Planar Organic Semiconductor-Materials for Flexible OLEDs

Organic light emitting diodes (OLEDs) have raised significant attention both in science and industry because of potential display and lighting applications. Benefiting from favorable features such as homogenous large-area emission, lightweight and ultra-thin applications, OLEDs are expected to become a competitive candidate in the future lighting market. In addition, unique characteristics such as flexibility in OLEDs allow for new concepts for the design of lighting products. To realize practical lighting application, the efficacy and stability of the flexible OLEDs require further improvement. Thus, the combined scientific efforts of researchers on both material synthesis and device fabrication are the prerequisite for successful research on these topics. Within this project, two teams from China and Austria will jointly develop novel materials for high performance flexible OLEDs. The research team led by Prof. Johannes Fröhlich at the Institute of Applied Synthetic Chemistry (Vienna University of Technology) will develop organic materials with desirable material characteristics with regard to stability and charge transport properties for OLEDs by designing rigid planarized molecular frameworks. The research team led by Prof. Dongge Ma in the Changchun Institute of Applied Chemistry (Chinese Academy of Sciences) will focus on developing high performance flexible OLEDs by combining both material and device engineering. As a result, the major goal of this project is to understand the relationships between material characteristics, device architectures and device performances. Hence, the intense collaboration of the material scientists in the Prof. Fröhlich group and the device engineers in the Prof. Ma group will trigger a valuable feedback loop in order to improve the OLED technology and, thus, to impact future lightning developments.

In recent years growing interest occurred in utilizing organic light emitting diodes (OLEDs) for display application, but increasingly also for solid state lighting, due to energy efficiency and the possibility of thin and flexible large area devices. Additionally, the possibility to process organic materials from solution using for example printing methods, can enable low production costs. The study of structure property relationships and their control is important for the development of new materials. This, together with advanced device engineering are important factors to generate improved devices. Lately, in my research group indolo[3,2,1-jk]carbazole (ICz) as new building block for organic electronics was introduced. ICz is a versatile building block having both weak electron donating and accepting properties, and materials utilizing it usually exhibit high triplet energies and good thermal stability, both beneficial for application in OLEDs. Within this project ICz should be used as molecular platform enabling to tune material properties with various modifications of the scaffold. In the first part of this project, sulfur and nitrogen atoms were introduced into the ICz scaffold allowing to increase electron donating or accepting strength, depending on the type of heteroatom used. A synthetic approach was developed for the selective incorporation of heteroatoms to specific positions within the scaffold. The photophysical and electrochemical properties can be fine-tuned by variation of the positions and the amount of the heteroatoms. Additionally, the nitrogen atoms introduced have large impact on the alignment in the crystalline state, which is a crucial factor for functional organic materials. The applicability of the nitrogen incorporated derivatives was investigated in a set of molecules which were utilized as emitters in OLEDs. The emission of these materials was tuned over a wide range by building block variation, proofing the possibility of property fine-tuning using these novel building blocks. Materials utilizing the ICz motif usually exhibit rather low solubilities. Therefore, in addition to the modification using heteroatoms, the attachment of alkyl chains was investigated. This significantly improved the solubility of the materials, rendering the applicability of ICz based materials for solution processing techniques. Using the alkylated ICz building blocks, materials were developed which were successfully employed in OLEDs processed from solution. Besides the improved processability also the stability of the materials was increased by the alkylation. Finally, we were able to show that the substitution pattern of the ICz motif has large impact on the interactions within thin films. It was possible to develop new materials which allowed the fabrication of OLED devices with increased performance compared to previously reported ones using similar materials.