9-Alkylidene-9H-Fluorene-Containing Polymer for High-Efficiency Polymer Solar Cells

A novel donor–acceptor copolymer containing 9-alkylidene-9H-fluorene unit in the main chain, poly[9-(1′-hexylheptylidene)-2,7-fluorene-alt-5,5-(4′,7′-di-2-thienyl-5′,6′-dialkoxy-2′,1′,3′-benzothiadiazole)] (PAFDTBT), has been synthesized and evaluated in bulk heterojunction polymer solar cells (BHJ PSCs). The polymer possesses a low band gap of 1.84 eV, a low-lying HOMO energy level (5.32 eV), and excellent solubility in common organic solvents. PSCs based on PAFDTBT and (6,6)-phenyl-C71-butyric acid methyl ester (PC71BM) demonstrate a power conversion efficiency (PCE) of 6.2% with a high fill factor (FF) of 0.70, which indicates that 9-alkylidene-9H-fluorene can be a very useful building block for constructing narrow band gap conjugated polymers for high-efficiency BHJ PSCs.