Control of growth and charge transport properties of quaterthiophene thin films via hexyl chain substitutions

Growth mechanism of quaterthiophene based organic thin films deposited by high vacuum deposition on Si:H(100) and SiO2 substrates have been investigated. Especially the influence of hexyl chain end-substitutions on the growth process and the electronic transport properties of organic thin films were studied in details. Highly crystalline films were obtained for both quaterthiophene (4T) and alpha, omega-hexyl-quaterthiophene (DH4T) based thin films. While unsubstituted 4T shows a typical island growth, an almost perfect layer-to-layer growth was found in the case of DH4T based films. It could be demonstrated that the change in the growth mode is directly related to the molecule structure, i.e. to the presence of hexyl chain substitutions on the 4T core, and leads to an increase of the grain size of one order of magnitude under the same evaporation conditions in the case of DH4T films. The characterization of the charge transport properties of thin films based on both molecules reveals one order of magnitude higher mobilities for the DH4T molecules. By using a simple model for charge transport in polycrystalline materials a linear dependence of the mobility on the grain size, independently from the molecule substitution, could be demonstrated. The results underline the importance of the control of the film morphology and give an impressive example of such a control in the case of hexyl end-substitutions of quaterthiophene. (C) 2004 Elsevier B.V. All rights reserved.