  The efficiency of GaAs nanowire solar cells can be significantly improved
without any new processing steps or material requirements. We report coupled
optoelectronic simulations of a GaAs nanowire (NW) solar cell with vertical
p-i-n junction and high band gap AlInP passivating shell. Our
frequency-dependent model facilitates calculation of quantum efficiency for the
first time in NW solar cells. For passivated NWs, we find that short-wavelength
photons can be most effectively harnessed by using a thin emitter while
long-wavelength photons are best utilized by extending the intrinsic region to
the nanowire/substrate interface, and using the substrate as a base. These two
easily implemented changes, coupled with the increase of NW height to 3.5 um
with realistic surface recombination in the presence of a passivation shell,
result in a NW solar cell with greater than 19% efficiency.
