Two linear-type pi-conjugated small molecules based on thiophene and benzothiadiazole, beta-DH5TB(1) and beta-DH5TB(2), were synthesized using the Suzuki coupling reaction to have beta-substituted solubilizing alkyl groups. The solubility of beta-DH5TBs was significantly improved by changing the direction of the alkyl group compared to alpha-substituted oligothiophene (alpha-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 beta-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 beta-DH5TB(2), indicative of good charge transport. The beta-DH5TB(2) film showed higher field-effect mobility (mu(FET)) of 6.28 x 10(-4) cm(2) V-1 s(-1) than beta-DH5TB(1) film. Owing to the improved solubility, two of the synthesized beta-DH5TBs could be used as donor materials in solution-processed organic photovoltaic cells (OPVs), whereas the alpha-DH5TB:PC71BM blend film showed no photovoltaic properties due to the poor solubility of alpha-DH5TB. The improved absorption and molecular alignment of the beta-DH5TB(2) yielded a device efficiency (0.98%) higher than that of beta-DH5TB(1). (C) 2012 Elsevier B.V. All rights reserved.