MANUFACTURINGMATERIALS
Keeping the wafers flat
Once the ingots have been created, they are cut into thin wafers and chemical doping is carried out to improve their efficiency. During this process the silicon wafers are passed into a diffusion furnace that operates at high temperatures of up to 900°C and with harsh chemicals for cleaning.
Rollers are used to move silicon wafers through the doping process. These are manufactured from fused silica material for their outstanding mechanical properties. The rollers are inert, have high mechanical strength, high purity and good wear characteristics, leading to a long life in a challenging high temperature environment. As a result the wafers maintain their shape and cell efficiency.
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As manufacturers look to increase the efficiency of the wafer cell they are using diffusion processes with increasing temperatures and using more aggressive chemicals and acids. This atmosphere is too extreme even for fused silica and new
materials for the process equipment are being developed by Morgan Technical Ceramics that can withstand these harsher environments. For example, an alumina/silica material for rollers is being created, which has improved chemical resistance, excellent thermal shock resistance and can be supplied with a smooth surface finish. The performance improvements associated with these materials are expected to lower costs by driving down the processing costs for each wafer produced.
Handling the wafers
Ceramic materials are also used in transporting the wafer from one diffusion furnace to another. In some cases up to five deposition processes take place and handling the wafers between these stages at high temperatures using steel carriers could contaminate the cells.
Large, high-precision, high purity ceramic bars and locator pins for lifting, stacking and aligning glass panels in PV processing provide a superior solution to stainless steel lifters. The ceramic pins are used as locators and separators between key components inside the PV reaction module chamber. High purity aluminium oxide formulations provide higher strength at elevated temperatures and eliminate buckling associated with metal equipment. Improved chemical stability of the handling equipment eliminates contamination on the surface of the glass in processing environments.
Thin film processing
In solar cell production, thin film deposition offers a simpler and more cost effective alternative to using silicon wafers. Thin films use less material and are much faster and simpler to manufacture than the complex and delicate process of slicing, dicing and placing of silicon wafers. However, the ratio of cost and cell efficiency must be improved. The cost of deposition must be reduced further and the efficiency of the resulting PV cells sufficiently increased to make solar cells manufactured via this process as inexpensive as those based on silicon wafers.
The first thin film technology to be extensively developed and manufactured was amorphous silicon. However, this technology suffers from low cell efficiencies and slow deposition rates. Two other thin film technologies which are being continuously developed are cadmium telluride (CdTe) and copper indium gallium diselenide
www.solar-pv-management.com Issue VI 2010
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