A solar energy cell, also called a photovoltaic cell, works by harnessing energy from the sun to generate electricity by means of the photovoltaic effect. As the sun is a virtually limitless source of energy, solar cells are an attractive technology, yet some limitations exist. One of those limitations has long been efficiency. Solar energy is not available at all times of the day and therefore must be stored for later use, which makes efficiency a concern for technology that’s costlier to build and develop in the presence of cheaper fuel sources. However, a new method for solar cell creation exists in the form of a substance known as perovskite. Perovskite solar cells are a newer type of solar cell that, with further development can make solar energy even more efficient while driving the costs of fabrication and development for solar cell technology down to manageable means for the average consumer.
What is a Perovskite Solar Cell?
A photovoltaic cell is comprised of two contacts, one positively charged, and one negatively charged, with a semiconductor material between. When light strikes the surface of the panel the electrons in the semiconductor panel are pushed free and collected by the circuit. This array of materials is known as a p-n junction for the positive-negative properties it possesses. Most photovoltaic devices today use a single interface, that is to say only one junction of positive and negative. In such a cell, light must be able to penetrate the bandgap of the cell material in order to free up electrons for the circuit. This means that only a limited amount of the light spectrum emitted from the sun can effectively stimulate the circuit.
Once such a system is built, however, the fuel is free – leaving only maintenance and upkeep costs. As the sun isn’t accessible by all areas at all times, this energy must be stored for later use, so efficiency is key. Incorporating perovskite materials into solar cell technology has shown exceptional promise, with one five-year study of the research demonstrating a jump to over 22% efficiency, making perovskite materials very interesting to researchers. To illustrate, only one paper on the method was published in 2009, however, that number soared to over 1100 publications in 2015. This level of interest from research and development communities is a staggering representation of the weight of promise perovskite material has to offer, provided we can develop it to its full potential.
A perovskite solar cell (PSC) uses a perovskite, usually compounded with other materials, as the active layer. A perovskite is actually any material with the same crystalline structure as that found within the mineral known by that name. The material is desirable in this instance because it’s cheap, abundant, and excels at absorbing light, using far less material to capture an equal amount of energy as compared to the current standard. It’s also far easier to work with, as traditional solar cells are costly and complex to produce. To push things even further, however, even the most basic of these cells today comes in at around 15% efficiency, yet projections for development are looking to move beyond 30% “in the next few years.”. The most recent certifications for PSCs cite a 22.1% efficiency. This coupled with the cheap availability of the materials and fabrication makes PSCs a very attractive alternative to standard silicon-based solar cell technology, even as advances in that field continue to drive costs of production down.
How is a PSC Different from a Standard Solar Cell?
The secret lies in something called the diffusion length, which refers to the distance charge-carriers such as electrons must travel before they are recombined in the circuit. The diffusion length is the distance these electrons must travel in order to recombine with the circuit. This is an important measurement because it places serious limitations on how efficient these cells can be made. “If the diffusion length is less than the thickness of the material, most charge-carriers will recombine before they reach the electrodes,” says Samuel Stranks, a researcher at Oxford. “You want a diffusion length that is two to three times as long as the thickness to collect almost all of the charges.”
So, in short, cells that are too thin collect too little light energy, while those that are too thick don’t allow the transfer of charge carriers through their material. While contemporary silicone makeups have addressed this by creating something called “multijunction” or “tandem” cells with multiple p-n junctions all tweaked to cover a different range of the spectrum of light, this process is complex, costly, and tricky to fabricate. The same techniques applied to perovskite materials which are far less costly are already yielding over 20% efficiency, and while the aforementioned silicone techniques have come close to 45% or more, they are far too costly to be economical. PSCs look to be able to supply nearly the same range of efficiency at a fraction of the cost from very abundant mineral sources. One of the largest arguments against solar power is the cost associated with the development and fabrication of the necessary materials, and perovskite materials are one approach to solving that problem.
All energy sources carry a significant disadvantage, and renewable fuels are not without their drawbacks. For PSC technology, there are several issues that must be addressed before the technology can be brought to bear for the end-user.
One major drawback of PSC technology is capped efficiency, projected to be around 31% by current estimates. In order for the technology to be viable, we must either find ways to make it more efficient or find ways to drive down the cost. While the prices of other sources for energy continue to be manageable this isn’t likely to occur very quickly. That said, solar power generation, in general, is estimated to be capped at around 33.7% according to the Shockley-Queisser limit, a theorem that applies physics to the science of solar cell efficiency to project the best-case scenario for sunlight harvest. Assuming an ideal bandgap and a single junction makeup, this means that roughly two-thirds of the energy falling on the cell is lost to other factors.
Another limitation of PSCs is their instability in the presence of a range of conditions, most notably moisture. Water or humidity will destroy perovskite materials in a PSC, and this is a cause for concern in a device that must be mounted out in full view of the sky where it will be susceptible to temperature and pressure changes, as well as precipitation and wind at velocity. Oxidation is also a concern as this will degrade perovskite over time, and high temperatures will have a similar effect. Researchers were able to increase efficiency for some blends to nearly 20%, however, in ambient conditions the material deteriorated to under 5% of its starting capacity. Proposed methods for addressing this glaring weakness in the material include encapsulation of the active layer of perovskite-blended material in a hydrophobic coating.
Another concern is lead ingredients bound within the material. Perovskites rich in toxic and highly soluble lead are the most efficient, so there is danger of this substance leaking into soil and groundwater if not properly encapsulated.
Finally, thermal stability is something of an issue in terms of material degradation as well, as perovskite material absorbs light readily and solar cells often get quite hot, creating internal stresses in the crystalline structure. Changing the arrangement of the blend and the use of polymers in the material have shown promise in mitigating this weakness, however, researchers have noted that more studies should be done to assess the best means for addressing all of the longevity and reliability concerns that plague the development of this technology.
Perovskite’s Future in Solar Technology
While the possibilities for the development of perovskite solar cell technology, it’s not likely that this will become a commonly-used material until more research is conducted and the material can be made more stable in standard use cases. Many chemical and mechanical processes are being identified with current research that shows promise with regard to shielding the material from temperature and humidity, as well as chemical or toxic leakage, without too great an impact on the efficiency or stability of the material within. In fact, while much of the development in silicon crystalline solar cells has plateaued, research and development of perovskite-based solar cell technology has continued to move progressively towards industrial application, with one company in the U.K. already planning to produce perovskite cells, with pilot launch of some models slated to hit the market in 2018, provided all goes well.