Stacking of Layered Halide Perovskite from Incorporating a Diammonium Cation into Three-Dimensional Perovskites

Quasi two-dimensional (2D) layered perovskites have been emerging as promising candidates for photovoltaic cells because they exhibit intrinsic stability and a higher tunability of optical properties compared to three-dimensional (3D) perovskites. However, since most 2D perovskites have bulkier groups as an organic space group, they will inevitably have a van der Waals gap between the inorganic layers and their crystal growth directions orient in a lateral direction. It also interrupts carrier transport across the conducting inorganic layer in the solar cell. Here, we presents the new homologous 2D layered perovskites, (HA)(A)PbI₃, where HA stands for the histammonium ((C₅N₃H₁₁)²⁺) as a diammonium cation and A stands for methylammonium (CH₃NH₃⁺) or formammonium (HC(NH₂)₂⁺). Since the ditopic HA has a diammoinium cation, it connects the inorganic slabs stacked in the vertical direction. The inorganic layer is stacked on the other layer to form a layered structure, which results in rigid and stable structures. These materials (1.64 eV for (HA)(FA)PbI₃ and 1.80 eV for (HA)(MA)PbI₃) have significantly lower band gaps than those of HAPbI₄ (2.20 eV). Compared to the pure 2D and 3D perovskites, these perovskites have a longer electron lifetime due to the vertical crystal structure and show improved environmental stability for perovskite solar cell application.