If you’re a homeowner who is comparing solar panels, there are really only a few features that obviously differentiate one panel from another, and efficiency tends to be one that manufacturers place front and center.
Solar panel efficiency refers to how much light a solar panel converts into electricity. The higher the efficiency, the more electricity you’ll get from the panel for the same amount of light. For a rooftop deployment with limited space, this can be a very important feature.
In the ongoing race to create increasingly cheaper and more efficient panels, solar manufacturers turn to different technologies to give them an advantage. One of the more common technologies that you’ll encounter is Passivated Emitter Rear Cell (PERC).
To understand what PERC is, we should first explain a little bit about how a solar cell works.
A conventional solar cell works by converting sunlight directly into electricity. Light is made of elementary particles called photons. Travelling at the speed of light, photons collide with the silicon in the solar cell. At that point, one of four things might happen to the photon:
As you can see, not every photon creates electricity in the cell: for residential solar panels, a high efficiency panel will convert around 20%, or a little more, of sunlight into power.
Solar cells are capable of capturing photons only in a specific energy range, which is why many photons simply pass through or are absorbed by the silicon without exciting an electron.
PERC is one technology used to improve that capture rate.
To improve the number of photons that are captured by a solar cell, PERC technology adds two additional layers at the rear of the cell. Those layers improve the movement of electrons in the cell, and also bounces light back into the cell, giving the cell a second chance to capture electrons that would otherwise simply pass through.
The absolute gains in efficiency from PERC will vary from manufacturer to manufacturer, but you can roughly expect an absolute increase of 1% efficiency in the cell. This means that if the solar panel was 19% efficient, the use of PERC might boost that panel to 20% efficiency.
So we’re not talking about a dramatic improvement in efficiency, but an incremental improvement. Silicon solar cells have been around since 1954, which means that the technology is now very mature. Future dramatic breakthroughs are likely to come from novel materials and processes, rather than silicon cell technology.
As with anything, there are tradeoffs with PERC. A positive aspect of PERC is that it’s a fairly mature technology, having been first developed in the 1980s. It’s also relatively cheap to implement because the additional rear layers are screenprinted, which is already part of solar manufacturing processes.
One possible downside to PERC is suspectibility to a type of degradation known as light and elevated temperature-induced degradation (LeTID). This type of degradation causes a loss of efficiency over time as the panel is subjected to light and heat.
The use of PERC can make a panel more suspectible to LeTID, even causing a 20% loss of efficiency in only 2 or 3 years, as noted by one researcher.
LeTID in PERC cells can be mitigated with the use of purer materials and better manufacturing processes. Some research suggests that current PERC panels do suffer some degradation in the real world, but the same research also argues that the use of PERC is worth it, even with the additional degradation factored in.
For one manufacturer, at least, the risk of LeTID is reflected in the power warranty. The REC TwinPeak 2 Mono, a PERC panel, comes with an 80.7% / 25 year power warranty. This means that after 25 years, the TwinPeak 2 Mono is guaranteed to still produce 80.7% of its original power output.
But this is the lowest warranty for panels offered by REC. The REC Alpha, a solar panel that uses heterojunction technology rather than PERC, comes with a significantly better 92% power warranty.
The use of PERC in solar panels is increasingly common and could reach 50% market share in a few years. Already, there are numerous manufacturers that currently sell PERC solar panels for the residential market. Here is a partial list of panels that incorporate PERC:
The main downside of a PERC panel is the possibility of increased degradation and loss of efficiency as the panel ages. However, because this degradation can be minimized with better manufacturing, and more and more manufacturers are introducing PERC panels, it seems likely that the industry will improve both the quality and cost of PERC over time.
As for the current state of industry, with so many manufacturers introducing this technology to their product lineup, the consumer has a lot of choice - especially when it comes to warranty.
Remember - because LeTID reduces the efficiency of the panel over time, look for a good power warranty with your panel. This will protect you from the risk of LeTID. Of the panels listed above, the best power warranty is offered by Hanwha Q.CELLS, which has a 83% power guarantee after 25 years.