How Metamaterials Can Be Programmed to Optimize the Efficiency of Solar PV Cells
Storytime. Rewind to the 1600s. Giordano Bruno was a physicist who first proposed the idea of the universe being infinite, with multiple parallel worlds, as well as several other foundational works that led to the heliocentric model. When he presented his findings, he was burnt at the stake. This happened during a time period when essentially everything scientific in nature that was dismissed by the Church was considered, well, “dark magic”.
Now, in a world where we can capture the energy of the Sun to power cities, and bend light backwards to see the world in 4D, the truth is, “magic” is just a placeholder for the scientific phenomena that humanity still has yet to explain using their limited cognition. The two biggest pieces of this puzzle — metamaterials and solar panels — were once upon a time, entirely magical.
But as technology progresses and emerges, so we evolve.
We’ve now reached a point where we can achieve extraordinary feats with thin, single-layered filaments that have millions of even tinier lenses and receptors that can bend the laws of physics, and to an extent, defy them. We are playing with magic, and it’s impacting us in incredibly beneficial ways.
Everyone we know has heard the buzz about renewable/alternative energy: Fossil fuels are non-renewable, and their production results in harmful repercussions to the environment, including greenhouse gas emissions, global warming, and pollution. Addressing how to tackle these issues with technology and innovation has become commonplace in most societies, with solar energy at the forefront of it all.
The Sun produces around 384.6 septillion watts per second, out of which 0.00000005% reaches the Earth through solar radiation. And that is enough to power the world’s energy needs uncountably many times over.
But what if I told you that the current way in which companies practice renewable energy production methods doesn’t even tap into 10% of the power potential that we have from solar?
That’s like studying for 250 hours spread over two weeks, and getting a C+ on your History test.
This is a representation of how much room we truly have for harnessing renewable energy to our convenience, minimizing risks and adverse effects of power consumption along the way.
Solar Photovoltaic (PV) cells are the center of attraction for major investors in the sustainable energy sector, being instrumental in reducing variable energy costs by up to 69% in six years, with no added greenhouse gas emissions.
If they’ve been so useful in promoting the transition to renewable energy, then what’s the real problem behind their technicality and setup?
Problems in a Nutshell…
Insolation — Solar panels reach their maximum capacity of generating electricity in regions of higher solar resource, which essentially means hotter, dryer places that are exposed to the solar wavelength range for longer time durations. Given that PV components of the cells also become productive in electricity generation in areas of higher solar intensity, irregular weather conditions, clouds, overcast skies, and similar unpredictable phenomena always stop solar panels in their tracks from maximizing their potential.
The current structure of PV cells are not automated to recognize these temporal changes, which is why they aren’t resistant or versatile to them. The power output we receive on average from solar panels is therefore, GREATLY reduced and narrow. In regions with varying weather patterns, the efficiency of PV cells solely depends on the time of day during which it can reach its maximum sunlight absorbance requirements, which follows a pretty funky trend when you have a couple of ‘fluffy white things’ or dull rainy afternoons blocking your way. After all, humans can only discern the true value of a light-absorbing device within the range of wavelengths that they can see, of course, without the help of other assistive technology.
The real deal: Tilt angle complications. Surprise(?) The Earth is round. Hence, the amount of sunlight that different regions receive is not only non-uniformly varied based on climate, but also on the angle to the ground level at which light rays are incidental. To put things in perspective, solar arrays facing the North would have practically no use in a region where light rays are incident at the most vertical possible angle when the Sun tends to shine from the South.
What sucks is that this in itself is varied based on the time of year. This means that the only way for such complications to be accommodated in the setup of solar energy systems is if automated algorithm-based trackers that use convolutional neural networks for aerial image processing, feature extraction (for weather pattern detection), and satellite mapping of geographically colder regions, are incorporated to remotely monitor the positions of the Sun, shifting the alignments of the cell arrays accordingly — like a magnet influencing the positions of a trillion little iron fillings in a dish. But such algorithms, although increasing the average solar energy output by around 35%, end up driving up the cost of already expensive cell materials by an even larger proportion.
3. Efficiency: Remember the C+ on your History exam (not like it actually happened, but for the sake of, you know…) it becomes necessary to gauge how emerging technology can be leveraged at our will to increase the productivity of solar cells, using details, data, and comparisons of different internal structures of photovoltaic cells.
To start off, that word has kind of been thrown around quite a bit. Where does ‘photovoltaic’ actually come into play here, and what does it comprise that requires technical enhancement? It’s crucial to understand this, to further comprehend the prospects of metamaterials in this use-case.
Photovoltaic: Photo + Volt + aic. In case you haven’t guessed yet, a PV cell is what converts solar energy to electrical energy within a solar panel. One solar panel would have millions of PV arrays, where each cell acts as its own circuit. PVs are usually made of a thin semiconductor material which standalone, is quite cheap. But assembling these parts manyfold, yields a significant uptick in the average fixed costs of solar panels.
PVs are made from wafers of silicon, which have certain impurities that provide electric charge, through the presence of ‘electrons’ and ‘holes’. The ‘holes’ are areas within the lattice structure of the silicon crystal where electrons are practically missing, leaving open spaces for ions to bond. As one hole is formed, the electron adjacent to it in the doped crystal structure becomes attracted to it due to differences in charge, causing it to ‘drift’ over to the hole and occupy its place. However, this causes a chain reaction with all the electrons following it, causing what looks like a movement of holes across the doped crystal, until one side entire crystal becomes a positively charged singular pole, also known as the P-type semiconductor. This hole movement occurs in the same direction as conventional current, which is how charge is believed to flow through a circuit. The same concept is used in piezoelectricity and other forms of electronics and sensing as well.
So now we have one positive side and one negative side, i.e, a P-type and N-type semiconductor, within the photovoltaic cell. What happens when all the electrons move over to one side, and all the holes move the other side? Well, you have a semiconductor layer with nothing but positively charged metal ions, so by the principles of electrostatic attraction, the electrons come rushing back to the N-type semiconductor (a negative-to-positive attractive force), causing the holes to now move in the opposite direction. This continuous movement is what generates the voltage that is used to heat up our homes, power our solar farms, and drive massive turbines.
Referring to the above diagram, a photovoltaic cell essentially acts as one big battery. It creates a dielectric interaction between the species of a doped silicon lattice (it could be other highly conductive materials as well), in order to generate a current which can then be passed through a circuit. The electrons in the N-type silicon layer, or the front-end semiconductor, get their energy to move around and move faster through the energy that is received by the Sun. This is why the amount of solar radiation a photovoltaic cell is exposed to heavily influences the amount of output power generated.
Keeping this in mind, how do we maximize the efficiency of a photovoltaic cell in a way that doesn’t exacerbate harmful environmental repercussions, drive up the costs of solar PV projects by factoring in remote AI programs like mentioned earlier, or reduce its compatibility in multiple geographical locations?
The answer, at least within the scope of this article, isn’t to change the internal circuitry, or the diffusion mechanisms within the PV cell.
Imagine being able to create a surface that can absorb more light by changing its angle at variable wavelengths that aren’t even visible to the human eye…
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Introducing Metamaterials: Next-Gen Material Tech
You know of the broken-pencil illusion? No? Here’s a quick pic:
The reason why the pencil appears broken underwater is, as most of you know, the refraction of light. According to what is known as Snell’s law, the refractive index of a material determines its ability to bend light rays passing to or from a different medium of a different optical density, for example, air and glass. Here’s how it works with two regular material mediums:
Since air has a lower optical density, when incident light rays pass through water, they bend towards the normal (the imaginary line perpendicular to both media), creating the distorted image of the pencil you see above, once the refracted light rays reflect into your eyes, creating a real depth and an apparent depth. The apparent depth is what you see due to refraction.
Now what if I told you that you could place an intermediary material that when transmitting light, creates a reverse image of the pencil, like this:
That’s a metamaterial in simple terms. That’s right; they aren’t materials made by Facebook.
Metamaterials are created by humans to exhibit properties that we usually don’t see in natural phenomena; in this case, it’s bending light completely backwards. This is in fact the key property that we can use to completely revolutionize solar tech, making sure that even countries with little to no sunlight, long winters, and low day-to-night ratios, such as in the polar regions, can receive adequate solar energy to power their homes.
Metamaterials can be fabricated to have negative refractive indices, which means that an incident solar ray can be bent at an angle such that it’s a mirror image of the refracted angle of the light ray in any natural medium, causing scattering and absorption of solar radiation by solar panels, in a way that can be manipulated by us to reach an optimal generative efficiency.
Piqued your interest? Check this out for some of the finer deets:
But let’s get into more of the HOW. We know our fundamentals, the problem scientists and major companies in the nanomaterial/solar industry are trying to solve, and what tools we can avail in doing so. But where does it all come together? What is the process?
Introducing lithography: A technique used to nanoimprint circuit components into a dielectric metamaterial surface, so that light absorbing properties can not only be exhibited, but spontaneously adjusted according to the generative requirements of the PV cell.
Electron-beam lithography is almost like the foundational ‘code’ that wires a metamaterial’s properties, factoring in the coating of the surface, the reflectance of each material layer, and the energy transforms lost through scattering and transmittance. It involves directing a focused electron beam onto the metamaterial layer, to pattern nanostructures onto a substrate that act as their own miniature light-absorbing cells.
Nanoimprinting is like stamping small identifiers onto a metamaterial substrate, using ‘templates’ per se, to press these patterns into the material.
To represent it visually, here is how regular flat meta-lens nanostructures are fabricated using different coating adjustments and modifications:
Note that this specific use-case of Titanium Dioxide film for the nanoimprinted multilayer was tested for the visible range and not subwavelengths, indicating a POSITIVE refractive index of 1.9, and exhibiting a proportional efficiency of around 43–55%. However, when we use metamaterials on solar panels, these wafers that become the surface instruction material for the PV cells themselves should be able to absorb wavelengths from the entire light spectrum, and therefore reach much higher target limits. Using the foundations of nanoimprinting, let’s explore those features.
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A Deep (and SOLAR) Dive into Nano-Perks
What attributes can we optimize through this nanofabrication technique? With regards to solar PV, lithography guarantees engineer autonomy over the design and structure of the metamaterial, specifically when it comes to absorbing light in the subwavelength region. By using negative refractive indices and imprinting nanostructures that harness subsequently bent light rays, metamaterials can tap into the power potential that is harbored by regions of the electromagnetic spectrum that otherwise wouldn’t be absorbed efficiently by PV cells.
Wait a sec. What does the subwavelength region look like for a photovoltaic cell? without metamaterial lithography?
If we were to layer PV cells with these prints that collectively sort of act as their own mini transformers at this point, we’d observe that there are many more material specifications and design requirements that metamaterials would use to reach these target efficiency/energy output goals, in order to project higher solar irradiances in the infrared region of the light spectrum.
Usually energy is lost through light transmission, via the reflection of infrared waves, instead of absorption. Along with nanoimprinting and lithography, metamaterials can harness the power of light absorption enhancement and trapping, which employs their negative refractive index to minimize reflection scattering across the surface of the PV array. This becomes a useful asset when considering the maximization of current generation for longer hours throughout the day, and during bleaker weather patterns when solar radiation is minimal and diffuse.
Light Absorption Enhancement and Trapping
By serving as surface multilayers on a PV cell, metamaterials become Antireflection Coatings. Let’s start off with a simple fact. In this case, any energy that you want to absorb, will be subject to some losses by reflection. To mitigate this, metamaterials can function as antireflection coatings by manipulating their refractive indices at the surface of the solar PV cell.
Traditional antireflection coatings use dielectric materials with a graded refractive index to reduce reflection:
At the surface of our metamaterials, instead of having a sudden change in refractive index at the surface, the refractive index gradually changes from that of the outer medium, which in the case of solar PV cells would be air, to the refractive index of the cell itself, which would now be varied by the metamaterial coating. The gradual change ensures that solar energy transmission through light waves is more streamlined, and energy isn’t lost in a split-second through a drastic change in refractive indices, which minimizes the energy lost to reflection.
To put it simply, if you were running super fast, and then all of a sudden started walking out of nowhere, instead of decelerating to a lower speed at a steady rate, you would feel significantly more exhausted in the first situation compared to the second, assuming you’ve been running for a long time.
Basically, bye bye reflection.
So the subwavelength structures we’re referring to here, are what the metamaterial uses to create the refractive index gradient, that can in turn be manipulated to harness solar energy in the infrared region. Antireflection coatings therefore optimize our solar irradiation prospects regardless of the surrounding environmental conditions.
So we can program these little things to capture light more efficiently. But using the concept of subwavelength structures in exercising the full potential of light, we can also program these little tings to capture light more precisely and selectively — another tool that optimizes our solar panel’s design.
Tailored Resonances and Spectral Selectivity:
Using electron-beam lithography, dielectric metamaterials can be programmed to have different resonances at specific wavelengths in the solar spectrum, covering every electromagnetic wave we commonly deal with in daily applications, from x-rays on the high-frequency end, to radio waves on the high-wavelength end.
In this scenario, resonance is when the metamaterial surface profile is subjected to an electromagnetic wave (from solar radiation) that matches the frequency it has been specifically tailored to. The metamaterial then absorbs solar energy from this incident oscillation and amplifies it — similar to how resonance works in optical fibers.
These resonances arise from the interaction between the metamaterial’s subwavelength structures and incident light. When the incident light matches the resonant frequency of the nanostructures embedded in the coating’s surface, it is absorbed more efficiently, due to increased resonance.
This can be reached at its maximum energy-absorbing potential when the metamaterial is tailored to satisfy the resonant mode of the wavelengths that we want to absorb in our solar PVs — which, as of now, is mainly higher subwavelengths as illustrated before. This phenomenon increases the absorption of light in the desired wavelength range, so enhancing energy conversion becomes single-pointed and focused, allowing for higher precision. This can be used to our advantage in designing solar cells that are resistant to tilt complications, because regardless of the position of the Sun, infrared waves from diffuse radiation can still:
- propagate towards the metamaterial coating,
- be completely singled out, and,
- be bent completely to strike the dielectric coatings of the PV cell at exact perpendicularity, AGAIN without losing any energy through transmittance
No doubt these literal nanofilms can help us generate truckloads of solar power.
But this is not all of the sweet stuff.
Why do you think certain salt solutions are a certain color? Well, if you were to pass light through that solution, only a certain area of the light spectrum would be absorbed, and that would correspond to the color that is subsequently reflected into our eyes. You can do this using a spectrophotometer, as it models the absorption spectra of the positive ions present in the solution, something that looks a little bit like this:
Same thing with the ions we use in dielectric solar PVs. Spectral selectivity means we can fabricate PV cells to be multilayered, each coating absorbing different regions of the solar spectrum and reflecting others using the concept of tailored resonance, within each layer. This is known as “tandem solar PV”. This means that we can use nanofabrication techniques to program metamaterial layers on a solar PV cell that, when stacked on top of each other like sheets of paper, can each cater to maximizing their absorptive efficiencies of different regions of the spectrum, therefore absorbing the ENTIRETY of the spectrum as a collective. Coupled with their refractive properties, all of this newly tapped into solar energy can now be directed to solar PV grids without fail.
When engineers consider how these properties should be harnessed in programming solar cells, the material of the nanostructures used in efficiently generating current from subwavelengths is important to consider. For example, plasmonic metamaterials calibrate their surface profiles in an entirely different way, therefore exhibiting different absorptive properties altogether, though that’s outside the scope of this article (food for thought though).
Material choice becomes an effective indicator of whether tandem solar cells, or a single-layered antireflection coating should be designed, (since one reflects certain wavelengths and one avoids reflection completely, while achieving different forms of metamaterial success). It all depends on the specificity of light absorption that is needed to amplify solar energy conversions and conversion EFFICIENCY, which is in turn, decided by how weather patterns and other external factors influence our decisions in programming such gamechangers. But the fact that using nanofabrication to manifest these options is a booming possibility right now, is a guiding light for future solar panel optimization.
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Pioneers of the Process
Let’s take a quick moment to spotlight a few of the frontrunners of this revolution:
BASF and Phomera Metamaterials Inc: Badische Aniline und Sodafabrik, as they say, CREATE chemistry. One of the world’s leading chemical and material manufacturing global lateral mergers, BASF’s investments into advanced dielectric metamaterials for photonic crystal research are scaling up off the charts. Their modus operandi behind secondary sector output is climate resiliency and planning, and their research into advanced metamaterial microfilms for solar energy capture and propulsion has been a hallmark in their recent endeavors within the solar-metamaterial microsphere.
BASF Venture Capital recently invested in Phomera Metamaterials Inc, an innovative metamaterials research company that has totally opened the world to new possibilities in solar energy conversions and transforms. Phomera’s BIPV scheme (Building Integrated Photovoltaics) now spans a range of applications where the surface effects of metamaterials in optical use-cases can be researched feasibly. Phomera and BASF’s recent partnerships has been fueled in conducting spectroscopic testing on adding lithographed metamaterial films to the encapsulation layers of solar panels.
Their findings have included surface effects along the affected regions of the PV grid, that simultaneously maintain a high level of light absorption, and an even higher energy conversion rates.
Programming these structures for incorporating a wider use of solar energy in our lives only unravels its possibilities further and further.
META: (Again, NOT FACEBOOK). META is an upcoming firm that has manufactured its own NanoWEB technology, a lightweight, high-efficiency solar microfilm that uses electron beams to capture incident solar radiation from any angle whatsoever, WITHOUT building larger solar panels, changing their configurations, or using computerized tracking systems to determine the optimal positioning of the PV cells. This groundbreaking collection of nanostructures led to their acquisition of Nanotech Securities, an instrumental stakeholder in advanced material substrate manufacturing for solar energy efficiency.
“Metamaterials are in essence the materials of the future. MTI is pioneering large scale affordable nanofabrication technology that can push the boundaries for crystalline silicon solar efficiency and create very thin form factors for solar cells.”, said Harry Atwater, Professor of Applied Physics and Materials Science at the California Institute of Technology.
MTI, another more prominently known manufacturer of nanotechnologies for applications in solar, used the foundations of META’s NanoWEB technology for their modernized PV designs, which just goes to show the extent to which collaboration in the scientific innovation sector carries us so far in discovering ways to improve humanity in the most intriguing possible manner.
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The greatest innovators of their time aren’t defined by their understanding of existing tech, but their abilities to use past creations to IDEATE newer ones that create newer opportunities, the limits of which could never be capped, come to think of it. There are many shapes and forms of existing technology that are often considered the next big thing, or the future. The novel intersection between programmable metamaterials and solar energy has now been welcomed to the same club, as we continue to climb the heap of manufacturing, synergetic, regulatory, and other challenges bit by bit, to reach a cleantech future that just 50 years go, could never be imagined.
Imagine living in Greenland, and still being able to self-sufficiently heat an entire apartment complex 72 times over, in a renewable fashion, using cute-looking silicon films and stamped wiring that can cost less than 2 dollars per panel.
Imagine having a solar farm that still powers your main generator during a drizzle.
Imagine solar-powered lights, incandescent, glimmering brightly possibly even an hour after sunset.
All sustainable. All efficient. All an earth-shattering breakthrough.
If Giordano Bruno were alive today, he would consider THIS pure magic.
It is the understanding of magic that gives us the motivation, power, and knowledge to pursue it.
Let’s see what the future holds for technology as a whole.
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