Nashua, NH — Solar photovoltaic (PV) technology is a recipe of highly engineered materials and components, sandwiched with specific functions and working together to harness sunlight and convert it into electricity. One of those layers is the encapsulation film which protects the solar cell and ensures its performance and reliability; its role is to provide optical and electrical transmissivity and keep out moisture.
Ethylene vinyl acetate (EVA) has been the solar module encapsulant material of choice since the 1980s. It is as field-proven as any solar PV technology component.
EVA still commands the vast majority of solar module encapsulation today: it has a proven track record and is a low-cost option, notes Alison Bennett, global technology manager for PV encapsulants at DuPont. But the emergence of both thin-film and high-efficiency cells means a greater need for an alternative to EVA. As the market expands for more thin-film and higher-efficiency crystalline silicon solar cells, so too will the usage of alternative encapsulant materials.
Time and technology march on, and EVA’s shortcomings have been revealed after years of evaluation. Additive materials to improve EVA’s crosslinking and adhesion properties can generate “free radicals” which contribute to physical deterioration and degradation of properties, starting with yellowing or discolouration. Among these is generation of acetic acid.
This acid might not be fatal to most silicon-based solar modules, which will technically still function even if the metal becomes coated. For thin-film technology, in particular copper-indium-gallium-(di)selenide (CIGS), however, any acid generated would be terminal. “Companies that are making CIGS modules are using alternative encapsulants to EVA,” said Fatima Toor, lead analyst for solar components at Lux Research.
Also encountering problems with EVA encapsulation materials are newer high-efficiency cells, which make more use of shorter wavelengths of light. Typically encapsulant materials have included UV blockers to protect them, but “that didn’t matter because the cell couldn’t use that light,” explained Bennett. But as new cell designs utilise more of the light spectrum, those UV blockers need to be removed, and “for EVA that can create a real problem.”
“Due to the inherent structure of the polymer, EVA does not have superior moisture resistance and electrical properties desired to protect the solar cell over the 25-year service life of the module,” summed up Brij Sinha, global strategic market manager for photovoltaic films at the Dow Chemical Company. “People are looking for something other than EVA.”
Ionomer materials have been employed as encapsulants in niche applications for several years. Chemicals and materials giant DuPont, which was already selling ionomer resin, began hearing about interest from PV module makers who wanted more varied encapsulant options, according to Bennett.
Ionomers are inherently more transmissive than EVA, and they’re also more adhesive to the other materials in the module stack (glass and the cell itself).
Another key performance benefit is in minimising potential induced degradation (PID), a well-known cause of solar panel failure. According to DuPont, its ionomers were “at least 25 times more effective” than standard EVA encapsulants in preventing PID. The company cited “no measurable power degradation” observed after 200 hours of accelerated durability testing, while EVA coated modules “lost significant power after only eight hours of testing.”
Dow Chemical, meanwhile, arrived at a similar conclusion but took another route. It began commercialising polyolefin-based encapsulant film in late 2010. That choice, explained Sinha, was due to the inherent stability of the polymer, superior moisture resistance, and significantly better electrical properties.
The company’s own figures show improved reliability of modules made with its polyolefin-based PV specific Enlight films vs EVA, under accelerated test conditions. It says its encapsulant offers the best volume resistivity – significantly better than ionomers, and up to 1000 times better than EVA. Higher volume resistivity means lower leakage current and longer-term performance. Like DuPont, Dow also promotes PID characteristics, which it too claims is “significantly better” than EVA. “Polyolefin based encapsulant films have superior moisture barrier and electrical insulation properties, enabling them to better protect the solar cell and the module, and allowing the module to generate more power over its service life,” Sinha said.
Comparing the Options
Lux Research’s Toor breaks down the numbers from her research:
- Volume resistivity: Both DuPont’s ionomer and Dow’s polyolefin (2 x 10-16 Ω-cm) possess two orders-of-magnitude better resistivity than EVA. This directly translates to improved potential induced degradation (PID) susceptibility of the PV modules. “1 x 1016 Ω-cm for polyolefin vs 2-2.5 x 1016 Ω-cm for ionomer is not dramatically different,” notes Toor.
- Moisture vapour transmission rate (MVTR): Both DuPont’s ionomer (0.3 g/m²/day) and Dow’s polyolefin (0.22 g/m²/day) are orders-of-magnitude improvements over EVA (3-10 g/m²/day).
- Sunlight absorbance: Ionomer edges polyolefin (1.49 refractive index vs 1.48). Ionomers have the best spectrum transmission, letting in most blue light. Polyolefins have good spectrum emission as well, though, and similar solar absorbance to EVA.
Polyolefins have a similar material density to EVA and are cost-competitive with it, points out Toor. There’s no acid formation, and no crosslinking (and subsequent free-radical formation). They also have the best MVTR of EVA alternatives, though ionomers are fairly close behind, and both are far better than EVA. And polyolefins offer the best volume resistivity (Ω/cm) of any of the encapsulation materials.
In terms of performance in the field, ionomers arguably get the nod between the two alternatives. DuPont points to recent data from Sandia Labs, showing ‘minimal’ power loss from a mounted module after 18 years of continuous service, and what it says is performance exceeding all typical power warranties offered today.
Reframing the cost conversation
Making a switch in any component or material is not a simple decision. Costs, manufacturing complexity (also boiled down to cost) and proven performance are also deciding factors.
EVA has a long track record, even with the above mentioned questions about long-term reliability. “Standard crystalline silicon module manufacturers are unlikely to switch away from EVA in the short term, since their lamination process and equipment is designed for EVA, and EVA has performed adequately in-field at an attractive $/W price,” admits Toor. It might be up to CIGS modules (specifically glass-glass CIGS module configurations) to give the alternative encapsulant materials their market stepping-stone.
But what really has to happen – and this is in all areas of solar PV, not just encapsulants – is a change in the conversation from purely low cost vs output ($/W) to a more performance-based metric ($/kWh), explains Toor. This trend resonates among project developers, and increasingly among Tier 1 module manufacturers such as Trina, Yingli and SunPower, she points out. A module can be packaged with low $/W materials, but that doesn’t guarantee that it will maintain its performance through its warranty period, say 80 percent performance over 20 years. A high $/W module packaging material, however, with more robust characteristics (MVTR and volume resistivity) will support those longer lifetimes, and that means lower $/kWh.
“The main driving force for quality-conscious module manufacturers is the reduction in PID, lower yellowing index, and improved moisture protection in-field,” Toor says. Both ionomers and polyolefins provide that benefit over EVA. Neither have the large-scale field-proven reliability behind them that EVA has, but they do have a low $/kWh roadmap. “Proving bankability and reputation of the new technology will take time in the PV industry,” she says.
Lead image courtesy DuPont