Energy is one of the hottest topics around at the moment. No matter what your stance on the topic of climate change, governments and organizations worldwide are sitting up and taking notice of their carbon footprints and emissions levels, and consumer demand for power sources is inexorably on the rise. Funding of research into sustainable sources and development of devices for harnessing them is also growing at an ever-accelerating rate, and materials science is at the forefront of the search for solutions to these challenges. Solar cells, fuel cells, and longer-lasting batteries are just a few of the major trends in materials research right now.
Solar power is in many ways the standard bearer of research for renewable energy (its cover girl, if you will), and the approaches to device construction are myriad: inorganic or organic, dye-sensitized, doped, solution-processed, bulk-heterojunction, zinc oxide, silicon, polymer, thin-film… I could go on, but I’ll spare you.
Polymer bulk-heterojunction (BHJ) solar cells are attractive because they fulfill a number of criteria that make them potentially useful for a massive number of applications. They can be manufactured through solution deposition methods, making them cheap to produce, they are thin, flexible, and can be made transparent, meaning that you could potentially turn your house and office windows into miniature solar farms. Or not so miniature – if you were to plaster the Burj Khalifa in solar cells, you would create a solar farm half a million square meters in area, which is about the size of Vatican City. Or, you could add portable free power sources to laptop cases, luggage, camping equipment, and even clothing.
So far, so carbon-footprint conscious, but at the moment there is one significant hurdle to overcome before use of such photovoltaic plastics becomes commonplace, and that’s that the efficiency of polymer solar cells is not yet competitive with devices formed from other materials (ca. 3-5% compared with 20% plus for crystalline silicon). This barrier is one which Gui Bazan, Thuc-Quyen Nguyen and their team at UC Santa Barbara have been working on. BHJ cells are typically a blend of an electron donor (for example a conducting polymer) and an electron acceptor (such as fullerene), which form interconnecting networks in the films. The morphology of these networks is important, as excitons must be able to reach boundaries between the phases quickly in order to generate charge, but those charges must also be able to travel freely through the network that carries them to reach the electrodes. By introducing an additive to the polymer blend used to form the cells, the team demonstrated that they could produce cells with better mixing of the phases, which led to smoother films, better performance, and improved efficiency.
This idea is beautifully expressed on the inaugural cover image for Advanced Energy Materials (a new section in Advanced Materials for all things energy). Against a backdrop of the Sun and Earth, we have mass production of sheets of plastic solar cells from solutions (the classic chemistry beakers are a nice old-school touch). On the left, the sparkling red drops of additive lead to a smoother film, as shown by the AFM underlayer of the image. This is a great mix of science and art, and a fitting way to launch a special section dedicated to such an important topic.