The development of novel small-molecule donor is crucial for achieving highly efficient small-molecule organic solar cells (SM-OSCs). In this study, two small-molecule donors, B3TR and B2, were designed and synthesized. In comparison to B3TR, the skeleton of B2 includes an additional benzo[1,2-b:4,5-b']dithiophene (BDT) unit, but lacks two alkylated thiophene units. Although both small-molecule donors exhibit similar absorption profiles in solution, B2 demonstrates stronger crystallinity and lower energetic disorder than B3TR. When blended with the non-fullerene acceptor BTP-eC9, the B2:BTP-eC9 film exhibits a smaller π-π distance and a more favorable bulk heterojunction morphology. Interestingly, with an increase in solvent vapor annealing (SVA) time, B3TR shows a significant redshift in the absorption edge in the B3TR:BTP-eC9 film and an upshifted HOMO level. In contrast, B2 does not exhibit redshifts in the absorption edge in the B2:BTP-eC9 film and maintains an unchanged HOMO level. Consequently, the as-cast B3TR:BTP-eC9-based device achieves a PCE of 2.36%, with an open-circuit voltage (VOC) of 0.870 V. After SVA treatment, the B3TR:BTP-eC9-based SM-OSC attains a PCE of 14.8% with a significantly reduced VOC of 0.809 V. In contrast, the as-cast B2:BTP-eC9-based device exhibits a significantly high PCE of 9.8% with a VOC of 0.876 V. After SVA treatment, the B2:BTP-eC9-based device produces a superior PCE of 17.1%, with a slightly reduced VOC of 0.861 V. This study indicates that the development of highly crystalline donor materials is one of the promising strategies for achieving high-performance SM-OSCs.