The optical properties and electrical properties of a series of low-band-gap conjugated copolymers, in which alkyl side chains were substituted at various positions, were investigated using donor-acceptor conjugated copolymers consisting of a cyclopentadithiophene derivative and dithienyl-benzothiadiazole. With substituted side chains, the intrinsic properties of the copolymers were significantly altered by perturbations of the intramolecular charge transfer. The absorption of poly[2, 6-(4,4-bis(2-octyl)-4H-cyclopenta-[2,1-b:3,4-b']dithiophene)-alt-4, 7-bis(4-octyl-thiophene-2-yl)benzo-2,1,3-thiadiazole] [PCPDT-ttOTBTOT (P2)], which assumed a tail-tail configuration, tended to blue shift relative to the absorption of poly[2,6-(4,4-bis(2-octyl)-4H-cyclopenta-[2,1-b:3,4-b'] dithiophene)-alt-4,7-bis (thiophene-2-yl)benzo-2,1,3-thiadiazole] [PCPDT-TBTT (P1)]. The absorption of poly[2,6-(4,4-bis(2-octyl)-4H-cyclopenta-[2,1-b: 3, 4-b']dithiophene)-alt-4,7-bis(3-octyl-thiophene-2-yl) benzo-2,1,3-thiadiazole] [PCPDT-hhOTBTOT (P3)], which assumed a head-head configuration, was blue shifted relative to that of P2. The electrical transport properties of field-effect transistors were sensitive to the side chain position. The field-effect mobility in P2 (mu 2 - 1.8 X 10(-3) cm(2)/V s) was slightly lower than that in P1 (mu(1) 4.9 X 10(-3) cm(2)/V s). However, the mobility of P3 was very low (mu(3) 3.8 X 10(-6) cm(2)/V s). Photoexcitation spectroscopy showed that the charge generation efficiency (shown in transient absorption spectra) and polaron pair mobility in P1 and P2 were higher than in P3, yielding P1 and P2 device performances that were better than the performance of devices based on P3. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 49:1821-1829, 2011