Surface energy control of soluble polyimide gate insulators by copolymerization method for high performance pentacene thin-film transistors

We have synthesized a series of surface energy controlled soluble polyimide (PI) gate insulators (KSPI-C1, KSPI-C3, and KSPI-C5) by one-step copolymerization method of the monomers 5-(2,5-dioxytetrahydrofuryl)-3-methyl-3-cyclohexene-1,2-dicarboxylic anhydride, 4,4-diaminodiphenylmethane, and 1-(3,5-diaminophenyl)-3-octadecylpyrrolidine-2,5-dione (DA-18-IM). The long alkyl chain containing DA-18-IM monomer was used to control the surface energy of PI polymers. Fully imidized PI polymers were found to be completely soluble in organic solvents such as N-methyl-2-pyrrolidone, dimethylacetamide, etc. Thermal gravimetric analysis exhibited that all polymers are stable up to more than 432 °C with only 5 wt.% weight loss. The glass transition temperatures of polymers are found to be between 208 °C and 242 °C by differential scanning calorimetry measurement. Thin films of all polymers could be fabricated at only 150 °C and a pentacene thin-film transistor (TFT) with KSPI-C3 as a gate dielectric was found to exhibit the highest field effect mobility of 0.53 cm2/Vs. Surface energy controlled PI polymers are promising candidates for gate dielectrics for organic TFTs.