Optimizing Dip‑Coating Parameters for High‑Performance Three‑State Electrochromic Devices

Abstract

We systematically varied dip‑coating conditions—nanoparticle size, lifting speed, precursor concentration, and dipping number—to modify fluorine‑doped tin oxide (FTO) with TiO₂ nanoparticles. The resulting electrodeposition‑based electrochromic cells, sandwiched with a gel electrolyte, achieve reversible transparent, mirror, and black states. By tuning the coating process, we obtained an optical contrast of 57 %, coloration/bleaching switching times of 6 s/20 s, and maintained only 27 % loss after 1,500 cycles. These findings provide a clear route to tailor multi‑state electrochromic devices for displays and smart‑window applications.

Background

Electrochromic (EC) materials change color under an applied voltage, enabling applications from smart windows to electronic displays. Transition‑metal oxides, conducting polymers, and electrodeposition‑based systems have been extensively explored for their high contrast, fast switching, and long cycling life. Among these, electrodeposition devices—where metals such as Ag or Cu are reversibly deposited on a transparent electrode—offer a simple sandwich architecture and low‑cost fabrication.

While many deposition methods exist, dip‑coating stands out for its scalability and precise control over film thickness and morphology. Prior work on WO₃ and TiO₂ films has shown that dip‑coating can enhance coloration efficiency and switching speed compared with spin‑coating, but its influence on multi‑state Ag/Cu EC devices remains underexplored.

Methods

Materials and Electrode Preparation

Commercial FTO glasses (25 × 30 mm, 2.2 mm thick, 10 Ω sq⁻¹) served as substrates. TiO₂ nanoparticles (average diameters 5–10 nm, 40 nm, 100 nm) were dispersed in terpineol/ethyl alcohol with surfactant and adhesive additives, ball‑milled for 50 min, and diluted to the desired concentration.

Dip‑Coating and Electrodeposition

FTO slides were immersed at 6000 µm s⁻¹ and withdrawn at 1000–3000 µm s⁻¹ (lifting speed). Precursor concentrations (TiO₂ : ethyl alcohol = 1:2, 1:3, 1:4) and dipping numbers (1, 3, 5) were systematically varied while keeping other parameters constant. After coating, the films were sintered at 500 °C for 30 min. Devices were assembled by sandwiching a DMSO‑based gel electrolyte (TBABr, AgNO₃, CuCl₂, PVB) between the TiO₂-modified and flat FTO electrodes.

Characterization

Film morphology was examined by FESEM and AFM; thickness and roughness were quantified. Electrochemical performance (transmittance/reflectance, optical contrast, switching time, cycling stability) was measured with a two‑electrode setup, applying +2.5 V (mirror) or –2.5 V (black) for 20 s, and 0 V for bleaching.

Results and Discussion

Effect of Nanoparticle Size

Devices with 5–10 nm TiO₂ exhibited the highest transparent‑state transmittance (61 %) and optical contrast (48 %). Increasing particle size to 100 nm reduced transparent transmittance to 46 % and contrast to 39 %, mainly due to thicker, rougher films (320 nm, 409 nm, 612 nm). Switching times were fastest for the smallest particles (6 s coloration, 20 s bleaching). Coloration efficiency peaked at 27 cm²/C for 5–10 nm and declined with larger particles.

Influence of Dip‑Coating Parameters

Higher lifting speeds produced thinner (≈300 nm) and smoother (≈40 nm roughness) films, leading to slightly higher transmittance and contrast but slower switching (up to 8 s). Lower precursor concentrations decreased film thickness and roughness, improving contrast (up to 57 %) and reducing coloration time to 6 s. Increasing dipping number thickened the film (up to 600 nm) and increased roughness (≈140 nm), which lowered contrast and prolonged switching.

Cycling Stability

All devices maintained <1 % transmittance loss after the first cycle. After 1,500 cycles, contrast loss was 27 % for 5–10 nm devices, 36 % for 40 nm, and 40 % for 100 nm. Devices coated at higher lifting speeds or with fewer dipping cycles exhibited the best longevity, attributed to smoother surfaces that facilitate Ag dissolution during bleaching.

Conclusions

By precisely tuning dip‑coating conditions, we achieved a reversible three‑state electrochromic device with 57 % optical contrast, 6 s coloration, 20 s bleaching, and robust 1,500‑cycle stability. The study demonstrates that film thickness and roughness—directly controlled by nanoparticle size, lifting speed, precursor concentration, and dipping number—govern the optical performance of electrodeposition‑based EC cells. These insights enable the rational design of high‑efficiency, multi‑state electrochromic devices for displays and smart‑window technologies.