Optical characteristics of GaAs nanowire solar cells

The reflectance, transmittance, and absorptance of GaAs nanowire (NW) arrays are calculated by solving Maxwell's equations using the finite element method. The model is compared with measurement results from well-ordered periodic GaAs NW arrays fabricated by dry etching. The model results are also compared with the reflectance measured from NWs grown by the Au-assisted vapor-liquid-solid method. The optimum NW diameter, periodicity (spacing between NWs), and length are determined to maximize absorptance of the AM1.5G solar spectrum and short circuit current density in a NW array solar cell. The optimum NW diameter, period, and length were 180 nm, 350 nm, and 5 μm, respectively, giving a photocurrent density from the NW of 27.3 mA/cm2 and corresponding to 91.3% absorption of the AM1.5 G solar spectrum. The photocurrent density saturated for NW lengths greater than 5 μm. A gold nanoparticle at the top of the NWs (used in the vapor-liquid-solid NW growth process) substantially reduced the optimum photocurrent density, while a polymer filling the space between NWs and a planar indium tin oxide contact had a relatively minor influence.