Mono-crystalline PERC modules experience typical first-year degradation of 3% (measured average 1.92%) due to boron-oxygen (B-O) complex defects, leading to significant power generation loss over the lifecycle;
while N-type TOPCon, utilizing phosphorus-doped wafers, avoids the BO-LID mechanism, achieving first-year degradation <1% (outdoor demonstration only 0.51%).
Yinchuan demonstration data shows: Under equivalent irradiation, TOPCon modules degrade less than 37% of PERC modules after 6000 hours.
TOPCon's tunnel oxide layer and poly-silicon passivation structure simultaneously suppress surface recombination, resulting in a lab light-induced degradation rate as low as 0.26%.
Lower degradation combined with 24%-26% conversion efficiency advantage enables TOPCon to achieve 3-5 year power gain covering the initial cost premium in large-scale power plants, reshaping high-efficiency module selection logic.
Causes
Formation and Activation of Boron-Oxygen Complexes
The core mechanism of LID is the formation of boron-oxygen complexes (B-O) under illumination. In P-type wafers doped with boron, boron atoms combine with interstitial oxygen to form unstable B-O defects:
· Formation Condition: Under illumination intensity >1 mW/cm², the boron-oxygen complex enters an active state (State B), causing minority carrier lifetime to drop from 1000μs to below 500μs.
· Temperature Influence: For every 10°C temperature increase, the B-O complex formation rate increases 2-3 times. For example, at 75°C, the LID degradation rate of PERC modules is 4.7 times that at 25°C.
· Oxygen Content Difference: Mono-crystalline silicon, grown using quartz crucibles, has a high oxygen content of 10-14 ppma, while multi-crystalline silicon from casting has only 1-2 ppma. This leads to 2-3 times higher LID degradation in mono-Si compared to multi-Si.
Process Parameter Amplification Effect on LID
Cell manufacturing processes directly affect the activity of B-O complexes:
·Sintering Temperature: When sintering peak temperature >850°C, hydrogen from the passivation layer diffuses into the silicon substrate, combining with boron to form reversible defects. Experiments show that for every 50°C increase in sintering temperature, the LeTID degradation rate increases by 0.8%.
·Metal Contamination: Iron (Fe) impurities combine with boron to form Fe-B pairs, which decompose into Feⁱ and Bⁱ⁰ under illumination, creating additional recombination centers. 1 ppm iron contamination can increase LID degradation by 0.5%.
·Insufficient Hydrogen Passivation: When hydrogen content in the passivation layer (e.g., AlOx/SiNx) is <1×10¹⁹ atoms/cm³, it cannot effectively passivate B-O defects. TOPCon requires 40% less hydrogen due to the absence of boron doping, improving defect regeneration efficiency.
Correlation Between Cell Structure and LID Sensitivity
Different cell structures show significant differences in LID response:
·PERC Cells: The rear passivation layer increases long-wavelength light absorption, leading to higher carrier concentration and enhanced B-O complex activity. Measurements show PERC LID degradation is 1.8 times that of conventional aluminum back surface field (Al-BSF) cells.
·TOPCon Cells: When the tunnel oxide layer (SiOx) thickness is controlled at 1.5nm, surface recombination velocity is <0.5 cm/s, suppressing defect activation. Lab data indicates TOPCon's LID degradation rate is 82% lower than PERC.
·Heterojunction (HJT) Cells: The amorphous silicon passivation layer introduces additional defects, but 90% of interface states can be repaired by hydrogen annealing, keeping LID degradation below 0.3%.
Environmental Factors and Dynamic Response of LID
Mechanisms of outdoor environment accelerating LID:
·UV Radiation: Ultraviolet light (280-320nm) induces oxygen vacancy generation, which combines with boron to form complexes. Zhangbei demonstration data shows, in regions with annual UV irradiation >2000 kWh/m², PERC modules experience an additional 0.7% LID.
·High Temperature and Humidity: Under 85°C/85%RH conditions, moisture penetration causes hydrolysis of boron-oxygen complexes, generating mobile ions and accelerating recombination center diffusion. Damp heat test (1000 hours) caused PERC module LID degradation of 1.2%.
·Mechanical Stress: Module encapsulation stress causes micro-cracks in wafers. Oxygen concentration gradients at crack tips trigger local B-O complex formation. During thermal cycling (-40°C~85°C) tests, cracked modules had 0.9% higher LID degradation than intact modules.
Data-Driven LID Prediction Model
Physics-based LID prediction requires integrating multi-dimensional parameters:
·Key Variables: Boron concentration (B), Oxygen concentration (O), Effective carrier concentration (Δn), Temperature (T).
·Empirical Formula: LID degradation rate (%) = 0.003×B×O×exp(-Ea/(kT)), where Ea=0.85eV (activation energy of boron-oxygen recombination), k is Boltzmann constant.
·Measurement Verification: Statistics on 1000 PERC cells show formula prediction error <±0.2%, can guide wafer doping process optimization.
Degradation Rate Comparison
Laboratory Light-Induced Degradation Test Conditions and Data
Standardized LID laboratory testing procedure:
·Illumination Dose: 5 kWh/m² (AM1.5G spectrum, 1000 W/m² intensity)
·Temperature Control: 25°C constant temperature
·Test Duration: Continuous illumination for 100 hours
Technology Improvement
Boron Doping Alternatives
Root Problem: P-type PERC cells suffer first-year degradation up to 3% (lab data) due to boron-oxygen complexes (BO-LID).
Solutions:
·Gallium (Ga) Doping: Replace boron with gallium as dopant, avoiding BO-LID reaction pathway. Gallium's segregation coefficient (0.35) is lower than boron's (0.8), requiring adjustment of thermal field distribution:
o Crystal growth temperature: 1450°C → 1520°C (reduces Ga volatilization)
o Radial temperature gradient: <5°C/cm (improves crystal quality)
o Measured effect: LID degradation reduced from 3% to 0.7%, but resistivity fluctuation ±12%.
·Indium (In) Co-doping: Boron-indium co-doping (B: In=10:1) further reduces oxygen solubility:
o Oxygen content: 10ppma → 5ppma
o Minority carrier lifetime: 500μs → 800μs
o Cost increase: Wafer price increased by $0.005/W.
Annealing Process:
·Low-Temperature Annealing (LTA):
o Temperature: 200°C → 300°C
o Time: 10 minutes → 30 minutes
o Effect: Activates hydrogen passivation, repairs boron-oxygen defects
o Data: PERC cell LID degradation reduced by 0.5%.
Passivation Layer Upgrade
Surface Passivation Technology:
·AlOx/SiNx Stack:
o Thickness control: AlOx 3nm + SiNx 80nm
o Surface recombination velocity: <10 cm/s (conventional PERC 20 cm/s)
o Lab data: Minority carrier lifetime increased to >1500μs.
Rear Passivation Optimization:
·SiNx Thickness Adjustment:
o Conventional: 120nm → Optimized: 150nm
o Effect: Reduces boron diffusion to rear, suppresses LeTID
o Result: LeTID degradation reduced from 1.17% to 0.3%.
Conversion Efficiency
Mass production efficiency reaches 25.4% (SunPower Maxeon 7), laboratory record 26.8%, approaching the 28.7% theoretical limit;
PERC is stagnant at 23.5%. TOPCon's temperature coefficient is -0.29%/°C, bifaciality 85%+ increasing energy yield by 20%, degradation rate <0.4% per year, 30-year power retention 87%.
Theoretical Limits
Physical Boundary of Mono-crystalline PERC
Mono-crystalline PERC cells, based on P-type wafers, have a theoretical efficiency limit of 24.5% (Shockley-Queisser limit).
This value is determined by silicon's bandgap (1.1 eV) and solar spectrum match.
In mass production, boron doping leads to boron-oxygen complexes (B-O) causing light-induced degradation (LID), with first-year efficiency loss of 2-3%.
