CIGS: Sodium for enhanced efficiency.
In the laboratory, CIGS solar cells achieve a level of efficiency of over 20% which is comparable to that of solar cells made from polycrystalline silicon. One element particularly influences the efficiency of CIGS cells: Sodium. Even small amounts of sodium in the CIGS absorber increase the efficiency of the solar cell.
But how is the sodium introduced into the absorber layer?
The conventional way.
In many CIGS modules, the soda lime glass substrate also acts as the sodium source. The temperatures that occur during the manufacture of the absorber reach approximately 500 °C. At this temperature, sodium diffuses out of the glass, through the molybdenum back contact and into the absorber. However, in this case, the sodium is not uniformly distributed. The age of the glass and the conditions of storage also influence the quality of sodium diffusion.
How to use our MoNa targets.
It's easy: You start with the substrate of your choice and apply the diffusion barrier. In the case of a glass substrate, for example, this may be a silicon nitride layer. If you use a steel film, a chromium-based barrier prevents the diffusion of iron. Now simply sputter a thin MoNa layer followed by - as usual - a layer of pure molybdenum.
You can control the sodium content in the absorber very simply by modifying the thickness of the MoNa layer and the sputtering conditions. And best of all: To deposit the MoNa layer, simply use the PVD system you already have for the sputtering process of the pure molybdenum layer.
The reason why our MoNa targets are so good.
A fine microstructure: Our sodium compound resides between the molybdenum grains. Because we use powder metallurgical methods to produce our MoNa, the sodium particles are particularly fine and evenly distributed. The electrical conductivity of a MoNa target is slightly lower than that of a pure molybdenum target, but still perfectly sufficient to perform sputtering in DC mode.
Pure material: Whereas sodium increases the efficiency of the CIGS cell, iron results in a loss of efficiency. To keep the iron content as low as possible, we take the greatest care to ensure that our processes are free from contaminants and that only top-quality input materials are used. We monitor the production path of our targets right from the raw materials stage and can guarantee the exceptional purity of our metal powders.
|MoNa 5 / 10 in at. %||Molybdenum|
|Elect. conductivity||10.05 MS/m / 4.9 MS/m||17.8 MS/m|
|Purity||99.97 %||99.97 %|
Our range includes MoNa sputtering targets with a sodium content of 5 % and 10 %. This corresponds to 1.3 and 2.6 % by weight. You can purchase our MoNa sputtering targets as planar targets or as rotary targets for even higher material utilization.
Just contact us! We're looking forward to your call.
MoNa assessed in research and practice.
EMPA: The Swiss Federal Laboratories for Materials Science and Technology (EMPA) have examined sodium doping in the CIGS process using our MoNa targets. Under the leadership of Professor Tiwari, the EMPA working group for Thin Films and Photovoltaics examined how to optimize the use of MoNa sputtering targets for the production of CIGS solar cells on stainless steel substrates. The sodium doping in the CIGS absorber from a MoNa layer increases the performance of the cell. At relatively low CIGS deposition temperatures of only 450 °C, the cells achieved an efficiency level of 14.4%. Our coating experts Dr. Christoph Adelhelm and Enrico Franzke took part in the research project. Find out the detailed results of this research work: The full article can be downloaded from the Elsevier site here.
Another research project examines the ideal sputtering parameters to dope the absorber with as much sodium as possible.
ZSW: Other research institutes have also investigated the effects of MoNa targets. The Baden-Württemberg Center for Solar Energy and Hydrogen Research (ZSW) has focused on the use of MoNa on flexible substrates. Read what it has to say. This article can also be downloaded from the Elsevier site here.