Impedance Spectroscopy of Organic Gate Dielectrics

This project of the Soft Matter Physics Department (chaired by Prof. Dr. Siegfried Bauer) and the Linz Institute for Organic Solar Cells (chaired by Prof. Dr. Nyazi Serdar Sariciftci) of the Johannes Kepler University is concerned with dielectric spectroscopy investigations of ionic impurities in polymer insulators employed as gate dielectrics in organic field effect transistors (OFETs). In OFETs, a current flowing between the source and drain electrodes is modulated via the electric potential applied to the gate electrode. A key issue defining the overall performance of such devices is given by the transport of charges within the organic layers involved. Cyclic sweeps of the gate voltage often reveal a hysteresis in the transfer characteristics in OFETs (the drain-source current vs. gate-source voltage) thereby unfolding an electrical instability of the transistor element. From one perspective, bistable transistors with a nonvolatile hysteresis in the transfer characteristics may be used in organic memory elements, but from another perspective, hysteresis free transistors are needed as components in integrated organic circuits. Therefore, understanding the causes of electrical bi- or instabilities in organic field-effect devices is of primary interest. The field is in its infancy and many of the fundamental problems are yet to be addressed. In the current joint project of the two laboratories we intend to investigate the influence of mobile ionic impurities on the dielectric behaviour of gate insulators in order to correlate the behaviour with the presence or with the absence of hysteresis in organic field effect transistors. The study will be performed on metal-insulator-metal (MIM), metal-insulator-semiconductor (MIS) structures and OFETs respectively. The dielectrics of interest are chosen from the different organic insulators already reported to be suitable as gate dielectric in OFETs: poly(vinyl alcohol) (PVA), poly(4-vinyl phenol) (PVP), poly(methyl methacrylate) (PMMA), poly(vinyl chloride) (PVC), poly(vinylidene fluoride) (PVDF), divinyltetramethyldisiloxane-bis(benzocyclobuthene) (BCB) and poly (a-methyl styrene) (PaMS). The selected dielectric materials have an inherent ionic impurity level owed to the particular fabrication route. Analysis of the dielectric response in MIM and MIS structures especially at low frequencies and under various testing conditions (e.g. temperature, humidity, ambient or inert atmosphere) corroborated with novel techniques of analysing ionic mobilities, charge carrier density and conductivity in polymer electrolytes displaying low charge carrier density will shed more light onto the influence of mobile ionic impurities in gate polymers on the performance of OFETs. In addition, we intend to outline ways for improving OFETs by cleaning the dielectrics, or by decreasing the mobility of ions by cross-linking. The project is not only of pure academic interest, it also provides important parameters to be considered as guidelines for the improvement and optimization of OFETs produced on an industrial scale.

Mobile ionic impurities in dielectric materials which are applied as gate insulators in thin film transistors impinge on the electronic performance of transistor devices. Uncontrollable voltage instabilities and unwanted hysteresis effects are regularly reported with various solution processed polymer interlayer dielectrics. The electrical properties of many insulating materials, used for organic electronic devices, and their effect on the device performance is not well understood. Polyvinylalcohol (PVA) has been chosen as model-polymer system as it is an important dielectric polymer in the field of organic electronics with known instability issues. In the course of the project a comprehensive analysis of the ionic impurities in PVA was conducted. Dielectric impedance spectroscopy and complementary methods, such as time-of-flight polarization experiments and thermal discharge current measurements, led to a much improved understanding of the electric and dielectric behaviour of PVA and similar solution- processed polymer dielectrics. Unfavourable effects caused by mobile ionic impurities, which limit electronic applications, can be reduced by dialysis cleaning of the polymer solution. Even better results can be achieved with vacuum-processed (instead of solution-processed) dielectrics where the purity of the material increases tremendously. Such high purity gate dielectrics like polyaniline and small molecule melamine generate hysteresis-free OFETs working at low voltages. High purity small molecule dielectrics in forms of various sugars (i.e. lactose, glucose, sucrose) and nucleobases (i.e. adenine, guanine, thymine and cytosine) have been employed for the fabrication of low-operating-voltage, hysteresis-free OFETs. The project research on small molecule dielectric materials of natural origin (i.e. nucleobases, sugars) led to discovery of small molecule natural semiconductors and bio- origin substrates for the possibility of truly natural, environmentally green organic electronics. Within the frame of the project many bio-origin semiconductors, (e.g. indigo, Tyrian purple and beta-carotene) and substrates (shellac, hard gelatin, caramelized glucose, bio-polymers, etc.) have been demonstrated for organic electronics. The project work started a novel area of research for bio-compatible, green and environmentally friendly organic electronics.