The impure phase in typical solution-processed two-dimensional/three-dimensional (2D/3D) perovskite heterostructures limit the performance and stability of perovskite solar cells. We overcome this limitation through a templated growth strategy utilizing excess lead iodide, which directs the formation of robust phase-pure 2D perovskite at the 3D perovskite surface. This strategy effectively suppresses phase segregation and ion interdiffusion at the 2D/3D perovskite interface, while promoting favorable energy alignment for charge extraction. The resulting 2D/3D devices achieved 26.8% efficiency (26.5% certified) and improved durability, retaining over 95% of their initial performance after 3000 h of continuous operation and maintaining nearly 100% of their initial performance throughout 90 days of outdoor testing, showcasing promising real-world stability.