Synergistic Effect of Fluorine Substitution and Thio-Alkylation on Photovoltaic Performances of Alternating Conjugated Polymers Based on Alkylthio-Substituted Benzothiadiazole-Quaterthiophene

Three low bandgap polymers (PSDTBT-DFDT, PSDTfBT-DFDT, and PSDTffBT-DFDT) based on fluorine and alkylthio-substituted benzothiadiazole-quaterthiophene alternating units are synthesized and applied in polymer solar cells. The effects of fluorination and thio-alkylation on photophysical, electrochemical, and photovoltaic properties of the polymers are comparatively investigated. The results indicate that PSDTfBT-DFDT and PSDTffBT-DFDT show better light-harvesting abilities, lower HOMO energy levels, and better morphology than that of PSDTBT-DFDT. Bulk heterojunction (BHJ) organic photovoltaic devices based on the three polymers as donor materials were fabricated. The power conversion efficiencies (PCE) of 6.94% for PSDTfBT-DFDT and 7.50% for PSDTffBT-DFDT were attained, respectively, both of which are higher than that of PSDTBT-DFDT (4.48%) due to the increase in the numbers of fluorine atoms. This work demonstrates that both fluorine substitution and thio-alkylation are efficient to improve photovoltaic performances of polymers.