19 July 2010
By Peter Brown, Lattice Materials, LLC
In the quest to improve efficiency and reduce costs associated with solar energy, what goes into a solar cell at the very beginning is just as important as the energy that comes out. When the process begins, highly purified silicon is paramount.
Highly purified silicon (Si, >99.999% purity) is a well-known material for applications such as semiconductors, solar cells, and infrared optical components, both transmissive (lenses) and reflective (mirrors). The physical characteristics of the silicon are important, but they vary from one application to another. Physical characteristics include optical transmission, resistivity, type (N or P), orientation, carrier lifetime, index of refraction and purity.
Looking at the table below, resistivity is the one characteristic that’s of primary importance across the board (with the exception of reflective optics).
|
|
Electronics |
Solar Power |
Transmissive |
Reflective |
|
Optical Transmission |
N |
N |
P |
N |
|
Resistivity |
P |
P |
P |
N |
|
Type (N or P) |
P |
P |
S |
N |
|
Orientation |
P |
S |
S |
S |
|
Carrier Lifetime |
P |
P |
N |
N |
|
Index of refraction |
N |
N |
P |
N |
|
Purity |
P |
P |
P |
P |
Relevance of common silicon specifications, by application
P=primary. S=secondary. N=no importance
Resistivity is important in solar power generation because it shows the degree to which a material tends to impede the flow of electrical current. (Note that it is related to, but not equivalent with, resistance, expressed in Ohms.) Resistance is measured for an individual electrical component; i.e., the voltage applied across the component, divided by the amount of current conducted with that voltage applied. Resistivity is a bulk characteristic of the material itself, regardless of how that material is eventually processed into an electrical component.
Low resistivity materials conduct electricity better than high resistivity materials. For example, silver, an excellent conductor, has a resistivity of 1.59×10-8 Ohms·cm, whereas glass, an insulator, has a resistivity of 1010 to 1014 Ohms·cm. The middle contains pure silicon, a semiconductor with a theoretical resistivity of 6.40×104 Ohms·cm when no dopants are added.
Dopants and resistivity
When Czochralski growth is used in solar wafer manufacturing, it must contain the proper dopant at the right quantity in order to produce silicon that can be used in solar wafers. Without the dopant, the silicon is useless for the solar industry.
Nearly all applications for silicon require the addition of dopants to produce the particular resistivity required. Dopants are either P type (typically boron) or N type (phosphorous, arsenic, or antimony). Dopants are added to the silicon as part of the crystal growth process, most commonly Czochralski growth (Cz). The amount of dopant added is very small, typically in the parts per billion to parts per thousand range. The following table lists some values of resistivity, and the dopant concentrations that produce these values:
|
P (Boron) |
N |
|||||
|---|---|---|---|---|---|---|
|
Resistivity |
ppba |
atoms/cm3 |
atoms/gm |
ppba |
atoms/cm3 |
atoms/gm |
|
100 |
2.6 |
1.35 EE+14 |
5.6 EE+13 |
0.86 |
4.2 EE+13 |
1.8 EE+13 |
|
10 |
27 |
1.35 EE+15 |
5.6 EE+14 |
8.8 |
4.4 EE+14 |
1.9 EE+14 |
|
1 |
290 |
1.45 EE+16 |
6.2 EE+15 |
96 |
4.8 EE+15 |
2.1 EE+15 |
|
0.1 |
5500 |
2.8 EE+17 |
1.2 EE+17 |
1550 |
7.8 EE+16 |
3.4 EE+16 |
|
0.005 |
410000 |
2.0 EE+19 |
8.6 EE+18 |
240000 |
1.2 EE+19 |
5.3 EE+18 |
The actual amount of dopant added to the silicon is very small. For example, a typical solar application calls for P type material, with resistivity > 1 ohm-cm. From the table, we can see that this resistivity corresponds to 6.2 x 1015 atoms of boron per gram of silicon. For a 50 kg charge of silicon in the Cz furnace, this doping level equals about 5.5 milligrams of boron dopant.
Next Page: The way it works
| < Prev Article | Next Article > |
|---|
