2, The Research Institute for Molecular Electronic Devices, Sakai, Osaka, Japan
Gate insulators can influence the electronic properties of organic field-effect transistor (OFET) and the typical requirements for gate insulators are low process temperature below 400 K, flexibility, high electrical insulation, high heat resistance, and high organic solvent resistance.
CYTOPTM (AGC Asahi glass) is a fluorine-containing polymer and has been used as gate insulators of top-gate OFET. A reason for this is that most organic semiconductors are not soluble to the fluorine solvent of CYTOP, and hence CYTOP can be formed on an organic semiconductor layer by a wet process with no damage to the semiconductor layer. Although a variety of CYTOPs having different terminal groups have been developed recently, OFET characteristics by using the CYTOPs have not been fully examined yet. In this presentation, we studied the electronic characteristics of OFETs with three types of CYTOPs and discussed the relationship between the electronic characteristics and the chemical structures of CYTOPs.
Top-gate bottom-contact OFETs with different CYTOP gate insulators were fabricated. Three types of CYTOPs were CTL-M, CTL-A and CTL-E. The polymer terminal groups of CTL-M, CTL-A, and CTL-E were -CONH-(CH2)n-Si(OEt)3, -COOH, and -COOMe. 2,7-Dioctylbenzothieno[3,2-b]benzothiophene (C8-BTBT) and C60-fused N-Methylpyrrolidine-m-C12-phenyl (C60MC12) were used as p-type and n-type semiconductors, respectively. The OFET characteristics of p-type and n-type OFETs were measured at different temperatures from room temperature to 150 K, and the gate bias stress test was also carried out in the OFETs. Interface localized-state distributions of the p-type and n-type organic semiconductors were determined from the temperature dependence of the transfer characteristics of p-type and n-type OFETs.
We found that the electrical stability, the field-effect mobility, and the interface localized-state density strongly depend on the polymer terminal groups of CYTOPs. We also found the relationship of acidity (for Lewis acids, acidity relates to the compound's ability to accept an electron pair) of the terminal groups and the OFET characteristics: higher acidity of the terminal groups of CYTOPs leads to higher density of interface localized states, and thereby to lower field-effect mobility and to poorer electrical stability.