Light Trapping with Nanosphere Arrays

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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.

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.

Reference:

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.