Each day the sun bathes the earth with enough energy to power the entire global community many times over, but we currently capture only a tiny fraction of it. The relatively high cost of the electricity produced by traditional solar panels may be partly to blame for solar energy's bit part. "I think your best bet for expanding the use of solar energy is to make it at least as cheap as fossil fuels," says Professor Jason Baxter, a chemical engineering researcher at Drexel University in Philadelphia who studies solar cells. Baxter is the author of a review paper, recently published in the Journal of Vacuum Science and Technology A, that examines possible ways to improve the price/performance ratio of one promising form of solar energy technology: dye sensitized solar cells.
Dye sensitized solar cells (DSSCs) are a type of nanostructured solar cell made from oxide nanoparticles and light-absorbing dye molecules. In traditional silicon photovoltaic cells, the silicon acts as both the light absorber and electron conductor, but in dye sensitized solar cells the two functions are separated into different components. Because the dye molecules donate electrons much like chlorophyll in green plants, DSSC technology is often described as artificial photosynthesis. DSSCs can be manufactured using low-cost materials and production methods, and can also be designed to be flexible and translucent. In 2010, the father of dye-sensitized solar cells, Michael Grätzel, was awarded the Millennium Technology Prize in recognition of his work.
The efficiency of Grätzel's first DSSCs, which he developed with his colleague Brian O'Regan and debuted in 1991, was 7 percent. Since then the efficiency of the cells has increased to 12 percent, but room for further improvement remains. The development of new combinations of dyes, anodes, and redox couples (chemical species that donate and accept electrons within the solar cell), may all lead to increased efficiency. While small efficiency gains might result from modifying one component at a time, large increases will likely require changing multiple cell components simultaneously, Jason Baxter writes in his review of the technology.
Improving the efficiency can help bring down the price per unit of energy produced, but it is only one measure of solar cell performance. Baxter also reviews efforts to lengthen the lifetime of the cells and lower the production costs. DSSCs will last longer, for example, if the modules containing the internal components of the cell are well encapsulated. And manufacturing costs for the cells can be lowered with high-throughput processes and the use of inexpensive, abundant materials.
Because DSSCs do not require expensive vacuum systems or high temperatures to manufacture, many research groups have the tools available to contribute to the study of the technology. The cells have already been added to commercial products like bags and backpacks, where they can recharge portable consumer electronics. With further improvements in efficiency and lifetime, and decreased manufacturing costs, DSSCs have the potential to be integrated into buildings or used in large outdoor energy-harvesting installations. By examining the multiple components of solar cell performance and staging a multi-pronged attack on the current cost premium for photovoltaics, researchers may help advance the date when the solar energy that reaches the Earth each day meets a much larger percentage of our energy needs.