We report a comparative study on the thermal- and microwave-assisted Stille-coupling polymerization of a benzodithiophene-based donor–acceptor polymer (PTB7). We synthesized three PTB7 polymers through Stille-coupling polymerization under microwave conditions (M-PTB7(32k) and M-PTB7(75k)) as well as thermal conditions (T-PTB7(31k)) for this study. The microwave-assisted polymerization is a powerful methodology to increase the M_n of PTB7 without broadening D in a short time. However, M-PTB7(32k) exhibited a lower PCE of 6.1% compared with that of T-PTB7(31k) (7.8%). Interestingly, the higher molecular weight of M-PTB7(75k) under microwave-assisted conditions resulted in a significant reduction in the device performance (PCE of 2.8%). The microwave-assisted polymerization method accelerated the formation of homo-coupled units that act as charge-trapping sites in PSCs comprising a blend of M-PTB7 and PC₇₀BM, which in turn resulted in a decrease in J_sc and FF values for M-PTB7.The increased abundance of these homo-coupled units under the microwave-assisted conditions was demonstrated by various optical spectroscopy and the model reaction study.