Balancing molecular structure, molecular weight, and processability in thiophene- and thienothiophene-based donor–acceptor polymers for scalable organic solar cells

Establishing reliable structure–processability relationships is essential for advancing organic solar cells (OSCs) toward scalable manufacturing. Herein, we present a comparative study of donor–acceptor (D–A) copolymers incorporating thiophene (T) and thienothiophene (TT) donor units, examining how donor and acceptor structure, polymerization outcome, molecular weight, operational solubility, and processing conditions are associated with film formation and device reproducibility under ambient blade-coating conditions. Six donor–acceptor copolymers (seven samples, including a TT3 batch prepared with a shorter reaction time, TT3*) were synthesized via Stille coupling and characterized with respect to molecular structure, molecular weight distribution, optoelectronic properties, thermal stability, thin-film morphology, and photovoltaic performance. TT-containing polymers showed distinct optical and processing behavior, but the present data do not isolate backbone conformation as the sole origin of these differences. Only the thiophene-based polymers T1, T2, and T3 could be processed into devices; among them, only T3 combined a high estimated degree of polymerization (DP n ≈ 25) with good film-forming ability, whereas T1 and T2 were obtained as short-chain materials (DP n ≈ 6). Among the investigated materials, T3 delivered the most reproducible device performance, reaching a power conversion efficiency of 2.69% in large-area (0.55 cm 2 ) flexible OSCs. Rather than establishing a single causal relationship between backbone structure and device efficiency, this study highlights the combined importance of molecular structure, attainable molecular weight, solution processability, and processing tolerance for scalable ambient fabrication.