A bithiophene imide-based polymer donor for alloy-like ternary organic solar cells with over 20.5% efficiency and enhanced stability†

Ternary organic solar cells (TOSCs) based on dual polymer donors offer enhanced absorption and stability by broadening spectral coverage and refining phase morphology. However, the inherent chain entanglement of dual polymer donors leads to sizable steric hindrance, hindering efficient mixing and posing challenges for further performance improvements. Here, we introduce a new polymer donor PBTI-FR, featuring a bithiophene imide (BTI) acceptor unit, which is specifically tailored to form a dual-polymer-donor TOSC with PM6 and L8-BO. Polymer donor PBTI-FR exhibits strong dipole moments and favorable miscibility with another polymer donor PM6, promoting a stable alloy donor structure. This alloy donor strategy not only reduces energy loss but also strengthens intermolecular interactions and fine-tunes film nanomorphology. Consequently, exciton dissociation and charge transport are improved, delivering a remarkable power conversion efficiency of 20.52%, among the highest values reported for OSCs, alongside an exceptional fill factor of 82.55%. Furthermore, the ternary devices exhibit excellent thermal stability, retaining over 92.2% of their initial performance after 1008 h of heating, underscoring the effectiveness of the dual-polymer-donor alloy design in countering performance degradation. This work highlights a versatile route for high-performance OSCs through the synergistic design of alloy donors with well-aligned energy levels and precisely tuned film morphologies, enabling both superior efficiency and stability.