‘These new pink materials convert light to electricity with only half the efficiency of commercially-available silicon-based solar cells, at only one quarter of the cost,’ said Yiying Wu, assistant professor of chemistry at Ohio State.
Typical DSSCs use dye containing ruthenium, which is red in colour, and metal oxides such as titanium oxide or zinc oxide which are white in colour which mix to give the solar cells their pink colour.
Wu’s research team is using zinc stannate, a complex oxide with tunable properties to increase the electricity produced. Zinc stannate belongs to a class of more complex oxides with tunable properties that opens up new possibilities for DSSCs in the future.
Colour determines the wavelength of light that a solar cell can capture, so adjusting the colour lets scientists optimise particular properties in how the device will function. So far in the development of DSSCs, scientists have achieved the best performance from red ruthenium dye.
Traditional solar cells look blue because of an anti-reflective coating, which boosts absorption of green light, which is the strongest in the solar spectrum. Wu’s materials don’t have that anti-reflective coating.
‘If you want to achieve the best efficiency, you need to consider both the voltage you can achieve and the current you can achieve,’ Wu said.
‘If you absorb a very broad range of wavelengths, that’s going to sacrifice voltage. And if your absorption energy threshold is very high, you can achieve high voltage, but you’ll sacrifice current. The idea is to find some balance.’
In DSSCs, dye molecules coat tiny metal oxide particles that are packed together into a thin film. The dye molecules capture light energy and release electrons, and the particles act like electrical wires to carry the electrons away to an electrical circuit. Wu is working on designs that incorporate tiny nano-wires that carry electrons directly to a circuit to minimise the electrons lost when travelling between particles.
Currently, commercially available silicon solar panels have 15 per cent efficiency; Wu has previously recorded 8.6 per cent efficiency with DSSCs containing particles and nano-wires of titanium oxide. By using zinc stannate particles but no nano-wires, Wu has achieved 3.8 per cent efficiency and is now working to combine the two approaches.
Wu’s team is also exploring the possibility of using nano-wires shaped like the branches of trees. ‘In our DSSC design, the dye-coated particles would provide the surface area, and the nano-trees would branch out in between them, to transport the electrons,’ said Wu.