Fe co-doping effect on fluorine-doped tin oxide transparent conducting films accelerating electrochromic switching performance

In this study, Fe co-doping into fluorine-doped tin oxide (FTO) films using horizontal ultrasonic spray pyrolysis deposition (HUSPD) is reported as a novel approach to improve their transparent conduction for high-performance electrochromic (EC) devices. To optimize their transparent conducting performance, we adjusted the Fe to F ratio to an atomic percentage of 0, 1, 2, and 3 at% during the film deposition. With the optimized Fe co-doping effect induced at Fe 2 at%, the resulting Fe-doped FTO films exhibited simultaneous improvements in the carrier concentration and Hall mobility which are, attributed to V_o induced by a Fe substitution, and the smooth surface morphology interfering with the growth of the FTO crystallites, respectively. As a result, the Fe-doped FTO films prepared at Fe 2 at% achieve a higher transparent conduction (4.4 ± 0.14 Ω/□ sheet resistance and 83.1% optical transmittance) than the other films with a figure of merit of 3.56 × 10⁻² Ω⁻¹. Hence, when these unique films are used in EC devices as a transparent conductive oxide, fast switching speeds (6.1 s coloration speed and 5.1 s bleaching speed) and a high coloration efficiency (CE) of 48.2 cm²/C occur as unique effects of the Fe-doped FTO films, accelerating the insertion of Li⁺ and electrons into the WO₃ films, which act as an active EC material and enhancing the electrochemical activity. Therefore, our study provides promising insight into unique transparent conductive oxide films for high-performance EC devices.