Benzodithiophene–Dithienylbenzothiadiazole Copolymers for Efficient Polymer Solar Cells: Side-Chain Effect on Photovoltaic Performance

A new series of low band gap D–A alternating polymers based on 4,5-bis((2-ethylhexyl)oxy)benzo[2,1-b:3,4-b′]dithiophene (BDT) and 5-fluoro-4,7-bis(4-alkylthien-2-yl)benzo[c][1,2,5]thiadiazole bearing different size of lateral alkyl substituents, namely, PfBB-n, n = 8, 10, 12, 14, and 16, was designed and synthesized for high-performance bulk heterojunction (BHJ) polymer solar cells (PSCs). PfBB-n-bearing linear alkyl side chains exhibited strong and controllable aggregation in both solution and solid states, which gives rise to a significant bathochromic shift of the absorption cut-off down to ∼780 nm in thin film. In addition, the strong and wide absorption (350–800 nm) of PfBB-n polymers can compensate for the relatively weak absorption of PC71BM, particularly in the 300–400 range nm to enhance light harvesting of such an active blend. BHJ solar cells based on PfBB-n:PC71BM blends as an active layer showed power conversion efficiency (PCE) in the range 7.8–9.7%. Because of the strong stacking interchain interactions, PfBB-12-based PSC exhibited aggregation-induced spectral broadening, superior structural order, higher exciton dissociation, higher and more balanced charge carrier mobilities, as well as reduced recombination losses. As a result, PfBB-12-based device afforded the best PCE of 9.7%, with the highest short-circuit current density (Jsc) of 16.6 mA cm–2 and open-circuit voltage (Voc) of 0.92 V among devices fabricated. These results demonstrate that the alkyl side chain of the polymer significantly affects the absorption, morphology, and electronic properties of the active blend of PfBB-n/PC71BM, which would provide an alternative useful tool to fine-tune the device performance. Our results also highlight that the electron-rich benzo[2,1-b:3,4-b′]dithiophene building block, BDT, is highly useful for the construction of low band gap D–A polymer for highly efficient PSCs.