Highly Efficient Layer‐by‐Layer Processed Quaternary Organic Solar Cells with Improved Charge Transport and Reduced Energy Loss

How to improve charge transport properties and reduce energy losses are the two main challenges of organic solar cells (OSCs). Herein, a new device fabrication strategy that combines the layer‐by‐layer (LBL) method and a quaternary active layer is demonstrated to realize highly efficient photovoltaic performance of the OSCs based on PM6:PM7/Y6:O1‐2F. The introduction of the second donor, PM7, forms a cascade energy‐level alignment with PM6 that facilitates effective charge transfer. The second acceptor, O1‐2F, which is poorly miscible with the donors, helps to reduce the miscibility between the donors and acceptors, and thus delivers a more desirable vertical phase separation of the active layer. Furthermore, the alloy acceptor of O1‐2F and Y6 can optimize the horizontal and vertical morphology of the active layer, forming an effective charge separation and continuous charge transport channel. As a result, the PM6:PM7/Y6:O1‐2F system shows a distinct increase in charge mobility and a reduced nonradiative loss Δ V nr of 0.231 V compared to the PM6/Y6 system, achieving a high‐power conversion efficiency of 18.23%. The results indicate that the quaternary LBL OSCs are promising for future large‐scale and industrial manufacture of the OSCs.