It is because the Shockley-Queisser limit predicts a maximum efficiency of ~33% for a single pn-junction solar cell with band-gap ~1.4 eV. This is a theoretical calculation and includes contributions from blackbody radiation, several carriers recombination rates, and spectrum losses. I suggest to check for literature to the Schockley-Queisser lmit.
It is because the Shockley-Queisser limit predicts a maximum efficiency of ~33% for a single pn-junction solar cell with band-gap ~1.4 eV. This is a theoretical calculation and includes contributions from blackbody radiation, several carriers recombination rates, and spectrum losses. I suggest to check for literature to the Schockley-Queisser lmit.
Though I am not an expert in this field, I intend to present my simple understanding of the issue here in a qualitative manner. So please correct me if I am wrong.
The p-n junctions in solar cells are designed with sufficient band gap so that the electrons in n-type material can absorb the energy from photons and jump to the other side, creating electric potential.
This phenomenon is governed by 3 simple rules,
1) At an instance one electron can absorb energy from only one photon and vice versa
2) An electron would absorb energy from a photon, only if that photon can give enough energy to the electron to overcome the band gap.
3). The amount of energy that an electron can absorb effectively from a photon is limited by the band gap energy (anything additional will be converted to heat).
Moreover, the energy content of a photon linearly depends on frequency (E=hf ,where h is Planck's constant). On the other hand the intensity of the frequencies of EM waves that we receive at the surface of the earth is different for different frequencies (as shown in figure below).
As a result, if the band gap is too high, the cell would be able to absorb energy only from high frequency EM waves (rule 2), whose intensity is low on the surface of the earth. In contrast, if the band gap is too low, the cell would be able absorb energy from low frequency EM waves as well, but the extra energy available in high frequency EM waves would be wasted (rule 3). Hence it is necessary to identify a middle ground to harness optimum solar energy. (Which I believe is 1.4-1.5 eV.)
Manaz has put it in the best way. As Manaz' picture shows the energy distribution from sun is maximum at around this wavelength(corresponding to 1.5-1.6 eV) in a 1 micrometer range. Hence for maximum absorption of energy the material should have an energy gap corresponding to approximately 1.5 eV.
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