Tracing the Family Tree: The Generations of Solar

What do you think of when you hear the word “solar”? Probably these guys, the big, bulky panels on rooftops.

Solar technology is at the forefront of our transition to renewable energy, the sun serving as an abundant resource. But advancements within the field of solar have progressed, as an ever-evolving field, far beyond these bulky solar panels.

It turns out these bulky solar panels are only the first generation of solar technology — which set me on a dive deep into the other three generations of solar technology. Development in the past few decades has never been more exciting!

Solar energy comes in two forms: photovoltaic (converting sunlight directly into electricity) and solar thermal (converting sunlight into heat, or thermal, energy). In this article, I’ll go over the current development and future potentials of photovoltaics.

Solar Panels 101

But first, a bit on the basics of how these solar panels work. The bulky things you see on roofs are typically made of silicon (silicon solar panels make up around 90% of the market share of solar technologies). These are first-generation solar cells — I’ll go more in-depth into this later on. Here is a diagram of the components of a typical solar panel.

Solar panels generate electricity through the Photovoltaic Effect, which is process that generates voltage or electric current in a photovoltaic cell when it is exposed to sunlight. “Photo” refers to light, and “voltaic” refers to the electric current produced!

The solar cell itself is made of two silicon wafers stacked on top of each other. Silicon, a semiconductor, has both properties of insulators (which don’t allow an electric current to flow) and conductors (which do allow an electric current to flow). Let’s take a closer look at the structure of the silicon atom.

In the middle of the atom is the nucleus. Silicon cells have 14 protons (as their atomic number is 14). This means they have 14 electrons, and 4 of those electrons are in the valence band. This is the outermost orbit, or energy level, that the electrons occupy in their normal state. Silicon likes to form 4 other covalent bonds with itself, so it shares electrons with neighbouring silicon atoms.

In this image, you can see how every silicon atom bonds with 4 other silicon atoms. These are covalent bonds, which is when atoms share electrons.

You may notice I said the valence band is the furthest orbit electrons occupy in their normal state. That’s because there’s another energy level above the valence band: the conduction band. In insulators, the distance between the top of the valence band and the bottom of the conduction band is very far, meaning electrons cannot move from one to another.

In conductors, they are touching or even overlap, which allows for electrons to move freely in the material. But if conductors conduct electricity so well, why don’t we choose them for solar panels? Because in conductors, the electrons are moving freely in all directions. In order for a current to form, electrons must flow in one direction.

In semiconductors, there is a gas between the two bands, but the distance between the two is very small — this distance is called the bandgap. The bandgap is very important for excitation of electrons (which I’ll explain in a minute), and it is why we choose semiconductors for solar panels.

Electron-Hole Pairs

So what happens when light strikes our semiconductor material? You may recall learning that light exists on the electromagnetic spectrum, which means it exists in different wavelengths. Light with shorter (higher frequency) wavelengths has a higher energy.

As you can see, the light that is visible to us is only a tiny portion on the electromagnetic spectrum! The shorter the wavelength (length of one wave), the higher frequency the light.

When photons of light with enough energy (or a high enough frequency) hit the semiconductor material, the energy of light can knock an electron from the valence into the conduction band! This is called excitation. When the electron is excited and moves into the conduction band, it leaves behind an empty space or “hole” that it used to occupy in the valence band, and can now theoretically move freely around in the semiconductor material.

Unfortunately, our electron won’t be free for long. Rather than conducting electricity, it will fall back into the hole in the valence band in a process called recombination. But if this is the case, how can solar panels made of semiconductors conduct electricity? The answer is doping.

Doping

Doping is the process where impurities are introduced into a material. Remember the bonds silicon formed with itself? If we add another element, or an impurity, into the silicon, it will bond with the silicon atoms. Solar cells are made of two main layers: first, the n-type layer (which is negatively charged), and second, the p-type layer (which is positively charged).

Here, silicon atoms are bonded to other silicon atoms. Each atom has 4 valence electrons — electrons are shared between atoms to fill silicon’s valence shell with 8 electrons.

In order to create the n-type layer, we inject phosphorus into a silicon wafer (which is just a thin slice of silicon). Phosphorus atoms have 5 valence electrons, so when it bonds with the other silicon atoms, there will be an extra free electron. This gives the n-type layer an overall negative charge.

On the left side is silicon doped with phosphorus atoms, which has five valence electrons. This results in a negative charge since there are extra electrons. On the right side is silicon doped with boron atoms, which results in a positive charge due to the presence of holes (an empty space unoccupied by an electron).

The p-type layer is created by doping the silicon wafer with boron — which has 3 valence electrons, leaving behind an extra hole. These holes are considered positive charge carriers, and can “move” when another electron fills its place.

P/N Junction, Drift, and Diffusion

The place where the n-type and p-type material meet is called the P/N junction, and it is what allows for the Photovoltaic Effect to occur. Now that there are “extra” electrons on the n-side thanks to doping, they begin moving towards the p-side in a process called diffusion, which is when charge carriers move from areas of high concentration to low concentration. The holes in the p-type material do the same thing. The holes from the p-type material begin mingling with the edge of the n-type side, and vice versa. This process results in a slight positive charge at the edge of the n-type material, and a slight negative charge at the edge of the p-type material, creating a built-in electric field!

Here, the holes have begun to diffuse (move from an area of high concentration to low concentration) into the n-type region (the red area). This is why there is a build-up of holes (indicated by the plus sign) at the n-side of the P/N junction (the yellow region) — the same happens for electrons at the p-side of the P/N junction.

As I’ve explained before, the electron-hole pairs generated when light hits the solar panel often recombine with themselves in their normal state. But this time, thanks to doping, we now have our built-in electric field at the P/N junction. Remember, opposites (positives and negatives) attract!

Here, a photon of light generates electron-hole pairs. The electron navigates towards the n-type region (which is blue) while the hole moves towards the p-type region. The electron flows through the circuit, the load, and returns to recombine with a hole in the p-type region.

This means that when new electron-hole pairs are generated, the electrons will now move towards the n-side (attracted by the slight positive charge at the P/N junction), and vice versa for the holes! Through this process, there is a build-up of electrons in the n-type side and holes from the p-type side, both created from electron-hole pairs. This process is called drift.

I like to think of it as a few soldier electrons from the n-type side guarding the border to the p-type side, directing the newly born electrons from the electron-hole pairs to move towards the n-type region. The p-type region, of course, has holes acting as its own army’s soldiers.

Finally, once enough electrons have built up in the n-side and enough holes have built up in the p-side, a potential difference is formed (resulting from the higher concentration of electrons in the n-side and holes in the p-side). Now all we have to do is connect a load to this solar panel, and the electrons will flow through the circuit until they reach the p-region, recombining once again with the holes there. This allows for the reaction to take place multiple times. You’ve got to admire the science behind it!

Problems/Advancements Within Solar:

Now that you know how the standard solar panels work, I’ll talk about a few problems and improvements being made within the industry. Here is a list:

• Efficiency Limit: The efficiency (the amount of energy you gain for the amount of energy you put in to a system) of solar panels is limited by the Shockley-Queisser (SQ) limit. Essentially, the SQ limit states that solar cells that are single-junction (having only 1 P/N junction) and that operate under normal light conditions (standard solar illumination) can achieve, at highest, an efficiency of around 33.7%.
• Intermittency: The amount of energy from the sun that can be converted into solar electricity varies depending on geographic location and time of the day. Some areas are sunnier, so solar panels in those regions will have a higher yield.
• Manufacturing: Creating typical silicon solar cells is a costly, complicated process today. It also requires a lot of heat and energy to purify and process the silicon for maximum solar panel efficiency.
• Unreliablity/Lack of energy storage: Solar power cannot be dispatched at any time of the day like fossil fuel energy can be. This is due to something called the Duck Curve, which shows that the amount of energy provided by solar panels does not correlate to the demand for energy as the day progresses.
• Higher costs: Installation and maintenance of solar panels can cost more than access to other forms of energy.
• Reflection: Sometimes, instead of reaching the P/N junction, light bounces off the surface of the solar panel and the solar cells can no longer convert that energy. This potential light is lost, resulting in a lower efficiency.
• Durability/Resistance to environment: The quality and efficiency of solar panels can be affected by rain, cloudiness, and shading. Some solar panels may also be affected by degradation.
• Environmental Impact: Solar panels can be difficult to recycle and can be made of toxic materials. They also require energy and certain materials to produce, which poses environmental concerns.

These are the main problems within the solar industry. Current research within the field of solar is focused on increasing the efficiency of solar cells, decreasing costs, and mitigating other problems (such as lifespan, environmental impact, etc.).

Just a disclaimer: Categorization of certain solar technologies isn’t strictly stated, especially between 3rd and 4th generations of solar technologies, but what I’ve included in this article include the distinctions I’ve gathered through my research.

1st Generation Solar

The first generation of solar panels includes polycrystalline, monocrystalline, and GaAs solar panels. I have already explained how they work above.

The top layer of the solar cell is the n-type material, with free electrons, typically thinner to allow photons to penetrate the solar panel. The bottom layer of the solar cell is the p-type layer, with extra holes, and is thicker and where absorption of photons happens. There is usually a glass layer to protect the solar cells from environmental conditions on top. There are metal fingers across solar panels to collect energy from the cells.

First generation solar cells currently make up 90% of the market share in solar technologies, and their costs have been driven down over time. They are stable and perform well, but conversion efficiency lies around 20–25% (while the theroetical efficiency is around 29.4%). The biggest problems with first generation solar cells include the loss of efficiency as heat, as well as the complex and energy-intensive manufacturing processes (such as silicon purification and doping) to create silicon solar panels which can drive up costs.

2nd Generation Solar

The second generation focuses around thin-film solar. Their main goal is to allow solar panels to be used more widely in various applications — they are flexible, lightweight, and thinner, decreasing the cost of materials since less is required, but oftentimes not as efficient as first generation solar cells.

Thin-film solar cells are flexible and lightweight.

Common examples include CIGS, CdTe, and a-Si (amorphous silicon) solar panels. The former two are the main thin-film photovoltaic technologies current on the market today. They are made by depositing one or more layers of the photovoltaic material onto a substrate.

I’ll take up CdTe solar cells as an example to explain thin-film solar. The CdTe structure includes a p-type CdTe layer and an n-type CdS layer. As they are technically two different materials, they form a heterojunction. Thin-film solar cells work according to the same principles as first generation solar cells — an electron knocked into the conduction band moves towards the heterojunction, travels through a circuit and a load, and returns to recombine with its hole.

Second generation solar technologies increase the applicability of solar cells, but face their own set of problems. For example, the CdTe layer and the CdS layer in a CdTe solar cell are full of grain boundaries (which serve as defects in the material), allowing for more recombination to happen (which, as you’ll remember, is when an electron falls back into a hole). This decreases the efficiency and power output of thin-film solar cells.

For this reason, the efficiencies of second generation solar cells are often lower. Another cause of this is because they have a voltage-dependent light-generated current. This means that if we try to increase the current in the solar cell, we sacrifice the amount of voltage we can gain. They also tend to have shorter lifespans compared to crystalline solar panels as they’re flexible and not as resistant to degradation. As well, some materials used for thin-film solar cells are toxic or expensive to manufacture. CIGS solar cells, for example, show high efficiencies in lab conditions, but involve a complicated manufacturing process that makes it difficult for them to transition onto the market.

3rd Generation Solar

This is where it get really exciting — at family dinners, these are the guys from the photovoltaic family that everyone wants to talk to. Third generation solar covers the most recent advancements in photovoltaic developments. You might’ve heard of some technologies listed here, such as perovskites and quantum-dot solar panels.

This generation focuses on improving where the 1st and 2nd generation lack, which includes surpassing the Shockley-Queisser limit, achieving lower costs, integration of new materials.

Dye-Sensitised Solar Cells (DSSC)

Also known as Grätzel cells, dye-sensitised solar cells are made of mesoporous (a material with pores of sizes from 2-50 nanometers) films of metal oxide nanoparticles (oftentimes TiO2 — titanium dioxide) coated in dye molecules, the latter of which have the ability to absorb light of a large range of wavelengths thanks to their unique properties. DSSCs don’t require a P/N junction. Instead, when a photon of light hits a dye molecule, it absorbs the light and an electron is excited. In a process called electron injection, this electron is transferred to the metal oxide material and joins its conduction band.

Here, the dye molecule absorbs light and transfers an electron to the titanium dioxide. The electron passes through the conductive glass, into the circuit and load, and returns to the counter electrode. Electrons are carried back by an iodide/triiodide solution, allowing the reaction to repeat.

Once the titanium dioxide has gained the electron, it now flows through the working electrode (typically conductive glass) into the circuit, through the load, and to the other side of the solar cell. When the electrons reach the counter electrode, they flow into an electrolyte solution, in which a redox mediator (chemicals that can transport electrons) accepts and transports the electron back to the dye molecule so the reaction can continue.

Pros:

• Low cost — DSSCs don’t cost much to manufacture, and are made from available materials. Maintenance costs are also low.
• Performance in low-light conditions — DSSCs perform very well in low-light conditions. They can function in cloudy environments.
• Flexible and lightweight — This can increase their applicability.

Cons:

• Efficiency — DSSCs have not demonstrated efficiencies higher than that of silicon solar cells.
• Degradation — DSSCs can be prone to degradation over time with exposure to light and moisture
• Safety risks — Since DSSCs include a liquid electrolyte that can be toxic, there are safety risks revolving around installation.

Perovskite Solar Cells

Perovskites are the hailed as the “future of solar technology” — after all, in just about a decade, their efficiency has increased from 3% in 2009 to 25% today! The term “perovskite” refers to a large class of materials with a particular structure following that of CaTiO₃, a natural compound existing in nature. Perovskites have an ABX-3 structure that looks similar to a diamond inside a cube, each of its vertices touching the face of the cube, connected to one central atom.

Here, the A and B atoms are cations (positively charged ions) and the X atoms are anions (negatively charged ions).

The perovskite layer is sandwiched between a hole transport layer and an electron transport layer.

Firstly, light passes through a transparent conductive oxide layer to reach the perovskite layer underneath. Just as in normal silicon solar cells, electrons in the perovskite cells are excited by photons from their valence band into the conduction band. This perovskite layer is sandwiched between a hole transport layer and an electron transport layer.

Once the electron-hole pairs are generated in the perovskite material, electrons are collected and transported to electrodes by the electron-transport layer, which is made from materials like titanium dioxide (TiO2), zinc oxide (ZnO), or fullerene derivatives (fullerenes are molecules made of 60 carbon atoms arranged together). The electrons run through the circuit, the load, and return to the hole-transport layer, which is typically made of a small organic molecule such as Spiro-OMeTAD (a derivative of spirobifluorene).

The properties of perovskites can be modified since a number of atoms in the ABX-3 structure can be modified. They can engineered to absorb a different spectrum of light, or with different bandgap properties, which allows for endless development — recent developments within perovskite technologies has thrust them into the spotlight of exciting solar development.

Pros

• Low cost — Perovskite solar cells can be manufactured using solutions processing, without as much heat. They can also be made into much thinner layers, which decreases the cost as well.
• Higher efficiency — As I’ve mentioned, perovskites can have tunable bandgaps and absorption spectrums. Their rapid increase in efficiency over the past few years makes them promising in the field of solar.
• Flexible and lightweight — This increases the applicability of perovskites.

Cons:

• Degradation/fragility — The biggest problem within perovskites is their their susceptibility to degradation. They have shown significant degradation under exposure to heat, light, and moisture, which affects their lifespan and may result in more complex manufacturing (since they require an encapsulation layer).

Quantum-Dot Solar Cells

Quantum-dot solar cells are another exciting development the intersection of nanotechnology and solar technology. On the nanoscale, materials can exhibit many different properties — for instance, quantum dots have tunable bandgaps and absorption properties and can therefore be engineered to absorb a larger range of incoming light.

As you can see here, the colour of quantum dots depends entirely on their size. At the nanoscale, properties of materials change significantly!

Just as in a normal solar cell, when light hits the quantum dots, electron-hole pairs are generated. But what makes quantum dots special is quantum confinement, which is when charge carriers (electrons or holes) are restricted within a small region of a material.

This is a highly simplified explanation, but essentially, quantum confinement allows for multiple “sub-bands” to form, which are discrete energy levels between the valence and conduction bands (here, discrete means not continuous; existing in a limited number of states). Essentially, rather than existing at any possible location between the valence and conduction band, the electron instead has sort of staircase of bands that it can exist in. This can decrease the chance of recombination, since electrons and holes cannot as easily interact each other on different steps of the staircase.

Here, QD stands for quantum dots. As you can see, rather than a continuous number of states or locations an electron can be present in, quantum confinement results in a discrete and limited amount of states (distinct lines) in which the electron can be present.

Part of what makes quantum-dot solar technology so exciting is the potential for multiple exciton generation, or MEG. This is when more than one electron-hole pair is created from one single photon — in most solar panels, excess energy (light with wavelengths above a material’s bandgap) is lost as heat. In quantum-dot solar cells, this heat can be transferred and continue generating more electron-hole pairs!

In a quantum-dot solar cell, the quantum dots are usually placed in between a hole transport and and electron transport material. This way, when electron-hole pairs are generated (a process facilitated by quantum dots’ large absorption capacity), the holes and electrons will separate and generate a potential difference and current.

Pros:

• Higher efficiency — Thanks to their broader absorption spectrum and tunable bandgaps, quantum dots can be much more efficient. They also have the potential to surpass the Shockley-Queisser limit thanks to the potential for MEG!
• Manufacturing — Quantum dots can be made through solutions processing, costs less and allows for more efficient manufacturing.

Cons:

• Toxic — Some materials, such as CdSe, used in quantum-dot solar cells can be highly toxic.
• Degradation — Quantum-dot solar cells show signs of degradation under light and moisture, which decreases their lifespan.

Multijunction Solar Cells

One limiting factor to the efficiency of solar panels is due to the limited bandgap of one single material’s bandgap — which means it can only absorb and convert a limited amount of wavelengths from incoming light. Multijunction solar cells combine different layers of materials with different bandgaps so the solar cells can absorb a larger range of the solar spectrum. This approach has yielded conversion efficiencies of up to 46%, and the theoretical efficiency of multijunction solar cells is around 87%!

The basic concept of multijunction cells is to absorb a larger range of solar radiation using different materials with different bandgaps.

Within a multijunction solar cell, there are multiple P/N junctions. The III-V multijunction solar cell is a popular one — it’s called III-V because its layers are made from elements from Group 13 and 15 on the periodic table. The top layer is made of GaInP, the middle layer made of GaAs, and the bottom layer made of Ge.

Each of them absorbs a different range of light from the solar spectrum, so the total voltage is the sum of the voltages produced by all three layers of the solar cell. It’s important to note that not all materials work well together — the materials selected must have similar lattice constants (crystalline structure), which facilitates flow of electrons between them.

Here, on the left side, the lattice structures of all three materials match. On the right, however, the lattice structures of the various materials are not compatible together, which restricts electron flow.

Currently, there is research done on optimizing the materials chosen and lowering manufacturing costs for multijunction solar cells. Scientists are also working on ensuring multijunction solar cells are operating at their maximum power point. Multijunction solar cells are especially exciting because they can combine various kinds of solar technologies to achieve higher-level efficiencies — for example, scientists have been working with silicon-perovskite multijunction solar cells.

Pros:

• Efficiency — Multijunction solar cells can surpass the Shockley-Queisser limit by optimizing usage of various materials. Higher efficiency means there is less space needed to produce the same yield compared to solar panels with lower efficiencies.

Cons:

• Costly — Since multijunction solar cells involve complicated steps to manufacture as well as the higher cost of more material for one solar cell, they can be more costly, which poses as a barrier to their widespread implementation.
• Material compatibility — Some materials don’t work well together since they have different lattice constants.

Singlet-fission Solar Cells

Singlet-fission solar cells also utilize the concept of generating multiple electron-hole pairs from one photon. To fully understand them requires diving deeper into quantum mechanics, which I unfortunately do not have space for in this article. I will, however, give an extremely simplified overview of my understanding of singlet-fission solar cells.

Here, one singlet exciton turns into two triplet excitons, which are then able to generate electricity through the photovoltaic effect.

As the term “fission” implies, singlet-fission solar cells involve splitting something into multiple parts. Essentially, an excited singlet exciton transfers its energy to a neighbouring ground-state molecule. “Ground state” in quantum mechanics (applied, in our case, to atoms) refers to the most natural and stable state in which an atom can exist. Due to the energy this ground-state molecule gains, it becomes two triplet-excitons (electron-hole pairs), both of which contain energy.

Through this reaction, the two electrons from the two electron-hole pairs move through the circuit, the load, and return to recombine with their holes. Do keep in mind this is a highly simplified explanation, but the most important thing to note is how MEG can occur in singlet-fission solar cells and increase their efficiency greatly!

Materials explored for singlet-fission solar cells include organic materials such as pentacene (C22H14) and tetracene (C18H12). They can also be made of other materials with solar applications, such as perovskites and quantum dots.

Singlet-fission solar cells are especially exciting because of the fast speed at which this reaction occurs, which helps prevents other causes of efficiency losses from lowering the efficiency. This may allow for the surpassing of the Shockley-Queisser limit for single-junction solar cells.

Pros:

• Efficiency — The ability to generate multiple electron-hole pairs from 1 photon gives them the ability to surpass the Shockley-Queisser limit. Their broader absorption range and the fast speed at which the reaction occurs can also increase efficiency.
• Manufacturing — Singlet-fission solar cells can be produced through solutions processing, which can lower production costs.

Cons:

• Material availability — There is a limited number of materials that can be used for singlet-fission solar cells.
• Cost/Ease of installation— Singlet-fission solar cells require more complex components, which can increase cost/decrease ease of installation.
• Degradation — Some singlet-fission solar cells show signs of degradation under exposure to light and other environmental conditions.

Ferroelectric Solar Cells

Ferroelectric solar cells use ferroelectric materials. Uniquely, though, they don’t require a P/N junction or any doping to induce the Photovoltaic Effect — which reduces their cost of manufacturing. Instead, ferroelectric materials have, on a microscopic level, separated positive and negative charges. As a result, they are polarized: this means one side is very slightly positive charged while the other is negatively charged, giving it an asymmetric nature structure. The polarization creates a “built-in” electric field, similar to the one created at the P/N junction, and allows for the Photovoltaic Effect to occur.

Due to polarization, there is a slight internal electric field.

When light hits the solar panel and electrons are excited from the valence to conduction band in the ferroelectric material, the internal electric field results in the build-up of electrons on one side and holes on another. Electrons flow from the electrode, through the circuit and load, and return to the other side so the reaction can continue.

One reason why ferroelectric solar cells are so revolutionary is because of their ultra-thin nature. Their lower manufacturing costs, driven down by the lack of need to create n-type or p-type doped layers, are especially exciting factors. Scientists are also looking into ways to increase efficiency, as ferroelectric solar cells aren’t able to absorb as much incident light as typical silicon solar panels. One approach is to layer multiple ferroelectric materials on top of one another, which has been shown to boost efficiency by a large factor — this approach has been shown to increase the output from the photovoltaic effect by 1000x!

Pros:

• Efficiency — Thanks to their ability to have engineered bandgaps and their inherent internal electric field, there is less recombination of electrons and holes and an increased efficiency. The theoretical efficiency of ferroelectric solar cells is much higher.
• Stability — Ferroelectric materials remain stable in the long-term even when exposed to environmental conditions, which increases their lifespans.

Cons:

• Absorption spectrum — Some materials, as I’ve mentioned above, don’t have as wide an absorption spectrum. This is why layers of ferroelectric materials are placed on top of each other.

Concentrated Photovoltaic

Instead of changing the solar cell itself, what if we try to change how much light is available to the cell itself? That is the aim of concentrated photovoltaic technologies — using mirrors or lenses, they manipulate light so more photons hit a solar cell, generating more electron-hole pairs. Some examples of concentrated photovoltaics include Fresnel lenses, parabolic mirrors, reflectors, and luminescent solar concentrators.

Here is an image of Fresnel lenses.

Fresnel lenses concentrates light from a larger surface onto a smaller point.

Here are parabolic mirrors, which concentrate light with their curved shape.

Most of these devices have the goal of concentrating light from various directions onto a single point, which largely increases the amount of light a solar cell is able the convert.

Another kind of concentrator is luminescent solar concentrators, which do not require lenses or mirrors. Instead, light passes through a layer of fluorescent dye or quantum dots painted on the surface of the solar panel (usually glass), which absorbs the light. It then fluoresces (which is when materials give off visible light after absorbing radiation that isn’t visible light), allowing for internal reflection onto a narrow solar cell.

Concentrated photovoltaic technologies can be combined with a number of other technologies to increase the total efficiency of the system.

Pros:

• Efficiency/Utilization of light — Light that otherwise would have been lost is able to be concentrated for the solar panel’s use, increasing the yield.

Cons:

• Cost — The cost of using extra materials for lenses or mirrors or other concentrating devices usually increases the total cost. This may also affect the ease of installation.

4th Generation Solar

Just when you thought it was over — nope! There’s still more to come in solar development! Family dinners only get more interesting. So far, the 4th generation is broadly categorized as the field of solar exploring novel technologies and materials to even better achieve the goals of the 3rd generation. One big area of research revolves around the applications of graphene within the solar industry.

Graphene-based solar cells

But first, an introduction to graphene. Long hailed as a “wonder material of the future,” graphene is formed by carbon atoms arranged hexagonally in a one-atom-thick layer. These 2D layers are stacked on top of each other, resulting in a material that is very flexible and lightweight, has very good conductive properties, has high strength and resistance, and is optically transparent. Graphene is the thinnest and strongest material ever created — it is calculated to be 200x stronger than steel and 5x lighter than aluminum!

Here is an image of a single layer of graphene, made of six carbon atoms arranged hexagonally.

With these properties, it’s easy to imagine the various applications of graphene in solar technology. Graphene has been researched as a conductive and transparent layer on top of PV cells to facilitate the transport of electrons (a job currently done by the metal “fingers” on a solar panel, which inhibit efficiency). They can also be applied to flexible and transparent solar cells, giving them widespread potential.

There is also research into graphene as a material for the solar cell itself. Graphene doped with silicon or perovskites, for example, have been experimented with, the latter of which surpassing an efficiency of over 20%. Unfortunately, problems facing applications of graphene in solar cells pertain to graphene as a material itself. It is currently expensive, complicated, and difficult to manufacture graphene.

Conclusion

That ties it up for the generations of solar technology! Innovation is essential to development and the transition of product onto market, especially with newer generations of solar technologies. Personally, I’m most fascinated by perovskite, ferroelectric, and singlet-fission solar cells, and I’m very excited to see where innovation takes them next.

The sun continues to be one of the greatest human resources. There is so much limitless potential in the field of solar energy, and new, exciting discoveries are being made every day. Here, the highest official efficiencies achieved for various solar technologies are recorded and updated periodically: https://www.nrel.gov/pv/cell-efficiency.html. Feel free to check them out!

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