Navy

Imprinting surfaces with carbon nanotubes for solar cell applications

Films which are surface imprinted with nanometer-sized particles to produce micro or nano-structured electron and hole collecting interfaces

Materials Energy

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Navy scientists have developed a process to imprint films with nanoparticles. The process may have application for the further development of organic solar cells as it can yield nano-structured electron and hole collecting interfaces.

At the crux of the layered cell is a uniform gap 26 of uniform width formed between a multiwalled carbon nanotubes (MWNT) stamp surface 44 and the surface of a polymerizable film layer 38. A sufficient amount of nanometer-sized particles including MWNTs 40 are provided to produce a charge separation interface.

The uniform gap is filled with a photoabsorbing conjugated polymer 46, which is a liquid, by using capillary action. This gap has a width of 60 and 100 nanometers, but constant throughout the length. The rationale for having a gap width of fewer than 100 nano-meters is that the photo-excited electrons of conjugated polymers can not diffuse very far before the photo-excited electrons recombine or are trapped. The diffusion distance for photo-excited electrons before the electrons recombine or become trapped is less than 100 nanometers. At distances greater than 100 nano-meters the photo-excited electrons absorb heat and do not generate electrical current. And when, for example, the width of the gap is well in excess of 100 nanometers, the photoexcited electrons within conjugated polymer are unable to bridge the distance between the carbon nanotubes and the titanium oxide 36. Overall, the current generation properties of a photovoltaic device 20 having a gap of uniform width which is less than 100 nanometers are substantially enhanced.

When the uniform gap is less than 60 nanometers there are not enough photo-excited electrons within the conjugated polymer to absorb a sufficient amount of light to generate electrical current.

This photovoltaic device includes a transparent electrode 34, which comprises a coating of at least one transparent metal oxide selected from the group consisting of SnO2:F, SnO2:In (ITO), and Au. Photoexcited electrons are designated 28.

The methods of imprinting further include compressing and thereafter, solidifying the MWNT stamp surface into the surface of the polymerizable layer.

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