STUDIES OF THE ELECTRONIC-PROPERTIES OF CONJUGATED POLYMERS - FROM DIMERIZATION TO CHARGE-TRANSPORT IN POLYMERIC TRANSISTORS

Electronic properties of conjugated polymers were studied theoreticall y and experimentally. The density-functional theory in the local-densi ty approximation and a self-consistent linear-combination-of-Gaussian- orbitals method were used to study the ground-state geometry and the e lectronic structure of undoped and doped trans-polyacetylene chains. A dimerized ground state of alternating single and double bonds is obta ined for the undoped case, although the alternation is weaker than ind icated by the experiments. The uniform dimerization is quenched at a d oping level of about 3 - 4 % per CH unit, suggesting an undimerized me tallic state at high doping levels. The possible polaron and soliton l attices were studied for a hole doping level of 20 %. A marginal solit on lattice was found to compete with the undimerized metallic state. T he experimental studies were performed mostly on thiophene derivatives , which are more stable and easily processable than trans-polyacetylen e. The semiconducting poly(3-alkylthiophenes), quinquethiophene and be ta-carotene, studied by using thin-film field-effect transistor struct ures, show a p-type behaviour and relatively low charge carrier mobili ties, typically less than 10(-3) cm2/Vs at room temperature for spin-c ast and vacuum-evaporated thin films. The mobilities in Langmuir-Blodg ett films are even lower and decrease with a decreasing number of mole cular layers. The results are consistent with charge transport by hopp ing. The polymer transistor was also used to study the photoluminescen ce quenching by the extra (bi)polarons accumulated in the channel. Sho rt-channel polymeric transistors were fabricated and studied experimen tally. In metal-island transistors islands of gold or copper act as ve ry small electrodes forming a random network of nanotransistors. Discr ete devices with sub-100-nm channels also show transistor operation. D espite the short channels, no qualitative difference in the transport mechanism is seen compared with the long devices, suggesting charge se lf-localization/trapping either directly or within a few nanometers af ter injection.