This study presents a novel mixed-cation lead mixed-halide perovskite solar cell with enhanced efficiency and stability. The perovskite layer is composed of butylammonium-Cs-formamidinium mixed-cation lead mixed-halide, which improves the crystallinity and stability of the 3D perovskite phase. The addition of butylammonium cations enhances the crystallinity of the 3D perovskite phase and induces the formation of new layered phases in the films. This results in "plate-like" layered perovskite crystallites that are interspersed between 3D perovskite grains, allowing for efficient charge transport and reducing charge recombination. The study shows that the addition of butylammonium significantly enhances the long-term stability of the devices. The cells sustain more than 80% of their "post burn-in" efficiency after 1,000 hours of operation under simulated full spectrum sunlight. With additional sealing, the lifetime is extended to nearly 4,000 hours. The study demonstrates that engineering heterostructures between 2D and 3D perovskite phases is both possible and can lead to enhanced performance and stability of perovskite solar cells. The devices achieved a power conversion efficiency of 20.6% (stabilized efficiency of 19.5%) from a narrow bandgap (1.61 eV) perovskite solar cell and of 17.2% (stabilized efficiency of 17.3%) from a wider bandgap (1.72 eV) perovskite solar cell. The study also shows that the addition of butylammonium greatly enhances the long-term stability of the devices. The results demonstrate that the BA/FA/Cs perovskite system is a promising candidate for stable perovskite solar cells.This study presents a novel mixed-cation lead mixed-halide perovskite solar cell with enhanced efficiency and stability. The perovskite layer is composed of butylammonium-Cs-formamidinium mixed-cation lead mixed-halide, which improves the crystallinity and stability of the 3D perovskite phase. The addition of butylammonium cations enhances the crystallinity of the 3D perovskite phase and induces the formation of new layered phases in the films. This results in "plate-like" layered perovskite crystallites that are interspersed between 3D perovskite grains, allowing for efficient charge transport and reducing charge recombination. The study shows that the addition of butylammonium significantly enhances the long-term stability of the devices. The cells sustain more than 80% of their "post burn-in" efficiency after 1,000 hours of operation under simulated full spectrum sunlight. With additional sealing, the lifetime is extended to nearly 4,000 hours. The study demonstrates that engineering heterostructures between 2D and 3D perovskite phases is both possible and can lead to enhanced performance and stability of perovskite solar cells. The devices achieved a power conversion efficiency of 20.6% (stabilized efficiency of 19.5%) from a narrow bandgap (1.61 eV) perovskite solar cell and of 17.2% (stabilized efficiency of 17.3%) from a wider bandgap (1.72 eV) perovskite solar cell. The study also shows that the addition of butylammonium greatly enhances the long-term stability of the devices. The results demonstrate that the BA/FA/Cs perovskite system is a promising candidate for stable perovskite solar cells.