Organic thin film transistors : electrical properties of organic/inorganic interfaces modified by self-assembled monolayers

First demonstrated in the middle of the 1980s, organic electronic devices have risen to prominence during the last decades. In some fields, as the smartphone display market, they have already overtaken their inorganic counterparts in their occurrence. Furthermore, new kinds of applications like medical skin sensors are emerging. However, such organic devices oftentimes still lack behind their inorganic counterparts regarding efficiency. In this line of thought, two crucial properties can be optimized to reach higher performance. These are the intrinsic electronic properties of the organic materials employed in the device and the crucial interface between the commonly employed inorganic electrode materials and the adjacent organic layer. In the scope of this work, the electrode/organic interface is studied in order to get a deep understanding of the mechanisms enabling the reduction of ohmic losses at such an interface and thus lifting device performance. To this end, organic thin film transistors are fabricated and their electrical behavior is evaluated. The respective interface of the electrodes is modified by self-assembled monolayers (SAMs) to tailor the energetic interface. SAMs are small organic molecules that are capable of self-assembly at an interface and will form a covalently bound monolayer. The modification of the electrode interface enables the tailoring of the interface energy barrier. This barrier is given as the difference between the electrode work function and the energetic position of the conduction orbital of the organic material. In order to assess the impact of the injection barrier at the electrode, the contact resistance of the transistors has been determined by means of the transmission line method. By correct choice of the SAM molecule and thus a minimized energy barrier, the thermal losses due to the contact resistance can be reduced by more than one order of magnitude. Furthermore, during the present work it is shown that utilizing the interface energy barrier, the performance difference of the transistors can be described.Based on existing frameworks in literature, a model is developed to predict the work function of an electrode subsequent to its modification without extensive calculation requirements. Thus, the choice of an optimal SAM molecule for a specific application can be facilitated. Finally, utilizing live in-situ measurements of the transistor properties during the growth of the organic active layer, guidelines for the fabrication of the optimal device for a given application are introduced. Besides the energetic alignment of the energy levels at the electrode interface, e.g. the thickness of the organic active layer is crucial