Rahul Dewan - Jacobs University

7. µC-SI SOLAR CELLS WITH 3-DIMENSIONAL SURFACE TEXTURE
the i-layer silicon. The largest loss in the p-layer is observed for the shorter wavelengths
(from 300 nm to 500 nm). The p-layer is almost transparent for longer wavelengths.
The absorption loss in the front zinc oxide layer for shorter wavelengths accounts for
almost 35% of the total absorption. In the wavelength range of 500 nm to 800 nm, the
absorption in the front zinc oxide is minimized. For longer wavelengths, the optical
loss in the aluminum doped front zinc oxide layer increases due to the free carrier
absorption. As compared to the theoretical absorption limit, between wavelengths of
500 nm and 950 nm, more than 18% of the incident light is lost due to reflection. For
such thin solar cells, the potential of minimizing the parasitic absorption is small as
compared to the gain that can be achieved by reducing the reflection and guiding the
light within the absorber layer. Therefore, a reduction of the reflection should be the
key point in designing thin solar cells, where the absorber is significantly thinner than
the absorption length for longer wavelengths. For such thin solar cells, the period and
height of the pyramid have to be reduced so that the diffracted light can interfere with
light from neighboring unit cells.
By increasing the thickness of the absorber layer to 2.5 µm [Fig. 7.11(b)], the
reflection of the solar cell is greatly reduced as compared to the theoretical absorption
limit. The quantum efficiency can be increased by reducing the parasitic absorption
in the front zinc oxide and silicon p-layer, particularly for the longer wavelengths.
The reflection losses in thicker solar cells are smaller than those of thinner solar cells;
therefore, efficient light management that reduces reflection loses is more beneficial
for solar cells with smaller absorber thicknesses.
7.3.1 Minimizing Losses in the Solar Cell
A better incoupling of the incident light into the silicon diode reduces the reflection and
concurrently enhances the quantum efficiency. Solar cells deposited on textured glass
or double textured TCO layers improve the incoupling of the incident light into the
silicon cell. Cross sections of both these structures are shown in Fig. 7.12. Fig. 7.12(a)
exhibits the cross section of a solar cell prepared on a textured glass substrate. After
patterning the glass, a 500 nm TCO layer is conformally deposited on the patterned
glass substrate. Through the patterning of the glass substrate, the average thickness
of the TCO layer is reduced by h g /3, where h g is the height of the pyramid texture.
Reducing the average thickness of the zinc oxide layer leads to a drop of the lateral
conductivity of the zinc oxide film. To compensate for the drop in the lateral conductivity,
the thickness of the zinc oxide layer was increased by h g /3. Therefore, we can
compare the relative change in the short circuit current between solar cells prepared
on patterned glass substrates and the devices discussed in the preceding section. The
FDTD simulation of the solar cell on a patterned glass substrate exhibits a gain of al-
106