In this work, a novel oligomer based on the introduction of alkylthio-thienylenevinylene thiophene (TVT-SR) side groups into benzo[1,2-b:4,5-b′]dithiophene (BDT) as a central donor unit, BDT(TVT-SR)2, is developed and used as the donor material for all-small molecule OSCs (SM-OSCs) with IDIC as the acceptor. BDT(TVT-SR)2 exhibits red-shifted absorption, a down-shifted HOMO energy level and higher hole mobility in comparison with its corresponding molecule H11 with alkylthiothienyl side chains. The power conversion efficiency (PCE) of the SM-OSCs based on BDT(TVT-SR)2:IDIC with thermal annealing (TA) at 110 °C for 10 min reached 11.1%. This TA treatment improves the film morphology, charge transport properties and thus device performance of the SM-OSCs. Besides, these devices show good morphological stability and photo-stability.