We report the photoexcitation dynamics of poly(hexylthiophene-2,5-diyl) (P3HT) as a solid film sandwiched between conducting films of a fluorine-doped tin-oxide-coated glass substrate (FTO) and an insulator film of poly(methyl methacrylate) (PMMA), from measurements of the effects of an external electric field on the absorption and photoluminescence (PL) spectra of P3HT. An Ag film was coated on the PMMA film; the FTO and Ag films served as electrodes to apply an external electric field to P3HT. According to the effects of a quadratic electric field, the lifetime of the emitting state of P3HT increased in the presence of that external field, resulting in an enhanced fluorescence quantum yield. An enhancement and quenching of fluorescence were observed also on applying an external electric field having the opposite polarity to the P3HT film between FTO and Ag films, which results from a field-induced change of both the population of the emitting state following photoexcitation and the rate of nonradiative decay at the emitting state. The dependence of the field-induced change of photoluminescence on the polarity of the applied external electric field between the FTO and Ag films was interpreted as an influence of an internal field located in the P3HT film sandwiched between FTO and PMMA films. These results of the effects of an electric field on the PL of a P3HT film sandwiched between FTO and PMMA films are compared with the effects on PL of P3HT sandwiched between a semiconductor film of TiO2 or a Sb2S3 film and a PMMA film.