Researchers at North Carolina State University have developed a new system for strengthening the connections between stacked solar cells which could improve the overall efficiency of concentrated photovoltaic technology and reduce the cost of solar energy production. The hardened connections could theoretically enable these cells to operate at concentrations of up to 70,000 suns while minimizing wasted energy.
Stacked solar cells are made up from several solar cells that are stacked one on top of the other. This arrangement allows up to 45 percent of the absorbed solar energy to be converted into electricity, a significant improvement over single-junction solar cells which have a theoretical maximum conversion rate of 33.7 percent. The trick is to ensure that the junctions between these stacked cells do not waste any energy in the form of heat which effectively "siphons off the voltage the cells produce."
The team at NCSU has discovered that by inserting a very thin film layer of gallium arsenide into the connecting junction of stacked cells they can eliminate energy loss without affecting solar absorption capabilities. Usually, at intensities above 700 suns, the solar cell’s connecting junctions start to break down causing considerable loss of energy – the more junctions, the greater the voltage loss.
"Now we have created a connecting junction that loses almost no voltage, even when the stacked solar cell is exposed to 70,000 suns of solar energy," says Dr. Salah Bedair, a professor of electrical engineering at NCSU and senior author of the paper on this research. "And that is more than sufficient for practical purposes, since concentrating lenses are unlikely to create more than 4,000 or 5,000 suns worth of energy."
This is not a technology you are likely to see in your home anytime soon. It is more geared for large scale solar power. Stacked cells are generally used in conjunction with optical concentration devices, often Fresnel lenses, mounted on a dual-axis solar trackers that keep the cell facing the Sun's rays during daylight. This brings to light potential challenges with elements of scalability and synthesis of the thin film layers.
"This [system] should reduce overall costs for the energy industry because, rather than creating large, expensive solar cells, you can use much smaller cells that produce just as much electricity by absorbing intensified solar energy from concentrating lenses, explains Dr. Bedair. "And concentrating lenses are relatively inexpensive."
It is conceivable to see how this more advantageous way of strengthening stacked solar cells could produce cheaper solar power if gallium arsenide thin film layers can be manufactured cost effectively and on a large scale.