Development of thermally stable FTO thin films on quartz substrates for carrier collection in semiconductor photoelectrodes

Abstract

The development of transparent conductive oxide (TCO) materials is crucial for establishing effective photogenerated carrier transportation in solar-to-chemical energy conversion processes. In this study, we utilized the spray pyrolysis technique to develop F-doped tin oxide (FTO) thin films on double-sided polished quartz substrates, to produce TCO with superior thermal durability. This has enabled the production of semiconductor-based photoelectrodes. We successfully fabricated FTO films with thicknesses of 650–750 nm on quartz substrates. Hall effect measurements revealed that the FTO prepared on quartz (FTO/quartz) had resistivity, carrier concentration, and Hall mobility comparable to those of commercial FTO-coated soda lime glass (FTO/glass). The light transmittance in the visible and near-infrared regions satisfied the TCO criteria. The thermal stability was assessed by post-annealing the samples in air at specific temperatures. Up to 700 °C in air, the variations in resistivity and transmittance were minimal. However, post-annealing above 800 °C eventually caused a substantial increase in resistivity. Microscopic and spectroscopic analyses indicated enhanced SnO₂ crystallinity owing to F-dopant elimination and film cracking. This yielded an increased resistivity. Furthermore, we fabricated a p-type CuBi₂O₄ semiconductor conjugated with FTO/quartz and evaluated the photoelectrochemical (PEC) properties. Unlike the use of conventional FTO/glass substrates, CuBi₂O₄/FTO/quartz maintained PEC functionality under simulated AM 1.5G solar illumination with H₂O₂ as a sacrificial reagent. This study has provided the potential for the development of semiconductor materials, including surface modification techniques requiring high-temperature treatments. Furthermore, it would contribute to material design strategies for efficient solar-to-chemical energy conversion systems.