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May 2011

Light Trapping with Nanosphere Arrays

Nanosphere coatings can boost the efficiency of thin film solar cells

Agustin Mihi

Art from the Advanced Materials cover representing the electric field intensity distribution in the nanospheres. Grandidier J, DM Callahan, JN Munday, and HA Atwater: Light absorption enhancement in thin-film solar cells using whispering gallery modes in dielectric nanospheres. Advanced Materials. 2011. Volume 23. Pages: 1272-1276. Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission.

A novel light-trapping scheme based on nanosphere coatings can boost the efficiency of thin-film solar cells, according to a research group at the Light-Material Interactions for Energy Conversion, an Energy Frontier Research Center. The work from the Atwater group at Caltech, which graced the cover of Advanced Materials, analyzes how incident light is concentrated in these beads and states under which conditions this light can be coupled into the active layer where it will be absorbed.

One of the major challenges in today’s photovoltaic research is producing highly efficient devices at low cost. To achieve this goal, one can use polycrystalline or amorphous materials, whose production costs are much lower than those of their single-crystal counterparts; however, these materials have poor electrical characteristics. Using thinner layers of these less expensive materials and combining them with advanced light-trapping schemes minimizes electrical problems and maximizes efficiency.

In their work, the researchers simulated the performance of a typical thin-film amorphous silicon solar cell structure with and without a nanosphere monolayer coating. An overall photocurrent enhancement was found due to the improved antireflective coating effect of the nanosphere array. Furthermore, certain wavelengths of light were transformed into guided modes in the semiconductor. For those frequencies the optical path lengths are significantly increased (light to spend longer times in the semiconductor) and lead to enhanced light absorption within the cell − higher than 100%. The fundaments behind this enhancement are resonances within the spheres, known as whispering gallery modes, that can significantly enhance its light-harvesting efficiency.

This concept has advantages over other absorption enhancement schemes because the coupling elements are lossless, meaning they do not introduce parasitic light-absorption-like metals. Also, this enhancement approach is easily fabricated, and their spherical geometry allows light to be efficiently coupled into the solar cell over a large range of incidence angles.

More Information

Grandidier J, DM Callahan, JN Munday, and HA Atwater. 2011. “Light absorption enhancement in thin-film solar cells using whispering gallery modes in dielectric nanospheres.” Advanced Materials 23, 1272-1276. DOI: 10.1002/adma.201004393.

Acknowledgments

This work was supported by the Department of Energy, Office of Science, Office of Basic Energy Sciences, through the Light-Material Interactions for Energy Conversion, which is composed of researchers from the California Institute for Technology, Berkley and the University of Illinois at Urbana-Champaign.

About the author(s):

  • Agustin Mihi is a Beckman Institute Postdoctoral Fellow at the University of Illinois at Urbana-Champaign. His research focuses on increasing the light harvested in thin-film solar cells by coupling photonic crystals. He is a member of the Braun Group and a participant of the Light-Material Interactions for Energy Conversion, an EFRC.

Small Particles Result in Big Efficiencies for Solar Cells

Layering nanospheres on top of thin silicon increases the amount of sunlight captured

Art from the Advanced Materials cover representing the electric field intensity distribution in the nanospheres. Grandidier J, DM Callahan, JN Munday, and HA Atwater: Light absorption enhancement in thin-film solar cells using whispering gallery modes in dielectric nanospheres. Advanced Materials. 2011. Volume 23. Pages: 1272-1276. Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission.

Cost vs. efficiency. It’s the epic battle when it comes to solar cells and turning sunlight into electricity. Many of the more affordable materials, which cost less than the traditional silicon, don’t capture as much sunlight, and hence produce less energy. Scientists turned the problem on its head, and instead of aiming at cost reduction in expensive solar technologies, they focused on boosting the efficiency of inexpensive solar cells. They found that coating very thin silicon films with a layer of tiny spheres allowed the material to absorb more sunlight. The easy-to-create coating does not ”steal” energy from the cell, so it can’t siphon off electrons and reduce the efficiency. Light-Material Interactions for Energy Conversion, a DOE Energy Frontier Research Center led by the California Institute for Technology, conducted this research.

More Information

Grandidier J, DM Callahan, JN Munday, and HA Atwater. 2011. “Light absorption enhancement in thin-film solar cells using whispering gallery modes in dielectric nanospheres.” Advanced Materials 23, 1272-1276. DOI: 10.1002/adma.201004393.

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.