New research out of Los Alamos National Laboratory, published in Nature Communications last week, has demonstrated a new material tailored at the nanoscale that provides great hope for use in next-generation, multiexciton solar cells. Multiexciton photovoltaics have been an enticing theoretical design to greatly boost efficiency but have not yet been experimentally realized. These new results using engineered PbSe/CdSe quantum dots may be one of the most promising leads to make multiexciton generation a reality in solar cell technology.
Solar cell efficiency is the holy grail for photovoltaic researchers. Silicon and gallium arsenide solar cells sit around 25-30% efficiency, but their relatively expensive manufacturing costs limit their ability to compete with fossil fuels.
Organic solar cells can be printed and rolled out like newspaper very cheaply, but their efficiencies max out around 5-10%.
Although silicon solar cells are already close to their theoretical 33% efficiency limit (the Shockley-Queisser limit), other cell types have the potential to break this limitation through physical mechanisms that are exciting researchers these days. One of the most intriguing methods is called multiexciton generation (others are tandem, intermediate band, and hot carrier extraction solar cells). In a regular solar cell, such as silicon, one photon of light excites one electron in the silicon. This electron has a lot of energy initially that could be useful if we could get to it, but it loses a lot of this energy to its surroundings quickly.
We could talk about why this happens for a long time, but you can generally think about it as the electron losing heat to its surroundings, just like a hot cup of coffee slowly cools because the air temperature around the cup is lower. By the time we are able to extract the electron from the silicon to a circuit to generate electricity, we have lost some of the energy from the photon that we could have used. This is partly why a traditional solar cell has a lower efficiency around 25%.
But what if we can find a way of using this energy that we normally lose to the environment? This is what multiexciton solar cells try to do (as do hot carrier cells, in a different way). In multiexciton cells, if we can confine the electron and limit its interaction with its environment for a long enough time (this is the hard part, as we’ll see!), it has a chance to collide with another electron and excite it as well!
Now we have two electrons for the price of one photon! We haven’t broken any basic physic laws like energy conservation – instead, the electron has given that excess energy to another electron instead of to the environment. In this way, one photon can lead to two (or more!) excited electrons, both of which are extracted, and we’ve doubled our efficiency! Sounds great, right? But is it easy to do? Unfortunately not…but researchers have definitely made progress finding materials that make it easier to achieve this phenomenon. Enter quantum dots!
I hope to do a whole post soon on the physics of quantum dots because they’re the foundation for a lot of cool new physics and engineering applications, like LEDs, medical sensors, and, pertinent here, solar cells. Physicists and engineers are still exploring new phenomena that occur when we move to the nanoscale and smaller, because quantum effects at that scale take over and lead to behavior we don’t expect in materials that we otherwise understand fairly well. You can imagine taking a big chunk of material, lead or titania or anything, and carve out a small ball of it only a couple nanometers in diameter – now you have a quantum dot! This material will behave very differently than its bulk counterpart from which you carved it out. More importantly, we’ve found new ways to create quantum dots with innovative structures. One such design combines two types of elements or compounds so that there is a core material at the center surrounding by a shell of a different material.
Or, we can put the quantum dots in solution and attach different ligands, strands of molecules and atoms, to them to change their properties.
The options are really endless and we as scientists are just pricking the surface in terms of the new physics and applications we can find. So I’m sure you’ll see more about them on this blog!
OK, now that my excitement has cooled off about quantum dots, we can get back to this new research article with some very promising results. The researchers used the core-shell quantum dot structure I mentioned above, using a lead selenide (PbSe) core and a thick cadium selenide (CdSe) shell around it.This structure is especially helpful for multiexciton generation because of how it leads to the electrons and holes being distributed throughout the dot (a hole is the absence of an electron after it is excited by a photon – the hole has positive charge but otherwise acts just like an electron – most importantly, it can also be extracted for energy applications). It so happens that these two core-shell materials interface such that electrons are spread evenly through both the core and shell, however holes are extremely localized just in the core PbSe (known as a quasi-type-II confinement structure).
Why does this hole localization help? Remember that there are two major factors we want to optimize to promote multiexciton generation through electron and hole bombardment. First, we want excited electrons (and holes) to cool as slowly as possible to give them a chance to collide. Second, we want these charge carriers to be close together to interact electrostatically so they attract to one another and collide. Having localized hole states in the core achieves both of these goals! Different localized hole states in the core have large differences in energy, making them cool more slowly. In addition, the localization will increase the Coulomb coupling between the holes. So this nanoengineered type of core-shell structure lends itself to charge carrier collisions and the generation of multiple excitons. Indeed, the researchers find that the multiple exciton generation is largely dominated by the action of the localized holes, not electrons, which end up having much denser energy levels compared to the holes. This smaller energy spacing leads to excited electrons being able to easily cool from environmental interactions.
So how did the researchers measure the amount of multiexciton generation, and what did they find? Researchers usually measure this through the a quantity called quantum yield, which is the measure of the number of excited electrons gained per incident photon. Experimentalists can measure this quantity through photoluminescence, in which they shine light on the material and then measure the intensity of light that is emitted from the material after the electrons fully relax and lose their energy to an emitted photon. Higher intensity at a particular wavelength indicates more excited electrons at that energy that are subsequently relaxing. Here, researchers plot 1-q, where q is the number of electrons per incident photon:The x-axis measures the ratio of the shell thickness to the overall diameter of the quantum dot. So, as the aspect ratio increases, the shell becomes thicker. The blue and red dotted horizontal lines are previous data in the literature for nanorods and PbSe quantum dots without the core-shell structure. For high aspect ratios, the core-shell structure shows dramatically higher quantum yields up to 1.7 excited carriers per photon for a 0.7 aspect ratio.
These results indicate that thicker shell structures assist in the multiexciton process, and it is not just the core itself that dictates this mechanism. The researchers explain this by the fact that thicker shells lead to higher energy holes being spread into the shell region, decoupling them from the localized core states. This leads to long-lived, lower energy hole states in the core that are ripe for collisions and the generation of multiple excited charge carriers. More theoretical and computational research will be extremely valuable in confirming this mechanism or understanding other reasons that this aspect ratio has such a significant impact on the quantum yield.
These results are extremely exciting for solar cell researchers – these are some of the highest levels of quantum yield ever reported, and the results provide specific reasons as to why. This last part is crucial as it gives paths for other researchers to improve upon the present results. The next steps will be to see if these multiply excited charge carriers can be extracted and lead to a significantly increased cell efficiency.
Cirloganu CM, Padilha LA, Lin Q, Makarov NS, Velizhanin KA, Luo H, Robel I, Pietryga JM, & Klimov VI (2014). Enhanced carrier multiplication in engineered quasi-type-II quantum dots. Nature communications, 5 PMID: 24938462