Impact of Transparent Conducting Oxide on the Performance of Dye-Sensitised Solar Cells for Indoor Applications

Abstract

Improving the performance of dye-sensitised solar cells (DSC) under artificial light sources and at low light intensities requires preserving the adequate electron transport properties in the sensitised photoanode that are characteristic of DSC operation at 1 sun. Besides, parasitic resistance and optical losses must be reduced. In this respect, the impact of the chemical and electrical properties of the transparent conducting oxide (TCO) has somehow been overlooked. Considering the systematic reduction of the electron diffusion length as the low quasi-Fermi level regime is approached, subtle variations of the properties of the TiO₂ photoanode and its interaction with the TCO substrate can compromise optimal performance under indoor illumination. In this work, the performance of DSCs fabricated with commercial fluorine-doped tin oxide (FTO) and indium tin oxide (ITO) substrates, as well as newly prepared ITO substrates of varying conductivity, has been analysed. Furthermore, a variation of the preparation conditions for the TiO₂-based photoanodes was conducted, with thermal treatments at two different annealing temperatures (450 and 550°C). Photovoltaic characterisation and analysis of the impedance response reveal a low conductivity of the substrate proves to be only adverse under 1 sun illumination, deteriorating the fill factor due to the series resistance voltage drop. In contrast, under low illumination intensity, all studied substrates show comparable performance, which can be attributed to the negligible voltage drop over the series resistance related to the significantly lower photocurrent. As a consequence, the conductivity of the TCO substrate is less critical when selecting a substrate for indoor applications. However, the choice of TCO affects the quality of TiO₂ photoanodes, leading to shorter diffusion lengths in some cases. Interestingly, the annealing temperature plays a critical role in homogenising the differences observed while also enhancing the diffusion length, ensuring efficient electron collection under low light conditions.