Quantum dots could help solar cells reach new levels of efficiency

One of the most important components of the solar cell is the material used to convert sunlight into electricity, which is usually a semiconductor. Defects in this material from manufacturing processes can reduce its efficiency, and in alternative energy forms such as solar power, efficiency is crucial. Professor Harley Johnson and his research group study how these defects can affect the properties of the material and decrease its efficiency in a solar cell—as well as how to make beneficial “defects” of their own with quantum dots.

Silicon is the most popular semiconductor material for consumer-grade solar cells. Because it comprises such a large portion of the solar market, even small changes in efficiency can mean drastic differences in industry prices. Using a novel infrared optical method developed by his collaborator Gavin Horn, Johnson is defining exactly how stress and defects alter the properties of a silicon wafer.

“We’re interested in understanding it at a level that is not being considered currently,” Johnson said. “And the way we’re doing it is to use a new inspection technique that lets us see the stress and the defects inside the material using an infrared optical method.  We’re really excited about it because we think it could teach us a lot of basic science, but also some very applied concepts that could be used by industry to make more efficient solar cells.”

Another way Johnson and his group, including graduate students Purnima Ghale, Tung-Wei Lin, Brian McGuigan, and Logan Rowe, and undergraduate Alex Kaczkowski, are trying to improve solar cell efficiency is by engineering new semiconductor materials: gallium arsenide embedded with indium arsenide nanoparticles, called quantum dots. The band gap energy for silicon is well-suited to the solar spectrum, which is what makes it an ideal semiconductor material for a solar cell. Gallium arsenide has a larger band gap that is matched differently to the solar spectrum. Embedding it with nanoparticles of indium arsenide gives it “states” in the bandgap, which allows it to absorb the energy of sunlight more efficiently. Theoretical estimates say that cells created with this strategy could have efficiencies upward of 60%, while current commercial silicon solar cells only have efficiencies around 20%.

There are still issues to be solved. In a solar cell, the absorption of light separates electrons from the material, and then the solar cell must get the electrons moving in a current in order to translate the absorbed energy into electricity. While the quantum dots allow for the cell to absorb more of the solar radiation, the electrons scatter off the quantum dots and make it difficult for the cell to produce electricity. On top of this, gallium arsenide and indium arsenide are more expensive than silicon, and manufacturing a solar cell with nanoparticles has a high cost.

“This kind of cell would be more suited to things like space applications, where the efficiency alone is really important,” Johnson said.

Besides applications in industry and manufacturing where efficiency is more of a concern than cost, Johnson believes that research in quantum dot solar cells is important in the search for higher efficiencies in solar energy.

“I think the hope here is really that we’ll learn a lot about the fundamental properties of how the more complex solar cell architecture works,” Johnson said. “These materials likely will not be the future of solar energy. But we might learn enough from studying them that we can make a breakthrough and transition to some other material that we haven’t yet discovered.”