Double current in solar cells

03 October 2010
Posted by Chris Smith.

At the moment solar power is significantly more expensive than fossil fuels and there are tens of thousands of researchers trying to reverse this. However this isn't really just Solar panela case of making the solar cells themselves cheaper because even now in the western world half of the cost of installing solar panels is in the installation. So the best way of making the electricity cheaper is to get more electricity per square metre of panel.

The problem is that different colours of light have different energies and a conventional solar cell can only be designed around a single energy called the band gap. This means that it can't absorb light with a lower energy than this, and any extra energy a light photon has is wasted as heat. This means that the theoretical maximum efficiency for a simple solar cell is about 31%.

One solution to this would be to find a material that would push two electrons around the circuit, if the energy of the photon was twice the band gap. This would mean that you could make the band gap a lower energy so that more colours of photon can be absorbed, without loosing the efficiency at higher energy photons. This effect has been observed in solid lumps of semi-conductor but is very inefficient - you need far more than twice the band gap energy to push 2 electrons.

Justin Sambur and collegues have been investigating this property in quantum dots, these are tiny lumps of material, in this case lead sulphide, which are so small that their energy levels start to look more like an atom's than a bulk material, but an atom whose properties you can engineer. They have built structures of lead sulphide nano-dots attached to titanium dioxide semiconductors. Although these absorb a tiny proportion of the light hitting them, as they are extremely thin, when they increase the energy of the photons to be more than twice the band gap, they see almost a doubling in the current produced, so they are producing multiple electrons per photon. Though it will take a lot more development to get a practical cell.

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