Photovoltaic component specification to help reduce solar cell manufacturing costs. John Baxter and Al Bousetta report.
A global push to reduce dependency on non-renewable energy sources is driving increased demand for photovoltaic (PV) technologies. However, the promise of solar power becoming a leading source of renewable energy is hampered by the high cost of solar cell production. To prosper, this market must become competitive with the cost of more traditional electric power. That means reducing the cost of solar power to achieve grid parity between solar power and traditional electricity.
One means of reducing costs is to match standards for component cleanliness and purity with the true process requirements for the production of solar cells. Thus far, the PV industry has looked to the semiconductor industry for standards but, in reality, UHP semiconductor standards are typically well beyond what is required for solar cell manufacturing.
Solar cell manufacturers are learning that when they utilise components rated for UHP semiconductor processing they are, in many cases, over-specifying their components. UHP components demand a price premium due to the highly controlled manufacturing and cleaning protocols used in their production, which may exceed those required for PV components. To date, the alternative to UHP components is 'non-qualified' components, which in the context of this article are defined as products that are not subjected to UHP manufacturing and cleaning protocols. But, such components introduce risks, such as downtime due to contamination and potential system integrity issues.
A solution to this dilemma is a component processing standard that is specifically designed for the PV industry. Swagelok Company has taken a first step towards reaching this type of standard by issuing a new Photovoltaic Process Specification. The specification matches testing, cleaning, and packaging steps for stainless steel components to the needs of the PV industry.
Swagelok's Photovoltaic Process Specification supports the industry's effort to establish specific guidelines to help facilitate market growth. The SEMI Photovoltaic Committee is also currently investigating and addressing specifications related directly to the PV fabrication market. Based on the company's work with several PV manufacturers, the Swagelok specification shares policies and procedures for the manufacturing of fluid system components at a level of cleanliness that meets - but does not unduly exceed - the requirements of today's PV manufacturers. The specification helps decrease total PV system costs compared to UHP semiconductor systems by reducing component manufacturing steps.
A high level of processing is required for fluid system components used in UHP semiconductor wafer manufacturing operations to minimise corrosion and particle generation. Small line widths and high device density drive the need for UHP gas and chemical delivery as any particles carried downstream may contaminate the wafer, resulting in increased scrap and operating expenses. Substrates employed in solar cell production are more tolerant of particles, which is, in part, why UHP processing standards are too stringent for PV manufacturing.
A clean processing environment is also critical to ensure proper adhesion between layers in thin film solar cells (TFSCs), particularly during the wiring process. Properly rated PV components provide the right level of contamination control for reliable TFSC production.
Safety is a further consideration related to corrosion and cleanliness. Certain gases used in PV manufacturing are highly reactive. If component connections corrode or otherwise deteriorate, these gases may escape to the atmosphere as fugitive emissions, creating a potentially dangerous work environment.
PV Manufacturing Protocols
The Swagelok Photovoltaic Process Specification outlines protocols for stainless steel component design, material selection, and manufacturing steps that are similar to, but in some cases less stringent than, those used for UHP semiconductor components. For example, surface finish requirements are relaxed in the PV specification. Minimising surface flaws and inclusions within a component reduces the total wetted surface area, which, in turn, improves purging and moisture removal. While these characteristics are important to maintain cleanliness in both semiconductor and PV processes, PV production systems are much more tolerant of small amounts of possible contamination that may result from less polished surfaces.
Under the specification, design promotes clean operation. Components must be able to be cleaned and purged quickly and easily, generate few particles in service, and contain minimal areas of entrapment. Sound component designs are of particular importance in TFSC production, which utilises a continuous manufacturing process. In such an environment, equipment must be very reliable to avoid shutting down the entire line. Component cleanliness specifications must therefore be commensurate with high reliability.
The areas in which UHP semiconductor and PV component manufacturing protocols differ most notably are during the end stages of the product manufacturing cycle, including the cleaning, verification and testing, and assembly and packaging steps. In these stages, the practices outlined in the Swagelok Photovoltaic Process Specification enable suppliers to reduce costs compared to UHP component manufacturing. The resulting savings may be passed on down the supply chain, helping to reduce the total cost of solar power generation. Further, during installation, end users may realise efficiencies based on a less stringent packaging requirement.
Cleaning
Each step in the manufacturing process introduces contamination. After each step, therefore, UHP semiconductor components must be cleaned thoroughly using organic solvents or alkaline- or acid-based cleaners.
Similarly, PV components must be also cleaned during manufacturing. However, the PV market collectively acknowledges that UHP semiconductor grade cleaning is not required for crystalline silicon (c-Si) or thin film PV technologies, the two most common manufacturing methods for producing solar cells. Compared to UHP component cleaning standards, the Swagelok Photovoltaic Process Specification reduces requirements for bath controls, including resistivity and bacteria levels. Rather than following today's stricter UHP purity standards, the Swagelok Photovoltaic Process Specification aligns PV component cleaning methods to the purity needs of the market.
UHP and PV components undergo a variety of tests during manufacturing to confirm product cleanliness and quality. Component producers are particularly concerned with verifying the corrosion-resistant properties of the chromium-enriched oxide surface layer of stainless steel components. These surfaces are enhanced during manufacturing through electropolishing and passivation processes that remove surface iron and smooth surfaces.
UHP standards typically call for advanced surface chemistry analysis techniques to confirm corrosion resistance. The techniques analyse a series of discrete points on a sample. Common testing methods include Auger Electron Spectroscopy (AES), Electron Spectroscopy for Chemical Analysis (ESCA), and Secondary Ion Mass Spectroscopy (SIMS). These tests are complex and are often performed by third-party labs, two factors that add cost to UHP component manufacturing operations.
The Photovoltaic Process Specification drives out those added costs by specifying an alternative testing method, known as the critical pitting temperature (CPT) test. CPT testing is based on ASTM G150: Standard Test Method for Electrochemical Critical Pitting Temperature of Stainless Steels. It evaluates the entire passivated surface of the sample by stressing a sample's chromium oxide surface layer to the point of failure to determine its resistance to localised pitting corrosion. The temperature at which the surface layer fails is known as the CPT. Because CPT testing analyses the full wetted surface area of a sample, it provides a better indication of how well a component will stand up to a harsh environment compared to surface chemistry analysis techniques.
John Baxter is Products and Technology Manager, Swagelok Company, Solon, Ohio, USA, and Al Bousetta is Marketing Manager, Gas Products, Santa Clara, California, USA. Swagelok Company. www.swagelok.com