Abstract
We investigated the photochromic (PC) and electrochromic (EC) properties of tin-doped nickel oxide (NiO) thin films for solution-processable all-solid-state EC devices. The PC effect is shown to be enhanced by the addition of Sn into the precursor NiO solution. We fabricated an EC device with six layers—ITO/TiO2 (counter electrode)/SnO2 (ion-conducting layer)/SiO2 (barrier)/NiO doped with tin (EC layer)/ITO—by a hybrid fabrication process (sputtering for ITO and TiO2, sol–gel spin coating for SnO2 and NiO). The EC effect was also observed to be improved with the Sn-doped NiO layer. It was demonstrated that UV/O3 treatment is one of the critical processes that determine the EC performance of the hydroxide ion-based device. UV/O3 treatment generates hydroxide ions, induces phase separation from a single mixture of SnO2 and silicone oil, and improves the surface morphology of the films, thereby boosting the performance of EC devices. EC performance can be enhanced further by optimizing the thickness of TiO2 and SiO2 layers. Specifically, the SiO2 barrier blocks the transport of charges, bringing in an increase in anodic coloration. We achieved the transmittance modulation of 38.3% and the coloration efficiency of 39.7 cm2/C. We also evaluated the heat resistance of the all-solid-state EC device and found that the transmittance modulation was decreased by 36% from its initial value at 100 °C. Furthermore, we demonstrated that a large-area EC device can be fabricated using slot-die coating without much compromise on EC performance.
Highlights
Chromic phenomena such as thermochromism, photochromism, electrochromism, and solvatochromism occur in a material by various external stimuli [1]
We investigated a hydroxide-ion-based all-solid-state EC device without any aqueous electrolyte included, which consisted of ITO/TiO2/SnO2/SiO2/nickel oxide (NiO) doped with tin (EC layer)/ITO
The hydroxide ions react with the NiO film, causing NiOOH that is colored into brown (Equation (2)) [30]
Summary
Chromic phenomena such as thermochromism, photochromism, electrochromism, and solvatochromism occur in a material by various external stimuli [1]. Photochromism (PC) can change its optical properties reversibly through interactions with sunlight or UV radiation, and electrochromism (EC) can be obtained through redox reaction under electric field [2,3]. The former phenomenon is commercially applicable in ophthalmics, sensors, optical memory, and switches, and the latter phenomenon is still extensively studied for the applications of car mirrors, smart windows, transparent displays, and solar protection. For the ion-conducting layer (solid-state electrolytes), ZrP, Ta2O5, TiO2, ZrO2, LiNbO3, LiAlF4, and LiBSO were usually employed [8,9]. EC devices with NiO and WO3 have great benefits in terms of energy savings because of low driving voltage (0–5 VDC), low power consumption, and memory effect by bistability even after the power supply is disconnected [14,15,16,17]
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