Development of semiconductor nanowire based solar cells

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Our proposed core/shell nanowire (NW) solar cell will consist of two main layers: a core GaAsSb NW and a radial GaAs shell. Using this material system a type-II bandgap alignment with GaAs is achieved, where electrons are confined in the GaAs shell and ho les in the GaAsSb core. When light is absorbed by the core/shell NW, electron-hole pairs are created. One key advantage of this type-II system is that the electron and hole are separated very efficiently even without any doping. Another advantage is that the electrons and holes only have to move across a very short distance (<100 nm) and therefore one can reduce the electron-hole recombination rate. Good optical absorption is achieved since the light is mainly absorbed along the direction of the NW. In pr inciple both the core (GaAsSb) and shell (GaAs) can absorb light and final dimensions will be optimized in terms of overall solar cell efficiency. Compared with Si based NW solar cells, GaAs has the additional advantage that it has a direct band-gap and t hus absorbs light much better than silicon. The thickness of material required to absorb 90 % of the power due to above-band-gap photons is 140 times smaller in GaAs than in Si (less material means lower cost). This "optical thickness" is only 900 nm for GaAs compared to 125 micrometer for Si. Even with 140 times shorter NWs, the solar cell efficiency for GaAs NWs is still expected to be a factor of two higher than for Si NWs, partly due to a better optimized bandgap to the solar radiation spectrum. In this proof-of-principle NW solar cell we target an efficiency of 10% by 2012, a factor of three higher than present state-of-the-art. Clean, renewable and high efficiency energy sources are today highly demanded in research strategies worldwide in order t o limit the green house effect on the climate and ensure the ever increasing energy need in the world. If successful our solar cell has the potential to be a good source of cheap renewable energy in the future.