A multi-junction solar cell is both the most efficient type of solar cell on the market but also the most expensive. These stacked cells convert up to 45% of the solar energy they absorb into electricity by stacking semiconductors with varying bandgaps on top of one another, allowing the cell to absorb differing wavelengths of solar radiation.
"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% 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."
The challenge is that the different materials in the layers are structurally incompatible, and so charges cannot pass through them to be collected. So heavily doped metals are used to create a tunnel junction between the various layers, adding significant expense and complexity.
Bedair and his team developed a simpler approach, using 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 to produce a two or three terminal GaAs/Si multi-junction solar cell.
"In a multi-junction solar cell the tunnel junction enables electric connectivity by acting as a metal-to-metal connection," said Bedair. "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."