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 p-i-n 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 p-i-n 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.
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