Dependence of Bias Stress on Hydrophobicity of Gate Insulator in Organic Thin-Film Transistors

Abstract

Organic thin-film transistors (OTFTs) have been paid great attention due to their potential for low-cost, large-area, and flexible electronics. Recently, the electrical performances such as field-effect mobility have come close to or even exceeded those of typical amorphous silicon transistors. Nonetheless, the operational instability of OTFT devices has been considered as a serious obstacle in use for practical applications. Among various factors, the surface properties of a gate insulator which determine charge trapping at the interface between an active layer and a gate insulator strongly influence the stability. In particular, the hydrophobicity of a gate insulator has been thought to play an important role when it comes to moisture in the atmosphere since it has been reported that the water molecules act as main trap sites and cause the instability of the OTFT. However, the effect of the hydrophobicity on the stability has not been elaborated so far because replacement of material for a gate insulator to modify the hydrophobicity is inevitably accompanied by the change in the other surface properties as well. In this work, we report the effect of the hydrophobicity of a gate insulator on the stability of the OTFT, in particular, the shift of the threshold voltage by the gate bias stress. For the investigation of the effect, OTFTs comprised of the same materials but having different hydrophobicity for a gate insulator were fabricated in a bottom-gate top-contact configuration using a solution-processed TIPS-pentacene as an active layer. Here, a hydrophobic fluoropolymer, CYTOP, was used for a gate insulator since the hydrophobicity can be modified without changing other surface properties using the aluminum-assisted method based on the reorientation of end functionalities described elsewhere. Depending on the hydrophobicity of the gate insulator, the value of threshold voltage shift after applying the gate bias stress of -20 V for 1000 s was found to significantly decrease from -13.42 V to 2.56 V on average. It was attributed to the suppression of the trap formation owing to water repellency as well as non-polar end functionalities of the hydrophobic interface. Our studies would help to understand the effect of the hydrophobicity on the stability toward highly reliable OTFT devices for a variety of applications including flexible electronics.