Thinking small for solar

Corinna Wu, Hideyuki Murata
2012 MRS bulletin  
O n rooftops all over the world, silicon solar panels are harvesting sunlight and converting it into gigawatts of electricity. But now, many groups are hard at work developing solar technology that eschews silicon and other inorganic materials altogether. Organic photovoltaics (OPV), which uses organic materials such as polymers as the active component, follows on the heels of fi rst-generation crystalline silicon and second-generation thin-fi lm PV. OPV promises to be lightweight, fl exible,
more » ... d cheaper to manufacture. Its unique properties also open a range of applications not possible with inorganic solar materials. Recently, new approaches to making OPV that use small organic molecules as the light-harvesting ingredient have posted some remarkable successes. Until now, it was unclear whether small-molecule OPV had the potential shown by polymer OPV technologies. But power conversion effi ciencies reaching 10%-near that of amorphous silicon-have encouraged companies and academic groups to plunge forward with their small-molecule OPV research. Organic PV is attractive because the overall cost of the electrical power generated-expressed as lifetime multiplied by effi ciency/manufacturing cost-can be extrapolated to become much lower than that of silicon or thin-fi lm PV. The critical price point is much less than $1/W, which is where "PV breaks even with some of the other conventional technologies that are used for power generation-namely, the burning of some kind of fossil fuel," said Bernard Kippelen, director of the Center for Organic Photonics and Electronics at the Georgia Institute of Technology. The production cost of today's silicon PV technology is around $1-2/W, Kippelen said, but the price of the raw material-pure, electronic-grade silicon-makes up a substantial portion of the cost of the device. "You can't go much beyond the cost of the materials that are needed to fabricate the cell unless someone comes up with a new process where you have a lowcost way to produce high-quality silicon," he said. "That would really require a breakthrough." The inorganic materials used in thin-fi lm PV, such as cadmium telluride, have similar limitations for breakthrough raw material cost reductions. On the other hand, organic molecules are typically cheaper and more abundant. The layers of materials in OPV can also be much thinner, so less material is needed. If a 100-nm-thick layer can absorb sunlight effectively, the amount of material that coats 10 m 2 would not be more than 1-2 grams, Kippelen said. OPV can also be manufactured at low temperatures, which reduces the energy needed and allows for the use of plastic substrates. Common methods are spin-coating, die-coating, and organic vapor phase deposition onto a substrate. The energy payback for OPV can be measured in months, whereas for silicon, it typically takes two years for a silicon solar panel to generate enough energy to recover its manufacturing cost. The ability to use plastic substrates also means that OPV can be lightweight and fl exible-as can some second-generation thin-fi lm inorganic cells. Structures might not need as much reinforcement to support the weight of such panels-all of which contribute to the overall cost. For the past two decades, OPV researchers have been hard at work boosting effi ciencies, an important component of the cost equation (see fi gure). The most advanced cells with organic molecules are the dye-sensitized ones, introduced by Michael Grätzel and Brian O'Regan at the École Polytechnique Fédérale de Lausanne, Switzerland, in 1991. Grätzel cells, as they are called, contain a thin layer of an organic dye to harvest sunlight and work, in principle, like an electrochemical cell to produce current. The best cell effi ciency is greater than 12%. However, the challenge for scaling up the technology and getting long operational lifetimes has been to make hermetic packages that seal in the electrolyte, Kippelen said. For solid-state OPV, polymer-based solar materials currently dominate. Konarka in Lowell, Mass., has been ramping up its production, making polymer-based PV in sheets up to fi ve feet wide. The company is also introducing PV in different colors to provide design options for architects, said Stuart Spitzer, Konarka's vice president of materials and engineering. Polymers are easy to synthesize by bulk methods, but because the molecular weight and purity can be hard to control, the photovoltaic properties can also vary from one batch to the next. That is why some researchers have been taking a different tack by synthesizing small organic molecules for the sunlightabsorbing layer. "Once you have a particular batch of a small-
doi:10.1557/mrs.2012.62 fatcat:2esqiuep7reo3caoivbeh4vhnm