| 概 要 |
1. Organic Electric Materials. Conjugated organic materials have extended rapidly their feasibility as practical semiconducting materials used in electronic devices such as OLED, OFET, OPVc, etc. In view of the intrinsic ability of semiconductor devices to modulate or switch electrical currents in these materials, the most important materials parameter is the charge carrier mobility. The key to the successful future of organic semiconductors is to realize a mobility equivalent to that of Si-based materials. Organic conjugated molecular crystals have been reported to often exhibit higher values of mobility (>10 cm2 V-1 s-1) than amorphous silicon (1~10 cm2 V-1 s-1) which is quite reasonable when considering the far higher mobility values reported for carbon-based graphenes, fullerenes, and carbon nanotubes.
2. Mobility Measurement Techniques. To date, the methodologies of time-of-flight (TOF), or field-effect-transistor (FET) are often the choices for the determination of the values of mobility in the organic materials. However under these methodologies, applied strong external electric field leads long range translational motion of carriers in the direct-current (DC) mode, and hence the transport of carriers is often disturbed by the presence of impurities, disorders and/or defects in the materials. Recently we have developed time-resolved microwave conductivity (TRMC) measurement for the determination of the mobility in the materials. Unlikely to the DC techniques of TOF and FET where the translational motion of charge carriers induced by the external electric field, an alternating-current (AC) method of TRMC is probing the motion of charge carriers on conjugated molecules and their assemblies without contacts (electrode-less). This has been applied to elucidate the intrinsic value of mobility free from the above mentioned “disturbing” processes, because the translational motion of charge carriers on the molecular materials is limited within the nm-scaled spatial area. The size of the area is depending on the frequency of the AC electric field and the mobility of charge carriers, and the “tuning” of the size by the frequency can deduce not only the value of mobility but also its dependence on the “shape” of the molecules and the conjugated molecular orbitals. In the present paper, we discuss comprehensively the mechanisms of charge carrier transports in the materials in view of what the determinant processes of the charge carrier transport by combining a variety of measurement techniques of charge carrier mobility in organic conjugated materials.
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