Total internal reflection (TIR) occurs when light traveling within a material with a higher refractive index hits the boundary with a material of lower refractive index at an angle greater than the critical angle. Instead of passing through the boundary, the light reflects entirely back into the material.
In solar cells, especially thin-film types, TIR is used to trap light within the cell structure. Light that enters the cell and strikes the rear surface at steep angles is reflected back into the cell due to TIR, instead of escaping. This internal bouncing increases the distance that light travels inside the absorbing material.
As the light travels a longer optical path due to multiple reflections, there is a higher probability it will be absorbed by the semiconductor material and converted into electrical energy. TIR is especially important in textured or layered designs, where surfaces are angled or patterned to direct light toward angles that favor TIR.
The cell’s back surface is often coated with a reflective material to assist TIR and prevent light escape. Some designs also use encapsulation layers with specific refractive indices to promote TIR at the front or rear interfaces. This mechanism helps retain photons inside the structure, enhancing absorption without increasing physical thickness.