Surface Plasmons
An illustration of how surface plasmon resonances
on metal nanoparticles scatter incident light into guided modes
of a thin semiconductor layer.
Excitation of metallic structures can lead to oscillation
of the conduction electrons, known as surface plasmons. Surface
plasmons roughly on the scale of the wavelength of the light can
improve the absorption and emission of light from thin planar semiconductor
layers by coupling the light with the waveguide modes of the semiconductor
layer. This provides an alternative way of providing light-trapping
for very thin silicon films that are too thin for conventional
light-trapping structures. Enhancing absorption using surface plasmons
also avoids the increase in surface recombination that occurs with
conventional light trapping methods due to the increased surface
area of the semiconductor layer. Another application is extraction
of light from light emitting diodes. In a light-emitting diode,
internally generated light is trapped in the semiconductor layer
by total internal reflection, and the planar semiconductor layer
acts as a waveguide. Surface plasmons can couple the emitted light
out of the waveguide before it is re-absorbed, potentially increasing
the efficiency.
Both metallic nanoparticles and fluorescing molecules
can be described classically by an electric dipole moment. Thus
the preferential radiation of a dipole into a semiconductor waveguide,
that allows metal nanoparticles to enhance absorption in semiconductor
layers, can also occur with a range of other types of dipoles in
ways that may be beneficial for photovoltaics. Such dipoles could
include, for example, dye molecules used for up or down conversion
of the energy of incident photons to an energy more suitable for
use by a photovoltaic cell. Using thin layers of dye molecules
radiating into the semiconductor would avoid the need for refractive
index matching of the frequency conversion material and the semiconductor.
Modelling has been performed which has shown that
the power emitted by a dipole (used to model a metal nanoparticle)
into a thin silicon waveguide can be a large fraction of the dipole’s
power. The fraction of power radiated into the waveguide was calculated
to be 75% for a 1µm thick waveguide and up to 86% light emitted
for a 160-170nm waveguide, for light of wavelength 850nm. At longer
wavelengths the fraction was even higher – up to 91% for
1100nm light and a 220nm thick waveguide. In addition to enhancing
the absorption in thin silicon layers, metal nanoparticles could
also be used to enhance absorption in other thin layers used in
novel photovoltaic structures, for example quantum wells or quantum
dots.
An evaporation technique has been developed to provide
a simple way of fabricating random arrays of metal nanoparticles.
Work is underway on the development of fabrication techniques for
small and large areas of ordered nanoparticle arrays. |
