In many CIGS modules, the lime-soda glass substrate is also the source of sodium. Temperatures of around 500°C occur during the manufacturing process for the absorber. At this temperature, sodium diffuses out of the glass via the molybdenum back contact into the absorber. The sodium distribution in the glass, however, is not homogeneous. The age of the glass and the storage conditions prior to coating also affect the quality of the sodium diffusion.
There is one key advantage of using molybdenum-sodium layers as the source of sodium: a diffusion barrier is first applied to any rigid or flexible substrate. In the case of a glass substrate, this may be a silicon nitride layer, for example. If steel foil is used, a chromium- or tungsten-based barrier prevents the diffusion of iron. A thin MoNa layer is then sputtered, followed by - as is normally the case - a layer of pure molybdenum as a back electrode layer.