According to New Scientist Environment :
Heating plastic solar cells can alter their structure in a way that boosts efficiency, new research shows. The US and Korean scientists behind the discovery say it could ultimately allow flexible, lightweight plastic cells to replace rigid traditional cells.
Solar cells are usually made from silicon, which is inflexible and relatively heavy.
By contrast, plastic solar cells could be more easily supported and wrapped around surfaces (see Pliable solar cells are on a roll). It might even be possible to spray light-collecting plastic onto a surface.
Plastic cells lag behind silicon in terms of efficiency, however, at best converting just 5% of solar energy into electricity compared with up to 40% for conventional cells. “To make plastic cells commercially viable, you need to reach about 8%,” says David Carroll of Wake Forest University in Winston-Salem, US. “That matches some silicon products already on the market.”
The best plastic solar cells are made from a light-absorbing polymer containing soccer ball-shaped carbon molecules called fullerenes. The fullerenes provide stepping stones in the plastic film for charge to hop across.
Space charge
The main efficiency-limiting factor is a kind of electrical “traffic jam” that occurs inside the plastic. “When you draw off the electrons freed when light hits these devices you leave behind an absence of electrons we call ‘space charge’,” Carroll told New Scientist. This presents a barrier to other electrons. “Charge isn’t mobile enough in these materials to fill the gap and everything gets blocked up,” he adds.
Carefully heating plastic cells seems to solve this problem. Working with colleagues Jiwen Liu and Manoj Namboothiry, also from Wake Forest University, and Kyungkon Kim from the Korea Institute of Science and Technology in Seoul, Carroll found that heating can introduce crystal patterns into the plastic to diffuse these jams.
“It’s possible to create little ‘highways’ that prevent space charge from building up,” Carroll says.
Crystal whiskers
By carefully heating finished cells to around 150ºC, Carroll and colleagues made the fullerene molecules form whiskers of crystal. These trigger crystallisation in the surrounding polymer as well. The fullerene-and-polymer crystal creates a network across the cell, allowing charge to move easily and preventing space charge blockages.
“Getting around space charge is a big step,” says Carroll. The heating process can increase efficiency from 5% to 6%. “Some performed as well as 7%,” Carroll told New Scientist. “We think we can probably push it up to 10%.”
This could pave the way for commercially viable plastic solar cells, he says. The research will appear in a forthcoming edition of the journal Applied Physics Letters.
- Oh…No…what does plastic made of? It’s the product of petrochemical!!
“One of the great appeals of plastics have been their low price as compared to other materials. However, in recent years the cost of plastics has been rising dramatically. The cause of the increase is the sharply rising cost of petroleum, the raw material that is chemically altered to form commercial plastics. As the cost of plastic hinges on the cost of petroleum, should petroleum prices continue to rise, so will the cost of plastic. In 2005, the higher price of plastic drove a number of plastic-toy manufacturers out of business.”
This is not really good when the performance is only about 7% Hmmm But it’s a good start hope more can be done and much faster!
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