How sputtering targets influence the manufacturing costs and efficiency of CIGS PLANSEE
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 target designs. With sodium doped molybdenum (MoNa), the company offers a new material alloy that increases conversion efficiencies.
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.
Sodium in the absorber layer concentrates defects at grain boundaries, which is a requirement for achieving high efficiencies. 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.
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27 January 2012
