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Bright Future for Solar Cells
How the next generation of photovoltaics will soak up the light their predecessors can't see...

Sat, 14th Jul 2012

Per-Anders Hansen
The World needs energy, and if we want to keep the climate stable, then it had better be clean energy. At the same time, the Sun is bathing us in thousands of times more[pic] energy than we currently consume, and we know how to turn the Sun's energy into electricity with solar cells.

But, the solar cells on the market today, although they are getting cheaper, are far from being as efficient as they could be in terms of their ability to turn light into electricity. In fact, the efficiency of the best cells is significantly less than 30%.

To address this problem of poor efficiency, we first need a crash course in what light is and how a solar cell works. Light consists of tiny energy particles called photons. For example, red light consists of “red” photons with a specific amount of energy. When these photons hit the solar cell, electrons inside the cell get knocked loose and, tapped off into a circuit, it's these electrons that give us the current.

But in order for a photon to knock loose an electron, it needs to have a minimum amount of energy. On the other hand, if the photon packs a bigger energy punch than this minimum, the surplus is just wasted as heat. And here lies the problem.

Sunlight contains photons with a wide range of energies. It can be divided into three parts. The visible light we can see; the high energy ultraviolet (UV) light that, amonst other things causes sunburn, and low-energy infrared (IR) light, which we experience as heat.

For silicon solar cells, red photons have just a bit more energy than is needed to knock loose an electron. Red photons are therefore used very efficiently by such silicon-based cells. On the other hand, blue and UV photons have much more energy and could knock loose two or even three electrons per photon respectively. But since we lose...
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