Molybdenum-sodium sputtering targets for CIGS thin-film solar cells

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.

Properties of our MoNa sputtering targets

Fine microstructure

The sodium compound that is added to the MoNa sputtering target lies between the molybdenum grains. As we produce MoNa by means of a powder metallurgical process, the sodium particles are finely and evenly distributed. The electrical conductivity of a MoNa target is somewhat lower than that of pure molybdenum targets but perfectly sufficient for sputtering in DC mode.

Pure material

Whereas small quantities of sodium increase the efficiency of the CIGS cell, other chemical elements such as iron or chromium cause a marked loss of efficiency. To keep the content of metal impurities in particular as low as possible, we take great care to ensure clean process management and the high quality of our source materials. We monitor the manufacturing route of our targets starting with the raw material, which means we can guarantee the exceptional purity of our metal powders.

Our range includes MoNa sputtering targets with a sodium content of 5% and 10%, which corresponds to 1.3 and 2.6 wt.%. Our MoNa sputtering targets are produced as planar targets and as rotary targets for an even higher material yield in the application.

 

MoNa sputtering targets as an object of research

The Swiss EMPA Institute has investigated sodium doping in the CIGS process with MoNa targets from Plansee. The EMPA working group for thin films and photovoltaics, under the leadership of Prof. Dr. Tiwari, investigated how MoNa sputtering targets can be used to optimum effect to manufacture CIGS solar cells. Back contacts made of MoNa enable sodium doping in the CIGS absorber and thereby enhance the performance of the cell. At relatively low separation temperatures, the cells placed on steel foil achieved an efficiency of 14.4%. Our coating experts Dr. Christoph Adelhelm and Enrico Franzke were involved in the research project. Read the detailed results of this research work. The complete article, entitled "Sodium-doped molybdenum back contacts for flexible Cu(In,Ga)Se2 solar cells - ScienceDirect", is available for download from the Elsevier publishing house.

A further publication deals with the optimum sputter conditions for doping the absorber with as much sodium as possible.