Organic thin film transistors (OTFT) are a key active devices of future organic electronic circuits. The biggest advantages of organic electronics are the potential for cheep production and the enabling of new applications for light, bendable or transparent devices. These benefits are offered by a wide spectrum of various molecules and polymers that are optimized for different purpose.
In this work, several interesting organic semiconductors are compared as well as transistor geometries and processing steps. In a cooperation with an industrial partner, test series of transistors are produced that are intensively characterized and used as a basis for later device simulation. Therefore, among others 4-point-probe measurements are used for a potential mapping of the transistor channel and via transfer line method the contact resistance is measured in a temperature range between 173 and 353 K.
From later comparison with the simulation models, it appears that the geometrical resistance is actually more important for the transistor performance than the resistance of charge-carrier injection at the electrodes. The charge-carrier mobility is detailed evaluated and discussed. Within the observed temperature range a Arrhenius-like thermal activation of the charge- carrier transport is determined with an activation energy of 170 meV. Furthermore, a dependence of the electric field-strength of a Poole-Frenkel type is found with a Poole-Frenkel factor of about 4.9 × 10E−4 (V/m) −0.5 that is especially important for transistors with small channel length. With these two considerations, already a good agreement between device simulation and measurement data is reached. In a detailed discussion of the dependence on the charge-carrier density and from comparison with established the charge-carrier mobility models, an exponential density of states could be estimated for the organic semiconductor.
However, reliability of OTFTs remains one of the most challenging hurdles to be understood and resolved for broad commercial applications. In particular, bias-stress is identified as the key instability under operation for numerous OTFT devices and interfaces. In this work, a novel approach is presented that allows controlling and significantly alleviating the bias-stress effect by using molecular doping at low concentrations. For pentacene as semiconductor and SiO2 as gate oxide, we are able to reduce the time constant of degradation by three orders of magnitude. The effect of molecular doping on the bias-stress is explained in terms of the shift of Fermi level and, thus, exponentially reduced proton generation at the pentacene/oxide interface. For transistors prepared in cooperation with the industrial partner, a second effect is observed that can be explained by a model considering a ferroelectric process in the dielectric and counteracts the bias-stress behavior.
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa.de:bsz:14-qucosa-167894 |
| Date | 12 December 2017 |
| Creators | Hein, Moritz |
| Contributors | Technische Universität Dresden, Fakultät Mathematik und Naturwissenschaften, Prof. Dr. Karl Leo, Prof. Dr. Henning Sirringhaus |
| Publisher | Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | deu |
| Detected Language | English |
| Type | doc-type:doctoralThesis |
| Format | application/pdf |
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