Benzodithiophene (BDT) and benzothiadiazole (BT) based small molecules have been synthesized through palladium-catalyzed Stille coupling reactions. The molecules with and without the conjugated thiophene side-chain were named as DRTBTTBDTT (2D) and DRTBTTBDT (1D), respectively. These molecules exhibited an optical band gap of around 1.8 eV. Solution-processed thin films of 1D and 2D showed that the conjugated thiophene side-chain oriented to produce a uniform 2D structure, which led to a relatively high hole mobility of 0.016 cm2 V−1 s−1, and this was one of the highest values for solution-processed small-molecule donors in organic solar cells (OSCs). With proper engineering of energy levels, the organic solar cells based on DRTBTTBDTT (2D) as donors exhibited a high Voc of around 0.9 V.