Synthesis and characterization of donor-acceptor type 4,4′-bis(2,1,3- benzothiadiazole)-based copolymers

A series of novel donor-acceptor type polymers based on 4,4′-bis(2,1,3-benzothiadiazole) were synthesized and characterized. Two soluble regioregular tail-to-tail and head-to-head coupled polymers, poly[7,7′-bis(3-octyl-2-thienyl)-4,4′-bis(2,1,3-benzothiadiazole)] poly[3TBB3T], and poly[7,7′-bis(4-octyl-2-thienyl)-4,4′-bis(2,1,3- benzothiadiazole)] poly[4TBB4T] were synthesized by FeCl₃-mediated oxidative polymerization. To further decrease the band gap of the polymers, vinylene spacers were incorporated into the polymer backbone by Stille coupling of the corresponding monomers and (E)-1,2-bis(tributylstannyl)ethene. A crystal structure of a monomer analog shows near planar arrangement of the aromatic units in the solid state. The optical properties of the monomers and polymers were investigated by steady-state absorption and photoluminescence spectroscopy. Cyclic voltammetry measurements indicate that the polymers could be employed as acceptor materials in polymer-polymer bulk heterojunction solar cells due to their low LUMO energy of about -4.0 eV. A maximum photovoltaic power conversion efficiency of about 0.3% was observed for a 1:1 blend of regioregular poly(3-hexylthiophene) (rr-P3HT) and poly[4TBB4T] and the origin of the moderate efficiency is discussed by interpreting the device current-voltage characteristics, external quantum efficiency and incident light intensity dependence of the power conversion efficiency.