Cathodic Deposition of TiO2 Electron Transport Layers on FTO and ITO Substrates
Shuya Fujita a, Hikaru Kobayashi b, Mikito Suto b, Ryuki Tsuji c, Seigo Ito c, Sho Kitano a, Hiroki Habazaki a
a Hokkaido University, Japan, 001-0021, Japón, Sapporo, Japan
b JFE Steel Corporation
c Department of Materials and Synchrotron Radiation Engineering, Graduate School of Engineering, University of Hyogo, 2167 Shosha, Himeji, Hyogo 671-2280, Japan
Asia-Pacific International Conference on Perovskite, Organic Photovoltaics and Optoelectronics
Proceedings of Asia-Pacific International Conference on Perovskite, Organic Photovoltaics and Optoelectronics (IPEROP23)
Kobe, Japan, 2023 January 22nd - 24th
Organizers: Seigo Ito, Hideo Ohkita and Atsushi Wakamiya
Oral, Hiroki Habazaki, presentation 055
Publication date: 21st November 2022

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 TiF62- 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.

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