There are p-type organic semiconductors with hole carriers, n-type organic semiconductors with electron carriers and ambipolar organic semiconductors with both hole and electron carriers. Among them, there are small molecule p-type organic semiconductors; rubrene and pentacene as acene series, dinaphthothienothiophene (DNTT) and benzothienobenzothiophene (BTBT) as heteroacene series, oligothiophene series and porphyrin series. A category of n-type small molecule semiconductor involves perylene tetracarboxydiimide (PTCDI) and tetracyanoquinodimethane (TCNQ), and fullerenes. A polymer-based organic transistor also has been developed by using polythiophene, polyfluorene, and a donor-acceptor type polymer. A mobility value obtained from a solution-processed OFET has been normally lower than that obtained from a vapor deposited one, thus solubility is incompatible with mobility so far. However, an excellent solution-processible device was recently developed by soluble organic materials with high mobility.4-7) Further synthetic and process developments of organic materials may provide an efficient solution-processible device.
One characteristic of organic semiconductors is flexibility. Liquid crystal organic semiconductors also receive attention, because they are more flexible than the usual crystalline and amorphous organic semiconductors.8,9) The liquid crystal organic semiconductor shows carrier mobility and self-assembly, in which the liquid crystal molecule spontaneously undergoes an orientation. Moreover, one can control the molecular orientation by applying an electrical field, thanks to flexibility of the liquid crystal molecule.