Improving solar module manufacturing with imaging

Jörg Schwartz, joerg.schwartz@photonics.com

In-line metrology systems developed by industrial vision specialist Basler Vision Technologies have been integrated into the production line of solar module manufacturer Sunfilm AG at its Großröhrsdorf plant near Dresden. The company is the first manufacturer to introduce large-scale production of silicon-based tandem junction thin-film modules, at a plant that went into production in April 2009 and uses Applied Materials equipment.

Sunfilm’s production facility manufactures tandem junction thin-film solar modules in Großröhrsdorf, Germany. The plasma-enhanced chemical vapor deposition system is important for depositing critical light-absorbing silicon layers on 5.7-m² glass substrates, with the quality of the coating being checked by optical metrology systems. Courtesy of Business Wire.

Thin-film solar modules use about 2 percent of the amount of silicon per watt of electricity produced compared with traditional solar cells fabricated using crystalline silicon wafers. Even more importantly, tandem junction cells’ photovoltaic devices offer higher efficiency by better utilizing the solar spectrum. They use more than one semiconductor material, each with a different bandgap – i.e., absorption peak – which means that larger parts of the solar spectrum are “harvested” or absorbed and converted into electrical energy. Sunfilm makes cells with two materials, a combination of amorphous and microcrystalline silicon. This combination offers more than 8 percent efficiency with a prospect of 10 to 12 percent in the longer term; solar cells using a single layer of amorphous silicon convert only about 6 percent of sunlight.

Making these cells requires several process steps because the production involves deposition of several thin layers on a transparent conductive oxide (TCO)-coated float glass substrate. Key elements are the absorber layers – i.e., amorphous and microcrystalline silicon – but equally important is the back contact, consisting of another conductive oxide and several metal layers, as well as protective and encapsulation/lamination layers. Accurate process handling is therefore absolutely essential, particularly for large sizes involved. Up to 5.7-m² substrates are processed in the factory, so metrology systems are important to support the “latest state-of-the-art process control,” as Dr. Wilhelm Stein, Sunfilm’s chief engineer noted.

Basler’s solution is based on the company’s experience in LCD inspection, built over the past 10 years and now adapted and optimized for thin-film applications. It is used to perform three different jobs in the solar cell production line. First, the glass is inspected (TCO-coated or -uncoated), and the cleaning process is checked. At this stage, edge defects are of particular importance because they can lead to glass breakage and significant production downtime.

Pictured is Basler’s imaging system, first developed for liquid crystal display manufacturing and now integrated into a solar cell production line that can process 5.7-m² glass substrates. The system helps improve efficiency and reduces the cost of solar module manufacturing.

Once the glass passes inspection, a second system checks the semiconductor coatings applied via plasma-enhanced chemical vapor deposition (PECVD). Here, pinholes are the biggest issue, as they will cause short circuits that reduce the efficiency of the photoactive layers. The detection system not only counts the number but also provides the distribution of the pinholes.

Finally, after the lamination process, the end product is checked for bubbles, scratches and delamination on the peripheral and photoactive areas. Lamination bubbles on the edge of solar modules can lead to penetration of moisture, degrading the lifetime of the product.

The metrology system has been fully integrated with the factory automation system via the SECS/GEM protocol. The semiconductor industry uses this interface for equipment-to-host communications. In an automated fab, it can start and stop equipment processing, collect measurements, change variables or select recipes for products – and now it has the vision to support greener energy more efficiently.