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Nanoparticle solar cells make light work

Cheap, printable photovoltaics could finally live up to its initial promise.

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Do you need power? Just print a solar cell.
Credit: Michael Graezel/Science/AAAS

A type of solar cell first discovered 20 years ago may finally become commercially viable thanks to reported improvements in Sciences This day1. This alternative design could lead to cheap, printable cells that would greatly boost the world’s use of solar energy.

Electrochemist Michael Grätzel of the Swiss Federal Institute of Technology in Lausanne devised the dye-sensitized nanocrystal (DSC) cell in 1991. It uses organic dye molecules to absorb sunlight, the energy of which then sends electrons to tiny nanoparticles of ceramic titanium dioxide. (titania) on which the dye sits. These electrons are collected by electrodes to generate an electrical current.

Titania is itself very cheap: in a larger grain form, it is the pigment in white paint. And the cells themselves should be easy to mass-produce. Grätzel and others have developed methods to ‘print’ nanocrystal solar cell arrays onto glass panels and metal sheets.

All of this makes DSCs seem like an attractive alternative to conventional photovoltaic cells, which are typically made from thin films or wafers of silicon and are relatively expensive to produce.

DSCs have previously achieved efficiencies of up to 11%, slightly less than commercial silicon PV cells, and are already being sold in small numbers. Cardiff, UK-based company G24 Innovations sells them in flexible plastic-mounted modules, and several other companies, especially in East Asia, market them in glass panels that can be integrated into buildings.

But the use of technology has been restricted so far. The dyes used to harvest sunlight contain atoms of ruthenium, an expensive metal. And due to their conversion inefficiencies, DSCs also tend to only produce low voltages (less than 0.8V).

To complete the electrical circuit and replace the electrons ejected from the dye, DSCs use a chemical compound to transport electrons from the second electrode. The above cells use dissolved iodine, which picks up an electron to form triiodide ions. The ions diffuse through the liquid between the electrodes until they reach the dye-coated titania particles.

But the triiodide ions are not a good match for the electron energies in the dye molecules: they waste energy transferring their electrons, resulting in low cell voltage and thus low power. The problem is that alternative electron carriers that are better suited for transferring electrons suffer from the fact that electrons can jump back onto them from the dyes, wasting absorbed solar energy.

Now, Grätzel and colleagues have found good alternatives to both expensive ruthenium dyes and voltage-limiting iodide mediators. “It’s a very good paper and a significant advance,” says Jenny Nelson, a specialist in nanocrystal and polymer solar cells at Imperial College London.

For the dyes, Grätzel’s team uses complex three-part molecules consisting of a group that readily loses electrons, a group that readily accepts electrons, and a bridging unit containing a light-absorbing group related to that of chlorophyll.

For the electron mediator, the researchers use organic molecules bound to cobalt atoms, which can switch between two states by the gain or loss of an electron. They tailored the dye by attaching bulky chemical groups that act as barriers, preventing unwanted backflow of electrons from the mediator into the dye.

The resulting DSCs have achieved record voltages (up to 0.97V) and efficiencies (up to 12.3%). If efficiency can be increased by up to 15%, the devices should become profitable competitors to silicon photovoltaic cells.

However, there are other problems to solve first. Notably, Grätzel’s cobalt mediator dissolves in acetonitrile, a highly volatile solvent that is unsuitable for use in practical devices, according to Gerrit Boschloo, a DSC expert at Uppsala University in Sweden, who first reported on cobalt mediators in 2010.two. He adds that the mediator currently used by the Lausanne team is probably not stable enough for long-term use.

Grätzel says he’s working on these and other improvements, for example tailoring the dyes to capture more of the red component of sunlight and testing new cobalt mediators that push the voltage even higher.

Credit: Michael Graezel/Science/AAAS

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Cite this article

Ball, P. Nanoparticle Solar Cells Make Light Work.
Nature (2011).

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