Tailoring Guest Symmetry for Efficient Charge Transport via Pre-structuring in Ternary Organic Solar Cells

Abstract Achieving an ideal bi-continuous network in organic solar cells requires balancing molecular self-assembly and film-formation kinetics. For strong-interaction polymers like D18, intense backbone interactions often lead to kinetically trapped, disordered states. Here, we present a molecular symmetry-breaking strategy using an asymmetric small-molecule donor, BC-A2O, as a guest regulator. Compared with its symmetric counterpart BC-A4O, BC-A2O features a rigid rhodanine hook and higher liquid-crystal Gibbs free energy difference. This strong thermodynamic drive enables BC-A2O to preferentially anchor onto D18 chains, establishing solution-phase interfacial pre-structuring that seeds a highly ordered fibrillar host network with optimized charge transport channels. Consequently, BC-A2O-based ternary devices achieve a benchmark efficiency of 20.53% (versus 19.46% for the binary baseline) with a fill factor of 81.16%. This work demonstrates the power of small-molecule donors in orchestrating hierarchical assembly for advanced photovoltaics.