news digest ♦ Solar
Monolithic targets with inner diameter coating molybdenum sputtering targets are used to deposit the back contacts in CIGS cells by magnetron sputtering.
Roth & Rau slashes the cost of solar cell production
Achieving 50 to 70 percent savings in silver, the process is developed for front and rear side coating with nickel. A tool is now available for the production of high efficiency heterojunction cells Meyer Burger Technology has announced that its Group member Roth & Rau AG has developed a process which uses inexpensive nickel in busbar metallisation for the electrical contacting of solar cells.
Image showing the upgraded rotary target
Monolithic rotary targets have no backing tube. They consist entirely of the thin film material.
Plansee says that thanks to these targets, CIGS manufacturers can considerably increase sputtering performance and consequently achieve greater throughput. The particularly high level of material utilisation also reduces the total cost of ownership in solar cell production.
When monolithic targets are used, the molybdenum is in direct contact with the cooling water inside the sputtering equipment. Additional particular additives (so-called inhibitors) have to be used to condition the cooling water for operation with monolithic targets. These stabilise the pH-value of the cooling water.
Plansee has now developed a protective coating for the inner diameter (ID) wall of the target, designed to make it even easier to use monolithic targets. This polymer-based layer ensures that the molybdenum no longer is exposed directly to the cooling water. The advantage is that CIGS manufacturers do not need to use the additional inhibitors and are therefore able to reduce their costs. Plansee points out that the ID coating does not noticeably impair the thermal conductivity of the rotary target.
The coating process includes both the front and rear sides of the cell within a production tool. Meyer Burger says this process, which is immediately available, significantly differentiates Roth & Rau from its competitors.
Depending on the contact technology, a saving of between 50 and 70 percent in expensive silver can be achieved and therefore significantly reduce the production cost of solar cells. A further benefit from the process is the fact that nickel is a readily available material. This means that solar cell manufacturers are not tied to specific suppliers as is the case with other materials.
The HELiA system, which was developed primarily for the production of high efficiency heterojunction cells, coats the solar cells with nickel in a shortened system configuration to form the front and rear busbars. This is achieved by means of a sputtering process. In contrast to other systems, the HELiA system permits simultaneous processing not only of the rear surface but also of the front surface on which there is a significantly greater potential for savings.
A further decisive benefit of this new process is the outstanding adhesion of the cell connectors to the front and rear surfaces of the solar cell as a result of the nickel metallisation in standard soldering processes.
The metallisation of the fingers can thus take place regardless of the electrical characteristics of the busbar and be optimised to match them. In this way, the metallisation of the fingers is de-coupled from the solderability of the busbar, thereby enabling the use of new pastes and metallisation processes that do not currently achieve reliable solderability. The
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www.compoundsemiconductor.net October 2012
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