Synthesis of Novel Selenium-based OFET Materials

Organic semiconductors offer a broad spectrum of potential new applications. The access of flexible, flat-panel, and low-cost devices based on Organic Light Emitting Diodes (OLEDs) and Organic Photovoltaic (OPV) cells has gained growing scientific and commercial interest. Especially Organic Field Effect Transistor (OFET) materials may be employed in innovative new devices such as flexible displays based on OLED technology, in bendable electronic paper, as sensors applicable in medical diagnostics and environmental monitoring, but also in everyday technology like radio frequency identification tags (RFIDs). However, most profoundly investigated materials feature low stability - due to their sensitivity towards air and light - and low solubility. In order to overcome these disadvantages and increase charge carrier mobilities in organic devices at the same time, strategies towards incorporation of heteroatoms (e.g. sulfur, selenium, nitrogen) into semiconductors have been established. Molecular engineering offers the possibility to trigger organic semiconducting properties: based on strong intermolecular interactions due to the heteroatom dense packing charge carrier mobilities can be enhanced. Based on preliminary results the goal of this project is on the one hand the investigation of compounds incorporating selenium and sulfur aiming for high stability and high charge carrier mobility. On the other hand the development of reliable synthetic pathways towards selenium-based materials is an essential matter of this project. However, due to a lack of available starting materials, reagents, and key intermediates up to now the methods evaluated in this project may also be beneficial for other disciplines. The obtained semiconductors will be analyzed with respect to heteroatom interactions given in single crystals. OFET thin-film device fabrication and investigation of the influence of the heteroatom on semiconducting properties will give insight in heteratom-based materials.

In recent years, organic semiconductors gained considerable scientific and commercial interest due to various advantages compared to their inorganic counterparts. Their unique properties allow the fabrication of flexible, large-scale, energy-efficient and low-cost electronic devices such as organic light emitting diodes (OLED), organic solar cells and organic field-effect transistors (OFETs). The study of structure property relationships and their control is important for the development of new materials. Intermolecular interactions strongly influence the charge carrier mobilities of these materials, which determine the efficiency of e.g. OFET devices. The aim of this project was to study the influence of heteroatom substitution on intermolecular interaction and use these insights to synthesize new efficient organic semiconductors. The introduction of electron donating and highly polarizable selenium instead of sulfur is supposed to strongly impact charge carrier mobility. Lately, in my research group bridged thienobenzothiophenes were introduces as new building blocks for OFET materials. Based on these preliminary promising data, we investigated annulated selenophenes as a new platform to control the molecular properties by variation of the molecular packing motifs. Due to a lack of available starting materials, reagents, and key intermediates up to now, the development of reliable synthetic pathways towards selenium-based materials was an essential part of this project. A new synthetic approach was developed for the selective incorporation of heteroatoms. The photophysical and electrochemical properties of the obtained bridged regioisomers showed a strong dependence on the number and position of the introduced heteroatoms. Furthermore, the experimentally determined frontal orbital energy levels follow the trend which was predicted by computation chemistry. Also, single crystals of the target compounds were obtained; the molecular structure and packing of the materials indicate strong intermolecular interactions of the bridged regioisomers depending on the number and position of the heteroatoms. These findings are crucial for the performance of the organic semiconductor. Finally, the obtained materials were tested toward their applicability in OFET devices. We were able to show that higher charge carrier mobilities could be obtained for selenium based semiconducting materials compared to their sulfur-based analogues.