Roll-to-Roll Sputtered ITO/Ag/ITO Multilayers for Highly Transparent and Flexible Electrochromic Applications

Abstract

Electrochromism refers to the phenomenon where optical changes in a material occur during electrochemical reactions under an applied voltage. Because of advantages such as high contrast and low operating voltage, electrochromic (EC) devices have been widely used in smart windows, switchable mirrors, optical shutters, and information displays.Such devices are usually fabricated on a rigid glass substrate to permit their use in automobile and building applications, which require long-term stability under climatic conditions. However, the use of soft polymer or textile substrates has increased the versatility of such technology, allowing the fabrication of novel flexible, stretchable, and wearable EC devices. Since the first flexible EC devices using WO3layers were developed in 1995 a wide variety of inorganic and organic materials has been investigated with respect to their compatibility with flexible substrates. However, most EC devices fabricated using these materials showed poor performances, especially in terms of coloration speed and long-term stability, which was mainly due to the electrical and mechanical properties of the flexible substrate. In addition, the large area and cost-effective roll-to-roll (RTR) coating process for transparent electrodes has been demonstrated for the fabrication of low cost flexible EC devices. To date, the most common transparent electrode for EC devices is indium-tin-oxide (ITO) due to its low resistivity, high transparency, and mature mass production via processes such as sputtering. Amorphous ITO(a -ITO) films can easily be coated on a flexible substrate using conventional DC sputtering at ambient temperatures, but the high sheet resistance and brittleness of thick a-ITO films prevents their use for flexible and transparent EC devices. Oxide-metal-oxide(OMO) multilayer films have been extensively reported in flexible optoelectronic devices as an alternative to a-ITOfilms. However, there are few reports regarding flexible and transparent EC devices fabricated using OMO multilayer electrodes. The development of a conductive layer with high electrical conductivity and stable mechanical flexibility is greatly needed for flexible EC device applications. In this paper, we report the electrochromic properties of poly(3-hexylthiophene) (P3HT) films coated on ITO/Ag/ITO (IAI) multilayers deposited on flexible PET substrates using an ambient temperature RTR sputtering system. By continuous sputtering of ITO/Ag/ITO layers on the flexible PET substrate, we realized highly transparent, flexible, and conductive electrodes for flexible and transparent EC devices. We characterized the coloration efficiency, response time, and long-term stability of the P3HT on the flexible substrate compared to the material prepared on glass substrates.