MANUFACTURINGMATERIALS
(CIGS), which has high, small-area cell efficiencies approaching 20% in the laboratory. Thin film cells are mostly manufactured by a chemical vapour deposition process at middle to high temperatures and under vacuum in diffusion furnaces. During the process chemicals are sputtered onto a glass panel to build up the layers of the cell.
Similarly to the silicon wafer manufacturing process, rollers are required, this time to move hot glass panels through the deposition process. CIGS and amorphous silicon processing take place at temperatures of 300 - 400°C, and polymer or steel rollers can be used.
However, at the higher temperatures of 600 – 800°C that are for example used in CdTe processes, these rollers will buckle.
The thermal stability of silica is exceptional; it has a coefficient of thermal expansion (CTE) of less than 1 x 10-6/°C, which is lower than any other ceramic material. This low CTE, combined with silica’s chemical compatibility with glass, ensures the glass panels remain totally flat during the manufacturing process and hence the PV cells maintain their shape and cell efficiency.
Rollers are typically available in a wide variety of standard dimensions, with outer diameters ranging from 15 - 110 mm, and in lengths ranging from 305 - 4100 mm. Custom designs and dimensions can also be manufactured.
Deposition without contamination Another application of ceramic materials is for the crucibles and boats that contain the material to be deposited. Morgan Technical Ceramics has developed PBN for use in manufacturing the photo absorption layer in thin film photovoltaic (TFPV) cells. It is an excellent material for crucibles and evaporation boats used in this process.
In the TFPV deposition process, precursor vapours are transported from a source vessel into a deposition zone onto a heated substrate to deposit the PV layer. In some instances, solid materials are melted and vaporised from ceramic crucibles or boats to form a flux that is deposited on the heated substrate. It is critical that the ceramic crucible or boat be dimensionally stable and chemically non-reactive to the molten source material.
PBN ceramic is an excellent material because of its high corrosion resistance and non-reactivity with
the source materials used in PV deposition.PBN is inert, has low wetting to alloys and is resistant to thermal shock, which gives it a long life. Furthermore, it can be manufactured in complex shapes and can also be used to coat graphite heating elements used for material vaporisation.
The PBN material is highly anisotropic (directionally dependent) in its thermal transport and very resistant to thermal shock. PBN is also an excellent electrical insulator. The material is stable in inert and reducing atmospheres up to 2800°C and in oxidizing atmospheres to 850°C.
Conclusion
Manufacturing solar cells, whether silicon wafer or TFPV, takes place in harsh environments and at high temperature. Ceramic assemblies provide robust, inert, and thermally stable solutions that allow solar cell manufacturers to achieve their increasingly demanding production targets and enable the manufacturing process to be optimized through reducing the risk of contamination and by using components that last longer.
Morgan Technical Ceramics works with leading players in PV cell manufacture in the US, Europe and Asia, supplying a wide variety of components for manufacturing solar cells. Use of its PBN material in the TFPV process is considered an important innovation that will make the goal of using solar energy to generate more of the world’s electricity more attainable.
As a result of using ceramic in their processes, solar cell manufacturers around the world are improving yield and process efficiencies, which will bring grid parity for renewable power close than ever before.
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www.solar-pv-management.com Issue VI 2010
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