Theoretical studies on the carrier tunability of oxidized oligothiophenes

A first principles density functional theory calculation in combination with the non-equilibrium Green's function technique has been performed to show the carrier tunability of a family of thiophene-1,1-dioxide (TDO) oligomers. Our theoretical results interpret how the molecular length dictates the nature of charge carriers that has very recently been observed through thermopower measurement (Nat. Chem. , DOI: 10.1038/NCHEM.2160). Our molecular level understanding dictates that with increasing the conjugation length through the molecular backbone, it is the LUMO orbital which is stabilized more and thus participates in resonance tunneling with the gold electrode. Thus the HOMO resonating channel, which is the major transporting channel for TDO1, is shifted to the LUMO resonating channel in the case of TDO4. This carrier switching from a hole to an electron is unique for these types of organic materials.