UCSD: Nanowires can boost solar cell efficiency

May 23, 2008 – Researchers at the U. of California/San Diego say they have created solar cells “spiked” with nanowires that could show the way to improved efficiency in thin-flim solar cells. The “proof-of-concept” project, published earlier this year in Nano Letters, describes the device as a “photodiode” rather than a “photovoltaic” since they did not measure how efficiently the device converted sunlight to electricity.

The paper describes a carrier transport polymer hybrid system with indium phosphide (InP) nanowires grown directly onto a metal oxide — without using a special substrate, e.g. gold nanodrops — and enveloped with a conjugated polymer, poly(3-hexylthiophene) (P3HT). “In this paper we used ITO, but you can use other metals, including aluminum,” said Paul Yu, a professor of electrical engineering at UCSD’s Jacobs School of Engineering, in a statement.

Figure 1:Schematic of the nanowire-polymer hybrid device. Top yellow layer is the gold electrode that attracts the holes; blue gradient is the P3HT polymer material that absorbs the sunlight; the yellow InP nanowires are grown directly on the green metal substrate (ITO). (Source: UCSD)

Including nanowires in the experimental solar cell increased the forward bias current (a measure of electrical current) by six to seven orders of magnitude vs. a polymer-only control device, the engineers found. And providing a pathway for electrons to the electrode can reduce inefficiencies seen in polymer-mixtures with current solar cells, noted Clint Novotny, author of the paper (now at BAE Systems), in the statement. Growing the nanowires directly on the electrode will “improve device performance, reduce the problem of reproducibility of contacts to nanowires, and provide a method of nanowire growth that does not use expensive substrates such as Si or InP,” they say in the paper.

Figure 2:SEM of n-type InP nanowire growth on ITO taken at a 45&deg. Scale bar is 500nm. (Source: UCSD)

The team notes that the nanowires provide “a very large number of junctions throughout the entire polymer matrix,” which enhances the likelihood of exciton disassociation. Carriers created in the nanowires also can go directly to the electrode without carrier hopping, creating “a more efficient and defined pathway for carrier collection.”

“In effect, we used nanowires to extend an electrode into the polymer material,” said co-author Edward Yu, a professor of electrical engineering at UCSD’s Jacobs School of Engineering.

Photon-absorbing electrons in the polymer material split apart from the holes at the interface of the polymer and the nanowire (the p-n junction). The electrons and holes travel along opposite directions in the nanowires, until the holes hit the end and travel through a thin polymer layer to the electrode. There are other experimental PV designs being examined that include nanowires or carbon nanotubes, the UCSD scientists note in the statement, but not electrically connected to an electrode, and thus do not minimize electron-hole recombination by providing a direct path from the p-n junction to the electrode.

Having a more efficient method for getting electrons to their electrode means that researchers can make thin-film polymer solar cells that are a little bit thicker, and this could increase the amount of sunlight that the devices absorb.

Future work would be needed to improve performance (overcome loss mechanisms, e.g. limited efficiency due to low-value short circuit current), and improving the polymer deposition technique to allow for more uniform layers (drop-casting resulted in a large gap between the nanowire tips and gold electrode; dip-coating or dry etching may offer improvements). Further experiments also will look at improving the interface between the nanowires and polymer (a leakage source), and overall charge transfer between the materials.

Novotny noted work can look ahead to eventually “incorporating millions or billions of nanowires in a single device,” but noted such technology is “at least a decade away” from becoming mainstream.

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