PID degradation photovoltaics: what is it & how is it detected?

Potential-induced degradation – PID for short – is a gradual power decline in photovoltaic systems that goes unnoticed for years and can cause up to 30% yield loss in affected systems. The good news: PID is reversible in early stages and reliably detectable with thermography. This article explains the causes, the damage pattern and the detection methods.

What is PID?

Potential-induced degradation (PID) denotes a power decline in crystalline silicon solar modules caused by electrical leakage currents between the semiconductor and the module frame. These leakage currents arise from the electrical potential between cells and frame – hence the name.

In practice PID primarily affects modules mounted near the negative pole of a string, where the potential relative to earth is most negative. For certain system configurations, particularly systems without a transformer, the risk is considerably increased.

Wie entsteht PID?

The mechanism is complex but can be simplified as follows: sodium ions migrate from the glass of the module structure into the anti-reflection coating of the solar cells and affect the charge carrier recombination. The result is a reduced short-circuit voltage and a reduced fill factor – both lead to measurable power loss.

PID is promoted by high system voltages (typical for larger strings), high humidity and high temperatures. Systems in humid climates – such as near the coast or in northern Germany – are therefore more frequently affected than systems in drier regions.

Damage pattern and yield loss

PID does not affect systems uniformly but shows a characteristic pattern: the most severely affected modules are at the end of the negative string half and show considerably more severe degradation than the positive ones. In the thermogram a gradually increasing temperature pattern from the string centre towards the negative end becomes visible.

The yield loss can be 20–30% in advanced cases – often without the operator knowing, because the loss develops gradually over years and is not identifiable in the monitoring without a comparison baseline. A 200 kWp system with 20% PID-related yield loss loses several thousand euros in revenue annually.

Wie wird PID erkannt?

The three common detection methods have different strengths and limitations:

Monitoring analysis: String monitoring can show anomalies but PID-related yield drops are often confused with normal degradation or other causes. A decline in the performance ratio over several years can be an indication but is not definitive.

Elektrolumineszenz (EL-Messung): EL measurement is particularly well suited to making PID visible directly at cell level. Affected cells appear darker in the EL image. Disadvantage: the system must be shut down and the measurement is time-consuming per module.

Thermographic inspection: Drone thermography detects PID as a characteristic temperature pattern across the entire string. The advantage over EL: full-coverage capture during live operation. Thermography is the fastest method for confirming suspected PID and identifying affected strings.

Thermografie als PID-Diagnose

In the infrared image PID shows a characteristic pattern that experienced thermographers immediately recognise: the modules at the negative end of a string show an elevated temperature that decreases towards the string start (positive end). The pattern is gradual and clearly distinct from local hotspots or bypass diode failures.

A standards-compliant inspection to IEC TS 62446-3 classifies PID patterns and indicates which strings are affected and how severe the manifestation is. This enables targeted prioritisation of remediation measures.

Which systems are particularly affected?

The risk is increased for systems with: high system voltages (1,000 V systems), transformerless inverters without potential equalisation, modules without PID resistance certification (particularly older models before 2013), humid site climate and long operating years without thermographic inspection. Ground-mounted systems are more frequently affected than rooftop systems because their system voltages and string lengths are greater.

Is PID reversible?

Yes, in early stages. When PID is detected before the degradation has progressed too far, the original power condition can largely be restored through suitable anti-PID measures. Common methods are potential equalisation at the inverter (negative earthing), anti-PID boxes that apply a positive voltage overnight and replacement of affected modules in advanced cases.

Early detection is decisive. Anyone who discovers PID only after 5 years of intensive degradation cannot undo the yield losses of the past years.

Prevention and protective measures

The PID risk can already be minimised at the planning stage: select PID-resistant modules (IEC 62804 certified), use transformer-based inverters or inverters with negative potential equalisation and optimise string lengths and voltages. For existing systems anti-PID boxes are the most economical retrofit solution.

Frequently asked questions