PERC Cells – Value Proposition
In the past, the value proposition of solar modules has been primarily about the lowest price (Rs/Wp) via the lowest manufacturing costs of conventional multi-Si modules. However, in the last few years, PV module cost had dropped at a faster pace than Balance of System (BOS) costs, resulting in PV modules accounting for a smaller and smaller percentage of overall PV system costs.
PV industry will continue driving down module cost, but its impact on overall system cost reduction has become smaller and smaller. High efficiency mono-PERC modules not only help to reduce module cost, but also leverage to reduce BOS cost.
In basic terms, higher efficiency modules mean fewer modules are required for a given system power output leading to less electrical and mechanical hardware required, as well as savings on land and labour.
Cost savings using high-efficiency mono-Si PERC modules on commercial and residential rooftop projects, can be more significant than on utility-scale projects. The chart below highlights those potential savings on a cost per-watt (US$/W) basis.
PERC Technology – A primer
PERC stands for Passivated Emitter Rear Cell, Passivated Emitter Rear Contact or even Passivated Emitter and Rear Cell. Currently, around one quarter of new solar panels are PERC and the figure may reach 50% in 20 20. The reason it has become popular is because it adds little to the cost of solar panels while increasing their efficiency by around one whole percentage point. It improves efficiency by allowing electrons to flow more freely and also by increasing the reflectiveness of the back of solar cells.
At its core, a PERC solar cell is simply a more efficient solar cell and PERC panels perform better than traditional panels in both low-light conditions and high temperatures. PERC technology boosts efficiency through the addition of a layer to the back of a traditional solar cell, which provides several benefits to the cell’s production.
PERC solar cells are an exciting technology because of the efficiency gains they provide over standard solar cells. Additionally, moving to PERC manufacturing doesn’t require a lot of modifications to existing cell manufacturing processes. It’s a relatively easy shift for manufacturers to start producing higher efficiency PERC cells at a low cost.
A PERC solar cell is not much different in construction from a normal solar cell. Both cells use silicon wafers to generate a flow of electrons using incoming solar radiation, and the overall construction is very similar. The main difference between PERC cells and normal cells is the integration of a back surface passivation layer, which is a di-electric material on the back of the cells that provides 3 main benefits that boost cell efficiency which are spelt out below:
1. Reflection of light back through the cell
A back-surface passivation layer has high internal reflectance and reflects light that passes through the silicon cell without being absorbed back into the silicon, giving the solar cell a second absorption attempt. This reflection of light means that more incoming solar radiation will end up being absorbed by the silicon cell, thus the cell becomes more efficient.
2. Reduced electron recombination
The addition of a back-surface passivation layer reduces “electron recombination” in the solar cell. Simply put, electron recombination is the tendency of electrons to recombine, which blocks the free movement of electrons through the solar cell. This inhibition of free electron movement leads to less-than-optimal cell efficiencies. In a PERC solar cell, electron recombination is reduced in order to bump up efficiency.
The atoms at the surface of a silicon wafer have ‘dangling bonds’ which can capture electrons and pull them back into the silicon crystal structure (a process called surface recombination). As a result, when an electron reaches the back surface of a conventional solar cell, it is likely to be captured and does not contribute to the current. However, in a PERC solar cell, the passivated film on the back surface reduces this effect by tying up the ‘dangling bonds’. Thus, an electron that strays too close to the back surface is allowed to continue on its way and the chance it will reach the emitter and contribute to the electric current is increased.
3. Reflecting Counter-productive Wavelengths
The sun emits light in different wavelengths and when the light reaches the cell, it generates electrons at various levels of the solar cell structure. When the sun shines on a standard solar cell, the shorter wavelengths below 500 nm (blue light) get absorbed by the solar cell (i.e., the blue light would knock off electrons), but the longer wavelengths above 620 nm (red light and infrared) may pass through the solar cell without being converted into useful energy.
A silicon wafer in a solar cell can only absorb light in wavelengths up to 1180 nanometers (nm), and higher-wavelength light waves pass through the silicon and are absorbed by the solar cell’s Al back contact, creating heat. When solar cells are heated, they operate at lower efficiencies.
PERC technology actually increases the cell’s ability to reflect longer wavelengths by introducing the di-electric passivation layer at the rear. The extra reflector layer in the PERC solar cell is effective at reflecting infrared wavelengths longer than 1000 nm for a second absorption phase, resulting in better response than standard cell technology across the whole spectrum, shown in below figure.
The back-surface passivation layer in PERC cells is specially designed to reflect light with a wavelength above 1180 nm, reducing the heat energy in the solar cell and consequently increasing efficiency. The longer wavelengths are especially present during mornings and evenings (sun under an angle) or during cloudy days.
PERC technology has a better ability to capture photons at low-light conditions or light at longer wavelengths, e.g., when the sun is at an angle (early mornings and evenings) or under cloudy conditions. At such times, a higher quantity of blue light (wavelengths between 450 to 495 nm) is absorbed by the atmosphere as it has a longer path to travel to the Earth’s surface than when the Sun is directly overhead (hence the sky is red/orange during mornings & evenings).
During low light (dawn and dusk) and weak light (cloudy) conditions, the shorter wavelength blue light gets absorbed by the atmosphere, allowing higher proportion of long wavelength light to reach Earth. In such situations, PERC solar cells improved spectral response at long wavelengths allows the cells to perform better than standard cells and maintain close to top efficiency at low light conditions, as illustrated by below figure. PERC modules are able to maintain close of 99% of its STC efficiency at 200W/m2 irradiance level, while a standard solar module’s relative efficiency drops to less than 96% at the same condition.
Blue light is generally converted to energy near the top of the cell, whereas red light (wavelengths between 620 to 750 nm) penetrates further through the cell and is converted to energy near the bottom. Red light is less easily absorbed by the Earth’s atmosphere and as a result, cells which capture more red light are generally more powerful (see figure below). Thus the ‘reflective’ properties of the PERC technology ensure increased absorption of red light, even in weak or diffuse light conditions, delivering better energy yields.
PERC technology improves the internal reflection of light at long wavelengths
The increase in sensitivity to light produced by the use of PERC technology is seen in the spectral response of a PERC cell (see below figure). As the graph shows, PERC technology increases the absorption of infrared light (wavelengths of between 1000 and 1180 nm) with this additional sensitivity resulting in increased current and cell efficiency.
PERC technology increases cell sensitivity to wavelengths between 1000 & 1180 nm
As a result, the energy output of a PERC module is higher than a standard module across a whole day, shown in below figure. A PERC solar panel installation is known to begin producing electricity earlier in the morning and stops later in the evening than an installation of conventional solar modules.
Summary of Benefits of Passivation
Helping electrons flow is not the only advantage of passivation. It also increases the amount of light reflected from the bottom of the cell. This is useful because if light missed hitting a silicon atom and knocking an electron free on its way in, reflection will give a second chance on the way out.
Almost all solar cells have a reflective layer on the bottom which is normally Aluminium. However, because the refractive index of silicon is higher than glass, the resulting reflectivity of around 89% isn’t as high as a normal Aluminium backed glass mirror. However, placing the passivated layer with a lower refractive index between the silicon and the Aluminium can increase reflectivity close to the maximum possible of 98%. This gives a boost to the panel’s efficiency.
Reflecting more light and reducing the amount absorbed by the back of the panel makes it a little bit cooler and this also results in an increase in efficiency.
PERC solar cell manufacturing
One of the biggest reasons why PERC technology can be so powerful is the minimal investment it takes to begin manufacturing PERC solar cells instead of standard monocrystalline solar cells. In order to produce a PERC cell, there are two additional manufacturing steps needed:
- Application of the back-surface passivation layer
- Laser/chemical etching to open small pockets in the passivation layer
In employing just two additional steps, the return is threefold:
- Electron recombination is significantly reduced;
- More light is absorbed; and
- Higher internal reflectivity is experienced.
These two steps don’t add significant costs to the solar cell manufacturing process and result in a higher quality, more energy-dense solar cell. A classic barrier to new solar cell technology is the cost of new equipment production, and PERC solar cells require very little monetary investment to upgrade to a better product.
Benefits of PERC technology for customers
Solar panels built with PERC technology allow for more energy-dense solar installations. This means that you can generate the same amount of energy using fewer PERC solar panels than they would with more standard solar panels. Consequently, by needing fewer solar panels for your installation, your costs can be reduced. Additionally, the fewer panels you need, the more flexibility you have on your roof to position your panels. If suitable roof space is limited, using PERC solar panels or any high-efficiency panel product can make a solar installation capable of the power you need a reality.
Reducing the number of solar panels you need has the added benefit of bringing down the balance-of-system (BOS) costs for your solar panel installation. BOS costs are generally any costs involved in solar installation components that are not the solar modules themselves. Inverters, racking and wiring all factor into your BOS costs, and the fewer panels you need, the fewer complementary components you’ll need as well.
Not all sunlight is absorbed through non-PERC solar cells (some light passes straight through). However, with a passivation layer on the rear side of a PERC cell, unabsorbed light is reflected by the additional layer back to the solar cell for a second absorption attempt. This process leads to a more efficient solar cell. This is great news for those across the spectrum of the industry.
PERC panels – Ideal for designers and architects
PERC technology lets solar cells absorb more light and gives them a higher internal reflectivity, providing end users with more possibilities for tilt and installation options. This is great news for those with unorthodox spaces or locations that might not initially seem the best for a solar PV installation, as the PERC tech allows for much greater array layout flexibility. Thus, panels incorporating PERC technology give more freedom to architects and designers, especially when dealing with spaces or locations that were once thought to be less than desirable for solar. PERC tech is ideal for rooftops, elevated ground, desert or snowy environments or on-water applications.
PERC panels have a higher energy density per square foot and perform well under low-light conditions and high temperatures. When considering total energy production, it is clear PERC panels are superior. Designers can utilize fewer panels to accomplish total output goals where footprint is limited, or they can dramatically maximize energy output if space is not a premium. It empowers designers to be more flexible and responsive to project objectives. This freedom also allows the option to drive down BOS costs. More is being achieved with less, which can trickle down to significantly reduced soft costs. This can be the difference between a client having sticker shock and not moving forward with project to one seeing a cost-effective and manageable system. Additionally, more attractive temperature coefficients make PERC a top performer in hotter climates, and less thermal loss is experienced. This allows end users to achieve superior performance from their systems throughout the year.
PERC leads to increased energy density and electricity generation for the system. Greater energy density significantly reduces the levelized cost of electricity (LCoE) for a solar PV installation. By effectively leveraging the modules with PERC technology, end users can obtain a much faster RoI. PERC panels also perform better under low-light conditions and high temperatures.
Reduced risk as it is a proven and advanced technology
PERC technology was first developed in Australia in the 1980s by scientist Martin Green and his team at University of New South Wales. However, manufacturers spent many years focusing only on the front side of a solar cell, and less attention was paid to taking advantage of production opportunities from the backside. Thus, the conventional technology went through incremental improvement, with better pastes to form front contacts, thinner contact fingers, optimized anti reflective coating… It took almost 30 years to the industry to relook at the PERC technology.
PERC cell technology has emerged in recent years, mainly due to economic reasons. For 30 years, the steady incremental improvements brought to the standard cell technology were economically and technically feasible. Now that the standard concept achieves its limits and that technical know-how is available along the value chain to introduce PERC technology, it can constitute a new viable platform to manufacture high power and high efficiency solar panels.
As anticipated by ITRPV, a body that gathers a set of manufacturers at the different steps of the value chain and that works on technology trends, the PERC technology will progressively take the biggest market share. Because this is the sense of history in the PV industry to get better and better, sooner or later, most of the panels installed will feature this technology.
PERC solar panels first appeared around 2012 but it took manufacturers several years to refine the process and bring down costs. Being that PERC technology is neither new or radically different from standard cells, there is reduced risk on the customer’s side of the table to back an advanced technology. By utilizing a proven technology and modifying the standard cell, there is no change in the inherent risk of the module and its performance. Customers should warm to the idea of panels manufactured with PERC technology. It sets a course for establishing dependable, long-term power output in a cost-effective manner for residential and commercial rooftop projects. Total power generation over the lifetime of the solar system is increased without dramatically boosting the cost per watt.
Higher Power Density
Mono-PERC modules have a higher power density than multi modules which means that fewer modules are required for each project or more power/kwh can be achieved with limited space as can be seen in the table below.
Slower Power Degradation
Extensive testing by the US National Renewable Energy Laboratory (NREL) has shown that mono-Si modules demonstrate lower long-term degradation rates than multi-Si modules, resulting in more electricity being generated over the long-term by mono-Si modules with the same power rating (Pmax) than by multi-Si modules.
In conclusion, the value proposition of using high-efficiency mono-Si PERC modules are:
- Higher efficiency/power density results in BOS cost savings (lower system cost.
- Mono-PERC modules demonstrate higher energy yield.
- Mono-PERC modules show a slower long-term degradation rate.
It is clear that lower LCOE can be realized by using mono-PERC modules.