New microscopically thin solar power cells could even be worn on clothing

US scientists have developed a new polymer-based solar power cell that uses nanotechnology to generate electricity, and which could be painted onto any surface and used to run low-power devices.

The cells, developed by researchers from the University of California Berkeley, use semiconductor technology and consist of nanorods dispersed in an organic polymer or plastic, in a layer only 200 nanometres thick. This is sandwiched between two electrode layers, with each layer applied to a surface in separate coats, making production relatively easy, say the researchers.

Currently, the technology has an electrical efficiency of only 1.7%, but the researchers are confident that this is a good beginning.

“Our efficiency is not good enough yet by about a factor of 10, but this technology has the potential to do a lot better,” said Berkeley Professor of Chemistry A Paul Alivisatos. “There is a pretty clear path for us to take to make this perform much better.”

“The beauty of this is that you could put solar cells directly on plastic, which has unlimited flexibility,” said Janke J Dittmer, a co-researcher on the project. “This opens up all sorts of new applications, like putting solar cells on clothing to power LEDs, radios or small computer processors.”

Alivisatos and his colleagues have already pioneered the production of nanocrystals and nanorods – chemically pure clusters of between 100 and 100,000 atoms. Alivisatos’ solar power nanorods are made from cadmium selenide, a semiconducting material, unlike conventional semiconductor solar cells, which are made of polycrystalline silicon, or crystalline gallium arsenide, in the case of the most efficient cells.

The researchers manufactured their nanorods in a beaker containing cadmium selenide, aiming for a diameter of seven nanometres, in order to absorb as much sunlight as possible. They also aimed to achieve the maximum length possible, achieving 60 nanometres. The nanorods act like wires, absorbing light of a specific wavelength, generating an electron.

The nanorods are mixed with a plastic semiconductor, called P3HT (poly-3-hexylthiophene). “All solar cells using plastic semiconductors have been stuck at 2% efficiency, but we have that much at the beginning of our research,” said graduate student and co-researcher Wendy Huynh. “I think we can do so much better than plastic electronics.”

“The advantage of hybrid materials consisting of inorganic semiconductors and organic polymers is that potentially you get the best of both worlds,” said Dittmer. “Inorganic semiconductors offer excellent, well-established electronic properties and they are very well suited as solar cell materials. Polymers offer the advantage of solution processing at room temperature, which is cheaper and allows for using fully flexible substrates, such as plastics.”

Nevertheless, there is room for improvement. Such enhancements include packing the nanorods closer together, perpendicular to the electrodes, and using minimal polymer, or even none, so that electrons are transferred directly to the electrode from the nanorods. The researchers also hope to ‘tune’ the nanorods to absorb different colours to span the spectrum of sunlight. An eventual solar cell might have three layers, each made of nanorods that absorb at different wavelengths, they say.

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