Why are \(\mathit{Si}\) and \(\mathit{GaAs}\) are preferred materials for solar cells?
Answer
The solar radiation spectrum received by us is shown in figure
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.
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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