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Lamination Key to Module Reliability

Author:ByMike Nowlan, Mark Willingham, Spire Corporation, Date:2016-7-8 10:29:15
Approaches used to address lamination process bottlenecks include using larger area laminators, multiple parallel laminators, stack laminators (multiple lamination chambers in a vertical stack), and multi-stage laminators, in which the process steps are split into different processing units.
Photovoltaic (PV) modules need to withstand the rigors of outdoor exposure in all kinds of climates for long periods - 25 years or more - to convert sunlight to electricity at a reasonable cost. One of the keys to module longevity is the lamination process, which encapsulates solar cells while attaching front and back protective sheets. The materials, process technology, and equipment described in this article have been proven by over 20 years of actual field experience for both crystalline silicon and thin film modules.
Ethylene vinyl acetate (EVA) based sheet materials have been the industry standard encapsulants since the 1980s, although there have been significant improvements in these materials over time. The base EVA is combined with a number of additives in the sheet extrusion process, including curing agents, UV stabilizers, anti-oxidants, and primers for glass adhesion. The additive package comprises a few percent of the final material. Early problems with EVA yellowing under combined UV and heat exposure have been resolved by replacing the original curing agent with a more stable alternative, while faster reacting curing agents have been developed to reduce process cycle times and increase the throughput capacity of lamination equipment.
Thermoplastic encapsulants such as polyvinyl butyral (PVB) and thermoplastic polyurethane (TPU) are also available in sheet form. While these materials do not require curing, their melting points and viscosities are higher than EVA, so the lamination process times are generally similar to EVA.
Because the encapsulants are in sheet form, a module laminate can be easily assembled in layers for processing in a vacuum laminator. The most common module construction uses tempered low-iron glass as the transparent front structural member, or superstrate, followed by a layer of EVA, interconnected solar cells, another layer of EVA, and a UV-stable plastic film as the back sheet, as shown in Fig. 1. An optional thin non-woven fiberglass sheet can be placed behind the cells to aid in air removal and to prevent cell motion when the EVA melts and flows during lamination.
Other module designs include double-glass, which use glass for both the front and back sheets, and flexible, which use flexible films for the front and back sheets. The former design is popular for thin film applications (CdTe and CIGS) that need an excellent moisture barrier to prevent film degradation, while the latter design is used where light weight portable modules are needed, and for roofing applications, where the roof provides the module's structural support.
Lamination process
The lamination process involves pumping the air out of the module layers in a vacuum chamber, heating the layers to melt the encapsulant, and pressing the layers together with a silicone rubber diaphragm to embed the cells in encapsulant and adhere the front and back sheets. EVA must be subjected to a temperature/time profile to obtain a minimum cure level of 80% for long term module reliability. The lamination process is qualified by performing visual inspections for voids, bubbles, back sheet wrinkles, and other defects; peel force measurements to determine the adhesion of the encapsulant to the various layers in the laminate; and gel content tests to measure the amount of EVA cross-linking. Gel content tests are done by extracting cured EVA from a sample module, weighing it, and soaking it in hot solvent (toluene) to dissolve the non-cured fraction of the material. Less accurate but quicker gel test methods include thermal creep measurements and differential scanning calorimetry (DSC).
 
Module laminators consist of a large area heated metal platen in a vacuum chamber. The top of the vacuum chamber opens for loading and unloading modules. A silicone rubber diaphragm is attached to the top of the chamber, and a set of valves allows the space above the silicone rubber diaphragm to be evacuated during the initial pump step and backfilled with room air during the press step. A pin lift mechanism is sometimes used to lift modules above the heated platen during the initial pump step, but most standard modules (including the design shown in Fig. 1) don't require it.
Platen temperature uniformities of ±5°C at the lamination temperature are sufficient for obtaining good laminations with acceptable gel content and adhesion across the module. While more uniform temperatures are available from some laminator suppliers, there is no real benefit to the module manufacturer. Note that the uniformity specification refers to unloaded (empty) platens. Once a room temperature module is placed on the platen, its thermal mass disturbs the platen temperature and it no longer meets this specification.
Active platen cooling systems are optional on some laminators. This option can be useful for special module designs to minimize bow or prevent edge delamination, but again, it is not needed for standard modules. Laminator throughput decreases for processes with active cooling, since additional time is needed to cool down and heat up the platen. On the other hand, active module cooling by convection or conduction on laminator output conveyors does not slow down the lamination process, and is useful to reduce work in process in high throughput production lines. That's because modules exit the laminator at elevated temperatures, typically 150°C to 160°C, and need to cool down to about 45°C or less before further processing and testing can be done.
Laminators are available with two types of cover opening systems: clamshell and vertical post. In the clamshell design, the cover is mounted on a hinge at the back of the laminator, which opens like the hood of a car. This leaves the laminator wide open on three sides, making it easy for an operator to load and unload modules manually. Automated belt-fed laminators, on the other hand, use the vertical post method, which lifts the cover horizontally above the process chamber. Because the cover does not need to travel much for belt loading, the chamber opening and closing times are reduced. As a result, most high throughput module lines use belt-fed laminators with vertical cover lifts, as shown in Fig. 2.
Process bottlenecks
The lamination process can be a bottleneck in module lines with throughputs of 50MW/year or more. Several approaches are used to address this issue, including larger area laminators, multiple parallel laminators, stack laminators (multiple lamination chambers in a vertical stack), and multi-stage laminators, in which the process steps are split into different processing units. In addition, the laminator can be used to do a lower temperature lamination process, which takes about one third the time of a full lamination plus cure process. In this case, curing is done after lamination in an in-line or batch process in a convection oven. The development of new faster curing and/or lower temperature curing encapsulant materials could have a significant impact on process throughput.
Conclusion
Module lamination is a key process step that directly impacts module reliability and lifetime, as it provides the weather barrier that protects solar cells from the environment. Sheet encapsulants allow for simple assembly of a variety of module designs (glass superstrate, double glass, and flexible), while providing good encapsulant thickness control with little material waste. Process control measures, such as peel force measurements and gel content tests, are critical to maintaining module quality in production. While a number of strategies may be used to alleviate bottlenecks in the lamination process for high volume production, the PV industry would benefit from the availability of faster curing encapsulants.