Organic PVs Promise Better Efficiency
Canadian researchers have founds ways to improve organic photovoltaics with a "club sandwich" structure. Organic solar cells are easy to put on plastic sheets for roll-to-roll processing, reducing costs. The researchers found out how to "flatten the lettuce" in the multilayered structure, making charges more uniform.
Alexander E. Braun, Senior Editor -- PV Society, 4/9/2009
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Researchers at Canada’s National Institute for Nanotechnology (NINT, Edmonton, Alberta) and the University of Alberta have engineered a novel approach to improve plastic solar cell (hybrid organic photovoltaics) performance. The research could improve the efficiency of plastic solar panels, which could prove to be a low-cost alternative to silicon-based solar cells.
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| In this bulk heterojunction photovoltaic device, the donor and acceptor materials are phase-segregated, resulting in a large interfacial surface area that improves charge transfer. |
Plastic solar cells are made up of layers of different materials, each with a specific function, in what is called a sandwich structure. Jillian Buriak, a professor of chemistry at Alberta, NINT principal investigator and member of the research team, uses a homely analogy to describe their approach: “Consider it as a multilayered clubhouse sandwich,” she said. “One layer absorbs light, another helps generate electricity, and others help separate the charges and get them to their respective electrodes. Generally, these layers don’t stick together well, so the electricity is never produced or doesn’t leave the device. We’re working on the ‘mayonnaise,’ ‘mustard,’ ‘butter’ and other ‘special sauces’ that will bring the sandwich together and make the layers work together, producing a better solar cell.”
Buriak sees a great future for organic photovoltaics. “If you’re making organic solar cells, you’re no longer working with beautiful silicon, a single material that can do many things, such as p-n junction photocells. Plastics don’t do it all, so you must layer different plastic materials with various organic polymers on top of each other — like mustard, mayonnaise and lettuce — one helping to improve the interface with the next layer.”
The accomplishment lies in taking known materials recognized as working in the making of solar cells and experimenting to improve them. “Solar cells are all about electric charges, and when you layer one on top of another there are hot spots,” Buriak said. “It’s like when you look at your sandwich and notice that the lettuce does not make an even contact with the bread and only touches on a couple of spots. Something similar happens with the plastic, where all these charges file through these hot spots, while the rest of the structure does nothing.”
The researchers found out how to “flatten the lettuce,” making charges more uniform, enabling the use of whole structures instead of a few hot spots that lead to bottlenecks. “We did surface chemistry to modify the property of the transparent conductive electrode with a very thin (&1 nm) layer of an organic molecule that itself has some electrical conductivity associated with it. We then grew the next plastic layer, ePEDOT, a conducting polymer, but we grew it electrochemically, instead of just slathering it on. This is like taking the fat molecules of butter and then growing the butter on the surface instead of just smearing it on. This led to a much more uniform contact between the transparent conducting electrode and the overlying layer.”
The group investigated the generation and characterization of a robust thienylsilane molecular layer on indium tin oxide (ITO) substrates. The researchers discovered that the molecular layer reduced the oxidation potential required for the electrochemical polymerization of 3,4-ethylenedioxythiophene. The resulting electrochemically prepared poly (3,4-ethylenedioxythiophene):poly(p-styrenesulfonate) (ePEDOT:PSS) films provided high uniformity in their coverage, exhibiting lower roughness and higher conductivity than analogous films fabricated with bare ITO. A relative improvement in the efficiency of 2,5-diyl-poly(3-hexylthiophene) (P3HT)/[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) bulk heterojunction solar cells was obtained when devices were formed on thienylsilane-modified ITO electrodes, rather than unmodified ITO control electrodes.
The efficiency is not exactly mind-boggling. The world record with silicon cells is 24-28%. Organic solar cells might never reach that level; they are at &10%, and will probably remain so. However, in the solar electricity world, widespread use depends on the price per kilowatt-hour and lifetime output. Because these &10% solar cells use considerably fewer materials, making them cheap to manufacture, the price per kilowatt-hour begins to get competitive. Organic solar cells can be very manufacturable, very easy to put on plastic sheets, and do roll-to-roll processing. Silicon requires high-purity material, which has a very high energetic cost, making it expensive.
“With polymers you can do anything you want,” Buriak said. “You’re not stuck with a single element that does what it wants. Polymers can be engineered to your needs, because with organic chemistry you can make just anything that you can think of from scratch and design into it what you want. Tailoring these interfaces with thin, subnanometer layers has profound effects on device performance. We hope that by taking a closer look at those interfaces, and spreading our ‘mayonnaise’ a little better, we’ll attain dramatic effects.”
The researchers have achieved a proof of principle, an important approach even though many improvements remain to be made. After two years of research the Canadian scientists obtained improvements of ~30% in the working model’s efficiency. The team is cross-disciplinary, with members from engineering physics and chemistry backgrounds all working toward the goal of cheap, manufacturable PV cells. The researchers predict that this multidisciplinary approach will enable plastic PV panels to be mass-produced in five to seven years, making solar energy available to everyone.
“The next generation of solar technology belongs to plastic,” Buriak said, adding that plastic solar cell material will be made cheaply and quickly and in massive quantities using inkjet-like printers. As for the future, Buriak said, “We’ve a lot of layers to fiddle with; we have to add other mustard and mayo layers. We’re tailoring each interface one at a time.”
