A Swedish company believes it can cheaply fabricate novel nanowire materials for direct use in highly efficient photovoltaic cells.
Conventional solar cells are made of slabs of crystalline silicon — where a greater thickness allows better light absorption but presents difficulties for electrons trying to escape and create charge.
This trade-off is largely overcome with nanowires, since their length maximises absorption, but their nanoscale width permits a much freer movement and collection of electrons.
‘They have a so-called direct band gap, which means their absorption coefficient is so much higher than that of silicon,’ said Lars Samuelson, chief scientific officer and co-founder of Sol Voltaics. ’So where you need 100 microns of silicon to absorb all the photons, you need just a couple of microns to absorb all the photons [with nanowires], so you can have very thin layers and multiple layers.’
While the potential of nanowires in PVs has been recognised for the better part of a decade, they have proved difficult to produce, requiring expensive reactors. Now Sol Voltaics believes it has found a solution to create nanowires that can be stored as a dry powder or liquid for later use by customers.
‘We can have a silicon wafer and grow these materials in dense arrays such as a grass field, or we have other techniques in which we do not even need a substrate and fabricate nanowires in an aerosol,’ Samuelson said.
Crucially, their fabrication methods can create heterostructured — or ‘striped’ — nanowires made of different layers of type- II-V and II-VI semiconductor materials. This effectively creates multi-junction individual nanowire solar cells, which when combined in whole devices could achieve overall efficiencies of up to 60 per cent, the company claims.
‘You can take care of different portions of the solar spectrum in different layers, which means in the top layer you absorb the UV and the blue, and in the next layer you absorb the green and so on sequentially. It means that for every layer you are losing very little in thermalisation [efficiency].’