Reversible Band Gap Narrowing of Sn-Based Hybrid Perovskite Single Crystal with Excellent Phase Stability

An intriguing reversible band gap narrowing behavior of the lead-free hybrid perovskite single crystal DMASnI₃ (DMA=CH₃ NH₂ CH₃ ⁺ ) from yellow to black is observed without phase transformation. We discuss the transformation mechanism in detail. More interestingly, the transformed samples in black can rapidly self-heal into yellow ones when exposed to deionized water (DI water). Contrary to other hybrid perovskites, DMASnI₃ crystals exhibit excellent water phase stability. For example, DMASnI₃ was immersed in DI water for 16 h and no decomposition was observed. Inspired by its excellent water phase stability, we demonstrate a potential eco-friendly application of DMASnI₃ in photo-catalysis for H₂ evolution in DI water. We present the first H₂ evolution rate of 0.64 μmol h^-1 with good recycling properties for pure DMASnI₃ crystals. After the narrowing process, the optical band gap of DMASnI₃ can be lowered from 2.48 eV to 1.32 eV. Systematical characterizations are applied to investigate their structures and optoelectronic properties. The reversible band gap narrowing behavior and outstanding electrical properties, such as higher carrier mobility and long carrier lifetime show that DMASnI₃ has a great potential for optoelectronic applications.