PID Effect-"Invisible Efficiency Killer" of PV Modules

I. Basic Definition and Classification

1. Core Concept

PID, short for **Potential Induced Degradation**, refers to a phenomenon where the performance of solar cells deteriorates continuously due to ion migration triggered by the potential difference between internal materials of PV modules under long-term high-voltage bias. First systematically proposed by SunPower in 2005, it is a core issue affecting the long-term reliability of PV modules.

2. Main Types (Classified by Degradation Mechanism)

II. Core Mechanism: Ion Migration is the Key

The essence of the PID effect is **electric field-driven ion migration and leakage current formation**, with the complete process as follows:

1. Establishment of Potential Difference：After the module frame is grounded, a high voltage difference (usually 1000-1500V systems) is formed between solar cells and the frame.

2. Formation of Conductive Channels：- Moisture absorption by encapsulation materials (e.g., EVA) in high-humidity environments (or condensation) forms a conductive medium required for ion migration.

3. Ion Migration：Under negative bias, mobile ions such as **sodium ions (Na⁺)** in glass and encapsulation materials migrate toward the cell surface.

4. Performance Degradation：Sodium ions accumulate on the surface of the cell's anti-reflection layer (SiNₓ), damaging the passivation effect and resulting in:

   • Decreased parallel resistance and increased leakage current

   • Reduction in fill factor (FF), open-circuit voltage (Voc), and short-circuit current (Isc)

   • Ultimately, significant attenuation of the module's maximum power (Pmax)

III. Key Influencing Factors (Three Essential Conditions for Occurrence)

1. Voltage Conditions(Necessary Condition):The higher the negative bias voltage of the module relative to the ground, the more significant the PID effect; generally, the more modules in series (higher system voltage), the greater the risk.

2. **Environmental Conditions(Accelerating Condition):

   • Humidity: Ion migration rate increases sharply when relative humidity exceeds 60%

   • Temperature: The PID rate approximately doubles for every 10°C increase; attenuation is fastest at 85°C

3. Module-Specific Factors:

   • Encapsulation Materials: Early EVA is prone to moisture absorption with poor PID resistance; POE materials perform better

   • Glass Type: Ordinary soda-lime glass contains a large amount of sodium ions; low-sodium glass can reduce risks

   • Cell Type: p-type cells are more sensitive to PID, while n-type cells (e.g., TOPCon, HJT) have stronger PID resistance

   • Encapsulation Process: Lamination quality and edge sealing directly affect moisture intrusion

IV. Hazards and Impacts

1. Power Generation Loss**: 10%-30% module power attenuation directly impacts the power plant's IRR (Internal Rate of Return)

2. Service Life Reduction: Long-term PID effect may cause irreversible damage such as hot spots and microcracks in modules

3. Increased Operation and Maintenance Costs: Additional resources need to be invested in testing, repair, and even module replacement

4. Safety Risks: Increased leakage current may cause local overheating of modules, posing fire hazards

V. Testing Methods and Standards

1. Laboratory Testing (Factory Verification)

• IEC TS 62804-1 标准: Specifies three testing methods to evaluate the PID resistance of modules

  • Dark-State Testing: Apply negative bias voltage (typically -1000V) to modules and maintain them in an 85℃/85%RH environment for 96 hours

  • Light-State Testing: Simulate actual operating conditions to evaluate PID effect under illumination

• Power Attenuation Criterion: A Pmax attenuation rate ≤ 5% before and after testing is considered qualified, and ≤ 2% is excellent

2. On-Site Testing (Power Plant Operation and Maintenance)

1. EL (Electroluminescence) Testing: EL images of PID-affected modules showblack spots/streaks，corresponding to leakage current areas

2. IV Curve Testing: Compare the actual Pmax of modules with the nominal value to calculate the attenuation rate

3. Leakage Current Monitoring: Install leakage current sensors to real-time monitor the leakage current of modules to the ground (normal value < 10μA)

4. Thermal Imaging Testing: Severe PID areas generate local overheating due to leakage current, which can be detected as abnormal hot spots via thermal imaging

VI. Solutions and Prevention and Control Strategies (Three-Level Prevention and Control System)

1. Module-Level Prevention and Control (Source Treatment)

• Adoptlow-sodium glassOptimize cell passivation processes to enhance surface resistance to ion contaminationPID-resistant encapsulation materials(e.g., POE/EVA blend films)

• Optimize cell passivation processes to enhance surface resistance to ion contamination

• Addmoisture-proof sealing layers at module edges to reduce moisture intrusion

• Conduct PID preconditioning(e.g., positive bias aging) before factory shipment to activate PID resistance performance

2. System-Level Prevention and Control (Design Optimization)

3. Operation and Maintenance-Level Prevention and Control (Long-Term Guarantee)

1. Regular Testing**: Conduct combined EL + IV + thermal imaging testing every 6-12 months to identify early-stage PID modules

2. Environmental Monitoring: Deploy temperature and humidity sensors in power plants and strengthen inspections during high-risk periods (plum rain season, summer)

3. Timely Repair: Use restorers to treat lightly affected PID modules and replace severely attenuated modules promptly

4. Cleaning and Maintenance: Regularly clean module surfaces to reduce local potential differences caused by dust accumulation

VII. Application of Machine Vision in PID Detection

As a manufacturer of photovoltaic inspection equipment, Shichuang Intelligence can provide the following PID-specific inspection solutions:

1. AI-EL Automatic Detection System**: Automatically identify PID features (black spots/streaks) with high-resolution EL imaging and deep learning algorithms, achieving a detection accuracy of over 99%

2. Online PID Monitoring Equipment: Integrate leakage current sensors and thermal imaging modules to real-time monitor module PID status and provide early warnings

3. PID Repair Effect Verification System: Compare EL images and IV curves before and after repair to quantify repair effectiveness (power recovery rate)

VIII. Industry Trends and Conclusion

1. Technological Evolution: Thanks to structural advantages, n-type cells (TOPCon, HJT) have significantly better PID resistance than p-type cells and have become the mainstream in the future

2. Standard Upgrade: IEC 62804-1:2025 adds light-state testing methods, which are more in line with actual operating conditions

3. Cost Optimization: The cost of PID-resistant materials decreases year by year and has become a standard configuration for modules (market share > 90% in 2025)