A flat, polished silicon wafer thick is lit at normal incidence by monochromatic…

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Energy from Physics, Ch The illuminated junction: generation, collection and the cell's characteristic

Problem

A flat, polished silicon wafer

180\,\mu\mathrm{m}

thick is lit at normal incidence by monochromatic

800\,\mathrm{nm}

light. At this wavelength silicon has refractive index

n = 3.68

and absorption length

1/\alpha = 12\,\mu\mathrm{m}

, and the finished cell's internal quantum efficiency is

\mathrm{IQE} = 0.90

. ((a)) Find the reflectance of the bare wafer, check that the light passing straight through it is negligible, and find the cell's external quantum efficiency. ((b)) Find the refractive index and the thickness of the ideal single quarter-wave antireflection coating for

800\,\mathrm{nm}

. ((c)) Silicon nitride, with index

n_c = 2.0

, is used instead, at a quarter-wave thickness for

800\,\mathrm{nm}

. A quarter-wave coating of index

n_c

on silicon in air leaves a reflectance

R = \left(\dfrac{n - n_c^2}{n + n_c^2}\right)^2

at the wavelength it is tuned to. Find the coating's thickness, its reflectance and the new external quantum efficiency.

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