Phonon sidebands and optical gain of light-emitting conjugated polymers

We study the interaction of excitons and optical phonons in light-emitting conjugated polymers with the variational approach. The optical transition matrix elements for the phonon sidebands in the absorption and emission spectra are calculated. The electron-phonon coupling constant is determined by fitting the theoretical oscillator strengths of the emission sidebands with the experimental photoluminescence spectra for the various poly(p-phenylene vinylene) derivatives. Using this coupling constant, we predict that the absorption one-phonon sideband is as important as the zero-phonon sideband for the ideal systems. Taking the experimental data of the peak absorption coefficient as input, we obtain the optical gain coefficient for a wide range of exciton densities. The gain is found to be determined by the exciton density n₀ by a general expression 1.98 × 10^-16n₀ cm², which is in good agreement with experiments.