Associate Prof. Michihisa Murata and one of his graduate students, Kazuki Ueda (3rd year PhD student), from the Department of Applied Chemistry at Osaka Institute of Technology (OIT), senior researcher Dr. Shintaro Kawano from the Osaka Research Institute of Industrial Science and Technology (ORIST), and Prof. John S.-C. Chuang from the National Yang Ming Chiao Tung University in Taiwan, have developed a new type of organometallic material that is highly conductive. The molecular design is based on the use of a specific thienothiophene ligand to synthesize nickel complexes. A composite film of this new material and polyvinylidene difluoride (PVDF) exhibited outstanding electrical conductivity, surpassing the highest values recorded so far for n-type films of organic and organometallic semiconductors prepared by a solution process.

The key points of this study are:
- Developing a new n-type organometallic material has led to the highest hitherto recorded electrical conductivity for a solution-processed film.
- The new material is stable under atmospheric conditions, which stands in sharp contrast to most n-type conductive organic materials.
- The new material exhibits good thermoelectric conversion, which suggests future applications in flexible thermoelectric generators.

While most organic materials are insulators, organic conductive materials can conduct electricity. Compared to inorganic materials such as silicon, organic conductive materials are characterized by light weight, large areas, flexibility, printability, and low thermal conductivity. These materials are thus expected to serve as indispensable materials for the development of advanced electronics and energy-harvesting technologies. Solid-state organic conductors are mainly composed of p- and n-type materials with hole and electron carriers, respectively. The performance of n-type materials has so far been lower than that of p-type materials, which is, at least in part, due to their instability under atmospheric conditions and difficulties associated with their synthesis. In general, solution-processed films of n-type organic materials exhibit low electrical conductivity (<10 S/cm), which needs to be improved if they are to serve in commercial applications.

Associate Prof. Murata, graduate student Ueda, and their co-workers focused on nickel complexes as n-type organic semiconductors, which are known to be stable under atmospheric conditions. In this study, they developed a new organometallic material, whose molecular design is based on the complexation of a specific thienothiophene ligand with a nickel core. The obtained new material can be accessed via a concise synthetic route and composite films of this material and PVDF can be readily fabricated (coating, drying, and annealing under atmospheric conditions). The resulting composite films exhibit the highest electrical conductivity (>200 S/cm) recorded so far for n-type films of organic and organometallic materials prepared by a solution process. In addition to the excellent electrical conductivity, which is approximately twice of the previously highest value, the film is stable under atmospheric conditions, easy to synthesize, and exhibits desirable thermoelectric properties.
The results have been published as a communication in the Journal of the American Chemical Society (JACS), which is one of the most prestigious journals in the field of chemistry;

URL: https://pubs.acs.org/doi/10.1021/jacs.2c07888（DOI:10.1021/jacs.2c07888）