Photovoltaic Module Light-Induced Degradation (LID) Testing: Standards, Procedures, and Evaluation

Photovoltaic module light-induced degradation (LID) testing quantifies the power loss that solar modules undergo during their initial exposure to light, providing the stabilized power values on which certification, warranty terms, and energy-yield predictions are based. Performed in accredited third-party laboratories in accordance with the IEC 61215 and IEC 60904 series of standards, the test exposes modules to controlled illumination and measures the resulting change in maximum power. The results are essential for design qualification, LeTID screening, and reliable performance modeling of both crystalline silicon and thin-film products.

Technical Overview

Light-induced degradation (LID) is a well-characterized phenomenon in which a photovoltaic (PV) module loses a measurable fraction of its output power within the first hours or days of exposure to light. The loss occurs without any visible damage and originates from metastable defects generated inside the solar cells by illumination. Because nameplate ratings, financial models, and energy-yield simulations all assume long-term stabilized performance, the magnitude of LID must be measured, documented, and, where possible, minimized through cell processing.

Degradation Mechanisms in Crystalline Silicon

In conventional p-type, boron-doped Czochralski (Cz) silicon, illumination activates boron-oxygen (B-O) complexes that act as recombination centers, reducing the minority-carrier lifetime and typically causing power losses in the range of 1% to 5%. Iron-boron pairs produce a faster but smaller initial drop in performance. B-O related LID is largely reversible: continued illumination or forward-bias current injection at moderately elevated temperature converts the harmful defect state into a stable regenerated state, which is why many modern cells are deliberately pre-regenerated before module assembly.

Light and Elevated Temperature Induced Degradation (LeTID)

Passivated emitter and rear cell (PERC) technologies introduced a slower degradation mode that appears under the combined influence of illumination and elevated operating temperature. Referred to as LeTID, this mechanism can progress over months or years of field operation and, if uncontrolled, may exceed the losses caused by classical LID. The 2021 revision of IEC 61215-2 therefore added a dedicated LeTID susceptibility test (MQT 21) to the module qualification sequence.

Thin-Film and n-Type Considerations

Amorphous silicon thin-film modules degrade through the Staebler-Wronski effect and require defined light soaking before a stable rating can be assigned, while cadmium telluride and copper indium gallium selenide products also exhibit transient behavior that must be stabilized before measurement. Although n-type technologies such as TOPCon and heterojunction cells show little B-O related LID, they can still display UV-induced degradation or mild LeTID behavior, so stabilization and verification remain necessary parts of the assessment.

Testing Samples

LID testing is normally performed on full-size production modules that are representative of the manufactured batch. Samples should arrive without significant prior light exposure, be free of transport damage, and be conditioned at 23 °C ± 5 °C before the initial measurement. Depending on the purpose of the study, mini-modules, laminates, or bare cells can also be evaluated, although formal certification under IEC 61215 requires complete modules.

P-Type Crystalline Silicon Modules

These are the primary candidates for classical LID evaluation because boron-doped Cz wafers are inherently susceptible to B-O defect formation. Testing verifies both the magnitude of the initial power loss and the effectiveness of any regeneration treatment applied during manufacturing.

PERC, TOPCon, and Heterojunction Modules

High-efficiency architectures are screened primarily for LeTID behavior in addition to conventional LID. Testing on these samples confirms whether firing and regeneration recipes have suppressed slow degradation modes that would otherwise surface in the field.

Thin-Film Modules

Technology-specific stabilization procedures defined in the IEC 61215-1 series are applied to thin-film samples, which may require cumulative irradiation doses well beyond those needed for crystalline silicon before a stable power value is reached.

Bifacial and Building-Integrated Modules

Bifacial products are characterized using the procedures of IEC TS 60904-1-2 so that rear-side contributions are handled correctly, and building-integrated modules are assessed as complete assemblies consistent with the IEC 61215 and IEC 61730 framework.

Testing Items

The principal measurand is the change in maximum output power (Pmax) between the initial and stabilized states, determined at Standard Test Conditions (STC: 1000 W/m², 25 °C cell temperature, AM1.5G spectral distribution). A typical LID test report includes:

• Initial and stabilized I-V curves with Pmax, short-circuit current (Isc), open-circuit voltage (Voc), and fill factor (FF)
• Calculated LID degradation rate for Pmax, Isc, Voc, and FF expressed in percent
• Stabilization history, including cumulative irradiation dose and module temperature records
• Visual inspection results before and after light exposure
• Electroluminescence (EL) images, where required, to confirm the absence of hidden defects
• Insulation and wet-leakage continuity verification for samples undergoing full qualification
• A statement of stabilization status in accordance with the criteria of the IEC 61215-1 series

Testing Methods

Initial Characterization

Each sample first undergoes visual inspection and an I-V measurement at STC following IEC 60904-1. The measurement is performed with a solar simulator meeting the class AAA requirements of IEC 60904-9, using reference devices calibrated in accordance with IEC 60904-2 and IEC 60904-6, with spectral mismatch corrections applied per IEC 60904-7 and temperature or irradiance translation performed per IEC 60891 where necessary.

Light Soaking and Stabilization

Modules are then exposed to sustained illumination, either from natural sunlight, a continuous or pulsed solar simulator, or, for crystalline silicon, an equivalent forward-bias current injection permitted by IEC 61215-1-1. Typical parameters include an irradiance level in the range of 800 W/m² to 1100 W/m² with the module temperature controlled, commonly between 25 °C and 60 °C, because degradation and regeneration kinetics depend strongly on temperature. Power is re-measured at defined checkpoints, and the module is considered stabilized when the change in Pmax between consecutive checkpoints falls within a small tolerance, commonly 1% to 2%, after a cumulative dose that typically reaches 10 kWh/m² to 20 kWh/m² for crystalline silicon and may extend to several tens of kWh/m² for thin-film technologies.

LeTID Susceptibility Testing

Where LeTID screening is required, modules are held at an elevated temperature of approximately 75 °C under sustained illumination or equivalent current injection for a prescribed duration on the order of tens of hours, consistent with MQT 21 of IEC 61215-2:2021 and the extended stress procedures of IEC 63209-1. The power measured before and after the exposure is compared against the power-retention limit defined by the standard, which is on the order of 5%.

Final Measurement and Data Evaluation

After stabilization or LeTID exposure, the I-V characteristic is re-measured under conditions identical to the initial measurement. The light-induced degradation rate is calculated as D_LID = (Pmax,initial − Pmax,stabilized) / Pmax,initial × 100%. Results are reported together with the measurement uncertainty, evaluated according to ISO guidance, so that small but meaningful differences between samples can be interpreted correctly.

Equipment and Facility Requirements

• A solar simulator classified per IEC 60904-9 for accurate STC characterization
• A temperature-controlled light-soaking chamber, high-intensity LED array, or qualified outdoor exposure station
• Calibrated primary or secondary reference devices with traceability maintained per IEC 60904-4
• Data-acquisition systems for continuous logging of irradiance and module temperature
• Supplementary instruments for EL imaging, insulation testing, and wet-leakage testing

Applications

• Design qualification and type approval under the IEC 61215 series, where stabilization precedes the final power measurement and MQT 21 addresses LeTID susceptibility
• Research and development support for evaluating regeneration processes, alternative dopants such as gallium, and firing-recipe optimization
• Factory quality control and outgoing inspection to confirm that shipped products meet declared stabilized power ratings
• Independent verification for project developers, lenders, and insurers during bankability due diligence
• Warranty claim support and validation of nameplate tolerance after field installation
• Technology benchmarking that compares LID and LeTID behavior across different cell architectures and suppliers
• Generation of reliable inputs for energy-yield simulation and performance modeling of PV plants

FAQ

What is the difference between LID and LeTID?

LID refers to the relatively fast degradation driven by light exposure alone, classically caused by boron-oxygen complexes in p-type Cz silicon, and it typically stabilizes within the first tens of hours of illumination. LeTID develops much more slowly under the combined action of light and elevated temperature, mainly affects PERC-type cells, and is evaluated separately at a controlled module temperature of roughly 75 °C.

How much LID is acceptable?

The IEC 61215 series does not impose a single universal LID limit; instead, it requires that modules be stabilized so that reported power reflects the true long-term state, and it defines power-retention criteria for the subsequent qualification tests. In commercial practice, well-regenerated crystalline silicon modules are commonly expected to show LID of 2% or less, with many premium products guaranteed at 1% or below.

Can LID be reversed?

B-O related LID is largely reversible. Continued illumination or controlled current injection at elevated temperature drives the defect into a regenerated, stable configuration, and many manufacturers apply this treatment during production. LeTID is also partially recoverable, but the recovery is slow at normal operating temperatures, which is why susceptibility testing before deployment is important.

How long does the test take?

A conventional LID stabilization for crystalline silicon typically requires on the order of 10 to 30 hours of exposure at approximately 1000 W/m², equivalent to a dose of 10 kWh/m² to 20 kWh/m², plus the time for interim and final I-V measurements. LeTID testing extends over several tens of hours at elevated temperature, and thin-film stabilization may require substantially longer cumulative doses depending on the technology.

Do n-type modules need LID testing?

Yes. Although n-type TOPCon and heterojunction modules are largely free of boron-oxygen LID, they can exhibit UV-induced degradation and some degree of LeTID-like behavior. Stabilization and verification testing therefore remain an integral part of their qualification and characterization.

Which standards govern LID testing?

The core framework consists of the IEC 61215 series for qualification and stabilization, IEC 61215-2:2021 for the LeTID susceptibility test, and the IEC 60904 series together with IEC 60891 for accurate electrical measurement. Supporting references include IEC TS 60904-1-2 for bifacial modules, IEC 61853-1 for power rating, IEC 63209-1 for extended stress testing, UL 61215 and the GB/T 61215 series as regional adoptions, and ASTM E1036 as an alternative method for module electrical performance measurement. All of these tests can be carried out by accredited third-party laboratories.