AbstractHerein, the synthesis of a novel isoindigo‐based conjugated polymer using Suzuki and Stille polycondensation reactions is reported. It is shown that the polymer obtained using various reactions possesses different performance in photovoltaic devices. In particular, polymers synthesized using Suzuki and Stille polycondensation demonstrate 2.7% and 4.1% efficiency in organic solar cells, and 12.6% and 15.1% in perovskite solar cells, respectively. The observed difference in the performances of devices is attributed to the appearance of defects in the polymer obtained via Suzuki reaction. Using high‐performance liquid chromatography it is demonstrated that the origin of the polymer quality deterioration is the poor stability of isoindigo‐based monomer under conditions of Suzuki polycondensation. Revealing this aspect allows one to figure out a more effective approach for the synthesis of high‐quality isoindigo‐based polymers for organic electronics.