Over 19% Efficiency Polymer Solar Cells Enabled by Selectively Tuning Bulkheterojunction Morphology via a Dual-Heating Strategy.

Photovoltaic performance of bulkheterojunction (BHJ)-based organic solar cells is critically governed by morphologies of donor:acceptor blends as light-harvesting layers. However, ideal morphological control remains challenging due to the systems' complexity. In this work, a sequential dual-heating (DH) strategy is presented to precisely tailor the BHJ morphology in a D18-Cl:Y6 system, achieving a remarkable 19.23% power conversion efficiency with enhanced device stability. The DH approach integrates a warm solution (WS) deposition and follow-on solvent-vapor annealing (SVA) by using carbon disulfide (CS₂). The results show that the WS process enlarges π-π distance, enhances photoluminescence, reduces energy loss, and accelerates hole transfer, while the subsequent SVA process increases π-π displacement, thereby reducing both bimolecular and trap-assisted recombination, balancing carrier mobilities, and accelerating hole transport as well. Sequential application of WS and SVA induces synergetic effects on the BHJ morphology by selectively suppressing lamellar ordering while enhancing π-π ordering, yielding simultaneous improvement in all key device parameters. The universal applicability of this approach is further validated through successful implementation in binary D18:Y6, D18-Cl:N3-BO, and ternary D18-Cl:D18:Y6 systems. The findings demonstrate this DH strategy as an effective pathway for precise BHJ morphology engineering, offering a new route for fabricating highly efficient and stable BHJ organic solar cells.