Photoreactive Materials for Thin Film Transistors

The past years have seen a strong increase in research on organic field effect transistors (OTFTs). This is mainly due to the promise of cheap and efficient device production (using, e.g., ink-jet or roll to roll printing), the possibility of realizing OTFTs on flexible substrates, and the vision of combining the electronic properties of organic semiconductors with the synthetic versatility of organic chemistry. The proposed research project focuses on novel materials and in-line production compatible patterning strategies for the fabrication of OTFTs (bottom gate architecture) with channel lengths in the sub-micrometer range. For that reason we will focus both on nanoimprint lithography for the fabrication of source and drain gold electrodes in the submicron meter scale, and on photolithography for the patterning of photosensitive conductive polymers serving as gate electrode. In preliminary investigations we have succeeded in the synthesis of a photosensitive polyaniline derivative which exhibits comparably high conductivities and also provides the possibility of UV patterning. This polymer will be investigated as material for polymer gate electrodes in OTFTs. Moreover, new photosensitive polymers will be synthesized and applied as polymer dielectrics in OTFT`s. The photo reactivity provides the possibility of (I) photolithographic patterning, e.g. for via holes in organic circuits, and (II) the UV-induced modulation of surface polarity which allows "tuning" of the growth of organic small-molecule semiconductors. A further goal of our work is to tune the surface properties of the source and drain gold electrodes using photoreactive organothiol self assembled monolayers in order to adjust the grain size of the deposited organic semiconductor with respect to improving the charge carrier mobility and to minimize the parasitic contact resistance. Moreover, dielectric and SAM materials that show promising results concerning OTFT performance will be applied on large areas as continuous layers (by means of slitcoating) and as patterned layers (by means of gravure printing). These experiments are done in a roll-to-roll pilot line. In this part of the project it is investigated if and how an application of these materials by large-area deposition techniques can lead to analogous results in terms of thin, pinhole-free and homogenous films.

The past years have seen a strong increase in research on organic thin film transistors (OTFTs). This is mainly due to the promise of cheap and efficient device production (using, e.g., ink-jet or roll to roll printing), the possibility of realizing OTFTs on flexible substrates, and the vision of combining the electronic properties of organic semiconductors with the synthetic versatility of organic chemistry.This research project focused on the investigation of novel photosensitive materials and in-line production compatible patterning strategies for the fabrication of organic thin film transistors (OTFTs). In the course of our research, photopatternable dielectric and electrode materials were developed and tested in OTFTs with the aim to realize complex organic integrated circuits. In this context, photosensitive polymers were synthesized, which can successfully be applied as intrinsically photopatternable dielectric material for OTFTs. Thin films of this polymer provide low surface roughness and can be patterned with resolutions in the low m-range without the need for any additional photoinitiator. In combination with appropriate patterning strategies, devices were obtained that show a high transistor performance at low operating voltage which promises the realization of highly integrated circuits with low energy consumption.A further focus of our research was the implementation of biodegradable and biocompatible materials in OTFTs and organic circuits. These materials represent a new niche in the organic electronics research, aimed to address the issues of cost and toxicity to humans and environment. Following this trend, we tested cellulose, the most abundant biopolymer on earth, as gate dielectrics in OTFTs. We could successfully demonstrate that highly robust and well-performing transistors can be realized using this biopolymer. Moreover, a straightforward approach for the photopatterning of cellulose has been developed, which paves the way towards the fabrication of complex organic integrated circuits using this green biopolymer.Besides photopatternable dielectric materials, also methods for the economical and efficient fabrication of electrically conductive structures were developed in course of this research project. In this context, an UV-induced cross-linking of gold nanoparticles with organic bisazides was investigated. This approach facilitates a sintering temperature below 150 C and thus the fabrication of electrically conductive structures on flexible polymeric substrates. The photo-lithographic patterning of these nanoparticles enabled OTFTs with channel lengths of 10 m and proper transistor performance on common polymeric substrates.