Unraveling Green-Solvent-Driven Morphology Evolution in an Unconventional Small-Molecule Donor/Polymer Acceptor Solar Cell with over 13% Efficiency

High-performance acceptor–donor–acceptor type photovoltaic materials, including small-molecule donors (SMDs) and polymer acceptors (PAs), have driven significant development in organic photovoltaics (OPVs). Nevertheless, the power-conversion efficiencies (PCEs) of SMD/PA systems still lag far behind those of mainstream systems, primarily due to the great challenge in achieving the optimal blend morphology. This work presents a multistep green-solvent processing strategy that can significantly enhance the efficiency of the SMD/PA system and achieve an outstanding PCE of 13.2% in the model blend (DR3TBDTT:PYFT-o). Systematic morphology investigation reveals that the sequential processing transforms the blend from an overmixed phase into a distinct bicontinuous fibrillar network while progressively enhancing SMD crystallinity. This favorable evolution results from the promoted self-assembly of the SMD during additive-assisted and solvent vapor annealing steps, followed by a synergistic molecular reorganization upon thermal annealing. Consequently, the hole mobility increases by 2 orders of magnitude, and symmetric charge transport and suppressed trap-assisted recombination are simultaneously obtained. The versatility of this strategy is validated across four additional SMD/PA systems, all of which break the 10% efficiency barrier. These findings highlight the critical importance of finely steering molecular assembly to enhance phase separation and donor crystallinity in SMD/PA blends, offering a valuable processing guideline for advancing high-performance photoactive systems in OPVs.