New Approach to Create Lower Cost, High-Efficiency Multi-Junction Solar Cells

Multi-junction solar cells are the most efficient kind of solar cells available currently in the market. Moreover, it is the costliest type of solar cell to make.

(Image credit: North Carolina State University)

As part of a proof-of-concept study, scientists from North Carolina State University describe a new method for developing multi-junction solar cells using off-the-shelf components, leading to highly efficient, lower cost solar cells for use in various applications.

Multi-junction or stacked solar cells are the most efficient cells available in the market today, turning up to 45% of the solar energy they harvest into electricity. The cells are made by placing semiconductors with differing bandgaps on top of one another, thus enabling the cell to gather differing wavelengths of solar radiation. However, these cells are much costlier to produce when compared to less efficient thin solar films.

We want to create high-efficiency solar cells at a reasonable cost. Silicon-based thin solar cells are very popular because the material has around 20% 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.

Salah Bedair, Distinguished Professor of Electrical and Computer Engineering, North Carolina State University

Bedair is also the lead author of the study.

However, one cannot just pile up different solar cells on top of each other—since the different materials are structurally incompatible, charges cannot travel through them to be collected. In order to overcome that issue in present multi-junction solar cells, a tunnel junction is formed between the various layers using heavily doped metals—adding considerable expense and intricacy to the multi-junction solar cell’s construction.

Bedair and his group created a simpler method, employing intermetallic bonding to join solar cells composed of different materials. In a proof-of-concept, the group placed 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 states. “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,” he added.

The paper titled “A New Approach for Multi-Junction Solar Cells from Off-the-Shelf Individual Cells: GaAs/Si” was presented at the IEEE Photostatic Specialist Meeting conducted on June 19th, 2019 in Chicago, IL. The co-authors of the paper are NC State graduate student Brandon Hagar and research assistant professor Peter Colter. The research was supported by the National Science Foundation under grant 1665211.

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