"We wanted to take advantage of solution-based nanocrystals in solar cells without the processing challenges and performance limitations of an organic phase," researcher Ilan Gur told nanotechweb.org. "The question thus naturally arose: can we make a donor-acceptor heterojunction cell simply using two types of nanocrystals? In the end, that's what we've made - a dual-nanocrystal solar cell."

The devices contained rod-shaped nanocrystals of cadmium selenide (CdSe) and cadmium telluride (CdTe). Spin-casting the nanocrystals from a filtered pyridine solution created thin films of the materials. Typically each layer was 100 nm thick, while the substrate was indium tin oxide glass coated with a 0.2 nm thick alumina layer. The scientists put down a CdTe film first, annealed it to remove residual solvent and then added a CdSe film.

The use of solution-processing techniques means that it would potentially be very low cost to manufacture such devices. The films are also flexible and suitable for deposition on "virtually any" substrate. The cells could combine the low-cost manufacture of organic solar cells with the broadband absorption, good transport properties and resistance to environmental degradation of solar cells based on inorganic semiconductors.

The researchers found that they could improve the properties of the devices by sintering the nanocrystals. They exposed the films to a solution of cadmium chloride in methanol and then annealed them at 400 °C in air. This process increased the photoconductivity of the films by about two orders of magnitude.

"We show that by sintering or fusing the nanocrystals together, we can produce cells with efficiencies approaching 3%," said Gur. "This is comparable to organic-based technologies, but these cells have the added advantage of being stable in air since they contain no organic components."

Conventional solar cells made from inorganic semiconductors contain a p-n junction between bulk-doped materials. Organic solar cells, on the other hand, contain a type II heterojunction and function by a donor-acceptor mechanism.

"Organic solar cells are very different from conventional systems - they rely on a different set of driving forces to extract electricity from the sun's energy," said Gur. "We've made a photovoltaic device from exclusively inorganic nanocrystals that appears to operate in the same way as the organic systems. This finding lends valuable insight into the operation and design of these donor-acceptor heterojunction solar cells."

Now the researchers plan to work on understanding the mechanism of operation of the cells and on increasing their efficiency.

The researchers reported their work in Science.