Tin-based halide perovskites (THPs) have emerged as promising candidates for both photovoltaics and near - IR light emitting applications thanks to their high carrier mobilities, low band gap, ideal for pairing with silicon in tandem solar cells^[1,2]. THPs present highly stable acceptor defects such as Sn vacancies and I interstitials, which result in a permanent population of holes: the material than behaves as a p-doped semiconductor, with self-doping densities as high as 10²² cm^-3 that can negatively impact device performances. As such, it is of critical importance to devise strategies to both quantify and control the dopant hole densities in THPs by compensating the oxidation state of Sn during material fabrication using additives such as SnF₂.

In this context, terahertz (THz) spectroscopy represents a powerful tool to characterize the carrier populations and dynamics in THPs. THz radiation is sensitive to mobile charged carriers^[3], as well as their coupling with lattice phonons^[4]: it can then be used to shed light on charge transport properties in THPs, as low frequency phonons in the THz range have been shown to limit their thermal and electrical conductivity. Moreover, as the doping density affects both transparency to THz radiation and carrier phonon coupling, THz absorption can be used as a sensitive, contactless probe to characterize the self – doping density even in samples where it has been brought down to levels comparable to background carrier densities suitable for device applications (e.g. 10¹⁵ cm^-3), and that could be challenging to characterize by traditional Hall effect measurements. Furthermore, time resolved THz spectroscopy after optical excitation of the material allows to follow the dynamics of photogenerated carriers with ps temporal resolution.

Here we develop a robust technique to study the doping hole density in FACsSnI₃ thin films by analyzing the static THz conductivity response, and characterize ultrafast carrier dynamics using time resolved THz spectroscopy. By supporting our results with DFT calculations, we also investigate the effect of doping concentration and defect states on optical phonon frequencies and their coupling to charge carriers. Finally, the possibility of studying the material polaronic response by combining THz and XUV absorption spectroscopy will be discussed.