A series of poly(3-hexylthiophene)s (P3HTs) and poly(3-butylthiophene)s (P3BTs) with predetermined molecular weights and varying polydispersities are prepared using a simplified Grignard metathesis chain-growth polymerization. Techniques were elaborated to prepare extremely high molecular weight P3HT (number-average molecular weight of around 280000 g mol(-1)) with a low polydispersity (< 1.1) without resorting to fractionation. Optimization of the annealing of a series of solar cells based on blends of poly (3-alkylthiophene)s (P3ATs) and [6,6]-phenyl C-61 butyric acid methyl ester indicates that the polydispersities, molecular weights, and degrees of conjugation of the P3ATs all have an important impact not only on cell characteristics but also on the most effective annealing temperature required. The results indicate that each cell requires annealing treatments specific to the type of polymer and its molecular weight distribution.