Theory of biexcitons in mixed and segregated stack charge-transfer solids, molecular crystals and π-conjugated polymers

We discuss biexcitons within three different theoretical models in one dimension with excitonic interactions. We develop a general physical picture for optical absorption from the one-photon allowed exciton to the biexciton and to the two-exciton continuum in one dimension. Based on this picture, we propose a criterion of biexciton stability. The validity of our criterion of biexciton stability is proved in the strong-coupling limits of models describing mixed and segregated stack charge-transfer solids, while studies by other investigators have shown that the criterion also remains valid within the Frenkel exciton model with moderate exciton-exciton binding. Based on the same criterion of biexciton stability, we present theoretical evidence for stable biexcitons within the Coulomb-correlated model for π-conjugated polymers. High-energy picosecond photoinduced absorption in π-conjugated polymers is explained as a transition from the optical exciton to the biexciton.