12.5
protective layer is deposited onto the back side prior to the emitter diffusion. This layer
would prevent the creation of an n-doped layer at the back.
Now the p-n junction in principle is finished; we only need to make the electrical
contacts at the front and the rear that allow us to connect the solar cell with an electric
circuit or with other solar cells in a PV module. In industry these contacts are usually
fabricated using screen printing processes followed by firing. These processes are
explained in detail in Chapter 14. Different screen printing pastes are used, which are
usually based on aluminium or silver as a metal. Ag paste is for the front contact
commonly printed on top of the passivation layer. As we have seen in Section 12.3, the
design of the front metal grid needs careful optimization between resistive and shading
losses. In contrast the rear side is totally covered with Al.
To make real contacts out of the screen printing pastes, the cell is put in a belt furnace
at 850 °C. This process is called firing; if both the top and front contacts are fired at the
same time, we call it co-firing. As a result of the co-firing process the front side Ag paste
etches away the underlying SiN layer, forming a direct contact with the emitter.
During the co-firing process Al atoms diffuse into the wafer at the rear side and act as
a p-type dopant, forming a p
+
layer at the rear of the device. Because of this layer, a back
surface field (BSF) is created, which enhances the performance of the solar cell as we
have seen in Section 12.3. However, during this process a eutectic layer is also created at
the rear side, which leads to high parasitic absorption and hence low internal reflection of
light that penetrates through the entire solar cell to the back. Usually, also Ag busbars are
screen-printed on the back side for the interconnections with other cells.
The last step is to create isolating grooves at the border of the solar cells. This for
example can be done with laser scribing. Isolating grooves are very important to prevent
leakage currents along the sides of the solar cells. Such leaking currents can be very
detrimental to the solar cell performance.
High-efficiency concepts
In the last section of this chapter we discuss three examples of high-efficiency concepts
based on crystalline silicon technology. As already discussed, different types of silicon
wafers with different qualities can be used. Naturally, to achieve the highest efficiencies,
the bulk recombination must be as small as possible. Therefore the high efficiency
concepts are based on monocrystalline wafers.
protective layer is deposited onto the back side prior to the emitter diffusion. This layer
would prevent the creation of an n-doped layer at the back.
Now the p-n junction in principle is finished; we only need to make the electrical
contacts at the front and the rear that allow us to connect the solar cell with an electric
circuit or with other solar cells in a PV module. In industry these contacts are usually
fabricated using screen printing processes followed by firing. These processes are
explained in detail in Chapter 14. Different screen printing pastes are used, which are
usually based on aluminium or silver as a metal. Ag paste is for the front contact
commonly printed on top of the passivation layer. As we have seen in Section 12.3, the
design of the front metal grid needs careful optimization between resistive and shading
losses. In contrast the rear side is totally covered with Al.
To make real contacts out of the screen printing pastes, the cell is put in a belt furnace
at 850 °C. This process is called firing; if both the top and front contacts are fired at the
same time, we call it co-firing. As a result of the co-firing process the front side Ag paste
etches away the underlying SiN layer, forming a direct contact with the emitter.
During the co-firing process Al atoms diffuse into the wafer at the rear side and act as
a p-type dopant, forming a p
+
layer at the rear of the device. Because of this layer, a back
surface field (BSF) is created, which enhances the performance of the solar cell as we
have seen in Section 12.3. However, during this process a eutectic layer is also created at
the rear side, which leads to high parasitic absorption and hence low internal reflection of
light that penetrates through the entire solar cell to the back. Usually, also Ag busbars are
screen-printed on the back side for the interconnections with other cells.
The last step is to create isolating grooves at the border of the solar cells. This for
example can be done with laser scribing. Isolating grooves are very important to prevent
leakage currents along the sides of the solar cells. Such leaking currents can be very
detrimental to the solar cell performance.
High-efficiency concepts
In the last section of this chapter we discuss three examples of high-efficiency concepts
based on crystalline silicon technology. As already discussed, different types of silicon
wafers with different qualities can be used. Naturally, to achieve the highest efficiencies,
the bulk recombination must be as small as possible. Therefore the high efficiency
concepts are based on monocrystalline wafers.