Wide and Narrow Bandgap Conjugated Polymers Synthesized by a Direct Arylation Polycondensation Method for Ternary Organic Solar Cells

In this study, we report the synthesis of two A1–D–A2–D-type conjugated polymers P139 and P141 with distinct optical bandgaps, prepared via direct arylation polycondensation. Both polymers incorporate a common wide-bandgap acceptor unit (A1) based on a carbazole-fused dithienoquinoxaline structure and a thiophene donor (D) unit. The key difference lies in their second acceptor unit (A2). P139 contains a diketopyrrolopyrrole-based moiety, while P141 features a difluorobenzotriazole-based fragment. Optical and electrochemical characterizations reveal that P139 exhibits a narrow bandgap of 1.34 eV with HOMO and LUMO energy levels at −5.25 eV and −3.68 eV, respectively. In contrast, P141 shows a wider bandgap of 2.00 eV, with HOMO and LUMO levels at −5.46 eV and −3.47 eV. Binary and ternary organic solar cells (OSCs) were fabricated using these polymers as donor materials in combination with a medium-bandgap non-fullerene acceptor (NFA-5). The optimized ternary device, based on a P139:P141:NFA-5 blend, delivered a power conversion efficiency (PCE) of 16.42%, significantly outperforming the binary devices, which achieved 12.12% (P139:NFA5) and 9.48% (P141:NFA5). The enhanced performance of the ternary OSC is attributed to broadened and complementary absorption, efficient energy transfer from P141 to P139, improved exciton dissociation due to an increased donor–acceptor interfacial area, balanced charge transport, reduced recombination losses, and improved charge extraction.