A facile method for enhancing photovoltaic performance of low-band-gap D–A conjugated polymer for OPVs by controlling the chemical structure

• DA-type polymer materials with different repeating units are modified molecular energy level.
• The structure–property relationships caused by the difference between the bithiophene and fused thiophene donor molecules were investigated.
• PPDToBT exhibited the best power conversion efficiency (PCE) of 3.9%.

Stille coupling polymerization has been used to synthesize a series of new crystalline conjugated polymers with low-lying HOMO level: PABToBT, PPDToBT, PQTDPT. All of these polymers exhibited sufficient energy offsets with respect to those of fullerenes to allow efficient charge transfer and low-lying highest occupied molecular orbital (HOMO), from −5.04 to −5.27 eV. As a result, the photovoltaic device comprising a PPDToBT/PC71BM (1:4) blend system exhibited excellent performance, under AM1.5 G irradiation, with a VOC of 0.878 V, a JSC of 8.7 mA cm−2, a FF of 0.51, and a promising PCE of 3.9%.

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