Note: This is the third article of a four-part series covering solar energy (photovoltaic) plastics (1) trends, (2) material advances, (3) process technologies and (4) applications.

System considerations such as reliability, overall energy production rate and competitiveness in the Levelized Cost of Energy (LCOE) are dominating solar energy or photovoltaic (PV) maturation as subsidized manufacturing is replaced by true grid parity.

Advances driven by large-scale volume manufacturing and overall system performance as measured by LCOE is the path forward. Since a reliable operation of PV modules over long periods of time (greater than 20 years) is essential for the economic success of solar-electric systems, two important R&D areas are solar cell encapsulation technologies as well as testing and reliability analysis of PV modules.

While the biggest PV producers of today push to optimize manufacturing and output, their fortunes could change sharply if a new kid on the block with a technology that dramatically cuts PV production costs and ramps up cell performance proves to be scalable. Globally, the solar industry is experiencing a period of heavy consolidation and international trade wars.

Automated adhesive layer framing of crystalline PV module.

Unlike conventional adhesive and sealant systems, this product has high initial bond strength, substantially reducing the dwell times in the production line. The adhesive makes it possible to attain significantly higher process speeds during automated frame bonding of crystalline solar modules.

The adhesive belongs to a new generation of elastic adhesives and sealants based on silyl-modified polymers. The patented one-component system combines immediate handling of the solar module, right after framing, with high-performance bonding/sealing.

Sag-resistant, the adhesive is applied warm and crosslinks by reacting with atmospheric moisture to form an elastic product. In addition to high initial tack for immediate handling, the adhesive also offers high flexibility over a wide temperature range from minus-20 to 100 degrees C.

The adhesive has excellent long-term resistance to hard weathering conditions, including UV resistance and damp heat resistance. It also easily compensates for differing rates of thermal expansion between substrates and provides excellent primer less adhesion to glass, various metals and many plastics.

Terostat MS 500's high adhesive strength.

Continuing, antireflective (AR) film advances are in advanced development. The eyes of moths, which allow them to see well at night, are also covered with a water-repellant, antireflective coating that makes them among the least reflective surfaces in nature, helping them to hide from predators in the dark. Researchers, using biomimicry are developing new antireflective technology.

Plastics Institute of America
Scanning electron microscope image of moth’s eye.

Photovoltaic processing system efficiency can be increased by reducing the reflections from the surface of solar cells. AR films and coatings are thus becoming essential components of PV module systems.

(a) With moth-eye AR Film (red acrylic), (b) without moth-eye and (c) upper image shows view with moth-eye; lower image shows view without moth-eye. Note the photographer's image reflected.

The Japanese team uses anodic porous alumina molds to imprint the microstructure of moth's eyes into acrylic resin providing a high-throughput, large-area/low-cost processing method of producing the film. The team estimates that the films would improve the annual efficiency of solar cells by 5-6 percent.

The efficiency improvement is attributed to an incident-angle characteristic, i.e., the ratio of diffuse solar irradiation to beam solar irradiation. Work continues to improve the film's durability and optimize it for use on different types of solar cells.

Nanoimprinted antireflective film.

3M has also introduced a new microreplication film for PV that uses a special surface geometry with patent-pending material combinations designed to minimize surface reflections letting more light into modules. The film increases power output by 5-10 percent. Not yet commercially available, the film is currently undergoing durability testing.

Finally, on another development front, solar/PV concentrator mirror film has evolved. "Cool Mirror," an all-polymeric multilayer optical film by 3M, is designed for low-concentration, crystalline silicon photovoltaic (LXCPV) installations. The film reflects only the wavelengths of light useful to solar cells and transmits IR light, substantially reducing the amount of solar energy that can degrade the performance of concentrator photovoltaic (CPV) systems.

When used in LXCPV systems, the film can significantly increase PV module power by increasing the amount of usable light on the module while limiting the amount of heat-generating IR light.

Left, a roll of 3M "Cool Mirror" film. Right, Cool Mirror building integrated photovoltaic panels.

3M uses its expertise in manipulating wavelengths to ensure the film reflects only the light that can be absorbed by solar cells and selectively transmits IR waves away from the module that would otherwise heat and degrade the module's energy output.

The new film of polyethylene terephthalate (PET) and fluoropolymers offers higher reflectivity and better UV stability than previous mirror films made of polyethylene naphthalate (PEN) and polymethyl methacrylate (PMMA) or acrylic.

The PET high-refractive index layers are easier to UV-stabilize than PEN, and incorporate fluoropolymers for greater UV stability. It has an average reflectance at normal incidence (700-1250 nanometers) of 89 percent minimum.