First we need to define Lambertian reflectance. Lambertian reflectance is a characteristic that describes the perfect matte or diffusely reflecting surface. The observed brightness of a Lambertian surface is the same for the observer from any angle of view. Next we can define Lambertian Rear Reflectors.
Definition of the Lambertian Rear Reflectors
A Lambertian back reflector is a unique rear reflector type which makes a random process for the direction of the light reflected. High reflection off the rear surface of the cell leads to reduction in the absorption of light in the rear cell contacts or light transmission through the rear, thus allowing the light to be reflected again and to bounce back into the cell for potential absorption. The process of randomizing the direction of reflected light permits more of the light being reflected to be totally reflected internally. This is because the light reaching the top surface of the cell at an angle which is greater than the critical angle for total internal reflection, is again reflected in the direction of the back surface.
Effect of Lambertian Rear Reflectors on light absorption
This method can be used to dramatically increase the light absorption, because the path length of the incident light on the surface can be improved by a factor equal to 4n2 (where n is the index of refraction for the used semiconductor). This can lead to an optical path length of nearly 50 times the physical thickness of the device and therefore this is an effective scheme for light trapping.
In actual devices, the front surface can be textured as well using schemes such as the random pyramids discussed before. Light incident at an angle less than the critical angle escapes the cell, but light incident at angle greater than the critical angle will be totally internally reflected within the cell.
For the short-circuit current of a silicon solar cell with and without light trapping. In case of no light trapping, the light makes only a single pass through the solar cell with a part of the light passing directly through the cell without being absorbed. In the case of ideal light trapping, the path length is increased by a factor of 4n2 and the short-circuit current is much higher. In real life, the level of light trapping is always within this range.