Design Rules of Crystalline Silicon 3

Continuing from Design Rules of Crystalline Silicon 2:

Besides managing charge carriers, the photons in a c-Si solar cell will also have to be managed.  The optical loss mechanisms are as shown in the diagram above.  A reduction of shading losses, as discussed in Design Rules of Crystalline Silicon 2, will have to be balanced by a reduction of resistivity losses.

The second loss of reflection at the front surface has been discussed in Light Trapping II - Anti-Reflection and Trapping Methods.  Using the Rayleigh film method, an intermediate layer (interlayer) can be placed between air and the silicon wafer.  The optimum value of the refractive index of the interlayer is as follows:

n₁ = √(n₀n_s)

where n₀, n_s are the refractive indexes of air and silicon.  Hence, the refractive index of the interlayer n₁ is about 2.1 if n₀ is 1 and n_s is 4.3.

Using the concept of destructive interference, the thickness of the interlayer for a light of 500nm can be calculated:

d = λ / 4n₁

where d becomes about 60nm.

An ideal material for the interlayer is silicon nitride (a-SiN), whose refractive index is between 2 and 2.2 for a wavelength of 500nm.

In addition, texturing of the front surface will improve the coupling of light into the wafer, thereby improving the absorption path length.  This will help absorb light with wavelengths above 900nm.

The texture on wafers can be done with wet-etching (anisotropic etching) techniques.  If a c-Si wafer at an initial 100 surface orientation is etched, textured surfaces with pyramid structures of 111 orientation will be created.  With a-SiN interlayer and textured surfaces, the wafer could be made to look dark blue or almost black (see diagram below).

Reference:
4.3 Design Rules of Crystalline Silicon, Delft University of Technology, https://www.youtube.com/watch?v=qmbrGk-c-P8