In addition to preparing two indolocarbazole-based random copolymers (named as r-PICTBT1 and r-PICTBT2), this work investigated their feasibility for bulk heterojunction polymer solar cells (PSCs). These copolymers consisted of commercially available 3,9-dibromo-5,11-dioctyl-5,11-dihydroindole[3,2-b]carbazole, 2,5-bis(trimethylstannyl) thiophene and dibromobenzo[c][1,2,5]thiadiazole by varying the feed in ratios via Stille cross-coupling reactions. The photophysical and electrochemical properties of the resulting copolymers could be fine-modulated easily by adjusting the feed ratios of monomers. Both copolymers in the thin film state exhibited two obvious peaks and a vibronic shoulder in the absorption spectra. Electrochemical experiments indicated that the highest occupied molecular orbital energy levels were -4.95, -5.00 eV; meanwhile, the lowest unoccupied molecular orbital energy levels were -3.38, -3.54 eV for r-PICTBT1 and r-PICTBT2, respectively. Bulk heterojunction PSCs composed of anelectron-donor copolymer blended with an electron acceptor [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) or [6,6]-phenyl-C71-butyric acidmethyl ester (PC71BM) at a weight ratio of 1: 1 or 1: 3 were investigated. Moreover, the r-PICTBT2/PC71BM-based (w/w = 1: 1) PSC performed the best with an open-circuit voltage of 0.54 V, short-circuit current of 6.83 mA/cm(2), fill factor of 0.44, and power conversion efficiency of 1.63%. (C) 2013 Elsevier B. V. All rights reserved.