Narrow Bandgap Donor-Acceptor Conjugated Polymer: Synthesis, Properties and Photovoltaic Application

New soluble, narrow bandgap bithiophene-based conjugated polymer poly[3-(1,4-dihydro-5,8-didodecanoxy-1,4-methano-naphthalen-6-yl)-2,2'-bithiophene-5,5'-diyl(2-cyano-1,2-ethenediyl)-1,4-diphenylene(1-cyano-1,2-ethenediyl)] (PBT-PDCN) has been synthesized by Knoevenagel polycondensation method. PBT-PDCN exhibits high solubility in common organic solvents. Optical studies show that the higher-energy absorption band of PBT-PDCN is red-shifted with increasing polarity of solvents from toluene to THF, and from solution to solid film. Electrochemical characterizations revealed that polymer PBT-PDCN has a low bandgap. The hole mobility was found to be 1.4 . 10(-4) cm(2) V-1 s(-1). Bulk heterojunction solar cells have been fabricated using blends of PBT-PDCN with PCBM in different ratios. The best solar cell performance obtained was based on the blend of PBT-PDCN with PCBM in 1: 3 weight ratio with the power conversion efficiency reaching 0.71%. The photocurrent action spectrum showed a broad curve between 400 nm and 700 nm, with an external quantum efficiency (EQE) maximum of 20% at 570 nm, matching the PBT-PDCN absorption spectrum very closely.