This study investigates the development of highly efficient and stable perovskite solar cells using a self-assembled butylammonium-Cs-formamidinium mixed-cation lead mixed-halide perovskite photoactive layer. The addition of long-chain alkyl-ammonium halides to the formamidinium-cesium-based perovskite precursor solution enhances the crystallinity of the 3D perovskite phase and induces the formation of new layered phases in the films. By carefully regulating the composition, "plate-like" layered perovskite crystallites are formed between the host 3D perovskite grains, which allows efficient charge carrier transport in the 3D perovskite phase while reducing charge recombination via grain boundary passivation. The optimized composition achieves a power conversion efficiency of 20.6% (stabilized efficiency of 19.5%) for a narrow bandgap (1.61 eV) perovskite solar cell and 17.2% (stabilized efficiency of 17.3%) for a wider bandgap (1.72 eV) perovskite solar cell optimized for tandem applications. Additionally, the addition of butylammonium significantly enhances the long-term stability of the devices, with cells sustaining more than 80% of their "post burn-in" efficiency after 1,000 hours of aging under simulated full spectrum sunlight in an ambient environment without encapsulation. With additional sealing, the lifetime extends to close to 4,000 hours. The work demonstrates that engineering heterostructures between 2D and 3D perovskite phases can enhance both the performance and stability of perovskite solar cells.This study investigates the development of highly efficient and stable perovskite solar cells using a self-assembled butylammonium-Cs-formamidinium mixed-cation lead mixed-halide perovskite photoactive layer. The addition of long-chain alkyl-ammonium halides to the formamidinium-cesium-based perovskite precursor solution enhances the crystallinity of the 3D perovskite phase and induces the formation of new layered phases in the films. By carefully regulating the composition, "plate-like" layered perovskite crystallites are formed between the host 3D perovskite grains, which allows efficient charge carrier transport in the 3D perovskite phase while reducing charge recombination via grain boundary passivation. The optimized composition achieves a power conversion efficiency of 20.6% (stabilized efficiency of 19.5%) for a narrow bandgap (1.61 eV) perovskite solar cell and 17.2% (stabilized efficiency of 17.3%) for a wider bandgap (1.72 eV) perovskite solar cell optimized for tandem applications. Additionally, the addition of butylammonium significantly enhances the long-term stability of the devices, with cells sustaining more than 80% of their "post burn-in" efficiency after 1,000 hours of aging under simulated full spectrum sunlight in an ambient environment without encapsulation. With additional sealing, the lifetime extends to close to 4,000 hours. The work demonstrates that engineering heterostructures between 2D and 3D perovskite phases can enhance both the performance and stability of perovskite solar cells.