Enhancing Perovskite Solar Cells With Graphene
For years, we had been using gravel to make roads. Must have been sad times. But then in the 1920s, we discovered and started using asphalt, which is better for reasons I don’t even need to explain. The same is now true for solar cells.
While traditional silicon solar cells remain at the top of the game in terms of efficiency, they have a new competitor that might overtake them soon. Silicon solar panels are very expensive, have high processing costs, and the raw material itself is costly both to your pocket and the environment.
That with not the best efficiency is enough motivation to try to make new tech, and after second-generation solar panels’ objective failure, researchers started hunting for new materials they could use in the third generation of solar panels. That’s when they came across perovskites 😉.
Perovskite Solar Cells
Perovskite solar cells (or PSCs because I don’t wanna have to type that over and over) have become the fastest-growing solar technology in history after going from 3% efficiency in 2006 to over 24% in 2019! Jeez 😦. And to put that into perspective, silicon solar cells have been in development for over 60 years, with over 90% in use and being manufactured ranging in efficiency from only 15–20% (efficiencies in labs are higher).
And they’re not just matching efficiency, they also have a ton of advantages over them. PSC’s are light, thin-film, cheap, a lot easier to make, can be transparent, have stronger solar absorption than silicon, and are growing at a fast rate and may become the first to overtake silicon.
And the first few of these also allow it to have a lot more versatility in where the solar cell can be placed. (Transparent + flexible + thin) > (Opaque and ugly + rigid and heavy + fat)
Stronger solar absorption is just a fancy way of saying they absorb light with more energy (for example violet, and UV rather than blue).
In a nutshell, all solar panels work around the same principle of electrons in semiconductors getting knocked loose from atoms after they absorb the energy in incoming light. These electrons can then be collected and the potential energy used to form a circuit, in which they rejoin the hole they left after they have lost the energy.
And in between that circuit, you can put a phone charger, a stove, or a house. Anything that you need to power.
That’s cute, but what even is perovskite?
Perovskite… isn’t even a material. Its a compound group. It’s just molecules following the chemical structure of Calcium Titanate or CaTiO₃. The structure is referred to as ABX₃. But, of course, it’s not that simple 🙄.
Imagine you’re building a house. You get the structure right (frame, windows door, everything), but the frame of your house is made of straw. Now, of course, that doesn’t work because the big bad wolf is gonna huff and puff and blow your house away. You need to use a specific or specific type of material for each part for the house to function.
The same is true for perovskite. For the compound to work, A needs to be an inorganic cation, B needs to be a divalent metal cation, and X needs to be a halide anion.
Definition Table — Inorganic: Has a carbon atom in the molecule; Divalent: Has 2+ valences; Halide: From the halogen group in the periodic table; Cation: Has a net positive charge; Anion: Net Negative charge.
But those are not the only parameters 😔. Going back to the house, if it has a wooden frame and dry walls and glass windows, but its the size of my wrist, it’s not a house anymore. You cant go in it. For the house to work, the size of everything also needs to be relative to each other, the people and furniture it’s going to hold.
For our perovskite, the size of each molecule should be relative to each other and can be described by this equation: [ ( Rₐ + Rₓ)] / [ √2 ( Rᵦ + Rₓ ) ] ⩰ 1
R here is the ionic radii. This equation is relatively simple and just describes the ratio of the sizes, and when they are put into it the answer needs to be extremely close to 1 for the perovskite to be a perovskite.
Now you know what perovskites are 😄!
What else is in the solar cell?
Holding a piece of perovskite under the sun isn’t gonna give you electricity🙄. Although it may give a nice tan.
The solar cell consists of a perovskite layer sandwiched between 2 electrodes. However, since perovskite is intrinsic, there is a separate n-type and p-type layer, or the HLT and the ETL, respectively. These are in between the electrode and the perovskite and act as a transport layer to carry electrons/holes to the electrodes.
How does the PSC work?
Finally, we’re getting into the juicy stuff. Some definitions to keep in mind:
Binding Energy: How tightly together parts of a molecule are held together
Free Charge Carriers: Electrons/ Electron Holes
Charge Recombination: When the electron just falls back in the hole and potential energy is lost 🙁
When light hits the perovskite, the photon absorption causes an electron to get knocked out of its bonds in the atoms. The free charge carriers are then quickly injected into the ETL/HTL through which they travel into the cathode/electrode.
The ETL and HTL create an internal electric field which is the driving force of the electron movement and also acts as hole/electron barriers to prevent holes going to the cathode and vice versa.
One of the advantages of perovskite is its low binding energy, which lowers the chances of charge recombination because the electrons move relatively fast, which is great because charge recombination is one of the biggest roadblocks in solar.
Another is that since the perovskite has both organic and inorganic components, it has strong interactions between covalent and ionic bonds which allow for a precise crystalline structure that increases conductivity and thus efficiency.
However, this also causes problems 🙁.
Disadvantages of PSC
Perovskite is extremely sensitive to water/moisture, and any exposure to the layer can cause some pretty big problems. The water causes the perovskite to literally start rotting and decomposing and as you can probably imagine that’s not too good for efficiency 😢.
And if we wanna see perovskites on rooftops and windows creating a clean room in a lab doesn’t apply here and we can’t drain out the moisture in the air so its a pretty big problem. PSCs lose 20% of their efficiency because of this stability issue.
But no need to trash the project, because we have nanotechnology to save the day 😉.
Nanotech x Solar Power
Nanotech is amazing and can be applied to literally any technology to make it better, and solar power is no exception. If you wanna learn more about nanotech in general, check out my article!
But this article is focused on a specific application, which is using a nanomaterial called graphene to make PSCs better.
Graphene
Graphene is a revolutionary nanomaterial that is essentially just a single atom thick layer of graphite. It has a list of amazing properties, including being the hardest (yes, harder than diamond), strongest, most pliable, most conductive, and one of the most flexible materials in the world.
But the property we care about more here is graphene’s hydrophobic nature.
Using graphene as a stabilizer material in PSC
The solution to the problem is adding a graphene layer between the perovskite layer and the ETL. Graphene is hydrophobic, which means it has a very weak affinity to moisture and does not let it pass through, keeping the perovskite layer pure.
But that’s not the only benefit, there are some extra perks 😉. The graphene layer also prevents diffusion between the perovskite, the cathode, and ETL. Diffusion is just loosely held particles transferring between the materials.
You can think of it like walking into an empty room with a steaming cup of coffee. When you walk in, the steam is going to be concentrated around the source, or the coffee, but it tends to spread and eventually the whole room will have even amounts of steam.
That basically what diffusion is, the tendency to spread, in a sense. In PSCs, the halide anions can diffuse into the cathode and zinc oxide (ZnO) particles from the ETL and particles from the cathode can also diffuse into the perovskite, which reduces purity and thus efficiency.
Adding the layer of the solid graphene prevents this diffusion and helps keep the ETL and the perovskite pure. So graphene basically helps even more in keeping high stability.
But stability is not the only way graphene can be used to enhance PSCs 😉. It can also be used as a dopant for the n-type layer and as a cathode.
Doping graphene into the ETL
One way to increase the efficiency of PSCs is doping graphene into the ETL, which is made of a metal oxide, usually ZnO. Doping is just intentionally manufacturing impurities into a material.
Doping graphene enhances the electrical properties and crystallinity of the ETL when acting as an electron carrier. The graphene network inside the metal oxide increases charge mobility (how fast electrons can move) and lowers charge recombination even further, increasing efficiency.
In one experiment by P.S. Chandrasekhar, the efficiency of the solar cell was increased by 47.5% after graphene was doped into the ZnO! That’s phenomenal!
Using Graphene as a conductive electrode
Conventional PSC cathodes are made of TCO, or a transparent conductive oxide. The most common ones are FTO (fluorine-doped tin oxide) and ITO (indium-doped tin oxide). Using a graphene electrode instead proves to be a lot better.
Compared to TCO, graphene electrodes show way better physical, chemical, and thermal stability, and are cheaper lighter flexible, and transparent. They also have higher efficiencies because of graphenes' higher conductivity and flexibility, especially against ITO.
ITO is brittle and cracks easily, and shows rapid loss of efficiency after 250 bends because the cracks cause it to start diffusing into the perovskite. This problem doesn’t occur with graphene 😉.
Key takeaways
- Perovskite Solar Cells are a new type of solar technology that has been progressing really fast
- They work by using a perovskite material as the photoconductive layer between 2 electrodes and p and n-type layers
- Graphene is a revolutionary material with amazing properties
- It can be used to enhance PSCs in three ways; adding a layer of graphene to preserve stability, doping graphene into the n-type layer, and using a graphene cathode instead of the conventional TCO
Authors note
Hey, I’m Ruben. Just a 15 y/o passionate about solving big problems. Hope you enjoyed reading this article 😄. If you wanna reach out to me or have any questions/comments feel free to shoot me a message on LinkedIn, I’ll be happy to connect!
