We report a new series of wide band-gap small organic molecules as donor-materials featuring an indaceno[1,2-b:5,6-b′]dithiophene (IDT) core and fluorinated thiophene linkers for solution-processed organic solar cells. Two isomeric aliphatic groups were introduced as side chains on the IDT units (coded I4FR-O and I4FR-EH). These donors have been systematically characterized and employed in bulk-heterojunction organic photovoltaic (OPV) devices cooperating with a PC70BM electron acceptor. Replacing the linear alkyl group (n-octyl) in the IDT core with a branched constitutional isomer (2-ethylhexyl) results in a remarkable improvement in the performance of OPV devices: the optimized power conversion efficiencies of 2.10% and 4.30% were achieved for I4FR-O and I4FR-EH, respectively. Such wide efficiency gap can be partly attributed to a preferable morphology in the donor-fullerene blends of I4FR-EHvia such side-chain engineering.