Light Trapping I - Absorption and Optical Losses

Consider light shining on a single absorbing medium of thickness d from the left (see diagram below), where the distance x starts from the left - the point of incidence of light.  2 simplifications are to be considered: that the reflection and scattering at the interfaces are ignored, and that the light is monochromatic (all photons having same wavelength/energy).  The absorption of a medium is represented by an absorption coefficient α, whose unit is cm^-1.

The reduction of light intensity as light passes through the medium, due to absorption, is described by Lambert-Beer's law:

I(λ, x) = I₀(λ)exp[-α(λ)x]

where I₀ is the full light intensity before entering the medium, and I is the intensity as light passes through the medium.

Lambert-Beer's law shows that more light is absorbed at the point of incidence/entry on the left, than on the right at the end.  It also shows that for absorption to be high, the absorption coefficient or the thickness d has to be large.  It must be noted that the absorption coefficient for each material is different at different wavelengths (see diagram below).

It can be seen from the diagram that Germanium has the lowest band gap and absorbs photons with low energies, where wavelengths are high.  Gallium arsenide (GaAs) has the highest band gap because it absorbs light with the highest photon energy, starting where wavelengths are small.  It can also be seen that for the visible spectrum from 300nm to 700nm, the absorption coefficient of Indium Phosphide (InP) and GaAs is much higher than that of silicon.  This is because InP and GaAs are direct band gap materials that have higher absorption coefficients.  This shows that silicon absorbs quite poorly compared to GaAs, and thicker absorber layers are required for the same fraction of light.

In general, the absorption of high energy photons like blue light is very much larger than low energy photons like red light.  This implies that blue light will not penetrate deeply into the absorber layer, because it will completely be absorbed once it touches the absorber layer (at the front/window layer).  Since the absorption of photons generates excited charge carriers, the diagram will also show the local generation profile of charge carriers based on the absorption coefficients and wavelengths.  More charge carriers are generated at the front layer.

This also implies that the EQE for blue light will be for charge carriers generated close to the front layer, while the EQE for red light will be for charge carriers generated throughout the entire absorber layer.

For optical losses, we consider a crystalline silicon c-Si solar cell with a p-type bulk and thin n-type layer on top (the yellow n-emitter) (see diagram below).  Metal contacts are located at the top and back.  Hence, the first optical loss mechanism comes from shading, where the top contacts block light from reaching the PV active layers.

The 2nd optical loss comes from the reflection at the front interface (between air and silicon) of the solar cell, because when light passes through an interface between 2 media with different refractive indices, light will be partly reflected and transmitted at the interface.

The 3rd optical loss comes from parasitic absorption losses in non-active PV layers, such as the green layer in the diagram.  This layer can be an anti-reflection coating or passivation layer for reducing defects at the surface of the emitter layer.  Photons absorbed at this layer will not contribute to carrier generation.  This means that this layer should preferentially have high transmissions for the spectral part utilised by the solar cell.

The last optical loss comes from the inability of the absorber layer to absorb all the light, and the light is transmitted.  This happens for thin film solar cells.



Reference:
3.3.4 Light Trapping I - Absorption and Optical Losses, Delft University of Technology, https://www.youtube.com/watch?v=FKNCF-bprhs