Today's photovoltaic technology, while certainly promising, offers very poor efficiency because of inherent issues in its working mechanism. Using carbon nanotubes, however, Cornell University researchers now hope to lead the way to the next generation of highly efficient solar panels.

How traditional solar cells work (and why they are so inefficient)

In a traditional, silicon-based solar cell, two silicon layers with opposite polarization — p and n, positive and negative — are placed next to each other. When photons hit the top part of the cell, which is designed to let the light through reflecting only a small portion of it, their energy frees the chemical bounds of both an electron and the corresponding electronic hole left by it, allowing them both to move freely.

The electric field in the cell will make the "freed" electron tend to approach the n layer and the electronic hole approach the p layer (see accompanying image). These charges moving in opposite directions generate an electric current.

The main issue with this approach is that a very precise amount of energy is needed to free an electron-hole pair: if the photon has lower energy, it will just go right through the cell; if it has higher energy, the remaining one will just be dissipated as heat. This fact alone accounts for nearly 70 percent of efficiency loss in a solar cell.

As a consequence, today's typical solar cell efficiency is placed somewhere between 12 and 14 percent, with some commercially available panels going up to 20 percent and (extremely expensive) cells for space applications reaching only a few more points.

Using carbon nanotubes for tomorrow's solar cells

Researchers from Cornell University have managed to tackle this issue by designing a simple carbon nanotube-based solar cell that, when hit by higher-energy photons, has a multiplying effect on how much current is produced rather than dissipate it as heat.

As the authors explained in the latest issue of the journal Science, the team built a tiny photodiode, a simple kind of solar cell, using a single carbon nanotube wired between two electrical contacts and close to two electrical gates, one negatively and one positively charged.

The researchers found that the very narrow structure of the nanotube forced the electrons to pass one by one, generating further electrons with the spare energy from the higher energy photons, in a nearly ideal energy conversion process that could be the key to higher efficiency solar panels.

Unfortunately, though, we will likely have to wait for nanotechnology mass production processes to develop further before this can be made into inexpensive and reliable devices available on the consumer market. Optimists, however, would say it's now just a matter of time.

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