The manufacturing cost reduction in terms of USD per Watt is one of the most important challenges for CIGS solar module producers. Especially the sputtering process - one of the most important techniques for material deposition in CIGS - plays a crucial role in efficiency enhancement.

Manufacturers of sputtering targets like PLANSEE High Performance Materials have two approaches to reducing CIGS module manufacturing costs: In order to lower the material consumption by better target utilization, PLANSEE has introduced the new monolithic and dogbone rotary target designs. With sodium doped molybdenum (MoNa), the company offers a new material alloy that increases conversion efficiencies.

New sputtering target designs for more material utilization

Changing the sputtering system from planar to rotary targets increases the target utilization from about 30% to 75%, which saves expensive raw material. Depositing molybdenum back contact by monolithic molybdenum rotary targets - which are completely made of molybdenum - gives additional benefits: The expensive bonding of the molybdenum material on a stainless steel backing tube is not required. In addition, a higher sputtering power of up to 30 kW/m can be applied. This creates a very high heat load which cannot be accommodated by bonded-type targets: Indium is used as bonding material which melts at 156 °C, thus the risk of de-bonding is eliminated with monolithic targets. A higher sputtering power results in a higher deposition rate and improved thin film properties, e.g. higher electrical conductivity.

In contrast to the high lateral homogeneity of thin films deposited by sputtering, a sputtered target generally has an erosion profile which is not homogeneous – a result of different plasma densities. As a consequence, the targets have to be replaced even if there is sufficient material on most of the target area. To overcome this limitation, targets with varying outer diameter (named “dogbone” for rotary targets) or thickness is a solution to extend the target lifetime and increase the target exchange interval.

Sodium doped sputtering targets for more efficiency

With record conversion efficiencies of >20 % in the laboratory, CIGS has reached the same efficiency level as polycrystalline silicon. Two elements have a big impact on the CIGS cell efficiency: iron (Fe) and sodium (Na). Whereas iron degrades the cell performance by introducing defects in the CIGS structure, sodium has the opposite effect: It concentrates defects at grain boundaries, which is a requirement for achieving high efficiencies.

Low iron levels are only achieved by a high purity of the primary materials as well as a high quality manufacturing process. Especially for molybdenum targets, the  content can vary significantly. The traditional way to introduce sodium into the CIGS absorber is by means of a soda-lime-glass substrate. During the manufacturing process sodium diffuses from the glass through the molybdenum back contact into the absorber layer, resulting in a sodium concentration in the order of 0.1 at%. However, reproducibility and lateral homogeneity of this process is low, and flexible substrates do not act as sodium source.

Solar cell producers now have an easy alternative at hand: By sputtering a layer of sodium doped molybdenum (MoNa), the amount of sodium in the absorber layer can precisely be controlled and reproduced. PLANSEE offers MoNa sputtering targets with high purity and a uniform and fine grained microstructure. Tests in cooperation with the Swiss EMPA institute have already proven their benefits in practice: With Na doped molybdenum layers the efficiency of CIGS solar cells could be significantly improved.

For more information on our thin-film materials for CIGS and CdTe solar cells please visit contact us at coating@plansee.com.