Hot Carrier Cooling Dynamics in Tin Tri-Iodide Perovskites Thin Films

Abstract

Hot carriers (HC) offer a potential route to surpass conventional efficiency limits in solar cells. Generated after light absorption, these high-energy carriers can significantly boost device performance if extracted before dissipating energy as heat. Although HC cooling has been studied in tin-based perovskites, interpreting these dynamics is challenging because high-energy, high-fluence pump pulses required to generate HCs have shallow penetration depths, confining excitation near the surface. In tin-based perovskites, surface properties differ from the bulk, and surface defects may strongly contribute to the optical response. Here, by using pump-push-probe ultrafast transient absorption spectroscopy, we photo-excite the semiconductor thin film with a low-energy pump, in a photon density range typical for solar cell operation, making relevant the bulk electronic properties, and selectively re-excite the photo-carriers only once they cool down, enabling direct tracking of HC relaxation in the relevant spectral region while minimizing surface contributions. We track the cooling process in FA0.85Cs0.15SnI3 (FA = Formamidinium) as a benchmark sample and extend the analysis to single-cation FA+, MA+ (MA = methylammonium), and Cs+ tin perovskites. Across all compositions, and at different doping densities, we find cooling timescales in the 150–600 fs time range. These results highlight the central role of phonon-mediated interactions and show that ultrafast cooling fundamentally limits hot carrier extraction in tin halide perovskites.