Development of a low-cost and high-speed pilot production line for CIGS manufacturing

There are two main strategies in the production of solar cells based on copper-indium-gallium-selenium (CIGS) compounds: evaporation and selenisation. The main drawback of both approaches is that the deposition process is slow, leading to low productivity. The only way forward to increase the productivity has been to gradually augment the size of each module.

However, this approach leads to very expensive production equipment and it is difficult to keep
the cell performance on these large substrates. The handling of large-area glass substrates, especially for temperatures above 400˚C, is highly complex with substrate bending causing yield problems. Maintenance in the large deposition chambers is also difficult, causing a decline in uptime. HIGHSPEEDCIGS explores a different path to high productivity CIGS production.

Micrograph of the CIGS solar cell layers (cross section)
Approach

The overall approach of the pilot production line is to have no manual handling of the substrates from cleaning and until the top contact is deposited onto the finished solar cell. Gee Kaye Ltd has developed an in-cassette cleaning of the 150 mm substrates and the same cassettes can be used when loading the substrates into the first sputtering station in the pilot production line.
The barrier layer and back contact is sputtered in two stand-alone metallisers before the substrate is transported by a robot into the main production tool. In the main production tool, all deposition and post-treatment of the cell from CIGS deposition until the top contact deposition takes place in continuous vacuum. The pilot production line does not use any wet processes.

In the project, different substrate types have been evaluated to find the best material for both performance and productivity. After the consortium decided on stainless steel as the most suitable carrier substrate, different barrier layers to avoid iron diffusion have been evaluated.
Various approaches to CIGS sputtering deposition have also been investigated, using different compound materials and both RF and DC magnetron sputtering. In addition, the consortium has evaluated different cadmium-free sputtered buffer layers to find the most suitable material that can be sputtered minimizing the detrimental effect on the absorber layer.

Results

During the project, the consortium decided on stainless steel as the most suitable material for the solar cell carrier. Based on Midsummer’s market research, solar grade stainless steel carriers are 2-3 times cheaper than equivalent soda lime substrates. From a cost perspective, it is a great advantage if stainless steel substrates – instead of soda lime glass – can be used as carrier. There are also many advantages in handling and automation with stainless steel. Soda lime glass is difficult to handle at high temperatures and is easily deformed. With Midsummer’s all-vacuum process, glass substrates that easily break would influence the uptime negatively. Midsummer has evaluated four barrier layers to avoid iron diffusion. The best performance was obtained with tungsten titanium- and tantalum nitride-barriers. With these barrier layers, similar short circuit current density and fill factor could be obtained as those on
soda lime glass, even though the open circuit voltage was still lower, most probably due to the lack of sodium diffusion from the soda lime glass. The next step will be to add sodium doping to the CIGS layer to improve the open voltage.
In the original EU application, the consortium planned to use a wet cadmium sulphide process for the buffer deposition. However, as the project evolved and the design of the pilot line took shape, the members realized that it would be almost impossible to integrate a chemical bath
in the production line and still maintain high productivity. Therefore, the development of a cadmium-free buffer layer was added. This work has been successful and a buffer layer deposition in vacuum is now included in the pilot line.
Future prospects
The project coordinator, Midsummer AB, plans to start pilot production on a slightly modified production line after the project end. Midsummer is also in the process of securing financing for an order-made all-vacuum production tool with a design that will reach the 10 s cycle-time goal of the EU project. Thanks to the support from the European Union, a novel approach to high speed, low-cost solar production will be transferred from idea to full-scale production.

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