Surface Crystallization Modulation toward Highly‐Oriented and Phase‐Pure 2D Perovskite Solar Cells

2D perovskites have shown great potential toward stable and efficient photovoltaic devices. However, the crystal orientation and phase impurity issues of 2D perovskite films originating from the anisotropic crystal structure and specific growth mechanism have demoted their optoelectronic performances. Here, the surface crystallization modulation technique is introduced to fabricate the high-quality 2D perovskite films with both vertical crystal orientation and high phase purity by regulating the crystallization dynamics. The solvent atmosphere condition is instituted during film processing, which promotes the formation of an oriented 2D perovskite layer in stoichiometric composition at the vapor-liquid interface and templates the subsequent film growth. The solar cells based on the optimized 2D perovskite films exhibit a power conversion efficiency of 15.04%, the record for 2D perovskites (with the perovskite slab thickness n ≤ 3 and high phase purity). The solar cells based on the highly-oriented and phase-pure 2D perovskite films also demonstrate excellent thermal and humidity stabilities.