Optical losses chiefly effect the power from a solar cell by lowering the short-circuit current. Optical losses consist of light which could have generated an electron-hole pair, but does not, because the light is reflected from the front surface, or because it is not absorbed in the solar cell. For the most common semiconductor solar cells, the entire visible spectrum (350 - 780 nm) has enough energy to create electron-hole pairs and therefore all visible light would ideally be absorbed.
Sources of optical loss in a solar cell.
There are a number of ways to reduce the optical losses:
- Top contact coverage of the cell surface can be minimised (although this may result in increased series resistance). This is discussed in more detail in Series Resistance;
- Anti-reflection coatings can be used on the top surface of the cell.
- Reflection can be reduced by surface texturing.
- The solar cell can be made thicker to increase absorption (although light that is absorbed more than a diffusion length from the junction has a low collection probability and will not contribute to the short circuit current).
- The optical path length in the solar cell may be increased by a combination of surface texturing and light trapping.
The reflection of a silicon surface is over 30% due to its high refractive index. The reflectivity, R, between two materials of different refractive indices is determined by:
where n0 is the refractive index of the surroundings and nSi is the complex refractive index of silicon. For an unencapsulated cell n0 = 1. For an encapsulated cell n0 = 1.5. The refractive index of silicon changes with wavelength and is given in the chapter on material properties.