Question

Explain why there is the optimized bandgap for single-junction photovoltaic cell, in which the cell efficiency initially increases as a function of bandgap, reaches a peak at certain value of bandgap, and then decreases as bandgap further increases.

Explain difference between superstrate and substrate configuration in thin-film photovoltaic cells.

Answer 1

There is a relation between bandgap and the wavelength of the light. As we increase the wavelength efficiency of the solar cell starts increasing and it attains a peak which corresponds to the wavelength of light falling on the cell (Typically 1.5 eV which corresponds to a wavelength of about 830 nm, in infrared). After that, the efficiency drops because we are again moving away from the matching wavelength. In physics, the Shockley–Queisser limit (also known as the detailed balance limit, Shockley Queisser Efficiency Limit or SQ Limit, or in physical terms the radiative efficiency limit) refers to the maximum theoretical efficiency of a solar cell using a single p-n junction to collect power from the cell where the only loss mechanism is radiative recombination in the solar cell.

The term superstrate refers to a solar cell configuration where the glass substrate is not only used as a supporting structure but also a window for the illumination and as part of the encapsulation. During operation, the glass is "above" the actual solar cell. Thus the name "Super"strate. On the other hand in substrate configuration it works as substrate only.

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