PERC Cell Technology: [All To Know About]

Solar PV systems have energy losses at different stages of converting solar energy to electricity.

These losses are associated with solar panels, inverters, cabling, and other electrical equipment that are used to build these systems.

Scientists and solar PV systems manufacturers are constantly trying to reduce these energy losses and improve the overall efficiency of solar panels.

We know a solar PV system is only as efficient as the cells that make it up. When a solar module or collection of modules is made from less efficient cells, the result is a PV system that often fails to meet its expectations.

Therefore, using better solar cells with higher conversion efficiencies is the first and most important step in reducing the losses of a solar PV system.

The standard aluminum back surface field (Al-BSF) design has accounted for over 90% of global solar cell production for decades.

This cell design is widely accepted as a cost-effective solution due to its extensive knowledge of manufacturing tools, materials, and methods.

However, cell technology has advanced significantly over the years, and efficiencies have increased.

A relatively newer cell design, PERC (Passivated Emitter and Rear Cell) has emerged as a better alternative to the conventional BSF design.

Although PERC technology has been known since 1989, commercial implementations have run into issues due to growing light-induced degradation.

PERC modules, as a result of continuous enhancements over time, are more efficient than standard solar cells by about 1%.

This number may seem not much. But in reality, this is a huge improvement since a small improvement in efficiency can lead to a large increase in overall energy production.

Considering that a traditional standard module typically has a 20% efficiency, a system utilizing PERC modules will produce about 5% more energy than a system using traditional modules provided all else is kept equal.

The passivated emitter and rear cell (PERC) design have a current and prospective commercial cell efficiency of 21–24% while requiring only minor modifications to typical Al-BSF processing.

This offers for the continued use of existing industrial equipment, materials, and processes.

The two main advantages of PERC's over the Al-BSF cell: reduced rear-surface recombination and improved rear-surface reflectivity that we are going to mention further in this post.

What Is PERC cell?

PERC cells are a type of monocrystalline silicon solar cell that was developed in response to the limitations of traditional cells. Traditional cells are made of a single silicon wafer, which absorbs sunlight and converts it into electrical energy.

However, traditional cells are limited in their efficiency by a phenomenon called rear surface recombination.

This is when electrons that are generated by sunlight on the rear surface of the cell recombine with holes, reducing the amount of electrical charge that can be extracted from the cell.

PERC cells overcome this limitation by adding a passivation layer to the rear surface of the cell.

This layer is typically made of silicon nitride and serves to reduce the number of electron/hole recombination events, increasing the amount of charge that can be extracted from the cell.

PERC cells also have an anti-reflective coating on the rear surface, which helps to reduce the amount of sunlight that is reflected away from the cell.

This coating is typically made of a material like titanium dioxide, and it can increase the cell's efficiency by up to 1%.

Overall, these two features (the passivation layer and anti-reflective coating) work together to increase the efficiency of PERC cells. In fact, PERC cells have a conversion efficiency of around 23%, compared to around 15-17% for traditional cells.

How does PERC increase energy efficiency?

PERC technology improves the overall performance of a cell by boosting a cell's light-capturing ability.

A typical solar cell is composed of two layers of silicon with distinct electrical properties: the base and the emitter.

When negatively charged particles (electrons) come into contact with the interface, a powerful electric field is generated, which draws them into the emitter.

Light entering the cell and releasing electrons from the silicon atoms generates the electrons. Electrons flow freely through the cell and contribute to the electrical current only if they reach the interface.

Different wavelengths of light generate electrons at different levels of the cell structure.

Shorter wavelengths (blue light) generate more electrons near the front of the cell, while longer wavelengths (red light) generate electrons at the back of the cell or even pass through the wafer without generating current.

The addition of PERC technology increases cell efficiency by reflecting back into the cell any light that has passed through to the rear without producing electrons.

As a result of this reflection, photons are effectively given a second chance to generate electricity.

The ability to capture longer wavelengths of light, such as when the sun is at an angle (early mornings and nights) or when it is cloudy, increases the energy yield of PERC cells.

When the Sun is directly overhead, a greater amount of blue light (wavelengths between 450 and 495 nm) is absorbed by the atmosphere because it has a longer path to the Earth's surface.

Blue light is generally converted to energy near the top of the cell, whereas red light (wavelengths 620 to 750 nm) penetrates deeper and is converted to energy near the bottom.

Because the Earth's atmosphere absorbs red light easier, cells that collect more red light are often more potent.

The PERC technology's reflecting properties boost red light absorption even in low or diffuse light conditions, increasing energy yields.

The silicon wafer does not absorb wavelengths over 1180 nm. These wavelengths are simply absorbed into the backside metallization of standard cells, raising the cell temperature and decreasing conversion efficiency.

Because the PERC layer reflects this light back through the cell and out of the panel, the amount of absorption by the aluminum metallization layer, and thus heat buildup within the cell, is reduced.

Because of the reduced absorption, the cell can operate at a lower temperature, increasing the energy yield.

What are the advantages of PERC technology?

PERC modules have several advantages over traditional solar cells:

Higher conversion efficiency

PERC technology allows for a higher conversion efficiency compared to traditional solar cells.

This is primarily due to the anti-reflective coating on the rear side of the cell, which allows more sunlight to be absorbed and converted into electricity.

The passivation layer also helps reduce electron recombination, allowing for a higher electrical charge to be extracted from the cell.

The increased conversion efficiency of PERC cells is particularly important for solar panel installations where space is limited, such as rooftop solar systems.

By generating more electricity per unit of area, PERC cells allow for more power to be generated from a given amount of space.

Reduced rear recombination

PERC cells also offer reduced rear recombination compared to traditional solar cells.

Rear recombination occurs when electrons on the rear surface of the cell recombine with holes, reducing the amount of electrical charge that can be extracted from the cell.

The passivation layer on the rear side of PERC cells helps to reduce this effect, increasing the overall efficiency of the cell.

Longer lifespan

Another advantage of PERC technology is that it can result in a longer lifespan for the solar cell.

This is because the anti-reflective coating on the rear side of the cell helps to reduce the amount of heat generated by the cell. Heat can degrade the performance of the cell over time, leading to a shorter lifespan.

By reducing the amount of heat generated, PERC cells are less likely to experience degradation over time and may have a longer lifespan compared to traditional cells.

More flexible design

PERC technology also allows for a more flexible solar panel design. The anti-reflective coating on the rear side of the cell means that the cell can absorb light from both the front and rear sides.

This opens up the possibility of using bifacial solar panels, which can generate electricity from both the front and rear sides of the panel, increasing overall efficiency.

PERC cells can also be used with solar trackers, which follow the sun as it moves across the sky, further increasing the amount of sunlight that hits the solar panel.

Overall, PERC technology offers several advantages over traditional solar cells, including higher conversion efficiency, reduced rear recombination, longer lifespan, and more flexible design options.

These advantages make PERC cells an attractive option for both residential and commercial solar installations.

What are the disadvantages of PERC technology?

However, there are also some disadvantages of PERC technology:

Light-induced degradation (LID)

One disadvantage of PERC technology is that it is susceptible to light-induced degradation (LID). LID occurs when the anti-reflective coating on the rear side of the cell enhances the effects of sunlight on the cell, causing a temporary reduction in efficiency.

This effect can be mitigated through careful design and material selection, but it is still a problem with this technology.

Potential-induced degradation (PID)

Another disadvantage of PERC technology is the potential for potential-induced degradation (PID). PID occurs when high voltages between the encapsulated solar cells and the front glass surface cause unintended migration of charge carriers, reducing the cell's performance over time.

PID can be more pronounced when high voltages are present due to lengthy string connections, and it is accelerated when there is high humidity and temperature in the environment.

The PID effect usually appears months after the PV system is installed, making it a serious problem that can significantly reduce the system's overall performance without warning.

The best way to avoid PID is to select solar modules that have been certified for PID resistance in accordance with IEC TS 62804.

Higher production costs

Another disadvantage of PERC technology is that it is more expensive to produce than traditional solar cells.

The extra steps required to apply the anti-reflective coating and passivation layer increase production costs, which can make PERC cells more expensive for consumers.

Limited availability

While PERC technology is becoming more widely adopted, it is still not as widely available as traditional solar cells. This can make it difficult to find PERC modules in some markets, which can limit its potential for widespread adoption.

In summary, while PERC technology offers several advantages over traditional solar cells, including higher conversion efficiency, reduced rear recombination, longer lifespan, and more flexible design options, it is also susceptible to LID and PID, which can reduce its efficiency over time.

Additionally, PERC technology is more expensive to produce than traditional solar cells and may not be as widely available in some markets.