150
A. Shah
With solar cells, we work in a particular regime of the diode—the photo-generated
current within a solar cell and the usual diode forward current flow in opposite
directions, as detailed in Sect. 3.4.2 of Chap. 3.
One can show that in solar cells V and E ext are reduced.
6 In fact, the higher is the
external voltage appearing at the contacts of the solar cell, the more V and E ext will
be reduced. For short-circuit conditions, the reduction will be zero. For open-circuit
conditions (V oc ) the reduction of V and E ext will be a maximum. An amorphous
silicon solar cell cannot function properly without a strong electric field within the
i-layer. There are two consequences of this (a) amorphous solar cells have to be
extremely thin (d i well below 1 µm) in order to have a high enough electric field E,
as E = V bi /d i , (where V bi is the built-in voltage and d i the thickness of the i-layer); (b)
amorphous silicon solar cells will have—with respect to their bandgap—particularly
low values of V oc ,
7 because at V oc the electric field breaks down and very strong
recombination in the i-layer results.
8
The reader should consult, at this point, Fig. 3.12 in Chap. 3, to visualize the
curves of current density versus voltage, for solar cells, in general, in the dark and
with illumination.
6.2.2 Fabrication of Amorphous Silicon Solar Cells
and Modules
The fabrication of amorphous silicon (a-Si:H) solar cells and modules—described
first for the example of an a-Si:H cell deposited on a non-transparent substrate (metal
or polymer
9 foil), involves the following steps:
1. Cleaning of the Substrate
2. Deposition of the back reflector (BR) acting as back contact
10 (e.g. silver)
3. Deposition of the a-Si:H layers, in a PE-CVD system
4. Deposition of the front contact (here zinc oxide)
5. Performing cell interconnection by laser scribing and deposition of further
metallic layers
6 To understand this, consider Fig. 3.12 in Chap. 3.
7 The reader should consult Fig. 3.17 of Chap. 3 in this context.
8 There are many reasons for the low value of V oc in a-Si:H solar cells. The “breakdown” of the
internal electric field described here is just one of them. The presence of bandtail states (see Fig. 6.6)
is another reason.
9 Polymer foils are mostly transparent to light, but one prefers to avoid letting the sunlight pass
through them, because they, in general, suffer from “yellowing” under the effect of UV light.
10 The back contact is usually textured, in order to increase the light-trapping properties of the cell
(see Chap. 4). Amorphous silicon (a-Si:H) solar cells have to be kept extremely thin (thickness below
0.2 µm), so as to maximize the internal electric field E int , and, thus, allow for satisfactory collection
of the photo-generated electrons and holes. Therefore, light-trapping is absolutely essential for
a-Si:H cells.
A. Shah
With solar cells, we work in a particular regime of the diode—the photo-generated
current within a solar cell and the usual diode forward current flow in opposite
directions, as detailed in Sect. 3.4.2 of Chap. 3.
One can show that in solar cells V and E ext are reduced.
6 In fact, the higher is the
external voltage appearing at the contacts of the solar cell, the more V and E ext will
be reduced. For short-circuit conditions, the reduction will be zero. For open-circuit
conditions (V oc ) the reduction of V and E ext will be a maximum. An amorphous
silicon solar cell cannot function properly without a strong electric field within the
i-layer. There are two consequences of this (a) amorphous solar cells have to be
extremely thin (d i well below 1 µm) in order to have a high enough electric field E,
as E = V bi /d i , (where V bi is the built-in voltage and d i the thickness of the i-layer); (b)
amorphous silicon solar cells will have—with respect to their bandgap—particularly
low values of V oc ,
7 because at V oc the electric field breaks down and very strong
recombination in the i-layer results.
8
The reader should consult, at this point, Fig. 3.12 in Chap. 3, to visualize the
curves of current density versus voltage, for solar cells, in general, in the dark and
with illumination.
6.2.2 Fabrication of Amorphous Silicon Solar Cells
and Modules
The fabrication of amorphous silicon (a-Si:H) solar cells and modules—described
first for the example of an a-Si:H cell deposited on a non-transparent substrate (metal
or polymer
9 foil), involves the following steps:
1. Cleaning of the Substrate
2. Deposition of the back reflector (BR) acting as back contact
10 (e.g. silver)
3. Deposition of the a-Si:H layers, in a PE-CVD system
4. Deposition of the front contact (here zinc oxide)
5. Performing cell interconnection by laser scribing and deposition of further
metallic layers
6 To understand this, consider Fig. 3.12 in Chap. 3.
7 The reader should consult Fig. 3.17 of Chap. 3 in this context.
8 There are many reasons for the low value of V oc in a-Si:H solar cells. The “breakdown” of the
internal electric field described here is just one of them. The presence of bandtail states (see Fig. 6.6)
is another reason.
9 Polymer foils are mostly transparent to light, but one prefers to avoid letting the sunlight pass
through them, because they, in general, suffer from “yellowing” under the effect of UV light.
10 The back contact is usually textured, in order to increase the light-trapping properties of the cell
(see Chap. 4). Amorphous silicon (a-Si:H) solar cells have to be kept extremely thin (thickness below
0.2 µm), so as to maximize the internal electric field E int , and, thus, allow for satisfactory collection
of the photo-generated electrons and holes. Therefore, light-trapping is absolutely essential for
a-Si:H cells.