Theory of optical absorption in doped conjugated polymers

The optical absorption of polarons and bipolarons in doped conjugated polymers is investigated theoretically. The Pariser-Parr-Pople model in the present paper includes long-range electron-electron (e-e) interactions, electron-lattice coupling of the Su-Schrieffer-Heeger type, and modulation in transfer integral lifting the degeneracy of the ground state. The calculations by mean of the single configuration-interaction method with unrestricted Hartree-Fock molecular orbitals show that a polaron (bipolaron) gives rise to two (one) absorption peaks below the optical gap even in the presence of the e-e interaction. The peak positions depend on the interaction strength.