Unprecedented energy efficiencies using a very promising solar cell design have been demonstrated by scientists in the US. 

In one year, the earth absorbs roughly 8000 times more energy from the sun than was consumed worldwide in 2008. However, due to the prohibitive cost and low energy conversion efficiencies of current solar cell technology, solar energy has yet to achieve widespread use. 

The microwire-array solar cell is a technology that drives down cost of solar cell production by using less silicon. This is achieved by aligning micron-thick silicon wires perpendicular to a dimethylsiloxane (PDMS) substrate (as opposed to the traditional flat layer of silicon), giving the cell the appearance of a bed of nails. However, efficiencies of only 3.4 per cent have so far been achieved with this design, despite a theoretical efficiency of 17 per cent. 

Now, Harry Atwater and his team at the California Institute of Technology, have demonstrated an efficiency of 7.9 per cent using their silicon microwire-array solar cell, by implementing an insulator that minimises surface recombination of charge carriers, aluminium oxide particles to scatter light between the wires and silver back reflectors to prevent loss of incident illumination into the substrate. 

Despite their already impressive results, the researchers state much higher efficiencies could be achieved. 'We know we were only absorbing about 50 per cent of the incident photons,' explains Nathan Lewis, one of the collaborators on the work. 'So, with device optimisation, we should already have efficiencies close to 15 per cent.' 

Frederik Krebs, an expert in solar cells at the Technical University of Denmark in Roskilde, is impressed by improvements the researchers have made to this solar cell design and praises the technical aspects of the work. 

'Here, they really have the dimensions [of the wire] bang on,' he says. 'So from that point of view, I think it's really nice.' 

Whilst this work is a big step forward, Lewis admits that there is still some way to go before the technology could be commercialised and the first thing the researchers plan to do now is to optimise their design by investigating device physics, such as the optimal wire diameter and spacing. 

Yuandi Li 

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