What solar module efficiency means—and why it matters
Published on 22 Nov, 2017 by Andrew Sendy
Ever since Bell Labs scientists Calvin Fuller, Gerald Pearson, and Daryl Chapin discovered the ability of crystalline silicon (c-Si) to turn sunlight into electricity in 1953, most commercial solar modules have been made of some form of the substance.
Initial solar modules were simple, one-junction structures (p-n) and had low efficiencies, meaning out of the total amount of sunlight hitting the module's surface, only a small percentage was converted to usable electricity.
And eight years after the Fuller-Pearson-Chapin trio discovered silicon's utility in converting solar energy to electricity, two physicists named William Shockley and Hans-Joachim Queisser established the Shockley-Queisser Efficiency Limit.
In essence, they told researchers trying to find the next solar module breakthrough that as long as they used one-junction structures, c-Si modules could reach no more than 35 percent efficiency.
Practically, what that meant was that the number of panels that would have to be installed to make solar power work for the majority of people would render it cost prohibitive except for the extraordinarily wealthy.
So that was the end of solar power as a potential source of endless electricity, right? Of course not.
Thanks to raw-material-production improvements (i.e., how module manufacturers harvest the silicon necessary to make their solar modules), efficiencies have continued to rise.
To be fair, no panel has directly clashed with the theoretical limit yet, but some more recent developments have suggested ways around the 46-year-old intellectual barrier. And new module structures—like perovskites, for example—are also being explored for future solar modules.
Once those new materials are ready for commercial development, the whole idea of limiting solar panel efficiencies may fade into the past like a bad dream.
So where does the state of solar panel efficiency stand today? Well, it depends on what kind of solar module you're talking about. Traditional polycrystalline silicon? Monocrystalline silicon?
For the purposes of this article, we are going to examine two most common c-Si technologies—monocrystalline silicon and polycrystalline silicon. Those two types of silicon-based modules make up around 90% of the world's module stock.
And then we'll tell you about the three most exciting solar module developments on the horizon that could just change the way we see solar modules today—and could revolutionize what you put on your roof in the future.
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Polycrystalline Solar Modules
Polycrystalline solar cells are some of the most popular in the module manufacturing industry. They are relatively easy to produce, have a much larger margin for errors in production, and are far less expensive to produce.
The challenge is that creating multiple crystals, by the nature of the process, is inherently imperfect. And once imperfections are introduced into solar modules, their efficiencies go down.
According to the National Renewable Energy Laboratory (NREL), which has tracked cell efficiencies since 1975, the current polycrystalline module efficiency record stands at 21.9 percent.
What that means for you is that though you will be able to install polycrystalline solar modules at a lower price per module, the overall costs might be balanced by the fact you will have to install more modules per square foot to reach the same electricity production as more efficient modules.
Monocrystalline Solar Modules
Unlike polycrystalline solar modules, monocrystalline solar modules are made from a single silicon crystal, meaning there are no flaws in the solar cells' surfaces. But such purity of surface does come at a premium cost.
And therein lies the biggest obstacle for monocrystalline solar modules for ordinary homeowners: price. The price difference between polycrystalline and monocrystalline modules can be fairly steep, putting them out of the reach of many residential solar customers. In fact, manufacturers of monocrystalline solar modules have abandoned the mass solar panel market in favor of higher-end projects, with clients who are willing to pay extra money more efficient solar panels.
NREL reports the current record for monocrystalline modules is 25.3 percent. While a difference of 4 percent may not seem like much at first, it does matter when the overall size of a project is taken into consideration. After all, the difference in price between having to install 10 higher efficiency panels vs 20 lower efficiency panels can't be underestimated. Those extra panels also require more space, which for homeowners with smaller roofs could be the difference between installing a solar array or not.
Beyond Traditional Silicon
As innovative as the solar industry has generally been over the past 60 years, the basic material—c-Si—hasn't changed. But advances in solar module structure, including new materials all together, are providing an exciting glimpse into what the future of module efficiencies could be. Some options that are moving toward general commercialization include:
Silicon Heterojunction Modules (HIT): You may well have at least heard of HIT technology even if you didn't realize it at the time. There's been a lot of publicity surrounding Tesla's Gigafactory in Buffalo, N.Y., which is where the company will be producing some of its highest-efficiency panels to support its residential solar installation business (formerly SolarCity). The solar panels produced in Buffalo will be Panasonic's HIT technology in mass production.
According to Panasonic, HIT solar modules reduce the amount of light reflected off the surface of the modules (i.e. capturing more sunlight), has multiple layers of silicon that reduce carrier loss and are bifacial—meaning they can capture sunlight on both its front and back. That gives HIT modules access to a wider array of raw solar energy than most traditional modules.
Efficiency: 26.6 percent
Multijunction Silicon Modules: Remember when we talked about the theoretical efficiency limit of silicon? It's important for us to remind you that the scientists who developed the theory were talking about single-junction solar modules. So it was natural for later scientists to ask: What if we added another junction?
This is where the magic is really happening. In laboratories across the country, researchers are making multijunction cells with efficiencies upward of 38.8 percent without concentrators. The problem is that multijunction solar modules aren't commercially available at the moment because it's not always easy to translate theory into practice.
If they ever get there? The sky could (literally) be the limit.
Efficiency: 38.8 percent (lab results only)
Perovskite solar cells: The most recent entry in the “replace silicon” sweepstakes are perovskite solar cells. Having come to the market only eight years ago, the technology is in its infancy and is not yet commercially available. So what has everyone in the industry buzzing about this technology?
Three words: Efficiency. Growth. Rate.
Original perovskite solar cells had efficiencies of 3.8 percent in 2009. In 2016, their efficiency was 22.7 percent—higher already than the most efficient polysilicon cells while being just as easy to produce and manufacture. Although unproven technologies often spark unrealistic dreams, perovskites have displayed the fastest efficiency-growth rate in this history of the solar industry, and they show now signs of slowing down.
Efficiency: 22.7 percent
As efficiencies continue to climb, the prices of installing solar will continue to come down, making it the first electricity source since the Industrial Revolution that you will be able to own yourself. And it seems new efficiency records are being set every other day in labs around the world—so stay tuned. We'll be updating this article periodically as new information becomes available.