## 1 Introduction  
Due to their high mobility, good transparency to visible light, good uniformity and reasonable electrical stability, metal-oxide thin-film transistors (TFTs) have attracted great attention in the field of active matrix devices such as liquid crystal displays (LCDs) and organic light-emitting diodes (OLEDs) [1; 2; 3; 4]. The gate dielectric plays an important role in TFTs because it manipulates the conductance of the semiconducting channel by accumulating charge carriers. Moreover, its electrical insulation to minimize a leakage current is another critical requirement for minimal static dissipation [5], which simultaneously affects the transfer performance and the stability and lifetime of TFT devices. Recently, considerable efforts have been devoted to fabricating metal-oxide-based TFTs with high dielectric constant (high-k) gate dielectrics. High-k dielectrics can increase the capacitive coupling between the gate and the active channel layer, which not only increases the driving current, but also reduces the operating voltage. Moreover, high-k materials are highly desirable for improving the electrical performance, reducing the size of the device, as well as reducing energy consumption [6]. Nowadays, a number of high-k metal-oxide dielectrics (such as Al\({}_{2}\)O\({}_{3}\)[7], ZrO\({}_{2}\)[8], HfO\({}_{2}\)[9], TiO\({}_{2}\)Y\({}_{2}\)O\({}_{3}\)[10], Ta\({}_{2}\)O\({}_{5}\) and CeO\({}_{2}\)) have been widely studied and used as an alternative to conventional SiO\({}_{2}\) dielectric layers (k = 3.9) with low leakage current density. Among these gate oxides, zirconium oxide (ZrO\({}_{2}\)) is one of the attractive materials because of its excellent physical properties, for example, high dielectric constant (23-29), good thermal stability, wide band gap (5-7 eV), and large transparency in the visible and infrared ranges [11; 12]. Various vacuum preparation technologies (such as sputter deposition) have been used to prepare metal-oxide films. However, these methods require high-vacuum conditions and a photolithography patterning process that leads to high costs and cumbersome fabrication procedures [2; 5; 13; 14]. To overcome these problems, alternative film-deposition methods have been proposed [15]. The solution-phase processes (such as spin-coating, spray-coating and ink-jet printing) possess many advantages of scalability, roll-to-roll manufacturing and low-cost fabrication processability. For instance, solution-processed ZrO\({}_{2}\) films has attracted much interest, since they have the advantages of low cost, easy chemical composition control and compatibility for large-scale roll-to-roll production [16]. [PERSON] et al. [17] fabricated high-quality ZrO\({}_{2}\) films by a combination of a solution-phase process and ultraviolet (UV) irradiation. The ZrO\({}_{2}\) films with 1 h UV curing showed a leakage current of 1.7 \(\times\) 10\({}^{-6}\) A/cm\({}^{2}\) at \(-\)3 V, a bandgap of 6.13 eV, and a high dielectric constant of 17.8. [PERSON] et al. [18] reported on solution-processed bottom-gate, bottom-contact indium-zinc-tin oxide (IZTO) TFTs with a mobility of \(>\)2 cm\({}^{2}\) V\({}^{-1}\) s\({}^{-1}\). The solution-processed ZrO\({}_{\rm x}\) film, with a thickness of 40 nm, showed a leakage current less than 10\({}^{-10}\) A/\(\upmu\)m.  
However, solution-processed TFTs suffer from severe bias-stress instability for device operation because of inherently rich and undefined defect states, such as pores and organic impurities [19]. For instance, [PERSON] et al. [20] fabricated a zinc-tin oxide (ZTO)/indium-gallium-zinc oxide (IGZO) dual-active-layered ZTO/IGZO TFT (DALZI TFT) and an unpassivated IGZO TFT, which exhibited voltage shifts of \(-\)1.86 V and \(-\)19.59 V, respectively, under negative bias illumination stress conditions (stress time = 1000 s). [PERSON] et al.