The photocatalytic activity of nanostructured semiconductor films can be exploited to make the anodes in solar cells. Of particular importance is the DSSC, which uses nanostructured titanium dioxide (TiO2) films modified with sensitising dyes. When exposed to light, electrons from the dye molecules are transferred to the conducting band in the TiO2 layer, thus producing a current. Such cells are appealing because dyed nanoparticles have a great potential for absorbing light and generating electrons and because they are cheap and easy to make.

Recently, scientists have become interested in using zinc oxide (ZnO) because of its wide direct band gap of 3.37 eV and high electronic mobility. In theory, a dense network of ZnO nanowires or nanorods should favour electron collection because the nanowires/rods provide more direct conduction paths for electrons.

Unfortunately, however, DSSCs made from upstanding ZnO nanorods so far suffer from a low "fill factor" – which is related to the maximum power the cell can supply – because of large surface recombination of electrons and holes. They also have a low photocurrent due to poor light absorption. This is because the dye loading is low and some photons fall on the gap between adjacent nanorods and so are not absorbed by the dye layer.

Now, Changyun Jiang and colleagues have replaced the upstanding ZnO nanorod structure with a "nanoflower" array instead (see figure). The random branches of the nanoflowers have a larger surface area and increased light-dye interactions, leading to better light absorption. Moreover, electron-hole recombination is reduced, resulting in a much improved fill factor and an energy conversion efficiency of 1.9% – compared to just 1% for the nanorod arrays.

The researchers grew their nanoflower arrays on transparent indium tin oxide glass using a low temperature hydrothermal technique. "This method is simpler and cheaper than other technologies such as silicon solar cells and inorganic thin film solar cells," Jiang told "The low temperature process also means that flexible solar cells can be made."

The team now plans to further improve the light-to-electricity power conversion efficiency of DSSCs based on its nanoflower electrode and develop solid-state DSSCs with ZnO nanoflower films fabricated on flexible substrates.

The researchers reported their work in Appl. Phys. Lett..