PV Module Testing for Today’s Reliability Challenges

Backsheets are one of the most important factors in PV module reliability. As the final layer of a PV module, they are an extremely important shield against environmental conditions such as UV light, rain and snow.

Backsheets first captured the attention of risk-averse PV module buyers and investors because of low-cost polyamide-based backsheets that proved to fail after only a few years in the field. Most backsheets that are poorly constructed or that use problematic materials will show signs of degradation such as yellowing, chalking and cracking after just a few years. Chalking refers to the accumulation of a powdery, chalk-like substance on the backsheet.

The image above shows the construction of a PV module backsheet. Backsheets are typically made of a polyester core laminated between a layer of weather protective film and a layer of adhesion-promoting film. Each layer of the backsheet plays a role in PV module reliability.

Backsheet degradation rightly triggers panic for some asset owners. When moisture ingresses into a PV module due to backsheet cracks, important internal components such as soldered connections can degrade more quickly because they’re exposed to the elements. Environmental exposure and moisture ingress can also trigger electrical arcs in the modules, or ground fault trips in the inverters, and other safety and performance issues over time.

For certain backsheet materials, this type of mechanical degradation only begins to impact performance after the expiration of the ten-year workmanship warranty. But no matter when degradation begins, the end result is reduced energy yield and financial underperformance.

Manufacturers and developers alike are under cost pressure, so there is always risk that new failure-prone backsheet materials will come to market and trigger problems. Our new Backsheet Durability Sequence (BDS) helps buyers and investors avoid products with poor-performing materials. It can identify backsheets that cause reliability issues in the field.

About Our New Test

Our goal for the BDS was to replicate the yellowing, chalking and cracking failure modes that have been observed in the field – and on a commercially reasonable testing timeframe. Our R&D engineers, with support from partners and research institutes, turned years of field exposure into an accelerated test stream that detects poor quality backsheets.

• We apply UV exposure to the rear side of the module for a duration that is long enough to mimic field failures. Testing the entire module instead of a backsheet coupon alone provides more instructive results to PV module buyers.
• During our factory witness process, we obtain a sample of the backsheet for long-term storage. If a backsheet performance issue arises in the field, our sample makes it easy for the asset owner to confirm whether the backsheet material in production was made of the same materials that PVEL tested.
• Our test replicates failures in the field for many different backsheet materials. The data and results give developers, financiers and asset-owners the data they need to invest in reliable PV modules.

PVEL’s Backsheet Durability Sequence

It’s official: Light and elevated Temperature Degradation (LeTID) has replaced Potential Induced Degradation (PID) as the newest, most novel degradation form that investors and asset owners must avoid. PVEL has seen LeTID reaching rates as high as 5% in the field, which can completely wipe out the profits from some PV power plants. While PV modules do recover from LeTID over time, recovery rates depend on environmental conditions. Energy shortfall is likely to drive financial losses in the meantime.

The LeTID phenomenon is found in advanced cell architectures like PERC, which has a rapid growing market-share. Both multicrystalline and monocrystalline PERC cells are impacted by this form of degradation.

Unfortunately, PVsyst does not model LeTID degradation or recovery, which is a problem because the initial results from PVEL’s LeTID testing show that there are wide ranges of susceptibility. It is vital to obtain detailed information for each module type and Bill of Materials (BOM). PVEL’s new LeTID test generates the data necessary to understand this degradation form and to help buyers avoid LeTID- susceptible modules.

The figure above shows LeTID degradation rates for different PV module BOMs. Thus far, PVEL has observed rates as high as 6%. Some research institutes report rates of 10% for select PERC modules.

About Our New Test

Our LeTID test is aligned with the draft IEC technical specification, which our team is helping to develop. A low current is applied to the PV module throughout the test to ensure that LeTID is induced, not Boron-Oxygen Light Induced Degradation, which is traditionally referred to as “LID”. While the use of higher temperatures can cause degradation rates to increase, they also have the potential to introduce other failure mechanisms that mask LeTID susceptibility. The test conditions are designed to approach maximum degradation slowly. Our weekly characterizations ensure that LeTID observations will not be missed due to recovery.

PVEL’s LeTID Sensitivity Test

Our PV Module PQP has always included a test for microcrack susceptibility. Our former dynamic mechanical load (DML) sequence combined mechanical and environmental stresses – dynamic loading, temperature changes, and moisture – in order to stress PV module components and reveal sensitivity to solder joint fatigue and cell corrosion. The test is important because cracking and breaking of solder joints and cells are one of the leading causes of underperformance in fielded PV modules.

A PV module under stress from dynamic mechanical loading during MSS testing at PVEL. 

About Our New Test

While our former DML test was among the industry’s most rigorous when it was first introduced, it became clear over time that we could improve it by adding static mechanical loading at the beginning of the test. The new static mechanical loading step  in our Mechanical Stress Sequence (MSS) simulates field representative loads that cause cell and interconnect cracking.

Basically, the static loading creates microcracks in a susceptible PV module, which are then further stressed by dynamic mechanical loading. The thermal and humidity freeze cycles are designed to force these cracks to reduce power output, just as environmental stress can cause microcracks to reduce performance in the field.

Reviewing our historic data also lead us to eliminate two rounds of humidity freeze from the test sequence. No significant differentiation occurred from HF 10 to HF 30, as the poor performers can be identified from the first round of humidity freeze cycles alone.

PVEL’s Mechanical Stress Sequence

Thermal cycling replicates and accelerates the temperature fluctuations that all PV modules experience from day to night in the field. Understanding the impact of temperature on PV module materials is important because they expand and contract as temperatures change. Each of these components can have a different “thermal expansion coefficient.” Differing expansion rates create stress that can break the bonds between the layers of a PV module. Poorly constructed PV modules will degrade quickly in the field because of this thermal stress.

But exactly how much does one need to stress a module in the lab to identify poorly constructed modules? According to NREL, 600 thermal cycles represents:

• 80 years in Honolulu, HI
• >50 years in Golden, CO
• >30 years in Phoenix, AZ

Our years of data demonstrate that 600 cycles – that’s 84 days in a climate chamber – is enough. As shown in the chart below, the last 200 thermal cycles produced very little variation in power degradation. This shift from 800 to 600 cycles also aligns our PQP to the IEC 63209 cross-sectional committee, where MSS is also being introduced. (Members of the PVEL team actively contribute to this committee.)

This scatterplot shows high-performing modules have degradation rates that do not fall dramatically after TC 600. Low-performing modules continue to degrade after TC 600.

In addition to these major changes, our PQP includes several more updates that enhance the Program overall:

• New biannual cadence for field exposure reports will allow for more frequent data delivery
• Field exposure reports will include capacity testing results to facilitate BOM to BOM comparison, and two albedos for bifacial modules to measure the range of bifacial gain
• New stabilization step following damp heat testing to better quantify degradation
• Optional PVsyst validation comparing field exposure data with PAN, IAM and LID measurements to improve modeling confidence
• Connector resistance measurements before and after thermal cycling, damp heat and our new MSS test to help buyers identify inferior connector components
• Additional characterization and reporting enhancements.