Evaluation of Initial Light-Induced Degradation (LID) of Photovoltaic Modules

Initial light-induced degradation (LID) evaluation measures the power loss that photovoltaic modules undergo during their first exposures to sunlight, a critical parameter for verifying nameplate ratings and predicting long-term energy yield. Performed in accordance with IEC 61215-2 and related international standards, this testing quantifies and stabilizes early-stage performance changes before modules are deployed or subjected to further qualification tests. This article explains the degradation mechanisms, sample requirements, test items, methodologies, and practical applications of LID assessment for solar panels.

Technical Overview

Light-induced degradation (LID) refers to the irreversible or partially recoverable reduction in photovoltaic conversion efficiency that occurs when solar cells and modules are first exposed to light or carrier injection. In conventional p-type crystalline silicon, the dominant mechanism is the formation of boron–oxygen (B–O) recombination-active defect complexes in boron-doped Czochralski-grown wafers. Under illumination, these complexes reduce the minority carrier lifetime, lowering the short-circuit current and fill factor, and typically cause a relative maximum power (Pmax) loss of 1% to 5% within the first 10 to 50 kWh/m² of accumulated irradiation before the module stabilizes.

Modern cell architectures introduce additional degradation phenomena. Passivated emitter and rear contact (PERC) cells may exhibit light and elevated temperature induced degradation (LeTID), a slower, hydrogen-related mechanism that can cause losses of several percent when not properly mitigated during manufacturing. Amorphous silicon thin-film modules undergo the Staebler–Wronski effect, with initial losses that can reach 10% to 25% before stabilization. In contrast, n-type technologies such as tunnel oxide passivated contact (TOPCon) and heterojunction (HJT) cells, as well as gallium-doped p-type wafers, show markedly lower B–O-related LID susceptibility.

LID evaluation is embedded in the international qualification framework. IEC 61215-2 defines the light-soaking stabilization procedure as test MQT 13, IEC TS 63202-1 specifies dedicated LID test methods for crystalline silicon modules, and the IEC 60904 series governs the electrical measurement of current–voltage characteristics. European (EN), American (UL), and Chinese national (GB/T) adoptions of these standards are technically aligned, allowing accredited third-party laboratories to deliver internationally comparable results.

Testing Samples

Crystalline Silicon PV Modules

Full-size production modules based on monocrystalline or multicrystalline wafers, including Al-BSF, PERC, TOPCon, and HJT architectures. Two or more representative modules are typically submitted so that one specimen can serve as a stored control for measurement reproducibility checks.

Bifacial Modules

Glass–glass or transparent-backsheet bifacial modules are exposed using front-side irradiation as the standard condition, with additional rear-side or simultaneous double-sided exposure protocols available when project-specific evaluation is required.

Thin-Film Modules

Amorphous silicon, cadmium telluride (CdTe), and copper indium gallium selenide (CIGS) modules are tested with extended stabilization sequences because their initial transient behavior is more pronounced and longer-lasting than that of crystalline silicon.

Cells and Mini-Modules

Individual solar cells, cut-cell strips, and laminated mini-modules are accepted for research and development purposes, manufacturing process screening, and evaluation of regeneration or annealing treatments at the component level.

Testing Items

The core deliverables of an initial LID evaluation include:

• Initial maximum power (Pmax) measurement at Standard Test Conditions (STC: 1000 W/m², 25 °C cell temperature, AM 1.5G spectrum per IEC 60904-3).
• Stabilized maximum power after controlled light soaking until the stabilization criterion is met.
• Initial light-induced degradation rate, calculated as the relative difference between initial and stabilized Pmax.
• Changes in current–voltage parameters, including short-circuit current (Isc), open-circuit voltage (Voc), fill factor (FF), and module conversion efficiency.
• Stabilization kinetics, documenting the exposure dose (kWh/m²) and elapsed time required to reach a stable power level.
• Electroluminescence (EL) imaging before and after exposure to identify crack propagation, shunts, or localized defects that develop during light soaking.
• Optional LeTID susceptibility screening at elevated temperature for PERC and other hydrogen-rich cell technologies.

Testing Methods

Initial Characterization

Each sample undergoes visual inspection, electroluminescence imaging, and an initial I-V curve measurement at STC. Measurements are performed with a solar simulator classified according to IEC 60904-9, calibrated reference devices traceable to IEC 60904-2, and irradiance and temperature corrections applied in accordance with IEC 60891.

Light Soaking Exposure

Modules are exposed to continuous or cumulative irradiation from a steady-state solar simulator or natural sunlight at an irradiance of at least 700 W/m², typically 1000 W/m², with the module temperature maintained in a controlled range around 50 °C. Following IEC 61215-2 test MQT 13, the exposure is interrupted at defined intervals of at least 5 kWh/m² for intermediate STC power measurements. A module is considered stabilized when the change in Pmax between two consecutive intervals remains within 2%.

LeTID Screening

For technologies susceptible to slow degradation kinetics, additional exposure is conducted at elevated cell temperatures, typically between 60 °C and 85 °C, to reveal degradation and recovery behavior that standard LID testing alone may not capture within a practical timeframe.

Final Measurement and Evaluation

Upon stabilization, a final STC I-V measurement and EL inspection are performed. The initial LID value is reported as the relative power loss between the initial and stabilized conditions, together with the full I-V parameter set, exposure history, temperature records, and measurement uncertainties, providing a defensible basis for nameplate verification and downstream reliability testing.

Applications

Initial LID evaluation serves multiple stakeholders across the photovoltaic value chain:

• Nameplate verification for manufacturers, ensuring that advertised power ratings reflect stabilized rather than freshly manufactured performance.
• Bankability assessments and technical due diligence for project developers, investors, and lenders evaluating module procurement.
• Quality control in cell and module production, confirming the effectiveness of LID regeneration and annealing processes.
• Research and development benchmarking of emerging cell architectures, doping strategies, and passivation schemes.
• Comparative module selection in tenders and auctions, where stabilized power differences directly affect levelized cost of energy calculations.
• Baseline establishment before IEC 61215 sequences such as thermal cycling, damp heat, and humidity-freeze testing, all of which require a stabilized starting point.
• Support for warranty claims, insurance assessments, and performance dispute resolution with objective laboratory evidence.

FAQ

What is initial light-induced degradation?

It is the power loss that occurs within the first hours to weeks of sunlight exposure, caused primarily by light-activated defect formation such as boron–oxygen complexes in p-type crystalline silicon or the Staebler–Wronski effect in amorphous silicon.

How much LID is considered acceptable?

Conventional p-type Czochralski modules typically show 1% to 3% initial LID, while well-processed PERC modules should remain within about 2% after regeneration. Values approaching or exceeding 5% generally warrant investigation of the manufacturing process or the presence of LeTID.

Which standards govern LID testing?

The principal references are IEC 61215-2 (test MQT 13, light soaking and stabilization), IEC TS 63202-1 for crystalline silicon LID test methods, and the IEC 60904 series for I-V measurement and solar simulator classification. EN, UL, and GB/T national adoptions are harmonized with these documents.

How long does an LID evaluation take?

Typical durations range from one to five days of light soaking for standard crystalline silicon modules, depending on how quickly the 2% stabilization criterion is met. LeTID screening and thin-film stabilization may require several additional days to weeks.

Can LID be eliminated?

B–O-related LID can be largely suppressed through regeneration treatments during cell manufacturing, gallium-doped or phosphorus-doped wafers, and advanced cell architectures such as TOPCon and HJT. Verified suppression is exactly what a professional LID evaluation confirms.

What is the difference between LID and LeTID?

LID occurs rapidly under illumination at moderate temperature, whereas LeTID develops more slowly and is accelerated by the combination of light and elevated temperature, making dedicated high-temperature exposure screening necessary for susceptible technologies.

Who should perform the testing?

Testing should be entrusted to accredited third-party laboratories operating under ISO/IEC 17025 quality management, with calibrated equipment traceable to international standards, to ensure that results are recognized by global markets and financial institutions.