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MIT project could boost solar cell power

Posted: 28 Nov 2008     Print Version  Bookmark and Share

Keywords:photovoltaic cells  computer modelling  chip-manufacturing  silicon films 

New ways of squeezing out greater efficiency from solar photovoltaic cells are emerging from computer simulations and lab tests conducted by a team of physicists and engineers at Massachusetts Institute of Technology (MIT).

Using computer modelling and a variety of advanced chip-manufacturing techniques, they have applied an antireflection coating to the front, and a novel combination of multi-layered reflective coatings and a tightly spaced array of lines—called a diffraction grating—to the backs of ultrathin silicon films to boost the cells' output by as much as 50 per cent.

The carefully designed layers deposited on the back of the cell cause the light to bounce around longer inside the thin silicon layer, giving it time to deposit its energy and produce an electric current. Without these coatings, light would just be reflected back out into the surrounding air, said Peter Bermel, a postdoctoral researcher in MIT's Research Laboratory of Electronics who has been working on the project.

"It's critical to ensure that any light that enters the layer travels through a long path in the silicon," Bermel said. "The issue is how far does light have to travel [in the silicon] before there's a high probability of being absorbed" and knocking loose electrons to produce an electric current.

The team began by running thousands of computer simulations in which they tried out variations in the spacing of lines in the grid, the thickness of the silicon and the number and thicknesses of reflective layers deposited on the back surface. "We use our simulation tools to optimise overall efficiency and maximise the power coming out," Bermel said.

"The simulated performance was remarkably better than any other structure, promising, for 2-micrometer-thick films, a 50 per cent efficiency increase in conversion of sunlight to electricity," said Lionel Kimerling, the Thomas Lord Professor of Materials Science and Engineering, who directed the project.

The simulations were then validated by actual lab-scale tests. "The final and most important ingredient was the relentless dedication of graduate student Lirong Zeng, in the Department of Materials Science and Engineering, to refining the structure and making it," Kimerling said. "The experiments confirmed the predictions, and the results have drawn considerable industry interest."

The team will report the first reduction to practice of their findings on Dec. 2 at the Materials Research Society's annual meeting in Boston.

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