During the design process of Solar cells, one of the main objectives is to reduce all types of losses in operation of solar cells. One of the key losses is the optical losses. Optical losses mainly affects the power produced from a solar cell by lowering the short-circuit current.
Optical losses consist of the light which potentially could have generated an electron hole pair, but actually does not generate any, since the light is reflected from the front surface, or as a result of the light not being absorbed by the solar cell. For the generally common semiconductor solar cells, the whole range of visible spectrum (350 – 780 nm) has adequate energy for creating the electron hole pairs and as a result all visible light would ideally be absorbed.
Sources of optical losses in a solar cell
There are numerous methods for reducing the optical losses which are described below:
- Minimizing the top contact coverage of the cell surface (although this process can result in an increase in series resistance).
- Using Anti-reflection coatings for covering the top surface of the cell.
- Using surface texturing for reduction of reflection.
- The width of the solar cell can be changed by making the solar cell thicker so as to increase absorption (although the absorbed light more than a diffusion length away from the junction has a low probability of collection and commonly will not contribute to the short circuit current).
- Increasing the optical path length of the solar cell by using a combination of both surface texturing and light trapping.
For top contact coverage the bottom can be covered with a metal, which allows for good conduction, but if the top is completely covered, then photons can’t pass through the opaque conductor and all the current is lost. If the contacts are placed only at the sides of the cells, then the electrons will have to travel a very long distance to reach the contacts. The internal resistance of silicone is fairly high which leads to high losses.
Refractive rate index equation
The reflection rate of a silicon surface is more than 30% as a result of its high refractive index. R which is the reflectivity between two materials of different refractive indices is determined by the equation below:
R= ( (n0−nSi) / (n0+nSi))2
n0: is the refractive index of the surroundings
nSi: is the complex refractive index of silicon.
For the unencapsulated solar silicone cell n0 = 1. For an encapsulated silicone cell n0 = 1.5. The refractive index of silicon varies with wavelength and depends on material properties.