This Solar Cell is highly efficient and can harvests more of sun’s energy

A silicon solar cell with silicon-germanium filter using a step-cell design (large) and a gallium arsenide phosphide layer on silicon step-cell proof-of-concept solar cell (small).

A silicon solar cell with silicon-germanium filter using a step-cell design (large) and a gallium arsenide phosphide layer on silicon step-cell proof-of-concept solar cell (small).

A team of researchers from MIT and the Masdar Institute of Science and Technology has developed new solar cell that includes two different layers of sunlight-absorbing material to harvest a broader range of the sun’s energy and that costs less than its counterparts.

The researchers call the device a “step cell,” because the two layers are arranged in a stepwise fashion, with the lower layer jutting out beneath the upper layer, in order to expose both layers to incoming sunlight.

The step cell is made by layering a gallium arsenide phosphide-based solar cell, consisting of a semiconductor material that absorbs and efficiently converts higher-energy photons, on a low-cost silicon solar cell.

The silicon layer is exposed, appearing like a bottom step. This intentional step design allows the top gallium arsenide phosphide (GaAsP) layer to absorb the high-energy photons (from blue, green, and yellow light) leaving the bottom silicon layer free to absorb lower-energy photons (from red light) not only transmitted through top layers but also from the entire visible light spectrum.  Such multilayered solar cells are typically expensive, but the researchers also used a novel, low-cost manufacturing process for their step cell.

The team’s step-cell concept can reach theoretical efficiencies above 40 percent and estimated practical efficiencies of 35 percent said MIT in a statement. Masdar Institute’s Ammar Nayfeh, associate professor of electrical engineering and computer science, and MIT’s Eugene Fitzgerald, the Merton C. Flemings-SMA Professor of Materials Science and Engineering is planning to start a company to commercialize the promising solar cell.

According to Fitzgerald the step cells might be ready for the PV market within the next year or two.

The team presented its initial proof-of-concept step cell in June at the 43rd IEEE Photovoltaic Specialists Conference in Portland, Oregon.

“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,” explains Sabina Abdul Hadi, a PhD student at Masdar Institute whose doctoral dissertation provided the foundational research for the step-cell. She added”By etching away the top layer and exposing some of the silicon layer, we were able to increase the efficiency considerably.”

The MIT team developed the GaAsP, which they did by growing the semiconductor alloy on a substrate made of silicon germanium (SiGe).

“Gallium arsenide phosphide cannot be grown directly on silicon, because its crystal lattices differ considerably from silicon’s, so the silicon crystals become degraded. That’s why we grew the gallium arsenide phosphide on the silicon germanium — it provides a more stable base,” explained Nayfeh.

The problem with the silicon germanium under the GaAsP layer is that SiGe absorbs the lower-energy light waves before it reaches the bottom silicon layer, and SiGe does not convert these low-energy light waves into current.

“To get around the optical problem posed by the silicon germanium, we developed the idea of the step cell, which allows us to leverage the different energy absorption bands of gallium arsenide phosphate and silicon,” said Nayfeh.

The step-cell allows for SiGe reuse since the GaAsP cell tiles can be under-cut during the transfer process. Explaining the future low-cost fabrication process, Fitzgerald says: “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. 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 SiGe can be removed and reused repeatedly, which significantly reduces the manufacturing costs.

“Adding that one layer of the gallium arsenide phosphide can really boost efficiency of the solar cell but because of the unique ability to etch away the silicon germanium and reuse it, the cost is kept low because you can amortize that silicon germanium cost over the course of manufacturing many cells,” added Fitzgerald

The step cell is expected to the fill gap between the super high-efficiency and low-efficiency industrial applications in the solar PV market.