Solar Power World

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Paul Dvorak, Contributing Editor to Solarpower Engineering

Grid parity–that’s the idea of getting solar power down to the cost of conventionally generated power. It’s happening thanks to recent thin films aimed at the production of PV panels.

PV production in a nutshell: Solar modules are secured together in a vacuum lamination process. It starts with about 36 octagon modules wired into a panel-sized array and sandwiched between a backsheet (on the bottom) for structural rigidity, an encapsulant atop the modules, and a frontsheet, often glass, on top of that. Workers place the sandwich of materials into the laminator which applies a vacuum to remove air. The laminator then heats the components to melt the encapsulant which secures everything in place. The entire process takes 12 to 30 minutes depending on encapsulant. The machines process several panels at one time. Upon their exit, a frame and junction box complete the panel.

The DOE says 2008 averages for U.S. electricity were 10.3¢/kWh for commercial facilities and about 11.4¢/kWh for residences. Some analysts say power from recent PV (photovoltaic) panels approaches these levels. Getting below the figures, however, will take further developments that may be in labs now.

Until those discoveries are commercialized, a better way to take cost out of solar power is by improving manufacturing efficiency. Material manufacturers have several ideas that focus on thin films. In a nutshell, these can speed PV panel production, solve some of its problems (wrinkling, blistering), and take weight out of panels to trim handling and labor costs. Here’s what several material suppliers are rolling out of their production facilities.

Think transmissibility
Any material used on PV panels had better tolerate the weather and resist UV discoloring. The question is, for how long can it do so? The manufacturer of one melt processable fluoropolymer says it provides superior weatherability and UV resistance. Manufacturer St. Gobain (www.saint-gobain.com) adds that it works well on flexible and lightweight solar modules. These transparent yet tough films can provide high resistance to chemicals and weathering, low flammability, resistance to stress cracking resistance, and good insulating properties.

The company’s thin-film product for PV manufacture is called LightSwitch and encompasses a front sheet and encapsulant. A frontsheet film is a surface treatment that provides adhesion to its accompanying Encapsulant film product. The two materials along with a backsheet provide a sandwich that secures the octagon solar modules and their wiring.

The Encapsulant film cushions and supports solar cells and circuitry, while maximizing transmission of sunlight for energy conversion. Good weathering properties are said to protect solar modules throughout their lifecycle. The Encapsulant film works well on flexible and rigid modules, providing good adhesion to other PV module components.

Efficiency increase of ETFE relative to glass: St. Gobain says its ETFE frontsheets are about 1.5 to 2% more transmissible than glass. A higher transmissibility rating means more photons get through to the solar cells, making them more productive.

The developer says these fluoropolymer films provide an alternative to conventional materials, mostly glass, that lets module manufacturers significantly increase production efficiencies and reduce overall system costs. “For instance, glass top sheets on a panel must be cut to size and carefully stored and handled for production,” says Saint Gobain Product Manager for Photovoltaics Nikhil Bhiwankar. “The more recent films, however, are delivered and stored on rolls which are easier to handle than glass.”

Glass is used in about 90% of panels manufactured today. Bhiwankar says swapping the glass for LightSwitch thin films pays off other ways as well. “For instance, glass has to be cut to precise dimensions after manufacturing and then tempered, which may change the dimensions a bit. The film, however, will be on a roll and unwound as needed right next to the laminator.”

Also, the glass frontsheet and its encapsulant are two separate products. “We can improve on that by combining the frontsheet and encapsulant films in one product, called FrontSheet Complete, so workers need not layup the two separately. Moreover, because there is no glass in the process, there is no loss due to breakage and handling. So production speeds up,” he adds. Lastly, because of the significantly lower weight of film over glass, shipping costs are significantly lower and labor on the job site is less strenuous.

Building better backsheets

Backsheets provide a cushion and structural rigidity to the sandwich of frontsheets and modules. Most backsheets are composites in about three layers which improve the performance of PV modules. They fail most often by cracking, discoloring, and delaminating.

Backsheet protect solar cells and their connections from the environment, provide electrical insulation, and contribute to aesthetics. The sheets must provide these properties for the life of the modules, which today can exceed 25 years.

One backsheet material, polyvinyl fluoride (PVF) films, comes layered on top and bottom of a PET sheet, and provides long-term durability in all-weather conditions, says manufacturer DuPont. The company adds that the material, Tedlar, is sufficiently reliable and durable to last the projected quarter century. This results in lower-cost photovoltaic power and improved returns for investments in solar energy projects.

Of course, a broad range of properties must be considered when designing a PV module. Testing backsheet materials for the stability of its properties under accelerated conditions is important for predicting long-term reliability. One challenge is to understand the correlation between the performance of the materials and the performance of the module. Studies like this can guide choices for construction materials to ensure long-term module performance.

The company has also released its Kapton polyimide films engineered for several thin films and flexible photovoltaic substrates. The company has developed products for Amorphous Silicon (a-Si, often associated with thin-film or flexible PV) modules and Copper Indium Gallium Selenide (CIGS) photovoltaic applications. “The films provide a combination of electrical, thermal, chemical and mechanical properties that withstand extreme temperatures and other demanding environments,” says Dupont’s Robert Schmidt.

Recent versions of Kapton include:

The test results are for several backsheet compositions. The blocks at the top list materials in several backsheets. A sheet of PVF on either side of a sheet of PET seems to perform best.

•PV9101 polyimide (50 μm) film for ease of manufacturing and mechanical performance. This 50-um thick and thicker films are easier to handle in a manufacturing process than are thinner films.

•PV9102 polyimide (38 μm) film for ease of manufacturing and increased productivity. Because this film is thinner, its cost per area is lower than previous films, which is interpreted as improved productivity.

•PV9103 polyimide film will be available later this year for maximum productivity. This 25-μm thick film is not as easy to process compared to the thicker films. But as a thinner film, it provides maximum productivity (lowest costs).

Paste on the left measures about 25 microns high and 79 wide for an aspect ratio of 0.32. Print-on-Print pastes on the right provide an aspect ratio of 0.475. Repeatable accuracy is key to the higher aspect ratio because the metallization grid is printed onto the solar cell twice. The slightest misalignment can result in wider features and a reduction in cell efficiency.

In both thin-film a-Si modules and CIGS applications, mechanical properties and dimensional stability of the substrates at deposition temperatures to 400°C are critical to producing cells with maximum efficiency and yields. The low-coefficient thermal expansion, high glass-transition temperature, and low shrinkage of Kapton polyimide films is said to help minimize stress at the interface with other construction materials, during processing, and at high environmental temperature in end use.

In CIGS manufacturing, higher temperatures result in higher “cell conversion efficiencies”, a constant goal for PV manufacturers. CIGS manufacturing temperatures range from 350 to over 550°C, while polymeric film substrates max out at around 500°C.

Tapes hold modules still

A lot of manual labor goes into a solar panel which makes the alignment of solar components before laminating absolutely critical. Shifting silicon wafers can break contacts and waste production time and material.

Conventional liquid adhesives provided some assistance but they dripped or oozed at the wrong time, or have complex cure cycles. One adhesive manufacturer has two ideas that it says will improve products and throughput.

CTE, DMA, Tg, and modulus, and TGA Isothermal weight loss are performed in a nitrogen environment.

One, a holding tape, provides a pressure-sensitive adhesive (PSA) option that help maintain material quality and process control during panel production. For example, MACtac Specialty Products’ says its Solarhold cell holding tape provides a high bond strength to many surfaces, in particular photovoltaic cells and EVA. Applied in strips or in sized pieces as speced by a particular manufacturing process, the tape holds silicon-cells in alignment until an ethylene vinyl acetate (EVA) film melts to encapsulate them. MACtac (www.mactac.com/technical) says these transparent adhesive tapes will not bubble, blister, discolor, or affect the cells’ performance during the melting or evacuation steps.

A worker applies MACtac Solarold tape by taking each piece off the supply roll and placing it on adjacent PV wafers. The precut pressure-sensitive adhesive eliminates variability that would come from each workers cutting pieces differently.

“Instead of just supplying rolls of tape, we die cut the tape to a particular shape on the roll, making it factory friendly,” says MACtac Solar Business Development Manager Steve Dominak. “This lets a user pick off the small square, an put it on the application. There are no additional steps that could damage the adhesive.”

About 75% of the crystalline cells are manufactured in Asia. “Previously, some manufactures there used traditional wire-insulation tapes to hold modules in place. It was wrong for the application. It blistered and yellowed at times because it was not rated for the high temperatures they were using it at,” says Dominak.

MACtac’s holding tape has a high-temperature acrylic-based pressure-sensitive adhesive on a 2.0-mil PET face. The Solarhold line comes with a kraft release liner. These quick-tack adhesives show strong resistance to movement and offer good continued use under aging and extreme temperatures, ranging from -40 to 350°F.

The Solarfast UV Cure Adhesive includes SF-1003, a 3.0-mil free-film adhesive, and SF-1005, a 5.0-mil version. Both are protected by a two sided, brown-kraft liner. The free-film adhesive initially bonds similarly to a typical pressure-sensitive adhesive, but the final bond is initiated through UV light.

“The adhesive, used mostly in roll-to-roll manufact-uring operations, does not have to stop under a UV lamp. As it moves under the UV light, molecules cross link transforming it from a thermoplastic to a thermoset material,” says Dominak. Once cured, the adhesive has extreme heat resistance, low creep, and excellent resistance to peel.

Tested and shown to resist more than 400°C while maintaining a secure bond, the adhesive on the new high-strength solar bonding tapes is an substitute for liquid or complex cure adhesives and simplifies manufacturing.

A brief glossary of thin-film terms

These terms are often encountered in discussions of thin films in solar-cell manufacturing.

A-Si: amorphous Silicon – a solar cell material that deposits relatively easily on thin films.

CTE: Coefficient of Thermal Expansion – thermal expansion is the tendency of a material to increase in volume in response to an increase in temperature. The expansion dimension divided by the change in temperature is the material’s coefficient of thermal expansion. It generally varies with temperature.

CIGS: Copper Indium Gallium Selenide – solar-cell material used in wafers, the blue octagons common to solar panels.

DMA: Dynamic Mechanical Analysis – a technique used to study and characterize materials, in particular the viscoelastic behavior of polymers. When applying a sinusoidal stress, measuring its strain allows determining the complex modulus. The approach can locate the glass transition temperature of a material and identify transitions corresponding to other molecular motions.

EVA: ethylene vinyl acetate – a thin film used in encapsulants, the sheet just under the front sheet and over the solar-cell wafers.

PET: polyethylene terephthalate – a good barrier against oxygen and carbon dioxide. One application is in the composite of backsheets.

PVF: polyvinyl fluoride – a thin film polymer often found in backsheets.

TGA: Thermo Gravimetric Analysis – testing that determines changes in weight in relation to change in temperature. Because many weight-loss curves look similar, they may require transformation before interpreting results. For instance, a derivative weight-loss curve can identify a point where weight loss is most apparent.

Tg: Glass Transition Temperature – in polymers, it is often the temperature at which a so-called Gibbs free-energy value exceeds the activation energy for the cooperative movement of 50 or so polymer elements. This lets molecular chains slide past each other when applying a force, which reduces the stiffness of thermoplastics. When a material reaches its glass temperature, its stiffness holds steady until its melt temperature. This region is called the rubber plateau.

UV: Ultra Violet – wavelengths of light shorter visible light, about 10 to 400 nm, and shorter than x-rays.