This paper reports a theoretical study of heterojunction solar cell efficiencies to include the effect of band - band impact ionization, the band - band Auger recombination and the p - n junction structure. We also study conditions under which configuration A (light encounters the large energy gap first) or configuration B (light encounters the small energy gap first) is the optimal heterojunction configuration for a solar cell, other conditions being kept fixed. Constant efficiency contour diagrams having the energy gaps as axes show that, subject to our assumptions, the best efficiencies are only of order 38% for a black body equivalent to one sun. The higher efficiencies are favoured by the smaller semiconductor widths. Open-circuit voltage, short-circuit current density and fill factor are also calculated for several sets of cell parameters.