Researchers Create Multi-Junction Solar Cells from Off-the-Shelf Components

Multi-junction
solar cells are both the most efficient type of solar cell on the market today
and the most expensive type of solar cell to produce. In a proof-of-concept
paper, researchers from North Carolina State University detail a new approach
for creating multi-junction solar cells using off-the-shelf components,
resulting in lower cost, high-efficiency solar cells for use in multiple
applications.

Multi-junction, or stacked, solar cells are currently the most efficient cells on the market, converting up to 45% of the solar energy they absorb into electricity. The cells are constructed by stacking semiconductors with varying bandgaps on top of one another, thereby allowing the cell to absorb differing wavelengths of solar radiation. However, these cells are much more expensive to produce than less efficient thin solar films.

“We want to create high
efficiency solar cells at a reasonable cost,” says 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% efficiency and the cells cost about 1/10^th 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.”

However, you cannot
merely stack different solar cells on top of each other – the different
materials are structurally incompatible, and so charges cannot pass through them
to be collected. To solve that problem in current multi-junction solar cells
heavily doped metals are used to create a tunnel junction between the various
layers – adding significant expense and complexity to the multi-junction solar
cell’s creation.

Bedair and his team developed a simpler approach, utilizing 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 says. “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.”

The paper, “A New
Approach for Multi-Junction Solar Cells from Off-the-Shelf Individual Cells:
GaAs/Si” was presented at the IEEE Photostatic Specialist Meeting held June 19
in Chicago, IL. NC State graduate student Brandon Hagar and research assistant
professor Peter Colter are co-authors of the paper. The work was supported by
the National Science Foundation under grant 1665211.

A patent application has been submitted for the work. The authors are interested in collaborating with potential academic and industry partners.

-peake-

Note
to editors: An abstract follows.

“A
New Approach for Multi-Junction Solar Cells from Off-the-Shelf Individual
Cells: GaAs/Si”

Authors: Brandon Hagar, Peter Colter, Salah Bedair, North Carolina State University
Presented: June 19, IEEE Photostatic Specialist Meeting, Chicago

Abstract:
We present a low temperature and low pressure approach to multi-junction solar cell fabrication combining the high efficiency multi-junction concept with the low cost of thin film technology in one solar cell structure. The intermetallic bonding approach presented bonds indium metal covering metal contacts of the respective solar cells. This approach avoids lattice mismatch and tunnel junction limitations in connecting solar cells of any material and permits bonding of commercial off the shelf devices. A two or three terminal GaAs/Si multi-junction solar cell bonded using this approach is demonstrated using an off the shelf Si solar cell.

This post was originally published in NC State News.