Lead and HTM Free Stable Two-Dimensional Tin Perovskites with Suitable Band Gap for Solar Cell Applications

Organic-inorganic hybrid perovskites have attracted great attention over the last few years as potential light-harvesting materials for efficient and cost-effective solar cells. However, the use of lead iodide in state-of-the-art perovskite devices may demonstrate an obstacle for future commercialization due to toxicity of lead. Herein we report on the synthesis and characterization of low dimensional tin-based perovskites. We found that the use of symmetrical imidazolium-based cations such as benzimidazolium (Bn) and benzodiimidazolium (Bdi) allow the formation of 2D perovskites with relatively narrow band gaps compared to traditional -NH₃ ⁺ amino groups, with optical band gap values of 1.81 eV and 1.79 eV for Bn₂ SnI₄ and BdiSnI₄ respectively. Furthermore, we demonstrate that the optical properties in this class of perovskites can be tuned by formation of a quasi 2D perovskite with the formula Bn₂ FASn₂ I₇ . Additionally, we investigate the change in band gap in the mixed Sn/Pb solid solution Bn₂ Sn_x Pb_x-1 I₄ . Devices fabricated with Bn₂ SnI₄ show promising efficiencies of around 2.3 %.