An ultra-thin solar cell that could provide a cheaper, lighter alternative to existing devices has been created by researchers in the US.
James Zahler from Aonex Technologies, together with colleagues from Harry Atwater’s group at the California Institute of Technology and researchers at EMCORE PhotoVoltaics made the device by replacing the relatively thick semiconductor substrate normally used in solar cells with a thin “wafer-bonded” substrate. This means the new device is considerable cheaper and lighter than conventional solar cells.
Zahler’s team says the photovoltaic characteristics of the new device are better than those of conventional cells deposited on bulk substrates. Such bulk substrates are normally several hundred microns thick, while wafers are just hundreds of nanometres thick.
Solar cells use layers of semiconductors to absorb photons from the Sun and convert them into electric current. They often use indium gallium arsenide (InGaAs), or a similar combination of elements, on top of indium phosphide (InP). These materials are good at conversing light to electrical energy, but are also relatively expensive.
Zahler and his co-workers found they could replace the regular, thick indium phosphide substrate with a very thin layer of the same material on top of a cheap, oxidized wafer of silicon – the wafer-bonded substrate. This trick could reduce manufacturing costs by up to half, they say.
The researchers implanted an indium phosphide substrate with helium and heated it so that the expanding gas sheared off a micron-thin layer. They then transferred this exfoliated layer onto the silicon wafer, before growing an indium gallium arsenide photovoltaic structure in the usual way.
“The transferred layer is just 900 nm thick,” team member Katsuaki Tanabe of Caltech told New Scientist. He adds they should be able to make hundreds of cells from the same amount of material currently used to make just one.
Tanabe also found that the cell had a 20% higher electrical output, thanks to the high reflectivity at the interface between the indium phosphide and the silicon. Moreover, the cell weighed only half as much as conventional device. This is because most of a cell’s weight lies in the substrate that the photovoltaic layer is grown on.
The team now plans to fabricate a solar cell containing four photovoltaic layers. Such cells could have a high efficiency, surpassing cells containing two or three layers significantly.
“I find these results impressive,” says Rolf Koenenkamp of Portland University in the US. “The prospects for a four-junction solar cell seem much improved, where the exfoliation technique can indeed be applied reliably and at a reasonable cost.”
German researchers at the University of Stuttgart have previously made low-cost, thin-film, silicon-based solar cells using single crystalline layers of silicon. No solar cell manufacturer has picked up on these concepts yet, but that could change with the publication of the new results.
Journal reference: Applied Physics Letters (DOI: 10.1063/1.2753751)