Accelerating F-doping in transparent conducting F-doped SnO₂ films for electrochromic energy storage devices

In this study, we designed a unique method for increasing F-doping concentration in F-doped SnO₂ (FTO) films, without the extra addition of NH₄F as a doping source, using NaOH acting as a functional additive during ultrasonic spray pyrolysis. The NaOH triggers a chemical reaction with HF, resulting in the presence of dissociated F⁻ acting as a doping source. To optimize the NaOH effect on the transparent conducting performance in the FTO films, we adjusted the volume percentages of the NaOH to 0, 1, 5, and 10 vol% during FTO deposition. Compared with other FTO films, the FTO film prepared with 5 vol% NaOH revealed enhanced carrier concentration (7.81 × 10²⁰ cm⁻³) generated by the increased F-doping concentration (3.57 at%) and high Hall mobility (27.18 cm²/(V S)) through smooth surface morphology. Such behaviors through the NaOH effect resulted in FTO films with decreased sheet resistance (5.3 ± 0.16 Ω/□), leading to improved electrochromic (EC) energy storage performances of fast switching speed (6.6 s for coloration speed and 8.4 s for bleaching speed) due to faster electrochemical kinetics at the active electrodes, high coloration efficiency (58.1 cm²/C) and high specific capacitance (65.2 F/g at 2 A/g) via enhanced electrochemical activity in the active electrodes that widens the transmittance modulation. Therefore, our study suggests a novel method to improve the transparent conducting performances of FTO films for application in EC energy storage devices.