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

Abstract

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 Vo 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.