University of Otago, New Zealand


Te Tari Hua-Ruanuku

Gordon Group Researchers

Tracey Clarke
Studies of Conducting Oligomers

In recent years, conjugated polymers such as polythiophenes have been extensively studied for their potential application in plastic solar cells and other electronic devices. However, the precise mechanism of conduction in conjugated polymers is still unclear. An increased understanding of the conduction mechanism could lead to a more rational design of polymers and therefore enhance the efficiency of the resultant solar cells.

The key to understanding the conduction mechanism of conjugated polymers is the structure of the charge-carrying species. For p-doped (oxidised) non-degenerate polymers, such as polythiophene, these are positive polarons and bipolarons. The structures of these species can be modelled using quantum chemistry and the accuracy of such methods tested by experimental measurements, such as electronic absorption and vibrational spectroscopy.

We have been studying a number of styryl-substituted terthiophenes, their corresponding sexithiophenes and the oxidation products of both using this combination of spectroscopy and quantum chemistry. Electronic absorption spectra measured during the oxidation process allow the identification and characterisation of the charged species that form and resonance Raman spectra of these species provide structural information. The electronic absorption spectra can be modelled using time-dependent DFT (density functional theory) to establish the electronic excitation processes involved and standard DFT methods have proven successful in vibrational spectra prediction. Examination of the sexithiophene polaron and bipolaron structures has revealed an unusual confinement of the charged defect by the styryl substituents that is not evident in unsubstituted and end-capped sexithiophenes

Structures of the terthiophenes and sexithiophenes studied.

Electronic absorption spectra of the neutral terthiophene (green), its corresponding sexithiophene (brown) and the sexithiophene radical cation (blue) and dication (red).