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

Explanation

The solar radiation spectrum received by us is shown in

The maxima is near 1.5 eV. For photo-excitation, `h ν gt E_(g)`. Hence, semiconductor with band gap `~1.5 eV` or lower is likely to give better solar conversion efficiency. Silicon has `E_(g) ~ 1.1 eV` while for GaAs it is `~1.53 eV.` In fact, GaAs is better (in spite of its higher band gap) than Si because of its relatively higher absorption coefficient. If we choose materials like CdS or CdSe `(E_(g) ~ 2.4 eV)`, we can use only the high energy component of the solar energy for photo-conversion and a significant part of energy will be of no use.
The question arises: why we do not use material like PbS `(E_(g) ~ 0.4 eV)` which satisfy the condition `hν gt E_(g)` for ν maxima corresponding to the solar radiation spectra? If 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 depletion region. For effective electron-hole separation, due to the junction field, we want the photo-generation to occur in the junction region only.