Light trapping Explained
As previously discussed different parameters affect the parameters of Solar cell design. Not just the requirement of absorbing all incident light affects the optimum device thickness. If the light does not get absorbed within a diffusion length of the junction for example, the light generated carriers are then lost to recombination. As a result, an optimal structure for the solar cell will typically have light trapping in which the optical path length is many times the actual device thickness, where the optical path length of a device defines the distance that the unabsorbed photon can travel through the material before it escapes out of the material.
This can be explained by the material thickness. A solar cell that has no light trapping features may have an optical path length of one, while the solar cell which has good light trapping may have an optical path length of 50, signifying that light bounces multiple times within the cell.
Equations for light trapping
Light trapping can be achieved through changing the angle at which light travels in the solar cell by making it incident on an angled surface. A textured surface can reduce reflection and also couple light implicitly into the silicon, therefore giving a longer optical path length than the physical material thickness. We can calculate the angle at which light will be refracted into the semiconductor material is, by the following equation as per Snell’s Law:
where θ1 , θ2 are the angles for the light incident on the interface relative to the normal plane of the interface and n1 and n2 are the refractive indices of the mediums.
Snell’s law equation above can be rearranged, so as to calculate the angle at which light enters the solar cell (the angle of refracted light):
The quantity of light reflected at an interface can be calculated from the Fresnel reflection equation. For light polarized parallel to the surface, the reflected light amount is:
R∥=tan2 (θ1-θ2) / tan2 (θ1+θ2)
For light polarised perpendicular to the surface the reflected amount is:
R⊥= sin2 (θ1-θ2) sin2 (θ1+θ2)
For unpolarised light the amount reflected is the average of the two as below:
RT= (R⊥+R∥) / 2
When light passes from a medium of high refractive index to a medium with low refractive index, TIR (total internal reflection) can possibly happen. The critical angle is the angle at which TIR occurs.
Using total internal reflection, light can be trapped inside the cell and makes multiple passes through the cell, thus allowing even a thin solar cell to maintain a high optical path length.