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)PbI, where HA stands for the histammonium ((CNH)) as a diammonium cation and A stands for methylammonium (CHNH) or formammonium (HC(NH)). 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)PbI and 1.80 eV for (HA)(MA)PbI) have significantly lower band gaps than those of HAPbI (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.