A new two-dimensional (2D) conjugated small molecule, namely DCA3TBDTP, with an alkoxyphenyl substituted benzo[1,2-b: 4,5-b']dithiophene (BDT) unit as the central core, octyl cyanoacetate as the end-capped groups and terthiophene as the pi-linked bridge, was designed and synthesized for solution-processed organic solar cells (OSCs) as an electron donor material, in which an alkoxyphenyl group was introduced as a weak electrondonating side chain of the BDT moiety. The DCA3TBDTP molecule exhibited good solubility, a deep highest occupied molecular orbital (HOMO) level (-5.25 eV), an appropriate optical band-gap (1.82 eV) and a high decomposition temperature (362 degrees C). By applying the simple solution spin-coating fabrication process, the bulk heterojunction (BHJ) OSCs based on DCA3TBDTP and [6,6]-phenyl-C-61-butyric acid methyl ester (PC61BM) exhibited a good power conversion efficiency (PCE) of 4.51% with a high open-circuit voltage (V-oc) of 0.90 V when thermal annealing at only 70 degrees C.