Light absorption enhancement in thin-film solar cells using whispering gallery modes in dielectric nanospheres.

DOI: 10.1002/adma.201004393 Dr. J. Grandidier , D. M. Callahan , Dr. J. N. Munday , Prof. H. A. Atwater Thomas J. Watson Laboratories of Applied Physics California Institute of Technology Pasadena, CA 91125, USA E-mail: [email protected] For thin-fi lm solar cells, light absorption is usually proportional to the fi lm thickness. However, if freely propagating sunlight can be transformed into a guided mode, [ 1 ] the optical path length signifi cantly increases and results in enhanced light absorption within the cell. [ 2 ] We propose here a light absorber based on coupling from a periodic arrangement of resonant dielectric nanospheres. It is shown that whispering gallery modes in the spheres can be coupled into particular modes of the solar cell and signifi cantly enhance its effi ciency. We numerically demonstrate this enhancement using full-fi eld fi nite difference time-domain (FDTD) simulations of a nanosphere array above a typical thin-fi lm amorphous silicon (a-Si) solar cell structure. The in-coupling element in this design is advantageous over other schemes as it is composed of a lossless material, and its spherical symmetry naturally accepts large angles of incidence. Also, the array can be fabricated using simple, well-developed methods of self assembly and is easily scalable without the need for lithography or patterning. This concept can be easily extended to many other thin-fi lm solar cell materials to enhance photocurrent and angular sensitivity. Thin-fi lm photovoltaics offer the potential for a signifi cant cost reduction [ 3 ] compared to traditional, or fi rst generation, photovoltaics usually at the expense of high effi ciency. This is achieved mainly by the use of amorphous or polycrystalline optoelectronic materials for the active region of the device, for example, a-Si. The resulting carrier collection effi ciencies, operating voltages, and fi ll factors are typically lower than those for single-crystal cells, which reduce the overall cell effi ciency. There is thus great interest in using thinner active layers combined with advanced light trapping schemes to minimize these problems and maximize effi ciency. A number of light trapping schemes have been proposed and demonstrated including the use of plasmonic gratings, [ 4 , 5 ]