Inorganic solar cells

Researchers at Berkeley Lab and the University of California, Berkeley, have developed what they claim are the first ultra-thin solar cells comprised entirely of inorganic nanocrystals that can be spun-cast from solution.

The dual nanocrystal solar cells are as cheap and easy to make as solar cells made from organic polymers and offer the added advantage of being stable in air because they contain no organic materials.

“Our colloidal inorganic nanocrystals share all of the primary advantages of organics — scalable and controlled synthesis, an ability to be processed in solution, and a decreased sensitivity to substitutional doping – while retaining the broadband absorption and superior transport properties of traditional photovoltaic semiconductors,” said Ilan Gur, a researcher in Berkeley Lab’s Materials Sciences Division.

In their labs, the researchers synthesised rod-shaped nanometre-sized crystals of two semiconductors, cadmium-selenide (CdSe) and cadmium-telluride (CdTe), then dissolved them in solution and spun-cast them onto a conductive glass substrate.

The resulting films, which were about 1,000 times thinner than a human hair, displayed efficiencies for converting sunlight to electricity of about 3%. This is comparable to the conversion efficiencies of the best organic solar cells, but still substantially lower than conventional silicon solar cell thin films.

“We obviously still have a long way to go in terms of energy conversion efficiency,” said Gur, “but our dual nanocrystal solar cells are ultra-thin and solution-processed, which means they retain the cost-reduction potential that has made organic cells so attractive vis-a-vis their conventional semiconductor counterparts.”

A solar cell that relies exclusively on colloidal nanocrystals has been anticipated theoretically in recent years. The US researchers have now demonstrated such a device and have presented a mechanism for its operation.

Unlike conventional semiconductor solar cells, in which an electrical current flows between layers of n-type and p-type semiconductor films, in these new inorganic nanocrystal solar cells, current flows due to a pair of molecules that serve as donors and receptors of electrical charges, also known as a donor-acceptor heterojunction. This is the same mechanism by which current flows in plastic solar cells.

The CdSe and CdTe films are electrical insulators in the dark but when exposed to sunlight undergo a dramatic rise in electrical conductivity, as much as three orders of magnitude. Sintering the nanocrystals was found to significantly enhance the performance of these films. And unlike plastic solar cells, whose performance deteriorates over time, ageing seems to improve their performance.

lan Gur is one of the researchers in Berkeley Lab’s Materials Sciences Division responsible for the development of the inexpensive process for mass-producing solar cell thin films.