Researchers super stack solar panels, increase efficiency

September 18, 2013

Researchers at N.C. State have developed a new way for improving overall efficiency of solar panels that will reduce the cost of solar energy production.

The stacked junction solar cell is more than just one cell. It needs to be connected together with other cells 
The stacked junction solar cell is more than just one cell. It needs to be connected together with other cells

The new technique improves the connections between layers of stacked solar cells, which allow them to operate at solar concentrations of 70,000 suns worth of energy, as opposed to the previous  commercially available concentrations of 500 to 1000 suns.

Peter Colter, a research assistant professor from the electrical and computer engineering department at N.C. State, explained how the process worked.

"The big improvement is that it removed tunnel junctions as the bottleneck," Colter said "A tunnel junction is used as the interconnection between multi-junction solar cells."

Tunnel junctions are what connect the layers of solar cells. Solar cells are made of multiple layers for more energy conversion efficiency.

Before the recent breakthrough, the tunnel junctions were a big limiting factor to the amount of energy a solar panel could absorb efficiently.

While multi-junction solar energy cells existed for several years, they were expensive and mainly used in space for satellites and Mars rovers.

"These multi-junction cells are commonly used for space satellites," Colter said. "All the television satellites have multi-junction cells on them these days. The main cost of a solar panel in space is launching it rather than the cost of the cells. It saves money by making them smaller."

Previous multi-junction cells such as the ones used in space were very limited by the amount of energy they could absorb. 

Commercially available solar cell modules can only absorb 31.8 percent of solar energy. The highest recorded multi-junction solar cell could absorb 44.4 percent. The new discovery hopes to significantly improve the efficiency of solar cells beyond that.

"The connecting junction was a problem because you can't use it with a very high solar concentration," said Salah Bedair, professor of electrical engineering at N.C. State and one of the main researchers on the project. "We solved the issue by doing some tricks." 

According to Bedair, it was recently discovered that by inserting a thin film of gallium arsenide into the connecting junctions of stacked cells, almost all the voltage loss can be eliminated without blocking solar energy.

"The stacked junction solar cell is more than just one cell. It needs to be connected together with other cells," Bedair said. "The connecting junction is very critical in multi-junction stacked cells. At the current energy absorption efficiency of 44 percent, we have difficulty using stacked solar cells over 800 suns. After that, higher concentration doesn't work very well. The achievement we made is that we allowed the connection junction to be used for 70K suns."

The recent development hopes to curb the high cost of producing the multi-junction cells.

"Rather than having a large area of solar cells that are very expensive, we can use smaller solar cell areas," Bedair said.

Currently, the solar energy cells are grown in a laboratory.

"The most economical way to fabricate these cells is to grow them monolithically from two different materials," Colter said "The growing process is similar to growing crystals in a lab but with different materials and equipment."

Colter and Bedair are working on a project that intends to increase solar concentrations from 1000 suns to 2000 suns.

"Our tunnel junction had at best shown a conductivity of 70,000 suns--the current carrying capacity," Colter said. "The tunnel junction will no longer be a limiting factor for the 2,000 suns project. We cut the cost by doubling the concentration it can operate at. Thus, tunnel junctions are no longer a bottleneck."