Synthesis and characterization of fluorinated benzothiadiazole-based small molecules for organic solar cells

Two small molecules, ß-DH5TB(1)-F and ß-DH5TB(2)-F, based on a thiophene donor and a fluorinated benzothiadiazole acceptor unit, respectively, were synthesized using palladium-catalyzed Suzuki coupling reactions. Their physical properties and photovoltaic performances were compared with those of the corresponding nonfluorinated small molecules. The optical, electrochemical, and thermal properties of the small molecules varied with the introduction of fluorine and the alkyl substitution pattern. In particular, the low-lying highest occupied molecular orbital energy levels of ß-DH5TB(1)-F and ß-DH5TB(2)-F resulted in higher open-circuit voltages of 0.72 V and 0.69 V, respectively, in organic photovoltaic cells, when compared with the non-fluorinated ß-DH5TB(1) (0.52 V) and ß-DH5TB(2) (0.60 V). The relationship between the alkyl substitution pattern and the photovoltaic properties was also investigated. The results show that ß-DH5TB(2)-F had a higher power conversion efficiency (0.86%) than ß-DH5TB(1)-F (0.66%) because of a higher short-circuit current and better molecular packing ability.