In the search for alternative energy sources, the direct use of solar energy is a particularly attractive option. The creation of efficient solar cells has therefore attracted much attention.

Dye-sensitized solar cells are a relatively new type of solar cell that offer significant advantages over conventional silicon-based solar cells in terms of cost and range of application. In such cells, light from the sun excites electrons in an organic molecule (the dye), which causes charge separation. To use the energy, these electrons must be transferred to an external circuit. In the final step of the process, after the electrons have given up some of their energy in the form of electricity in an external circuit, the electrons are returned to the solar cell by a counter electrode that usually contains platinum nanoparticles.

Much research effort has been invested in optimizing dyes and designing better support materials, but most dye-sensitized solar cells still rely on the use of platinum — a rare and expensive metal — for their counter electrodes. Now, Seung Il Cha and co-workers from the Korea Electrotechnology Research Institute1 have developed a counter electrode that contains carbon nanotube ‘micro-balls’ in place of platinum nanoparticles.

A critical feature of the counter electrode is the efficiency with which charge is transferred to the electrolyte. To obtain efficiency similar to that of a platinum-based electrode, a carbon electrode must have a large surface area. This can be achieved using a thick electrode layer, but increasing the thickness of the electrode also allows less light into the cell. “This is important for solar cells that collect light through the counter electrode,” explained Cha, “such as those used in building integrated photovoltaics and flexible solar cells.”

Fig. 1: (upper) Transparent electrodes for solar cells prepared using carbon nanotube micro-balls. (Lower) Scanning electron microscopy image of the carbon nanotube micro-balls.

The solution is to deposit a layer of micro-balls of carbon nanotubes onto a transparent substrate (Fig. 1). The micro-balls of carbon provide the high surface area, while the gaps between the balls allow light to pass through to the cell.

Carbon is far less expensive than platinum, and thus achieving performance even close to that of platinum-based counter electrodes would have a significant effect on the price–performance ratio. “In the future, we hope to be able to use this type of counter electrode in a supercapacitor or battery,” says Cha.