You’ve seen them on wristwatches, pocket calculators, traffic signals and maybe even on top of campus buildings — futuristic-looking, sleek panels of metal facing the sun. Solar cells are becoming integral to our lives as the technology used to harness arguably the cleanest energy source available —the sun.
Solar cells are made off special materials called semiconductors (such as Silicon). When sunlight shines on these materials, it knocks of electrons within them. By controlling the direction of flow of these electrons, an electric current can be set up in the semiconductor. In a nutshell, this is how solar cells convert the energy of the sun to commercially usable electrical energy.
For decades, solar energy has been viewed as a promising candidate in global efforts to reduce pollution and take the pressure off non-renewable sources of energy. According to the U.S. Energy Information Administration, solar cell users across the planet save 75 million barrels of oil and 35 million tons of gasoline each year.
Although the global solar energy generation is growing, its share in the energy market remains quite small, less than one percent. This is mainly due to poor efficiency and robustness of devices that run on solar energy. To improve the commercial viability of devices powered by solar energy, it is essential to improve these parameters of solar cells.
Provision of continuous power output and the ability to store energy for future use are important challenges faced by scientists in solar energy production.
“Conventional solar cells do not store energy,” said Hongrui Jiang, a professor in the Electrical Engineering Department at the University of Wisconsin-Madison, whose research group is studying solar cells, solar energy harvesting and storage, among other areas.
“Of course, there are other devices, such as capacitors, that can store electricity. But this means we need separate devices for energy conversion and storage,” Jiang said.
Combining such devices is known to affect the efficiency of solar devices.
Jiang’s group recently developed a much-improved solar cell that combines conversion and storage of energy in a single device. The cell exhibited impressive performance stability over multiple charging-discharging cycles. The superior performance of this type of solar cell is attributed to the presence of nanocomposites —(materials approximately 100 nanometers in size in at least one dimension of length)—in the cell electrodes.
These materials have favorable dielectric properties that aid in energy conversion and possess very high surface area, which improves the energy storage capacity of the cell.
“The current storage capacity of our device is less than half that of supercapacitors. So, we are testing new materials to improve this aspect of the device,” Jiang said.
Another area that the group is working on is the optimization of the thickness and morphology of the nanocomposites, which could significantly affect electrical properties of the solar cell.
Less than a block away from Jiang’s laboratory, assistant professor Michael Arnold and his research group in the Materials Science and Engineering Department have been developing Carbon-based materials that have exceptional properties for next-generation electronics devices. Arnold’s group recently reported the fabrication of novel solar cells that are based on materials known as Carbon nanotubes.
Nanotubes are cylindrical tubes of Carbon and a few nanometers wide. To put things in perspective, a strand of human hair is roughly 80,000-100,000 nanometers wide. So, these nanotubes are really small!
Carbon nanotubes are easy to prepare, they possess interesting electronic properties and are mechanically robust. So, it is advantageous to use them as components in electronic devices. Although nanotubes have been used previously in electrical applications, their roles have mainly been as passive electrode materials or to aid in electronic conduction.
In the solar cells developed by Arnold’s group, the nanotubes drive more than 60 percent of the power conversion. By optimizing the thickness and alignment of the nanotubes, the researchers obtained a power efficiency of 1.02 percent, which is the highest ever reported in a cell. The fact that nanotubes were of uniform size and shape was critical to the results.
Although both groups are still optimizing their respective devices, the future looks promising for solar cells. A coordinated research effort involving materials chemists, electrical engineers and device makers is pushing the envelope to develop highly efficient, robust devices powered by solar energy.
