What is the light and elevated temperature induced degradation behavior of Polycrystalline Solar Panels

When polycrystalline solar panels are exposed to prolonged sunlight and high temperatures, a phenomenon called Light and Elevated Temperature Induced Degradation (LeTID) occurs. Unlike temporary efficiency losses from heat, LeTID causes permanent damage to the silicon wafer structure. Here’s what happens at the atomic level: boron-doped silicon cells develop recombination-active defects when heated above 50°C under illumination. These defects trap charge carriers, reducing the panel’s ability to generate electricity. Studies show efficiency drops of 6-12% in untreated polycrystalline panels after 1,000 hours under 75°C and 1 sun irradiance. The root cause lies in the interaction between metal impurities (like copper or iron) and crystalline defects in lower-quality silicon. During manufacturing, cheaper polycrystalline wafers accumulate more impurities at grain boundaries compared to monocrystalline ones. When heated, these impurities migrate and cluster, creating electron-hole recombination centers. A 2023 study by NREL found that panels with iron concentrations above 10¹² atoms/cm³ degraded twice as fast as cleaner samples. Polycrystalline Solar Panels using standard phosphorus-doped (p-type) silicon are particularly vulnerable. The boron-oxygen complex formation accelerates under heat and light – a double whammy for outdoor installations. Real-world data from desert solar farms reveals 3-5% annual performance decline in affected panels versus 0.5-1% in LeTID-resistant designs. Manufacturers combat this through three key strategies: 1) Hydrogen passivation: Injecting hydrogen atoms during production to neutralize defects. Tier-1 suppliers like Tongwei now use advanced hydrogenation techniques that reduce LeTID losses to under 2% in accelerated aging tests. 2) Alternative dopants: Switching to gallium-doped (n-type) silicon eliminates boron-related degradation. These panels maintain 98% initial efficiency after 2,000 thermal cycles. 3) Controlled cooling: Post-production thermal annealing at 200-300°C redistributes metal impurities away from active regions. Field data shows this extends LeTID onset from 2-3 years to over 8 years in standard operating conditions. Installation practices matter too. Panels mounted with 10-15 cm rear ventilation gaps show 8°C lower operating temperatures compared to flush-mounted units. This simple design change slows defect formation kinetics – every 10°C reduction cuts LeTID progression rate by half, according to Fraunhofer ISE’s 2022 thermal modeling research. Advanced monitoring detects early warning signs. Electroluminescence imaging can spot LeTID hotspots when efficiency drops by just 2%. Some inverters now incorporate algorithm-based degradation alerts by tracking voltage-current curve deviations as small as 1.5%. The industry’s moving toward standardized testing protocols. IEC TS 63202-4:2023 specifies 192 hours at 75°C cell temperature under 1,000 W/m² light exposure as the benchmark for LeTID resistance. Panels surviving this test with <2% power loss receive TÜV Rheinland’s LeTID Certification – a growing differentiator in procurement contracts. For existing installations, regenerative annealing offers a fix. Applying controlled reverse current at 50-70°C for 6-12 hours can recover up to 90% of lost efficiency. Solar O&M companies now deploy mobile annealing units that complete this process without removing panels from arrays. The bottom line? LeTID isn’t a dealbreaker but demands informed choices. Specifying LeTID-tested polycrystalline panels, ensuring proper thermal management, and scheduling preventive maintenance can maintain system ROI above 90% throughout the 25-year lifespan. As material science advances, next-gen poly panels using upgraded metallurgical silicon (UMG) promise to cut LeTID risks while keeping costs 20% below monocrystalline alternatives.