September 7th, 2016, Published in Articles: EE Publishers, Articles: Energize, Articles: Vector, Featured: Energize

by Mike Rycroft, features editor, EE Publishers

 

Modern solar PV equipment is designed for reliable operation over the full lifetime of the product. In spite of this manufacturing defects and premature failures still occur which can affect the performance of a product.

Reliability and quality are designed and built in to modern solar PV equipment. Mass production techniques, although controlled, and poor quality control can still introduce manufacturing defects into the product, and field installation as well as transport can result in damage, all of which can shorten the lifespan of products.

One key factor of reducing the costs of photovoltaic systems is to increase the reliability and the service life time of the PV modules. Today’s statistics show degradation rates of the rated power for crystalline silicon PV modules of 0,8%/year [1]. Although modern products are designed to make use of higher quality materials and mechanised manufacture, price competition has resulted in thinner and less material being used in the manufacture of panels. In addition there is evidence that some manufactures have reverted to using lower quality materials to lower prices.

Premature failure of panels can have a major financial implication for PV installations, as the major life cycle cost is capital. A PV module failure is an effect that either degrades the module power which is not reversed by normal operation or creates a safety issue.

A purely cosmetic issue which has neither of these consequences is not considered as a PV module failure. A PV module failure is relevant for the warranty when it occurs under conditions the module normally experiences [1].

Typically failures of products are divided into the following three categories:

• Infant failures
• Midlife failures
• Wear-out failures

Fig. 1 shows examples for these three types of failures for PV modules. Besides these module failures, many PV modules show light-induced power degradation (LID) immediately after installation. The LID is a failure type which occurs anyhow and the rated power printed on the PV module’s label is usually adjusted by the expected standardised saturated power loss due to this failure.

LID: Light-induced degradation
PID: Potential induced degradation
EVA: Ethylene vinyl acetate
J-box: Junction box

Fault and failure occurrence

Detailed studies of in-service failure over the full lifetime of panels are not available as most installations are recent, and suppliers are reluctant to release such figures. Reports of infant mortality studies, i.e. failure on installation, give figures between 1 and 2% of all panels installed [3]. Several simulation studies with accelerated lifetimes have been undertaken, but on a limited number of panels.

BP Solar has reported a failure rate of 0,13% over an eight year period for Solarex c-Si panels and Sandia National Laboratories has predicted a failure rate of 0,05% per annum based on field data [4]. However these are short term early life figures and no figures on late life failures for large scale installations are available.

Major defects and failures

Failures can be divided into performance and safety related failure types. Safety related failures could result in damage to property or injury to personnel. Performance related failures result in a loss or drop in output power.

Defects occur in the following areas:

• The wafers or cells in crystalline PV products
• The encapsulation
• The glass base
• Internal wiring
• Frame and fittings
• The amorphous layers in amorphous PV

Wafer or cell faults

Deterioration of the efficiency of the cell is normal over the life of the cell and is not regarded as a fault or failure unless the rate of degradation exceeds the normal limits. The majority of wafer or cells faults will be cracking of the wafer and damage to connections and conductors. Smaller faults arise from anti-reflective coating (ARC) damage and cell corrosion. Light induced degradation in amorphous solar panels is a known effect and is not necessarily regarded as a failure. Potential induced degradation is a new phenomenon which has appeared as a result of increasingly higher voltages used in PV systems.

Anti-reflective coating delamination

An anti-reflective coating (ARC) increases the capture of light and, therefore, increases module power conversion. ARC delamination occurs when the anti-reflective coating comes off the cell’s silicon surface. This is not a serious defect unless there is a lot of delamination [2]. Research has shown ARC properties to be a causative factor in PID.

Cell cracking

Cracks in PV modules are ubiquitous. They may develop in different stages of the module’s lifetime.

During manufacturing in particular, soldering induces high stresses into the cells. Handling and vibrations in transport can induce or expand cracks [4]. Finally, a module in the field experiences mechanical loads due to wind (pressure and vibrations) and snow (pressure).

Micro-cracks may be caused or aggravated by:

• Manufacture
• Transport
• Installation
• In-service stress (thermal and otherwise)

Crystalline wafers have increased in size and decreased in thickness over the years, increasing the potential for breakage and cracking. Cracks in solar cells are a genuine problem for PV modules as they are hard to avoid and, up to now, basically impossible to quantify in their impact on the efficiency of the module during its lifetime. In particular, the presence of micro cracks may have only a marginal effect on the power of a new module, as long as the different parts of the cells are still electrically connected.

As the module ages and is subjected to thermal and mechanical stresses, cracks may be introduced. A repeated relative movement of the cracked cell parts may result in a complete separation, thus resulting in inactive cell parts. For this special case a clear assessment of the power loss is possible. For a 60 cell, 230 W PV module the loss of cell parts is acceptable as long the lost part is smaller than 8% of the cell area [3].

Micro-cracks are cracks in the silicon substrate of the PV cells which can often not be seen by the naked eye. Cracks can form in different lengths and orientation in a solar cell. The wafer slicing, cell production stringing and the embedding process during the production process causes cell cracks in the photovoltaic cells. The stringing process of the solar cells has an especially high risk for introducing cracks [1].

There are three different sources of micro-cracks during production; each has its own occurrence probability:

• Cracks starting from the cell interconnect ribbon are caused by the residual stress induced by the soldering process. These cracks are frequently located at the end or starting-point of the connector, because there is the highest residual stress. This crack type is the most frequent.
• The so called cross crack, which is caused by machinery pressing on the wafer during production.
• Cracks starting from the edge of the cell are caused by the cell impacting against a hard object.

Once cell cracks are present in a solar module, there is an increased risk that during operation of the solar module short cell cracks can develop into longer and wider cracks. This is because of mechanical stress caused by wind or snow load and thermo mechanical stress on the solar modules due to temperature variations caused by passing clouds and variations in weather.

Micro-cracks may have various origins and result in rather “soft” outcomes such as yield-reducing shattering of parts of the affected cell up to more severe impacts involving decreases of the short circuit current and cell efficiency. Visually, micro-cracks may appear in form of so called “snail trails” on the cell structure. However, snail trails – as a long-term impact sign – can also be the result of chemical process causing the surface of the cell to change and/or hot spots.

Depending on the crack pattern of the larger cracks, the thermal, mechanical stress, and humidity may lead to “dead” or “inactive” cell parts that cause a loss of power output from the affected photovoltaic cell. A dead or inactive cell part means that this particular part of the photovoltaic cell no longer contributes to the total power output of the solar module. When this dead or inactive part of the photovoltaic cell is greater than 8% of the total cell area, it will lead to a power loss roughly linearly increasing with the inactive cell area [1].

Cracks potentially grow over a longer operational time and thus extend their malicious impact on the functionality and performance of a PV module, potentially triggering hot spots as well. Undetected, micro-cracks can result in a less than expected field lifespan. They differ in size, location on the cell and impact quality.

Micro-cracks can be detected in the field before installation and over the lifetime of a project. There are different quality testing methods to identify micro cracks of which electroluminescence (EL) or electroluminescence crack detection (ELCD) testing is one of the most applied method. EL testing can detect hidden defects that were before untraceable by other testing methods, such as infrared (IR) imaging with thermal cameras, V-A characteristic and flash testing [1]. Some manufacturers recommend regular inspection of installed panels over the lifetime [3].

Encapsulation faults

A solar panel is a “sandwich”, made up of different layers of materials (Fig. 3).

Encapsulating materials are used to:

• Resist heat, humidity, UV radiation and thermal cycling
• Provide good adhesion
• Optically couple glass to cells
• Electrically isolate components
• Control, reduce, or eliminate moisture ingress

The most common material used for encapsulation is ethaline vinyl acetate (EVA). Failure of the encapsulant can result in failure or deterioration of the PV module.

Adhesion failure

The adhesion between the glass, encapsulant, active layers, and back layers can be compromised for many reasons. Thin-film and other types of PV technology may also contain a transparent conductive oxide (TCO) or similar layer that may delaminate from an adjacent glass layer.

Typically, if the adhesion is compromised because of contamination (e.g. improper cleaning of the glass) or environmental factors, delamination will occur, followed by moisture ingress and corrosion. Delamination at interfaces within the optical path will result in optical reflection (e.g., up to 4%, power loss, at a single air/polymer interface) and subsequent loss of current (power) from the modules [1].

Acetic acid production

EVA sheets react with the moisture to form acetic acid that speeds up the corrosion process of the inner component of PV module components. This can also result from EVA aging process, and can attack silver contacts and affect cell production. For permeable back-sheets, this is not a problem because the acetic acid can escape. However, for impermeable back-sheets, this defect can cause substantial power losses over time.

Encapsulant discolouring

This will result in some loss of transmission and therefore reduced power. The discolouring is due to bleaching oxygen, so with a breathable back-sheet the centre of cells discolour while outside rings remain clear. This can occur due to poor crosslinking and/or additives in the EVA formulation.

Without concentration it takes five to ten years to see discoloration and longer to start appreciably reducing output power. It is not the EVA itself that discolours, but additives in the formulation. This defect can prevent some light from reaching the panel [5].

Delamination

Delamination is the separation of the encapsulant from the glass or cell. Delamination can be between superstrate (glass), substrate (back-sheet) and encapsulant or between encapsulant and cells. Delamination from the front glass can occur due to poor EVA adhesion or poor glass cleaning procedures during the fabrication process. This defect can prevent some light from reaching the panel. The problem can become more serious if humidity accumulates in the void and creates short circuits near the solder wires.

Delamination from the cell is most likely caused by poor cross-linking or contamination of the cell surface. This defect can be serious because when an air bubble is created in the laminate, there is the possibility for humidity accumulation and short circuits. Delamination from the insert occurs if the EVA did not adhere well to the insert during fabrication.

The new pathways and subsequent corrosion following delamination reduce module performance, but do not automatically pose a safety issue. The delamination of the back sheet, however, may enable the possibility of exposure to active electrical components. When a module is constructed with glass front- and back-sheets, there may be additional stresses enhancing delamination and/or glass breakage.

Back-sheet defects

The back-sheet of a module serves to both protect electronic components from direct exposure to the environment and to provide safe operation in the presence of high DC voltages. Back-sheets may be composed of glass, or polymers, and may incorporate a metal foil.

Most commonly, a back-sheet is made up of a laminate structure with a highly stable and UV resistant polymer, often a fluoropolymer on the outside, directly exposed to the environment, an inner layer of PET, followed by the encapsulant layer [1].

When a rear glass is used instead of a back-sheet, it may fail by breaking. If the module is constructed as a thin-film device on the back-sheet (substrate CIGS), then this presents a significant safety hazard in addition to significant or, more likely, complete power loss for that module. There may be a small gap along the cracks and some voltage which is capable of producing and sustaining an electric arc.

If this happens in conjunction with failure of a bypass diode, the entire system voltage could be present across the gap creating a large and sustained arc which is likely to melt glass, possibly starting a fire. However, if a glass back-sheet were to break in a typical crystalline Si module, there would still be a layer of encapsulant to provide a small measure of electrical isolation.

Delamination from the EVA can occur due to poor adhesion between the EVA and the back-sheet or if the adhesion layer of the back-sheet is damaged by UV exposure or a temperature increase.

Front side yellowing is caused by a degradation of the polymer used to promote the adhesion of the specific back-sheet to the encapsulant. Yellowing is often associated with worsening mechanical properties. With this defect, it is likely that the back-sheet might eventually delaminate and/or crack [3].

Air-side yellowing is a sign of UV sensitivity that can be accelerated by high temperatures. This defect also occurs in some back-sheets as a result of thermal degradation. Yellowing is often associated with worsening mechanical properties. With this defect, it is likely that the back-sheet might eventually delaminate and/or crack [3].

Hot spots

Hot-spot heating occurs in a module when its operating current exceeds the reduced short-circuit current (I_sc) of a shadowed or faulty cell or group of cells. When such a condition occurs, the affected cell or group of cells is forced into reverse bias and must dissipate power.

If the power dissipation is high enough or localised enough, the reverse biased cell can overheat resulting in melting of solder and/or silicon and deterioration of the encapsulant and back-sheet [5].

Conductor ribbon and joint failures

Solar cells are equipped with two basic elements, the front and the rear contacts, allowing delivery of current to the external circuit. Current is carried by buss strips that are soldered to the front and back contacts. A failure of the string ribbon is associated with loss of output power. Interconnection breaks occur as a result of thermal expansion and contraction or repeated mechanical stress. Moreover, thicker ribbon or kinks in ribbon contribute to breaking of interconnections, and result in short-circuited cells and open-circuited cells.

A critical part of the module is the solder joint interconnections. They consist of many materials bonded together including the solder, bus-bar, ribbon and the silicon wafer. These materials possess different thermal and mechanical properties. In bonding, the assembly develop thermo-mechanical reliability issues which are caused by differences in the bonded materials’ coefficient of thermal expansion. The solder provides a connection between the electrode and ribbon.

The PV module temperature varies according to local weather which in turn affects the rate of solder interconnection degradation. In a lifetime prediction modelling analysis it was reported that for the same type of c-Si PV modules located in various weather conditions, lifetime was shortest in a desert followed by those in the tropics.

Although the use of soldering process in the assembly of solar cells in PV modules has the advantage of yielding products which possess high reliability at minimal production cost, the technology occurs at high temperature with inherent potential to produce shear stress in the silicon wafer. Failure and degradation of solder joints causes an increase in series resistance, which leads to loss of power.

Module lifetimes

All of the above faults contribute to the degradation and ultimate failure of PV panels. PV modules are designed to last for 20 years or more, and new modules undergo accelerated test programmes that simulate the effects of heat, humidity, temperature cycling, UV radiation and other factors [5]. The results of test programs conducted by Kohl are shown in Fig. 7 [7].

A normalised power level of 0,8 is usually taken as end of life for a PV panel. It can be seen from the test curves that the panels deteriorate rapidly after this point.

In the early 1990s, ten year warranties were typical. Today, almost all manufacturers offer 20 to 25 year warranties. But a 25 year warranty does not mean the project is protected. One needs to ask the following questions:

• Will the module supplier be around in 15 years when problems are encountered?
• Does the supplier fund an escrow account to ensure that if it is gone, the project will be protected?
• Does the supplier simply rely on IEC qualification tests to make claims about long-term durability?
• If the supplier has only been around for five years, how can it claim that the modules last for 25 years?

The increase in length of warranties is promising, but an investor or developer must carefully review the company providing it [4].

References

[1]    IEA: “Review of Failures of Photovoltaic Modules”, Task 13 external final report, IEA-PVPS, March 2014.
[2]    Dupont: “A guide to understanding solar panel defects: from fabrication to fielded modules”, www.dupont.com
[3]    M Kontges, et al: “Crack statistics of crystalline photovoltaic modules”, 26th European Photovoltaic Solar Energy Conference and Exhibition, 2011.
[4]    E Fitz: “The bottom line impact of PV module reliability”, Renewable Energy World, March 2011.
[5]    J Wolgemuth et al: “Failure modes of crystalline Si modules”, PV Module Reliability Workshop 2010.
[6]    M Zarmai: “A review of interconnection technologies for improved crystalline silicon solar cell photovoltaic module assembly”, Applied Energy, 2015.
[7]    M Koehl et al: PV reliability (Cluster II): Results of a German four-year joint project – Part I, results accelerated ageing tests and modelling of degradation, 25th EU-PVSEC, 2010.

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