Graphene-on-SiC - ISOM

TECHNOLOGY SOLAR CELLS
Figure 2 : Light trapping in thick solar cells by total internal reflection of weakly absorbed light (a). Scanning electron microscope images of a plasmonic
nanoparticle array (b) and a diffraction grating (c) that are both fabricated on a solar cell. (d) shows how a plane wave front incident on a solar cell (left)
is modified by the presence of a plasmonic nanoparticle (center) and a dielectric gratig (right) on the rear surface of the solar cell
larger than the critical angle for total internal reflection at the
absorber-air interface). We work with substrate-based structures
that support a continuum of optical modes and feature either
quantum well or dot absorber layers. We employ two light
trapping approaches to enhance the absorption efficiency
of long wavelength light in our quantum well/dot solar cells:
plasmonic light trapping and the addition of a diffraction grating
on the surface of the solar cell.
Powerful plasmonics
Light trapping is possible using plasmonic nanoparticles – tiny
metallic structures that interact with the incoming light and
exhibit local oscillations in the density of their free electron
gas. One consequence of this phenomenon is that incoming
radiation is scatterred into the solar cell at angles outside the
escape cone.
Good light trapping with plasmonic structures is possible by
exceling in two areas: maximizing the scattering cross-section
of the plasmonic nanoparticles, and realising a high efficiency
for the coupling of scattered light into the substrate. it is vital
to maximise the scattering cross-section, because this ensures
that the nanoparticles interact with most of the light incident on
the solar cell and randomise its direction.
Meanwhile, it is critical to achieve a high coupling efficiency of
scattered light into the substrate of the solar cell, because this
minimises reflection or transmission losses at each encounter
between the weakly absorbed light and the plasmonic particles.
it is also important to address parasitic losses, which are
inherent to the resonant metallic nanostructures. We form our
plasmonic nanoparticles by depositing a thin silver film on the
surface of a finished solar cell and annealing it in a nitrogen
atmosphere. nanoparticle arrays result from the difference in
thermal expansion between the metallic layer and the substrate
(see figure 2).
the fabrication process is relatively easy, since it does not
require modification of the solar cell fabrication process and is
scalable to large areas. Last year we reported an
8 percent enhancement in the efficiency of a quantum dot
solar cell through the addition of plasmonic light trapping. the
improvement in power predominantly came from an increase in
short-circuit current density (J sc
) by 5.6 percent − the opencircuit
voltage (V oc
) also went up, but just by 0.9 percent.
A good indicator of the effectiveness of a light trapping strategy
is the enhancement in path length. this is defined as the ratio
between the average distance travelled by weakly absorbed
light in a solar cell featuring light trapping, to the distance
travelled in a planar solar cell. Based on experimental results
from our plasmonic quantum dot solar cell, we calculate
that this path length enhancement is approximately two at a
wavelength of 1000 nm. We can explain why this value is much
smaller than expected from a good light trapping structure:
Our solar cells are fabricated on n + substrates, so free carrier
32 www.compoundsemiconductor.net April/May 2013