Order versus Mobility in Organic Thin Film Structures

For the past forty years inorganic semiconductors like silicon and gallium arsenide have been the backbone of microelectronic device applications. However, there is a growing research effort in the field of "organic electronics" at the cross point of organic chemistry, condensed matter physics, materials science and device physics. In order to improve the semiconducting, conducting and opto-electronic properties of organic electronic materials novel synthesis methods and self-assembling techniques as well as single crystal growing techniques are used. In addition to their unique electronic and optical properties these materials possess good mechanical properties. Consequently, this field attracted the attention of global research and development in academia as well as in industry due to the promising opportunities for new device applications. As a result of intense research efforts, the list of organic electronic devices, realized today, includes diodes, photodiodes, photovoltaic cells, light emitting diodes, lasers, field effect transistors, electro-optical couplers and modulators and all-organic integrated circuits. People even speak about it as a key technology of 21st century. Conjugated polymers combine properties of classical semiconductors with the inherent processing advantages of plastics and therefore play a major role in low cost, large area optoelectronic applications. Unfortunately, polymers are commonly highly disordered in the solid state. Consequently, carrier transport is dominantly influenced by localization resulting in very low charge carrier mobilities. Therefore, an important part of proposed research aims toward significant improvement in the performance of organic devices and deeper understanding of physical processes using epitaxially grown thin films of small molecular systems, in which highly ordered or even crystalline structures can be obtained. In addition, well-ordered small molecule systems allow an easy and reproducible investigation of their anisotropic transport properties. The goal of this project is to investigate the properties of highly ordered small molecule thin films for practical applications in organic electronic and photoelectronic devices using electrical and photoelectrical characterization of the field effect transistor methods. The field induced charge carriers will be investigated in the dark as a function of the temperature in standard FET electrical characterization. Afterwards, photoinduced charge carriers in these devices will be characterized using opto-electrical characterization methods. The Structure/Morphology versus mobility relationships will be also established.

During this cluster project our part has been concentrating on the importance of order on the mobility on the device characteristics and performance in organic thin film semiconductor devices. Our studies revealed the following results: a.) The mobility of charge carriers in an organic thin film field effect transistor (OFET) and organic solar cells is a sensitive function of the organic thin film nanomorphology. b.) The mobility of charge carriers in an organic field effect transistor is a critical function of the interface to the organic dielectric used in such devices. That interface determines even the ambipolar character of an OFET system. c.) The mobility of the charge carriers in an OFET is much higher when the crystallinity and long range order of the first few nanometers close to the organic dielectric/organic semiconductor interface is higher. The How Wall Epitaxy (HWE) reveals ordered organic thin films with higher crystallinity which in return gave record charge carrier mobilities in organic field effect transistors.