Perovskites. If you can't pronounce it, don't sweat it. But no matter how you say it, pervoskites are an amazing material that you should know about. Here I'll cover two different technologies in which perovskites can offer improved performance: displays and solar panels.
I know what you're thinking, what the heck do the materials that make my TV awesome, and the ability to convert sunlight into electricity have in common? More than you might think. As a class of materials, perovskites have been around for decades, but it's not been until recently that scientists have discovered they are VERY useful for light harvesting and electricity generation. Think about it, your TV emits light when powered by electrons, and solar cells absorb light and convert it into electrons. At a basic level it's the same process, just in reverse. Let's dive in.
Perovskite quantum dots for displays
You already probably know that some TVs contain quantum dots, which are great because they emit really pure colors and enable more efficient and colorful displays. What
you may not know is that there are currently only two types of quantum dots that are used in displays - cadmium selenide (CdSe), and indium phosphide (InP). It turns out, perovskite quantum dots have some properties that are even better than both of the commercially available solutions, in particular in the pure colors they emit. The full-width-half-maximum (FWHM) is a property of emissive materials that is used to characterize the width of their emission peak. The table below shows the comparison of a few different materials (smaller FWHM=better).
Not only can pervoskite QDs be tuned throughout the entire visible spectrum, but they also have the distinct advantage of a more narrow peak width (for some colors even more narrow than an LED). The result: Impressive color.
Perovskite quantum dots are still in the R&D stage, so no one is using them in commercial displays, but they might be soon. While the optical properties look great, there are two major drawbacks to implementing them in current display technology.
- Most flavors of perovskites contain lead, and electronics makers have done a lot of work to get rid of lead-containing materials in their products (solders are a great example). It would be a big hurdle to implement an optical component that contains lead.
- The stability of these materials is not yet sufficient to survive for years (or decades) in a TV. No one wants to buy a TV that changes color and dims over time!
Rest assured scientists are working to develop lead-free perovskites and improve the stability of these materials so that someday perhaps they will find their way into your living room.
Perovskite solar cells
If you work in the area of solar energy, you have almost certainly heard of perovskite solar cells. If you aren't familiar, here's a crash course. Perovskites burst onto the scene of photovoltaics in 2013 with efficiencies approaching 15% (impressive for a new material). But the truly impressive thing about this technology is how quickly it has progressed in efficiency in such a very short time. Thanks in part to the special electrical properties, and simple materials and methods used to create these solar cells (see C&E News video), many labs started experimenting, and the record efficiency skyrocketed to the current record of ~22%. The chart below from National Renewable Energy Laboratory (NREL) tracks progress of research-cell efficiencies over time. As you can see, solar technology takes a long time to improve, with many technologies inching along at only a few % increase every decade. But perovskites (red circle with yellow fill) have jumped much faster than any other technology on the chart. And who knows where they'll stop.
Link HERE for higher quality image
When the two collide...
You may have also noticed another technology rising rapidly on this chart- quantum dot solar cells (open red diamond in the lower right corner). While this technology started below 5% efficient, it has risen to a respectable 13.4% in less than a decade. The quantum dots used in these cells are not typically that same CdSe or InP quantum dots
used in displays, but rather PbS or perovskite quantum dots due to their ability to absorb a more broad range of wavelengths from the visible to infrared. Perovskite quantum dots have some unique advantages of their their thin-film brethren, in particular they have been found to be more thermally stable, a property that may allow them to be better suited for long term use. It should be pointed out, the champion quantum dot solar cell device on the above chat (13.4%) was only recently added, and hasn't even been published yet. Look for an update to this blog soon with details about this device from the team at NREL that I had the good fortune of working with when I was a post doc. [EDIT Oct 31: NREL just published their paper about the perovskite QDs with a unique ligand exchange step which results in improved performance. Read about their record breaking device here.]
A thought experiment
Perhaps some day your TV, phone, tablet, solar panels, and LED lights will all contain quantum dots. I enjoy the following thought experiment: A photon strikes your perovskite solar cell on your rooftop, which is then converted into electricity. That electricity travels to your TV or charges your phone where a perovskite quantum dot display awaits the electrical energy input to create color and light for your eyes to see. Photoreceptors in your eye convert that photon to an electrical signal in your brain, informing you of the color you are witnessing. Certainly not all of these energy conversion events are highly efficient, but with perovskites perhaps we can inch a tiny bit closer to perfection.
Thanks for reading!