Note: This is the second article of a four-part series covering solar energy (photovoltaic) plastics (1) trends, (2) material advances, (3) process technologies and (4) applications.
A variety of films and plastic coatings have been developed to allow more flexible and mobile power systems to be built. These barrier layers protect against oxygen and water and are highly resistant to damaging ultraviolet light.
Most solar cells are commonly designed to operate outdoors for 25-plus years, and a typical solar module includes:
- superstrate or frontsheet
- layer of encapsulant
- array of connected cells
- layer of encapsulant
- substrate or backsheet
- plus various adhesive layers and bonding tapes
Thin-film solar panels also contain barrier films, such as aluminum, sputtered films or high-performance polymeric films to prevent moisture vapor ingress into the module. Other layers can also be included that control reflection or concentrate solar rays.
Thin-film solar is either rigid or flexible. These are made by depositing thin layers of photovoltaic (PV) material using various deposition methods onto an array of thin substrates such as glass, metal or plastic.
Key components for both crystalline silicon and thin-film modules include glass, pastes/screens, junction boxes and cables, encapsulants, frames, backsheets, frontsheets, adhesives, ribbons and thin-film feedstock. Plastics are finding other PV applications beyond sheets and films.
For example, polyethylene terephthalate (PET) and polyphenylene oxide/polystyrene (PPO/PS) are being used to replace metal junction boxes. PPO/PS is also being used for PV connectors and glass filled PPO/PS for a solar module frame.
In automating fabrication, solar module manufacturers have turned to adhesives to reduce assembly time and production costs. Adhesives have proven themselves to be fast and cost-effective automated assembly alternatives, able to bond dissimilar materials (plastics to metals, etc.) and reliably withstand harsh environmental conditions over the long service life of a solar module.
In solar electronic material advances carbon nanotube (CNT) antennas have the potential to enhance PV efficiency. Using CNT, researchers at MIT have found a way to concentrate solar energy 100 times more than a regular PV cell.
Single-walled carbon nanotubes (SWNT) are used to form antenna-like structures to improve PV energy harvesting. The nanotube antennas or funnels are made of "fibrous rope" about 10 micrometers long and 4 micrometers thick containing about 30 million carbon nanotubes.
The light-concentrating antennas are made by forming a fiber with shells of successively larger-bandgap SWNTs radiating outward. The inner layer of the antenna contains nanotubes with a small bandgap, and nanotubes in the outer layer have a higher bandgap.
This is important because excitons like to flow from high to low energy. In this case, it means the excitons in the outer layer flow to the inner layer, where they can exist in a lower (but still excited) energy state. SWNTs were isolated and dielectrophoretically spun into largely homogeneous solid core-shell fibers. The antennas were possible because new separation methods allow sorting of SWNTs by their optical properties.
When light energy strikes the SWNT material, excitons will flow to the center of the fiber where they are concentrated. An exciton forms when a photon is absorbed by a semiconductor. These structures can boost the number of photons captured and funnel energy from light into a solar cell.
By capturing and focusing light energy in this way, much smaller and more powerful solar arrays are possible. The MIT team is now working on ways to minimize the energy lost as excitons flow through the fiber, and on ways to generate more than one exciton per photon. Currently, the nanotube bundles lose about 13 percent of the energy they absorb, but the team is working on new antennas that would lose only 1 percent.
MIT
MIT engineers built tiny CNT channels (left); CNT filament absorbs solar energy then re-emits photons (right).
Continuing, front sheets provide a protective barrier for the interior components of the photovoltaic module. Critical requirements include low water vapor permeation, excellent UV resistance, a high degree of visible light transmission, service life up to 25 years and, in some cases, increased mechanical impact strength.
Flexoskin is a new multilayer barrier film for front barrier sheet in flexible photovoltaics. The new multilayer 350 µm (micron) film was commercialized by Evonik. The PMMA (polymethylmethacrylate, acrylic) based Flexoskin consists of several active and compound layers to ensure the barrier, and an outward-facing, fluorine-free overlay to protect lower layers of the film.
In addition to weatherproofing, ultraviolet (UV) resistance and excellent adhesion to conventional encapsulating materials, the film is highly transparent with 87 percent light transmission.
Others are similarly developing multilayer frontsheet materials for flexible PV applications. Mitsubishi Plastics has developed a multilayer front sheet for flexible PV that consists of the following layers:
- A fluorine-based layer on the surface
- Adhesive layers sandwiching a silica deposition film barrier layer
- A polyester film layer
- An ethylene vinyl acetate (EVA) encapsulant layer. The EVA layer comes in direct contact with the solar cells and prevents ingress of moisture with permeation of 10-4g/m2/day (grams per square meter per day).
Mitsubishi has established a plant in Tsukuba, Japan, with capacity to produce 16 million square meters of the film annually.
Finally, SilTrust's transparent silicone encapsulant technology commercialized by Momentive Performance Materials helps ensure PV modules' long-lasting performance in harsh outdoor environments, while improving light-to-electricity conversion yield. The refractive index (RI) of the new encapsulant closely matches the RI of glass, reducing energy-dissipating reflection of sunlight away from the solar cell.
UV radiation with a wavelength below 380 nanometers is blocked by typical EVA formulations due to UV absorbers. Silicones, on the other hand, are transparent throughout the AM 1.5 solar spectral irradiance spectrum and as such transmit about 3.1 percent more light to the PV cell's surface.
Momentive Performance Materials
SilTrust silicone improved light transmittance (white) vs EVA (yellow).
Mechanical stresses in a module are an important cause of electrical connection failures. SilTrust encapsulant can be used to surround the fragile solar cells of the PV module with a flexible, stress-dissipating silicone matrix that adheres well, yet does not pass on mechanical stress the cell may suffer due to harsh environmental conditions. This lower mechanical stress helps extend the solar cell life span.
The graph above shows that at 80 degrees C, which is about the maximum operating temperature for a PV module, a SilTrust silicone module is theoretically virtually stress free, whereas the EVA encapsulated module theoretically shows significant stress.
At roughly 30 degrees C, the thermal stress in an EVA module is estimated to be approximately double that of a SilTrust silicone encapsulated module. Modules built with SilTrust encapsulation are less prone to suffer from failures resulting from mechanical stress not only because of the more favorable mechanical properties of silicone, but also because of their much lower module manufacturing temperature of 85 degrees C compared to higher than 140 degrees C for EVA.
Upon cooling, stress builds in the module because encapsulation materials shrink more than the connectors, cells, glass and back sheet due to differences in thermal expansion coefficients.
