Investigation of the Impact of Cs/Sn Ratio on Material Properties in Coevaporated CsSnI₃ Libraries

Tin-halide perovskites represent a potential non-toxic alternative to lead-based perovskite solar cell absorbers showing similar fundamental optical and electrical material properties as the lead-based halide perovskites. However, solar cell peformance of Sn-based halide perovskites so far has been much lower. This has been attributed to the oxidation of Sn²⁺, strong (intrinsic) p-type doping, and rapid crystallization in the precursor solution which results in poor thin film morphology. Inorganic halide perovskites are attractive from a thermal stability point of view, however, tend to struggle with the presence of a competing non-photoactive non-perovskite delta-phase.<br/>In our study we use a vacuum-based co-evaporation approach to study the influence of a varying Cs/Sn composition ratio on the phase stability as well as the optoelectronic properties of CsSnI₃.<br/>CsSnI₃ perovskite thin films were deposited with a lateral chemical gradient to obtain thin film libraries with a Cs/Sn ratio from 0.9 to 1.1. We find that the evaporated perovskite crystallizes in the orthorhombic gamma-phase with different preferred orientations in case of excess Sn and/or Cs content. Similar to previous results for CsPbI₃[1] we find that Cs-rich regions show significantly better stability of the γ-phase than the Sn-rich sample.<br/>With respect to the optoelectronic properties we find a strong blue shift of the absorption onset from 1.35-1.45 eV with increasing Cs-content, whereas the room temperature photoluminescence peak position stays constant at 1.33 eV. The photoluminescence quantum yield shows a distinct maximum with a rather high value of 0.6% for the stoichmetric samples, falling of by a factor of 2-3 toward both off-stoichimetric sides. Despite the high PLQY minority carrier lifetimes as measured by TRPL are relatively low with about 2-3 ns, indicating a how equilibrium carrier density above 10¹⁷cm^-3 in the samples.<br/>These results indicate that defect-related non-radiative recombination still pose a major challenge for obtaining high performance in inorganic Sn-based halide perovskite solar cells.<br/>[1] Pascal Becker et al., Adv. En. Materials 2019, 9, 1900555