Material Thickness

While the reduction of reflection is an essential part of achieving a high efficiency solar cell, it is also essential to absorb all the light in the silicon solar cell. The amount of light absorbed depends on the optical path length and the absorption coefficient. The animation below shows the dependence of photon absorption on device thickness for a silicon solar cell.

The device simulated is a cell with no front surface reflection losses so that all incident light enters the cell. The electronic properties are assumed to be perfect (infinite diffusion length) so that all light generated carriers are collected. Finally, the light only passes once through the cell. In reality, thin cells are usually designed with a reflector on the rear so that light makes multiple passes across the cell and the absorption is increased. In the case of ideal lambertian light trapping the path length is effectively increased by 4n2 [1]. For silicon with a refractive index of 3.5, light trapping increases the path length by a factor of ~50. See light trapping page.

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For silicon material in excess of 10 mm thick, essentially all the light with energy above the band gap is absorbed. The 100% of the total current refers to the fact that at 10 mm, all the light which can be absorbed in silicon, is absorbed. In material of 10 µm thick, only 30% of the total available current is absorbed. The photons which are lost are the orange and red photons.