Initial light-induced degradation (LID) testing quantifies the power loss that photovoltaic (PV) modules undergo during their first exposure to light, before their electrical output stabilizes. Performed in accordance with IEC 61215 and IEC 63202-1, this light-soaking test is a critical step in design qualification, factory quality control, and independent verification of nameplate ratings. This article explains the degradation mechanisms, sample requirements, test procedures, acceptance criteria, and typical applications of initial light aging tests for PV modules.
Technical Overview
Initial light aging, commonly referred to as the light-induced degradation (LID) test or light-soaking preconditioning test, evaluates the temporary or permanent reduction in module output power that occurs when a newly manufactured photovoltaic module is first exposed to sunlight or an equivalent artificial light source. For many module technologies, the maximum power (Pmax) measured immediately after production does not represent long-term field performance, because specific defect mechanisms activated by illumination cause the power to drift downward (or occasionally upward) until a stabilized state is reached. Initial light aging testing deliberately accelerates and documents this transient behavior under controlled laboratory conditions.
The principal degradation mechanisms addressed by the test include:
- Boron–oxygen (BO) complex formation in boron-doped p-type Czochralski-grown crystalline silicon, historically the most common cause of LID, typically producing 1–3% relative power loss, and up to approximately 5% in unoptimized material.
- Light and elevated temperature induced degradation (LeTID), which affects certain passivated emitter and rear contact (PERC) designs and develops over longer exposure times at elevated temperatures, sometimes exceeding several percent if not controlled during cell processing.
- Iron–boron pair dissociation in silicon, which can cause partial recovery effects during the early hours of exposure.
- Metastability effects in thin-film technologies, such as the Staebler–Wronski effect in amorphous silicon (substantial initial degradation) and light-soaking-induced performance increases in some copper indium gallium selenide (CIGS) devices.
The test is governed primarily by the IEC 61215 series (design qualification and type approval), which defines light-soaking and stabilization procedures for the initial characterization of modules, and by IEC 63202-1, which specifies a dedicated LID test method for crystalline silicon modules. Supporting measurement standards include the IEC 60904 series for current–voltage (I-V) measurement and solar simulator classification, and IEC 60891 for temperature and irradiance correction of I-V curves. Manufacturers, system developers, and independent engineers rely on accredited third-party laboratories to conduct these tests so that nameplate ratings, energy-yield models, and warranty terms are all referenced to stabilized rather than "fresh" module performance.
Testing Samples
Crystalline Silicon Modules
Full-size crystalline silicon modules, including monocrystalline and multicrystalline products based on p-type (aluminum back-surface-field or PERC) and n-type (TOPCon, heterojunction) cell architectures, are the most frequently tested sample category. P-type Czochralski silicon with boron doping is particularly susceptible to BO-LID, while gallium-doped p-type and most n-type products typically exhibit LID below 1%, making them valuable benchmarks for comparison.
Thin-Film Modules
Amorphous silicon, microcrystalline silicon tandem, cadmium telluride, and CIGS modules require mandatory light-soaking or other preconditioning (such as current injection or thermal annealing, as permitted by the IEC 61215 stabilization procedures) before their initial power measurement. Because thin-film devices exhibit pronounced metastability, an unstabilized flash test can significantly overstate or understate actual operating performance.
Bifacial and Emerging Designs
Bifacial glass–glass modules and new cell concepts can also be evaluated, with careful attention to rear-side irradiance shielding during exposure so that the front-side dose remains the controlled variable. Emerging technologies such as perovskite-silicon tandems may show their own transient behaviors, and adapted light-soaking protocols are used during development to characterize them.
Sample Condition and Handling
The validity of an LID test depends strongly on sample history. Key requirements include:
- Modules should be as-manufactured and not previously exposed to significant light; ideally they are shipped in light-tight packaging directly from the production line.
- Samples must be stored in the dark at controlled room temperature prior to testing, with storage duration and conditions documented.
- A minimum of one full-size module is required for a standalone LID determination, although duplicate samples are strongly recommended to assess measurement and material variability.
- Visual inspection per IEC 61215-2 must confirm the absence of pre-existing major visual defects before testing begins.
Testing Items
A complete initial light aging evaluation typically reports the following items:
- Initial electrical parameters at standard test conditions (STC: 1000 W/m², 25 °C cell temperature, AM1.5G spectrum): Pmax, Isc, Voc, fill factor, and current–voltage curve shape.
- Light-induced degradation of maximum power, expressed as relative percentage loss from the initial value.
- Stabilized power output and the cumulative radiant dose (in kWh/m²) required to reach stabilization.
- Power-versus-dose degradation curve, illustrating the transient behavior and any recovery effects.
- Electroluminescence (EL) images before and after exposure to reveal cracks, broken cells, or solder bond changes that may develop during soaking.
- Optional LeTID assessment at elevated temperature for susceptible PERC-type products, and optional low-irradiance performance checks on stabilized samples.
Testing Methods
Overall Test Sequence
The test follows a defined sequence that separates transient degradation from permanent defects:
- Receipt inspection, dark storage verification, and visual examination of the sample.
- Baseline I-V measurement at STC using a calibrated solar simulator and reference device, together with baseline EL imaging.
- Controlled light-soaking exposure with the module held at a specified temperature and monitored dose.
- Periodic I-V measurements at defined dose intervals (for example, every 5 to 10 kWh/m²) until the stabilization criterion is satisfied.
- Final I-V measurement, final EL imaging, calculation of relative degradation, and issuance of the test report.
Light-Soaking Exposure Conditions
Modules are exposed using steady-state or pulsed solar simulators, or dedicated light-soaking chambers, under conditions designed to accelerate the relevant degradation mechanism without introducing unrelated failures. Typical parameters include:
- Irradiance on the module plane of approximately 800 to 1100 W/m², with spectral content reasonably matched to the terrestrial reference spectrum so that the targeted defect mechanisms are properly activated.
- Module temperature controlled in the range of about 50 to 60 °C for conventional BO-LID testing, ensuring uniformity across the module and preventing uncontrolled thermal gradients.
- Elevated-temperature conditions of approximately 75 °C for LeTID screening, where applicable.
- Spatial non-uniformity, temporal stability, and spectral characteristics of the light source qualified in accordance with IEC 60904-9.
- Cumulative dose tracking with calibrated irradiance sensors; BO-LID stabilization commonly requires on the order of 10 to 20 kWh/m², while LeTID procedures may extend to 100 kWh/m² or more.
Electrical Characterization and Correction
At each measurement point, the module is characterized at STC in accordance with IEC 60904-1, using a reference device calibrated per IEC 60904-2 and a solar simulator meeting IEC 60904-9 classification requirements. Measured I-V curves are translated to STC using the correction procedures of IEC 60891, with measured temperature coefficients applied so that degradation is not confounded by ambient drift. The module should be allowed to equilibrate thermally before each flash or steady-state measurement.
Stabilization and Acceptance Criteria
A module is considered stabilized when the change in maximum power between consecutive dose intervals falls within the criterion defined by the applicable standard, commonly 1% for crystalline silicon and up to 2% for thin-film technologies, subject to a maximum permitted dose cap. Evaluation against specification is then performed as follows:
- For design qualification under IEC 61215, modules must remain free of major visual defects and the power degradation limits defined in the standard (typically 5% relative for individual test sequences) must not be exceeded.
- For procurement and bankability purposes, tighter limits are customary; many commercial specifications cap initial LID at 1–2% for p-type products and below 1% for n-type products, with LeTID frequently limited to a few percent over the specified procedure.
- Any power increase observed in metastable thin-film devices must be documented so that the stabilized rating, rather than the fresh or annealed state, is used for rating purposes.
Reporting
A professional test report issued by an accredited third-party laboratory or an in-house accredited facility should include:
- Complete sample identification, production date, storage history, and test dates.
- Description of equipment, simulator class, irradiance and temperature conditions, and dose intervals.
- Tabulated I-V parameters and relative Pmax change at each interval, with a plotted degradation curve.
- Before-and-after EL images and visual inspection results.
- Stabilized power, total LID percentage, stabilization dose, and measurement uncertainty statements.
Applications
Initial light aging testing supports nearly every stage of the PV value chain. Typical applications include:
- Design qualification and type approval of new module models under the IEC 61215 series, where proper stabilization is a precondition for meaningful pass/fail power measurements in subsequent climate and mechanical tests.
- Research, development, and process validation, such as evaluating gallium doping, regeneration and annealing steps, firing recipes, and anti-LID treatments for PERC production lines.
- Factory quality control and statistical process monitoring to confirm that shipped products meet declared LID specifications and positive power tolerances referenced to stabilized values.
- Independent verification and incoming inspection by project developers, EPC contractors, and asset owners, often executed by accredited third-party laboratories as part of bankability due diligence.
- Energy-yield modeling and financial modeling, where stabilized power ratings prevent systematic overestimation of first-year and lifetime production.
- Warranty enforcement and technology comparison, providing objective evidence of degradation behavior across competing module technologies.
FAQ
What is initial light-induced degradation (LID) in PV modules?
LID is the reduction in module output power that occurs during the first hours to days of light exposure, caused mainly by light-activated defect mechanisms such as boron–oxygen complexes in p-type crystalline silicon or metastability effects in thin-film devices. The power stabilizes once these mechanisms saturate, and the stabilized value, not the fresh measurement, represents real operating performance.
How long does the test take?
It depends on the technology and test conditions. At approximately 1000 W/m², conventional BO-LID stabilization typically requires on the order of 10 to 20 kWh/m², corresponding to roughly 10 to 40 hours of controlled exposure including measurement intervals. LeTID screening at elevated temperature and thin-film stabilization procedures may require several days of cumulative dose.
What is the difference between LID and LeTID?
LID develops rapidly at moderate temperatures under illumination and is dominated by boron–oxygen defects in p-type Cz silicon. LeTID (light and elevated temperature induced degradation) develops more slowly, is promoted by elevated temperature, and has affected certain PERC-type products; it requires longer, hotter exposure protocols to detect and is treated as a separate evaluation in most test programs.
Which standards apply to initial light aging testing?
The primary references are the IEC 61215 series, which defines light-soaking and stabilization procedures within design qualification testing, and IEC 63202-1, which specifies a dedicated LID test method for crystalline silicon modules. Measurement quality is ensured through IEC 60904-1, IEC 60904-2, and IEC 60904-9 for I-V characterization and simulator classification, and IEC 60891 for curve correction.
Do thin-film modules need light soaking before rating?
Yes. Because amorphous silicon, CdTe, and CIGS devices exhibit pronounced metastability, IEC 61215 requires preconditioning by light soaking, current injection, or controlled annealing, followed by verification that the power has stabilized, before the initial rating measurement is accepted.
What sample condition is required for a valid test?
Samples must be as-manufactured, preferably never exposed to meaningful light after production, transported in light-tight packaging, and stored in the dark before testing. Any prior exposure alters the measured degradation, so sample history must be documented and considered when interpreting results.
What degradation level is acceptable?
Under IEC 61215 design qualification, power degradation limits (typically 5% relative per test sequence) must not be exceeded. In commercial practice, specifications are stricter: initial LID of 1–2% is commonly required for p-type modules, below 1% for modern n-type and gallium-doped products, and defined caps are applied to LeTID where applicable.