V-shaped solar cells could lead to better efficiency

Dec 19, 2007 By Miranda Marquit feature

“In solar cells,” Peter Peumans tells PhysOrg.com, “the goal is always higher efficiencies. Higher efficiencies usually mean lower cost.”

As solar cells continue to be more important as renewable energy sources, affordable techniques for producing solar cells will be in demand. Peumans, a scientist at Stanford University, and his colleagues Seung-Bum Rim, Shanbin Zhao, Shawn R. Scully and Michael D. McGehee describe one such technique to increase solar cell efficiency: v-shaped cells. The results of their findings are reported in Applied Physics Letters: “An effective light trapping configuration for thin-film solar cells.”

Peumans explains that he and his peers used organic solar cells to develop their technique. Organic solar have an active layer made out of molecules, such as pigments or polymers. They are low-cost and flexible. However, as Peumans points out, “organic solar cells typically have low efficiencies.”

A traditionally designed organic solar cell consists of a film layer of the light absorbing material spread on top of some sort of substrate. The Stanford team found that if they took a traditionally designed solar cell and then bent it to form a v-shape, it was possible to significantly increase the efficiency of the cell. “It’s about light management,” Peumans says. “This is a pretty simple solution.”

Peumans goes on to explain that most organic solar cells are made on planar substrates. “When the light hits it, there is only one bounce – only once chance for the light to be absorbed.” The v-shape, he continues, creates an environment in which the light can bounce around. “Every time the light bounces, it has a chance to be absorbed into the cell.”

Organic solar cells are defined mainly by a “thin film of organic material sandwiched between two electrodes,” Peumans explains. This is what makes them low cost and flexible. For the most part, due to their low efficiency, they are not realistically considered for energy generation on a large scale. However, the technique developed at Stanford has the potential to change that. “We were able to increase the efficiency by 52 percent,” he says. “The same cell generates more electrical power.”

Organic solar cells are not the only technology that could make use of this efficiency-boosting technique. “While this works particularly well on organics,” Peumans says, “it can be applied to other thin film solar cell technologies as well.” He explains that they evaluated the potential for increased efficiencies in thin film silicon solar cells. “For many solar cell technologies, it would make sense to adopt this approach.”

Peumans says that there is already a company working on this technique, “trying to improve the concept and trying to get even higher efficiency out of it.” He also points out that he and his colleagues performed a cost analysis on their method. “Companies would have to figure it out on a case by case basis,” he concedes, “but the numbers we looked at indicated that you could produce solar modules at a lower cost per installed Watt.”

While organic cells would likely need further development to become practical for large-scale electrical power generation uses in grids, the idea of a v-shaped solar cell could be incorporated into existing then film solar cells.

“This is potentially a very simple application technique,” Peumans says. “It could be a way to increase the efficiency of different types of thin film solar cells.”

Copyright 2007 PhysOrg.com.
All rights reserved. This material may not be published, broadcast, rewritten or redistributed in whole or part without the express written permission of PhysOrg.com.

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User comments : 16

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gopher65
4.5 / 5 (2) Dec 19, 2007
I wonder if it is possible to create something kinda sorta like a one way mirror? Where the solar panel is upside down (or inside out rather), and sphere shaped, and the light enters it and bounces around for a while being slowly absorbed? They're be some leakage, but like the article says, the more bounces, the more light you end up being able to convert.
Valentiinro
3.3 / 5 (3) Dec 19, 2007
One way mirrors also absorb some light though, less light would get through to be bounced around. Meh.
gopher65
1 / 5 (1) Dec 19, 2007
That's why I said "kinda, sorta, like a one way mirror." I don't know if it is possible, that's why I asked;).
SDMike
2.7 / 5 (3) Dec 19, 2007
How about a thin highly transparent "cover" angled so that energy bouncing off the solar cell is beyond the critical angle as in:
"When light is incident upon a medium of lesser index of refraction, the ray is bent away from the normal, so the exit angle is greater than the incident angle. Such reflection is commonly called "internal reflection". The exit angle will then approach 90° for some critical incident angle %u0398c, and for incident angles greater than the critical angle there will be total internal reflection." http://www.liveph...gle.html
This the radiant energy is "trapped" within cell.
billyswong
2 / 5 (3) Dec 19, 2007
but with such a cover, sunlight shined sideway onto the the solar cell with an angle will then reflect away right at the beginning, never reaching the cell.
Ashibayai
1.5 / 5 (2) Dec 19, 2007
Do you think a concave parabolic shape coupled with such a cover could offset the loss of reflected light?
weewilly
2.7 / 5 (3) Dec 19, 2007
Now this is something I can get into. Being an amateur solar energy experimenter I have kept up on this topic for years. We are now talking about something I have advocated for awhile now with a parabolic shape. This would take some serious lab work beyond my capabilities but if these two basic ideas could be optimized then there are much stronger possibilities of acceptance and usage. I believe the angle of incidence is 60 degrees from the perpindicular angle made to the sunlight on mirrored surfaces. At least it is within my lab abilities. One of my students designed a solar oven that reached heat levels of close to 300 degrees F., using SolidWorks 3D CAD. He is 11 years old. Some youngster out there will make use of "thinking outside of the box" technology to kick start photovoltaic efficiencies and enivitably their use on earth. Good Article.
nilbud
3 / 5 (4) Dec 20, 2007
Damn kids, we'd better learn some old monkey tricks to amuse them or its medical experiments for the lot of us.
SDMike
1 / 5 (1) Dec 20, 2007
Using a solar tracker ensures the angle of incidence is ~90 all the time. This also increases the efficiency of the cell regardless of the add-ons. Cells on the roof are nice but active pointing is nicer.
ontheinternets
1 / 5 (1) Dec 23, 2007
I sometimes wonder how the cost of conveniently-shaped reflectors compare to that of the solar cells. Mirrors and/or lenses used to concentrate the sunlight used in combination with wasteful one-way filters could in theory add up to a practical compromise, depending on how the costs compare. I'm assuming too that a solar cell has a limit to the amount of light it can have thrown at it before bad things happen. I know much smarter people have thought about this before, but I've never come across the/an explanation for why reflectors are not employed more often.
earls
1 / 5 (1) Dec 23, 2007
3d > 2d
philipt18
not rated yet Dec 24, 2007
How different is this from the paper presented in 2004 by a Chinese team ( http://dx.doi.org...4.11.005 )?
earls
1 / 5 (1) Dec 26, 2007
phil, there's a million different valid angles to the problem, and they will work, it's just that the cost of implementation, promotion, and establishment of each and every individual solution may or may not materialize due to a variable of factors.
mrlewish
not rated yet Dec 27, 2007
Basically what we are talking about is sunlight being reflected back onto another solar cell and visa versa in a "V" shape? And if so why is this news. Yes ontheinternets there are solar concentrators in the works also.
Ashibayai
1 / 5 (1) Dec 31, 2007
"How different is this from the paper presented in 2004 by a Chinese team ( http://dx.doi.org...4.11.005 )?"

Because they're in China, and haven't filed for a patent in America?
sly
1 / 5 (1) Jan 06, 2008
Check this out http://www.cv21.co.jp/