As more industries continue to adopt alternative sources of green energy for commercial markets, there has been significant growth in next-generation, flexible solar panel technologies for a wide range of indoor and outdoor end uses. Flexible solar panels – or solar foils – tend to be less expensive, more lightweight, and easier to install than traditional, rigid solar panels, making them ideal for consumer electronics, outdoor advertisements, portable charging devices, and surfaces with curvature like buildings, boats, buses, camping gear, or other automotive applications. While research continues to improve efficiencies in the solar energy conversion and manufacturing processes for this new disruptive technology, polyester PEN film has been identified as an ideal solution for the encapsulation of solar cells. For flexible, thin-film solar cell designs, Kaladex® PEN film functions as a transparent, UV-resistant encapsulant with the heat resistance to withstand high-temperature, thin-film deposition processes in manufacturing. If you have any initial questions about Kaladex® PEN film from Tekra, contact our team today.
Thin-Film Solar Cell Design with Transparent PEN Film Encapsulation
One emerging, thin-film solar technology is the flexible, dye-sensitized solar cell (DSSC). The core components of a DSSC include two electrodes, typically a photoanode coated with photovoltaic material and a counter electrode, with an electrolyte between them. Together, these components catalyze a chemical reaction designed to mimic the process of photosynthesis by converting light to usable energy. The exterior side of each electrode works as a protective encapsulant that insulates the cell’s internal components from environmental factors such as moisture and long-term UV exposure. Many flexible solar panels, including DSSCs, feature bifacial designs that can convert light energy from either side and require both to be transparent.
As shown in the flexible DSSC design below (Figure 1), each electrode is made up of a UV-resistant PEN film encapsulant, a transparent conducting oxide (TCO) thin film, and a catalyst material. The transparent conducting oxide is a thin-film material that offers a low-resistance electrical contact between the catalyst and external circuitry which carries the generated electrical current. The front contact carries the produced electrical current out of the cell to an external receiver, while the back contact carries the current back into the cell to complete the circuit.
Figure 1. Dye-sensitized solar cell (DSSC) design featuring Kaladex® PEN film protective encapsulation
In addition to dye-sensitized solar cells (also known as Grätzel cells), PEN films can be used as a flexible replacement for glass encapsulants in many other thin-film solar technologies such as cadmium telluride (CdTe), perovskite, amorphous silicon, quantum dot, organic, and more. Due to their superior properties for harsh environment applications, PEN films are widely regarded as the more durable and effective option compared to PET film for flexible solar panels.
Ideal UV-Resistance Properties for Optimized Longevity and Performance
Solar panels primarily use visible light for energy conversion, which features wavelengths ranging from 380 to 700 nanometers on the electromagnetic spectrum. However, a major concern in flexible solar panel designs is longevity due to the long-term exposure to UV radiation from sunlight. Polyester PET film, a common low-cost encapsulant material in solar cells, will yellow and crack over time after prolonged UV exposure as it fails to block the most harmful UVA rays on the spectrum (315 – 400 nm). This compromises the film’s barrier properties and decreases light transmission to the cell, shortening the longevity of the solar panel. However, Kaladex® PEN film inherently screens non-visible UV radiation below 380 nm, allowing solar cells to capture the full potential of the visible light spectrum for energy conversion while minimizing the adverse effects of long-term sun exposure.
Figure 2. Kaladex® PEN films are an ideal, transparent encapsulant for flexible, thin-film solar cells, allowing the full spectrum of visible light through for optimal energy conversion while blocking harmful UV rays to maximize cells’ longevity.
Exceptional Moisture Barrier Properties to Protect Against the Elements
Another important property impacting longevity for a solar cell’s encapsulation is moisture resistance. Solar panels must withstand the demanding elements of their outdoor environment, whether it be rain, sleet, or snow. With a moisture permeability of just 4 g/m2/24 hr. (ASTM E398 test method), Kaladex® PEN films offer exceptional moisture barrier properties outperforming polyester PET, polyimide, and even specialized BOPP films. This results in excellent durability without the need for fluoropolymer encapsulants like PVF or PTFE that are far more expensive and harmful to the environment.
Heat Resistance to Withstand High-Temperature, Thin-Film Deposition Processes
The manufacturing of thin-film solar cells typically involves a deposition or “sputtering” process to coat the encapsulating substrate with a thin film of transparent conducting oxide such as ITO or ZnO which functions as a low-resistance electrical contact. The high temperatures required for many deposition techniques often present challenges for standard polyester PET films. Polyester PEN films like Kaladex® 2000 or Kaladex® 2000L offer superior heat resistance properties at demanding processing temperatures. Kaladex® PEN films have a glass transition temperature (Tg) of 121°C and a melting temperature (Tm) of 265°C, compared to 78°C and 254°C for PET films respectively. Kaladex® PEN films also offer improved dimensional stability and low shrinkage at elevated temperatures, as shown in Figure 3 below:
Figure 3. Kaladex® 2000L has lower shrinkage properties than Kaladex® 2000 for improved dimensional stability at elevated processing temperatures.
Excellent Mechanical, Chemical, and Dielectric Properties for Flexible, Thin-Film Solar Panels
Kaladex® PEN films have superb tensile strength and stiffness (Young’s modulus) properties for improved processing and stability over standard PET films, with the additional benefit of greater environmental protection for harsh environments in outdoor end uses. Additionally, PEN films have good dielectric properties that act as an insulator for solar panels’ internal circuitry, including a higher breakdown voltage than PET films. These durable materials also have excellent resistance to chemicals, such as those found in the cell’s electrolyte or household cleaners.
Tekra Offers Custom Converting of Kaladex® PEN Films for Solar Panel Applications
Tekra is U.S.-based, authorized distributor and converter of Kaladex® PEN films from Mylar® Specialty Films (formerly DuPont Teijin Films™), with the ability to slit or sheet large rolls to custom sizes and set up custom stocking solutions for our customers. The two most common Kaladex® PEN products offered by Tekra are Kaladex® 2000 standard PEN film and Kaladex® 2000L low shrinkage PEN film for applications requiring even greater dimensional stability at high processing temperatures.
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