A Universal Approach for Potassium‐Passivated 2D Perovskites

2D hybrid halide‐layered perovskites have been compelling candidates for photovoltaics and optoelectronics owing to their remarkable electrical and optical properties. However, high‐quality crystals are usually obtained either through complicated and energy‐consuming procedures or using toxic solvents, which remains a significant challenge to the crystal synthesis. To address the issue, a facile, universal, ecofriendly, and energy‐saving approach to synthesize the Ruddlesden−Popper‐ and Dion−Jacobson‐phase 2D‐layered potassium‐passivated perovskites at room temperature is reported. By involving potassium iodide to the reaction, potassium ions can assist both the crystal growth and surface passivation, improving their stability and passivated trap states. Potassium‐passivated (R‐MBA)₂PbI₄ exhibits superior optoelectronic features than pristine (R‐MBA)₂PbI₄ as well as outstanding strain stability. The findings herein may not only improve synthetic methods to obtain high‐quality crystals, but also motivate more fundamental investigations for potential optoelectronic and flexible applications.