Low-temperature fabrication of TiO2 electron transport layers by solid-state electrochemical oxidation for selenium photovoltaics

In this study, titanium oxide (TiO 2 ) prepared via a solid-state electrochemical oxidation process was characterized and applied in photovoltaic devices employing a selenium light-absorbing layer with a bandgap of 1.9 eV, making it promising for indoor photovoltaic (IPV) applications. This process enables the oxidation of titanium metal thin films at relatively low temperatures, facilitating the formation of electron transport layers that may be suitable for future application to flexible substrates. The fabricated TiO 2 thin films were employed as electron transport layers in selenium-based photovoltaic devices. Ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS), and transmittance spectroscopy revealed a conduction band offset of −0.42 to −0.47 eV with selenium, resulting in the formation of a cliff-type junction. This band alignment is comparable to that reported for sputter-deposited ZnMgO and TiO 2 electron transport layers used in high-performance Se thin-film solar cells. Au/MoO 3 /p-Se/Te/n-TiO 2 /ITO photovoltaic devices achieved a power conversion efficiency of 3.5% after heat and light soaking treatment. These results demonstrate the feasibility of solid-state electrochemically oxidized TiO 2 as an electron transport layer for selenium photovoltaics and establish solid-state electrochemical oxidation as a low-temperature and solvent-free fabrication route for future flexible optoelectronic devices.