I’ve been working with conjugated materials for over 15 years. I was trained as a Ph.D. chemist at USC Columbia, where my thesis involved the production of this class of molecules, and I carried that knowledge on to both NASA and the faculty at Ga Tech. Conjugated materials are organic molecules which are (almost without exception) highly colored. Beta-carotene, the bright orange pigment found in carrots, is a conjugated material; so are many of the dyes that we use for textiles and for other applications. One very tricky aspect of conjugated materials is that you never know how the molecules are going to “line up”, once they’re a solid. Do all of the molecules just pack on top of each other however they can, sort of like pouring a bunch of peculiarly-sized objects into a box? Do they try and keep some semblance of order, stacking on top of each other or at right angles to each other? My research over the last ten years has delved deep into this area, because it’s important. The manner in which conjugated molecules “line up” influences their behavior as an actual physical device.
Take phthalocyanines as an example. These molecules are a class of blue dye used in blue jeans, blue paint, and many other consumer products. These materials (because they conduct electricity and are highly colored) are great candidates for the active layer in solar panels. Their deep color captures and absorbs much of the incoming sunlight, which is then converted to electricity to be siphoned off and used to power appliances. These dyes have a lot going for them: they’re inexpensive, they can be printed into a very thin layer that can bend back and forth (unlike the clunky silicon solar panels), and they do a great job at capturing light. The problem with these dyes – and it’s a general problem with conjugated materials, a problem I’ve studied all of my career – is that the efficiency of the solar panel is deeply dependent on how the different dye molecules organize. If they don’t all line up correctly, the electric current will be dissipated and the solar panel won’t be very effective.
I’ve tried all manner of tricks and manipulations in the past in order to force conjugated molecules to adopt the arrangement that I desired. I know just how hard it can be! That’s why I was excited to read about work recently published in the very prestigious journal Nature Chemistry that describes a new strategy for organizing these blue organic dyes into a well-defined structure. The researchers used acids and bisphenol compounds called catechols to make a nice, tidy sheet of dye molecules. When stacked on top of each other, these sheets form a three-dimensional lattice through which the electric current can move. I worked for several years on a similar strategy while at Ga Tech, and I recognize the brilliance in this new journal articles design.
Instead of trying to be too “strict” with the molecules – forcing them into positions where they don’t want to be, which implies high reaction temperatures and unstable structures – the chemists designed the molecule to be a type of free-floating chain of overlapping islands. While non-productive electrical routes were often made as the sheets overlapped in the wrong fashion in the third dimension, the clever molecular design of the dyes meant that the desired orientation was the lowest energy orientation. In other words, the precise overlapping that gave the best results for charge transfer was also the most stable. So, all the chemists had to do was mix these sheets together and then wait. After a bit of back-and-forth, the molecular sheets settled into the position which they found to be the most comfortable – which was precisely the position the chemists needed to make the best quality solar cell.
This type of approach is opposite to ones I’ve tried myself in the past, and I easily see it’s strengths. It takes more molecular design work up front (to ensure that the most stable conformation is the one that will give the best solar panel performance), but after that, the molecules are free to scuttle back and forth. Eventually, they nestle into their preferred orientation, and the result is a superior performance from the solar panel.
This discovery is doubly exciting, as not only do the sheets form perfect charge transfer channels for the solar panel, but the dye molecules themselves absorb almost all of the incoming light. This is pretty rare for an organic compound, as most conjugated materials absorb only specific regions of the visible spectrum. The phthalocyanines take in most of the light, and thanks to the clever design of the molecular sheet, they are able to produce a superior framework for a solar cell. As someone who’s worked all his career with conjugated materials, I really admire this article. It makes numerous strides towards low cost, high efficiency solar panels – a goal that I think is very important, and which will become even more important as time goes on.
The source of this article can be found at:
Spitler, E.L.; Dichtel, W.R. “Lewis acid-catalysed formation of two-dimensional phthalocyanine covalent organic frameworks”. Nature Chemistry, 2010.