Design, Synthesis, and Photovoltaic Characterization of Thiophene‐Linked Phenazine Copolymers for Solar Energy Conversion

ABSTRACT Side‐chain engineering represents an effective molecular strategy for tuning the optoelectronic properties and solid‐state organization of conjugated polymers for organic photovoltaic applications. In this study, two donor–acceptor copolymers, PBDT‐DT‐BODFPz and PBDT‐DT‐EHDFPz, were synthesized via Stille polymerization using benzodithiophene (BDT) as the electron‐donating unit and difluorophenazine (DFPz) as the electron‐accepting moiety, differing only in their branched alkyl side chains. The polymers were systematically characterized by UV–vis, CV, TGA, AFM, and photovoltaic device measurements to investigate the influence of side‐chain structure on their material and device properties. Although both polymers possessed identical conjugated backbones, distinct differences in their solid‐state behavior were observed. PBDT‐DT‐EHDFPz exhibited a larger bathochromic shift in the thin‐film absorption spectrum and a slightly higher HOMO energy level compared with PBDT‐DT‐BODFPz, suggesting differences in intermolecular interactions and solid‐state molecular organization. Bulk heterojunction solar cells fabricated using polymer:PC 70 BM active layers exhibited power conversion efficiencies (PCEs) of 4.24% and 4.14% for PBDT‐DT‐BODFPz and PBDT‐DT‐EHDFPz, respectively, significantly higher than that of the reference polymer system. AFM analysis revealed that PBDT‐DT‐BODFPz formed smoother and more homogeneous blend‐film morphology, which correlated with its slightly improved photovoltaic performance.