The solar sell

Materials World magazine
1 Jan 2015

Make photovoltaics cheaper and they shall buy. But how do we cut manufacturing costs? Eoin Redahan reports

Another day, another record in solar cell efficiency. But while it is nice to read that Fraunhofer’s multi-junction cell can convert 46% of solar light into electrical energy, it’s largely irrelevant to the rest of us. 

The device uses III-IV compound semiconductor materials, many of which are too expensive for widespread use in photovoltaics. So gallium and indium may squeeze the most out of the sun, but the barrier to widespread solar panel uptake is not conversion efficiency. It is cost.

In recent decades, the cost of solar panels may have dropped, but it must fall further for the industry to compete with fossil fuels. Silicon is by far the most widely used semiconductor in solar cells, but increasing raw material prices and processing costs have led manufacturers to seek lower cost alternatives. So, what else is out there?

Carbon-based nanomaterials

At the University of Wisconsin-Madison, researchers are concentrating on depositing carbon nanotubes on substrates for two reasons – carbon nanotubes are superb electrical conductors and very little material is needed. Lead researcher Mike Arnold adds that carbon nanotubes are powerful light absorbers, quite stable and inexpensive, due to the abundance of carbon.

Arnold and his colleagues are also varying the diameters of the nanotubes to capture different wavelengths of light including ‘all the colours of the rainbow, as well as the ones in the spectrum we can’t see.’ As with a lot of carbon nanotube research, UW- Madison’s work is still in the exploratory stage.

The polymer solution

While we wait for carbon nanotube photovoltaics to become 2D reality, other contenders are emerging, such as semi-conducting polymers. In theory, these materials are light and flexible, and can be mass-produced cheaply.

However, up to now scientists have been stymied by the morphology of the polymer donor–fullerene acceptor mixture. A team from Hong Kong University of Science and Technology and North Carolina University claims to have overcome this through temperature-controlled aggregation in a new class of semi-conducting polymers.

While the 10.8% conversion efficiency recorded may seem modest next to silicon-based photovoltaics, these organic solar cells are compatible with existing methods of mass production, such as roll-to-roll processing, which is a bit like newspaper printing.

Metal organic frameworks

In dye sensitised solar cells (DSSCs), optically excited dyes couple with semiconductors, such as titanium dioxide, to convert energy into electrical current. Advocates of DSSCs say this light absorbing technology could capture more of the sun’s energy than silicon-based solar cells if multiple dyes and different molecular systems were used.

To help these dyes absorb more light, researchers at Sandia National Laboratories are exploiting the ordered structure and versatile chemistry of metal organic frameworks (MOFs). According to Sandia’s Mark Allendorf, the porosity of MOFs allows them to place a second dye in the material’s pores to cover additional parts of the solar spectrum.

The sun machine: 

US$1.2bln – US Department of Energy (DOE) investment in Sandia’s dye-sensitised solar cell research 

100GW the amount of sunlight hitting Earth at any one time*

40% University of New South Wales researchers, in Australia, have converted two fifths of the sunlight hitting a photovoltaic system into electricity. Unlike many other tests, this mark was recorded outdoors. 

US$0.06 the US DOE’s goal price for solar electricity per kWh. At the moment, the cost is about $0.30

150m number of kilometres that photons travel from the sun to Earth*

* Stats taken from UW Madison