Solar

Materials Engineering Enhances Solar Performance

Issue 5 and Volume 2.

The manufacture of photovoltaic (PV) cells and modules mandates designed-in materials engineering to reduce development time, streamline production, and optimize performance.

Cost and performance are the two primary concerns of solar engineers, and the effort is ongoing to minimize cost and maximize performance. While considerations such as physical size, number of cells (whether crystalline silicon or thin-film), required output, and peak capacity are essential concerns for a given installation, the development of a solar product to meet the specifications is, among other things, a materials challenge. The question is: what materials must be designed-in and how are they to be configured to achieve the desired performance, including sunlight-to-power efficiency, under prescribed environmental conditions and cost limitations?

Historically, rigid solar cells have incorporated crystalline silicon wafers and are classified as generation 1 products. Today, there is great interest in generation 2 modules, in which a thin-film process is employed utilizing a rigid substrate, such as glass, or a flexible substrate, typically a polyester or polyimide material. Much research and development is also focusing on concentrator technologies, which involve advanced thin film processes intended to achieve higher conversion efficiencies of 25%-42% vs. 12%-25% for conventional rigid silicon and thin film applications.

Figure 1. Rigid solar module with silicon cells.

In developing a solar product, the operative word is designed-in, for many of the key components that make up a particular module — bus bars, contacts, blackout/protective film, back sheet laminate, junction box attachment, wiring, packaging, or array interconnections — depend on the proper selection, conversion, and assembly of materials. Often the design process is complex, and at times, custom materials must be developed and tested.

Role of the materials converter

A materials converter takes volume materials and converts them into finished parts for an end product. In the case of solar, such parts include foil tapes slit to specification for bus bars, thermal conductors and insulators, electrical insulators, adhesives for bonding layers, die-cut parts on production rolls, foam tapes and precision cast foams, edge seals and edge delete tapes, and a host of other materials that are cut, shaped, and formed to meet design and assembly requirements for solar modules, or panels.

Figure 1 is a diagram of a rigid solar module with rigid silicon PV cells. Shown are the parts and materials incorporated in an average assembly. Figure 2 is an illustration of a representative thin film product. The actual materials used for a given application depends on the design of the module. In Figure 2, for example, the specification may require that a blackout material be applied to the face of the glass to prevent pre-energizing of the cells. On the other hand, where solar activation is not a concern, a clear protective film may be called for instead. The choice of a particular back sheet material — usually a laminate — may vary with the application as well.

Figure 2. Rigid solar module with thin-film cells.

The trend toward thin film is a boon for solar manufacturers. Whereas a manufacturer may be able to turn out 100 rigid silicon modules a day, the same manufacturer — with the bulk of the engineering and materials supplied by a converter — may be able to produce as many as 4,000 modules during the same timeframe based on thin film technology.

Materials converters who serve the solar industry are distinguished by the added services they offer and the degree of involvement with customers above and beyond the production of parts that meet particular manufacturing and performance specifications. For example, they often participate with their customer in the design and development process to ensure the specification of materials and parts that result in the most functional and cost-effective solution for the application. In such instances, the knowledge of the converter is brought to bear with regard to the performance requirements of the materials and manufacturing capabilities of the solar manufacturer. Depending on the material, specifications may include temperature resistance; performance at upper temperature limits; shear, tensile, and peel strength; outgassing; dielectric strength, thermal conductivity; slitting widths, and tolerances. For example, typical requirements may include temperature resistance of 180°C with an upper limit of 200°C; peel adhesion of 37 oz. per inch; Z axis resistance of 56mohms; and shielding effectiveness of 80dB to 95dB from 1GHz to 18GHz (Fig. 3.)

For the converter, maintaining a state-of-the-art test facility is necessary as well. The facility enables testing to governing specs — typically UL specifications 1703 and 746, as well as others, in the U.S. — not only to confirm incoming materials supplied by vendors, but also to certify that performance specs are met by the materials and parts being delivered to the solar manufacturer. For global applications, customers may mandate UL specs as well. IEC standards may also apply to ensure both performance and safety in the design and manufacture of solar modules.

Converters are sometimes called upon for independent third-party testing services. In these instances, the end user either lacks the capability to perform particular tests or is looking to meet deadlines with offsite support. Occasionally, the end user wishes corroboration of results obtained in-house.

In introducing a new material to a given process, the converter may choose to retain the proprietary rights to the product, or may release the rights to the end user. In some instances, converters create proprietary materials and composites in anticipation of a future need or application or to achieve a performance or cost objective.

Ongoing research is also devoted to developing improved materials for existing end user requirements. In such instances, where a notable advancement has resulted and been validated through qualification testing, the end user may decide that performance improvements outweigh the cost of changing production.

Optional services

While converters tend to concentrate on materials engineering and on producing sub-assembly products that meet end user specifications, some companies offer extended services at the end of the development cycle. Some converters also offer prototyping, production-run manufacturing, and final assembly services, such as kitting and inventory control.

Figure 3. Fabrico materials lab showing slitting of foil tape.

Manufacturing challenges and solutions

Some of the typical solar panel design and manufacturing challenges where engineering evaluation and materials recommendations can be required by a converter are discussed below.

Bus bars can provide several challenges to solar panel manufacturers. The selection of the proper adhesive is often critical. Inconsistent adhesion can cause hot spots that can lead to arcing and affect panel performance. Here, the converter can recommend adhesives with the appropriate crosslink capabilities. With a strong understanding of adhesives and their capabilities, a converter can help the panel manufacturer overcome issues that occur during vacuum lamination.

Junction box sealing is another area where a converter’s understanding of adhesives and their capabilities is invaluable. Water vapor transfer rate (WTVR) is a concern for every panel manufacturer. Results from moisture can include internal contamination or separation of the junction box from the panel. A converter with the right expertise can work with the manufacturer to overcome WVTR issues without sacrificing production speed. For example, a double-sided acrylic cast foam tape can offer the 100% adhesion retention within a wide range of moisture and temperature parameters.

In addition to selecting the right materials, a converter can often help the solar panel manufacturer by discovering how to process unique materials by customizing the manufacturer’s process or equipment. Using the converter’s materials processing knowledge can help a manufacturer gain a competitive advantage and cost-effectively handle a unique material.

Conclusion

Converters play an essential role in the manufacture of solar panels. Such companies are more than parts manufacturers inserted between the supplier of materials and the manufacturer of the solar panels. Not only do converters produce the parts needed to assemble a module, but they also often participate in initial materials engineering. Many converters also have the capability to develop materials to meet unique physical and performance requirements. They are also capable of performing testing services that check incoming materials as well as products being delivered to customers.

Most materials converters are problem solvers as well, from coming up with interleaf materials for flex panels to specifying foam tapes — some exceedingly thin — that stick under changing environments with factors such as moisture, temperature, and vibration, while providing the desired dielectric performance. Converters must also be aware of contamination, which can come from many sources, not just the environment.

As third generation solar modules become an increasing challenge for manufacturers, especially with the inroads being made in concentrator technology, converters will be called upon not only for design and materials development and testing to ensure compliance with governing standards, but also to provide expertise and support to optimize costs, minimize rejects, and meet manufacturing schedules. Only with that level of assistance can solar cells evolve to the point of competing with, as opposed to augmenting, conventional methods of power generation.

Rick Traver received his Bachelor’s degree in business administration from California State U. and is a technical field representative at Fabrico, 4175 Royal Drive, Suite 800, Kennesaw, GA 30144 USA; ph.: 678-202-2700; [email protected]