Why are Si and GaAs are perferred materials for solar cells?

Explanation

The solar radiation spectrum is shown below.

1.5 eV has the maximum intensity of radiation.
To obtain the electrical energy in a solar cell the energy of the incident photon must be `hv gt E_(g)` (where `E_(g)` is band gap energy). So the band gap for solar cell which is about 1.5 eV or less, works at maximum capacity for solar energy conversion.
Silicon has `E_(g)` about 1.1 eV while for GaAs it is about 1.53 eV. In fact GaAs is better than Si because of its relativer higher absorption coefficient.
The band gap `E_(g)` of CdS or CdSe is about 2.4 eV and if we use such materials in solar cell then only the high energy part of the solar energy can be converted into light energy and a significant part of energy will be no use.
The question arises, why we do not use material like PbS `(E_(g)=0.4eV)` which satisfy the condition `hv gt E_(g)` for v maxima corresponding to the solar radiation spectra.
It we do so, most of the solar radiation will be absorbed on the top layer of solar cell and will not reach in or near the deplection region.
For effective electron hole seperation, due to the junction field, we want the photo generation (formation of e and h occuring after light incident) to occur in the junction region only.