The Roles of Organic Spacer Molecular Structures in Modulating Crystallization Toward High‐Performance Quasi‐2D Perovskite Solar Cells

Organic spacers play a crucial role in governing the optoelectronic properties of quasi‐2D perovskites; however, a limited understanding of their structure‐performance relationships hampers progress in enhancing device efficiency. Here, we systematically explore how the chlorine substituent position on commonly used benzylamine organic spacer affects quasi‐2D perovskite solar cell performance. Combining experimental and theoretical calculations results, we demonstrate that the structure of the organic spacer influences both the formation energy of low‐dimensional perovskites and the interactions between the organic spacer and the 3D perovskite framework. Our findings reveal that the introduction of a meta‐chlorine substitution into benzylamine results in a higher formation energy for 2D perovskites and improved interaction with 3D perovskites, leading to the formation of a well‐structured film with reduced defects. Correspondingly, the optimized meta‐chlorine benzylamine device achieves a peak efficiency exceeding 20% and exhibits significantly improved long‐term light, heat and humidity stability.