Investigation of the effects of surface modifications on the growth of pentacene and on the electrical properties of organic thin film transistors (2011)

In the past few years organic semiconductors have seen a great interest in research and industry. In particular, the possibility of using printing techniques for large-scale production and the opportunity of flexible substrates open up new application areas for polymer electronics. Displays on flexible substrates show a possible application employing organic light emitting diodes (OLEDs) and organic thin-film transistors (OTFTs). The first commercial products are available for a few years. Objective of this work is the systematic study and optimization of organic thin film transistors based on the organic semiconductor pentacene. Improvement is needed in particular with regard to charge carrier mobility, stability and reproducibility of the properties. In the course of the dissertation, the effects of the morphology of the semiconducting pentacene-layer on the electrical properties of the organic thin film transistors on a silicon substrate with silicon dioxide as a dielectric have been investigated systematically. The morphology was influenced by changes in production parameters. In particular it was shown that the crystal size has no effect on the charge carrier mobility in semiconductors. Taking into account the parameters of deposition rate and surface free energy simulations of the crystallization has been developed based on the diffusion-limited aggregation, which is in a good agreement with the morphological studies. The relationship has been found that a modification of the substrate surface, which leads to a change of the surface free energy, improves significantly the crystallinity of the pentacene-layer, but also the charge carrier mobility and the switch-on characeteristics of transistors. Also, the electrical stability to influences such as water and degradation of transistors improved. These findings have been transferred successfully to other substrates and dielectrics. By optimizing the surface-modification and the production parameters thin-film transistors with not purified pentacene can be prepared reproducibly with a charge carrier mobility of up to 1 cm2/Vs and a switch-on voltage of about 0 volts. In these transistors, mechanical pressure measurements have been performed. Mechanical pressure changes reversibly the electrical properties of transistors in subthreshold area. This effect can be attributed to the ratio between the bulk and the thin-film phase in mixed crystalline pentacene layers, as can be observed in bending tests. Investigations of the field gradient using the scanning electron microscope show an inhomogeneous field distribution in the transistor channel, which is caused by the different conductivity of grains and grain boundaries. Based on these results, the electrical transistor model is extended to the transition regions between the work areas and thus achieve a better simulation of the curves. Finally, the investigation results are discussed and attributed to the existing structures in the traps. A model that takes into account the distribution of charge carriers in the traps and in the HOMO as a function of pre-state, serves to explain the hysteresis in the input characteristics and the drain current curve for a step change in the gate-source voltage. An analysis of the possible causes for adhesion sites shows which factors to optimize the electrical properties must be considered.