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 Snx Pbx-1 I4 . Devices fabricated with Bn2 SnI4 show promising efficiencies of around 2.3 %.