Plastic materials are being evaluated by several leading research institutions for broadened commercial solar cell applications due to their advantages of significant cost-savings and flexibility. Concentrated photovoltaic (CPV) solar cells use materials such as glass and plastic to concentrate solar radiation onto small, high-efficiency photovoltaic cells. However, the jury is still out in many camps on whether plastics are truly reliable over typical 20 year warranty periods for solar-related products.
Several CPV manufacturers such as Amonix use plastic-based fresnel lenses for concentrating solar radiation, but many competitors like Solenergy have retained glass-based materials for optics in their systems. Many companies are designing CPV systems for enhanced reliability and longevity and are conducting more aggressive accelerated performance evaluations that extend beyond the typical 20-year forecast period; thus increasing the stringency of requirements for plastics used in CPV systems.
Evonik Industries, who manufacturers various polymer materials including acrylic and silicone-on-glass materials for the CPV industry, has been focused on improving their UV-resistant and temperature-stabilized acrylic product line. This company offers a variety of options and protection systems for solar cells such as UV absorbers, stabilizers, and encapsulants, which are implemented to reduce degradation due to heat, humidity, and exposure to the elements.
CaseWestern Reserve Universityhas provided independent verification on performance of one of these products and reported its findings at the US Department of Energy Energy Efficiency and Renewable Energy (EERE) 2011 Solar Reliability Workshop. It found that during natural weathering of Evonik’s Acrylite, which is utilized in Low-Concentration PV systems, transmission was only slightly affected after a decade of lifetime testing, while high humidity exposure did not affect the amount of light transmission when tested in the extreme heat of Arizona and Florida. This supports Evonik’s claim that their lenses do not discolor over a decade of estimated use in Arizona, and virtually without any “yellowing” in Florida’s higher humidity environment, which are both major performance milestones in the industry.
Plastics are not only being developed for optics in CPV systems but also as the main active component in next-generation solar cells.
IMEC, a leading international electronic materials R&D center based in Belgium, recently developed an inverted bulk heterojunction architecture for polymer-based solar cells that enhances cell light management and enables more robust devices. The inverted cell structure increases the light capture capability within the organic layers of the device, compared with conventional polymer-based solar cells. Remarkably, IMEC’s latest inverted solar cell has achieved a certified conversion efficiency of 8.3 percent, which is the highest ever recorded for this type of polymer cell architecture, topping its prior generation device result by 1.4 percent.
Since IMEC is not a manufacturing hub, it will likely license the technology to organic photovoltaics developers for future production. The inverted cell architecture may also be implemented in tandem cell structures, which are essentially two cells in a device able to absorb light from different portions of the overall light spectrum. Thus, efficiencies of conventional organic photovoltaic cells could be improved, which still dramatically lag behind common silicon solar cells on the market.
Plastic materials have shown major progress in performance for solar cell applications in the last year but still face many hurdles to gain significant traction in this competitive industry regarding performance compared to more pricey yet proven alternatives. Moreover, as more solar cell companies reduce or stall R&D efforts due to poor financial performance, the integration of various plastics into many commercial applications will be delayed accordingly.