Preparation and performance of fast-response ITO/Li-NiO/Li-WO₃/ITO all-solid-state electrochromic devices by evaporation method

Highlights

• All-solid-state ECD was prepared by e-beam and resistive heating evaporation.

• The ECD has four layers with no electrolyte layer.

• The ECD has a fast response with coloring and bleaching time of 8.6 s and 1.7 s.

• The ECD has good stability with no attenuation after 1000 cycles.

• High coloration efficiency, which reaches 87.5 cm² C⁻¹, is obtained.

Abstract

In this study, all-solid-state electrochromic device (ECD) with a novel four-layered structure of glass/ITO/Li-NiO/Li-WO₃/ITO was prepared by e-beam and resistive heating evaporation. Tungsten oxide (WO₃), nickel oxide (NiO) and indium tin oxide (ITO) were used as electrochromic layer, complementary layer and electrode layer, respectively. Lithium was used as electrolyte ion and evaporated on the complementary layer directly, replacing the normally used independent electrolyte layer. Since the lithium ions can transfer between WO₃ and NiO layer directly during the electrochromic process, the ECD shows a fast response speed with coloring and bleaching time of 8.6 s and 1.7 s, respectively, due to the reduction of the overall resistance of the device. The unique structure of ECD may further advance the development in the practical applications.

Introduction

Electrochromism is a phenomenon that materials can change their optical properties (transmittance, absorbance and reflectance) reversibly under an applied potential. S.K. Deb [1] first demonstrated electrochromic devices (ECDs) based on tungsten oxide (WO₃) in 1969. In 1984, the concept of smart windows was proposed by C.M. Lampert [2] and C.G. Granqvist [3], which can adjust the incident light to save building energy consumption. Apart from that, ECDs have attracted attention of researchers due to their great potential in anti-glare rearview mirror, spacecraft thermal control system, display and other multifunctional devices [3], [4], [5], [6].

All-solid-state devices are attracting much interest for practical application in recent years due to their stable performance, easy processing, continuous production and low recession. In a typical solid-state ECD, the electrolyte layer is essential to provide active ions such as H⁺ or Li⁺ as conducting ions. Liu et al have prepared a seven-layer ECD with the structure of ITO/NiO_x/Ta₂O₅/LiNbO₃/Ta₂O₅/WO₃/ITO, which can achieve an optical modulation approximately 52.5%, and its response time of coloring and bleaching is 21 s [7]. Song et al fabricate an all-thin-film ECD using LiTaO₃ as the electrolyte with an excellent transmittance modulation of 67%, but the coloring and bleaching response time is as long as 85 s and 42 s, respectively [8]. Normally, the ionic conductivity of the electrolyte layer is lower than 10^-6 S cm⁻¹, which may slow down the response speed, and thus limits their practical application. In this paper, an ECD with a novel four-layered structure of glass/ITO/Li-NiO/Li-WO₃/ITO is designed and prepared by e-beam and resistance heating evaporation method, using lithium directly as the active ions, replacing the electrolyte layer to reduce the overall resistance of the whole device. The ECD shows very high coloration/bleaching response speed of 8.6 s and 1.7 s, respectively and optical modulation range of 32% at wavelength of 517 nm. The results provide a new strategy for further development of all solid state ECDs.