"We have a very short negative electrode providing a remarkable photocurrent density – the amplitude of which is generally proportional to the length up to the point all the light is absorbed," Craig Grimes of Pennsylvania State told nanotechweb.org. "If we could maintain such properties with increasing nanotube length, we should be able to achieve low cost solar cells of tremendous efficiencies – basically up to the theoretical limit."

Grimes and colleagues made the nanotubes by sputtering 500 nm-thick titanium films onto fluorine-doped tin oxide-coated glass substrates. This use of a transparent substrate enabled front-side illumination of the cells. The researchers anodized the films at 12 V in an electrolyte of hydrofluoric acid and acetic acid. Finally, the team annealed the films in oxygen to induce crystallinity. The final product was transparent arrays of titania nanotubes that had pore diameters of 46 nm, wall thicknesses of 17 nm and which were 360 nm long.

"Highly-ordered titania nanotube arrays have remarkable charge transfer and photocatalytic properties," said Grimes. "The remarkable charge transfer properties of the arrays beg the application to dye solar cells. The nanotube-arrays can be basically viewed as 'electron highways'."

Next, the team treated the arrays in a solution of titanium chloride, in order to improve their charge transfer properties. They attached molecules of a ruthenium-based dye to the nanotubes by immersing the array in a solution of the dye overnight. To form a dye-sensitized solar cell, the team added an electrolyte and a counter electrode consisting of a conductive glass slide sputter coated with platinum.

For an active area of 0.25 sq. cm, the cells typically exhibited a photocurrent density of 7.87 mA/sq. cm, with an overall conversion efficiency of 2.9%. This efficiency was five times higher than nanotubes not treated with titanium chloride. The nanotubes also had better recombination characteristics than the nanoparticulate titania typically used in dye-sensitized cells.

Grimes believes that low cost, high efficiency solar cells are in everyone's best interest. "The Earth's oil supply will be depleted in our lifetimes and the Earth's coal supply will be depleted in our grandchildren's time," he said. "To supply the energy needs of 6–10 billion people in a manner they are accustomed to is a big, big challenge that will not be met without a tremendous, focused effort. It truly behooves us to begin serious consideration of what we are going to do."

Now the researchers are working to increase the length of the transparent nanotube arrays. They are currently prevented from making nanotube arrays longer than 360 nm because they cannot make good-quality titanium films thicker than 500 nm. "Our biggest challenge is simply finding a suitable means for titanium film deposition from which we make the nanotube arrays," said Grimes.

The researchers reported their work in Nano Letters.