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The primary driver of solar cell research development is the quest to enhance efficiency while reducing costs.
Passivated Emitter and Rear Cell (PERC) solar cells achieve greater efficiencies than conventional solar cells, which appear to be reaching their physical limits for efficiency. By using a more-advanced cell architecture, PERC solar cells are prime candidates for the next generation of solar panels.
The technology allows manufacturers to upgrade existing cells with a bit of tooling and achieve higher efficiencies. While the production method calls for more steps, the boost in efficiency enables significant decrease in costs at both the cells level and the system level.
Two main steps are applied to a conventional back surface field (BSF) solar cell to produce a PERC solar cell. First, a rear exterior passivation film is applied to the cell. Second, lasers or chemicals are used to create small pockets in the film and allow for more light absorption.
These two steps supply three benefits: lower electron recombination, more light is absorption and greater internal reflectivity.
When sunlight strikes non-PERC solar cells, some of it passes through. However, with a PERC cell, the passivation layer reflects unabsorbed light back to the solar cell for a second absorption pass, which results in a more efficient solar cell.
Like for any new technology, a big challenge for PERC technology determining how to scale it up while maintaining fidelity. There are also two primary technical difficulties associated with PERC technology.
The first is Light Induced Degradation (LID), an effect that causes a cell to lose a percentage of its production after first exposition to light. The second challenge to PERC technology is Probable Induced Degradation (PID). This type of defect can totally damage the performances of a solar panel.
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Light Induced Degradation (LID)
At best, LID can neutralize the efficiency boost provided by the PERC solar cell enhancements. At worst, LID can push efficiency below that of conventional ASF cells.
The two best-known mechanisms behind LID are the activation of boron–oxygen (B–O) complexes and the dissociation of iron–boron (FeB) pairs.
B-O LID is driven by significant oxygen and boron levels in the wafer, with monocrystalline silicon being most vulnerable due to its high oxygen concentration. Conversely, multicrystalline silicon is much less vulnerable due to lower oxygen concentrations.
In FeB LID, the shift in the Fermi level during illumination triggers a neutralization of the iron ions, which consequently separate from the boron atoms. Interstitial iron causes a much greater charge carrier recombination under operating conditions. The resulting dissociation of iron-boron pairs causes a severe efficiency loss in cells, particularly those with a high iron concentration.
Potential Induced Degradation (PID)
In an ideal world, solar panels should maintain optimum performance for 20 years. However, most sustain peak performance for 2 or 3 years. Possible Induced Degradation (PID) is what causes this less-than-ideal efficiency drop. PID can cause a power loss of up to 30 percent and solar panels to last for a relatively short amount of time.
PID is triggered by minor, unwanted currents between the two sides of a solar cell. These currents are facilitated by the migration of sodium ions from the glass plate through the encapsulation and the Anti-Reflective Coating into the cell.
The presence of these ions creates an effective shunt path across the cell and cuts down on power output. The impact is cumulative with the passage of time.
Heat, humidity, and adverse voltage are known to cause PID, but it is not clear why this happens in the first place or in only certain PERC solar panels.
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