Solar Cell Uses New Materials to Harvest Energy from More of the Spectrum

Now that solar energy is maturing as a technology and prices are coming down, researchers are trying to find ways to make solar cells work even more efficiently at even lower costs. One of those ways is to create technology that can harvest more of the sun's energy than current cells, which don't use the full solar spectrum.

One of the latest research efforts is from a cross-disciplinary team from MIT and the Masdar Institute of Technology, where researchers have taken a layered-material approach to developing a solar cell that allows it to harvest energy from different parts of the solar spectrum.

The team -- which included principal investigators Ammar Nayfeh, associate professor of electrical engineering and computer science at the Masdar Institute; and Eugene Fitzgerald, a professor of materials science and engineering at MIT -- created the technology by layering a gallium arsenide phosphide-based solar cell on a low-cost silicon solar cell. Gallium arsenide phosphide is a semiconductor material that absorbs and efficiently converts higher-energy photons.

While the Masdar team was mainly responsible for developing the proof-of-concept solar cell, researchers at MIT contributed the gallium arsenide phosphide to the project. The two materials could not be combined on one cell because the crystal lattices differ considerably from silicon's and will degrade the silicon crystals if grown directly on the material, Nayfeh said. That's why the gallium arsenide phosphide was developed on a substrate made of silicon germanium.

However, this posed an additional problem that itself inspired the step-cell concept, which paves the way for a low-cost fabrication process that can also drive the cost down of these new, more efficient cells, Nayfeh said. Silicon geranium absorbs lower-energy light waves before they reach the bottom silicon layer and does not convert them into current. To get around this problem, "we developed the idea of the step cell, which allows us to leverage the different energy-absorption bands of gallium arsenide phosphate and silicon," he said.

The cell creates a step between the top layer and the bottom layer, exposing the silicon layer, which allows the top gallium-arsenide-phosphide layer to absorb the high-energy photons from blue, green, and yellow light. This leaves the bottom silicon layer free to absorb not only lower-energy photons -- like those from red light -- transmitted through top layers -- but also the entire visible light spectrum, said Sabina Abdul Hadi, a PhD student at Masdar Institute whose doctoral dissertation provided the foundation for the work.

"We realized that when the top gallium arsenide phosphide layer completely covered the bottom silicon layer, the lower-energy photons were absorbed by the silicon germanium -- the substrate on which the gallium arsenide phosphide is grown -- and thus the solar cell had a much lower efficiency," she said. "By etching away the top layer and exposing some of the silicon layer, we were able to increase the efficiency considerably."

In fabricating the cell, researchers removed and reused the silicon-geranium template to create a solar cell in which the gallium arsenide phosphide cell tiles are directly on top of a silicon cell. This allows for reuse of the silicon-geranium cells since the gallium arsenide phosphide cells can be undercut during the transfer process.

READ MORE ARTICLES ON SOLAR ENERGY:

"We grew the gallium arsenide phosphide on top of the silicon germanium, patterned it in the optimized geometric configuration, and bonded it to a silicon cell," MIT's Fitzgerald said. "Then we etched through the patterned channels and lifted off the silicon germanium alloys on silicon. What remains then, is a high-efficiency tandem solar cell and a silicon germanium template, ready to be reused."

Because the tandem cell is bonded together rather than created as a monolithic solar cell (where all layers are grown onto a single substrate), the silicon-geranium layer can be removed and reused repeatedly, which significantly reduces the manufacturing costs, he added.

In tests researchers found that the cell has the potential to reach 35% efficiency, a significant increase in the current efficiency of solar cells. Combine this with its low fabrication cost, and Fitzgerald said the step cell could fit well to fill an existing gap in the solar photovoltaic market between super high-efficiency and low-efficiency industrial applications.

Elizabeth Montalbano is a freelance writer who has written about technology and culture for more than 15 years. She has lived and worked as a professional journalist in Phoenix, San Francisco, and New York City. In her free time she enjoys surfing, traveling, music, yoga, and cooking. She currently resides in a village on the southwest coast of Portugal.