The engineering of alkylthio side chains is a smart and effective strategy to improve the photovoltaic properties of donor materials for organic solar cells (OSCs). In order to further exploit the capability of alkylthio chains in tuning the physicochemical and photovoltaic properties of small-molecule donor materials, two small molecules DTS-BTT-S and BDTT-BTT-S with alkylthio end groups were designed and synthesized. The DFT calculation results reveal that introducing alkylthio end groups in DTS-BTT-S and BDTT-BTT-S effectively down-shifted the HOMO energy levels in comparison with their analogous molecules with alkyl end groups. The DTS-BTT-S based OSC device exhibited a higher V-oc of 0.840 V, with a PCE of 3.65% without the need of extra-treatments of the active layer. The optimized device based on BDTT-BTT-S showed a higher PCE of 5.21%, with V-oc = 0.823 V, J(sc) = 10.93 mA cm(-2) and FF = 57.2%.