[Editors] MIT finds ways to boost solar cell efficiency

[Elizabeth Thomson]

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Boosting the power of solar cells
--New MIT research could lead to higher output, lower cost
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For Immediate Release
MONDAY, NOV. 24, 2008

Contact: Elizabeth A. Thomson, MIT News Office
E: thomson at mit.edu, T: 617-258-5402

Photo Available

CAMBRIDGE, Mass. — 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 MIT.

Using computer modeling 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 percent.

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 physics department 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  
optimize overall efficiency and maximize the power coming out,” Bermel  
said.

“The simulated performance was remarkably better than any other  
structure, promising, for 2-micrometer-thick films, a 50 percent  
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. A paper on their findings has been accepted for publication in  
Applied Physics Letters.

The work is just a first step toward actually producing a commercially  
viable, improved solar cell. That will require additional fine-tuning  
through continuing simulations and lab tests, and then more work on  
the manufacturing processes and materials. “If the solar business  
stays strong,” Kimerling said, “implementation within the next three  
years is possible.”

The MIT Deshpande Center selected the project for an “i-team” study to  
evaluate its business potential. The team analyzed the potential  
impact of this efficient thin solar cell technology and found  
significant benefits in both manufacturing and electrical power  
delivery, for applications ranging from remote off-grid to dedicated  
clean power.

And the potential for savings is great, because the high-quality  
silicon crystal substrates used in conventional solar cells represent  
about half the cost, and the thin films in this version use only about  
1 percent as much silicon, Bermel said.

This project, along with other research work going on now in solar  
cells, has the potential to get costs down “so that it becomes  
competitive with grid electricity,” Bermel said. While no single  
project is likely to achieve that goal, he said, this work is “the  
kind of science that needs to be explored in order to achieve that.”

In addition to Kimerling, Bermel and Zeng, the work was done by John  
Joannopoulos, the Francis Wright Davis Professor of Physics, and by  
research engineer Bernard A. Alamariu, research specialist Kurt A.  
Broderick, both of the Microsystems Technology Laboratories;  
postdoctoral associate Jifeng Liu; Ching-yin Hong and research  
associate Xiaoman Duan, both of the Materials Processing Center.  
Funding was provided by the Thomas Lord Chair in Materials Science and  
Engineering, the MIT-MIST Initiative, the Materials Research Science  
and Engineering Center Program of the NSF and the Army Research Office  
through the Institute for Soldier Nanotechnologies.

--END--

Written by David Chandler, MIT News Office
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