Interfacial carbon quantum dot thin film impacts on morphological and chemical structures of fluorine-doped tin oxide films

Morphology and chemical bonding states, including surface roughness, film density, F-doping, and oxygen vacancy (V_O) concentration directly affect the electrical and optical properties of fluorine-doped tin oxide (FTO) films. In this study, morphology and chemical bonding states of FTO films are engineered simultaneously by introducing a carbon quantum dot (CQD) thin film as an interface layer during ultrasonic spray pyrolysis deposition. The abundant oxygen-containing surface functional groups and large surface free energy of the CQD thin film promoted (200) oriented crystal growth and accelerated nucleation of FTO, resulting in smooth and dense FTO film morphology. The abundant carboxyl surface functional groups on the CQDs generate active F⁻ species that are effectively doped into SnO₂. Formic acid, formed by dissociation of the carboxyl group, can serve as a strong reducing agent for extracting oxygen atoms from the SnO₂ lattice, thereby increasing the V_O concentration. The accelerated F-doping and V_O concentration result in the generation of additional free electrons. The effects of the optimized CQD on the morphological and chemical structures of FTO films are demonstrated by their superior electrical (sheet resistance of ∼5.7 Ω/□) and optical properties (visible transmittance of ∼85.9 %) compared with bare FTO. Moreover, CQD/FTO used as transparent conducting electrodes in electrochromic (EC) application exhibited improved EC performances, including fast switching speeds (5.3 s and 2.3 s for bleaching and coloration, respectively) and high coloration efficiency (81.1 cm²/C) compared to that of bare FTO.