Stress-dissipative strong bimodal molecular packing towards efficient and highly stretchable organic photovoltaics†

The development of flexible organic photovoltaics (OPVs) faces significant challenges due to the intrinsic brittleness of conjugated polymer donors and the non-ideal active-layer morphology, with conventional systems typically exhibiting <10% elongation at break. Although small-molecule solvent additives demonstrate potential in regulating polymer crystallization dynamics, systematic investigations into their role in mechanical enhancement and the corresponding mechanism remain underexplored. This study presents a strategic molecular engineering approach employing 1-Chlorohexadecane (Cl-16C) to reconfigure the molecular orientation of PM6 films. Cl-16C induces a remarkable transition from face-on to bimodal molecular packing, enabling multidirectional crystalline domain formation. This engineered microstructure enables energy dissipation through molecular reorientation, crystalline domain twisting, and physically crosslinked crystalline regions, thereby effectively suppressing crack propagation. The optimized PM6/BTP-eC9 OPV devices achieved a high power conversion efficiency of 19.7% alongside significantly improved mechanical stability, with Cl-16C processed PM6 films demonstrating a 3.7-fold enhancement in the elongation at break (37%) compared to the untreated counterpart (10%). This approach establishes a generalizable method to improve the elongation at break of organic semiconductors, bridging the critical gap between photovoltaic performance and mechanical reliability in flexible OPVs.