New approach stacks up for multi-junction solar cells

A new approach to creating multi-junction solar cells with off-the-shelf components could lead to photovoltaic devices that are more efficient and cheaper to make.

multi-junction solar cells
(Credit: NREL/Public Domain)

This is the claim of researchers from North Carolina State University (NC State) whose proof-of-concept paper – A New Approach for Multi-Junction Solar Cells from Off-the-Shelf Individual Cells: GaAs/Si – was presented on June 19 at the IEEE Photostatic Specialist Meeting in Chicago.

Multi-junction solar cells – which convert up to 45 per cent of the solar energy they absorb into electricity – are made by stacking semiconductors with varying bandgaps on top of one another. This allows cells to absorb differing wavelengths of solar radiation but makes them more expensive to produce than less efficient thin solar films.

“We want to create high-efficiency solar cells at a reasonable cost,” said Salah Bedair, Distinguished Professor of Electrical and Computer Engineering at NC State and lead author of the research. “Silicon-based thin solar cells are very popular because the material has around 20 per cent efficiency and the cells cost about 1/10th what a multi-junction solar cell costs. And other low cost, lower efficiency materials are gaining popularity as well. If we could create stacked solar cells using this existing technology we would be well on our way to reaching our goal.”

multi-junction solar cells
(Image: NC State)

In current multi-junction solar cells, heavily doped metals are used to create a tunnel junction between the various layers, which adds expense and complexity to the multi-junction solar cell’s creation.

Bedair and his team are said to have developed a simpler approach that utilises intermetallic bonding to bond solar cells made of different materials. In a proof-of-concept, the team stacked an off-the-shelf gallium arsenide solar cell on top of a silicon solar cell.

“In multi-junction solar cells the tunnel junction enables electric connectivity by acting as a metal-to-metal connection,” Bedair said in a statement. “In our system, indium serves as a shortcut to that. The existing metal contacts of the individual cells are covered with indium films. The indium films bond to themselves easily at room temperature under low pressure. The result is a solar cell made of two different materials that is mechanically stacked and electrically connected.

“With this technique we are able to take advantage of inexpensive, off-the-shelf solutions without having to develop all new technology. Manufacturers could simply tweak their existing products slightly to increase their efficiency in multi-junction solar cells, rather than having to create new products.”

A patent application has been submitted for the work.