For example, GreatCell of Switzerland, a spin-off from the Ecole Polytechnique Fédérale de Lausanne, now offers small-scale production of dye-sensitized solar cells (DSCs) in collaboration with STI of Australia. The active components in these cells are nanoparticles of titanium oxide - a semiconductor with a large bandgap of 3-3.2 eV - formed into a thin layer on a glass substrate. Incident light excites organic dye molecules absorbed onto the titanium sites in the nanoporous film. This causes electrons to be released into the semiconductor layer to generate an electrical current.

According to a new report, Nanomaterials for Next-Generation Energy Sources, these DSC modules convert about 7% of the incident solar energy into electricity. That compares with up to 18% for conventional solar panels based on costly monocrystalline silicon. However, the DSC modules are able to absorb sunlight over a wide range of angles, which means that a module installed on a building facade can generate as much energy as a silicon version that is 12% efficient.

DSCs are also produced by US start-up Konarka, which exploits a low-temperature materials processing technology to enable the use of low-cost polymers for the top and bottom surfaces of the photovoltaic cell. To improve efficiency, Konarka is working with fellow start-up Evident Technologies to introduce quantum-dot structures into the active region. This will allow the absorption of incident photons over a wider range of the solar spectrum. The companies claim that this approach will yield low-cost solar cells that offer better performance than today’s best silicon-based versions.

Konarka is building on technology originally developed for the US Army, which believes that lightweight DSCs could replace the heavy and bulky batteries that soldiers must carry to power increasingly sophisticated electronics equipment. As a result, the company is one of the main beneficiaries of a $6 m R&D programme launched in February by the Defense Advanced Research Projects Agency (DARPA), which ultimately aims to demonstrate a photovoltaic device with an efficiency of 50%.

Konarka is also one of several US start-ups to have attracted venture capitalist funding for the development of nanostructured solar cells (see table). “Many VCs have now hired specialists to assess both the short-term and long-term viability of emerging energy technologies,” says the report. “They will be pressing start-ups to demonstrate low-cost manufacturing processes as well as high-performance nanostructured designs.”

Nanomaterials for Next-Generation Energy Devices evaluates the potential of nanostructured materials in the development of photovoltaics, fuel cells, hydrogen-storage systems, supercapacitors and rechargeable batteries. For more information see