ANALYSIS
T
he photovoltaic industry seems at the moment to be in one of those knock- down, drag-out contests typical of the
start-up phase of a disruptive technology. Not just the business, but also the variety of technologies themselves – monocrystalline silicon versus polycrystal, bulk materials versus thin film, thin-film silicon versus CdTe (cadmium telluride) versus CIGS (copper-indium-gallium selenide).
At present, crystalline silicon panels account for the vast majority of the market; however, they remain expensive and are unlikely to ever be able to compete with electricity from the grid without subsidies. Thin-film panels have the potential for lower cost production, since the active layers are deposited directly onto molybdenum coated glass.
Vacuum deposition, including co-sputtering and co-evaporation, and non-vacuum based methods are possible, including electroplating. Sputtered films can be either deposited in layers and then alloyed through thermal annealing cycle or co- sputtered using a mixed target.
Chipworks has recently completed a detailed structural analysis of two commercially available CIGS panels, namely the Wurth WSK0020 and the Avancis PowerMax 100 FB. Broadly speaking the panels have a similar structure. They are both made as a sandwich of layers between two glass sheets. The bottom sheet provides a substrate and the top provides protection from the external environment. The optically-active CIGS stack is deposited on the bottom glass substrate. The CIGS stack is comprised of a bottom molybdenum layer, a copper-indium-gallium-selenide (CIGS) layer, and a top aluminum-doped ZnO transparent conductive oxide (TCO) layer.
The bottom molybdenum layer is the cell anode and helps reflect any unabsorbed light back into the active layers. The top aluminum-doped ZnO layer, as well as being the electrically conductive cell cathode, is also transparent to allow the sunlight to penetrate into the CIGS layers. Scribe lines are cut into the layers of the CIGS stack to form the individual cells. A layer of transparent polymer material is applied over the CIGS stack followed by the top glass.
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Figure 1 CIGS Solar Panel Structure
Figure 1 is a schematic diagram of two individual cells in a typical CIGS solar panel. It turns out that, in terms of light to electricity conversion efficiency, the optimum width of CIGS cells is around 5 to 10 mm so panels are typically formed using banks of such cells in series. Scribe lines, denoted P1, P2, and P3, are used to create the interconnect
Figure 2 CIGS Solar Panel Cleaved Edge
www.solar-pv-management.com Issue III 2010
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