Understanding hydroscopic properties of silk fibroin and its use as a gate-dielectric in organic field-effect transistors

Silk fibroin (SF) has attracted great interest as gate dielectric in organic field-effect transistors (OFETs), owing to outstanding characteristics such as high dielectric constant, transparency, flexibility, and solution processability. In this report, we investigated the relationship between the structural properties of SF films and their performance as SF gate dielectrics in OFETs. Solvent vapor treatment with water or methanol altered the structural properties of the SF films, which adopted a beta-sheet structure; accordingly, both the surface energy and areal capacitance of the SF films were reduced. Notably, atmospheric water contributed to the increased capacitance of the SF film, especially before the solvent vapor treatment. The growth characteristics of pentacene on the SF films were determined by the surface conditions of the films; in particular, Stranski-Krastanov (layer-plus-island) growth mode with mixed standing-up/lying-down orientation of pentacene on the SF film was observed before solvent vapor treatment, whereas Volmer-Weber mode with standing-up orientation dominated after solvent vapor treatment. Pentacene OFETs based on the untreated SF film exhibited a higher on-current compared with the devices based on the solvent vapor-treated SF film. The hydroscopic characteristics of the untreated SF film enhanced its capacitance, thereby inducing accumulation of hole carriers. The present results show that the structural characteristics of SF films have a marked impact on the electrical properties of OFETs based on SF gate dielectrics. In particular, the water uptake capability of SF is a key factor in the electrical properties of organic electronic devices using SF materials.