Photocurrent Altered with Nanoparticles

Michael A. Greenwood

Sprinkling nanoparticles over the active face of photodetectors and solar cells could be a relatively easy and inexpensive way to enhance the performance of these devices.

But exactly what effect do these tiny particles have, and what size, material and geometry are needed to accomplish these improvements?

Researcher Naomi J. Halas and her colleagues at Rice University in Houston recently reported that even small amounts of nanoparticles distributed over the light-collecting face of a photodiode can significantly augment — or suppress — the photocurrent that is generated.

Photocurrent images in nanoamperes are shown for light incident at wavelengths of 532 (a), 633 (b), 785 (c) and 980 (d) nm for silicon photodiodes functionalized with gold nanoshells with a 96-nm core radius and a 20-nm shell thickness. Scale bar = 2 μm. Courtesy of Naomi J. Halas.

Halas’ team used silicon p⁺n photodiodes, to which they applied five nanoparticles that varied in size, geometry and plasmon resonance frequency. The group used solid silica nanospheres (radius = 60 nm); solid gold nanospheres (radius = 25 nm); and three sizes of gold-and-silicon nanoshells with radii of 62, 81 or 116 nm.

The researchers obtained a snapshot of the photocurrent generated by the nanoparticles by scanning them with a WiTec optical microscope with 532-, 633-, 785- and 980-nm-wavelength laser sources. The laser power was monitored with a Thorlabs photodiode, and a Stanford Research Systems photomultiplier tube measured the reflected light.

In the images, the nanoparticles appeared as bright spots if the scientists increased the photocurrent and as dark spots if they curbed the photocurrent at that spot.

The researchers found that silica nanospheres exhibit a consistent and uniform enhancement of the photocurrent over all of the wavelengths studied, which was attributed to the nonresonant scattering properties of these particles.

A maximum photocurrent enhancement of 20 percent was observed with the larger nanoshells at 980-nm excitation. Both small and large nanoshells suppressed the photocurrent at wavelengths of 532 and 633 nm and enhanced the photocurrent at 785 and 980 nm. The strongest photocurrent suppression (30 percent) was achieved with 116-nm nanoshells at a wavelength of 633 nm.

The researchers noted that, although the nanoparticles could be ulitized for long-wavelength sensitization of silicon photodetectors and for silicon-based solar cells, there is a trade-off, with a loss of photocurrent in the shorter wavelengths.

Nano Letters, February 2008, pp. 624-630.