Stability of bipolarons in conjugated polymers

The bipolarons appear to be the main entity for charge collection in conducting polymers. But their existence is challenged by the Coulomb repulsion. We study the stability of a bipolaron (BP) as a unction of the strength of the long range Coulomb interaction U with and without impurities for both weak and strong electron-phonon coupling (alpha). We find that in a free state the BP is stable only for small values of U for the weak coupling limit but stays stable until relatively large ti in the strong coupling case. Being bound to a dopant, the bipolaron becomes stable in a wide range of U. Free BPs near the metal-polymer contact also are stabilized due to a screening by image charges. These results are important for charge injection at polymer-metal interfaces in the context of organic light-emitting diodes. (C) 1998 Elsevier Science B.V.