Solution-processable oligothiophenes with solubilizing β-alkyl groups for organic photovoltaic cells

Two linear-type π-conjugated small molecules based on thiophene and benzothiadiazole, β-DH5TB(1) and β-DH5TB(2), were synthesized using the Suzuki coupling reaction to have β-substituted solubilizing alkyl groups. The solubility of β-DH5TBs was significantly improved by changing the direction of the alkyl group compared to α-substituted oligothiophene (α-DH5TB). The relationship between the alkyl substitution pattern and the photovoltaic properties was investigated by measuring the optical and electrochemical properties and the film morphology. Among them, the 4-hexyl substituted β-DH5TB(2) showed the lowest optical band gap and HOMO level together with a red-shifted broad UV absorption. In addition, a high degree of molecular alignment was also observed in β-DH5TB(2), indicative of good charge transport. The β-DH5TB(2) film showed higher field-effect mobility (μFET) of 6.28×10−4 cm2 V−1 s−1 than β-DH5TB(1) film. Owing to the improved solubility, two of the synthesized β-DH5TBs could be used as donor materials in solution-processed organic photovoltaic cells (OPVs), whereas the α-DH5TB:PC71BM blend film showed no photovoltaic properties due to the poor solubility of α-DH5TB. The improved absorption and molecular alignment of the β-DH5TB(2) yielded a device efficiency (0.98%) higher than that of β-DH5TB(1).