Institute for Materials Chemistry and Engineering, and Graduate School of Engineering Sciences, Kyushu University, Kasuga, Fukuoka, 816-8580, Japan
Conjugated polymers, such as polyphenylenvinylenes, polythiophenes and polyfluorenes, have been widely used for organic light-emitting diodes (OLEDs) and organic photovoltaic devices (OPVs). Even though the first discovery of electroluminescence in p-conjugated polymers was done on precursor-route PPV films, PPV derivatives with long side-chains soluble in organic solvents have been used in the development of polymer LEDs, because the preparation of homogenous amorphous ultrathin films has been one the most important requirements for the attainment of high performance polymer LEDs. It should be noted that high carrier mobility has not been considered to be the key issue in the course of the development of PPV derivatives for polymer LEDs.
In contrast, high carrier mobility is one of the most important requisites in the material design of p-conjugated polymers for field-effect transistors (FETs). Among main-chain p-conjugated polymers, regioregular head-to-tail poly(3-hexylthiophene) (P3HT) has been known to give one of the best FET performances. The field-effect mobility of 10-1 cm2V-1s-1 was achieved and the origin of the high mobility has been assumed to the presence of well-developed intermolecular p-p stacking of thiophene rings. Thus it is believed that highly organized intermolecular p-p interaction is one of the most important key issues for the design of conjugated polymers for high-mobility FETs. On the other hand, some experimental results, however, revealed the importance of charge transport along well-developed linear p-conjugation polymer chains.
In the case of OFETs made of vacuum-sublimed small molecules, molecules using p-conjugation units composed of building blocks, such as benzene, thiophene, fluorene, and phenylenevinylene, have been utilized. Thus, it is worth to find out common understanding in molecular design concept of both p-conjugated polymers and aromatic small molecules for OFETs. In this talk, we show the FET characteristics of both vacuum-sublimed oligomer films and conjugated polymer films via wet-processes, which have the common fluorene, thiophene and phenylenevinylene units. The aim of this study is to elucidate the rolls of inter-molecular charge transport along p-p stacking of conjugated rings and the intra-molecular charge transport along conjugated oligomeric or polymeric chains in OFETs.