To-date, a large scale use of photovoltaic devices for electricity generation has not developed because the available technology is prohibitively expensive. However, a recent letter from the Swiss Federal Institute of Technology describes a photovoltaic cell system for which a commercially realistic energy conversion efficiency is claimed (B. O’Regan and M. Grätzel, Nature, 1991, 353, (6346), 737). Colourless, optically transparent films of titanium dioxide, displaying the fundamental absorption edge of anatase (band gap 3.2 eV) are deposited on conducting glass sheet. These 10 μ m films, which consist of particles with an average size of 15 nm and a particle surface roughness factor of 780, gave linear photo-current response up to full sunlight.

The subsequent deposition of a monolayer of the trimeric ruthenium complex dye RuL₂(μ -(CN)Ru(CN)L’₂)₂ (where L is 2,2’bipyridine-4,4’dicarboxylic acid and L’ is 2,2’-bipyridine) onto the titania results in a deep brownish red coloration of the film. The high surface area of the semiconductor film and the ideal spectral characteristics of the dye lead to a high proportion of the incident solar energy flux being harvested in a cell employing this system in the photoanode; the counter electrode consisting of conducting glass coated with a few monolayers of platinum. The very fast electron injection observed with dyes such as this tri-ruthenium complex, combined with their high chemical stability, makes these compounds look attractive for practical development.

Exceptionally high efficiencies for the conversion of incident photons to electrical current are claimed, with the device harvesting 46 per cent of the incident solar energy flux. The overall light to electric energy conversion yield is 7.1 to 7.9 per cent in a simulated solar light and 12 per cent in diffuse daylight. With current densities greater than 12 mA/cm² and with at least five million turnovers being achieved without decomposition, practical applications may be feasible. The technology described thus seems to represent a significant advance in photovoltaic cell technology.