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Midsummer flexes its muscles with CIGS on stainless steel

A novel sputtering manufacturing process coupled with the use of stainless steel substrates enables high efficiency flexible modules without any toxic cadmium in the buffer layer
Swedish firm Midsummer has developed a high speed process for manufacturing CIGS solar cells.

The new technology utilises sputtering of all layers in the solar cell structure. The completely dry, all-vacuum process means less stringent requirements for use in cleanrooms etc.

Midsummer recently achieved a 15 percent active area efficiency on an entire 225 cm2 solar cell using this sputtering technology.

Midsummer flexible CIGS solar cell

One key achievement is the fact that the buffer layer was sputtered - normally it is deposited with chemicals or by atomic layer deposition. Also, the CIGS layers were sputtered from compound CIGS targets resulting in a very short cycle time (eliminating the need for selenising the solar cells for several hours).

By using sputtering in all processing steps, the process cycles in the manufacturing of solar cells can be drastically shortened and the solar cells can be made with a cadmium-free buffer layer.

Cadmium and its compounds are highly toxic and exposure to this metal is known to cause cancer and other illnesses. Avoiding cadmium in the manufacturing process is desirable for the sake of the production staff and it makes it generally easier to commence manufacturing of thin film CIGS solar cells.

In the manufacturing process, the CIGS absorber is deposited on a stainless steel substrate, along with electrodes on the front to collect current. The cells are then connected in series and covered by a protective layer of plastic to form a flexible solar module.

The cells are made on 156 x 156 mm substrates stamped out from 0.3 mm thick ordinary ferritic stainless steel. This means the modules can be made without glass. The CIGS solar modules are therefore much lighter, flexible and can be made frameless, to suit applications where traditional silicon solar cells cannot be used, such as on structures that are uneven, moving or weak.

Applications include floating modules, vehicles, landfills, portable power generation and membrane roofs on factories, offices and other structures that are not strong enough for traditional glass modules.

“Most photovoltaic experts consider thin film flexible solar modules to be the future of solar energy, and I agree,” says Sven Lindström, CEO, Midsummer. “Our unique process makes thin film CIGS solar cells even more commercially attractive by making it possible to manufacture solar cells fast, efficiently and cost effectively even in small volumes.”

Midsummer has its roots in the optical disc manufacturing equipment and the photo mask industries.

The company’s DUO turn-key system is a scalable and compact manufacturing system for solar cells with a 5 MW annual production capacity. The heart of Midsummer’s photovoltaic production system, the Duo line is a sputter tool that deposits all the layers forming the finished cell. The Midsummer DUO Line is the most cost effective way to start CIGS (copper, indium, gallium and selenide) solar cell manufacturing.

Midsummer’s turnkey manufacturing equipment has a small footprint and are scalable allowing for the small-scale production of solar cells and modules.

Flexible CIGS solar modules are gaining market share thanks to its high efficiency, low weight, flexibility and durability. There have been significant improvements in CIGS cell efficiency, with Midsummer achieving aperture area efficiency in excess of 15.8 percent in an all-vacuum process.

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