Perovskite solar cells and organic thin film solar cells have attracted recent attention as new-generation solar cells. These solar cells consist of three main layers: electron transport layer, hole transport layer, and absorber layer. The electron and hole transport layers play an important role in effective charge separation and enhancing solar energy conversion. The most typical material of the electron transport layer is TiO₂, which is an n-type semiconductor. The TiO₂ layer has often been formed by spray pyrolysis at high temperatures of ~500°C. The necessity of high-temperature deposition is one of the limitations of applying the TiO₂ electron transport layer to flexible solar cells. In this study, an attempt was made to prepare the TiO₂ layer on FTO and ITO transparent conductive oxide substrates by cathode deposition from aqueous electrolytes. The cathodic deposition method has the advantage of the cost-effective preparation of the TiO₂ layer on large conductive substrates at relatively low temperatures. After deposition, hot water treatment was conducted to obtain a crystalline TiO₂ layer at relatively low temperatures.

The cathodic deposition was conducted at a constant current density in aqueous electrolytes containing K₂TiF₆ and K₂SO₄. Hydrogen evolution reaction occurs during the cathodic polarization, and the pH of the electrolyte in the vicinity of the cathode increases. Then, the hydrolysis of TiF₆^2- ions is promoted, inducing the deposition of TiO₂. Thin uniform TiO₂ layers of 20-100 nm thickness were successfully prepared on FTO substrates. The deposited TiO₂ layer was amorphous, but crystallization occurred after hot water treatment at 80°C. When the ITO substrate was used, the colored deposited layer was obtained because of the dispersion of metallic Sn nanoparticles as a consequence of the reduction of ITO. We found that the anodic polarization could effectively remove the metallic nanoparticles, and the deposited layer became colorless.

The deposition involves the hydrogen evolution reaction, often introducing pinhole defects or partial detachment of the TiO₂ layer on the FTO substrate. We introduced a nitrate reduction reaction instead of the hydrogen evolution reaction to increasing the pH in the vicinity of the cathode. Because of the suppression of gas evolution during the deposition, the TiO₂ layer more adherent to the substrate was successfully obtained.