Constructing Bulk Homojunction for Intrinsic Photo-Charge Utilization in Organic Solar Cells

The photo-charge generation from localized Frenkel excitons (EXs) reduces the theoretical power conversion efficiency (PCE) of organic solar cells (OSCs) below Shockley-Queisser limit as extra energy is required for EX splitting at heterojunctions. Though developing the intrinsic photo-charge pathway based on intermolecular charge transfer (ICT) excitons is expected to break the efficiency bottleneck, its current contribution to charge photogeneration is negligible due to the difficulties in spontaneous ICT dissociation. To amplify the intrinsic photo-charge utilization, a concept of bulk homojunction (BHOJ) within non-fullerene acceptors (NFAs) is proposed. Under this structure, the EX converts to ICT in NFA grains, whereas the anisotropic molecular orientation at grain boundaries drives ICT splitting. To fulfill the morphological requirements, a co-solvent strategy is taken: a good main solvent, chloroform, guarantees the NFA's crystallinity, and a poor auxiliary solvent, acetone, helps the anisotropic grain formation. Taking advantage of the BHOJ, a fivefold growth of short-circuit current density (J_SC) is observed in Y6-only OSCs. More importantly, its compatibility with cost-effective printing, fabrication, and semi-transparent devices is also confirmed in model devices, even with the existence of bulk heterojunction (BHJ) structure. This work highlights the BHOJ as a general morphological pursuit for future OSCs' improvements.