Synthesis and characterization of a series of low-bandgap copolymers based on cyclopenta[2,1-b:3,4-b′]dithiophene and thienopyrroledione for photovoltaic applications

A series of low bandgap copolymers based on cyclopenta[2,1-b:3,4-b′]dithiophene and thienopyrroledione have been synthesized and characterized. High average molecular weight copolymers could be synthesized by microwave assisted polymerization methods with significantly reduced polymerization times. The copolymers exhibited broad absorption in the visible region of the electromagnetic spectrum (∼400–700 nm), which is a prerequisite for photovoltaic applications. Copolymers P2 and P3 containing thiophene spacers showed a broader absorption with a slightly blue shifted absorption maxima compared to that of P1 due to the loss in the coplanarity of the polymer backbone. All copolymers possess fairly low bandgaps (∼1.7 eV) and deep highest unoccupied molecular orbital (HOMO) levels. Bulk heterojunction (BHJ) photovoltaic devices fabricated from the copolymers and PC71BM gave power conversion efficiencies as high as 3.10% for the P1 devices under illumination of AM1.5G 100 mW/cm2.