Ultrafast Non-Equilibrium Electron and Spin Dynamics in Perovskite Nanocrystals

Understanding and control of ultrafast non-equilibrium processes in semiconductors is crucial for making use of quantum states, opening opportunities to surpass traditional limits in optoelectronic devices for energy harvesting, light emission, and quantum technologies. In this paper, we first demonstrate our work in slowing down hot carrier relaxation through strategies involving electronic structure management, phonon structure management, and decoupling carriers from defects. These approaches effectively engineer carrier-phonon, phonon-phonon, and carrier-carrier (defect) interactions. Subsequently, we demonstrate the alignment of the transition dipole moment in self-assembled nanoplatelets, resulting in polarised electroluminescence with a high degree of polarization comparable to single nanocrystals.<br/>First of all, we introduce new perovskite nanocrystals, formamidinium tin iodide nanocrystals (FASnI3 NCs), where quantum confinement significantly influences the electronic structure. The evolution in electronic structure from a continuous band structure to separate energy states is directly observed with decreasing nanocrystal size. The appearance of separate energy levels slows down the cooling of hot carriers by two orders of magnitude at low injected carrier densities. We attribute the slowed carrier cooling to a phonon bottleneck effect, where the discrete energy level structure effectively suppresses carrier cooling by optical phonon emission, leading to significant enhancement in cooling time. Importantly, this slow cooling is observed in the limit of low-intensity illumination, making it practically relevant. In addition to manipulating the electronic band structure, we demonstrate the management of the phonon band structure by introducing tin into lead halide perovskites. Increasing the tin content leads to screened Fröhlich interaction, suppressed Klemens decay, and reduced thermal conductivity (acoustic phonon transport), contributing to slowed relaxation mediated by the hot phonon bottleneck effect. To further control ultrafast non-equilibrium processes on a timescale of tens to hundreds of femtoseconds, we decouple hot carriers from sub-bandgap defects via sodium doping, resulting in a decreased energy loss rate during the thermalisation process. The control over non-equilibrium electron dynamics we achieved offers new insights into the intrinsic photophysics of perovskite nanocrystals, with direct implications for hot carrier solar cells.<br/>In addition to adjusting the electronic and phonon structures, we also achieve the modulation of exciton fine structure, along with the precise alignment of the transition dipole moment in self-assembled nanoplatelets. This results in a substantial number of excitons recombining at a specific energy level within the triplet manifold with minimal relaxation to other triplet states, leading to polarised electroluminescence with a high degree of polarisation approaching that of single nanocrystals. Our approach addresses the critical challenge of translating the high degree of polarisation found in photo-excited individual nanocrystals to an electrically driven film level, opening up a new frontier for enabling spin-related technologies through precise control of dynamics at the fine-structure level.