French researchers have found that increasing PV system voltage significantly worsens potential-induced degradation (PID), with encapsulant conductivity playing a key role in module performance. They also identified a new degradation mechanism involving partial discharges and electrical treeing, highlighting additional reliability risks for medium-voltage PV modules.
Researchers at the Liten research institute of the French Alternative Energies and Atomic Energy Commission (CEA) have investigated how increasing PV system voltage affects potential-induced degradation (PID) in solar modules, focusing on the relationship between encapsulant electrical conductivity and module performance.
The scientists said this is the first study to investigate PID in passivated emitter and rear cell (PERC) solar modules at voltages as high as 6,000 V and to identify partial-discharge-induced electrical treeing as an additional degradation mechanism in medium-voltage PV modules. “PERC modules were chosen even though they are progressively being phased out; this cell technology has been well studied and is therefore a logical choice for investigating medium-voltage degradation phenomena,” they explained.
Their analysis considered four encapsulant materials – ethylene-vinyl acetate (EVA), polyolefin elastomer (POE), thermoplastic polyolefin (TPO), and ionomer – under controlled electric field, temperature, and humidity conditions. For the tests, they used 20 cm × 20 cm glass-glass PV mini-modules, each containing a half-cut bifacial PERC cell.
Conductivity was measured using a parallel-plate electrode configuration with gold-coated polymer samples, applying voltages of up to 3,000 V through alternating two-hour polarization and depolarization cycles. The researchers conducted two experimental campaigns. The first examined PID at 1,500 V, 3,000 V, and 6,000 V using nine TPO-based modules, while the second compared the PID resistance of the four encapsulants at 3,000 V. All modules underwent accelerated aging for 96 hours at 85 C and 85% relative humidity.
Current-voltage (I-V) measurements under standard test conditions were performed before and after aging to quantify electrical performance losses on both sides of the bifacial modules. Electroluminescence (EL), photoluminescence (PL), and external quantum efficiency (EQE) measurements were used to characterize degradation mechanisms.
The analysis showed that encapsulant electrical conductivity strongly depends on electric field and temperature, significantly influencing PID. For example, TPO conductivity increased with applied voltage, while raising the temperature from approximately 20 C to 80 C increased conductivity by two orders of magnitude. Further PID testing revealed progressively greater power losses with increasing voltage magnitude, particularly on the rear side of the modules. At 6,000 V, maximum power losses reached 13.5% on the front side and 19.5% on the rear side.
The researchers also found that front-side degradation was primarily associated with sodium-ion migration, while rear-side degradation was mainly attributed to charge accumulation and polarization. Following testing at 6,000 V, rear-side power losses increased to 26% during dark storage but fell to 9.8% after illumination, indicating partial recovery.
A comparison of the four encapsulants at 3,000 V revealed a strong correlation between electrical conductivity and degradation severity. TPO exhibited the highest rear-side power loss at 23.6%, followed by EVA at 21.7%, ionomer at 13.0%, and POE at 4.6%. Power losses increased approximately linearly with the logarithm of conductivity, at rates of 1.8 and 4.9 percentage points per tenfold increase in conductivity on the front and rear sides, respectively.
POE demonstrated the greatest resistance to PID, although residual fill-factor losses suggested possible metallization-related degradation. EL and PL imaging confirmed the electrical performance trends. The researchers also observed partial-discharge-induced electrical treeing exclusively in ionomer-based modules at 3,000 V, particularly near metallic cell ribbons and glass-encapsulant interfaces. They identified this phenomenon as a new PID degradation mechanism, referred to as PID-discharge (PID-d).
The authors attributed this phenomenon to locally enhanced electric fields associated with reduced encapsulant thickness, ribbon geometry, and possible insulation weaknesses. The discharges appeared to cause localized cavities and delamination, potentially compromising long-term electrical insulation.
Overall, the findings highlight the importance of low-conductivity encapsulants, adequate insulation thickness, and optimized module design for mitigating PID and partial-discharge risks in medium-voltage PV systems. “This study demonstrates that moving toward higher system voltages is not without risks for PV modules; however, these risks can already be effectively quantified and identified through appropriate laboratory testing and material qualification,” the researchers concluded.
Their findings are presented in the study “Degradation of photovoltaic modules at medium voltage with different types of encapsulants,” published in Solar Energy.
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