The solar cell industry has been looking for a hot new material to use for some time now. Silicon solar cells are reaching the limits of their optimization and efficiency increases for traditional materials for thin film solar cells have stagnated a bit. Luckily, there’s a new competitor on the block generating a lot of excitement – perovskite-based photovoltaics.
The excitement mainly surrounds the fact that these solar cells can be solution-processed at low temperatures, which promises low costs and deposition onto flexible substrates so that they can be used for a wide range of environments (phones, windows, or more creative places I can’t think of!). The current research challenge is to get energy efficiencies (the percent of energy from incoming light converted to electrical energy – this is always what we want to maximize in designing a solar cell) of these types of cells in a range that makes them commercially viable – say, in the 20 percent area to start. A new study in Journal of Physical Chemistry Letters has demonstrated an efficiency close to that range, but, more importantly, has delineated the key characteristics influencing efficiency that will allow researchers to focus on optimizing performance.
Perovskite refers to a general crystal structure (illustrated above) that can be synthesized using many different combinations of elements that fit together with the general form of ABX3, where A, B, and X each stand for a different type of element. This means there will be 3 X atoms for every A or B atom (ignoring disorder or doping for the moment!). Here, the authors used methylammonium (CH6N) for A, lead (Pb) for B, and iodine (I) for X, so we have a the perovskite (CH6N)PbI3, which has previously shown the highest efficiencies for any perovskite solar cell.
Starting from this basic structure, the authors then fiddled with the amount of iodine in the samples of the material they used. This is known as substitutional doping – in place of a small percent of iodine, they put in their bromide (Br) or chlorine (Cl). Then, they see how this changes the properties of the material – absorption of light, solar cell efficiency, etc. From the image below, you can see that the pervoskite actually changes color as you add Br – from black to red to yellow! Adding Br also increases the band gap of the system, which determines the minimum energy light required to excite an electron that can then be extracted for electrical use (bottom right).
Figure courtesy of 
Ok, so that gives us some basics about how changing the chemical composition affects the perovskite’s electronic structure, but what about its use as a solar cell? Well, here comes the most important of the study. When the researchers added chlorine as a substitutional dopant to the system, the open circuit voltage (Voc), short circuit current (Jsc), and the effiency (η), all are increased (purple bars) compared to no Cl (grey bars). Voc and Jsc are both the crucial parameters of a solar cell that dictate efficiency, so you want to operate a solar cell at a particular point that has both high Voc and Jsc. The authors also tested placing the perovskite on different support substrates – remember, the perovskite part is in solution, so it must be placed on a more robust substrate. Here, they test an aluminum oxide (Al2O3) compared to a TiO2 substrate, and found the Al2O3 always performs better (blue-green bars along the bottom row).The authors used a technique called impedance spectroscopy to explore why the Cl dopants and the Al2O3 substrates led to such an increased efficiency. They found the main cause to be a reduced recombination rate in these materials compared to the undoped perovskite and the TiO2 substrate. Recombination is the process in which an excited electron (excited from the incoming light), loses energy and can no longer be extracted to use for energy. We want to reduce this rate as much as possible so we can extract as many excited electrons for energy. So, for some reason, the chlorine and Al2O3 substrate reduce this time and make it easier to extract them from the perovskite.
These kinds of studies are great because they 1) demonstrate the potential of a new material, and 2) open up so many new questions for other researchers to explore. In particular, since I do computational work, am excited to try to model some of these materials, play around with the levels of chlorine, and see what new interesting physics I can find. Whatever the reasons, this study gives hope that a new, cost-effective material could be very promising for next-generation solar cells!
Suarez, B., Gonzalez-Pedro, V., Ripolles, T., Sanchez, R., Otero, L., & Mora-Sero, I. (2014). Recombination Study of Combined Halides (Cl, Br, I) Perovskite Solar Cells The Journal of Physical Chemistry Letters, 1628-1635 DOI: 10.1021/jz5006797