The earth benefits from an impressive 125,000 terawatts (TW) of solar energy. While the future energy needs of the planet will undoubtedly be met with a combination of technologies, many believe that solar – in the form of photovoltaic (PV) cells - is the only renewable energy source with the capacity to make a significant impact on global energy production.
As a result, the race is on to push the performance of PV cells to a level where the total cost of the electricity generated is as cheap (if not cheaper) than that from carbon-based sources. Some predict that grid parity, as it is called, could be achieved in some locations within as little as a few years.
Most of the effort in this direction is now centred on thin film deposition, rather than wafer-based modules, although there is still discussion around the relative merits of both. The main arguments in favour of thin film are that it uses less material, and is much faster and simpler than the complex and delicate process of slicing, dicing and placing of silicon wafers. This means that if the cost of deposition can be reduced, and the efficiency of the resulting PV cells increased sufficiently, the goal of grid parity will be achieved.
What most commentators and manufacturers do agree on is that, for significant increases in efficiency, all components of the equipment and all steps of the process must be considered; there is no one panacea that will achieve the goal in a single step.
Thin film deposition process
Thin film deposition has been used for some years for a variety of applications, including semiconductor and optical components, decorative and low-emissivity architectural glass, and most recently in the manufacture of flat screens for TVs and computers. In solar cell production, the process offers a simpler and cheaper alternative to using silicon wafers.
Manufacturers continue to experiment with various materials and refinements of the thin film deposition process for solar cells based on silicon and other materials. The direct band-gap semiconductors cadmium telluride (CdTe), copper indium diselenide alloy (CuInSe2) and copper indium gallium diselenide alloy Cu(InGa)Se2, have high optical absorption coefficients (>105cm-1) are now emerging as the most popular materials for the photo absorption layer in thin film photovoltaic (TFPV) cells. More than a dozen companies worldwide are already actively producing these cells, or are in a start-up phase.
Creation of the TFPV layers can be achieved by various methods; using a physical or chemical vapour deposition processes, particle sintering or electro-deposition for example. Reports suggest that the best results are achieved using high temperature (up to approx 500C) deposition and post-growth anneal of the TFPV layers.
While the processes are complex, and manufacturers continue to research, develop and refine, the essential features remain - high temperatures, aggressive and corrosive process materials.
Quality is key
To date TFPV cells have only achieved approximately 20% efficiency (which is the current benchmark for Crystalline Silicon PV cells manufactured in production quantity) over small areas and under laboratory conditions. In production quantities and large panel sizes the best efficiencies that manufacturers currently achieve is in the range of 10-12%.
In the push towards achieving the goal of grid parity, the manufacturing challenge is to create reliable and consistent process conditions that can reproduce laboratory quality in large quantities.
This is an area where manufacturers of PV cells and their equipment suppliers can benefit from the huge investment in materials research that has already been done over the years in the manufacture of semiconductors and flat screens, both of which have been through large-scale, fast ramp-ups in volume manufacture.
Ceramic – the perfect choice
Technical ceramic materials feature high hardness, physical stability, extreme heat resistance and chemical inertness. As such, they are highly resistant to melting, bending, stretching, corrosion and wear – and ideal for use in environments of extreme heat and aggressive chemicals, like that of TFPV deposition.
Morgan Advanced Materials, a division of the Morgan Crucible Company plc, is a world leader in specialist engineering of ceramic components. A global business, the company is working with leading players in PV cell manufacture in USA, Europe and Asia, supplying a wide variety of components for both silicon-based and non-silicon based thin film solar cells.
Non-silicon thin film solar cell manufacture
In an application borrowed from the manufacture of architectural glass, fused silica rollers are used to move the hot glass panels through the deposition process. The thermal stability of silica is exceptional; it has a coefficient of thermal expansion (CTE) of <1 x 10-6/C – lower than any other ceramic material. This low CTE combined with its chemical compatibility with glass make fused silica rollers an ideal choice for ensuring the glass remains perfectly flat during the process.
Morgan Advanced Materials are supplying precision machined fused silica rollers for use in continuous flow TFPV deposition machines from its locations in Fairfield, NJ, USA and Yixing, China.
In TFPV deposition equipment, precursor vapours and gases are transported from a source vessel through a deposition zone onto a heated glass substrate to deposit the PV layer. Morgan Advanced Materials produces a number of components used in this part of the TFPV process.
In some instances, solid materials are melted and vaporized from ceramic crucibles or boats to form a flux that is deposited on the heated glass substrate. It is critical that the ceramic crucible or boat be dimensionally stable and chemically non-reactive to the molten source material. Pyrolytic boron nitride (pBN) ceramic is an excellent material for this application due to its high corrosion resistance and non-reactivity with the source materials used in PV deposition. Morgan Advanced Materials’ Hudson, NH USA site supplies pBN crucibles and evaporation boats made via a chemical vapour deposited (CVD) process that are ideal vessels due to the ultra-high purity nature of the CVD pBN material. Further, Morgan Advanced Materials provides pBN-coated graphite heating elements used for material vaporization.
In other configurations, vaporized precursor materials are transported from the source to the deposition zone via a vapour distribution manifold. The manifold is formed from a perforated tube made of ceramic because it is one of the few materials with the chemical stability to operate without problems with these very toxic, hazardous chemicals, at high temperatures (above 500C).
Morgan Advanced Materials produces these tubes in mullite and in alumina, at its specialist extrusion facility in Waldkraiburg, Germany. Tubes are up to 2.5m (100inches) long x 105 mm (4inches) diameter, with multiple vapour exit points, for uniform deposition across the glass. They are extruded, fired in large kilns, and then precision machined to achieve final product tolerances within +/-0.15mm (0.005inch).
Silicon-based thin film solar cell manufacture
Oerlikon Solar, a European manufacturer of thin film deposition equipment for PV panel production, is using precision-engineered, high-purity ceramic bars in some of its higher temperature thin film deposition machines, for lifting, stacking and aligning components and the glass panels inside the chamber.
The ceramic is semiconductor-grade 99% alumina, chosen for its excellent thermal and chemical stabi