Nanotechnology and Solar Power

What is there not to like about solar energy? There is ample capacity, for example, in one day enough energy comes from the Sun to meet the planetâ''s energy needs for one year. Its lifetime is indefinite. There are little to no negative environmental impacts from using it. And, the technology obstacles are not insurmountable.

The thing not to like is cost, despite industry analysts predicting a boom for solar companiesâ'' bottom line. To give you a comparison to other energy sources electricity produced by solar (or photovoltaic) cells costs about $0.30 per kilowatt hour (kWh), while electricity from wind costs about $0.05 per kWh and from natural gas about $0.03 per kWh. Thatâ''s 10 times the cost to get electricity from solar cells than from natural gas.

But back in 2005 in a report written by George Crabtree and Nathan Lewis for the US Department of Energy, it is predicted that we can expect to see the price for generating electricity from solar cells drop to $0.02 per kWh inâ'¿wait for itâ'¿20â''25 years' time.

Over the last 30 years the price for photovoltaic electricity has decreased by a factor of 20 mainly due to incremental improvements to single-crystal silicon solar cells, and the 20-25 year timeframe that Crabtree and Lewis predicted is based on the idea that these incremental improvements to creating cheaper crystalline materials will eventually lead to a cheaper kWh price point.

The slow development predicted in this model could be accelerated dramatically to as early as 2015 if nanotechnology can deliver in improving photovoltaics.

This kind of leap in technology will require moving beyond first-generation solar cells (single-crystal silicon wafers) and second-generation solar cells (thin-film semiconductors), which while cheaper to produce lag significantly in their efficiency to first-generation technology.

Instead a third-generation solar cell will need to be developed that can exceed the 32% Shockley-Queisser Limit.

One of the nanotechnologies being experimented with to overcome this limit has been quantum dots.

One area of research with quantum dots is to use them for achieving photon multiplication, which involves making multiple electronâ''hole pairs for each incoming photon. This moves electrons from the valance band into the conduction band. Victor Klimov at Los Alamos National Laboratory has been able to use quantum dots to achieve up to seven electron-hole pairs per incoming photon, and Klimov claims that this could lead to solar cells with efficiencies of up to 40%.

Another area in which quantum dots could be used is by making so-called â''hot-carrierâ'' cells. Typically the extra energy supplied by a photon is lost as heat, but with â''hot carrierâ'' cells the extra energy from the photons result in higher-energy electrons which in turn leads to a higher voltage.

As appealing as these solutions are they are estimated to be still 10-15 years off from commercial use. While industry gurus may be touting â''green technologyâ'', which in turns leads to the general public expecting to be getting solar-powered electricity off the grid in the near future, the reality is that is still somewhat far off. It will require a huge commitment to developing these technologies, and despite recent national initiatives and subsidies in Germany, Spain and Japan in solar power there remains a huge gap between where we are now and when we can expect electricity that comes from the Sun.

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