LeTID

What is LeTID

LeTID (Light and Elevated Temperature Induced Degradation) is a degradation mechanism affecting PERC (Passivated Emitter and Rear Contact) solar cells under combined exposure to light and elevated temperatures (typically 60-90°C). Unlike LID (Light-Induced Degradation), which occurs within the first 100 hours of light exposure, LeTID develops slowly over months and years, eventually causing 1-5 percent loss of module capacity.

The mechanism involves hydrogen-related defects formed during cell manufacturing that become recombination-active under heat and light. The exact atomic-level mechanism is still researched, but is associated with hydrogen passivation processes used in PERC cell production.

For Indian solar, LeTID is a particular concern because Indian summer conditions (cell temperatures of 60-80°C during noon) accelerate the mechanism; Solar Performance in Indian Winters, What to Expect covers the same PERC temperature coefficient from the opposite seasonal angle. Modern Indian PERC manufacturers implement LeTID mitigation through controlled hydrogen passivation, optimised cooling profiles and dedicated regeneration treatments. TOPCon n-type cells largely eliminate LeTID due to different material composition.

Why LeTID matters

Long-term yield. LeTID can reduce 25-year energy yield by several percent, one of several degradation mechanisms that a realistic generation estimate needs to account for; see Solar Yield in India, kWh-per-kW Benchmarks by State for how module degradation is factored into yield projections.

Warranty enforcement. Excessive LeTID may trigger warranty claims.

Indian climate sensitivity. Hot conditions accelerate LeTID.

Cell technology choice. N-type silicon cells avoid the mechanism.

Manufacturing differentiation. LeTID control distinguishes quality manufacturers.

How LeTID works

1. PERC cell manufacturing introduces hydrogen for passivation of rear surface.
2. Under combined light and heat exposure, hydrogen-related defects activate.
3. Defects act as recombination centres, reducing minority carrier lifetime.
4. Reduced lifetime lowers voltage and current, reducing module power.
5. Degradation proceeds over months and years until plateau.
6. Partial recovery possible through thermal regeneration in dark.

Technical deep dive

LeTID timeline

Magnitude by mitigation level

Test methods

Comparison with LID

Mitigation strategies

1. Controlled hydrogen passivation during cell manufacturing.
2. Optimised firing temperature profile.
3. Carrier injection regeneration treatment.
4. Process audits at cell fabrication.
5. Choice of n-type silicon (eliminates mechanism).

Real-world applications

Long-term yield modelling.

Performance warranty enforcement.

Cell technology selection.

Indian climate-specific module specification, the kind of judgment call Heaven Green Energy’s residential installation teams make when comparing PERC and TOPCon options for a hot-climate rooftop.

Common LeTID problems

Yield underperformance in second to fifth year.

Confusion with normal aging.

Difficulty distinguishing from soiling losses.

Warranty disputes.

Best practices

Specify LeTID-tested modules per IEC TS 63209.

Choose n-type TOPCon for highest LeTID resistance.

Monitor yield curves for first 5 years.

Use Tier-1 manufacturers with documented LeTID control.

Indian market context

Major Indian PERC manufacturers implement LeTID mitigation.

Transition to TOPCon n-type largely eliminates LeTID concerns.

ALMM-listed PERC modules generally control LeTID.

PM Surya Ghar warranty implicitly covers LeTID through linear performance.

Standards and certifications

Key takeaways

LeTID is slow degradation in PERC cells caused by combined light and heat exposure, manifesting over months and years. Total impact 1-5 percent depending on cell manufacturing quality. Modern controlled PERC limits LeTID to under 1 percent. TOPCon n-type cells largely eliminate the mechanism. Indian climate conditions accelerate LeTID. Specifying LeTID-tested modules and Tier-1 manufacturers is the practical mitigation for ongoing PERC installations.

Further reading

For how LeTID plays out in real projects, these guides go deeper:

• How Long Do Solar Batteries Last in India?
• Solar Shading Analysis Software 2026: Guide
• Solar Inverter Derating in Indian Summer, Explained
• Solar Inverter Failure: 20 Questions Answered

Sources

• IEC TS 63209, Extended stress testing of PV modules.
• IEC 61215, Module qualification.
• IS 14286, Indian module standard.
• ITRPV Roadmap on LeTID.
• Kersten F., Engelhart P., Ploigt H.-C., et al., A new mc-Si degradation effect, IEEE PVSC.
• Eberle R., et al., Modelling of LeTID, Energy Procedia.
• Manufacturer LeTID test reports.
• IEA PVPS Task 13 Report on PV degradation.
• NREL PV degradation studies.