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.

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Dramatic advances in perovskite solar cells (PSCs) and the blossoming of wearable electronics have triggered tremendous demands for flexible solar-power sources. However, the fracturing of functional crystalline films and transmittance wastage from flexible substrates are critical challenges to approaching the high-performance PSCs with flexural endurance. In this work, a nanocellular scaffold is introduced to architect a mechanics buffer layer and optics resonant cavity. The nanocellular scaffold releases mechanical stresses during flexural experiences and significantly improves the crystalline quality of the perovskite films. The nanocellular optics resonant cavity optimizes light harvesting and charge transportation of devices. More importantly, these flexible PSCs, which demonstrate excellent performance and mechanical stability, are practically fabricated in modules as a wearable solar-power source. A power conversion efficiency of 12.32% for a flexible large-scale device (polyethylene terephthalate substrate, indium tin oxide-free, 1.01 cm ) is achieved. This ingenious flexible structure will enable a new approach for development of wearable electronics.