Synthesis of Donor–Acceptor polymers through control of the chemical structure: Improvement of PCE by planar structure of polymer backbones

A new acceptor unit with high solubility octyloxy-diphenylquinoxaline was synthesized with octyloxy-dibenzophenazine. The new D–A type polymers poly(carbazole-octyloxy diphenylquinoxaline) (PCzPDTQ) and octyloxy-dibenzophenazine) (PCzFDTQ), which adopted carbazole as a donor, were polymerized through a Suzuki coupling reaction. PCzPDTQ and PCzFDTQ dissolved in organic solvents and showed high thermal stability. Due to the appropriate LUMO energy levels, an effective charge transport and a low band gap (2.06 and 1.97 eV) were observed in PCzPDTQ and PCzFDTQ. According to X-ray diffraction measurements, a single broad diffraction peak was detected at approximately 20.5°. The π–π stacking distances (dπ) for PCzPDTQ and PCzFDTQ were 4.4 and 4.3 Å, respectively. When PCzPDTQ and PC71BM were blended in a 1:3 ratio, The open-circuit voltage (VOC), short-circuit current (JSC), fill factor (FF) and power conversion efficiency (PCE) were 0.85 V, 6.5 mA/cm2, 32.2% and 1.8%, respectively. The efficiency of PCzFDTQ tended to improve as PC71BM increased. At a 1:6 ratio of PCzFDTQ to PC71BM, Voc, Jsc, FF and PCE were 0.85 V, 5.5 mA/cm2, 44.9% and 2.1%, respectively. In terms of FF, 39% of the increase was observed in PCzFDTQ.