Dye-sensitized solar cells are a promising low-cost alternative to silicon-based solar cells. However, while the principle behind these dye-based cells is simple — photosensitive dyes absorb sunlight and generate free electrons — many technical challenges must be overcome to make these cells as efficient as possible.

Now, Wan In Lee and co-workers1 at Inha University and the Korea Institute of Science and Technology in Korea have fabricated highly efficient dye-sensitized solar cells by including tiny, porous titanium-dioxide spheres into the cell structure.

“One important part in a dye-sensitized solar cell is the titanium dioxide photoelectrode,” says Lee. “It must absorb dye molecules, transport photogenerated electrons, and allow diffusion of electrolytes.”

An ideal photoelectrode should be full of pores, providing a high overall surface area that absorbs as many dye molecules as possible. This can be achieved by building the electrode from tiny particles.

However, in most electrodes prepared previously, only very small pores are present, and the cell performance is degraded by crystal boundaries or defects at which free electrons can recombine with the electrolyte or dye molecules. The problem can be compared to driving through a complicated network of city streets —there is a high likelihood of getting lost or stuck somewhere.

Fig. 1: A new design for the electrode on dye-sensitized solar cells includes spheres of porous titanium dioxide. Pores within the spheres help to absorb dye molecules, while the larger pores between the spheres provide a channel for the flow of charge carriers.Copyright © Wan In Lee (2009)

Lee and co-workers solved this problem by building electrodes from relatively large, porous titanium-oxide spheres. The tiny pores in each sphere help the absorption of dye molecules, while the larger pores between spheres act as ‘highways’ for the charge carriers to travel along without interference (Fig. 1).

The researchers prepared two types of electrode, one containing well-defined spheres of similar sizes, and another containing varied, deformed spheres. Both electrodes absorbed more dye than an electrode prepared from commercial nanoparticles. Surprisingly, the electrode consisting of deformed spheres provided higher conversion efficiency than the well-defined spheres, attributed to better contact between neighboring particles.

The best cell fabricated by the researchers converted 10.52% of incident light into electricity. According to Lee, this is close to the world’s best conversion efficiency achieved for dye-sensitized solar cells by the Sharp Corporation (11.1%).

“Considering that we are not experts in fabricating high efficiency cells, the achieved result is quite high,” he says. “We believe that our spherical titanium dioxide electrodes could replace current nanoparticle-based electrodes.”