Low-bandgap quinoxaline-based D - A-type copolymers: Synthesis, characterization, and photovoltaic properties

Three classes of quinoxaline (Qx)-based donor-acceptor (D-A)-type copolymers, poly[thiophene-2,5-diyl-alt-2,3-bis(4-(octyloxy)phenyl-quinoxaline- 5,8-diyl] P(T-Qx), poly{4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b′] dithiophene-2,6-diyl-alt-2,3-bis(4-(octyloxy)phenyl-quinoxaline-5,8-diy} P(BDT-Qx), and poly{4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b′] dithiophene-2,6-diyl-alt-(5′,8′-di-2-thienyl-2,3-bis(4-octyloxyl) phenyl)-quinoxaline-5,5-diyl} P(BDT-DTQx), were synthesized via a Stille coupling reaction. The Qx unit was functionalized at the 2- and 3-positions with 4-(octyloxy)phenyl to provide good solubility and to reduce the steric hindrance. The absorption spectra of the Qx-containing copolymers could be tuned by incorporating three different electron-donating moieties. Among these, P(T-Qx) acted as an electron donor and yielded a high-performance solar cell by assuming a rigid planar structure, confirmed by differential scanning calorimetry, UV-vis spectrophotometer, and density functional theory study. In contrast, the P(BDT-Qx)-based solar cell displayed a lower power conversion efficiency (PCE) with a large torsional angle (34.7 ) between the BDT and Qx units. The BDT unit in the P(BDT-DTQx) backbone acted as a linker and interfered with the formation of charge complexes or quinoidal electronic conformations in a polymer chain. The PCEs of the polymer solar cells based on these copolymers, in combination with [6,6]-phenyl C70 butyric acid methyl ester (PC ₇₁BM), were 3.3% [P(T-Qx)], 1.9% [P(BDT-Qx)], and 2.3% [P(BDT-DTQx)], respectively, under AM 1.5G illumination (100 mW cm^-2).