Effects on Photovoltaic Performance of Dialkyloxy-benzothiadiazole Copolymers by Varying the Thienoacene Donor

A series of four donor-acceptor alternating copolymers based on dialkyloxy-benzothiadiazole (ROBT) as an acceptor and thienoacenes as donor units were synthesized and tested for polymer solar cells (PSCs). These new polymers had different donor units with varied electron-donating ability (thieno[3,2-b]thiophene (TT), dithieno[3,2-b:2′,3′-d]thiophene (DTT), benzo[1,2-b:4,5-b′]dithiophene (BDT), and naphtha[1,2-b:5,6-b′]dithiophene (NDT)) in the polymer backbone. To understand the effect of these thienoacenes on the optoelectronic and photovoltaic properties of the copolymers, we systematically analyzed and compared the energy levels, crystallinity, morphology, charge recombination, and charge carrier mobility in the resulting polymers. In this series, optimized photovoltaic cells yielded power conversion efficiency (PCE) values of 6.25% (TT), 9.02% (DTT), 6.34% (BDT), and 2.29% (NDT) with different thienoacene donors. The introduction of DTT into the thienoacene-ROBT polymer enabled the generation of well-ordered molecular packings with a π-π stacking distance of 3.72 Å, high charge mobilities, and an interconnected nanofibrillar morphology in blend films. As a result, the PSC employing the polymer with DTT exhibited the highest PCE of 9.02%. Thus, our structure-property relationship studies of thienoacene-ROBT-based polymers emphasize that the molecular design of the polymers must be carefully optimized to develop high efficient PSCs. These findings will help us to understand the impact of the donor thienoacene on the optoelectronic and photovoltaic performance of polymers.