superstrate configuration. This means that the layer that is passed first by the incident
light in the solar cell is also deposited first in the production process. Also the term p-i-n
layer refers to this superstrate configuration, as it indicates the order of the depositions:
because in thin-film silicon holes have a significantly lower mobility than electrons, the p
layer is in front of the n layer and hence the p layer is deposited first. As the generation
rate is then higher close to the p layer, more holes can reach it.
In superstrate thin-film silicon solar cells, glass is usually used as a superstrate
because it is highly transparent and can easily handle all the chemical and physical
conditions in which all the depositions are carried out. Before the p-i-n layers can be
deposited, a transparent front contact has to be deposited. Usually transparent conducting
oxides are used, that we introduced in Section 13.1.
Another possible configuration is the substrate configuration. There, either the
substrate acts like a back contact or the back contact is deposited on the substrate.
Consequently, no light will pass through the substrate. Thin-film silicon solar cells
deposited in the substrate configuration are also called n-i-p cells as the n layer is
deposited before the i layer and the p layer.
Usually, thin-film silicon solar cells have no flat interfaces, as shown in Figure 13.15
(a), but nanotextured interfaces, as illustrated in Figure 13.15 (c). These textured
interfaces scatter the incident light and hence prolong the average path length of the light
through the absorber layer. Therefore, more light can be absorbed and the photocurrent
can be increased. This is an example of light management that we discussed in Section
10.4.3.
Often, the p layer is not made from amorphous silicon. Instead, materials with a
higher bandgap are used, such as silicon carbide or silicon oxides in order to minimize
parasitic absorption mainly in the blue part of the spectrum. Usually, boron is used as a
dopant. Also for the n layer, silicon oxides are often used, but sometimes still a-Si:H is
utilized with phosphorus being the main dopant. The n-SiO x :H is very transparent.
Therefore it can also be used as a back reflector structure when its thickness is chosen
such that destructive interference occurs at the i-n interface, minimizing the electric field
strength and hence parasitic absorption in the n layer. Additionally, several tenths of nm
thick TCO can be used for the same purpose. Further, a metallic back reflector that also
acts as the electric back contact is used. Because of its attractive cost, aluminium is mainly
used for this purpose. However, silver that has a higher reflectivity can also be used, but it
is more expensive.
The bandgap of hydrogenated amorphous silicon is in the order of 1.75 eV, hence it
only is absorptive for wavelengths shorter than 700 nm. The highest current densities
light in the solar cell is also deposited first in the production process. Also the term p-i-n
layer refers to this superstrate configuration, as it indicates the order of the depositions:
because in thin-film silicon holes have a significantly lower mobility than electrons, the p
layer is in front of the n layer and hence the p layer is deposited first. As the generation
rate is then higher close to the p layer, more holes can reach it.
In superstrate thin-film silicon solar cells, glass is usually used as a superstrate
because it is highly transparent and can easily handle all the chemical and physical
conditions in which all the depositions are carried out. Before the p-i-n layers can be
deposited, a transparent front contact has to be deposited. Usually transparent conducting
oxides are used, that we introduced in Section 13.1.
Another possible configuration is the substrate configuration. There, either the
substrate acts like a back contact or the back contact is deposited on the substrate.
Consequently, no light will pass through the substrate. Thin-film silicon solar cells
deposited in the substrate configuration are also called n-i-p cells as the n layer is
deposited before the i layer and the p layer.
Usually, thin-film silicon solar cells have no flat interfaces, as shown in Figure 13.15
(a), but nanotextured interfaces, as illustrated in Figure 13.15 (c). These textured
interfaces scatter the incident light and hence prolong the average path length of the light
through the absorber layer. Therefore, more light can be absorbed and the photocurrent
can be increased. This is an example of light management that we discussed in Section
10.4.3.
Often, the p layer is not made from amorphous silicon. Instead, materials with a
higher bandgap are used, such as silicon carbide or silicon oxides in order to minimize
parasitic absorption mainly in the blue part of the spectrum. Usually, boron is used as a
dopant. Also for the n layer, silicon oxides are often used, but sometimes still a-Si:H is
utilized with phosphorus being the main dopant. The n-SiO x :H is very transparent.
Therefore it can also be used as a back reflector structure when its thickness is chosen
such that destructive interference occurs at the i-n interface, minimizing the electric field
strength and hence parasitic absorption in the n layer. Additionally, several tenths of nm
thick TCO can be used for the same purpose. Further, a metallic back reflector that also
acts as the electric back contact is used. Because of its attractive cost, aluminium is mainly
used for this purpose. However, silver that has a higher reflectivity can also be used, but it
is more expensive.
The bandgap of hydrogenated amorphous silicon is in the order of 1.75 eV, hence it
only is absorptive for wavelengths shorter than 700 nm. The highest current densities