PID (Potential Induced Degradation)

PID Mechanism

1. High voltage between cells and grounded frame.
2. Humidity + heat enable ion migration through encapsulant.
3. Ions accumulate at cell-glass interface.
4. Shunt resistance decreases → Pmax drops.
5. Voc drops; FF drops.

Prevention

• PID-free / PID-resistant modules: certified per IEC 62804.
• Recovery systems: nightly reverse-voltage application (some inverters).
• Grounding strategy: PID-N (negative ground), the specific bonding and grounding scheme is what actually gets documented on the module frame bonding detail and the project’s electrical drawing set.
• Encapsulant quality: high-quality EVA, POE backsheets.

Detection

• IR thermography during operation.
• I-V curve tracing.
• Power output monitoring (sudden drops).

Why String Position Matters

PID is not evenly distributed across a plant, it concentrates wherever the voltage-to-ground stress is highest, which in a long series string is almost always at the bottom of the string (for negatively grounded systems) or the top (for positively grounded systems). A 1500V central-inverter design puts far more modules at high voltage-to-ground than a string-inverter layout at 1000V, which is one reason PID risk is a factor in string-sizing decisions and not just a module-datasheet checkbox. Humid, hot climates accelerate the ion-migration mechanism described above, so the same module and string length that shows negligible PID in a dry climate can lose meaningful output within a couple of operating seasons in a coastal or monsoon-heavy site, this is why field monitoring during the first year, not just the module’s IEC 62804 certificate, is what actually confirms whether PID is under control on a given plant.

Key Takeaways

• PID = leakage-current-driven module degradation.
• Can cause 20-50% power loss in months on unmanaged systems.
• TOPCon and HJT more PID-resistant than PERC.
• Specify PID-resistant modules per IEC 62804.
• Use PID-recovery systems on long strings in humid/hot climates.

Related Reading

PID losses that go undetected in year one show up later as a gap between modeled and actual generation, which is exactly the risk a bankable yield report is built to price in, see P50 / P90 / P99 in Solar Yield Reports for how uncertainty from module-level effects like PID factors into the exceedance-probability numbers lenders rely on. Because PID risk is highest in long, high-voltage strings, it’s worth reviewing alongside the degradation rate assumptions used in the underlying yield simulation, since both feed the same 25-year output forecast. On the electrical side, the grounding scheme chosen to mitigate PID (PID-N or PID-P) has to be documented consistently with the plant’s module frame bonding and grounding details, which is where our Electrical CEIG Drawings work overlaps with module-quality decisions made earlier in procurement.