Note: This is the second article of a three-part series covering plastics in solar energy (photovoltaic) (1) trends, (2) material/process advances and (3)applications.
To understand the advances being made in photovoltaic (PV) cells, it's important to understand the basic components of these cells. From front to back layer, solar modules include the following elements:
- super-strate or frontsheet
- layer of encapsulant
- array of connected photovoltaic cells
- layer of encapsulant
- substrate or backsheet
- plus various adhesive layers and bonding tapes
Plastics institute of America
Basic solar cell layered structure.
For polymer-based organic PV cells, scientists have long believed the key to high efficiency rested in the purity of the cell's two domains, namely acceptor and donor. To improve cell efficiency, many researchers have focused on tweaking the backbones of existing donor and acceptor materials or on designing new ones.
Now, however, improvement in the core array of connected photovoltaic cells has come from a new path to a more efficient organic PV surface. Scientists using Berkeley Lab's Advanced Light Source (ALS) have shown an alternate and possibly easier route forward.
Researchers led by North Carolina State University found that size matters for highly-efficient polymer/organic PV cells. The team showed that impure domains, made sufficiently small, can lead to improved performance in polymer-based organic PV cells. Organic PVs are of great interest as a potential source of renewable and economically viable electric energy.
Materials that blend polymer chains with organic fullerenes, processed from solution, have achieved power conversion efficiencies surpassing 8 percent, with the record being held by blends of PTB7 (a polymer) and PC71BM (a C70-based fullerene). The addition of a solvent, diiodooctane (DIO), increases efficiency, but the precise origin of the performance enhancement was not well understood.
Using ALS technology, thin films of PTB7:PC71BM made from chlorobenzene (CB) solution, with and without the addition of the solvent DIO were studied. The study showed that films without the solvent additive had a dominant acceptor domain size of about 177 nanometers (nm).
Berkeley Lab
Basic solar cell layered structure.
The addition of the solvent shrank the acceptor domain size to approximately 34 nm while preserving the film's composition and crystallinity and resulted in an efficiency gain of 42 percent. The results showed the DIO additive does not affect crystallinity or composition, but rather helps to better disperse fullerene in a "saturated" polymer/fullerene matrix.
Because the decrease in fullerene aggregate size increases the interfacial area between the phases, one explanation for the measured increase in photocurrent in the polymer is that direct access of the polymer chains to fullerene agglomerate facilitates efficient electron/hole charge separation.
The above molecular view of a polymer/fullerene solar film shows an interface between the acceptor and donor domains. Red dots are PC71BM molecules and blue lines represent PTB7 chains. Excitons are shown as yellow dots, purple dots are electrons and green dots represent holes.
Continuing, let's review improvements in single-structure backsheet development. Backsheet materials for PV modules serve several purposes such as providing electrical insulation, environmental protection and structural support. These functions are essential for modules to be safe for people working near them and for the structures to which they are attached.
The PV backsheet market has been growing at a compound annual growth rate of over 20 percent and will reach $1.37 billion in global market size by the end of this year.
Backsheets are usually comprised of a multisheet laminate and provide a range of functions including the following:
- Puncture and abrasion resistance
- Protection from moisture vapor ingress
- Electrical insulation
- Ultraviolet (UV) stability and moisture stability over the life of the module
- And can improve efficiency through optimized internal reflection
Key backsheet characteristics are peel strength, water vapor transmission rate (WVTR) and dielectric strength. Enlight BEC (back encapsulant composite) film developed by Dow Solar Solutions is a single structure (a 2-in-1 solution) that serves as both the back encapsulant and backsheet for rigid crystalline silicon (c-Si) solar panels.
The Enlight Back Encapsulant Composite films feature a three-layer coextruded polyolefin-based composite containing an advanced encapsulant layer integrated into a backsheet. The innovative coextrusion technology produces a smooth composite film. The resulting film has three seamlessly integrated layers: outer, bonding and encapsulant.
The coextrusion process creates excellent lamellar entanglement at the molecular level of the components. This eliminates the possibility of interlayer adhesion problems that can come with the more traditional lamination process of a separate back encapsulant and backsheet layers.
The film offers significant processing and performance advantages. It eliminates delamination, improves electrical insulation and offers higher levels of moisture resistance. High adhesion between the layers is maintained even after severe thermal treatment. The coextruded film can meet high voltage requirements.
Dow Solar Solutions
Enlight BEC (back encapsulant composite) backsheet film placement (left) and extruded laminate structure (right).
Finally, let's take a look at protective backsheet technology. TPNext developed by DuPont Photovoltaic Fluoromaterials is a proprietary laminate consisting of Tedlar polyvinyl fluoride (PVF) film, polyethylene terephthalate (PET) film and a unique extrusion coated tie-layer. The tie layer ensures adhesion between the backsheet materials and the EVA.
The adhesion has to survive initial adhesion measures such as humidity-free cycling, damp heat and thermal cycling. The technology is designed to enhance the quality and manufacturability of c-Si-based modules by:
DuPont
Layered structure of TPNext backsheet film.
- Improving adhesion to encapsulants
- Improving resistance to ultraviolet light
- Enabling faster throughput in the backsheet production process
The laminate technology:
- Reduces the use of organic solvent-based adhesives
- Delivers superior and consistent adhesion to EVA encapsulants
- Provides improved UV resistance
Taiflex Scientific has a exclusive global license agreement with Dupont for TPNext materials and backsheet manufacturing technology. It is offering its Solmate backsheet powered by the fully commercialized Dupont TPNext laminate and expects with the help of this technology to achieve 30 percent of the global solar cell backsheet market by the end of this year. Both Taiflex and Dupont will cooperate to market the product globally.
