Jan 042011
== Summary == A diagram showing the types of c...
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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.

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