Novel donor–acceptor type conjugated polymers based on quinoxalino[6,5-f]quinoxaline for photovoltaic applications

Abstract

The field of polymer solar cells has undergone tremendous advancement in terms of power conversion efficiency in the past decade. However, there is still an urgent requirement to further enhance device performance for achieving large-scale commercialization. Here, we report the design and synthesis of three novel conjugated polymers alternatively copolymerized by a newly developed 2,3,8,9-tetrakis(3-(alkoxy)phenyl)-6,12-di(thiophen-2-yl)-2,3,8,9-tetrahydroquinoxalino[6,5-f]quinoxaline (DTNQx) acceptor unit and a conventional two-dimensional alkylthienyl-substituted benzodithiophene (BDT) unit. These polymers possess an identical main chain but different side chains. The structure–property relationship is systematically studied. All polymers exhibit an optical bandgap around 1.7 eV and a HOMO energy level around −5.16 eV. Compared with the quinoxaline unit, fused quinoxaline DTNQx leads to a downshifted HOMO energy level in the resulting polymers by 0.04 eV while keeping the bandgap unchanged. By changing the side chain length and by introducing branched side chains, polymer properties including absorption spectra and hole mobilities could be finely tuned. Under optimal conditions, polymer P2 shows the highest hole mobility of 2.7 × 10⁻⁴ cm² V⁻¹ s⁻¹ and the best photovoltaic performance, with J_sc = 9.1 mA cm⁻², V_oc = 0.88 V, FF = 48%, and PCE = 3.8%.