## 1 Introduction  
Organic field-effect transistors (OFETs) have attracted great attention because of their potential applications in flexible and bendable displays with high field-effect mobility, low-temperature processability, and flexibility [1; 2]. Although high-performance OFETs have been consistently reported in the past decade, most of the studies have focused on devices employing rigid inorganic dielectrics, such as silicon dioxide (SiO\({}_{2}\)) and aluminum oxide [3; 4; 5]. Therefore, polymer dielectrics, such as poly(methyl methacrylate), poly(vinyl alcohol), polyimide, and poly(4-vinylphenol), have been extensively studied as alternatives to inorganic dielectric layers in OFETs because the former offer the advantages of solution processes and the need for flexible device applications [6; 7; 8; 9]. However, single layers of such polymer dielectrics have some limitations with regard to working in low-voltage driving OFETs. Realization of low-voltage OFETs necessitates the use of high-capacitance (C\({}_{i}\)) gate dielectrics. In general, C\({}_{i}\) can be expressed as C\({}_{i}\) = \(k\)-\(\varepsilon_{0}\)-\(d^{-1}\), where \(k\) is the dielectric constant, \(\varepsilon_{0}\) is the permittivity of the dielectric material, and \(d\) is the dielectric thickness. From this equation, it is clear that the use of a high-\(k\) gate dielectric film or a reduction of the thickness of the dielectric layer is necessary for increasing C\({}_{i}\). With only a few exceptions, polymer dielectrics generally have low \(k\); thus, the thickness of the polymer layer must be reduced to a scale of at least a few nanometers to increase C\({}_{i}\). However, the insulating properties of polymer dielectrics degrade severely with a reduction in film thickness [10]. Therefore, it is highly challenging to develop novel gate dielectric materials that simultaneously are thin and exhibit good dielectric properties with high \(k\) for application to low-voltage OFETs.  
On account of its outstanding electrical, mechanical, and chemical properties, graphene has been intensively studied for application as a semiconductor, electrode, and encapsulation layer in OFETs [4; 11; 12; 13; 14]. In particular, graphene derived from graphene oxide (GO), which is referred to as reduced GO (rGO), has attracted much attention for application to flexible devices because of its thinness, excellent electrical properties, and mechanical flexibility [15; 16; 17]. Most researchers have focused on achieving good electrical properties of rGO; however, it is notable that GO has potential to be used as a gate dielectric layer of OFETs since it possesses insulating properties and a very high \(k\)[18; 19; 20]. Nevertheless, neither the dielectric properties of GO-based materials nor their application as dielectric layers of OFETs has been studied satisfactorily. To enable the use of GO as a dielectric layer of OFETs, its surface properties need to be modified because the hydrophilic hydroxyl and epoxide functional groups on the GO surface can function as trap sites and reduce the device characteristics. Several studies have attempted to passivate the hydrophilic surface of gate dielectrics with hydrophobic polymers to realize high-performance and reliable OFETs [21; 22].  
Here, we report on the feasibility of using GO as the gate dielectric layer of low-voltage OFETs. We attempted to modify the surface of GO by overlaying it with the hydrophobic polymer polystyrene (PS). Through atomic force microscopy (AFM), X-ray diffraction (XRD), and two-dimensional grazing incidence X-ray diffraction (2D-GIXD) analysis, we confirmed that the GO/PS bilayer gate dielectric reduced the surface roughness and increased the crystallinity of the organic semiconductor. We also found that the GO/PS bilayer gate dielectric resulted in a reduced gate leakage current, which, in turn, drastically enhanced the electrical properties of OFETs within a low operating voltage of \(-\)5 V.