In-situ production and characterisation of organic transistors

Electronic devices based on organic materials are generally expected to be of great future importance. Some of the advantages of this type of "plastic electronics" are their flexibility, cheap large area production and lightweight. Even the production of green, disposable electronics is envisioned. Nevertheless, there are still great challenges, which have to be overcome to bring this technology to the market. The challenges are the reproducibility, stability and electron mobility, to mention just a few. In the proposed project we claim to make considerable progress in the fabrication, characterization and optimization of novel organic model devices. To reach this goal the collaboration of two research institutes, the Institute of Solid State Physics at the Graz University of Technology and the Institute of Surface Technologies and Photonics at the Joanneum Research Forschungsgesellschaft in Weiz is suggested. In both institutes a long-time expertise on the fabrication and characterization of organic films and organic electronic devices exists. In this project the applicants will produce and investigate novel organic model thin film transistors (OTFT). The novelty of this project is the unique combination of surface science methods for the film characterization with in-situ studies of the electrical properties of OTFTs during film preparation. It is known that the morphology, structure and chemical composition of the very first layers mainly determine the functionality of an organic transistor. The organic layers, as well as the contact electrodes, will be prepared by physical vapor deposition and the films will be characterized by a variety of surface analytical techniques: Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), thermal desorption spectroscopy (TDS), work function measurements (F), atomic force microscopy (AFM), etc. In this project we will explore promising novel materials; both for the active layer (e.g. PTCDI, rubrene, phthalocyanine) as well as for the dielectric layer (e.g. PVP, PVCi, BCB, PaMS). The main issue will be the optimization of the charge carrier transport as a function of the thickness, morphology and structure of the semiconductor layer and electrode layer in top and bottom contact geometry. Although the main focus of this project is put on organic thin film transistors, the gained findings will be also of relevance for the production of other organic electronic devices, e.g. solar cells, light emitting diodes and sensors. Areas of application will be the energy and entertainment industry, but applications in medicine, for green technology and food processing industry can also be anticipated.

In our modern world, electronic devices and gadgets are abound everywhere and influence our daily life considerably. For the future such equipment will play an even more important role in the society, think just about the so-called internet of things (IoT). Wearable electronics, smart objects and biochips are some catchwords. Consequently, electronic devices based on organic material, possibly even biodegradable, will become increasingly important. In this project, we focused on the manufacturing and characterization of organic model transistors, in order to better understand the complex relationship between the growth of the ultrathin organic films and the electrical properties of such devices. For this purpose, the preparation of the active organic layer and the characterization of the transistor was performed in-situ under ultrahigh vacuum conditions. This allowed unprecedented measurements of the performance of organic transistors, in particular with regard to reproducibility and the correlation between film morphology and electrical properties. To this end, in situ Auger Electron Spectroscopy and Thermal Desorption Spectroscopy, as well as ex situ Atomic Force Microscopy were applied to determine the chemistry, thermal stability and morphology of the dielectric gate substrates and the semiconducting organic layers. A special sample holder was developed allowing the cooling and heating of the organic devices, thus enabling the study of the transistor performance in-situ as a function of film thickness, sample temperature and substrate modifications. In particular, organic transistors with pentacene and epindolidione as the active semiconducting material, and silicon dioxide, as well as spin-coated capping layers PVCi and PNDPE as dielectric substrates, were prepared and characterized. The main findings can be summarized as follows: The transistor properties, mainly characterized by the charge mobility and threshold voltage, were governed by a subtle interplay between the grain size and density, the molecular ordering in the grains, and the overlap between the organic film and the gold contacts. Deposition of the film at elevated temperature increased the grain size and the molecule ordering, thus increasing the charge mobility; however, at the same time dewetting effects diminished the overlap between the organic film and the gold source and drain contacts, thus decreasing the attainable drain current and consequently the mobility. With an elaborate deposition technique, by utilizing the specific layer growth modes at different substrate temperatures, this restriction could be overcome and excellent mobilities for the pentacene/silicon dioxide system at comparably low film thickness could be obtained.