Novel benzo(1,2-b:4,5-b’)dithiophene-based donor–acceptor conjugated polymes for polymer solar cells

Two new donor–acceptor (D-A) alternative copolymers, poly{4-(5-(4,8-bis(dodecyloxy)-6-methylbenzo[1,2-b:4,5-b’]-dithiophen-2-yl) -4–dodecylthiophen-2-yl)-2-dodecyl-7-(4-dodecyl-5-methylthiophen-2-yl)-2H-benzo[d] [1‐3] triazole} (P1) and poly{4-(5-(4,8-bis(dodecyloxy)-6-methylbenzo -[1,2-b:4,5-b’]dithiophen-2-yl)-4-dodecylthiophen-2-yl)-2-dodecyl-9-(4-dodecyl-5methylthiophen-2-yl)-6,7-dimethyl-2H- [1‐3] triazolo[4,5-g]quinoxaline} (P2), were designed and synthesized. Also, the effects of the structural variation on their properties, especially their UV and electrochemical behaviors, were studied. The polymers were verified by NMR and characterized by TGA, UV–vis absorption and electrochemical cyclic voltammetry. It was found that P1 exhibits stronger π-π stacking and aggregation in solid film related to in solution state than those of P2. All the polymers show narrow band gaps, with the electro-chemical band gaps (E _g ^EC ) 1.45 eV for P1 and 1.61 eV for P2. Interestingly, the polymer P2 exhibits power conversion efficiency (PCE) of 2.2 %, while the copolymer P1 shows an improvement PCE of 2.9 %. It indicates that mildly tailoring the acceptor segments of the D-A copolymer is a facile and promising approach for active layer engineering to developing high performance polymer solar cells.