  In this paper, we have investigated the optical properties of GaAs nanocone
(NC), inverted nanocone (INC), and hourglass (HG) shaped solar cells by
performing Finite Difference Time Domain (FDTD) simulations. The different
structures are compared for their light absorptance, reflectance,
transmittance, and photogeneration with traditional cylindrical nanowires. The
variation of the different optical properties with the sidewall angle is
observed for all the structures. It is seen that the best performance of NC,
INC, and HG is obtained at a sidewall nanocone angle of ~ 7{\deg}, ~ 4{\deg},
and at a sidewall bottom nanocone angle of ~ 9{\deg} respectively, with the NC
having the best overall performance followed by INC, except for very large
angles where the performance of HG is better than INC. The optical properties
of these structures were also observed for different sun angles; the HG and INC
structures show improvement in absorption for slightly larger sun angles
attributed to their better light trapping properties. Finally, a period study
is done to observe the effect of dense and sparse cell packing on optical
properties and photogeneration profiles of the nanostructures and it is
observed that the sparsely spaced nanowires has a better and more evenly
distributed photogeneration profile, which will greatly improve the carrier
extraction of the radial junction solar cells.
