Tuning Morphology of Active Layer by using a Wide Bandgap Oligomer‐Like Donor Enables Organic Solar Cells with Over 18% Efficiency

Abstract A wide bandgap oligomer‐like donor CNS‐6‐8 is synthesized and incorporated into the host PM6:Y6:PC₇₁BM system to tune the morphology of the active layer for better device performance. Due to the good miscibility of CNS‐6‐8 with both host donor (PM6) and acceptors (Y6 and PC₇₁BM), an optimized morphology is achieved with the appropriate phase separation size and enhanced crystallinity, which ultimately leads to more efficient exciton dissociation, charge transport, and lower nonradiative energy loss. As a result, the quaternary device achieves an improved efficiency of 18.07%, with a simultaneously increased open circuit voltage of 0.868 V, fill factor of 78.8%, and the comparable short‐circuit current density of 26.43 mA cm⁻². This work indicates that the favorable 3D interpenetrating network morphology of Y6 containing blend films can be optimized by introducing small amount of a specific molecule with high crystallinity, thus providing an effective strategy to achieve better photovoltaic performance for state‐of‐the‐art Y6 analogs‐based organic solar cells.