Effect of a TiN alkali diffusion barrier layer on the physical properties of Mo back electrodes for CIGS solar cell applications

Cu(In,Ga)Se2 thin-film solar cells have attracted strong interest in the photovoltaic community due to their high efficiency and demonstrated industrial relevance. As the most commonly used back electrode for CIGS solar cells, molybdenum (Mo) is typically deposited on soda-lime glass substrates by magnetron sputtering. During the high-temperature CIGS absorber formation process step, alkali (Na) atoms diffuse from the soda-lime glass substrate into the CIGS absorber via the Mo contact, a process that is known to improve the cell efficiency. However, Na diffusion from soda-lime glass sheets is an uncontrolled process, which adds to batch-to-batch variations due to fluctuations in the glass quality and process parameters such as the CIGS formation temperature. Hence, a diffusion barrier layer between the glass substrate and the Mo back electrode is required to prevent this uncontrolled impurity diffusion. In this study, a TiN diffusion barrier layer is deposited by reactive magnetron sputtering of a metallic Ti target, using various N2 flow conditions. It is observed that the adhesion, microstructure, and morphology of the Mo films get significantly improved by the introduction of a TiN barrier layer, which in turn leads to better cell efficiencies. Hence, the TiN/Mo bilayer design developed in this work seems to be a good choice for enhancing the efficiency of CIGS solar cells.