Quantum molecular dynamics study of polaron recombination in conjugated polymers

We examine the dynamics of polaron recombination in conjugated polymer systems using mixed quantum classical molecular dynamics. The model treats the particle-hole pair as a fully correlated two-particle quantum mechanical wave function interacting with a one-dimensional classical vibrational lattice. This description allows a natural evolution of the particle-hole wave function from the polaron limit to the exciton limit, and we have performed real-time simulations of the coupled nuclear and electronic dynamics associated with the scattering of polarons into exciton states. We use these simulations to calculate cross sections for exciton formation as a function of spin state, and explore the variation of these cross sections with respect to changes in the magnitude of the particle-hole Coulomb interaction and the effective masses of the quasiparticles. Our results indicate that for an optimal choice of parameters the electroluminescence quantum yield may be as high as 59%, substantially greater than the 25% predicted by simple spin statistics. We interpret these results in a diabatic framework, and suggest strategies for the design of organic systems for use in electroluminescent devices.