With an increased focus on alternative sources of cheap, abundant, clean energy, solar cells are receiving lots of attention. Researchers are now on the brink of improving the efficiency of solar cells through nanowires..

The dye sensitized solar cell (DSSC) is one of the most important developments in photovoltaics in the last two decades.

Excitonic solar cells, such as organic, hybrid organic and inorganic solar cells are promising devices for inexpensive, large-scale solar energy conversion. DSSCs are an exciting variant of the most efficient and stable of the excitonic photovoltaic devices.

Untreated TiO2 absorbs light only in the UV region, but when the surface becomes modified with dye molecules, these can absorb light in the visible range and then transfer the excited electron to the particle. Back in 1991, Graetzel et. al came up with the methodology to dye-sensitize colloidal TiO2 film as a way to fabricate low-cost, high-efficiency solar cells (link to article).

Central to today’s DSSCs is a thick titanium dioxide (TiO2) nanoparticle film that provides a large surface area for the adsorption of light-harvesting molecules. One drawback of nanoparticle DSSCs is their reliance on trap-limited diffusion for electron transport, a slow mechanism that can limit device efficiency, especially at longer wavelengths.

To improve electron transport in these solar cells, while maintaining the high surface area needed for dye adsorption, two researchers have designed alternate semiconductor morphologies, such as arrays of nanowires and a combination of nanowires and nanoparticles, to provide a direct path to the electrode via the semiconductor conduction band. Such structures, that increases the rate of electron transport, may provide a means to improve the quantum efficiency of DSSCs in the red region of the spectrum, where their performance is currently limited.
The paper is titled “Dye-sensitized solar cells based on semiconductor morphologies with ZnO nanowires” and will be published in the March 23, 2006 edition of Solar Energy Materials and Solar Cells.

Professor Eray Aydil from the University of Minnesota, co-author of the paper together with Jason Baxter from the UC Santa Barbara, explained to Nanowerk:

“Detailed research into the way DSSCs work has shown that transport and recombination of electrons in the nanoparticle TiO2 network are coupled. Researchers from NREL have shown that this interdependency may be due to transport limited recombination; that is any increase in transport rates also result in an increase in the recombination rate with no net change in the cell performance.”
“However, the use of single crystal nanowires may allow electron transport via extended states in the conduction band rather than by a series of hops between trap states” says Aydil. Thus the interdependency between the transport and recombination may be removed in the case of nanowire DSSCs.

In addition, if nanowires achieve electron transport rates that are significantly faster than transport rates in nanoparticle films, significant flexibility in choosing the hole-transport medium could be gained since faster recombination rates could be tolerated.
“One could also make thicker films and increase optical density in the regions of the solar spectrum where the dye absorption decreases” says Aydil. “These possibilities are the driving force behind pursuing nanowire-based dye sensitized solar cells.”
ZnO has nearly the same band gap and electron affinity as TiO2, making it a possible candidate as an effective DSSC semiconductor. While little work has been done on large-scale, template free growth of TiO2 nanowires, ZnO can readily be grown in a variety of morphologies and by several different processing methods.

By Michael Berger, Copyright 2006 Nanowerk LLC

Gold nanowires 20 nm thick and 200 nm to 2000 nm in length with absorptions from the near to mid-IR will improve solar cell efficiencies, optics, and nanoelectronics.

A revolutionary new gold nanoparticle manufacturing method has been developed that results in gold nanowires that are upwards of 2 microns in length and only 20 nm in diameter. These ultra-long devices exhibit tremendous photothermal properties, converting up to 90% of incident light energy to heat. Their tunable optical absorption range is from 1 to 10 microns.

Nanowires are an extension of the technology currently employed by Nanopartz™ for nanorods. Gold nanorods have recently found huge successes in cancer therapy. Gold nanorods are also used for blood testing in diagnostics; as optical contrast agents in imaging; in material science, optics, negative refractive index materials such as the “Harry Potter Cloak;” and for improving the density of optical data storage in compact disks.

With tunable absorptions in the near to mid-IR, solar cell manufacturers can use these devices to improve the efficiencies of their devices since current devices do not absorb well at these wavelengths. Their shapes lend themselves to be a component in better optical devices like polarizers, filters, and negative refractive index materials. Many scientists have employed these devices as wires in nanocircuitry.
Nanopartz™ is also experts in conjugating surface coatings to these gold nanoparticles. These conjugation provide the mechanical linkage necessary for many of these applications.

These devices are now available for evaluation at pre-production quantities. “We are very excited at the potential applications of these gold nanoparticles,” said Christian Schoen, President of Nanopartz™.