Solar Panel LeTID and LID: What Manufacturers Should Know

Solar modules are expected to deliver reliable electricity for decades, but their performance can change during the early stages of operation. Two important degradation mechanisms that solar manufacturers, project developers and module buyers should understand are Light-Induced Degradation (LID) and Light and Elevated Temperature-Induced Degradation (LeTID).

Both phenomena can affect photovoltaic module output, but they arise from different mechanisms and may respond differently to cell processing, material properties and operating conditions.

Understanding LID and LeTID helps manufacturers improve cell quality, evaluate degradation risks, select appropriate testing procedures and support long-term solar project performance.

What Is Light-Induced Degradation (LID) in Solar Panels?

Light-Induced Degradation, commonly known as LID, refers to a reduction in solar cell performance that occurs after exposure to light. It is particularly associated with certain crystalline silicon cell materials and manufacturing processes.

In conventional boron-doped p-type silicon, illumination can activate defects associated with boron and oxygen in the silicon wafer. These defects can increase carrier recombination, reducing the cell's ability to convert sunlight into electricity.

As a result, a module may experience an initial reduction in power output after being exposed to sunlight.

The magnitude of LID depends on factors such as wafer material, oxygen concentration, doping characteristics, cell processing and stabilisation procedures. It should not be assumed that every module experiences the same level of degradation.

Main Causes of LID

Several factors can contribute to LID in crystalline silicon solar cells:

• Boron-oxygen-related defects: In susceptible p-type silicon, illumination can activate defects that reduce minority-carrier lifetime.
• Wafer material properties: Oxygen concentration and other material characteristics can influence susceptibility.
• Cell manufacturing processes: Processing conditions can affect defect formation and the resulting electrical performance.
• Initial operating exposure: Some degradation becomes apparent during the first period of light exposure before performance stabilises.

LID is generally associated with an early performance change, although the magnitude and progression depend on the cell technology and material system.

What Is Light and Elevated Temperature-Induced Degradation (LeTID)?

Light and Elevated Temperature-Induced Degradation, or LeTID, is a degradation mechanism observed in crystalline silicon solar cells under illumination and elevated-temperature conditions.

LeTID became an important concern as high-efficiency passivated cell technologies, including certain PERC designs, gained wider adoption. The mechanism is associated with changes in recombination-active defects within the silicon and can result in a measurable loss of cell or module power.

Unlike conventional LID, which is often associated with an initial light-induced change, LeTID can develop over a longer period under favourable operating conditions. In some cases, the degradation can later partially recover, depending on the material and operating history.

The underlying microscopic mechanisms are complex and can vary with cell structure, material properties and processing conditions.

Main Causes of LeTID

The exact mechanism is not fully captured by a single universal explanation, but important contributing factors include:

• Elevated cell temperatures: Higher operating temperatures can influence defect reactions and degradation kinetics.
• Illumination: Light exposure is part of the conditions associated with LeTID in susceptible cells.
• Silicon material and defect chemistry: The wafer's material characteristics influence how degradation-related defects develop.
• Cell manufacturing processes: Hydrogen-related processes and other fabrication conditions can affect LeTID susceptibility.
• Operating history: The combination of light exposure, temperature and time can influence the observed degradation pattern.

LeTID should therefore be evaluated using suitable testing protocols rather than inferred from module technology labels alone.

LID vs LeTID: Key Differences

Feature	LID	LeTID
Full form	Light-Induced Degradation	Light and Elevated Temperature-Induced Degradation
Main trigger	Light exposure in susceptible cells	Light exposure combined with elevated-temperature conditions
Typical behaviour	Often an initial performance loss	May develop over an extended period
Commonly discussed technology	Certain conventional p-type silicon cells	Particularly associated with some PERC cell designs
Possible recovery	Depends on the defect mechanism and cell technology	Partial recovery may occur under certain conditions
Key mitigation approach	Material selection, cell processing and stabilisation	Process optimisation, defect control and dedicated testing

The table provides a general comparison. Actual degradation behaviour depends on the cell material, manufacturing process and test conditions.

How Do LID and LeTID Affect Solar Module Performance?

LID and LeTID can influence the electrical performance of solar modules in several ways.

1. Reduction in Power Output

Both mechanisms can reduce a module's maximum power output. If the reduction is not accounted for in system modelling, the actual energy yield may be lower than expected.

2. Changes in Electrical Parameters

Degradation may affect parameters such as open-circuit voltage, short-circuit current, fill factor and maximum power. The relative change in each parameter depends on the underlying mechanism.

3. Impact on Energy Yield

Even a relatively small power reduction can matter across a large solar installation. For utility-scale projects, manufacturers and developers should evaluate degradation assumptions using appropriate module test data and performance models.

4. Warranty and Performance Expectations

Manufacturers must distinguish between initial stabilisation, degradation mechanisms and long-term performance loss when establishing product specifications and warranty commitments.

Buyers should review the applicable product documentation rather than assuming that every module follows the same degradation curve.

Which Solar Cell Technologies Are More Susceptible?

Susceptibility to LID and LeTID depends on the cell material, design and manufacturing process.

Mono PERC Solar Cells

Some PERC cells, particularly certain p-type designs, have been associated with both LID and LeTID. The actual risk depends on wafer material, hydrogen management, cell processing and stabilisation procedures.

N-Type TOPCon Solar Cells

N-type TOPCon Solar cells use a different material and passivated-contact architecture from conventional p-type PERC cells. They are generally less susceptible to the classic boron-oxygen-related LID mechanism associated with boron-doped p-type silicon.

However, this does not mean that TOPCon modules are immune to all light-induced degradation or other performance changes. Their behaviour depends on material properties, processing, contacts and module construction.

HJT Solar Cells

Heterojunction technology (HJT) uses a different cell architecture and material-processing approach. Its degradation behaviour differs from that of conventional PERC cells, but the specific module's performance should still be evaluated through appropriate reliability testing.

Important: Cell technology alone is not sufficient to establish a module's degradation rate. Manufacturers should use measured results from representative products and validated testing methods.

How Can Solar Manufacturers Reduce LID and LeTID?

Solar Manufacturers can reduce degradation risks by controlling material quality, optimising production processes and validating module performance through appropriate testing.

1. Select Suitable Silicon Wafer Materials

Wafer characteristics can influence the formation and behaviour of degradation-related defects. Manufacturers should establish incoming material specifications and monitor relevant parameters through supplier quality programmes.

2. Optimise Cell Processing

Cell processing can affect defect formation, hydrogen behaviour and passivation quality. Process windows should be validated for the specific cell architecture and material combination.

3. Evaluate Stabilisation Procedures

Appropriate stabilisation procedures can help manufacturers determine the module's initial stable power level and distinguish early degradation from subsequent performance changes.

Stabilisation conditions must be appropriate for the technology and the applicable test method.

4. Conduct Dedicated Reliability Testing

Manufacturers should evaluate LID and LeTID using recognised test methods applicable to the relevant cell and module technology.

Testing may include controlled illumination, elevated-temperature exposure and electrical performance measurements before, during or after the test. The exact conditions should follow the selected standard or protocol.

5. Maintain Production Traceability

Traceability across wafers, cells, production batches and finished modules helps manufacturers investigate abnormal results and identify recurring patterns.

Consistent documentation supports root-cause analysis, corrective action and continuous process improvement.

6. Verify Module Performance

Power measurements should be performed under controlled conditions using suitable calibration and measurement procedures. Manufacturers should compare initial and post-test results to understand the magnitude of any performance change.

Where relevant, independent laboratory testing can provide additional confidence in product performance claims.

How Are LID and LeTID Tested?

Testing helps manufacturers quantify degradation under controlled conditions and compare results against defined acceptance criteria.

A typical evaluation process may involve:

1. Initial characterisation: Measure baseline module or cell performance using a suitable electrical test procedure.
2. Controlled exposure: Apply the illumination, temperature and duration specified by the selected test method.
3. Intermediate measurements: Where required, monitor performance changes during the exposure period.
4. Post-test characterisation: Measure the electrical output again under comparable conditions.
5. Data analysis: Calculate the measured change in power and relevant electrical parameters.
6. Reporting: Document test conditions, equipment, sample identification, results and deviations.

Testing should distinguish LID-related behaviour from LeTID and other degradation mechanisms wherever the protocol permits.

IEC 61215 is an important series of standards for the design qualification and type approval of terrestrial photovoltaic modules. Manufacturers should check the applicable edition, relevant test provisions and any additional technology-specific procedures needed to assess LID or LeTID. Passing a general module qualification programme should not automatically be interpreted as proof that every degradation mechanism has been fully characterised.

Why LID and LeTID Matter for Solar Project Developers and Buyers

Although LID and LeTID are often discussed in the context of cell manufacturing, they also matter when selecting modules for commercial, industrial and utility-scale projects.

More Accurate Energy Yield Estimates

Reliable degradation data helps project developers create realistic generation forecasts and financial models.

Better Product Comparisons

Buyers can compare manufacturers using documented test results, initial stabilised power ratings, degradation assumptions and applicable warranty terms.

Lower Performance Uncertainty

Understanding degradation mechanisms can help developers identify potential risks before procurement and avoid relying solely on nominal efficiency ratings.

Improved Long-Term Planning

Documented performance data supports maintenance planning, asset monitoring and evaluation of long-term project economics.

Conclusion

LID and LeTID are important degradation mechanisms that solar manufacturers need to understand when developing and evaluating photovoltaic modules. Although both can reduce electrical output, they differ in their underlying mechanisms, typical progression and testing requirements.

Material selection, process optimisation, appropriate stabilisation, traceability and dedicated reliability testing can help manufacturers identify and manage these risks.

For project developers and buyers, the most useful approach is to evaluate verified module performance data and technology-specific reliability evidence rather than relying on efficiency ratings alone. These measures support informed procurement decisions and more realistic expectations for long-term solar energy generation.

Bluebird Solar recognises the importance of module quality, reliable manufacturing processes and performance validation in the development of dependable solar energy solutions.

Frequently Asked Questions

1. What is the difference between LID and LeTID in solar panels?

LID is a light-induced performance reduction commonly associated with certain silicon material defects. LeTID is associated with light exposure and elevated-temperature conditions and may develop over a longer period in susceptible cells.

2. Do TOPCon solar panels experience LID?

N-type TOPCon cells are generally less susceptible to the classic boron-oxygen-related LID mechanism found in certain p-type silicon cells. However, other light-induced degradation mechanisms may still occur, so product-specific test data is important.

3. Are PERC solar panels susceptible to LeTID?

Some PERC cell designs, particularly certain p-type designs, have shown susceptibility to LeTID. The extent depends on the silicon material, cell processing and other manufacturing factors.

4. Can LeTID reverse over time?

Partial recovery has been observed in some cells under certain operating conditions. However, recovery is not guaranteed and should not be assumed when estimating module output or evaluating warranty performance.

5. How can manufacturers measure LID and LeTID?

Manufacturers can compare electrical performance before and after controlled exposure using suitable test procedures. Testing conditions should be selected to evaluate the relevant degradation mechanism and technology.

6. Do LID and LeTID affect a solar module's lifespan?

They can reduce power output and affect long-term energy yield. Their effect on service life depends on the magnitude of degradation, the underlying mechanism and the module's overall reliability.

7. What should buyers check before selecting solar modules?

Buyers should review the module's technology, applicable certifications, product and performance warranties, initial stabilised power rating, degradation specifications and relevant reliability-test results.