Evolution of the electronic structure of cyclic polythiophene upon bipolaron doping

Electronic structures of undoped and doped cyclic polythiophene (PT) are studied using modified σ-bond compressibility model. Cyclic PT doped with odd number of bipolarons creates an aromatic polyene backbone containing (4n+2) π-electrons and the system is driven towards the quinoid form. Consequently, we find an insulator-metal transition for dopant concentration ≥ 14 mol % and a ∼ 0.8 eV redshift in Fermi energy at 30 mol%. For even number of bipolarons, we propose here that the form having two singly occupied degenerate orbitals will be stable in a sufficiently large cyclic PT. PACS numbers : 79.60Fr, 63.20.Kr, 71.20Nr. Typeset using REVTEX