Significantly increasing open-circuit voltage of the benzo [1,2-b:4,5-b′]dithiophene-alt-5,8-dithienyl-quinoxaline copolymers based PSCs by appending dioctyloxy chains at 6,7-positions of quinoxaline

Two novel 5,8-disubstituted benzo[1,2-b:4,5-b']dithiophene-alt-2,3-di(4-methoxylphenyl)- 6,7-dioctyloxy-5,8-dithienyl-quinoxaline copolymers of PBDTDT-(Qx-2)-O and PBDTDT(Qx-2)-T were designed and synthesized, in which the substituents are branch dioctyloxy and di(octylthienyl), respectively. Their optical, thermal, electro-chemical and photovoltaic properties were investigated. Compared to the reported analogues, both one-and two-dimensional (1D and 2D) copolymers exhibited outstandingly increasing open-circuit voltages (V-oc) about 0.91-0.95 V in their bulk hetero-junction polymer solar cells (PSCs) with [6,6]-phenyl-C-71-butyric acid methyl ester (PC71BM) acceptor. The maximum power conversion efficiency (PCE) of 6.31% with V-oc of 0.95 V, short-circuit current of 10.82 mA cm(-2) and fill factor of 61.4% was obtained in the 2D-conjugated polymer of PBDTDT(Qx-2)-T/PC71BM-based device. The PCE value is 1.42 times higher than that of the 1D-conjugated polymer of PBDTD(Qx-2)-O/PC71BM-based device. Our work demonstrates that Voc values of the quinoxaline-type copolymers based PSCs can remarkably be increased by appending dioctyloxy chains at 6,7-positions of quinoxaline. (C) 2014 Elsevier B.V. All rights reserved.