A scanning electron microscope image shows the rigid pillar-like bristles of the FLUENCE rake, which is used to apply light-harvesting polymers to a solar cell. The distance between the pillars is 1 micrometer, about one-hundredth the diameter of a human hair. Credit: Z. Bao et al, Nature Communications
When commercialized, this advance could help make polymer solar cells an economically attractive alternative to those made with much more expensive silicon-crystal wafers. In experiments, solar cells made with the tiny rake double the efficiency of cells made without it and are 18% better than cells made using a microscopic straightedge blade.
Polymer-based photovoltaic cells are much cheaper than silicon because they’re made of inexpensive materials that can be simply painted or printed in place. They are also flexible and require little energy to manufacture. While small, lab-scale samples can convert >10% of sunlight into electricity, the large-area coated cells have very low efficiency – typically converting < 5%, vs 20-25% for commercial silicon-based cells.
Polymer cells combine 2 types of polymers: A donor, which converts sunlight into electrons, and an acceptor, which stores the electrons until they can be removed from the cell as usable electricity. But when this mixture is deposited on a cell’s conducting surface, the 2 types tend to separate as they dry into an irregular assortment of large clumps, making it more difficult for the cell to produce and harvest electrons. The SLAC/Stanford researchers’ solution is “fluid-enhanced crystal engineering,” or FLUENCE, originally developed to improve conduction of organic semiconductors.
As polymers are painted onto a conducting surface, they are forced through a slightly angled rake containing several rows of stiff microscopic pillars. The rake is scraped along the surface at the relatively slow speed of 25-100 micrometers/s. The large polymer molecules untangle and mix with each other as they bounce off and flow past the pillars, ultimately drying into tiny nanometer-sized crystals of uniform size with enhanced electrical properties.
To achieve the polymer patterns they wanted for the solar cells, the researchers made the pillars in the rake much shorter and more densely packed than those used earlier for organic semiconductors. 1.5 micrometers high and 1.2 micrometers apart. “Ideally, the two types of photovoltaic polymers should be close enough to each other for electrons to move quickly from donor to acceptor, but not so close that the acceptor gives back its electrons before they can be harvested to electricity,” said Yan Zhou, a Stanford researcher on Bao’s team.”Our new FLUENCE rake achieves this happy medium. https://www6.slac.stanford.edu/news/2015-08-12-microscopic-rake-doubles-efficiency-low-cost-solar-cells.aspx
Recent Comments