Sputtering targets affect cost and efficiency of CIGS
Changing the sputtering system from planar to rotary targets increases the target utilisation from about 30 to 75%
The sputtering process is one of the most important techniques for material deposition in CIGS manufacturing.
With innovative sputtering targets, CIGS producers can significantly reduce their production costs.
According to Plansee, a manufacturer of sputtering targets, most of the players in the market have two approaches to reduce CIGS module manufacturing costs. The first is to improve the target utilisation and the second is to use an alloy with increased conversion efficiency.
To address these problems, Plansee have introduced new monolithic and dogbone rotary target designs. With MoNa the company offers a new material alloy that increases conversion efficiencies.
Monolithic rotary target
Plansee says that changing the sputtering system from planar to rotary targets increases the target utilisation from about 30 to 75%, which saves expensive raw material. Depositing the molybdenum back contact by monolithic molybdenum rotary targets - which are completely made of molybdenum - gives additional benefits: The expensive bonding of molybdenum on a stainless steel backing tube is not required. What's more, 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 only 156 °C . This eliminates the risk of de-bonding 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 normally 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.
Two elements have a big impact on the CIGS cell efficiency: iron (Fe) and sodium (Na). 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 needed 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 iron 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, 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 sodium doped molybdenum layers the efficiency of CIGS solar cells could be significantly improved.