Potential Induced Degradation (PID): an introduction

In recent years, news about severe quality issues with PV panels have become more and more in quantity. There are increasingly often reports about “mysterious” output yield losses which are linked to potential induced degradation (PID).

Even though this problem has been known for years, only in recent years there has been more research about the causes and mitigation possibilities for PID-affected panels.

Similarly, new testing methods as well as the drafting of international PID testing standards have increasingly drawn attention in the PV industry. This article introduces the background of PID and explores related impact factors and ways to reduce PID.

What is Potential Induced Degradation?

Potential Induced Degradation (PID) refers to the phenomenon of power output losses from a solar PV module. It results from the leakage of electrical current from the cell of a solar PV module to the panel frame.

This drives negative ions to migrate away from the semiconductor to other components of the module – such as glass and frame – while positive ions migrate toward the semiconductor from those components.

This ion migration impacts the electrical characteristics of the solar cell and leads to a power output degradation.

What are the main PID impact factors?

Basically, the factors making PID possible exist on all solar modules and systems, yet it depends on a range of (unforeseeable) factors leading to a module either being affected by PID or not.

The main factors accelerating PID are environmental factors, material factors and system factors.

Environmental factors that may cause PID are humidity and temperature the solar panels are exposed to.

Increasing temperature and humidity can accelerate the output performance degradation of a solar panel and there is not much – except for a deliberate choice of system installation location – a PV system operator can do about this.

Material component factors relate to the quality and material property of certain cell, glass, encapsulation types etc. potentially leading to PID.

Solar glass: research has indicated that Quartz glass is much less susceptible to PID than soda-lime (SL) glass with its high concentration of sodium which is a highly mobile ion in SiO2.

The effects of sodium as a material component factor are linked the moisture the SL glass is exposed to, causing it to leach the sodium.

PID can thus potentially be reduced by using glass types with low/ no sodium ingredient concentration.

Encapsulants: in terms of encapsulant material property and quality moisture is an important related impact factor as well.

As moisture increases conductivity, those panels laminated with encapsulants with higher moisture permeability properties are much more prone to PID, such as for example Ethylene Vinyl Acetate (EVA) and Polyvinyl Butyral (PVB).

To reduce the potential of PID, manufacturers need to use highly impermeable encapsulant material.

Solar Cell: the anti-reflective (AR) coating on a solar cell helps to increase the amount of light absorbed into the cell, which results in higher currents.

However, AR coating has been determined as an either weak or strong potential impact factor causing PID – depending on the material used.

The widely used silicon nitride (SiNx) AR coating for example is such a PID accelerating cell factor as its layers can accumulate highly mobile sodium ions.

System factors refer to PID impact factor related to the grounding in a system design.

The whole system voltage which is determined by the quantity of solar PV modules put in a string, sun irradiation and temperature.

Improper grounding leading to negative cell bias is a significant PID impact factor that can be avoided by proper grounding.

PID signs and troubleshooting

PIDs are in most cases indicated by abrupt yield losses. PID-spurred reduction in shunt resistance (Rsh) causes the Maximum Power Point (MPP) and the Open Circuit Voltage (VOC) to decrease.

To detect PIDs in PV system, standard voltmeters can be used to measure and compare the VOCs of modules from opposite ends of an array.

There can be either irreversible PIDs in form of damaging electrochemical reaction or reversible PIDs in form of polarization effects where static charges accumulate on the surface of the cells. The latter effect can be soothed by reversing the potential voltage applied.

PID testing and standards

So far there are no industry-wide standards to test solar panels on PID, so manufacturers presenting PID “certifications” offer in fact non-standard test reports from testing labs/ bodies which have different methods of PID testing.

The problem of defining a global and reliable PID standard lies in the lack of real-world simulation programs/ machines allowing to make exact predictions about the performance of a module in a certain environment.

In the framework of PID testing, the procedures of many labs involve comparative visual inspection, I-V measurement and EL testing prior to and after the actual PID testing.

Based on a currently discussed IEC 62804 standard, the PID testing itself involves a test run of 96 hours with the tested panel’s applied voltage corresponding to the maximum system voltage (as per panel datasheet) and exposed to a chamber environment with temperature and humidity levels of 60°C (± 2°C tolerance) and 85% (± 5% tolerance) respectively.

After having run the test, the modules must not show any major defects and shall have no power losses greater than 5%.

Conclusion

Even though PID neither be fully controlled nor curtailed, there are multiple ways to reduce the danger of PID to a solar module.

While environmental factors are hard to control, the application of proper material (e.g. glass with low/ no sodium concentration, impermeable encapsulant etc.), proper grounding of the module as well as PID testing of the module before shipment can reduce that risk significantly.

It remains to be seen what effects new PID standards and certifications thereof will have on the real-world performance and longevity of solar panels.

Sources:

1) http://solarenergy.advanced-energy.com/upload/File/White_Papers/ENG-PID-270-01%20web.pdf
2) http://www.solon.com/export/sites/default/solonse.com/_downloads/global/article-pid/Pingel_et_al_PID.pdf