# Research Progress on Flexible Oxide-Based Thin Film Transistors
## 3 Approaches to Flexible Oxide-Based TFTs
### Flexible TFTs on Different Kinds of Substrates
#### 3.1.2 PEN Flexible Substrates  
On PEN substrates, the films appear in a slightly white color. Compared with PET, the maximum process temperature of PEN is higher (Tg \(\approx\) 180 \({}^{\circ}\)C). In the past, PEN has been utilized in the production of electronic items such as audio-visual accessories and videotape. Besides, PEN provides better barriers for oxygen and carbon dioxide, which makes it interesting for applications in flexible displays. [PERSON] et al. used RF sputtering to achieve a-IGZO thin films on PEN at room temperature, which featured high transmittance and good performance after 6 h of oxygen-annealing followed by 6 h of vacuum-annealing [152]. Furthermore, a 130 nm thick Al\({}_{2}\)O\({}_{3}\) was deposited as a gate insulator by [PERSON] et al. employing an atomic layer deposition (ALD) method at a growth temperature (Ts) of 150 \({}^{\circ}\)C [81]. Bending tests were performed to investigate the change in device properties. Oxide TFTs based on the dual active layer architecture were developed by [PERSON] et al. [153], as shown in Figure 5a. Devices with this dual-active-layer exhibited an improved performance and stability under gate bias stress when compared to their single-layer counterparts. A top-gate structure IGZO TFT fabricated on PEN is shown in Figure 5b [86]. A cool-off type adhesive was used to laminate PEN from glass, which is convenient for the debonding process. There were two buffer layers deposited on the PEN before sputtering the S/D electrodes to achieve a smooth surface, low moisture permeability, and strong protection from mechanical damage. This device with a 2:1:2 (In:Ga:Zn) IGZO channel composition exhibited a high performance such as a mobility of 7.83 cm\({}^{2}\) V\({}^{-1}\) s\({}^{-1}\), a threshold voltage of 1.93 V, and a subthreshold swing of 0.24 V dec\({}^{-1}\). As discussed  
Figure 5: Schematic of the TFTs fabricated on PEN. (**a**) Dual active layer structure. (**b**) Top-gate structure [86]. (**c**) IZO:F-TFTs with fluorine-doped [124]. (**d**) a-IGZO TFTs with nanocomposite dielectric [49]. (**e**) A-IGZO-TFT with double-layered barrier layer structure [154]. (**f**) A-IGZO-TFT formed using a flash lamp annealing process [155]. in Section 2.1, the solution-process technology is widely used in the production of flexible devices, due to the low process temperature and the inexpensive fabrication of large-areas without using vacuum deposition techniques. In 2013, [PERSON] et al. fabricated flexible fluorine-doped indium zinc oxide (IZO:F) TFTs by spin-coating an aqueous solution containing metal fluoride precursors, as shown in Figure 5c [124]. After 2 h of annealing at 200 \({}^{\circ}\)C, these TFTs showed a mobility of 4.1 cm\({}^{2}\) V\({}^{-1}\) s\({}^{-1}\). [PERSON] and co-workers adopted the Al\({}_{2}\)O\({}_{3}\) nanoparticles to incorporate into a polymer as a nanocomposite dielectric on a-IGZO TFTs by means of a solution process. Compared to inorganic dielectrics, TFTs with nanocomposite dielectrics had a higher flexibility and exhibited a relatively high mobility of 5.13 cm\({}^{2}\) V\({}^{-1}\) s\({}^{-1}\), as shown in Figure 5d [49]. An IGZO-TFT with inorganic/organic barrier layers as a buffer layer is illustrated in Figure 5e. A 3 mm-thick spin-coated organic layer and a 50 nm-thick ALD Al\({}_{2}\)O\({}_{3}\) inorganic layer were fabricated before depositing onto the active layer. TFTs with the proposed structure exhibited a saturated mobility of 15.5 cm\({}^{2}\) V\({}^{-1}\) s\({}^{-1}\), an on/off ratio of 2.2 \(\times\) 10\({}^{8}\), and a subthreshold swing of 0.2 V dec\({}^{-1}\)[154]. The dual gate structure of the a-IGZO, shown in Figure 5f, formed a TFT using a flash lamp annealing process under mechanical stress, as presented by [PERSON] et al. [155]. This device showed a threshold voltage of 0.26 V, a subthreshold swing of 0.37 Vdec\({}^{-1}\), an on/off ratio of 10\({}^{8}\), and the drain current was \(\sim\)6 \(\times\) 10\({}^{-5}\) A. In 2014, [PERSON] et al.