Anion Exchange‐Induced Crystal Engineering via Hot‐Pressing Sublimation Affording Highly Efficient and Stable Perovskite Solar Cells

Crystalline, dense, and uniform perovskite thin films are crucial for achieving high‐power conversion efficiency solar cells. Herein, a universal method of fabricating highly crystalline and large‐grain perovskite films via crystal engineering is demonstrated. Anion exchange of Cl − and I − , and annealing perovskite films, in an ultraconfined and uniform temperature enclosed space with saturated MAI (or FAI) vapor using hot‐pressing sublimation technology are conducted. This process ensures a rapid crystal growth rate due to fast exchange between the gas phase and the crystalline film to reduce vertically oriented grain boundaries. The generation of the commonly observed PbI 2 phase is also suppressed due to the chemical equilibrium state during the thermal annealing process. Using this approach, pinhole‐free perovskite films with preferred crystal orientation and micrometer‐scale grains are obtained, leading to a high steady‐state efficiency of 22.15% based on mixed‐cation perovskite composition. In addition, devices based on different perovskite compositions all exhibit enhanced photovoltaic performance based on the crystal engineering method. The device (without encapsulation) has an efficiency loss of about only 4% after 2520 h of aging in ambient conditions and retains 87% of its initial efficiency after 1000 h of continuous 1 Sun light soaking, thus demonstrating considerably improved stability.