  Effect of geometrical and structural parameters on the efficiency of the
tandem solar cell based on the III-V nanowire array on silicon is studied by
the means of coupled opto-electrical simulations. A close to realistic
structure, consisting of AlGaAs core-shell nanowire array, connected through a
tunnel diode to a Si subcell is modelled, revealing the impact of top contact
layer, growth mask and tunnel junction. Optical simulation of the tandem
structure under current matching condition determine optimal geometrical
parameters of the nanowire array. They are then used in the extensive
electrical optimization of the radial junction in the nanowire subcell. Device
simulations show the necessity of high doping of the junction in order to avoid
full shell depletion. The influence of bulk and surface recombination on the
performance of the top subcell is studied, exposing the importance of the good
surface passivation near the depleted region of the radial p - n junction.
Finally, simulations of the fully optimized tandem structure show that a
promising efficiency of 27.6% with the short-circuit current density of 17.1
mA/cm^2 can be achieved with reasonable bulk and surface carrier lifetime.
