Perovskite solar cells (PSCs) have experienced an outstanding advance in power conversion efficiency (PCE) by optimizing the perovskite layer morphol., compn., interfaces, and charge collection efficiency. To enhance PCE, here we developed a new method i.e., engineering a compositional gradient thinly at the rear interface between the perovskite and the hole transporting materials. We demonstrate that charge collection is improved and charge recombination is reduced by formation of an engineered passivating layer, which leads to a striking enhancement in open-​circuit voltage (VOC)​. The passivation effect induced by constructing an addnl. FAPbBr3-​xIx layer on top of the primary (FAPbI3)​0.85(MAPbBr3)​0.15 film was proven to function as an electron blocking layer within the perovskite film, resulting in a final PCE of 21.3​%. Our results shed light on the importance of the interfacial engineering on the rear surface of perovskite layers and describe an innovative approach that will further boost the PSC efficiency.