Conjugation Pathway of Benzobisoxazoles in Polymer Donors Mediates the Charge Management and Enables Organic Solar Cells with Record Certified Efficiency

Charge management plays a pivotal role in achieving high-performance bulk heterojunction (BHJ) organic solar cells (OSCs). In this study, two efficient polymer donors are designed, P[4,8]BBO and P[2,6]BBO, by regulating the conjugation pathways of benzobisoxazoles (BBO) through 4,8- and 2,6-linkages, respectively. Comparing to P[2,6]BBO, the isomer of conjugation pathway has been proved to enable P[4,8]BBO a shallower highest occupied molecular orbital (HOMO) energy level of -5.20 eV, significantly enhanced luminescence efficiency, and reduced aggregation property. These improvements lead to a dramatic increase in device efficiencies from 2.6% for P[2,6]BBO:eC9-2Cl to 19.0% for P[4,8]BBO:eC9-2Cl. The combined characterizations show that a better comprehensive charge management can be reached in P[4,8]BBO:eC9-2Cl-based OSCs, yielding a significantly higher short-circuit current density (JSC) and fill factor (FF) parameters compared to P[2,6]BBO:eC9-2Cl-based ones. Furthermore, P[4,8]BBO demonstrates good applicability and can achieve an impressive efficiency of 19.4% in all-polymer solar cells with a third-party certified efficiency of 19.1%. This work highlights the critical role of conjugation pathway isomerism in mediating polymeric properties and advancing the development of high-performance multifunctional photovoltaic materials.