Paper Explores Plasmonic Energy Conversion for Photovoltaics and Photocatalytics

While costs for some solar photovoltaics (PV) have dropped sharply over the past few years, the search for PV modules that are both low-cost and highly efficient continues. An important focus of that quest is on developing lower-cost, more-efficient methods of attaining electron-hole separation. Plasmonic energy conversion, which generates "hot" (highly energetic) electrons in plasmonic nanostructures through the electromagnetic decay of surface plasmons, is one promising solution. It offers potentially high conversion efficiencies (greater than 22 percent) while keeping fabrication costs low, given the right materials, architectures, and fabrication methods.

César Clavero, an Environmental Energy Technologies Division researcher at Lawrence Berkeley National Laboratory, recently surveyed the research on those topics, to determine the current state of the technology. The resulting paper, "Plasmon-induced hot electron generation in nanoparticle/metal oxide interfaces for photovoltaic and photocatalytic devices," was published in the 30 January 2014 issue of Nature Photonics.

The review, which covered fundamentals of hot electron generation, injection, and regeneration in plasmonic nanostructures, found that two key factors promote high conversion efficiencies: fast hot-electron injection before recombination and optimum carrier regeneration. The research also suggests that by combining multiple metals and conducting oxides, the devices will be able to generate electricity from more spectrums of solar radiation, thereby increasing electricity production.

Clavero found that material, size, and shape of the plasmonic nanostructures are the most important design factors affecting the localized surface plasmon resonance (LSPR) electron-generation processes. The literature also suggests that using plasmonic energy conversion could solve the problem of efficiency decreases at higher temperatures, which affects conventional PV cells, because the efficiency with plasmonic structures actually increases with temperature.

While titanium dioxide has been the most-used semiconductor for plasmonic energy conversion, Clavero suggests that the valence bands of zinc oxide, cerium oxide, and silver bromide could also make them efficient electron acceptors. Further studies will need to determine the most efficient semiconductor material.

This work was supported by the Department of Energy's Office of Energy Efficiency and Renewable Energy, Office of Building Technology.

A Q&A with Cesar Clavero on Plasmonic Energy Conversion

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.