The performance of single-junction solar cells in thermal equilibrium is limited by the conversion of excess photon energy to heat through hot-carrier—phonon interactions [1]. However, hot-carrier photovoltaic devices, a future-generation technology for solar power conversion, could overcome this limitation. If the temperature of the charge-carrier gas is elevated compared to that of the crystal lattice, extraction of the photoexcited charge carriers with above-bandgap energies may be possible, boosting the energy conversion efficiency. Metal-halide perovskites, and in particular tin-iodide perovskite semiconductors, have been investigated in recent years as potential candidates for such technology, owing to the long hot-carrier cooling times observed in these materials [2]. Reported nanosecond-long cooling dynamics could allow for efficient extraction of hot carriers high above the bandgap, pushing the efficiency limit up to 66% PCE [3,4].

Unfortunately, tin-iodide perovskites suffer from tin-vacancy formation. These hole-donating point defects lead to significant electrical doping of the materials. The ocean of cooled, dopant holes could lead to rapid lowering of hot-carrier temperatures through cold-carrier—hot-carrier scattering events, hindering the application of these materials in hot carrier devices operating under solar illumination [5].

In this talk, we present the results of our investigation of carrier cooling dynamics in tin-iodide perovskites, performed using a novel pump-push-probe terahertz spectroscopic technique [6]. We show that, when applied to high-mobility materials, this technique has high sensitivity which enables the investigation of hot-carrier dynamics at lower excitation densities (below hot-phonon bottleneck threshold) than alternative spectroscopic methods. Our study, performed at these low hot-carrier densities, reveal ultrafast, sub-picosecond cooling dynamics in doped tin-iodide perovskite semiconductors, originating from cold-carrier—hot-carrier scattering. Our results are directly relevant to the applicability of these materials in hot-carrier devices, for which the fast thermalisation of the photoexcited carriers with cold ocean of dopant holes could hinder the efficiency of above-bandgap extraction, highlighting the need for further doping suppression methods in lead-free perovskites.