The photophysical properties and solar cell performance of the classical donor–acceptor copolymer PCDTBT (poly($N$-9′-heptadecanyl-2,7-carbazole-$alt$ -5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole))) in relation to unintentionally formed main chain defects are investigated. Carbazole–carbazole homocouplings (Cbz hc) are found to significant extent in PCDTBT made with a variety of Suzuki polycondensation conditions. Cbz hc vary between 0 and 8 mol% depending on the synthetic protocol used, and are quantified by detailed nuclear magnetic resonance spectroscopy including model compounds, which allows to establish a calibration curve from optical spectroscopy. The results are corroborated by extended time-dependent density functional theory investigations on the structural, electronic, and optical properties of regularly alternating and homocoupled chains. The photovoltaic properties of PCDTBT:fullerene blend solar cells significantly depend on the Cbz hc content for constant molecular weight, whereby an increasing amount of Cbz hc leads to strongly decreased short circuit currents J. With increasing Cbz hc content, Jdecreases more strongly than the intensity of the low energy absorption band, suggesting that small losses in absorption cannot explain the decrease in J alone, rather than combined effects of a more localized LUMO level on the TBT unit and lower hole mobilities found in highly defective samples. Homocoupling-free PCDTBT with optimized molecular weight yields the highest efficiency up to 7.2% without extensive optimization.