Sn and mixed Sn-Pb iodide perovskites (ASnI₃, ASn_0.5Pb_0.5I₃) provide an excellent opportunity to yield lower toxicity and efficient Pb-free perovskite photovoltaics, close-to-ideal bandgaps in single-junction solar cells and near-infrared absorption in all-perovskite tandem solar cells. Despite a growing interest in these perovskite variants, the sensitivity of Sn(II) to oxidation under operational conditions remains the main challenge towards their widespread adoption. Additionally, their strong p-type character arising from degradation-related defects (i.e., Sn(IV) states, Sn(II) vacancies) further hamper their potential, causing charge carrier losses. It is therefore imperative to gain detailed knowledge of their chemical decomposition pathways to boost their stability, as well as to design strategies to fine-tune their electrical properties. This talk will cover our work on these avenues, unveiling the degradation mechanisms of both Sn and Sn-Pb perovskites under ambient conditions and presenting novel molecular doping routes to harness carrier density in Sn-Pb perovskites.

Firstly, the key role of perovskite native iodine (I₂) on determining stability will be unveiled. Previously, we identified a cyclic degradation pathway where I₂, formed from Sn perovskite exposure to ambient conditions, aggressively degrades the material to SnI₄, which then evolves back to I₂ in the presence of H₂O and O₂.^1,2 We observe this process to govern the degradation of Sn-Pb perovskites as well, highlighting the need to identify perovskite compositions with innate I₂ resilience. Specifically, we reveal the inconspicuous role of A-site cations in these oxidation pathways, finding that Cs-rich compositions present rates of I₂ and SnI₄ generation one order of magnitude lower than their methylammonium (MA)-rich analogues. We ascribe the enhanced resistance of Cs-rich Sn-Pb perovskites against oxidative stress vs MA compositions to stronger I₂ adsorption at the surface of perovskite in the latter, mediated by the polarising power of the MA cation.

Secondly, we address the electrical aspect by providing novel molecular doping design rules based on the crucial role of host-dopant interactions.³ By leveraging an n-type molecule, namely n-DMBI-H, we discern a unique dative bonding mechanism between Sn atoms in Sn-Pb perovskite surfaces and the dopant, followed by the dissociation of an electron-donating hydride from n-DMBI-H. Arising from the higher Lewis acidity of Sn, we find this surface interaction to mediate charge transfer, providing an effective way to compensate the p-type character of Sn-Pb perovskites (nearly one order of magnitude reduction in charge carrier density).

These insights offer a comprehensive roadmap to developing stable, efficient and adaptable Sn and Sn-Pb perovskite solar cells and beyond.