Junction Operation of GaAs Wire Array Solar Cells
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Junction Operation of GaAs Wire Array Solar Cells Adam F. Halverson1 and Loucas Tsakalakos1 1 GE Global Research, 1 Research Circle, Niskayuna, NY 12309, U.S.A ABSTRACT Wire array solar cells benefit from enhanced coupling of light into the active area of the device, significantly decreased collection lengths due to radial charge separation and collection, and easier access to grain boundaries for passivation which may enable future deposition on nonwafer substrates. We report on an analysis of the junction operation of wire array based GaAs solar cells through temperature and light intensity dependent current-voltage analysis and compare these data to matched planar devices. We see evidence of non-ideal recombination pathways indicated by activation energies for generation-recombination that are significantly less than the band gap of GaAs. We observe voltage shifts in the wire array devices at low temperature and high light intensity that we posit can be explained by electron accumulation in the window layers of the devices. INTRODUCTION Among single planar junction photovoltaic technologies, devices fabricated in the III-V materials systems, specifically GaAs, have demonstrated the highest conversion efficiencies [1]. GaAs has an optical band gap that is both direct and ideally matched for absorption of the solar spectrum incident on Earth. In addition, the material system benefits from large carrier mobilities, an extensive system of lattice matched alloys for device fabrication and tuning, and the ability to easily dope the materials [2]. One drawback of this material system is the considerable expense of the base materials and fabrication processes, leading this material system to be uncompetitive for large scale deployment due to high dollar-per-watt costs compared to competing technologies [2]. A challenge then, is to determine if there are alternative fabrication techniques or device structures that can mitigate these costs and lead to a more competitive technology. This paper describes one such approach: wire array solar cells. Figure 1 shows a schematic of idealized planar and wire array solar cells. The fabrication of these cells proceeds nearly identically to that of a planar junction device, containing only additional lithographic and etching steps in the intermediary stages to produce a pillared structure upon which subsequent layers are deposited. The idealized wire array solar cell has several theoretical benefits that should improve performance beyond that of a planar junction. First, the dimensions of the wire array can be chosen to enhance scattering of incident light, and thus off-normal incident light is coupled into the device more efficiently than a planar film [3] . Such techniques typically result in a loss of efficiency versus a planar device for normally incident light, but increase the conversion efficiency for light incident from other angles, resulting in a greater net power generation over a day of outdoor illumination. Second, the threedimensional junction allows for electron-hole pa
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