Ultrafast glimpses of the excitation energy-dependent exciton dynamics and charge carrier mobility in Cs ₂ SnI ₆ nanocrystals

Advancements in photovoltaic research suggest that tin-based perovskites are potential alternatives to traditional lead-based structures. Cs₂SnI₆, specifically, stands out as a notable candidate, exhibiting impressive performance. However, its complete potential remains untapped primarily owing to the limited understanding of its photophysics. In light of this, this study aims to bridge this knowledge gap. To commence our study, we first executed theoretical investigations to locate the energetically diverse excitons within the Brillouin zone. Building on this knowledge, we then utilized transient absorption spectroscopy to investigate their temporal evolution. Herein, we observed the formation of high-energy excitons even when the incident photon energy was below the necessary threshold, which is quite distinctive and intriguing. Of particular interest is the generation of ultraviolet (UV) domain exciton using visible photons, which implies that Cs₂SnI₆ has the potential for efficient solar light harvesting. Tracking the kinetics revealed that this unique finding arises due to the intertwined formation and decay pathways undertaken by the different excitons, aided by intervalley scattering and phonon absorption processes. In addition, we found that the decay of the UV exciton was unusually slow. Transient mobility investigations were undertaken to probe the carrier transport behavior that further established hot carriers (HCs) in Cs₂SnI₆ to be highly mobile and susceptible to polaron formation. Overall, our findings demonstrate that Cs₂SnI₆ is a strong candidate for HC-based photovoltaics because it possesses all the prerequisites desired for such applications.