Material Thickness in Solar Cells
Another parameter we need to study and consider in our process of designing Solar cells to have maximum efficiency is the Material thickness. While reducing the reflection level is an important part for having a high efficiency for the produced solar cell, it is also really important to absorb all the available light which is incident on the surface of the silicon solar cell. The quantity of photon energy or of incident light absorbed depends on two factors which are the optical path length and the absorption coefficient. This highlights the direct relationship between the photon absorption and material thickness for the layers of the silicon solar cell.
If we simulate a device which resembles a cell which has no losses from front surface reflection, so consequently all incident light on the front surface enters into the cell. The electronic characteristics are assumed to be perfect so it has an infinite diffusion length, so all the light generated carriers are fully collected. Lastly, the light passes through the cell only once. In real life, cells made from thin layers are typically designed with a reflecting surface (a reflector) on the rear of the cell so that light can make multiple passes across the cell and the rate of absorption is subsequently increased. In other cases like trapping of ideal lambertian light, the path length is efficiently increased by a factor of 4n2. For silicon which has a refractive index of 3.5, light trapping can increase the path length by a factor of nearly 50.
Material Thickness and absorption rates
For silicon material which has a material thickness above 10 mm thick, nearly all the incident light which has an energy above the band gap energy levels are absorbed. This means 100% of the total current is absorbed which highlights the fact that at 10 mm, all the incident light which can be possibly absorbed in silicon cells, is actually absorbed. In the materials which have a material thickness of 10 µm thick, only 30% of the total available current can be absorbed. The lost photons lost are normally the orange and red photons.
One of the other factors that can affect the material thickness in the material selection process is absorption depth of the material the thickness of the semiconductor material, as we need to increase the absorption of light incident on the surface and decrease reflectivity so as to maximize efficiency of solar cell design as highlighted above.