Molecular Diffusion Enables Gradient Energy‐Level and Defect Passivation for Efficient and Stable n–i–p Perovskite Solar Cells

Perovskite solar cells (PSCs) exhibit promising power conversion efficiencies and strong feasibility for large‐scale manufacturing. However, the energy‐level mismatch between the perovskite layer and charge transport layers impedes charge carrier collection at their interfaces and compromises device performance, an issue that is further exacerbated by the high density of defects on the perovskite surface. Herein, we report a facile molecular diffusion strategy that achieves the synergistic effects of gradient energy‐level alignment and defect passivation at the upper interface of the perovskite layer. The functional molecule (2‐(9H‐carbazol‐9‐yl)ethyl)phosphonic acid (2PACz) reduces the surface roughness of the perovskite layer, fosters improved contact with the hole transport layer, and thereby facilitates hole injection and collection under gradient energy‐level configuration. Additionally, the incorporation of the 2PACz molecule passivates surface defects on the perovskite layer, prolonging charge carrier lifetime and mitigating nonradiative recombination. Consequently, the PSCs modified with 2PACz exhibit a notable efficiency enhancement, rising from 19.17% to 22.45%. More importantly, the unencapsulated 2PACz‐modified devices retain 82% of their initial efficiency following 2064 h of aging under inert conditions. This work thus presents a novel strategy for further boosting the performance and stability of PSCs.