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July 2012

Gold Shines in New Solar Cell Design

Tiny gold particles improve efficiency of thin solar cells

Bryce Sadtler

Gold nanoparticles increase the photocurrent produced in plasmon-enhanced solar cells. The maximum increase in current occurs at a wavelength of light of 613 nanometers near the plasmon resonance of the gold nanoparticles. The inset shows a schematic of the DSSC, where a thin layer of the ruthenium dye (red) covers the titanium dioxide particles (grey) and the gold nanoparticles (yellow) are embedded within the pores of the titanium dioxide particles.  The solar cell is built on a conductive glass substrate called ITO (blue) to allow light to enter the cell.

Researchers at the Center for Energy Nanoscience, or CEN, have demonstrated dramatically increased efficiency in thin solar cells by using small gold particles to concentrate light into the cell. New types of solar cells that leverage cheaper materials and fabrication methods could make solar energy more affordable. However, most of these new designs do not yet show high enough efficiency to compete with conventional silicon-based solar technology. One strategy to decrease cost but keep efficiency high is to make the solar cell material thinner, and concentrate sunlight into the cell. 

The thickness of a conventional silicon solar cell is balanced so that it is thick enough to absorb as much sunlight as possible but thin enough for the electrical charge to be collected efficiently to generate electricity. Light absorption excites electrons into a more mobile state. The excited electrons travel through the thickness of the silicon where they are collected by electrical contacts to produce a light-generated current, or photocurrent. A measure of the efficiency of a solar cell is the fraction of the incoming light that is converted into photocurrent.

Dye-sensitized solar cells, or DSSCs, are one new type of solar cell that can be made both thin and from cheap materials. However, there are challenges in making high-efficiency DSSCs. In a DSSC, a ruthenium-based dye absorbs the light, exciting electrons within the dye. The excited electrons must then transfer from the dye to a porous film of titanium dioxide particles, which carry the electrical charge to produce photocurrent. Both incomplete light absorption by the ruthenium dye and poor transfer of electrons from the dye to the titanium dioxide lower the efficiency of the DSSC. The CEN researchers showed that they could increase the photocurrent produced by the DSSC by taking advantage of a property of small gold particles called a "plasmon resonance."

Plasmons arise from the interaction of electromagnetic waves, such as visible light, with small metal particles with sizes ranging from 5 to 100 nanometers in diameter. To provide a sense of scale, the thickness of one sheet of paper is about 100,000 nanometers. The excitation of electrons by light at a particular frequency results in resonant oscillations of the free electrons, such that they collectively move back and forth within the metal nanoparticle. The process is similar to the standing waves created in a trough of water when the right pressure is applied across the water’s surface. Because the oscillation of electrons within the metal particle create high electromagnetic field intensities near its surface, the plasmonic particle is very effective at collecting light and focusing it into a small region, much like an antenna.

Professor Stephen Cronin of the University of Southern California, who led the team of researchers, says, "One of the main problems limiting the efficiencies of DSSCs is related to the very small thickness of the dye monolayer.  The plasmonic nanoparticles couple light very effectively from the far field to the near field at the absorbing dye molecule monolayer, thereby increasing the local electron-hole pair, or exciton, generation rate significantly."

The CEN scientists embedded gold nanoparticles within the porous titanium oxide layer of the DSSC and found that the photocurrent doubled compared to a similar cell made without the gold particles. The photocurrent was measured as a function of the wavelength of light, and the team found that the largest current enhancement occurred near the plasmon resonance frequency of the gold nanoparticles. This result indicates that the plasmonic gold particles enhance light absorption in the ruthenium dye to create more excited electrons and increase photocurrent. While the observed efficiencies in the plasmon-enhanced DSSCs are still not comparable to that of silicon solar cells, further optimization may improve their efficiency. 

Plasmon absorption enhancement in solar energy materials is also being pursued by other Energy Frontier Research Centers. Scientists at the Light-Material Interactions in Energy Conversion Center have shown that a nanostructured silver film can both serve as the electrical contact to collect excited electrons and lead to increased photocurrent in thin silicon solar cells. Researchers at the Center on Nanostructuring for Efficient Energy Conversion have demonstrated enhanced photocurrent in iron-oxide-based devices containing embedded gold nanoparticles. Thus, plasmonic absorption enhancement could be a powerful strategy for achieving high-efficiency, low-cost solar energy devices.

More Information

Hou W, P Pavaskar, Z Liu, J Theiss, M Aykol, and SB Cronin. 2011. “Plasmon Resonant Enhancement of Dye Sensitized Solar Cells.” Energy and Environmental Science 4(11):4650-4655. DOI: 10.1039/C1EE02120F

Deceglie MG, VE Ferry, AP Alivisatos, HA Atwater. 2012. "Design of Nanosctructured Solar Cells Using Coupled Optical and Electrical Modeling." Nano Letters 12(6):2894-2900. DOI: 10.1021/nl300483y

Thomann I, BA Pinaud, Z Chen, BM Clemens, TF Jaramillo, ML Brongersma. 2011. "Plasmon Enhanced Solar-to-Fuel Energy Conversion." Nano Letters 11(8): 3440-3446. DOI: 10.1021/nl201908s
 

Acknowledgments

This research was supported by the Center for Energy Nanoscience, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Basic Energy Sciences.  

About the author(s):

  • Bryce Sadtler is a Beckman Postdoctoral Scholar at the California Institute of Technology and a member of the Light-Material Interactions in Energy Conversion, an Energy Frontier Research Center. His research interests are in light-driven processes for directing the morphology of inorganic structures and the design of nanoscale materials for energy conversion and storage.

Curing Laziness in Solar Cells

Adding tiny particles of gold boosts the ability to capture sunlight and create electricity

In the refined solar cell, ruthenium-based dye (red) covers titanium dioxide particles (grey) and gold nanoparticles (yellow) are embedded within the porous titanium dioxide particles. The refined cell creates 240 percent more electricity with the same amount of light than the same cell without the gold particles.

With cheaper materials and manufacturing, dye-sensitized solar cells are an attractive way to turn sunlight into electricity. However, the cells are inefficient, with little improvement over the last two decades. The very best dye-sensitized solar cells turn about 11 percent of the light they capture into electricity, while traditional silicon-based cells convert about 20 percent. Scientists refined the cell by adding gold particles. When visible light hits the particles, they are able to very efficiently collect the light and concentrate it within the solar cell. This results in more electrical current being generated by the solar cell. The refined cell creates 240 percent more electricity with the same amount of light than a similar cell but made without the gold particles. This discovery could pry open the doors for more affordable solar technologies. The Center for Energy Nanoscience, led by the University of Southern California, conducted the research.

More Information

Hou W, P Pavaskar, Z Liu, J Theiss, M Aykol, and SB Cronin. 2011. “Plasmon Resonant Enhancement of Dye Sensitized Solar Cells.” Energy and Environmental Science 4(11):4650-4655. DOI: 10.1039/C1EE02120F

Deceglie MG, VE Ferry, AP Alivisatos, HA Atwater. 2012. "Design of Nanosctructured Solar Cells Using Coupled Optical and Electrical Modeling." Nano Letters 12(6):2894-2900. DOI: 10.1021/nl300483y

Thomann I, BA Pinaud, Z Chen, BM Clemens, TF Jaramillo, ML Brongersma. 2011. "Plasmon Enhanced Solar-to-Fuel Energy Conversion." Nano Letters 11(8): 3440-3446. DOI: 10.1021/nl201908s
 

Disclaimer: The opinions in this newsletter are those of the individual authors and do not represent the views or position of the Department of Energy.