In this report, we have studied the influence of the nature of the end groups (bromine or hydrogen) of regioregular poly(3-hexylthiophene) (P3HT) polymers on the performance of polymer solar cells made with blend films of P3HT and 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61 (PCBM). Films and devices were studied before and after annealing at 140°C for 2 h. The effects of the end-group type on the properties of pristine polymers and blend films were examined using optical absorption and emission spectroscopy, transient absorption spectroscopy, and measurements of photovoltaic device performance. It was observed that hydrogen end groups result in slightly higher absorption coefficients, higher photoluminescence intensities, faster and less dispersive charge recombination, and superior solar cell performance (notably a higher fill factor) compared to bromine end groups. The results are attributed to more-ordered polymer-chain packing in blend films made with hydrogen-capped P3HT, on account of the smaller size and weaker electrostatic interactions resulting from hydrogen compared to bromine. Some influence of the C - Br group on exciton quenching may also be present. The effect of the bromine end group on solar-cell performance became more pronounced with reducing incident-light intensity. Comparison of the polymer transport characteristics in organic field-effect transistor configuration indicated that the bromine end group enhances hole trapping. © 2007 American Chemical Society.