What exactly is a perovskite?
Before we dive deep into the world of the perovskite semiconductor, we must be aware of the fact that they have been around for a really long time. One hundred and eighty years to be exact. However, with the recent boom in the semiconductor industry researchers, have begun to see the vast potential of this class of materials called perovskites. To put things in perspective, the first perovskite solar cell was developed in 2009 by Tsutomu Miyasaka and had a power conversion efficiency of 3.8% (Miyasaka, 2009). Fast forward to today, the highest efficiency produced by a single junction perovskite solar cell is 25.2% (NREL). The time frame of this growth in power conversion efficiency is almost negligible compared to the time passed since its discovery. It is exactly this steep growth in the power of this technology that has earned it the name, ‘The wonder material’. Another way to look at it is that perovskites took 10 years to reach the same point silicon PV took 30 years to reach. Simply put, the growth rate curve of perovskites is far steeper than that of silicon PV.
“So what exactly is a perovskite?” is a question we find ourselves at the receiving end of more times than we can count. This question in itself has an inherent flaw, perovskite isn’t one specific compound with defined elemental ratios. It is a class of materials that have an ABX3 compound structure. Here A and B are cations(a positively charged particle) with a stark difference in size whereas X is an anion(a negatively charged particle). The arrangement of these ions in their crystal structure is what gives this class of materials their special semiconducting abilities.
Now, if we began a detailed discussion on how and why perovskites are impressive semiconductors, that would make a separate article of itself(or two). An overwhelmingly short summary is as follows:
- Perovskites can be made in a lab with readily available and cheap chemicals at relatively low temperatures. There is no need of an extensive mining and extraction process. In fact, they can be manufactured by low-cost and quick methods like inkjet printing and roll-to-roll coating.
- Perovskites in a single junction setting have a theoretical efficiency of 33%, 4% more than their rival; silicon
- They have a broad absorption spectrum, fast charge separation, long transport distance of carriers, and a long carrier lifetime. What all this jargon adds up to is the fact that the base stats of perovskites are far higher than that of silicon as a semiconductor. This enables perovskites to be made in a thin film format, drastically reducing material usage and hence overall cost. This thin film format allows the production of highly flexible solar cells which have several unique applications.
With all these benefits and extensive research, you may be wondering, “Why don’t I see any perovskite solar modules in use?”. The answer to that question would make at least 5 articles of its own. However, a sincere effort to answer that question is as follows.
The other side of the coin
The 25% efficiency perovskite cell mentioned before was made in a leading Korean research lab with sophisticated equipment and expensive materials (NREL). For example, gold electrodes are used in cells to decrease power losses. Gold is obviously extremely expensive and cannot be used in commercial cells. The perovskite layers which produce the best efficiency, contain small amounts of lead, a toxic element that wreaks havoc when released into ecosystems. However, it gets worse. The perovskite active layer and the cell itself is inherently unstable material. This works on two levels. The perovskite layer begins to degrade due to its loose ions at around 90o C, a temperature most solar panels operate above. Other significant materials used in these cells are Spiro-OMeTAD and Titanium dioxide, unfortunately, both these materials react with the perovskite layer in the presence of UV light, heat, humidity, and even oxygen. You may start noticing a trend here, the perovskite solar cell is harmful to itself and that is where this technology’s biggest hurdle lies. Another major problem is the efficient manufacturing of perovskite cells. Traditionally they are made in the lab with a vapor deposition process which requires higher temperatures and works for only small area cells. Producing efficient perovskite solar cells with uniform layer thickness using commercial methods is a very difficult task and even a minor change in the thickness of the layer causes a drastic drop in performance. One research group produced perovskite cells with an R2R manufacturing process achieving a power conversion efficiency of 11% subsequently losing 30% of this efficiency in 93 days (Zuo, 2018). In contrast to silicon solar cells which generally have an efficiency of 16% to 20% and a lifetime of about 20 years, perovskites still have a long way to go.
But it’s not all bad news
Yes, perovskites do have a long way to go before they start competing with traditional silicon PV technology. However, there is an international effort taking place too fast-track the maturation of this technology. Some researchers at Iowa State University are trying to replace the unstable organic cations in the cell with inorganic cations such as Cesium, thereby increasing its thermal stability. To tackle the lead problem, researchers have attempted to replace it with Tin. However, the Tin cation is very unstable and leads to rapid decrease efficiency of the cell. They further went ahead and tweaked the crystal structure of the perovskite layer and this has shown promising results in terms of stability. (Li, 2020). Researchers from the Gwangju Institute of Science and Technology developed a creative way to get rid of the loose ions in the perovskite layer by vacuum treating the cell. This vacuum treatment pulls out these defects from the cells sandwich layer and absorbs them with a solvent on the top layer greatly increasing the stability of the perovskite cell. (Kim). An alternative, VNPB, is being explored as a potential replacement for the unstable Spiro-OMeTAD as a hole transporting layer. VNPB is used in organic LEDs and has shown great potential in perovskite cells (Dunn, 2020). These are just a few examples of the research taking place globally towards perfecting this technology. Many companies are devising clever ways to use perovskites such as energy-generating windows, this is discussed in the next and final section;
The Way Ahead; Towards Commercialization
At this point in time, perovskites cannot compete with silicon solar cells on a macro scale, for example, in applications like rooftop solar and large solar farms like in Rajasthan. However, their unique properties are being exploited in other fields. Swift Solar, a California-based startup is focusing its efforts on developing perovskite sheets for trucks and other large vehicles to run air conditioning units and other electronic devices (Solar panels are more efficient than you’ve heard. This material could make them even better., n.d.). Regular silicon cells are rather heavy and thick and would severely hinder the performance of the vehicle. In this case, the advantage of the physical attributes of perovskite cells outweighs the disadvantage due to their lower efficiency and short lifespan. Internet of Things, an increasingly popular subject in the world of communications and networks, is another place where perovskites find their use. Perovskites are used in sensors and in powering sensors, an essential commodity for future Internet of Things applications. Perovskite cells can work in low light conditions in thin cell formats making them the perfect choice for small sensors in buildings and homes. While we are on the topic of buildings, the ARC Center of Excellence in Exciton Science is exploring the possibility of using perovskites to convert regular widows to translucent solar cells. Though this is still in research stages, it could soon be used to substantially power buildings across the world. A novel application of perovskites is in tandem solar cells, where a perovskite cell architecture is coated on a regular silicon cell to make two series solar cells. On a small area solar cell this application has achieved about 28% efficiency and has the potential to reach even 30% efficiency, Oxford PV, a company that branched out of an Oxford University lab, is leading this development (Cholteeva, n.d.). Finally, there are many companies working on a manufacturing line for perovskite solar cells. Microquanta Semiconductor, Hunt Perovskites, BackbonePV are a few examples. Most of them have already developed and tested pilot factories for large-scale production and can possibly begin production 2 to 3 years from now. They will definitely not directly compete with solar PV right off the bat, but it took silicon PV 40 years to mature, and hence we must be patient with this technology because it clearly has the potential to change the world of energy.
References
Cholteeva, Y. (n.d.). Record-breaking solar perovskites. Retrieved from https://www.power-technology.com/features/record-breaking-solar-perovskites/
Dunn, C. (2020). Semi-Transparent Perovskite Solar Cells with a Cross-Linked Hole Transport Layer. Nano Energy. doi:Yu, J. C., Sun, J., Chandrasekaran, N., Dunn, C. J., Chesman, A. S. R., & Jasieniak, J. J. (2020). Semi-Transparent P10.1016/j.nanoen.2020.104635
Kim, H. (n.d.). GIST Excellence. Retrieved from GIST: https://www.gist.ac.kr/en/html/sub06/060202.html?mode=V&no=195558
Li, M. (2020). Tin halide perovskite films made of highly oriented 2D crystals enable more efficient and stable lead-free perovskite solar cells. ACS Energy Letters. doi:10.1021/acsenergylett.0c00782
Miyasaka, T. (2009). Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells. Journal of the American Chemical Society. doi:10.1021/ja809598r
NREL. (n.d.). NREL Efficiency Chart. Retrieved from https://www.nrel.gov/pv/assets/pdfs/best-research-cell-efficiencies.20190802.pdf
Solar panels are more efficient than you’ve heard. This material could make them even better. (n.d.). Retrieved from grist.org: https://grist.org/energy/solar-panels-are-more-efficient-than-youve-heard-this-material-could-make-them-even-better/
Zuo, C. (2018). One-Step Roll-to-Roll Air Processed High-Efficiency Perovskite Solar Cells. Solar Energy Materials and Solar Cells. doi:doi:10.1016/j.nanoen.2018.01.037
