A: Materials of smaller band gap (‘Pbs’) not used for solar cell because radiation is absorbed by top layer of cell
B: In Ge and Si, the energy liberated due to hole electron recombination is mostly in the form of heat 
 

Materials like PbS (where band gap is 0.4 eV) which have band gaps less than 1.5 eV -  are not as frequently used in solar cells as Si, or GaAs, because in solar cells made of PbS, where incident photons' energy is a lot higher than the band gap, most of the photons incident are absorbed by the top layer of the cell and do not reach the junction (electron-hole separation, of course, needs to occur in the depletion region). 

In Si and Ge due to their band gap recombination of hole and  electron produce heat.