Preparation of tin-doped indium oxide films and application in all-solid-state electrochromic devices based on phosphate proton conductors

Tin-doped indium oxide (ITO) films are often used as transparent electrodes for optoelectronic applications such as electrochromic devices (ECDs) and solar cells. Preparing ITO films below intermediate temperatures (≤ 350 °C) has become a challenge that must be solved to expand their range of applications (such as all-solid-state ECDs based on phosphate proton conductors and flexible optoelectronic devices). This work aims to develop a practical technique for preparing ITO films at intermediate temperatures. The performances of ITO films prepared by sol-gel and magnetron sputtering methods at these temperatures were compared. The scattering effect of the organic residues in the film, such as the C–H and C=C groups, hinders the substitution of Sn⁴⁺ with In³⁺, the grain growth and the carrier migration, leading to poor carrier concentration and mobility. These factors hindered the preparation of highly transparent and conductive ITO films obtained by sol-gel method. By combining room temperature magnetron sputtering with heat treatment under an O₂ followed by a reducing atmosphere below 350 °C, a carbon-free ITO film with an average visible light transmittance of 87.0% (with substrate) and a resistivity of 2.3×10^-4 Ω·cm was obtained. Further experiments investigated the feasibility of applying the obtained film in all-solid-state ECDs with ITO/tungsten oxide/phosphate proton conductor/nickel oxide/ITO structure. This ECD exhibits good electrochromic capabilities, particularly demonstrated by a bleached state transmittance of up tp 76% at 650 nm, which endows it with outstanding transparency characteristics. This work not only clarifies the limitations and causes of the intermediate temperatures preparation of ITO films by sol-gel method, offering a theoretical reference for the selection of ITO film preparation technologies, but also lays a foundation for the development of novel substrate-free all-solid-state ECDs.