Hybrid nanostructure with extreme light absorption looks promising for photovoltaics

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Posted on July 2, 2013
The core-shell hybrid nanowire consists of a metal core wrapped with semiconductor thin films. Three different resonances excited at different wavelengths are shown. (b) The fraction of absorbed above-band gap photons in the silicon shell for a wide variety of configurations. Credit: Mann and Garnett. ©2013 American Chemical Society

The core-shell hybrid nanowire consists of a metal core wrapped with semiconductor thin films. Three different resonances excited at different wavelengths are shown. (b) The fraction of absorbed above-band gap photons in the silicon shell for a wide variety of configurations. Credit: Mann and Garnett. ©2013 American Chemical Society

In photovoltaics, there is generally a trade-off in terms of semiconductor thickness, with thicker semiconductors offering better photon absorption and thinner ones offering higher charge carrier extraction efficiency. In recent years, scientists have begun investigating semiconductor nanowire solar cells, which tackle this tradeoff through morphology-dependent resonances that significantly enhance the absorption compared to a planar film.

Now, somewhat counterintuitively, scientists have theoretically found that thin semiconductor films wrapped around metal nanowires have substantially better light absorption properties than solid semiconducting nanowires, despite the fact that they use less semiconducting material. At the same time, the metal core acts as a contact to efficiently extract charge carriers. By confronting the semiconductor thickness trade-off and offering exceptional performance, the nanostructures might become ideal building blocks for inexpensive photovoltaic and solar fuel applications.

A paper on the new devices by Sander A. Mann and Erik C. Garnett at the Center for Nanophotonics at FOM Institute AMOLF in Amsterdam, The Netherlands, will be published in a future issue of Nano Letters.

“The greatest significance to our work is that we provide a design for nanowire building blocks that incorporates both excellent light trapping properties and a local metal electrode contact (for current extraction),” Garnett told Phys.org. “Silver nanowire networks have already been used as high performance transparent electrodes and we expect that by coating them with thin semiconducting shells we will be able to make high-efficiency solar cells using cheap materials. It has now been observed in a number of papers that nanostructuring a material can increase light absorption even while using less semiconductor material. However, this paper takes the next step and starts thinking about how to design such structures with integrated electrical contacts.”

Read more at: Phys.org