# NiO-Based Electronic Flexible Devices
## 3 NiO-Based Flexible Devices
### Electrochromic Devices  
Electroch micromism is the reversible color change of an electrochromic device (ECD) under an applied voltage. NiO is among the transition metals reportedly displaying the best electrochromic properties as anodic material, though associated drawbacks may be a low optical modulation, short-term stability, and slow switching time. These pitfalls can be overcome by modulating the NiO properties through synthetic strategies. A facile solid-state strategy for the preparation of NiO particles [105] foresaw a solid-state calcination of nickel acetate dihydrate at 600-900 \({}^{\circ}\)C, followed by NiO ink preparation by mixture with solvents (chlorobenzene, a 2:1 isopropanol-water mixture, or chloroform) sonication and sedimentation. According to this procedure, the ink was subsequently spin-coated on cleaned ITO glass and flexible Ag/PET substrates. The latter, in particular, was achieved by subsequently coating AgNanoWire (AgNW), poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT: PSS, 5 wt% in ethanol) and NiO ink onto the PET substrate. Analogous films were synthesized using WO\({}_{3}\) particles. Devices were built using a NiO film as working electrode and PEDOT: PSS/AgNW/PET or WO\({}_{3}\)--coated PEDOT: PSS/AgNW/PET as the counter electrodes. The NiO and NiO/WO\({}_{3}\) ECDs on ITO substrate exhibited a transmittance variation of \(\Delta\)T = -84% at 700 nm between \(-\)3.0 and 0 V. The flexible NiO/WO\({}_{3}\) device achieved \(\Delta\)T = -38% at 700 nm between 2.0 V-0 V for, indicating a potentially use in smart windows and optical devices.  
The properties of NiO can be modified by doping, for instance with tungsten to achieve Ni\({}_{1-x}\)W\({}_{x}\)O, which is suitable as anode in electrochromic devices. In this framework, the  
Figure 4: SEM image of a flexible NiO ReRAM device on flexible PET substrate. (Reproduced from [104], Copyright 2021, Elsevier).  
employment of doped oxides required a preliminary optimization procedure to determine the most suited Ni\({}_{1-x}\)W\({}_{x}\)O composition. To this purpose, the electrochromic properties of Ni\({}_{1-x}\)W\({}_{x}\)O were evaluated by varying the W atomic percentage in a range between 0 and 20% and performing measurements in lithium perchlorate and propylene carbonate. Enhanced cycle stability with moderate optical modulation was obtained for a Ni\({}_{1-x}\)W\({}_{x}\) oxide film with at x = 0.024 [106]. Flexible electrochromic devices employing Ni\({}_{1-x}\)W\({}_{x}\)O were fabricated on a PET substrate with monolayered graphene as a platform, thus creating stacked layers of PET, ITO, graphene, WO\({}_{3}\) and Ni\({}_{1-x}\)W\({}_{x}\)O thin films. In particular, the flexible electrochromic device was achieved by growing a single layered graphene on Cu foil (99.8%) by rapid thermal chemical vapor deposition (RTCVD). A procedure of DC magnetron sputtering at RT and subsequent post-annealing process at 250 \({}^{\circ}\)C was used to fabricate highly crystalline ITO (c-ITO) on graphene/Cu foil with a thickness of 60 nm, which was afterwards continuously transferred onto a PET substrate. The c-ITO/graphene/PET electrode showed a sheet resistance of \(\sim\)45 \(\Omega\)/sq and a transmittance of \(\sim\)92% at a wavelength of 550 nm. The flexible EC devices were fabricated by laminating the WO\({}_{3}\) film and optimizing the Ni\({}_{0.976}\)W\({}_{0.024}\)O film deposited on the c-ITO/graphene/PET substrate, using Li-based polymeric solid-state electrolyte. The flexible EC device showed stable cycling performances, maintaining an optical modulation of \(\Delta\)T \(\approx\) 40%. In addition, the flexible EC device showed stable optical modulation, with a \(\sim\)2% reduction after 1000 cycles.