Light-induced degradation (LID) in crystalline silicon solar cells is a critical issue that affects long-term performance and reliability. Two primary mechanisms dominate LID: boron-oxygen (B-O) complexes and iron (Fe) contamination. Understanding these mechanisms and implementing stabilization methods such as regeneration and pre-treatment are essential for mitigating efficiency losses.
**Boron-Oxygen (B-O) Complexes**
B-O complexes are a leading cause of LID in boron-doped p-type silicon solar cells. Under illumination, these complexes form metastable defects that increase carrier recombination, reducing minority carrier lifetime and cell efficiency. The degradation process involves the following steps:
1. **Formation of B-O Defects**: When silicon containing boron and oxygen is exposed to light, the energy from photons breaks weak bonds, creating B-O complexes. These complexes act as recombination centers.
2. **Recombination Activity**: The B-O defects introduce energy levels within the bandgap, enhancing Shockley-Read-Hall recombination. This reduces the open-circuit voltage (Voc) and fill factor (FF).
3. **Reversibility**: Unlike permanent degradation, B-O defects can be partially or fully reversed through thermal annealing or high-intensity illumination at elevated temperatures.
Studies show that the extent of degradation depends on boron and oxygen concentrations. Cells with higher oxygen content exhibit more severe LID. For example, a typical efficiency loss of 1-4% relative is observed in standard boron-doped Czochralski (Cz) silicon.
**Iron (Fe) Contamination**
Iron is another significant contributor to LID, particularly in multicrystalline silicon (mc-Si) and lower-purity Cz-Si. Fe impurities introduce deep-level traps that degrade carrier lifetime. The mechanism involves:
1. **Dissociation of Fe-B Pairs**: In p-type silicon, iron typically forms pairs with boron (Fe-B). Under illumination, these pairs dissociate into interstitial iron (Fei), which is highly recombination-active.
2. **Recombination Centers**: Fei introduces deep energy levels near the mid-gap, significantly increasing recombination rates.
3. **Reversibility**: Unlike B-O defects, Fe-related degradation can be reversed by dark annealing, where Fe-B pairs re-form, reducing recombination activity.
The impact of Fe contamination depends on its concentration. Even trace amounts (below 1e12 cm-3) can cause measurable efficiency losses. Advanced purification techniques during silicon processing help minimize Fe contamination.
**Stabilization Methods**
**Regeneration Techniques**
Regeneration methods aim to neutralize or reverse LID effects, particularly for B-O defects. Key approaches include:
1. **Illumination at Elevated Temperatures**: Exposing solar cells to high-intensity light at temperatures around 75-200°C accelerates the passivation of B-O defects. This process, known as regeneration, permanently deactivates the recombination centers.
2. **Thermal Annealing**: Heating cells to 150-300°C in the dark can partially recover performance by dissociating B-O complexes. However, this method is less effective than illuminated annealing.
3. **Fast-Firing Regeneration**: A rapid thermal process during cell manufacturing can pre-stabilize cells, reducing initial LID effects.
For Fe contamination, regeneration involves:
1. **Dark Annealing**: Storing cells in the dark at room temperature or slightly elevated temperatures allows Fe-B pairs to re-form, reducing Fei concentration.
2. **Low-Temperature Illumination**: Controlled light exposure at low temperatures can accelerate Fe-B pair reformation.
**Pre-Treatment Methods**
Pre-treatment strategies focus on preventing LID before it occurs. These include:
1. **Reduced Oxygen Content**: Using silicon with lower oxygen concentrations minimizes B-O defect formation. Float-zone (FZ) silicon, which has negligible oxygen, exhibits no B-O LID.
2. **Alternative Dopants**: Gallium-doped silicon avoids B-O LID entirely, as gallium does not form light-sensitive complexes with oxygen.
3. **Gettering Processes**: High-temperature gettering during wafer processing removes Fe impurities, reducing Fe-related LID. Phosphorus diffusion gettering is particularly effective.
4. **Hydrogen Passivation**: Incorporating hydrogen during cell processing passivates defects, including B-O complexes and Fe impurities. Hydrogenation can be achieved through SiNx:H coatings or post-deposition annealing.
**Comparative Effectiveness of Methods**
| Mechanism | Stabilization Method | Effectiveness | Notes |
|-----------------|-------------------------------|--------------|----------------------------------------|
| B-O Complexes | Illuminated Annealing | High | Permanent deactivation |
| B-O Complexes | Thermal Annealing | Moderate | Temporary recovery |
| B-O Complexes | Gallium Doping | Complete | No B-O formation |
| Fe Contamination| Dark Annealing | High | Reforms Fe-B pairs |
| Fe Contamination| Gettering | High | Reduces Fe concentration |
**Conclusion**
LID in crystalline silicon solar cells is primarily driven by B-O complexes and Fe contamination. While B-O defects are reversible through illuminated annealing, Fe-related degradation can be mitigated by dark annealing and gettering. Pre-treatment methods, such as alternative dopants and hydrogen passivation, offer proactive solutions. Implementing these stabilization techniques ensures higher long-term efficiency and reliability in silicon photovoltaics. Continued research into advanced passivation and material engineering will further reduce LID impacts in future solar technologies.