In his review article published in Nature Photonics, Cesar Clavero, a researcher in the Environmental Energy Technologies Division, examines plasmonic energy conversion, a phenomenon that has only been known about for a few years. Clavero examines the speculation that plasmonic energy conversion could be harnessed in a new generation of photovoltaic materials that could be far more efficient at converting solar energy into electricity than what's currently in the marketplace.

What is plasmonic energy conversion?
In plasmonic energy conversion, light from the sun, in the form of photons, are trapped in plasmonic nanostructures on the surface of a specially designed thin film. The photons of light of certain wavelengths form "surface plasmons" within these nanostructures.

Some of the time the light is just re-emitted as photons and radiated back to space. However, at other times, in a non-radiative process, the energy captured in the surface plasmons can be transferred to "hot electrons" and injected into a semiconductor to form an electric current. It has only been under a decade or so that researchers have thought this process could be harnessed into a more efficient way of generating electricity from solar energy.

Various research teams have observed this process taking place in particles of silver or gold deposited in tube nanostructures of titanium dioxide, however the use of other materials such as conducting oxides would extend the range of applicability of this technology

What's the difference between this process and how an electric current is generated in conventional photovoltaic panels on the market today?
In a conventional PV panel, photons in the sunlight that have high enough energy are absorbed by electrons in the semiconductor film that forms the photovoltaic panel.

The process forms an "electron-hole pair." The electrons become mobile, resulting in the electric current, and the positively charged "holes" in the lattice of the semiconductor material maintain the overall charge balance of the material. This process has a theoretical maximum energy conversion efficiency that cannot be exceeded by simple improvements to material.

Why is plasmonic energy conversion promising as a method of achieving higher efficiency of energy capture than in conventional semiconductor-based PV materials?
The physics of the plasmonic energy conversion process is fundamentally different from that of the photoelectric effect that generates current in the conventional PV panels.

In plasmonic energy conversion, the process takes place on nanostructures, at the nanoscale. A surface plasmon-based photovoltaic material would be much thinner-instead of micrometers thick, it would be nanometers thick. This opens the possibility of PV panels with coatings that are much thinner and therefore considerably less expensive to manufacture than today's panels, yet much more efficient at trapping energy.

Also, in the review article in Nature Photonics, I suggest that a wide range of metal oxides could use plasmonic energy conversion to capture energy from a broader range of wavelengths of the solar spectrum than are currently captured by conventional PV devices. Capturing energy across the whole solar spectrum-visible and infrared light helps increase the efficiency of these devices.

What are the barriers to exploiting plasmonic energy conversion in solar photovoltaic devices?
The field is in its infancy and there is much we don't know about what materials are best at generating hot electrons from the solar spectrum, how to build and optimize nanostructures for maximum efficiency, and so on.

But there are great opportunities for Berkeley Lab to explore a groundbreaking new field that could lead to the fabrication of much more efficient, cheaper solar PV devices. This research direction has the potential to cause a great leap in the use of solar photovoltaic technology to generate electricity.

What do you think are the next steps to advance this field?
A great window of opportunity has opened in the field of plasmonic energy conversion.

The use of new plasmonic materials such as semiconductors and conducting oxides, combined with new architectures such as multijunction plasmonic solar cells, will allow us to further push the energy conversion limits while keeping low fabrication costs. Also, fundamental studies shining light onto the hot-electron generation, injection and regeneration processes will be key to advance this field.