Question

why Pbs (Eg= 0.4eV) is not used for fabricating solar cell.?

Answer 1

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).