Nanotechnology May One Day Revolutionize Solar Power

Imagine a future in which rooftops can be laminated with inexpensive, ultra-thin films of nano-sized semiconductors that will efficiently convert sunlight into electrical power and provide virtually all of our electricity.

This future is a step closer to being realized, say researchers at the U.S. Dept. of Energy’s Lawrence Berkeley National Laboratory. They have developed the first ultra-thin solar cells comprised entirely of inorganic nanocrystals and spin-cast from solution. These 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 and a fourth-year graduate student in UC Berkeley’s Department of Materials Science and Engineering.

In a recent paper, Gur, along with co-authors Paul Alivisatos, Neil Fromer and Michael Geier, describes a technique whereby rod-shaped nanometer-sized crystals of two semiconductors, cadmium-selenide (CdSe) and cadmium-telluride (CdTe), were synthesized separately and then dissolved in solution and spin-cast 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.

Most commercial solar cells today are made from silicon. Like many conventional semiconductors, silicon offers excellent, well-established electronic properties. However, the use of silicon or other conventional semiconductors in photovoltaic devices has to date been limited by the high cost of production—even the fabrication of the simplest semiconductor cell is a complex process that has to take place under exactly controlled conditions, such as high vacuum and temperatures between 400

Unlike the conventional semiconductor solar cells, in which an electrical current flows between layers of n-type and p-type semiconductor films, with 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.

According to the Energy Foundation, if the available residential and commercial rooftops in this country were to be coated with solar cell thin films, they could furnish an estimated 710,000 megawatts of electricity across the United States, which is more than three-quarters of all the electricity that this country is currently able to generate. Because of its favorable sunlight levels, California is considered a prime candidate for this technology.

For more information go to www.cchem.berkeley.edu/~pagrp/ .

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