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No-Jargon Research

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Building Better Annihilators

By Andrew Pun

Everyone loves rainbows! Dan likes them so much they’re up on his Twitter banner right now. And everyone likes rainbows because they’re beautiful and made up of many colors. 

And outside of the colors of a rainbow we can see, there’s a variety of other types of light we can’t see. Light that is purple-er than purple is called ultraviolet (or UV), and light that’s red-er than red is called infrared. One interesting thing about different colors of light is that they’re not created equally. Purple-er light actually caries a lot more energy than red-er light. This is why UV light is dangerous - it actually has enough energy to give you a sunburn, and it’s what we need sunscreen (and the ozone layer) to protect us from.

We in the Congreve and Campos labs are interested in converting light from one color to another, and recently have been involved in the process of turning low energy (red-er) light into higher energy (purple-er) light, using a process called photon upconversion. There are a lot of different reasons why scientists are interested in upconversion, but one of the main reasons is to make more efficient solar cells. (Stay tuned for the next installment to hear more reasons!)

We need to make solar cells as efficient as possible in order to make solar energy cheaper than energy from fossil fuels. Unfortunately, even the most expensive solar panel you can buy can’t turn all the light from the sun into energy. A lot of the energy the sun gives off is that invisible, low energy, infrared light, and a lot of that light doesn’t have enough energy to make electricity when it hits a solar panel, and goes straight through the panel and is wasted. If we could turn that low energy light into higher energy light, we could turn that wasted light into usable electrical energy, giving us more efficient solar cells.

To do this, we use a process called upconversion. The particular form we study involves two kinds of compounds, a sensitizer and an annihilator. The sensitizer absorbs the low energy light and gives it to the annihilator. Two annihilators then fuse the low energy light together, giving off high energy light. Here, we want to talk about two ways we made the process better.

Currently available annihilator molecules work decently but have a couple of key drawbacks. First, it’s hard to pick the exact color of light that comes off the annihilator. A molecule called rubrene can turn infrared light into orange light, but what if you want that orange light to be a little more yellow, or more red? We wanted to make a molecular system where you could easily choose the color of light that gets made. The second major drawback of common annihilator molecules is that they’re really unstable. If you leave them in air and light they’ll fall apart and turn colorless. I think most people would agree that you don’t want to make a solar panel out of something that gets destroyed by sunlight.

To solve these problems, we used a molecule that’s been studied for a long time, called a diketopyrrolopyrrole, or DPP for short. Unlike previously used annihilators, DPP is extremely stable in light and air, so much so that a type of DPP is commercially available as pigment red 254, nicknamed Ferrari Red. It’s so stable it’s used as paint for high end sports cars like…Fiats. This stability is really apparent when we deposit a small amount of one of our DPPs (compound 3), onto a glass slide. This is compared to rubrene, the old school annihilator. After leaving these glass slides on a lab bench for 2 hours, there’s no visible changes to the DPP, while the rubrene has degraded into nothing.

Another great thing about DPP is that we can easily make small changes to the molecule, which lets us control the exact color of light that comes out from upconversion. One of the best things about working in upconversion is that it’s very obvious when it’s occurring! When we shoot an infrared (low energy) laser into a solution containing a sensitizer and our DPP annihilators, we can see all the different colors of upconversion we can make. More details on this work can be found in the paper itself here (https://pubs.acs.org/doi/10.1021/jacs.8b11796).

The second way we went about making better annihilator molecules was inspired by the process of upconversion itself. We mentioned that after the sensitizer hands off the low energy to the annihilators, two of those annihilators meet and fuse the energy into higher energy light. Instead of having two separate annihilators, why not link them together? These new annihilators made of two old annihilators are called dimers (di = two), while the original annihilator is just a monomer (mono = one).
 
After doing our usual measurement, mixing sensitizer with annihilator and shooting with an infrared laser, we were pleased to see nice bright yellow-orange light come out. What was even better was that our dimer was noticeably brighter than our monomer, as we’d hoped.
 
To confirm that this fusing process was occurring within individual dimers, we did a study where we lowered the concentration of dimers in solution and measured the intensity of high energy light out, and did the same with the original monomer. The monomer shows a huge drop off in high energy light out as the concentration drops, whereas the brightness of the dimers only decreases a little even as you decrease the concentration of the dimers by quite a lot - more than a factor of 10! This showed that the monomers were having trouble meeting up, but the dimers had their buddy next to them the whole time, confirming our hypothesis that linking two annihilators together into a dimer would give us more efficient upconversion.
 
The fact that these dimers are such efficient annihilators even at low concentrations allows us to use them for a lot of stuff! For example, for in vivo applications you can imagine you want to avoid putting any more outside chemicals into the body than you would need. More details about this dimerization study can be found in the paper here. Stay tuned to this no jargon section for more, including some of these different applications of upconversion.

Efficient Blue and White Perovskite LEDs via Manganese Doping

Loosely inspired by XKCD’s Thing Explainer, in the following hopefully coherent narrative we’re going to try to break down our recent paper in a non-jargony way. Wish us luck! Paper link: https://doi.org/10.1016/j.joule.2018.08.005

​First, let’s talk about color. This picture represents the colors you can see. For your phone/TV/FancyWatch/whatever to show them, you need a red, green, and blue LED (circles in the picture).  By powering them at different levels you can make any color between the three circles. In that way, with three LEDs, you can make almost every color. So every screen has red, green, and blue emitters.

​We decided to study perovskites as our light emitter. These materials are awesome – they’re bright, have sharp colors, and are inexpensive to make. Mixing a few materials together in the right ratios and you get the structure above at left – a lattice of lead atoms surrounded by halides, with cesium in the voids. For this work, we’re making nanocrystals – tiny crystals on the nanoscale surrounded by insulating ligands. At right, you can see what the nanocrystals actually look like at the nanoscale using an electron microscope. They’re roughly 20 nanometers wide – 500,000 of them would fit across your fingernail. We were hopeful that these tiny crystals would be powerful enough to act as the light emission engine of our LEDs.
 
A lot of smart people have done very cool research on these materials and have shown that red and green LEDs can be very efficient.  But as we mentioned above, you need red, green, AND blue LEDs to access all the colors. But blue LEDs are way worse – over 100x worse! This is a huge problem, and we set out to discover why they were so bad and how we could fix them.
 
We immediately found the big issue: these blue materials aren’t very good at emitting light, which turns out to be a bit of a problem in a light emitting diode. We measure how good materials are at emitting light using a measurement called photoluminescence quantum yield or PLQY. 0% means the material emits no light whatsoever, and 100% means it emits light perfectly. We find that red and green perovskites have PLQYs of 70-90%. Our blue materials? 9%. Think about it like pouring a glass of beer. Red and green materials barely spill a drop, while blue materials are getting beer everywhere except the glass, making a giant mess and wasting most of the beer. We wanted to figure out why we were making a mess and how to fix it.
 
Then it got weird. Unrelated to LEDs, several groups had started adding manganese as a dopant to these nanocrystals to do some cool magnetic stuff. The Mn takes energy from the perovskite and emits it with an orange color. But all the groups noted that the amount of light coming out of the perovskite increased rather than decreased! Going back to the beer analogy, this would be like a friend asking you to split the little beer you had left in your glass, but somehow after sharing you had more beer in your glass than when you were drinking alone! Like I said, weird. We wondered if we could take advantage of this bonus brightness.

So we made some nanocrystals. The right two vials above have LOTS of manganese, and you can clearly see its orange emission. But if we look at the three left vials (which have 0, 0.1%, and 0.2% Mn) we see that as we added the dopant, we got much brighter materials with the nice pure blue we wanted. Success!

​Of course, we can formalize things a little bit. We measured the PLQY and saw that it went up to 28%. So when our manganese friend asks to split the beer, they’re really making sure a lot less spills, and there’s plenty available for both glasses. (What’s actually happening? We suspect that the Mn helps repair defects in the crystals, but research in that area is ongoing, by us and others).

To measure our LEDs, we use a metric called external quantum efficiency or EQE. That’s the number of photons that we get from the device, divided by the number of electrons we put into it. Perfect materials would be 100% - one photon for every electron. In reality, in these structures a lot of the light is trapped, and so the best you can do is about 25%. That’s the efficiency of the OLEDs in cell phones, for example, and red and green pervoskites are starting to approach this value. At this blue wavelength, however, perovskites are much lower. In fact, our devices without Mn (black line in the picture) reached a maximum efficiency of 0.50%, which was the highest for perovskite nanocrystals at the time. When we add Mn, we see the value shoots up to 2.12% due to the improved performance from the perovskite nanocrystals. These devices are bright and efficient, showing that blue has the potential to be competitive with red and green!

​Finally, we decided to make a white LED that could eventually be used for room lighting. To do that, we add downconverters in front of our LED. These perovskites absorb the blue light and re-emit it as red or green. When properly mixed, the three colors together look white. The middle pictures are our downconverters under room light and UV light, showing how good they are at downconversion. When the whole system is built, we see a blue, red, and green emissive peak, which is properly mixed to give white, as shown in the picture.
 
We hope this shows that blue perovskites are not the dumpster fire they are often assumed to be. They have the potential to be just as good as red or green, and when we pair all three together, have a lot of promise to make the next generation of emissive TVs and phones.
 
Again the paper can be found here: https://doi.org/10.1016/j.joule.2018.08.005. Thanks for tagging along on this journey! Any questions can be addressed to congreve@stanford.edu.