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Why are Si and GaAs are preferred materials for solar cells? |
Answer» Solution :The solar radiation spectrum received by us is SHOWN in Fig.  The maxima is near 1.5 eV. For photo-excitation, hν > Eg . Hence, semiconductor with band gap ~1.5 eV or lower is likely to give better solar conversion efficiency. Silicon has Eg ~ 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 (Eg ~ 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 (Eg~ 0.4 eV) which satisfy the condition hν > Eg 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. |
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