CsPb₂Br₅-assisted direct crystallization of the 3D perovskite phase for highly efficient and stable solar cells

Improving the crystallization quality of perovskite films is crucial for both the efficiency and stability of perovskite solar cells. However, the solid-state transition of formamidinium lead iodide perovskite thin films from the δ to α phase at high temperatures typically introduces a significant number of lattice defects, which in turn leads to increased carrier recombination rates and reduced humidity stability. Directly growing the α-phase perovskite phase from a precursor solution still remains a challenge. In this work, a low solubility, halogen-rich CsPb₂Br₅ crystal was introduced into the perovskite solution to regulate the crystallization behavior of the perovskite layer. It was demonstrated that the CsPb₂Br₅ nanocrystals pre-precipitated and induced the direct crystallization of the α-phase perovskite during the solution processing. The direct formation of the α-phase instead of the δ-phase effectively enhances the crystalline quality and reduces the defect density of the perovskite films. As a result, the corresponding device exhibits an enhanced power conversion efficiency of 25.55%, and the module efficiency with 14 cm² of active area reaches 22.35% (certified PCE is 21.7%). Thanks to the higher crystalline quality, these unencapsulated devices exhibit excellent operational stability at the maximum power point (∼92% efficiency retention after 1000 h).