Reducing the Doping and Defect Density in Mixed Sn-Pb Halide Perovskite by SnF₂

Last years mixed Sn-Pb perovskites have been applied in single- and multi-junction solar cells as absorbers with a low bandgap energy down to 1.2-1.3 eV, reaching power conversion efficiencies of 23%. However, the performance is still limited by tin oxidation resulting in p-type self-doping. To counteract doping, a plethora of additives has been explored, including SnF₂. To shed light on the functionality of this additive, we prepared mixed Sn-Pb perovskite thin films (Cs_0.25FA_0.75Sn_0.5Pb_0.5I₃) with varying amounts of SnF₂ ranging from 0 to 20 mol% in the spin-coating solution. The dark conductivity of corresponding thin films was analyzed via steady state microwave conductance techniques. A large decrease of more than one order of magnitude in conductivity from ∼85 to <∼2 S/m was observed on going from 0 to 1-2 mol% SnF₂, while no further measurable reduction occurs for higher SnF₂ concentrations. Moreover, we demonstrate that the minimum SnF₂ addition required to achieve this reduction in conductivity is not absolute but is highly dependent on the initial quality of the SnI₂ precursor. In addition, we also investigated the dynamics of laser-induced excess carriers as function of the SnF₂ concentrations using time-resolved microwave conductivity. Although the charge carrier mobility is not affected, the charge carrier lifetime progressively increases with higher SnF₂ concentrations up to 10 mol% from ∼102 to ∼442 ns. By fitting the laser intensity-dependent photoconductivity signals with a model, the main kinetic parameters affecting the photocarrier dynamics can be extracted. From the results we conclude that both the doping but also the defect density concomitantly decreases with increasing SnF₂, with the most prominent changes between 0 and 2 mol%. Via additional measurements of optical and opto-electronic properties we also noticed a slight reduction in the Urbach energy and optically active, shallow trap states upon SnF₂ addition. In contrast, no significant structural and/or morphological changes of the perovskite films on SnF₂ addition could be discerned. Therefore, it is inferred that only a minuscule fraction of the mixed Sn-Pb perovskite is modified by SnF₂, in contrast with the tremendous improvement in the charge carrier properties. We attribute this improvement to the fact that SnF₂ scavenges the oxidized Sn⁴⁺ from the spin-coating solution, leading to reduced self-doping. To suppress doping and reduce the crystal defect density, our work underlines the importance of the quality of the SnI₂ precursor in combination with the optimal SnF₂ concentration on the perovskite opto-electronic properties.