Effects of fluorination on the properties of thieno[3,2-b]thiophene-bridged donor–π–acceptor polymer semiconductors
Thieno[3,2-b]thiophene-bridged polymer semiconductors, P(BDT-TT-HBT) and P(BDT-TT-FBT), combining a benzo[1,2-b:4,5-b′]dithiophene donor unit and a benzothiadiazole or fluorinated benzothiadiazole acceptor unit, respectively, were designed and synthesized. The introduction of fluorine substituents remarkably influenced the molecular architecture, optical, electrochemical, and morphological properties of the polymers, as well as the optoelectronic performance of the devices made from these materials. The introduction of fluorine substituents on the benzothiadiazole unit not only down-shifted the HOMO energy level of the organic semiconductor but also enhanced the intra- and intermolecular interactions of the resulting conjugated polymer. As a result, the open-circuit voltage and mobility of corresponding devices based on the fluorinated polymer were enhanced markedly. Power conversion efficiencies of the polymer solar cells based on P(BDT-TT-HBT) and P(BDT-TT-FBT) reached 4.37% and 3.56%, with open circuit voltages of 0.72 and 0.81 V, respectively. The fluorinated polymer exhibited much higher mobilities (4.1 to 6.3 times) than the non-fluorinated polymer, reaching 0.017 cm2 V−1 s−1.