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Molecular Design Concept for Organic Semiconductor Devices
Prof. Tetsuo Tsutsui has extended his study on material design of organic materials for electronics application from the viewpoints of materials chemistry towards the utilization of active functionality of organic compounds for elctronics as early as 1980. Prof. Tsutsui started his basic study on organic electroluminescent (EL) materials and devices more than twenty-five years before, and continued extensive studies in both basic research on organic EL materials and application oriented development for organic EL devices. His extensive research activities have largely contributed to the commercialization of high-performance organic EL displays today.
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Prof. Tetsuo Tsutsui
Kyushu University
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The research achievement of Prof. Tsutsui is the establishment of the basis of material design concept for the development of organic semiconductor devices, such as organic EL, organic photovoltaics, and field-effect transistors, as described below.
1.Preparation of π-conjugated polymers and their electronic properties
Prof. Tsutsui investigated intrinsic electronic properties of high-purity &pi-conjugated polymer films and pointed out the key importance of the elimination of impurities and structural defects incorporated in the polymer films for the basic understanding of the intrinsic properties of main-chain π-conjugated polymer systems. Specifically, he focused his attention to polyarylenevinyles, which were used as representative ?-conjugated polymers in 1980s. He developed a new route for the preparations of high-purity polyarylenevinylene films using organic solvent-soluble precursors. Spin-cast precursor polymer films prepared from organic solvents were converted to high-quality ultra-thin π-conjugated polymer films via the acid-catalyzed solid-state conversion. Detailed observations of optical and electronic properties of high-quality polyarylenevinlyne ultra-thin films thus prepared revealed that the intrinsic conductivity of polyarylenevinylenes was less than 10-15 Scm-1 under the absence of absorbed oxygen and water. In addition to the elucidation of the profound effect of impurities on electronic properties of π-conjugated polymers, the role of chemical and structural defects on skeletal chains of π-conjugated polymers were also clarified.
The basic understandings of intrinsic electronic properties of &pi-conjugated polymers had been quite informative for the understanding of basic working mechanism of high-performance organic thin-film EL devices in 1990s.
2.Organic EL materials and devices
From an early stage of studies in 1980s and 1990s to an application-oriented research and development stage in recent 10 years, Prof. Tsutsui has made variety of key contributions. His several key break-through issues are described below.
Prof. Tsutsui started his research on organic EL devices as early as 1983. Inspired by the proposal of high-performance double-layer structure EL devices by Dr. Ching Tang in 1987, he proposed another double-layer device structure and a triple-layer device structure, and was successful in explaining the intrinsic roles of multilayer structures in high performance organic multilayer EL devices. He gave one of the earliest proposals on the basic concept of molecular design of hole-transporting, electron-transporting and emissive materials for multilayer EL devices.
Prof. Tsutsui established the concept of the "carrier balance" as early as 1993 and proposed the simple but quite useful formulation expressing power and quantum efficiencies of organic thin-film EL devices, which have been extensively used in every aspect of investigations concerning device performances since then. The EL quantum efficiency is composed of four basic factors, a coupling out factor, a carrier balance factor, an efficiency of production of excited states, and a molecular luminous efficiency.
High-performance organic thin-film EL devices are composed of organic multilayers with the thickness of less than a quarter of the wavelength of visible light sandwiched between a transparent indium-tin-oxide (ITO) anode and a reflective metal cathode. Thus optical interference effect reveals and this aspect is intimately related with device performance of EL devices. Prof. Tsutsui firstly pointed out the importance of the so-called micro-cavity effect in thin-film EL devices as early as 1991, and demonstrated the possibility of controlling emission spectra and spatial distribution of emitted light. Moreover, he pointed out the fact that the optical couple-out factor in thin-film EL devices was intrinsically low, and proposed some practical methodology for removing this limitation.
3.Organic thin-film transistors
Based on the full knowledge concerning materials design and device design for organic EL devices, two-terminal diodes, Prof. Tsutsui has extended his research field to three-terminal devices, organic field-effect transistors (FETs) and elucidated the similarity and dissimilarity in molecular design concept between two class of organic materials for EL devices and FETs. Two major attainments are pointed out here. One is the appearance of ambipolar transistor behaviors in both p- and n-channel operations in typical organic semiconductor materials such as pentacene and phthalocyanines. Thus importance of bipolar nature of high-purity organic semiconductors for FETs, giving a quite clear contrast to the case of monopolar hole or electron transporting materials for EL devices, are generally accepted. Another attainment is the demonstration of the importance of molecular orientation and intra-chain carrier transport in FETs using vitrified liquid crystalline oligomers.
In summary, Prof. Tsutsui has contributed the establishment of the basis of materials design concept of organic semiconductor materials always seeking intimate relations with the design concept of high-performance organic devices, for about 30 years. Especially, Prof. Tsutsui devoted his research efforts on both basic and application-oriented studies of high-performance organic thin-film EL devices for more than 25 years, and has played a leading role of the development of unprecedented organic functional devices, high-performance organic EL displays, which are now commercially available. All those achievements in his studies on organic semiconductor materials and devices have been receiving international reputation, and hence were judged to be worthy of Chemical Society of Japan Award.
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