Rahul Dewan - Jacobs University

5. µC-SI SOLAR CELLS WITH TRIANGULAR TEXTURE
1 .0
Q u a n tu m E ffic ie n c y
0 .8
0 .6
0 .4
0 .2
P e rio d
1 0 0 n m
9 0 0 n m
F la t C a s e
0 .0
3 0 0 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
W a v e le n g th [n m ]
Figure 5.3: Comparison of quantum efficiency for solar cell on smooth substrate with
triangular structures of height 400 nm and periods of 100 nm and 900 nm.
λ min = 300 nm to λ max = 500 nm. The dashed line represents the short circuit current
of a solar cell on a flat substrate. For larger texture periods the short circuit current
is comparable to the solar cell on flat substrates. Due to the small penetration depth
in this range of the spectrum, light trapping is negligible. For smaller and very small
periods an enhancement of the short circuit current is observed. The enhancement of
the short circuit current is caused by an improved incoupling of the light. Compared
to that of a solar cell on a smooth substrate, enhanced short circuit current in this
spectral range is increased from 2.9 mA/cm 2 to 3.6 mA/cm 2 . This increase in the short
circuit current is observed for periods smaller than 200 nm. For such small periods
the wavelengths of the incident light is much larger than the period of the triangular
grating, so that the triangular grating acts as an effective refractive index gradient. The
refractive index linearly increases from a refractive index of zinc oxide to the index of
microcrystalline silicon. As a consequence the reflection at this particular interface is
reduced and more light is coupled in the solar cell. If the grating period is smaller than
λ/2n, where λ is the wavelength of the incident light and n the refractive index of the
grating, the behavior of the grating can be described by an effective refractive index
gradient. The effective refractive index, n eff , at the front zinc oxide/silicon interface
can be calculated based on
68
n eff (h) = n ZnO × W T ex (h)
P Unit
+ n Si × P Unit − W T ex (h)
P Unit
(5.1)