Transition Metal Oxides in Organic Light Emitting Diodes

Organic light emitting diodes (OLEDs) exhibit several specific properties such as an extremely thin design and a wide viewing angle, making them favorable for the application in display technology and general lighting. The development of OLEDs is strongly driven by the prospect of low-cost production of largearea applications in the future. Accordingly, their performance was considerably enhanced in terms of efficiency and lifetime over the past years. The introduction of transition metal oxides (TMOs) in OLEDs is regarded as a promising concept for further improving their properties due to their technological compatibility with organic layers and their high thermal stability. The first results from the insertion of TMOs in OLEDs indicate their versatile application as neat functional layers and electrochemical dopants of organic semiconductors. On the other hand, the knowledge of their electronic properties and the mode of operation in OLEDs is very limited so far. In this context, it becomes apparent that fundamental mechanisms such as the electrochemical doping of organic semiconductors or the charge generation in interconnecting units of stacked OLEDs are not yet completely clarified. Thus, this work focuses on the physical and technological correlations arising from the application of transition metal oxides in organic light emitting diodes. First, the electronic structure of molybdenum oxide (MoO3) and tungsten oxide (WO3) is analyzed by photoelectron spectroscopy and Kelvin probe. It is demonstrated that both TMOs exhibit comparably deep-lying energy levels. Moreover, their electronic structure indicates them as intrinsically n-doped semiconductors. The functional principle of the efficient hole injection by neat layers of TMO is then studied by the examination of the interfaces between the TMO and its adjacent layers. As a result, a new model of hole injection by thin TMO layers is developed. The concept of electrochemical doping in OLEDs represents a very importan