Ceramic materials help manufacturers of thin film photovoltaic cells achieve greater efficiency

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.

The earth benefits from an impressive 125,000 terawatts (TW) of solar energy. 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 centered on thin film deposition, rather than wafer-based modules, although there is still discussion around the relative merits of both. The main arguments in favor 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 vapor deposition processes, particle sintering or electro-deposition for example. Reports suggest that the best results are achieved using high temperature (up to approx 500°C) 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 Technical Ceramics, 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 Technical Ceramics 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 vapors and gases are transported from a source vessel through a deposition zone onto a heated glass substrate to deposit the PV layer. Morgan Technical Ceramics 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 Technical Ceramics' Hudson, NH USA site supplies pBN crucibles and evaporation boats made via a chemical vapor deposited (CVD) process that are ideal vessels due to the ultra-high purity nature of the CVD pBN material. Further, Morgan Technical Ceramics 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 vapor 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 500°C).

Morgan Technical Ceramics 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 vapor 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 stability as an alternative to stainless steel, which has a tendency to buckle and bend at high temperatures.

Morgan Technical Ceramics is able to produce consistent flatness of less than 0.01mm over the 1.2m length of the bar and parallelism of less than 0.05mm, with a polished mirror finish. In fact, these tight specifications are well within the capability of the company's specialist manufacturing facilities at Rugby, UK, where skills have been honed and refined through years of supplying critical components for the semiconductor, aerospace, laser and other demanding industries.

Ceramic pins, also made of high-grade alumina, are used as locators and separators between key components inside the TFPV deposition reaction module chamber. Shaped like a drawing pin, the component is about 15mm in length with tightly controlled dimensions to enable consistent deposition of the thin film layers within the reaction module.

Morgan Technical Ceramics' Stourport plant, also in the UK, produces several thousand of these pins per month, and is expecting to double its production volume within the next 12 months.

Summary

In all these examples, two things are key. First, in these applications the very high quality engineering ceramics are not operating any where near the boundaries of their thermal and chemical stability. TFPV manufacturers are free to continue experimenting with higher temperatures and different thin film materials, safe in the knowledge that these components of the system will not have any adverse effect on the efficiency of the process or the finished PV panel. In such a rapidly developing market, this level of reliability is vital.

Second, Morgan Technical Ceramics has proven ability to produce consistently high specification components of this kind in large volume, and to be able to react quickly to sudden increases in demand on both sides of the Atlantic.

The manufacture of PV cells using thin film deposition processes is one of the fastest moving and most exciting manufacturing industries of our time. The global market is growing at a rate of 50% per year and estimates are that growth will continue at this rate until 2010, then increase even more rapidly for a couple years before settling down' to a steady 25% year on year growth. Clearly, the race is on, and the big money is there for PV manufacturers who can perfect their processes fast and take the lead.

Proven in other sectors, technical ceramics can make an important contribution to helping this roller-coaster of a developing industry achieve its goals of consistent quality in both the process and the finished product, for better PV cell efficiency in volume productions, and ultimately, parity with other sources of grid energy.

About Morgan Technical Ceramics

Morgan Technical Ceramics (MTC) has comprehensive range of Ceramic materials, from which its products are manufactured. Supplying to a variety of demanding markets, MTC has established an enviable reputation for providing value-added solutions through world-class research and development, innovative design and, perhaps most important of all, application engineering.

Morgan Advanced Ceramics, together with Morgan Electro Ceramics forms Morgan Technical Ceramics, a division of the Morgan Crucible Company plc. From manufacturing locations in Australia, North America, Europe and Asia, Morgan Technical Ceramics supplies an extensive range of products, including ceramic components, braze alloys, ceramic/metal assemblies and engineered coatings.

For more information on Morgan Technical Ceramics visit www.morgantechnicalceramics.com

Featured Product

Solar FlexRack - TDP Turnkey Tracker

Solar FlexRack - TDP Turnkey Tracker

Solar FlexRack TDP™ Turnkey Trackers are the next generation, single-axis solar tracker solution, complete with comprehensive services and support. What sets this tracking solution apart is its updated tracking technology bundled with a full suite of best-in-class project services and support for commercial and utility-scale solar customers. The TDP Turnkey Tracker offers the lowest total cost solution by providing the only tracker in the industry that includes full design, installation, commissioning, and support services bundled in one contract. Solar FlexRack's TDP Solar Tracker solution is supported by an in-house team of experts that includes mechanical, structural, civil, electrical and geotechnical engineers and geologists. To learn more about Solar FlexRack's TDP Turnkey Tracker, one of the most cost-effective and comprehensive tracker solutions available, please visit: http://solarflexrack.com/products/tracker.