A difluorobenzothiadiazole-based conjugated polymer with alkylthiophene as the side chains for efficient, additive-free and thick-film polymer solar cells

Two donor–acceptor (D–A) conjugated polymers with alkylthiophene as side chains, i.e., poly[(5,6-difluoro-2,1,3-benzothiadiazol-4,7-diyl)-alt-(3,3′′′-di(2-(2-decyltetradecyl)thiophene-5-yl)-2,2′;5′,2′′;5′′,2′′′-quaterthiophene-5,5′′′-diyl)] (P-2T) and poly[(5,6-difluoro-2,1,3-benzothiadiazol-4,7-diyl)-alt-(2,5-bis(3-(2-decyltetradecyl)thiophene-5-yl)thiophen-2-yl)thieno[3,2-b]thiophene] (P-TT), are designed and synthesized. Both polymers exhibit temperature-sensitive aggregation in solution. Polymer solar cells (PSCs) based on them are fabricated with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as the acceptor material. The devices based on P-TT deliver power conversion efficiency (PCE) up to 9.96% while no additive is used. Moreover, the photovoltaic performance of P-TT is insensitive to the polymer:PC71BM ratio and active layer thickness, and PCEs are all above 9% with the donor : acceptor (D : A) weight ratio of 1 : 1–1 : 1.8 (wt/wt) and film thickness of 200–360 nm. The high photovoltaic performance of P-TT is ascribed to its high and balanced charge carrier mobilities (4.51 × 10−3 and 3.44 × 10−3 cm2 V−1 s−1 for holes and electrons, respectively), owing to the favorable face-on packing of the conjugated backbones as well as the desired morphology of the blend films. Our study demonstrates that alkylthiophenes are one kind of promising synthon of oligothiophene based D–A conjugated polymers, which can serve as side chains for tuning the aggregation of the polymer in solution and the packing mode of the conjugated backbones in the film state.