In this work, the effects of the spacer between benzo[1,2-b;3,4-b′]dithiophene (BDT) and dialkoxybenzothiadiazole (ROBT) in polymers were investigated for applications in organic solar cells. Polymer PBDT–2T–ROBT has a bithiophene (2T) spacer between the BDT and ROBT units, whereas PBDT–ROBT is a direct copolymer of BDT and ROBT. The polymer/PC70BM solar cells using both polymers were fabricated and optimized via polymer/fullerene ratio, solvent, and solvent additives. The spacer has significantly improved solar cell performance from 1.28% (Voc =0.77 V, Jsc = 3.13 mA/cm2, FF = 53.11%) to 5.11% (Voc = 0.66 V, Jsc = 13.33 mA/cm2, FF = 58.12%). The X-ray diffraction (XRD) spectra show the PBDT–2T–ROBT/PC70BM blended film is semicrystalline, whereas the PBDT–ROBT/PC70BM film is amorphous. This indicates that the spacer facilitates polymer organization for higher carrier mobility in the film. Atomic force microscopy (AFM) topography and phase images show that PBDT–2T–ROBT/PC70BM films form fibrillar networks, ...