Abstract and subjects
The quantum efficiency of solar cells, like that of any photon detector, is
dictated by the ability to absorb photons to create conducting carriers, and the
efficiency to drive such carriers to electrodes for collection. Having a medium
that enables full photon absorption in a short length, together with a long
carrier lifetime that allows photogenerated carriers to reach electrodes before
recombining is ideal but is not always realistic. For example, silicon
photovoltaics, despite being a major player in the solar cell market, suffer
from their low absorption coefficient, thus requiring a thick absorbing layer
that impairs the efficiency with which photogenerated carriers are collected.
Radial
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silicon nanowires (NWs) have been proposed as a candidate for
reducing the optical absorption length and required processing purity in
silicon-based solar cells without compromising their quantum efficiency and
yet reducing the overall cell cost. In this scheme incident light propagates
along the axial dimension of the NWs and thus has a greater chance of being
absorbed when the NW length extends beyond 10 μm due to interarray light
scattering effects. At the same time, the core-shell radial
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structure leads electrical current flow along submicron radii, enabling rapid
collection of most photogenerated carriers, as the transport length is typically
less than the diffusion lengths of minority carriers.
Since the first discussion on the device operation of a radial NW geometry
for photovoltaic cell, much work has been done to experimentally realize the
advantages of this NW array system. In the
current work discussed in the chapter we perform finite-difference timedomain
(FDTD) simulations to investigate the absorption process in arrayed radial NWs. The goal of this work is to gain insight on absorption processes in
NW arrays and to develop strategies for enhancing absorption efficiency. The
effects of light scattering and the material filling ratio (ratio of the cross-sectional
area that is occupied by the nanowires to the total area of the array)
at different NW spacings will be discussed. Evolution of absorption with NW
length, particularly in the long-wavelength range (700-1100 nm) will be
shown to illustrate the advantages of NWs as opposed to conventional planar
structures. In addition, actual NW geometries after shell overgrowth using
chemical vapor deposition (CVD) for different NW lengths and spacings have
been studied.