Two small molecule (SM) donors, named CBTSeHR and CBTSeEHR, have been constructed by introducing selenophenes and chlorine atoms simultaneously into the conventional benzodithiophene-terthiophene-rhodanine architecture. In comparison with the non-chlorinated and thiophene based counterpart, the new SM donors exhibit a red-shifted absorption band, lower highest occupied molecular orbital (HOMO) energy levels, stronger intermolecular packing and higher molecular crystallinity. When combined with non-fullerene molecule FO-2Cl, the optimized all-small-molecule organic solar cells (ASM-OSCs) based on CBTSeEHR give a moderate power conversion efficiency (PCE) of 9.30%. Notably, CBTSeHR based devices achieve an outstanding performance of 12.10% with a high open-circuit voltage (V-oc) of 0.881 V, short-circuit current densities (J(sc)) of 18.76 mA cm(-2) and fill factor (FF) of 0.732, benefiting from the deep HOMO level, good light-harvesting ability and favorable morphology in the active layer. These findings provide an effective strategy to develop the medium band-gap SM donors for high-performance ASM-OSCs.