Rahul Dewan - Jacobs University

2.3. Solar Cell Material Properties
]
-1
A b s o rp tio n C o e ffic ie n t [c m
P h o to n E n e rg y [e V ]
3 .0 2 .5 2 .0 1 .7 1 .5 1 .1
1 0 5
1 0 4
1 0 3
1 0 2
1 0 1
c -S i
µc -S i:H
a -S i:H
1 0 -1
1 0 0
1 0 1
1 0 2
1 0 3
1 0 0
1 0 4
4 0 0 5 0 0 6 0 0 7 0 0 8 0 0 9 0 0 1 0 0 0 1 1 0 0 1 2 0 0
W a v e le n g th [n m ]
Figure 2.9: Comparison of the optical absorption coefficients of microcrystalline silicon
(µc-Si:H) and amorphous silicon (a-Si:H). Also shown is the curve for single crystal
silicon (c-Si) as a reference. The corresponding penetration depth for the materials are
also highlighted on the right hand side axis. This figure is adapted from Fig. 2.2 in Ref.
[44].
P e n e tra tio n D e p th [µm ]
called the extinction coefficient. The absorption coefficient, α[cm -1 ], is directly related
to the extinction coefficient as
α = 4πκ
λ
We briefly discuss the optical properties of the solar cell materials in this section.
(2.22)
2.3.1 Optical Properties of a-Si:H and µc-Si:H
The optical absorption coefficients of the absorber materials in thin-film silicon solar
cells are shown in Fig. 2.9. Along with the curves for a-Si:H and µc-Si:H, the absorption
coefficient for single crystal silicon (c-Si) is also depicted as a reference. On the right
hand side of the axis, the penetration depth for each material as a function of the
incident wavelength is also highlighted. The top of the x-axis is also shown in terms
of the photon energy, where the band gaps of a-Si:H (1.7 eV) and µc-Si:H (1.1 eV) are
also marked.
Due to the lack of long range order, the law of momentum conservation is relaxed
in a-Si:H and as a result, a-Si:H acts like a direct semiconductor [26]. Therefore the
absorption coefficient of the material is determined by the availability of electronic
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