Plasmonic device converts light into electricity

Nov 09, 2011 by Lisa Zyga feature
Surface plasmons on the top electrode in the MIM device can increase the current from the top electrode so that it is greater than the current from the bottom electrode, generating a positive net current. Image credit: Wang and Melosh. ©2011 American Chemical Society

(PhysOrg.com) -- While the most common device for converting light into electricity may be photovoltaic (PV) solar cells, a variety of other devices can perform the same light-to-electricity conversion, such as solar-thermal collectors and rectennas. In a new study, engineers have designed a new device that can convert light of infrared (IR) and visible wavelengths into direct current by using surface plasmon excitations in a simple metal-insulator-metal (MIM) device.

The researchers, Fuming Wang and Nicholas A. Melosh of Stanford University, have published their study on the new device in a recent issue of .

“The greatest significance thus far is to show an alternative method to rectennas and PV devices for IR and visible conversion,” Melosh told PhysOrg.com. “The conversion efficiencies aren't amazingly high compared to a PV in visible, so it’s not going to replace PVs, but it could be used for energy scavenging later on.”

The new device’s MIM architecture is similar to that of a rectenna. However, whereas rectennas operate with long-wavelength light such as microwaves and radio waves, the new device operates with a broad spectrum of infrared to .

When the MIM device is illuminated, incoming photons are absorbed by the top and bottom metal electrodes. Upon absorption, each photon excites an electron in the metal into a higher energy state so that it becomes a “hot electron.” About half of the hot electrons travel toward the metal-insulator interface, where they may be collected by the other electrode. However, photon absorption in the upper and lower electrodes generates currents with opposite signs, so a net DC current is achieved only if the absorption is larger at one electrode than the other.

Electron transmission in MIM devices (a) with and (b) without surface plasmon excitations. (c) The measured photocurrent in a device with surface plasmons (black line) is higher than in a device without them (red line). Image credit: Wang and Melosh. ©2011 American Chemical Society

This ability to maximize current from one electrode while minimizing it from the other is one of the biggest challenges for MIM devices. To do this, researchers can change the thicknesses of the electrodes. However, there is a tradeoff, since in a thicker electrode, more photons are absorbed but fewer electrons reach the interface due to increased scattering.

Wang and Melosh’s solution is to use a prism to excite surface plamons (SPs) on the metal surface of the electrodes when under illumination. The SPs, which are small electron oscillations, can create a higher concentration of hot electrons in one electrode by efficiently coupling to light. The SP coupling efficiency depends on several factors, such as the thickness of the electrode, the type of metal used, and the wavelength of incoming light.

“SPs are excited by incident light when the photon and SP wave vectors match with each other,” Wang said. “For actual applications, it's more realistic to use nano-grating patterns on one electrode to excite SPs. By simply controlling the pitches of these gratings, SPs can be excited at any specific wavelength. As a result, energy conversion efficiency could be enhanced in the optical band from infrared to visible.”

The engineers calculated that these SP-enhanced MIM devices made with silver electrodes can achieve a power conversion efficiency as high as 4.3% for light with a 640-nm wavelength. Devices with gold electrodes have a maximum efficiency of 3.5% for light with a 780-nm wavelength. Both devices also have good theoretical efficiency across the entire solar spectrum – up to 2.7% for the silver-electrode device. The engineers also calculated that SPs can make silver devices almost 40 times more efficient than without the SPs for infrared light.

In addition, the researchers fabricated a gold-alumina-gold device, with the top gold layer being slightly thicker than the bottom gold layer. Their experiments confirmed that light hitting the top layer excites SPs on the surface, which cause more to be transmitted from the top to the bottom electrode.

Although the resulting photocurrent that the researchers measured was smaller than the theoretical calculated value, they hope to increase the photocurrent in the future by using more effective coupling methods for SPs, optimizing metal thicknesses, and other strategies. Ultimately, the device could prove useful due to the wavelengths at which it operates.

“It can work in the IR better [than other devices that convert light into DC], which can be used for energy scavenging,” Melosh said.

The devices other advantages include easy fabrication and the possibility for being realized on flexible substrates.


From now on, you can follow Physorg on Google+ too!

Explore further: Study reveals new characteristics of complex oxide surfaces

More information: Fuming Wang and Nicholas A. Melosh. “Plasmonic Energy Collection through Hot Carrier Extraction.” Nano Letters, DOI: 10.1021/nl203196z

4.6 /5 (16 votes)

Related Stories

Device can heat home, save money

Apr 19, 2011

(PhysOrg.com) -- A new polymer-based solar-thermal device is the first to generate power from both heat and visible sunlight – an advance that could shave the cost of heating a home by as much as 40 percent.

Plugging in Molecular Wires

Feb 11, 2009

(PhysOrg.com) -- Plants, algae, and cyanobacteria (blue-green algae) are masters of everything to do with solar energy because they are able to almost completely transform captured sunlight into chemical energy. This is in ...

Recommended for you

Tough foam from tiny sheets

16 hours ago

Tough, ultralight foam of atom-thick sheets can be made to any size and shape through a chemical process invented at Rice University.

Graphene surfaces on photonic racetracks

Jul 28, 2014

In an article published in Optics Express, scientists from The University of Manchester describe how graphene can be wrapped around a silicon wire, or waveguide, and modify the transmission of light through it.

Simulating the invisible

Jul 28, 2014

Panagiotis Grammatikopoulos in the OIST Nanoparticles by Design Unit simulates the interactions of particles that are too small to see, and too complicated to visualize. In order to study the particles' behavior, he uses ...

Building 'invisible' materials with light

Jul 28, 2014

A new method of building materials using light, developed by researchers at the University of Cambridge, could one day enable technologies that are often considered the realm of science fiction, such as invisibility ...

User comments : 3

Adjust slider to filter visible comments by rank

Display comments: newest first

ashraf fazili
not rated yet Nov 09, 2011
It appears to be a reverseable process. So far we were getting light from electrisity, now we shall be getting electricity from the light.
Cave_Man
2 / 5 (2) Nov 10, 2011
This sounds idea for space, you can set up a power plant close to the sun where the concentration of photons is higher.

I didn't read the whole article though, I just really like space.
Cave_Man
not rated yet Nov 15, 2011
After reading the whole article I think the only future of this WILL be direct solar energy collection. Given the low efficiency of the current material tech, and the fact that I don't know what the hell a plasmon is nor what excitation entails I believe this line of research could see interesting applications in FUTURE energy production.