Synthesis and characterization of dimethyl-benzimidazole based low bandgap copolymers for OPVs

The advantage of dimethyl-2H-benzimidazole compared to the benzothiadiazole moiety of PCDTBT is to improve the solubility of the polymer while keeping the 1,2-quinoid form to lead coplanarity of the backbone. New random copolymers, to broaden the absorption range for the wider coverage of the solar spectrum, were synthesized by Stille coupling reactions to generate PCPPDTMBIs (or PCPPBBTMBIs). The solid films of PCPPDTMBIs show absorption bands with two maximum peaks at about 408–417 and 613–640 nm. The PCPPBBTMBIs show two maxima peaks at about 440–444 and 627–650 nm which are red-shifted about 5–30 nm as compared to PCPPDTMBIs caused by the introduction of bithiophene units. The device with PCPPBBTMBI7:PC71BM blend demonstrated a VOC value of 0.64 V, a JSC value of 2.12 mA/cm2, and a FF of 0.34, leading to the efficiency of 0.46%.