4 Solar Cells: Optical and Recombination Losses
83
the year compared to the situation, where the modules are mounted with a fixed
angle.
3. Double layer coating: If the reflections are to be further reduced over a wide
range of wavelengths and angles, it is necessary to stack several layers with different refractive indices and thicknesses, one above the other. Figure 4.7 illustrates
the reflection of different ARC stacks. Basically, an attempt is made to keep the
reflectance low at the wavelength of light where the greatest amount of light
energy hits the earth. This means the reflection should be low between 550 and
900 nm (see Chap. 2, solar spectrum). The double layer for which data is given
in Fig. 4.7 uses two different ARC layers SiN x and SiO x . Double layers are used
in newer cell concepts (see Chap. 5, Sect. 5.4.2).
4.1.5 Wafering and Its Effects on Light Trapping
Texturing is one of the most important ways to improve light trapping. The wafering
process has a significant influence on the texture and, thus, on the efficiency of the
solar cell. Therefore the different wafering processes will now be described in more
detail here. In our discussion here we start out with silicon ingots,
8 which are cut into
thin wafers with a thickness of 120–200 μm. Today two processes are used in mass
production: the slurry process and the cutting process with diamond wire. Figure 4.8
illustrates the two processes.
1. Slurry process: In the slurry process, a brass coated steel wire with a diameter of ~100 μm pushes silicon carbide grains, which are about 20–30 μm in
size, through the ingot. The silicon carbide (SiC) grains are harder than silicon.
The slurry itself is a liquid containing oil, glycol and SiC grains. This process
was mainly used for multicrystalline material until 2017. Process time is about
5 h.
9 The process is illustrated in Fig. 4.8a.
2. Diamond wire: In the diamond wire process, a steel wire with a diameter of
~60 μm is covered with small diamonds, which have a size of 5–20 μm, and which
are attached to the wire by means of a nickel layer. This wire saws the silicon
ingot into wafers. This process is mainly used for monocrystalline material. It
is very efficient, process time is 1–2 h, saw damages are less pronounced and
kerf loss is smaller than in the slurry process. In the diamond wire process, the
wire is run in the so-called Pilgrim Mode.
10 In this way, the service life of the
wire can be increased. Wire consumption is about 0.5 m/wafer. The process is
8 Manufacturing of ingots and silicon crystals will be described in Chap. 5.
9 In semiconductor industry this type of wafering is still used.
10 In Pilgrim mode, the diamond wire is fed forward and backward through the silicon, similar to
cutting wood with a hand saw. In the forward cut one feeds more wire than in the backward cut.
In this way, fresh wire is added. In forward cutting, for example, 600 m of wire is used for cutting
and in reverse cutting less wire is used, for example 580 m of wire. So 20 m of fresh wire is fed per
cycle into the cutting process.
83
the year compared to the situation, where the modules are mounted with a fixed
angle.
3. Double layer coating: If the reflections are to be further reduced over a wide
range of wavelengths and angles, it is necessary to stack several layers with different refractive indices and thicknesses, one above the other. Figure 4.7 illustrates
the reflection of different ARC stacks. Basically, an attempt is made to keep the
reflectance low at the wavelength of light where the greatest amount of light
energy hits the earth. This means the reflection should be low between 550 and
900 nm (see Chap. 2, solar spectrum). The double layer for which data is given
in Fig. 4.7 uses two different ARC layers SiN x and SiO x . Double layers are used
in newer cell concepts (see Chap. 5, Sect. 5.4.2).
4.1.5 Wafering and Its Effects on Light Trapping
Texturing is one of the most important ways to improve light trapping. The wafering
process has a significant influence on the texture and, thus, on the efficiency of the
solar cell. Therefore the different wafering processes will now be described in more
detail here. In our discussion here we start out with silicon ingots,
8 which are cut into
thin wafers with a thickness of 120–200 μm. Today two processes are used in mass
production: the slurry process and the cutting process with diamond wire. Figure 4.8
illustrates the two processes.
1. Slurry process: In the slurry process, a brass coated steel wire with a diameter of ~100 μm pushes silicon carbide grains, which are about 20–30 μm in
size, through the ingot. The silicon carbide (SiC) grains are harder than silicon.
The slurry itself is a liquid containing oil, glycol and SiC grains. This process
was mainly used for multicrystalline material until 2017. Process time is about
5 h.
9 The process is illustrated in Fig. 4.8a.
2. Diamond wire: In the diamond wire process, a steel wire with a diameter of
~60 μm is covered with small diamonds, which have a size of 5–20 μm, and which
are attached to the wire by means of a nickel layer. This wire saws the silicon
ingot into wafers. This process is mainly used for monocrystalline material. It
is very efficient, process time is 1–2 h, saw damages are less pronounced and
kerf loss is smaller than in the slurry process. In the diamond wire process, the
wire is run in the so-called Pilgrim Mode.
10 In this way, the service life of the
wire can be increased. Wire consumption is about 0.5 m/wafer. The process is
8 Manufacturing of ingots and silicon crystals will be described in Chap. 5.
9 In semiconductor industry this type of wafering is still used.
10 In Pilgrim mode, the diamond wire is fed forward and backward through the silicon, similar to
cutting wood with a hand saw. In the forward cut one feeds more wire than in the backward cut.
In this way, fresh wire is added. In forward cutting, for example, 600 m of wire is used for cutting
and in reverse cutting less wire is used, for example 580 m of wire. So 20 m of fresh wire is fed per
cycle into the cutting process.