olar cells can either be made from inorganic compounds, which perform well but are costly to fabricate, or polymeric organic compounds, which are cheap but have poor energy efficiencies. Conventional inorganic solar cells can routinely convert 10% of the light falling on them into electrical energy, and some can even reach as high as 30%, but they require complex processing methods like those used to manufacture computer chips. On the other hand, organic polymers are not limited in this way, but they struggle to reach efficiencies of 2%.

Inorganic materials perform better because they can transport charges to the electrodes more quickly, thereby reducing the losses that accrue from a recombination of negative electrons and positive holes. In contrast, electrons move relatively slowly down a conjugated polymer, and along the way they are prone to meet up with destructive entities such as oxygen. One answer is to blend into the polymer small molecules with conjugated systems; these allow electrons to find quicker routes to the desired destination. Even so, they may still guide the electron to a dead end.

Paper #3 shows how it is possible to combine the two types of semiconductor, organic and inorganic, to produce a hybrid material which is easy and cheap to make and has high efficiency. This milestone paper comes from the group of Paul Alivisatos of the Department of Chemistry at the University of California, Berkeley.

The inorganic components are nanorod-sized particles of cadmium selenide (CdSe) and the organic ones are fibers of the conjugated polymer poly(3-hexylthiophene) (P3HT), in which the backbone of the polymer consists of directly bonded thiophene units with each molecule having a hexyl group attached. The best energy conversion was achieved with nanorods having radii of 7 nm and lengths of 600 nm, and it is these which act as the electron transport material, while the P3HT does the same for the positive charges. What also adds to the energy-gathering efficiency of CdSe and P3HT is that they have complementary absorption spectra in the visible spectrum, which means they can gather photons over the wavelength range of 300 to 720 nm, thereby covering most of the solar spectrum.

The solar cell itself consists of a film of the material sandwiched between an aluminum electrode and a transparent conducting electrode. This latter is made of another conducting polymer, poly(ethylene dioxythiophene) doped with polystyrene sulfonic acid, mounted on an indium tin oxide glass substrate. The key to its efficiency lies with the nanorods of CdSe which comprise 20% of the composite. What the Berkeley group needed was a solvent that would permit the mixing of the P3HT polymer and the CdSe nanorods, and this turned out to be a mixture of pyridine and chloroform. From such a solution they were able to spin-cast a uniform flexible film at room temperature.

Commercial applications are now on the horizon. The inventions underlying the work have been licensed by Nanosys Inc., which is based in Palo Alto, California, and they have a team working on the design of solar cells. They also have a cooperative agreement with Matsushita Electric Works. Meanwhile, Alivisatos continues to explore: "We have not yet exhausted the possibilities that arise for solar cells when organic and inorganic nanoscale components are combined," he tells Science Watch. " Presently we are working on some new designs that better organize the pathways for charge migration within the devices, using tetrapods instead of rods, and using liquid crystals." It remains to be seen whether these will improve on current efficiencies, but there is every reason to expect that CdSe/P3HT itself will transform the solar power industry.

Readers wanting more information about the CdSe/P3HT system should consult other papers from the Berkeley group, such as that in Physical Review B (see W.U. Huynh, et al., 67[11]: art. no. 115326, 2003), which is on charge transport within CdSe/P3HT photovoltaic cells, and that in Advanced Functional Materials, (see W.U. Huynh, et al., 13[1]: 73-79, 2003), which is about controlling the morphology of the nanocrystal-polymer composites.

Dr. John Emsley is based at the Department of Chemistry,
Cambridge University, U.K.