The work presented is focused on the electro-optical properties of III-V nanowire array solar cells. We begin with a study of the modal and material dispersion in a singlediameter nanowire array. A dual- diameter nanowire array structure is then proposed as a broadband absorber and a solar spectrum divider. Next, a microscopic carrier transport model is adopted to further study the internal device physics. Nanowire array solar cells with both axially and radially arranged p-n junction were studied in terms of the currentvoltage relationship, cell efficiency, and the response to the surface recombination with the inclusion of active region analysis. The optical properties of a nanowire array are evaluated by solving Maxwell’s equations. In the carrier transport model, the continuity equations are solved along with the Poisson’s equation. Recombination mechanisms include the radiative recombination, Shockley-Read-Hall recombination, Auger recombination, and the surface recombination.