Current solar cell technologies are dominated by silicon. Limited primarily by silicon’s inherent properties, these solar cells convert only 15–20% of solar energy into electricity. Solar cells made from III-V compound semiconductors (materials that contain group III and group V elements) have much higher efficiencies, due to their better optical (absorption) and electrical (charge mobility) properties. Furthermore, with different III-V materials on the same device, researchers can produce multiple junctions or heterostructures to cover a broader range of the solar spectrum. In these devices, researchers have demonstrated efficiencies of over 40% under concentrated solar energy.1 The additional benefits afforded by nanotechnology may further enhance solar cell performance. Nanowire research has emerged as a quickly growing field. Much excitement stems from the unique electronic and optical properties of semiconductor nanowires. Nanowires of a particular substance may behave quite unlike much larger ‘bulk’ samples of the same material. Nanowires are of great interest in photovoltaics because of their large surface area, high aspect ratio (long and thin), intrinsic antireflection effect (which increases light absorption), and ability to direct light absorption with specifically designed arrays.2 More importantly, core-shell nanowires, which operate based on radial p-n junctions, represent a revolution in photovoltaics by decoupling light absorption from carrier collection pathways: light is absorbed vertically, whereas carriers are separated radially. This separation eliminates the once-fundamental trade-off between light absorption and carrier collection. By incorporating the superior photovoltaic properties of III-V semiconductors into nanowire structures, researchers expect to achieve efficiencies similar to today’s best solar cells, with significantly less material. Because nanowire-based cells use less material than planar devices, such changes can ultimately reduce costs. Figure 1. Scanning electron microscopy (SEM) image of gallium arsenide/aluminum gallium arsenide (GaAs/AlGaAs) core-shell nanowires.
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