In this study, cyclic poly(3-hexylthiophene-2,5-diyl) (c-P3HT) with a controlled M-n was synthesized by the intramolecular cyclization of alpha-bromo-omega-ethynyl-functionalized P3HT via the Sonogashira coupling reaction. The effect of the cyclic structure, which does not have terminal groups of polymers, on the photoelectric conversion characteristics was investigated in comparison to linear P3HT (l-P3HT). c-P3HT was successfully synthesized with M-n approximate to 17,000, dispersity approximate to 1.2, and regioregularity approximate to 99%. The hole mobility was determined to be 5.1 x 10(-4) cm(2) V-1 s(-1) by time-of-flight (TOF) experiment. This was comparable to that of l-P3HT of 5.6 x 10(-4) cm(2) V-1 s(-1). Organic solar cell systems were fabricated with each polymer by blending them with [6,6]-phenyl-C-71-butyric acid methyl ester (PC71BM). The l-P3HT:PC71BM system showed a dispersive TOF photocurrent profile for electron transport, whereas a nondispersive profile was observed for c-P3HT:PC71BM. In addition, an amount of collected electrons in c-P3HT:PC71BM was greater than that in l-P3HT:PC71BM for TOF experiments. The photoelectric conversion characteristics were improved by using c-P3HT rather than l-P3HT (power conversion efficiency [PCE] = 4.05% vs 3.23%), reflecting the nondispersive transport and the improvement of electron collection. PCEs will be much improved by applying this cyclic concept to highly-efficient OSC polymers. (c) 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 266-271