In recent years, photovoltaics has attracted attention as a clean energy source. Although silicon-based materials are the mainstay of solar cells, organic–inorganic hybrid perovskite crystals are a candidate alternative. Perovskite solar cells can be lightweight and flexible devices manufactured by easy methods, and some have been developed to achieve photoelectric conversion efficiencies comparable to silicon-based materials. However, one of the organic cations present in perovskite crystals, methylammonium (MA), is prone to decomposition and desorption, leading to device instability. To address this issue, composition control using additives and optimizations of device structures through passivation techniques have been actively explored. In addition, encapsulation with films and improvements in film fabrication techniques have been pursued to achieve uniform thin-film formation. Various additives have been studied, including organic molecules such as guanidinium (GA) and ethylammonium, as well as alkali metal cations like cesium (Cs). Among these, dimethylammonium (DMA) has a larger molecular radius and possesses high molecular symmetry. The introduction of DMA into formamidinium (FA)-based perovskite crystals has been reported to limit the motion of FA, and to promote crystal growth in Cs-based perovskite crystals. In this study, the effects of DMA addition to MA-based perovskite crystals were investigated, and the photovoltaic properties of solar cells fabricated using these crystals were reported. Additionally, first-principles calculations were also carried out to evaluate performance in conjunction with experiments.