Cheaper, More Efficient Solar Cells-A new type of material could allow solar cells to harvest far more light.

簡評：
這是去年的文章,專講"光子晶體概念在太陽電池上的應用".去年三月發表的.這裡重新貼上,已和之前那一中一英兩篇講"鎖光太陽電池"文章做個對照.
前年這個時候,誰把光子晶體技術和太陽電池放在一起提出,就被嗤之以鼻以為是天方夜譚或科幻題材;去年這個時候,提出光子晶體太陽電池的人雖然已經被認同技術有可行性,但是卻是旁門左道的方法;今年,就在上個月,MIT的另個教授提出所謂"鎖光"概念,卻被財團認可,再也沒有人會認為這是天方夜譚或是旁門左道--我們的世界迫切需要高效率的太陽電池.
只要是可行的技術,就會被認可;只要提升總轉換效率3%,所能造成的效應等於傳統製造程序上所有的cost down可行性加總.因此,我們該好好思考如何方能把我們多年前far-seeing的光晶太陽電池專利實踐出來.
太陽電池和LED在設計上是相反的概念.在LED上,我們的需求是把P-N Junction創造出的光盡可能放出,而在太陽電池我們不能放過任何一道可能進入元件的光線.在LED中我們需要在極小的面積中創造足夠的面積增加電子電洞的結合率,在太陽電池中我們要盡量減少電子電洞的再結合率.由此發想,太陽電池的PN結要非常平整,以減少recombination rate,而LED的PN結要粗糙來增加combination rate.
在外型方面,兩者都要有粗糙面以減少光子在高低不同折射率介質傳遞時發生的全反射角損失以及折射損失;另外我們都要在元件的其中一端增加反射面,就LED來說是要確保光能穩穩地射往照明方向,就太陽電池來說是要確保光子能在元件中不斷地反射與折射,而不會直接穿透元件.採用光子晶體,可以輕易地在LED或太陽電池元件上造成這種效果.
當然,我們所提出的光晶太陽電池還有另外一種特性,是MIT的教授沒有注意到的部分,就先賣個關子.日後有機會再說.
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Much more efficient solar cells may soon be possible as a result of technology that more efficiently captures and uses light. StarSolar, a startup based in Cambridge, MA, aims to capture and use photons that ordinarily pass through solar cells without generating electricity. The company, which is licensing technology developed at MIT, claims that its designs could make it possible to cut the cost of solar cells in half while maintaining high efficiency. This would make solar power about as cheap as electricity from the electric grid.


The effort uses a type of material called a photonic crystal that makes it possible to "do things with light that have never been done before," says John Joannopoulos, a professor of physics at MIT who heads the lab where the new designs for solar applications were developed. Photonic crystals, which can be engineered to reflect and diffract all the photons in specific wavelengths of light, have long been attractive for optical communications, in which the materials can be used to direct and sort light-borne data. Now new manufacturing processes could make the photonic crystals practical for much-larger-scale applications such as photovoltaics.

StarSolar's approach addresses a long-standing challenge in photovoltaics. Silicon, the active material that is used in most solar cells today, has to do double duty. It both absorbs incoming light and converts it into electricity. Solar cells could be cheaper if they used less silicon. If the silicon is made thinner than it is now, it may still retain its ability to convert the photons it absorbs into electricity. But fewer photons will be absorbed, decreasing the efficiency of the cell.

MIT researchers developed sophisticated computer simulations to understand how thin layers of photonic crystal could be engineered to capture and recycle the photons that slip through thin layers of silicon. Silicon easily absorbs blue light, but not red and infrared light. The researchers found that by creating a specific pattern of microscopic spheres of glass within a precisely designed photonic crystal, and then applying this pattern in a thin layer at the back of a solar cell, they could redirect unabsorbed photons back into the silicon.

Today's solar cells already reflect some of the light that passes through the silicon. But the photonic crystal has distinct advantages. Conventional solar cells are backed with a sheet of aluminum. The photonic crystal reflects more light than the aluminum does, especially once the aluminum oxidizes. And the photonic crystal diffracts the light so that it reenters the silicon at a low angle. The low angle prevents the light from escaping the silicon. Instead, it bounces around inside; this increases the chances of the light being absorbed and converted into electricity.


>Better solar: In conventional solar cells (a), light (dashed line) enters an antireflective layer (yellow) and then a layer of silicon (green) in which much of the light is converted into electricity. But some of the light (solid arrows) reflects off an aluminum backing, returns through the silicon, and exits without generating electricity. A new material (represented by the dots in [b]) makes it possible to convert more of this light into electricity. Instead of reflecting back out of the solar cell, the light is diffracted by one layer of the material (larger dots). This causes the light to reenter the silicon at a low angle, at which point it bounces around until it is absorbed. The light that makes it through the first layer is reflected by the second layer of material (smaller dots) before being diffracted into the silicon.
Credit: Peter Bermel

As a result, the photonic crystal can increase the efficiency of solar cells by up to 37 percent, says Peter Bermel, CTO and a cofounder of StarSolar. This makes it possible to use many times less silicon, he says, cutting costs enough to compete with electricity from the grid in many markets. The savings would be especially large now, since a current shortage in refined silicon is keeping solar-cell prices high and slowing the growth of solar-cell production.

The company plans to work with existing solar-cell makers, applying its photonic crystals with a machine added to the solar-cell makers' assembly lines, Bermel says. But StarSolar needs to choose a large-scale manufacturing technique that will allow it to produce the photon crystals inexpensively. What's needed is a way to cheaply arrange two materials in an orderly three-dimensional pattern. For example, microscopic spheres of glass would be arranged in rows and columns inside silicon. Currently, techniques such as e-beam lithography can be used, but that's too slow for large-scale manufacturing.

Shawn-Yu Lin, professor of physics at Rensselaer Polytechnic Institute, has developed a method for manufacturing eight-inch disks of photonic crystal--a measurement considerably larger than what can be done with conventional techniques. The method, which employs optical lithography similar to that used in the semiconductor industry, works best for a type of solar cell that concentrates light onto a small chunk of expensive semiconductor material. Such a device would require a relatively small amount of photonic crystal compared with conventional solar cells. Lin says the technique could be applied for more-conventional solar panels, although it would be expensive.

Another potentially less-expensive method, called interference lithography, creates orderly patterns in the photonic-crystal materials. The method is fast and uses machines that are far less expensive than those used for conventional optical lithography. It also requires fewer steps than Lin's existing process, so he says it could be far cheaper. Such methods have been developed by Henry Smith, professor of electrical engineering at MIT, who was not involved with the StarSolar-related work. Smith says his interference-lithography method could be used to build templates for imprinting photonic-crystal patterns on large areas.

Another promising technique is self-assembly, in which the chemical and physical properties of material building blocks are engineered so that they arrange themselves in orderly patterns on a surface. For example, Chekesha Liddell, professor of materials science and engineering at Cornell University, has engineered building blocks in the shape of peanuts and the caps of mushrooms that line up in rows because of the way they fit together and the tug of short-range forces between them. She says this could be useful for assembling photonic crystals for solar cells.

With such approaches available, Bermel says that StarSolar hopes to have a prototype solar cell within a year and a pilot manufacturing line operating in 2008.