A Multifunctional 2D‐Conjugated BDT Polymer Interlayer Enables Over 20% Organic Solar Cells

Abstract The bicontinuous active layer morphology plays a crucial role in affecting the charge transport/recombination in organic solar cells (OSCs). However, the conventional bulk heterojunction (BHJ) blending typically results in an uncontrollable vertical phase distribution, hindering further improvement in power conversion efficiency (PCE). Here, we designed a two‐dimensional conjugated polymer donor PBDB‐tvt by incorporating a long‐conjugated side chain, chlorinated alkylthio‐thiophene‐vinyl‐thiophene (tvt), onto the benzodithiophene (BDT) unit. The extended structure up‐shifts energy level, enhances optical absorption, and improves charge transport. Interestingly, PBDB‐tvt shows selective solubility in common processing solvents, making it suitable for sequential deposition. By using it as the interlayer between the electrode modification layer and bulk heterojunction, we constructed a hybrid device (functional modification layer/BHJ) configuration. The tailored structure not only brings improved phase distribution but also enhances light utilization in the short‐wavelength region, which leads to a simultaneous increase of photovoltaic parameters, including open‐circuit voltage, short‐circuit current density, and fill factor. As a result, the best device achieves a maximum PCE of 20.3%. This contribution highlights the pivotal role of a multifunctional interlayer in enhancing the light absorption and controlling the active layer morphology, providing a feasible method to further improve the photovoltaic performance of OSCs.