Three-dimensional (3D) metal-halide perovskite solar cells (PSCs) have demonstrated exceptional high efficiency. However, instability of the 3D perovskite is the main challenge for industrialization. Incorporation of some long organic cations into perovskite crystal to terminate the lattice, and function as moisture and oxygen passivation layer and ion migration blocking layer, is proven to be an effective method to enhance the perovskite stability. Unfortunately, this method typically sacrifices charge-carrier extraction efficiency of the perovskites. Even in 2D-3D vertically aligned heterostructures, a spread of bandgaps in the 2D due to varying degrees of quantum confinement also results in charge-carrier localization and carrier mobility reduction. A trade-off between the power conversion efficiency and stability is made. Here, by introducing 2D C6H18N2O2PbI4 (EDBEPbI4) microcrystals into the precursor solution, the grain boundaries of the deposited 3D perovskite film are vertically passivated with phase pure 2D perovskite. The phases pure (inorganic layer number n = 1) 2D perovskite can minimize photogenerated charge-carrier localization in the low-dimensional perovskite. The dominant vertical alignment does not affect charge-carrier extraction. Therefore, high-efficiency (21.06%) and ultrastable (retain 90% of the initial efficiency after 3000 h in air) planar PSCs are demonstrated with these 2D-3D mixtures.