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Why are \(\mathit{Si}\) and \(\mathit{GaAs}\)  are preferred materials for solar cells?

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The solar radiation spectrum received by us is shown in figure
Answer for Why are  mathit{Si} and  mathit{GaAs}  are preferred materials for solar cells?
The maxima is near \(1.5\mathit{eV}\) . For photo-excitation. \(\mathit{hv}>E_g\) . Hence, semiconductor with band gap \({\sim}1.5\)   \(\mathit{eV}\)  or lower is likely to give better solar conversion effleiency. Sillicon has \(E_g{\sim}1.1\)   \(\mathit{eV}\)  while for \(\mathit{GaAs}\)  it is \(-1.53\mathit{eV.}\)  In fact, \(\mathit{GaAs}\)  is better \((\) in spite of its higher band gap \()\) than \(\mathit{Si}\)  because of its relatively higher absorption coefficient. If we choose materials like CdS or CdSe \(\left(E_g-2.4\mathit{eV}\right)\) . we can use only the high energy component of the solar energy for photo-conversion and a signiflcant part of energy will be of no use. The question arises: why we do not use material like \(\mathit{PbS}\left(E_g{\sim}0.4\mathit{eV}\right)\)  which satisfy the condition \(\mathit{hv}>E_g\)  for \(v\)  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.
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