Entire books have been devoted to the niche discipline of polymeric photovoltaic solar cells, also known as organic solar cells. It is not the intent of this section to cover the expansive research in this area but to provide the reader with an overview of the application’s current feasibility and limitations as it relates to polymeric packaging.
Like CPV, polymeric photovoltaics are a cost-reduction technology. Specifically, the manufacturing costs can be reduced by eliminating inorganic chemistry, sourced from precious Earth metals, and using polymers, mainly sourced from petroleum by-products.
Polymeric substrates are flexible and can be formed into a number of different geometries, increasing the commercial applications for PV. If polymer packaging is used, the modules are also flexible. Polymeric photovoltaics have been integrated into automobile components, such as the hoods and roofs, and personal apparel, including backpacks and blankets. The former has public-sector applications, while the latter has military applications.
The use of polymers eliminates the need for Restriction of Hazardous Substances (RoHS) and Waste Electrical and Electronic Equipment (WEEE) exemption discussed in Chapter 2. Polymers can be easily formulated to exclude these restricted inorganic elements and brominated flame retardants, because these chemicals are not part of the inherent chemistry of the polymer chains. Instead, they are small molecular additives placed in the commercial formulations. Formulation of compliant materials simply requires manufacturers to avoid restricted additives. Additive substitution is a challenge for polymer manufacturers, but it is not an impossibility. In contrast, certain semiconductor chemistries used for PV cells require restricted elements in their structure. These restricted substances cannot be excluded without completely reformulating the cell chemistry.
The polymeric PV module is a multilayered structure. The specific chemistry used in each layer is proprietary, but there are some materials that have been commonly used in various academic and commercial pursuits. The superstrate cover is either glass or a transparent polymer depending on if the module needs to be rigid or flexible, respectively. The next layer is indium tin oxide (ITO), a cathode layer. A polymer insulates the cathode from the active layer, commonly composed of poly-3,4-ethylenedioxythio- phene-polystyrenesulfonate (PEDOT-PSS). The photoactive substrate contains a combination of electron donors, highly conjugated polymers, and electron acceptors, dopant molecules (e. g., nanomaterials). The photoactive layer is sandwiched with another insulating layer, commonly sodium fluoride, and an anode. The anode is commonly an opaque metal (e. g., aluminum, silver, or gold).
There are a number of technical limitations for polymeric photovoltaics. Most importantly, the efficiency (2% to 7.9%) is two to five times lower than commercial inorganic formulations. This lower efficiency requires a larger surface area to get the same amount of power. Even with reduced costs, this new technology commonly cannot be competitive in high-power applications, such as residential and commercial installations.
There have been significant reliability issues due to the polymeric packaging used for polymeric solar cells. Their performance will decrease exponentially in the presence of water and oxygen. Some devices will not operate longer than a few hours when exposed to the air. In effect, polymeric solar cells have more stringent water vapor transmission rate (WVTR) and oxygen transmission rate (OTR) specifications (10-7 to 10-6 g/m2/day, 10-7 to 10-3 cm3/ m2/day) than food packaging (10-1 to 10[10] g/m2/day, 10-1 to 103cm3/m2/day) and thin-film PV cell applications (10-4 to 10-3 g/m2/day, 10-4 to 10-3 cm3/m2/day).
Packaging options are limited because polymer manufacturers have focused on providing packaging for inorganic PV cells, which constitute a larger segment of the photovoltaic market.
These requirements are not insurmountable; light-emitting diodes (LED) have similar requirements. Historically, LED technology has used silicones for encapsulation; however, new commercial laminate structures are a cheaper alternative. These laminates are constructed from a transparent layer of aluminum oxide (Al2O3) or silicon oxide (SiOx) sandwiched between thermoplastic polymers. Rollprint sells polyethylene terephthalate-coated aluminum oxide and silicon oxide films under the trade name ClearFoil®. The addition of inorganic layers improves the permeant barrier characteristics by dropping the ingress rates by at least an order of magnitude. For example, ClearFoil® exhibits a lower WVTR and OTR (0.025 to 1.55 g/m[11] 1 [12]/ day, 0.062 to 0.62 cm[13]/m2/day) than a laminate of polyethylene terephtha – late and polyethylene (4.65 g/m2/day, 69.77 cm3/m2/day) [28]. Some of these films have been commercialized specifically for organic solar cell applications. As an example, Ceramis® is a multilayered polylactic acid (PLA), silicon oxide film sold by Alcan Packaging (Asheville, North Carolina).
Even though these transmission rates are still above the specification, these improvements have translated into longer service lives when metallized foils are used for encapsulation. Lungenschmied and coworkers extended their polymeric solar cell service life from 6 hours when packaged in polyethylene terephthalate to 6000 hours when packaged with metallized polyethylene naphthalate [29]. However, how these barrier properties age due to weathering remains unknown.
Because polymeric solar cells remain in their developmental infancy, chemical and material scientists will need to develop a new class of polymers and compounds to make this a viable commercial option. In April 2010, 3M announced that they were working on a next-generation product composed of polyethylene terephthalate and polyethylene naphthalate that would specifically address the 10-6 g/m2/day WVTR specification [30].
