A series of poly[9-(heptadecan-9-yl)-9H-carbazole-2,7-diyl-alt-(5,6-bis-(octyloxy)-4,7-di(thiophen-2-yl)benzo-[1,2,5]-thia-diazole)-5,5-diyl] compositions containing various ratios of 3,6-carbazole was synthesized for testing in a polymer solar cell. An appropriate amount of 3,6-carbazole units incorporated into the copolymer improved intermolecular charge transport, whereas excess amount of 3,6-carbazole units temporarily seized on the partial negative charge generated in the conjugation breaks. We extensively studied the effects of the incorporated 3,6-carbazole units on the intermolecular interactions, which can affect nongeminated recombination in bulk heterojunction-polymer solar cells. These properties were investigated using photocurrent- and light intensity-dependent measurements and electrochemical impedance spectroscopy. (c) 2014 Wiley Periodicals, Inc.