We report a photoluminescence study of poly(3-alkylthiophene) films. The quantum yield of photoluminescence of amorphous films is higher than that of semicrystalline films. Temperature dependence measurements reveal an intrachain non-radiative decay channel (NRDC) with an activation energy of 0.015 eV in semicrystalline films. The quantum yield increases with increasing alkyl chain length, suggesting that an interchain NRDC is also responsible for the low quantum yields in these materials. We find that the confinement parameter gamma of the polaron-exciton increases when the film order increases, indicating that the two sublevels are closer to the band edge in ordered films owing to a strong intermolecular interaction. The value of gamma for poly(3-dodecylthiophene) is lower than that for poly(3-hexylthiophene), indicating a larger tight-binding energy in the former.