Stacking Solar Cells Is A Neat Trick To Maximise Efficiency

Solar power is already cheap and effective, and it’s taking on a larger role in supplying energy needs all over the world. The thing about humanity, though, is that we always want more! Too much, you say? It’s never enough!

The problem is that the sun only outputs so much energy per unit of area on Earth, and solar cells can only be so efficient thanks to some fundamental physical limits. However, there’s a way to get around that—with the magic of tandem solar cells!

More and More And More

Solar cells are constrained by something called the radiative efficiency limit, also known as the Shockley-Queisser limit. It defines the maximum possible efficiency of a solar cell based around a p-n semiconductor junction. It states that a single solar cell can only have an efficiency of around 32% for the most popular material, silicon. Even with some other semiconductor material with a higher band gap, peak efficiency could only go as high as 33.7%.

A multi-junction solar cell developed by NREL. It glows red due to gallium indium phosphide in the top layer. Credit: NREL

The physics involved is complex, and the limit comes down to multiple factors. For a start, not all photons in sunlight have enough energy to excite an electron to the bandgap of the solar cell material, so they don’t contribute to energy generation.

Meanwhile, photons with excess energy above the bandgap level waste some of it as heat. Excess heat tends to reduce the efficiency of the cell due to other effects, while it’s also a fact that not every electron excited in the cell structure contributes to current flow. Work through all these limitations, and you can pencil out that your silicon-based solar cell will never achieve more than 32% at best.

The thing about the limit is that it only applies to a single cell in isolation. The concept of a tandem solar cell is that you stack multiple solar together, each tuned to different wavelengths of light. The idea is that by using different semiconductor materials for the different cells, you can generate electricity more efficiently from different wavelengths of light.

In theory, the idea has great promise. Where an ideal single-junction cell has a maximum efficiency of 33.16% in theory, a tandem or multi-junction solar cell with “infinite” junctions could hit an efficiency of up to 86.8%. Of course, we don’t have infinite types of semiconductor materials to use to make a cell that good, but it’s indicative of the gains that can be made.

A schematic for a multi-junction solar
cell, indicating how each layer is tuned to suit different wavelengths of light. Credit: Ncouniot – Fraunhofer Institute for Solar Energy Systems, CC BY-SA 3.0

In reality, we’re still a ways off the theoretical limits, as you might expect. The highest performing single-junction silicon solar cells were capable of achieving 26.81% efficiency, as of last year. In contrast, the most capable quadruple multi-junction cells have achieved efficiencies of over 47% under concentrated sunlight.

In real-world conditions, triple-junction cells have achieved efficiencies of 39.5%, still a huge gain over conventional single-junction panels. This isn’t just lab technology, either. Commercial tandem cells are available, and while expensive, they can achieve efficiencies of over 30%. Common customers for this technology are often those working in the space industry, looking to power satellites.

It bears noting as well that these efficiency gains are actually much greater than they sound on paper. Take a 20% efficient solar cell for example, sized at one square meter. Nominally, the sun puts out around 1000 watts of energy per square meter as it shines on Earth on a bright day. So, our 20% efficient panel will output 200 watts under ideal conditions. If we bump its efficiency up by just 10%, though, to 30%, we get 300 watts output instead. That’s not a 10% improvement—it’s a 50% improvement! Scale that out to a whole solar array, and you can see the benefits to be had from installing more efficient panels.

Perovskites and More

Of course, you might imagine that this improved efficiency comes at a cost, and it does. Namely, cost! Manufacturing a solar cell with multiple types of semiconductor junctions is far more expensive then simply churning out simple silicon-based panels. Popular semiconductor materials for multi-junction cells include indium gallium phosphide (InGaP), gallium arsenide (GaAs), germanium (Ge), and indium gallium arsenide (InGaAs). Known as III/V semiconductor materials, they all have different band gaps, which allows them to convert photons of different energy levels more efficiently into electricity. However, these materials can be very expensive, and aren’t really suitable for mass production of consumer-grade solar cells.

A record-breaking perovskite-silicon solar cell developed by a team at HZB. Credit: Johannes Beckedahl/Lea Zimmerman/HZB

However, the most talked-about tandem configurations involve pairing silicon with perovskites, a class of materials that have shown remarkable photovoltaic properties. Perovskites are cheaper to produce and have tunable bandgaps, which means their light absorption properties can be adjusted for optimal performance when layered on top of silicon.

In one example from Helmholtz-Zentrum Berlin, scientists created a perovskite solar cell layer that was tuned to capture photons from the blue part of the visible light spectrum. Meanwhile, the conventional silicon solar cell underneath was left to capture the red and near-infrared light.

This improves efficiency, because the perovskite could be tuned with a band gap suitable for the higher energy blue light. If the blue light was instead captured by the silicon layer, the extra energy above the band gap would just be wasted, being turned into heat.

Scientists at KIT developed a cell using two tuned perovskite layers atop a standard silicon solar cell layer. Each layer has a different band gap, with the top layer capturing energy from the most energetic photons. Weaker photons travel through to be converted in the lower layers. Credit: KIT paper

It’s actually possible to use multiple layers of perovskites too. Each layer can be tuned to a different band gap, with the highest-energy layer on top and silicon on the bottom. Researchers at the Karlsruher Institut für Technologie have been experimenting with this very concept, producing a triple-layer perovskite-perovskite-silicon cell.

Their design achieved a record efficiency of 24.4%, as revealed in a new paper published this year. Cells using this type of design aren’t yet achieving the 30%+ results of other tandem designs, but the hope is that multi-layer perovskite designs could continue to improve in efficiency in future.

As the world continues to wean itself of fossil fuels, demand for solar cells will continue to rise. At the same time, the more efficiency we can get out of each cell, the less we’ll require to meet our energy needs. Expect development to continue at a rapid pace in this area as companies vie to develop the perfect multi-junction solar cell that can be manufactured en masse. Ultimately, these improved panels will be a key tool in the fight against climate change.

51 thoughts on “Stacking Solar Cells Is A Neat Trick To Maximise Efficiency

      1. Hail with a sufficient diameter to cause that sort of damage only occurs once in 30 years or less, where I live. Also you only need replace the panels, the fixings, wiring, inverters etc etc are usually undamaged.

        1. Normally it’s rare in where I live but last year Michigan had 2 separate major hailstorm weeks apart. One of them wiped out all of Davison area car dealer’s car stock. If the climate keeps getting weirder, we can expect more of baseball sized hail storms.

        1. I bet you a nickel that if you ever looked at what all is subsidized, you’d find some things on there that are inconvenient for your point of view. Say, various parts of the chain leading to your local gas pump – sure, there’s a bit of a tax on the fuel at the end, but there’s also quite a bit of subsidization and tax breaks if you should happen to be a company that wants to drill for oil or get involved in corn ethanol.

          1. Im glad work is being done to make a cheaper version of these more efficent cells. I am curious id these upper cell layers will hold up as well as silicon cells.

            If it all is worth it depends on the price premium. Not all solar installations are constrained on area.

        2. I’m assuming you’re taking about the $7 trillion in subsidies (according to the IMF, if they can be trusted) that went to fossil fuels in 2022? Because I couldn’t agree with you more. But please, elaborate.

        3. You mean subsidized like having having multiple wars and a permanent presence in the middle east for decades? Wait, that was for oil. Subsidies for solar pale in comparison.

      2. Solar panels are generally built much much tougher than modern car windscreen and those are not known for breaking easily… So it has to be one heck of a hailstorm, absolutely can happen but it won’t be common. We have had hail getting to around marble size since we had our panel and many smaller doses, other than being covered in diesel gunge mixed with dust (again) they are still good as new.

        And even after 30 odd years a modern panel is expected to be barely degraded in output from brand new, they really don’t need anything but a good cleaning. The most likely thing to fail within that 5 years will be your battery/inverter electronics, a relatively easy component to replace, and again there expected lifespan is way more than 5 years…

        1. Not any more, most recent ones are built soley for power density and come with warnings about abnormal stresses and highlight the significant likelyhood and risks of (and lack of guarantee against) micro cracks.

      3. Ah, yes, that must be why everyone lives in windowless concrete bunkers and all cars older than five years must have been parked indoors to avoid being totaled by the explosive lightning-ice meteors we get on a regular basis.

    1. Im sure installers are still ripping people off, but i bought some panels for off grid that were $0.50 per watt. That IS pretty cheap. Supporting equiptment are really becoming a large fraction of the cost at this point.

  1. Some confusion about percentage and percentage point?

    “bump its efficiency up by just 10%, though, to 30%, we get 300 watts output instead. That’s not a 10% improvement—it’s a 50% improvement!”

    0.3 / 0.2 = 1.5 (i.e. 50% increase)

  2. Perovskite cells last time I checked still have rather limited lifespan compared to silicon ones. So I hope these layers are easily separable to refurbish the cells if the concept ever gets mass produced – be a massive waste to throw away or recycle the perfectly good silicon cells underneath.

    1. Incredibly limited, though improving, and the records are held by cells in the 1mm x 1mm size region, and commercialisation of a process is 99% of the effort to make a product.

  3. “If we bump its efficiency up by just 10%, though, to 30%, we get 300 watts output instead. That’s not a 10% improvement—it’s a 50% improvement! ”

    No, sorry. This confuses two uses of ‘percent’. An increase from 20% to 30% is not a 10% improvement, it’s a 50% improvement, just as you’d expect. The increase is 10 *percentage points*, not 10 percent.

    1. By the principle of charitable interpretation, it should be reasonable to believe the author is trying to say exactly what you have condescended to rephrase.

      At the least, it makes the most sense for the author to have bothered to write this if the point was to highlight how the improvement looks better when viewed as an improvement over the previous panels rather than relative to a nonexistent ideal panel.

      1. Yes, of course, the charitable thing is to assume the author didn’t intend to mislead, and I don’t think that. But it does sound as though they believed that a 10% improvement at the input led magically to a 50% gain in output.

        1. As a self-admitted uneducated barbarian, I found the concept that panels collecting 10% more solar energy would result in a 50% increase in power production very understandable.

          I value clear and accurate language (and the readers of this blog are one of the few places where I find that any more), so the point is appreciated.

    2. Massive pedantry fail.
      X “percent” means exactly the same thing as X “Percentage points”. There is no difference, the writer has not confused two different uses of the word, there are no different uses.

      The confusion here is that the two figures refer to percentages *of different things*. First is the percentage of the incoming light energy captured. Second is the percentage change in electrical output. These are related but different parameters, which is why the numbers are different.

      1. You of course are free to believe what you like, but ‘percentage points’ and ‘percentage’ are two distinct things, and are being confused by the author.

    3. The real point would have been to compare the price of such technology to find out if the 30% efficient panel are 50% more expensive than the 20% one. If they are, the point is moot. If it’s not 50% more expensive, then it makes sense to choose the former for the latter. In the end, the ROI is what matter, not the efficiency of the panel (unless you are surface limited, which is rarely the case).

      1. Surface limited is I’d suggest the default position for most. Even the big installations only have so much land to build on and most domestic/mobile scale deployments can only fit a handful or two of panels total in their space. There might be lots of extra space out there in the world you could theoretically cover, but you actually need permission, buy access rights and land etc – which you won’t get, or ends up costing even more to get than the better performance panels.

        Do wonder what the total cost of ownership would be for these – normal cells are pretty well understood and virtually immortal compared to the best the Perovskites have managed to my knowledge. So while the initial speed to return your investment (be that fiscal or embodied energy) may be better I would be surprised if they end up working out as well in the medium/long terms.

      2. I tend to find that while I may have plenty of surface area, such as an entire empty field, I’m obviously not just going to set the panels on the ground and walk away, and that’s where the necessary quantity of panels begins to matter. I have to think about how much they will weigh, how high far apart they need to be, how tilted, how I’ll get between them in the future, how I’ll wire them together and how much voltage drop that will represent, what do I need to do about wind and weather, etc. It adds to the cost enough to make a difference.

        1. Exactly. Raw dollars per watt is becoming less and less important. I remember when I was a student, a dollar per watt was this magical unattainable goal, and we all speculated that “holy crap that would make grid scale photovoltaics viable!”

          Now we have blown through that barrier and the price keeps dropping, to the point that the hardware to which we bolt the photovoltaics becomes a significant percentage of total cost. Then add in the land, more metres of expensive copper cable etc etc etc.

  4. Thank you Lewin for working to explain what is going on at the leading edge of solar cell technology.

    I’m sure there were as many trolls writing dismissive and tangential letters-to-the-editor back when internal combination engines were slowly replacing horse drawn wagons.

  5. Another “cheap” way to increase efficiency is with bifacial solar cells. Basically the “rear” of the panel can also generate electricity. It doesn’t work for every application, but for e.g agrivoltaics where the panels are often mounted quite high, there’s lot of ambient light from below that can be harvested.

  6. If excess light is converted to heat, which decrease the efficiency, then ia the hewt not preventing u from getting closer to the theoretical. How about look to build a reflector, that can dynamic be control so as cell approches maximum output or to keep heat stable you start reflecting photons, to cool them. Whats the best efficient that can be achive if buid an experimental cell that pull away heat from excess photos. As 80percent photons, dont end up band gap brigding so how out those 80percebt photons were want them so dont incrase the heat and decrease the efficiency. Any research or focus on were 80percent photons can be directed avoid degregation of the ideal theoretical. As clearly more parameters involved we have yet to model.
    Were need to focus on other areas to imorove.

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