Nanostructures for Improving the Spectral Response of Photovoltaic Cells

A. Shoji Hall, M. Faryad, G.D. Barber, L. Liu, M. Solano, T.S. Mayer, A. Lakhtakia, P. Monk, T.E. Mallouk
The Pennsylvania State University, US

Keywords: solar, photovolatic


Solar energy conversion devices (photovoltaic cells and solar fuel systems) must be both efficient and inexpensive in order to be competitive with electric power or fuels derived from fossil resources. In order to meet this challenge, much effort has been devoted to improving light absorption. The strategies explored include texturing the front and/or back surfaces of photovoltaics to trap more photons, anti-reflection coatings, and device configurations that concentrate sunlight. Recently, interest has been growing in using both dielectric and metallic nanostructures for light trapping in photovoltaics and liquid-junction solar cells. We showed several years ago that photonic crystals - periodic arrays of high dielectric spheres or inverse opal structures made by colloidal assembly and replication - could enhance the spectral response of dye-sensitized sensitized solar cells. In addition to acting as dielectric mirror back reflectors, photonic crystals can be coupled directly to the active layer of these devices. In the latter configuration, the photonic crystal extends the response of the cell into the red by slowing the propagation of light in that spectral region. The same effect extends the spectral response of photocatalysts fabricated as inverse opals. We have also recently demonstrated improved spectral utilization in spectrum-splitting photovoltaic cells made from dye-sensitized TiO2, which absorbs well in the visible, and single crystal Si, which is most efficient in the near-IR. Sub-wavelength periodic texturing of the backing metallic layer of a thin-film solar cell can be used to launch surface plasmon-polariton (SPP) waves, to create a high electric field in the region of the semiconductor close to the metal. This effect however leads to modest gains in light harvesting because gratings interfaced to uniform dielectrics couple only one linear polarization of incident light, and because the enhanced electric field extends only a short distance into the semiconductor. We have recently proposed from modeling studies that a grating interfaced to a dielectric with a periodically varying refractive index (i.e., a photonic crystal) should support multiple SPP modes, which should be excited by incident light of both sand p-polarization states. This finding has now been verified experimentally by measuring the reflectivity of such structures. With appropriate grating design we observe surprisingly strong coupling of s- as well as p-polarized incident light over a wide range of angles and free space wavelengths. This talk will discuss strategies for incorporating these new metallodielectric nanostructures into thin film solar cells.