How Quantum Dots Could Make the Most Efficient Solar Panel

Ever heard of a quantum dot? What about an artificial atom? Though it might sound like technobabble from Star Trek (which of course I’m a fan of), this is a real piece of nano-technology that has the potential to revolutionize fields like medicine, consumer electronics, and especially solar energy. Actually, quantum dot technology has been in TVs for years, so why isn’t it already in my solar panels? Well, there are a few challenges… some which might sound familiar to you if you’re in-the-know when it comes to cutting edge solar tech. If we can engineer around these challenges, then these little devices could have a BIG impact – breaking the limits of our current solar panels, and ushering in a new and radically more efficient age of solar energy.

What is a Quantum Dot?

What even is a “quantum dot” (QD)? While it sounds like a mcguffin that Stan Lee and Jack Kirby would’ve thought up in the mid-sixties, it is in fact the real scientific term. But it’s also unhelpfully nondescript, so let me explain. Quantum dots are microscopic, crystalline semiconducting particles that could make our solar panels much more efficient. How much more? Well the theoretical maximum conversion efficiency for single-junction solar cells is around 30% efficient. We can add more junctions, like 6 of them, to bump up that theoretical max efficiency to 51.4%.¹ The National Renewable Energy Laboratory (NREL) calculated the theoretical max efficiency of quantum dot solarcells (QDSCs) at a staggering 66%!² And they’re (theoretically) much easier to pull off than multijunction cells. If we can make them work, then quantum dots could change everything.

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That might make you wonder, like it did me, how do they do this? Quantum dots are so small that their electrons can only move in very specific ways. They’re ‘locked in’ at very specific band gaps, which means they only produce very specific wavelengths of light (or in other words they emit very specific colors). We call this phenomenon ‘quantum confinement.’² This is really strange when you think about it: these vials contain exactly the same material,³ down to an atomic level. Yet, they’re glowing vibrantly in extremely different colors. The only thing differentiating them is their size.³ The biggest and reddest ones tip the scale at 10 nanometers, while the smallest and bluest quantum dots are just 2 nanometers in diameter.²

For comparison a human hair is 80,000 to 100,000 nanometers thick. Your blood cells are 10,000 nanometers across. Even a single strand of DNA is about 2.5 nanometers wide.⁴⁵ Quantum dots are so small that, for almost all intents and purposes, they’re considered zero-dimensional. And at this size things start to get weird, and the laws of physics seem to start to break, like with the quantum confinement we mentioned a moment ago. To really understand how quantum dots are breaking these laws to make photovoltaic cells more efficient, we have to brush up on these laws of physics and how they work with solar cells.

How Solar Works

At its core, a solar cell is a crystal with a positive (p) side and a negative (n) side, forming a p-n junction. The sun shoots out photons and eightish minutes later they hit our cell. If that photon has an energy level greater than or equal to the band gap energy of the cell, it’ll transfer its energy to an electron, knocking the electron free from its home and creating an electron-hole pair. If not, then that energy is wasted. The ‘free’ electron and hole want to rejoin and/or settle down, but if they remain free long enough they might pass through the electrode and bam! We got free energy from the sun.⁶

Even though we tried to simplify that, it’s still a little complicated. You don’t have to be an engineer to notice that there’s some big “ifs” and “mights” in that explanation. Solar panels actually aren’t all that efficient. Most commercial panels can only convert 15-23% of the light that hits them into usable electricity.⁷⁸ And solar panels won’t get much more efficient, at least not the way we currently understand them. All the way back in 1961, scientists William Shockley and Hans-Joachim Queisser calculated that the maximum possible efficiency for single junction solar cells to be about 30%. Thankfully, with modern advances in materials and engineering, that max efficiency now stands at a mighty… 33.7%. Aw, man.⁹

QDSC: The Shockley-Queisser Limit Breaker

Why is the Shockley-Queisser limit so low? In addition to all the hoops to jump through we mentioned earlier, there’s a lot of factors that limit how much juice we can squeeze out of a photon. Luckily there might be a few loopholes. For instance, Shockley and Queisser assumed there’s only one p-n junction. However, we could always add more semiconductors to create more band gaps. Remember that band gaps only generate energy from a photon if the photon has the same or greater energy level – they’re picky eaters.¹⁰¹¹ So combining a bunch of them together to form what’s called a multi-junction cell is a feasible way to break the Shockley-Queisser limit. It’s kind of like making a wider net to catch more energy levels of photons… or maybe it’s more like making a net with a bunch of back up nets behind it so less photons slip through? I’m stretching that metaphor … anyway, this works great until you look at the price tag. Multi-junction PVs are expensive and hard to make (at least for now). For reference a typical solar panel costs well less than $1 per watt. For instance, an American made panel typically costs between $0.50 – $0.80 per watt.¹² However, even mere two-junction cells are estimated to cost around $4.85 – $7.17 per watt right now depending on the materials.¹³ That cost will drop over time as manufacturing gets perfected, but it’s why multi-junction cells aren’t really commercially viable at the moment.¹⁴

And this is where quantum dots come back in. Since quantum dots are teeny-tiny semiconductors, we can use them to cheat something that works just like a multi-junction cell, potentially for less than a $1 per watt.¹⁶¹⁷¹⁸ That’s right, quantum dots are relatively cheap and easy to make, we can grow them by just mixing some high temperature solutions, and we can control how big they get with heat or time. Remember that the size of the dots changes how the electrons are confined, which changes their band gap and hue.¹ In a normal cell your absorption layer absorbs the same band gap of light. However, quantum confinement lets us cram a spectrum of different band gaps, even into the infrared spectrum, all precisely tuned to capture different wavelengths of light. At just a few nanometers in size, we can fit more quantum dots in a given space than semiconductors, which further helps to push their efficiency.¹⁵¹⁶

But wait, there’s more! Multiple exciton generation (MEG) is another way quantum dot solar cells can get around the Shockley-Queisser limit. In a normal cell, we get one hole-pair (or exciton) per photon, if we’re lucky. But quantum dots can create two or more excitons for each photon. Even though the mechanics of MEG are well understood, there’s not yet a consensus on why it works. Anyway, you don’t’ have to be a quantum physicist to understand that two excitons for the price of one is going to radically increase the efficiency of our solar cell.¹⁷¹⁸ At the very least, it doubles the chance that something is going to make it to the electrode.

Cutting Edge Quantum Dots

So with all the potential of quantum dots, you probably won’t be surprised to hear that we’re learning new things about them almost every day.

A team of researchers from Korea University and the Ulsan National Institute of Science & Technology are trying to combine quantum dots with perovskites to get the best of both worlds. Perovskites are great, as we’ve previously covered, but most of them are made primarily of lead and other toxic chemicals, which has raised concerns. That’s why researchers have been leaning toward the less toxic tin–lead halide perovskites (TLHP), but they’re less powerful than their lead cousins and more prone to defects.¹⁹

To fix this the researchers added perovskite quantum dots (PQDs) to the cell, which both made the perovskites more stable and upped the maximum voltage of the cell. But now they had a new problem with ligands, which is the material the nano-crystals are suspended in. They were slowing down the charge transport. You can sort of think of it like having a very fast car, but hitting every red light on the way home. Doesn’t matter what the speed limit is on the road if you have to stop at every block. The researchers treated their cells with a little isopropyl, which sort of loosened up the ligands and made everything gel together, creating better, smoother highways for the charge to ‘drive’ though. As a result their perovskite quantum dot cell was more efficient and faster, showcasing improved open-circuit voltage and a record-breaking conversion efficiency for tin–lead halide perovskites.²⁰

Meanwhile, a totally different team of scientists from several Korean universities and institutions just developed a flexible quantum dot solar cell based on all-inorganic cesium-lead iodide (CsPbI3) or ‘black’ perovskite. They can deposit a coat of these perovskite quantum dots quickly and at room temperature using a simple layer-by-layer process to create a flexible cell. The “room temperature” thing is particularly important, as you’ll soon see, since quantum dots don’t like too much heat. Ordinarily it takes some heat-treating to connect the perovskite quantum dots to the base layer of your cell, but this also increases the risk of defects. Being able to do this at room temperature should take a lot of the headache out of manufacturing them.²¹

And these cells weren’t slouches either, the team reported power conversion efficiency of 12.70% under standard testing conditions.²²²³ Commercial solar panels right now tend to hit around 20% so the technology has a ways to go, but 12.70% is still a new record for flexible quantum dots PVs.⁷

And this is all just from the last few months of 2024, but it’s not all sunshine and rainbows for quantum dots though. Despite the explosion of innovation, they are still a ways off. Despite their huge potential there’s still some serious issues that need to be dealt with.

The Challenges

I only briefly mentioned what quantum dots are made from, and that’s because most formulations contain harmful metals like lead, cadmium, arsenic and mercury.²⁴
This has raised some concerns about toxicity, but the construction and recycling of quantum dots is comparable to standard solar. Where health and safety standards have been well established to deal with contaminants. Again, like standard solar, once in operation quantum dots don’t present issues because they’re locked into the layers of that solar sandwich. What we don’t know is what the package of quantum dots and nanoparticles does to the human body.²⁵²⁶

Due to their small size, quantum dots could be inhaled or otherwise enter the body. An early study suggests that once inside, they can accumulate in organs with a lot of blood flow like the liver and kidneys, where they can cause inflammation and harm.²⁷ Though these questions are outstanding and more research is needed. For what it’s worth, a team from the Aligarh Muslim University in India has already begun work on making non-metallic, non-toxic quantum dots.²⁸

Durability remains the biggest issue for quantum dot solar cells. Much like perovskites, quantum dots are sensitive and can degrade when exposed to air, moisture, and high temperatures (kind of like me). All things that a solar panel is expected to face for at least a decade or two.³ They also don’t like being illuminated too much or getting hit with a lot of ultraviolet light. Needless to say, a solar cell that can get a sunburn is not ideal. Again, I can relate. You also have to be careful about what material you pair with them. A solar panel is basically a sandwich filled with lots of different materials all aiding in the collection of sunlight or the protection of the sunlight-collectors. Quantum dots can react poorly with some materials and degrade even faster.²⁹

One of the big advantages of current-gen solar tech, and the reason we see it (at least so far) still in use over less expensive or more efficient technologies, is their longevity. A solar panels’ number one job is to change sunlight into electricity, but job number two is to do so reliably for years. While quantum dot solar cells might be radically more efficient, they are super-radically less durable, which means today’s more basic solar tech has the edge (at least for now).³ This isn’t a problem when it comes to quantum dots other applications like medicine or high-def television screens. A bit ironic then that solar is the field where quantum dots have the most revolutionary potential, but also face the biggest hurdles.

The potential for quantum dots to revolutionize the energy landscape makes them tantalizing. And the fact that they’re already in QLED TVs make it feel like they’re right around the corner for solar. And much like perovskites, if researchers can find an affordable way to make quantum dots more durable, then our solar panels are about to double in efficiency … but that’s a pretty big ‘if.’ And if not? Well… quantum dots can still make our TVs, phones and especially medical devices better, and that’s still pretty neat.³⁰