Recently, the isoindigo unit has been successfully introduced in the field of organic electronics for constructing both donor–acceptor (D–A) small molecules and polymers. Isoindigo has strong electron-withdrawing character due to its two lactam rings. Their broad absorption spectra, high extinction coefficients, and appropriate energy levels inspired us to modify their chemical structures and explore their PV performance further. It has been recognized that the insertion of one thiophene as a spacer group between D–A units can enhance performance of the resulting polymers. However, there are only few reports on the insertion of bithiophene as a spacer group. Thus, we conducted a systematic study on solar cell performance of D-(π)n-A-type poly[2,7-bis-9-(1-decylundecylidene)fluorene-alt-N, N'-(2-octyldodecyl)-6, and 6'-π-isoindigo] (P1, P2, P3) as a function of the number of thiophene spacer units. P1~P3 were synthesized via a Suzuki coupling reaction. As the spacer increased from a polymer, the UV-Vis absorption maximum (λmax) red- shifted, and the band gap decreased to 1.83, 1.71, and 1.64 eV, respectively. The highest occupied molecular orbital (HOMO) level of the polymer tended to increase (from approximately -5.93 to -5.48 eV) as the electron-donating characteristics increased due to the implementation of the spacer. When bithiophene was used as the spacer, the tilt angle of the polymer was the lowest. In addition, the facile intermixing due to better accessibility with the PC70BM led to an increase in the hole mobility and Jsc. In contrast to P1 and P2, when the orientation of the P3 thin films was blended with PC70BM in an X-ray diffraction (XRD) measurement, an increased intensity of the (100) peak was observed because the structure of the active layer was converted to an edge-on rich structure. When an inverted polymer solar cell device was fabricated through a solution process, it was found that the polymer that contained bithiophene spacers showed superior performance reaching PCE. P3 exhibited a power conversion efficiency of 4.2%, with a Jsc value of 8.5mA/cm2, a Voc value of 0.80V, and an FF value of 58.9% at 1:2 ratio with PC70BM.