Photovoltaic Module Initial Light Soaking Test: LID Preconditioning, Standards, and Procedures

The initial light soaking test is a mandatory stabilization and preconditioning procedure defined in the IEC 61215 series for photovoltaic modules, designed to reveal early-stage light-induced degradation (LID) and establish a stable power baseline before certification. By exposing new modules to controlled irradiance and temperature and tracking maximum power over defined dose intervals, the test verifies nameplate ratings and ensures fair comparison between products. This article explains the technical basis, applicable sample types, test items, methods, and practical applications of the PV module initial light soaking test.

Technical Overview

Initial light soaking, also referred to as light preconditioning or LID stabilization, is the controlled exposure of a newly manufactured photovoltaic (PV) module to simulated or natural sunlight before its electrical performance is characterized and before the module enters the qualification test sequence. The procedure is defined in the IEC 61215-2 standard as a stabilization test (commonly designated MQT 19, "light soaking") and is supported by technology-specific requirements in IEC 61215-1-1 through IEC 61215-1-4. Its fundamental purpose is to bring the module to a repeatable, stabilized performance state so that all subsequent measurements and stress tests are referenced to a meaningful baseline.

The underlying physics differs by technology. In p-type crystalline silicon, particularly boron-doped Czochralski (Cz) wafers, illumination activates boron–oxygen complexes that reduce minority carrier lifetime and typically cause 1% to 3% maximum power loss within the first few kilowatt-hours of exposure. In amorphous silicon, the Staebler–Wronski effect can degrade power by 10% to 30% before the material stabilizes. Thin-film technologies such as CdTe and CIGS exhibit transient metastabilities that both degrade and recover with light, temperature, and dark storage. More recently, light and elevated temperature induced degradation (LeTID) observed in PERC and other cell architectures has reinforced the need for carefully controlled initial exposure conditions.

Without initial light soaking, nameplate power ratings could overstate real-world output, test-to-test comparisons would be irreproducible, and downstream reliability tests such as damp heat and thermal cycling would start from inconsistent baselines. The test is therefore a cornerstone of design qualification, factory quality control, and bankability assessment performed by accredited third-party laboratories.

Testing Samples

Crystalline Silicon Modules

Full-size crystalline silicon modules in standard 60-cell, 72-cell, or half-cell formats, including p-type PERC, TOPCon, heterojunction (HJT), and n-type designs, are the most common test articles. These modules usually stabilize quickly, with the majority of boron–oxygen related degradation occurring within the first few kilowatt-hours of exposure per square meter.

Thin-Film Modules

Amorphous silicon, microcrystalline tandem, CdTe, and CIGS modules require extended light soaking because of their larger and slower degradation kinetics. For these technologies, the applicable part of the IEC 61215 series may additionally specify dark storage and annealing steps to distinguish irreversible degradation from reversible metastability.

Mini-Modules, Coupons, and Cell-Level Specimens

Research and development programs frequently use electrically interconnected mini-modules or encapsulated cell coupons to screen new materials, dopant schemes, anti-LID regeneration processes, and encapsulant formulations before committing full-size modules to lengthy stabilization campaigns.

Bifacial and Emerging Architectures

Bifacial modules and emerging technologies such as perovskite and perovskite-silicon tandems may be tested with modified procedures that account for rear-side illumination, ionic metastability, or unencapsulated sample handling, always with the measurement conditions clearly documented.

Testing Items

The initial light soaking campaign typically comprises the following measurement and inspection items:

• Initial current–voltage (I–V) characteristic measurement at Standard Test Conditions (STC: 1000 W/m², 25 °C cell temperature, AM1.5G spectrum) per IEC 60904-1.
• Controlled light soaking exposure with continuous or periodic monitoring of irradiance and module temperature.
• Intermediate I–V measurements after defined equal-dose exposure intervals, translated to STC according to IEC 60891.
• Calculation of maximum power (Pmax) degradation percentage relative to the initial value at each interval.
• Verification of the stabilization criterion, typically a change in Pmax of no more than 1% to 2% between two consecutive equal-dose exposures, as specified by the applicable edition of the standard.
• Electroluminescence (EL) imaging before and after exposure to reveal cell cracks, broken fingers, or localized degradation patterns.
• Visual inspection per the IEC 61215-1 criteria to document any appearance defects induced by exposure.

Testing Methods

Equipment and Test Conditions

Exposure may be performed using natural sunlight, pulsed or continuous solar simulators, or dedicated light soaking chambers equipped with xenon, halogen, or LED sources. The light source must meet the spatial uniformity, spectral match, and temporal stability requirements of IEC 60904-9, typically class BBA or better for soaking and class A equipment for I–V measurement. Typical exposure conditions are an irradiance in the range of roughly 800 to 1200 W/m² with the module temperature held within approximately 40 °C to 60 °C, commonly regulated near 50 °C, ensuring that temperature-driven artifacts do not mask or exaggerate photo-induced power changes.

Exposure Procedure

The module is first characterized at STC to record its initial electrical parameters (Isc, Voc, Pmax, and fill factor). It is then mounted in the soaking station with temperature sensors attached to the rear surface, and irradiance is verified with a calibrated reference device per IEC 60904-2. Exposure proceeds in increments of a defined dose, commonly 5 kWh/m² for crystalline silicon and substantially higher cumulative doses for thin-film technologies. After each increment, the module is returned to 25 °C and remeasured, and the resulting Pmax values are plotted against cumulative dose.

Stabilization Criteria and Evaluation

A module is considered stabilized when the Pmax change between two successive equal-dose intervals falls within the tolerance stated in the applicable standard, at which point the last measured value is adopted as the stabilized power for reporting, labeling verification, and use as the baseline in the subsequent test sequence. If the module continues to degrade beyond the specified total dose, the test reports the degradation trend and the stabilized value is determined according to the technology-specific rules of the standard.

Distinguishing LID from LeTID

Because LeTID progresses noticeably only at elevated temperatures, some programs complement the standard initial light soaking with higher-temperature illuminated exposure to separate conventional LID from slow LeTID mechanisms. Documenting both the degradation magnitude and the recovery or saturation behavior provides a more complete picture of long-term field performance.

Applications

Initial light soaking results are used across the entire PV value chain:

• Type approval and design qualification under the IEC 61215 series, where a stabilized baseline is a prerequisite for subsequent stress tests.
• Nameplate verification and power rating accuracy checks for module manufacturers and independent certification bodies.
• Factory incoming inspection, batch sorting, and acceptance testing by project developers, EPC contractors, and asset owners.
• Research and development of LID mitigation strategies, including gallium co-doping, hydrogen passivation, and rapid thermal regeneration processes.
• Energy yield modeling and financial due diligence, where stabilized rather than flash-test power is required for accurate performance ratio predictions.
• Bankability studies and extended stress testing programs that compare degradation signatures across cell technologies and production batches.

FAQ

Why must new modules undergo light soaking before performance measurement?

Many PV technologies lose a measurable fraction of their power during the first hours to days of illumination. Measuring an unstabilized module overstates its field output, so light soaking brings the device to its stabilized state and makes ratings, comparisons, and reliability tests reproducible.

How much power loss during initial light soaking is considered acceptable?

The acceptance thresholds depend on the applicable standard and technology. Modern p-type crystalline silicon modules typically show 1% to 3% LID, while stabilized thin-film products may exhibit larger early losses that are accounted for in their rated power. The key compliance requirement is that the reported rating reflects the stabilized condition rather than a fixed universal percentage.

Can natural sunlight be used instead of a solar simulator?

Yes, natural sunlight is permitted when irradiance exceeds a defined minimum, spectral conditions are monitored, and module temperature is measured, provided the procedures and instrumentation comply with the relevant clauses of the IEC standards and are documented in the test report.

How long does the test take?

For crystalline silicon modules, stabilization is often achieved after approximately 5 to 15 kWh/m² of cumulative dose, which translates to a few hours to several days depending on irradiance level and equipment capacity. Thin-film modules may require significantly longer exposure plus dark storage intervals.

Is initial light soaking the same as LeTID testing?

No. Initial light soaking addresses conventional LID under moderate temperature conditions defined in IEC 61215-2, whereas LeTID evaluation involves extended illumination at elevated temperatures. The two mechanisms interact, so many laboratories offer combined screening protocols that characterize both.