Question 24.3: Nonreflective Coatings for Solar Cells and Optical Lenses Go...
Nonreflective Coatings for Solar Cells and Optical Lenses
Goal Calculate interference effects in a thin film when there are two inversions.
Problem Semiconductors such as silicon are used to fabricate solar cells-devices that generate electric energy when exposed to sunlight. Solar cells are often coated with a transparent thin film, such as silicon monoxide (\mathrm{SiO} ; n=1.45) to minimize reflective losses (Fig. 24.9). A silicon solar cell (n= 3.50) is coated with a thin film of silicon monoxide for this purpose. Assuming normal incidence, determine the minimum thickness of the film that will produce the least reflection at a wavelength of 552 \mathrm{~nm}.
Strategy Reflection is least when rays 1 and 2 in Figure 24.9 meet the condition for destructive interference. Note that both rays undergo 180^{\circ} phase changes on reflection. The condition for a reflection minimum is therefore 2 n t=\lambda / 2.
Solve 2 n t=\lambda / 2 for t, the required thickness:
t=\frac{\lambda}{4 n}=\frac{552 \mathrm{~nm}}{4(1.45)}=95.2 \mathrm{~nm}
Remarks Typically, such coatings reduce the reflective loss from 30% (with no coating) to 10% (with a coating), thereby increasing the cell’s efficiency because more light is available to create charge carriers in the cell. In reality, the coating is never perfectly nonreflecting, because the required thickness is wavelength dependent and the incident light covers a wide range of wavelengths.