630
22 Light-to-Electricity Conversion
n
+
p
p
+
E F
metal
back
surface
front
surface
base
metal
Fig. 22.60 Increase of the carrier collection efficiency by a back surface field. Adapted from [1886]
22.4.4 Design Refinements
In order to collect electrons most efficiently, a back surface field is used (Fig. 22.60). A higher-doped
region at the back contact creates a potential barrier and reflects electrons back to the front contact.
An important point for optimization is the management of the reflection at the solar cell surface.
First, a dielectric antireflection (AR) layer (or multi-layers) can be used. These layers should have a
broad AR spectrum. Additionally, a textured surface reduces reflection (Fig. 22.61d), giving reflected
photons a second chance for penetration (Fig. 22.61c). The reflectance of bare Si, 35%, can be reduced
to 2%. An AM0 efficiency of over 15% was reached using textured multi-crystalline cells. Alkaline
KOH-based etches attack Si (001) anisotropically and yield pyramidal structures (Fig. 22.61b) with
{111} facets. Recently an acidic HF/HNO 3 -based process has been established [1887], resulting in a
worm-like surface pattern on multi-crystalline silicon wafers (Fig. 22.61a) with superior anti-reflection
properties.
During its course over the sky during the day, the sun changes its angle towards a fixed solar cell.
10
A tracking mechanism can optimize the angle of incidence during the day and increase the overall
efficiency of the solar cell (Fig. 22.62).
22.4.5 Modules
In order to cover a large area and supply certain values of output voltage and current, several solar
cells are connected into modules. Arrays are built up from several modules (Fig. 22.63). If solar cells
are connected in parallel, the total current increases; if they are connected in series, the output voltage
increases. We note that in partially shadowed modules the reverse characteristics of solar cells are
important [1892, 1893]; local breakdown can lead to hot spots and irreversible degradation.
10 We are of course aware that the earth rather rotates around the sun.
22 Light-to-Electricity Conversion
n
+
p
p
+
E F
metal
back
surface
front
surface
base
metal
Fig. 22.60 Increase of the carrier collection efficiency by a back surface field. Adapted from [1886]
22.4.4 Design Refinements
In order to collect electrons most efficiently, a back surface field is used (Fig. 22.60). A higher-doped
region at the back contact creates a potential barrier and reflects electrons back to the front contact.
An important point for optimization is the management of the reflection at the solar cell surface.
First, a dielectric antireflection (AR) layer (or multi-layers) can be used. These layers should have a
broad AR spectrum. Additionally, a textured surface reduces reflection (Fig. 22.61d), giving reflected
photons a second chance for penetration (Fig. 22.61c). The reflectance of bare Si, 35%, can be reduced
to 2%. An AM0 efficiency of over 15% was reached using textured multi-crystalline cells. Alkaline
KOH-based etches attack Si (001) anisotropically and yield pyramidal structures (Fig. 22.61b) with
{111} facets. Recently an acidic HF/HNO 3 -based process has been established [1887], resulting in a
worm-like surface pattern on multi-crystalline silicon wafers (Fig. 22.61a) with superior anti-reflection
properties.
During its course over the sky during the day, the sun changes its angle towards a fixed solar cell.
10
A tracking mechanism can optimize the angle of incidence during the day and increase the overall
efficiency of the solar cell (Fig. 22.62).
22.4.5 Modules
In order to cover a large area and supply certain values of output voltage and current, several solar
cells are connected into modules. Arrays are built up from several modules (Fig. 22.63). If solar cells
are connected in parallel, the total current increases; if they are connected in series, the output voltage
increases. We note that in partially shadowed modules the reverse characteristics of solar cells are
important [1892, 1893]; local breakdown can lead to hot spots and irreversible degradation.
10 We are of course aware that the earth rather rotates around the sun.