Perovskites, The PV Technology Of The Future

Perovskites are on the rise in the solar industry, being both high performing and low cost. They are the most rapidly growing advancement within photovoltaics. Where they were only 3% effective at converting UV light to electricity back in 2009, today they are reaching efficiencies over 25%. Perovskites may have their barriers to commercialization, but they’re likely worth fighting for.

What Are Perovskites?

Perovskites are any materials that have the same crystal structure as calcium titanium oxide, the first ever discovered perovskite crystal. Since silicon isn’t involved with the usage or production of perovskites, they have the potential to become the superior thin-film solar manufacturing technology and play a dominant role in enhancing all photovoltaic cells. With their compositional flexibility, perovskite crystals can have a variety of unique characteristics.

Intro to Band Gaps

To understand the more technical advantage of Perovskites as compared to counterparts, we first must be introduced to band gaps. A band gap is the space between the valence and conduction band of an electron. It is the “energy range in a solid where no electronic states can exist”. Let’s break down what that means.

You can think of it as electrons sitting in a region called the valence band, and when these electrons are hit by photons, some get excited and use that energy to jump up to a higher region called the conduction band. Once an electron is in the conduction band, it is free to move and do work on a circuit, providing energy, which of course is crucial for the functioning of solar. Therefore, we want to get as many electrons up into that conduction band as possible, because that equates to more electrons being able to provide energy.

The space in between those two regions is called the band gap, and it’s crucial to efficiency. You want to set that band gap perfectly to optimize the ability for electrons to move from the valence band to the conduction band, ie absorb the maximum number of photons from the sun’s radiation. If you set it to be too large, you won’t generate a lot of electrons because few photons have the high amount of energy required to move all that way up. However, if you set it too low, then despite the fact that a lot of electrons will in fact be generated, most of the energy will be lost in the form of heat. An ideal solar cell has a direct band gap of 1.4 eV, and a direct band gap means that an electron can directly emit a photon. Some materials, though, do not have direct band gaps.

Band Gaps in Silicon vs Perovskite

Silicon is the most common solar-cell substrate material, and yet it is also quite inefficient at absorbing light because it has an indirect band gap. This makes it so that each time the material absorbs a photon it requires a specific kick in vibrational energy called a phonon at the same time. This is double the effort and therefore it’s a lot less likely to absorb the photon now because of the added work of having to absorb a phonon.

To address this, silicon is made very thick to increase the chances of absorption. The downside here is that thickening the silicon is expensive. The other problem with materials having indirect band gaps is that the absorption spectrum depends more heavily on the temperature as opposed to that of a direct band gap material. In lower temperatures there are fewer phonons, and therefore the chances of a photon and phonon being simultaneously absorbed are reduced. That makes solar energy less reliable and more dependent on external factors.

Where silicon is hundreds of microns thick, thin film solar cells that are direct band gap materials absorb light in a far thinner region and can be made with an active layer less than 1 micron thick. One of these alternatives can be — you guessed it — perovskites. These can be tuned to match the sun’s spectrum while also having similar properties to silicon. Their production processes take a fraction of the cost and energy and their crystallographic structure makes them highly efficient at converting sunlight to electricity.

Other Benefits

Perovskites’ ability to have an array of different band gaps means they can be used in a tandem solar cell, where cells of varying band gaps are stacked on top of each other. This enables higher module effectiveness because it means the solar spectrum can be divided into multiple bands that can be more efficiently converted by separate devices. Perovskites are additionally lightweight, flexible, and transparent. The pair of low cost production and higher efficiency with unique characteristics here and there is an ideal combination for a solar cell.

How is Perovskite Made?

Within lab spaces, perovskite cells are made by depositing chemicals like lead iodide and methylammonium iodide — which crystallize into perovskite — onto a substrate like plastic or glass. This can be done through deposition methods such as spin-coating, inkjet, spraying, or even painting. Being able to simply paint perovskite onto a substrate is an amazing synthesis approach because of its easy application. This is called solution processing, the same practice used for newspaper printing.

The film thickness can be controlled by varying the number of solution droplets on the substrate. Perovskite manufacturing allows the potential for huge scalability. Other approaches usually require expensive deposition equipment, but perovskite equipment is cheap and available, contributing to the low cost. No high-cost machinery or facilities are needed, and although the majority of thin-film photovoltaics require complex engineering processes, perovskites are defect-tolerant and easy to produce.

How Perovskite Works in Solar Cells

You’ve probably heard of the numerous amount of layers that go into making a solar cell, each one playing its own special role. What, then, does the perovskite specifically do?

Solar cells all require semiconductors, the material between electrical insulators and metallic conductors there to convert energy from light into electricity. Sunlight makes electrons excited in a semiconductor material, so they flow into conducting electrodes and produce current. Perovskite is an excellent semiconductor.

Perovskite solar cells work similarly to conventional solar panels, where a semiconductor absorbs solar energy and initiates electron flow, AKA current, which can be captured by wiring and converted into electricity to power homes, buildings, devices, etc. When the perovskite layer is exposed to sunlight, it will absorb photons to produce electron-hole pairs called excitons. The excitons form free electrons and holes because of the difference in the exciton binding energy, also called free carriers.

These free electrons and holes are collected by an electron transport material like titanium dioxide and a hole transport material like Spiro-OMeTAD. Electron transport layers have high electron mobility and block the holes so that only electrons can flow through. Hole transport layers do the opposite and block electron flow. Solar cells require holes to be collected at one contact and electrons at the other contact so that a net voltage and current can be achieved. This optimizes the performance and stability of the devices.

Downsides

Perovskite solar cell technology definitely has the potential to be impactful but there are still some barriers to commercialization.

One major problem is the life span of perovskite cells, which decreases in the presence of moisture, in return damaging metal electrodes because of the decay products. Encapsulation to prevent this can significantly increase the price and weight. The most stable perovskite cells in lab testing can only survive about 4,000 hours of continuous light.

The next issue is scaling. The larger in size perovskites get, the harder it is to produce a uniform layer, and the more pronounced defects and pinholes become. Ultra-high efficiency ratings have mainly been achieved in small cells, but this isn’t plausible for widespread panels since they need to be a lot bigger. The crystallization step of the synthesis process can be rapid and uncontrollable, making thicker films contain small grains.

However, there are tons of promising advancements here. For example, researchers recently added ammonium chloride to increase the solubility of lead iodine — a common precursor material used to create perovskite — which enabled the lead iodine to be more evenly dissolved. This subsequently resulted in a more uniform perovskite film with reduced defects and increased efficiencies and life spans. Findings like this give hope that such hurdles can be overcome.

The last major issue is toxicity. Perovskite contains lead salts, which can pollute the environment if leaked during the life cycle of solar cells. Something called PbI is one of the breakdown products of perovskite which is potentially carcinogenic. Substitutions are being looked into like tin but these are much more inefficient and the highest rate of conversion has been around 6%.

Conclusion

In conclusion, perovskites will no doubt have a huge impact on the solar energy industry. Once we get past a few barriers, their path to commercialization will be accelerated.

Let’s look at an overview of what we learned!

• Perovskites are any materials that have the same crystal structure as calcium titanium oxide
• Band gaps, the region between an electron’s valence and conduction band, must be optimized to increase the efficiency of solar cells
• Where silicon has an indirect band gap, perovskites have a direct band gap, making them cheaper, simpler, and more efficient
• Perovskites can be tuned to match the sun’s spectrum and used in tandem solar cells
• Perovskite cells are made by depositing chemicals onto a substrate like plastic or glass which is very easy to do and does not require expensive equipment
• Perovskites can act as the absorbing layer in a solar cell, sandwiched between an electron transport layer and a hole transport layer
• Downsides include life span, scaling to larger sizes, and toxicity

Thank you so much for reading this! I’m a 15-year-old passionate about sustainability, and am the author of “Chronicles of Illusions: The Blue Wild”. If you want to see more of my work, connect with me on LinkedIn, Twitter, or subscribe to my monthly newsletter!