Additive-Free Sequential Thermal Evaporation of Near-Intrinsic Pb-Sn Perovskites

To boost the efficiency of perovskite solar cells beyond the limit of a single-junction cell, tandem cells are employed, requiring low bandgap materials. This is realized by partially substituting lead(II) (Pb²⁺) with tin(II) (Sn²⁺) in the perovskite structure. In this work, a scalable method is presented to produce formamidinium lead tin iodide (FAPb_0.5Sn_0.5I₃) films by sequential thermal evaporation (sTE) of PbSnI₄, which is an alloy of SnI₂ and PbI₂, and FAI, in vacuum. Annealing at 200 °C yields a highly oriented and crystalline layer comprising grains over 1 µm on average. Photoconductance measurements reveal carrier lifetimes exceeding 2 µs and mobilities ≈100 cm²/(Vs). Structural analysis confirms that, while interdiffusion is abundant even at room temperature, the complete conversion requires high temperatures. Although the incorporation of Cs⁺ into the A-site of the perovskite increases the grain size, charge carrier dynamics are reduced. A comparison between the sTE films and spin-coated samples of the same composition demonstrates the superior photoconductance of the sTE films, without the need for any additives. Overall, this study showcases the potential of sTE for producing high-quality low band gap (LBG) perovskite materials.