlich interaction plays an essential role in ionic crystals, we also need to consider the strong phonon anharmonicity of halide perovskites that may lead to the formation of an unconventional polaron. In this review article, we discuss the uniqueness of perovskite semiconductors from the viewpoint of electron-phonon interactions. We review the experimental results and the proposed models concerning the effective carrier mass and carrier mobility. Finally, we briefly explain two physical phenomena related to strong electron-phonon interactions: strong anti-Stokes photoluminescence and slow hot-carrier cooling.