Unveiling Trapping Mechanisms in Tin-Halide Perovskites Sidestepping Dopant-Mediated Recombination

Tin halide perovskites (THPs) are promising low-bandgap materials for photovoltaic and light-emitting applications^1-2. In lead-based perovskites, iodine chemistry plays a major role in forming deep hole traps. However, in THPs, tin chemistry dominates³, with tin vacancies acting as shallow defects and influencing intrinsic doping, while tin interstitials, iodine vacancies, and surface Sn(IV) serve as deep electron traps^4.<br/>Due to the intrinsic doping density, carrier trapping processes are overshadowed by the fast and efficient radiative recombination of photogenerated carriers with dopant holes^5,6. As a result, even though trapping occurs, its impact on recombination dynamics might be indistinguishable from the rapid pseudo-monomolecular recombination with dopant carriers. Therefore, solely examining fast dynamics does not provide a comprehensive understanding of trap states' effects. Variations in carrier lifetime and radiative efficiency cannot be unambiguously attributed to changes in trap or doping density. This poses a significant challenge in thin-film optimization, as the effects of compositional engineering cannot be directly assessed.<br/>To directly investigate processes unaffected by doping and dopant-mediated recombination, we use Transient Absorption Spectroscopy to study sub-bandgap and band-edge transitions over time scales from ultrafast (fs) to slow (μs). This approach clarifies the distinct roles of doping- and trap-mediated processes in determining optoelectronic properties.<br/>The dynamics of sub-bandgap trap states at early times reveal two distinct processes: a slow component lasting longer than 1 nanosecond and a fast population of shallow traps lasting tens of picoseconds. The latter repopulates the band-edges and is particularly significant as it may impact the apparent carrier cooling. Contrarily, in time scales of microseconds, we detect long-lived trap states. By separately modulating doping and defect density with additives and intentional air exposure, we identify the spectral features and dynamics of Sn(IV) and tin interstitial (Sn_i) trap states. These findings are supported by simulations of photoexcited carrier dynamics, which replicate the decay of the transient absorption spectroscopy signal to explain the experimental data.<br/><br/>REFERENCES:<br/>1. Yuan, F. <i>et al.</i> Bright and stable near-infrared lead-free perovskite light-emitting diodes. <i>Nat. Photonics</i> <b>18</b>, 170-176 (2024).<br/>2. Wang, L. <i>et al.</i> 14.31 % Power Conversion Efficiency of Sn-Based Perovskite Solar Cells via Efficient Reduction of Sn⁴⁺. <i>Angew. Chemie</i> <b>135</b>, 1-5 (2023).<br/>3. Zhou, Y., Poli, I., Meggiolaro, D., De Angelis, F. & Petrozza, A. Defect activity in metal halide perovskites with wide and narrow bandgap. <i>Nat. Rev. Mater.</i> <b>6</b>, 986-1002 (2021).<br/>4. Ricciarelli, D., Meggiolaro, D., Ambrosio, F. & De Angelis, F. Instability of tin iodide perovskites: Bulk p-doping versus surface tin oxidation. <i>ACS Energy Lett.</i> <b>5</b>, 2787-2795 (2020).<br/>5. Poli, I. <i>et al.</i> High External Photoluminescence Quantum Yield in Tin Halide Perovskite Thin Films. <i>ACS Energy Lett.</i> <b>6</b>, 609-611 (2021)