How does Solar Cell efficiency relate to Band Gap/ Isc/ Voc?
Not a full answer but you also have to consider the spectrum of the illumination. Higher bandgap gives higher voltage and thus more energy per electron released by a photon. 

However if the bandgap is higher than the photon's energy, the photon won't release an electron. 

This is exploited in double-or multi-layer solar cells, where different layers have different bandgap energies to exploit the greater energy at the blue end of the spectrum.