Importance of Lamination Foil in Solar Module Degradation and Life

Ethyl Vinyl Acetate (EVA) film accounts for approximately 5% of module cost, but it is one of the critical Bill of Materials (BoM) for the longevity of modules without excessive yellowing, delamination, cracking, Potential Induced Degradation (PID) and snail trail.  Primary function of the EVAs is to provide mechanical support to cells and ensure maximum transmittance of incident sunlight to the cell over the lifetime of modules and to fulfill these functions, EVA or any other substitute of EVA is required to have properties given in the table below.

The reliability and durability of the module is dependent on its BoM, design and process control in manufacturing.  While it is important to have good quality of individual BoMs, it is equally important to ensure the compatibility of various BoMs.  It is because of this reason that if one material of BoM is changed then either a few or all tests in type test (IEC 61215) must be repeated.  Type test does not guarantee quality, performance over time and reliability of modules.  It only qualifies the design and assures that there will not be infant mortality provided modules are manufactured using a set of BoMs with proper process control.  It is also a fact that IEC certified modules also exhibit PID, glass breakage, delamination, finger and busbar corrosion and snail trails in the field.   It is because unlike laboratory tests in the field all the environment stresses like temperature, humidity, mechanical load, irradiation, voltage are present simultaneously in a complex manner.   Changes in the BoM may also contribute to internal stresses, and we would analyze how small changes in EVA can negatively impact the module reliability and performance. 

Optical Coupling

The usable solar spectrum for silicon cells is 290-1100 nm.  The cured EVA sheet is required to have high transmittance (>90%) in the wavelength range of 380-1100 nm.  However, the transmittance in 290-380 nm is lower.  To avoid reflection of light at the interface of two media the refractive index is required to be graded.  In solar module the light travels from air (Refractive index:1) to glass having refractive index of 1.5 and then it falls on solar cells having silicon nitride anti refractive coating of refractive index, 2.05.  Thus, for maximum light coupling the refractive index of lamination should be between 1.5-2.05.  The refractive index of EVA is in the range of 1.48-1.5.  Similarly, an alternative to EVA, Polyolefin Elastomer (POE), has a refractive index in the range of 1.49-1.51.  POE has matching transmittance in the 380-1100 nm for the thickness it is used in PV industry though its transmittance in the 290-380 nm ultraviolet range is lower than EVA.  The power in the UV region (280-400 nm) is about 4.5% of the total. 

Electrical Properties

The most important electrical properties for EVA are electrical resistivity and dielectric breakdown voltage.  Higher electrical resistivity is required for PID resistance.  PID occurs when there is a high electrical potential between the module frame and solar cells in the presence of high humidity and temperature, which generates leakage currents through the module packaging and drives cations (notably sodium) from the glass into the solar cells.  PID leads to enhanced recombination and shunt formation.  The PID effect can lead to significant power losses and is more severe on the negative pole of a string.  PID can be mitigated using high-volume resistivity encapsulants.

Dielectric breakdown voltage determines the gap between the cells and the frame.  The frame is at ground potential and the cells can be at system voltage i.e., 1000 or 1500 V.  The gap between the frame and module should be sufficient to withstand the voltage and hence high breakdown voltage of encapsulant is important.

Adhesive Strength

Good adhesion between glass, cells and bus bars is required for resistance to break and tear.  The encapsulant should provide good mechanical support.  In this respect, the Vinyl Acetate (VA) content of EVA is critical.  The VA content of EVA should be in the 28-33% range.  VA Content within this range ensures strong adhesion and improves durability.  Lower than 28% VA reduces the adhesive strength and more than 33% makes EVA more flexible, but also softer and stickier, which can create problems during lamination process, such as improper curing or handling difficulties. Higher VA content may increase the material’s tendency to absorb moisture, which may lead to electrical insulation issues and promote corrosion.  Softer EVA does not resist mechanical stress effectively in thermal cycling.  In this regard, thickness of EVA is also important.  Too thin EVA may not provide sufficient mechanical protection, leading to an increased risk of micro-cracks in the cells.  Excessive EVA thickness on the other hand can cause issues with the lamination process, potentially leading to uneven encapsulation or air bubbles, which may result in lower durability.  Higher thickness reduces the transmittance also.  Thinner EVA impacts electrical strength negatively.  Gel content, which reflects the cross-linking of EVA, during curing process is monitored for the optimum lamination.  If the gel content is too low, The EVA may remain soft and flexible, which reduces its ability to protect the solar cells from mechanical stress.   Gel content basically is an indicator of extent of cross linking.  Higher cross linking makes the polymer more rigid.  Since the extent of thermal expansion coefficient is dependent on the rigidity of the structure, poorly cured EVA will have higher coefficient of thermal expansion.  In such cases, under cyclic thermal load, the cells will contract and expand excessively leading to higher fatigue loading on the ribbons and mechanical failure may occur. This may lead to module warping or cell breakage, especially in harsh environments. The optimum gel content is in the range of 75-90%.  Gel content below 65-70% poor adhesion, increased moisture permeability leading to cell corrosion, delamination and reduced mechanical and electrical performance.  Peroxide compounds are added to EVA for cross linking and peroxide content is also critical for EVA performance.  Unused peroxide may lead to yellowing and long-term reliability issues. Gel content more than 90% is harmful too as it leads to brittleness, which makes module more prone to cracking under thermal and mechanical stress.  Keeping EVA film thickness constant, if the cell thickness varies, which may happen because of change of the cell supplier, the response of same EVA may be different.

Water Vapor Transport Rate (WVTR):

Crosslinking the copolymer chains (VA) during PV module lamination, the EVA sheet is transformed into a transparent, dimensionally stable encapsulation material. This crosslinking is initiated via a free radical reaction, using an organic peroxide or peroxycarboxylic acid as radical initiator (“crosslinker”).  One of the reasons of discoloration of EVA is due to interactions between the peroxide curing agent and the UV absorbers or phosphite (antioxidant) compounds.  EVA Yellowing rate is affected by Formulation, Processing, UV Filtering, and Air/moisture Permeability of Superstrate.  Under prolonged exposure to heat, UV light and moisture, EVA can degrade through a process called hydrolysis in which acetic acid is formed as byproduct.  This byproduct may lead to corrosion of fingers, busbars, delamination, yellowing.  For this reaction the presence of moisture is a must.  The moisture ingress takes place primarily from the backside and to a lesser extent from the edges.  Earlier the modules had a back sheet and its WVTR was higher than glass-glass modules of today.  Because of its higher WVTR it was easier for moisture to equilibrate with environment.  Moisture is used to ingress and egress easily from polymeric back sheets and hence acetic acid concentration does not build up within module.  Glass has lower permeability for moisture and hence moisture ingress from the back side is minimal, but ingress takes place from edges.  Once the acetic acid is formed, the glass-glass structure prevents its escape from modules and acetic acid concentration builds up.  In view of this, a different polymer chemistry is being tried in place of PVA.  Polyolefin Elastomers (POE) as an alternative laminate material for glass-glass modules is, therefore, developed.  POE has higher electrical resistivity and lower moisture permeability.   POE is more expensive than EVA and hence a combination of EVA and POE is used for glass-glass bifacial modules.  The argument in favour of using EVA is that glass has lower WVTR and hence formation of acetic acid is limited in glass-glass modules having EVA-POE combination.  Some module suppliers prefer to use the tried and tested EVAs for frontside encapsulation, and a single sheet of POE between the cells and the rear glass as a PID prevention strategy.   The POE lamination process takes place at lower temperature and pressure but for a longer duration.  It does not require cross linking.  Thus, glass-glass modules with EVA-POE combination of lamination in glass-glass modules easily pass the type test (IEC 61215).

Yet another defect observed in the field after a few months of operation in field is snail trail/track.  Snail Trails are a visible effect, and it is a discoloration of the silver paste of the front metallization of solar cells. In a PV module the effect looks like a Snail Track, and it occurs at the edge of the solar cell and along usually invisible small cell cracks. The discoloration itself is reported to have no influence on the performance of the PV module. But the visualized cell cracks can reduce the PV module power. The choice of EVA and back sheet type seems to be important for the Snail Track occurrence. It is hypothesized that water vapor coming through the back sheet dissolves silver particles which migrate into the encapsulation on top of the grid finger, where a chemical reaction within the encapsulation foil results in the typical observed coloring.

In one of the recent studies involving TOPCon and HJT modules in a Glass-Back sheet configuration, it is found that PERC modules remain stable, with a maximum power loss (Pmax) of only 1–2%rel after 1000 h of damp heat testing, regardless of the BOM used. However, TOPCon modules experienced significant degradation with a drop in Pmax ranging from 4 to 65%rel after the same testing duration. Despite utilizing POE, with a significantly lower water vapour transmission rate than EVA and devoid of acetic acid formation, a severe performance decline was observed in a specific POE variant within TOPCon modules, with a loss of power up to 65%rel.

Storage Conditions and Shelf Life

The shelf life of EVA is 6 months if it is stored at 0-30 deg.C and 60%RH condition.  POE is not very superior to EVA in this regard and has almost similar storage conditions and shelf life.

Thus, EVA plays a very critical role in the degradation and life of module.  One must be very careful in choosing the supplier as the chemical formulation plays an important role in the performance of modules in the field.  Apart from its chemistry, the process parameters of lamination, and combination with other module materials also impact the module performance in field.