An energetic study of the excitation spectrum in doped polythiophene is presented within the framework of the Hückel approximation. The calculations show that localized states like polarons and bipolarons are produced by doping, the formation of doubly charged bipolarons being energetically more favorable than the formation of two polarons charged singly. It is then shown from the analysis of available magnetic and optical data that the bipolarons are the dominant charge states while the features due to the polarons appear under certain circumstances at low doping levels.

• Received 30 July 1986

DOI:https://doi.org/10.1103/PhysRevB.35.626