Dieter Spiehl

Analysis of the mobility of printed organic p-channel transistors depending on the transistor geometry and orientation

The organic thin film transistor (OTFT) is an elementary part of most organic electronic products. A cost efficient and fast way to produce these circuits is by the use of mass printing techniques like flexography or gravure printing. A huge amount of OTFTs is produced via flexography and via spin-coating as a reference. The morphology of the printed layers and the electrical performance of the produced OTFTs are investigated. Based on these investigations, the dependence of the mobility on the transistors source/drain geometry is analysed and explained. Analysed geometry parameters are the transistors channel length, channel width and its orientation compared to printing direction.

Influence of moisture on the electrical properties of solution processed multilayer high-k ZrO2-capacitors

The dielectric performance of ZrO2 thin films – which were manufactured by solution deposition of a single-source precursor – strongly depends on the surrounding atmosphere. Capacitors were constructed and impedance measurements were carried out under different environments. The electrical performance under an inert atmosphere in a glove box could be explained with a capacitor and a parallel resistance, whereas the behaviour in ambient air was more complex. Accordingly, further measurements were carried out in a vacuum chamber which allowed flushing with specific components of ambient air. Nitrogen and oxygen did not strongly influence the device characteristics, whereas the presence of water leads to considerable deviations. This effect could also be monitored by X-ray photon spectroscopy which indicated a change in the O 1s peak attributed to surface hydroxylation. Although thicker ZrO2 films were obtained by repeated deposition of thinner layers, the overall deposit appeared uniform and no evidence of the individual layers could be detected by Scanning Electron Microscopy. The multilayer characteristic of the film only appears by X-ray reflectometry (XRR), revealing a density of 4.4 g cm−3, which is only 75% of the density of monoclinic ZrO2 and must be attributed to porosity within the films. The corruption of the dielectrics in the presence of moisture is thus an effect in the bulk and it is not restricted to the film surface.

Printing of Ultrathin Nanoparticulate Indium Tin Oxide Structures

This contribution focusses on three printing techniques: inkjet printing, flexographic printing and gravure printing for the manufacture of ITO structures based on nanoparticular ITO inks. The quality and stability of nanoparticular inks depends mainly on the dispersing step and is reflected by the achieved particle size distribution. The particle size distribution of the ITO inks has a distinct influence on the optical and electrical properties of the deposited ITO structures. The optimization of nanoparticular ITO inks concerning rheological behavior, wetting behavior and resulting material performance is presented. Inkjet printing was used as a low speed method to manufacture structures in the range of 30 μm. It is shown how coffee-staining, a frequently arising problem in inkjet printing, can be prevented by different strategies. Nanoparticular ITO structures with layer thicknesses clearly below 1 μm were printed. Gravure and flexography printing are well-established industrial printing processes with...

Organic thin-film transistors for circuits in a foundry: process, charge transport phenomena and device library

For the development of circuits consisting of organic thin film transistors (OTFT) with satisfying yield, a stable and reliable process is necessary. This can be achieved by eliminating failure mechanisms and understanding the charge transport phenomena in the individual device. Following the way of a charge through the device, we start with the investigation of the influence of the Schottky barrier height and contact morphology on the device performance by finite-elements simulations. It could be verified that the charge injection limiting contact resistance can be decreased by two orders of magnitude by reducing the thin oxide layer at the source and drain contacts and improving the semiconductor layer morphology at their vicinity. Second, we present an analytical closed-form solution of the OTFT channel potential used for Monte-Carlo charge transport simulations and compute current-voltage and transient response characteristics out of it. In a next step, the influence of the deposition process on the layer interface is investigated. Therefore, velocity distribution measurements of the charge carriers lead to a simulation model with varying disorder, depending on the layer surfaces and deposition techniques. Afterwards, leakage currents through the gate dielectric can be described by a poor conducting semiconductor model in the finite-elements framework. Leakage currents increase power consumption in circuits and, what is more critical, can lead to a total failure of the OTFT. However, they can be influenced by the number of deposited dielectric layers and charge injection supporting self-assembled monolayers at the source and drain contacts. These findings lead to circuit building blocks for an organic device library whereupon still existing performance fluctuations can be coped with Monte-Carlo circuit simulations.

Improved contact resistivity and intra-die variation in organic thin film transistors

We studied the processing-related influence on contact resistivity of organic thin-film transistors in top gate architecture which are placed and oriented differently over flexible substrates. Appropriate plasma treatment reduces degradation of the source and drain contacts, increases effective contact surface for self-assembled monolayer treatment, and thus better injection. Increasing the semiconductor film thickness reduces the contact resistivity until a certain critical thickness. By these means, the contact resistivity has been reduced by two orders of magnitude. We did a mass characterisation of 366 solution-processed transistors on six samples that lead to a modified transfer line method in which we permutated the transistors to extract the contact resistivities. Thus, the intra-die dependency of the contact resistivity on the distance from the centre of the sample, the orientation of the transistor, its width, the pre-processing of the samples and on the semiconductor layer thickness has been ana...