Page 180 - A Comprehensive Guide to Solar Energy Systems

182 A COmPrehenSIVe GUIDe TO SOlAr enerGy SySTemS

Table 9.1 Some Material Properties of Crystalline Silicon at Temperature 300 K
Thermal Thermal Expan-
Atomic Density Density Lattice Constant Melting Point Conductivity sion Coefficient
−1
−6
28
5 × 10 m −3 2328 kg m −3 0.5431 nm 1415°C 150 W m K −1 2.6 × 10 K −1
Intrinsic Carrier Relative Maximum Electron Maximum Hole
Energy Bandgap Concentration Permittivity Mobility Mobility
1.12 eV 1 × 10 m −3 11.9 0.143 m V s −1 0.047 m V s −1
−1
2
−1
16
2
• microcrystalline silicon (µc-Si) with grain size below 1 µm; and
• nanocrystalline silicon refers to a range of materials around the transition region from
microcrystalline to amorphous phase.
The unique properties of Si and SiO 2 enabled the development of integrated circuit
technology that has been the basis of present-day microelectronics. many fabrication
tools have been developed and are used in silicon devices technology. Details about par-
ticular techniques like diffusion, photolithography, ion implantation, chemical wet and
dry processes, and so on can be found in ref. [1].
The silicon energy bandgap determines the ultimate efficiency of PV cells made from
c-Si; this value is 29.4%. As already explained in Section 8.4.2, c-Si solar cells have to be fabri-
cated from wafers of multi-crystalline or mono-crystalline silicon. In the following sections,
the technological processes from preparing pure silicon, to silicon wafer fabrication, to cell
design and fabrication, and finally to PV module design and fabrication will be discussed.

9.2.1 Semiconductor Silicon Manufacture Technology
In nature, silicon occurs only in the form of oxides and silicates. Silicon is produced by the
carbothermic reduction of silica. The so-called metallurgic-grade silicon is produced in a
graphite crucible from silicon dioxide (SiO 2 ) of high-quality lumpy quartz, by reduction
with carbon (metallurgical coal) in an arc furnace, as illustrated in Fig. 9.1. The reduction
process takes place at approximately 1800°C, according to
+
SiO 2 +2C→Si+2CO SiO 2 + 2C → Si 2CO
The liquid silicon of purity of approximately 98% is collected by drawing it off at the
bottom of the crucible. The reduction process is described in more detail in refs. [2,3].
To obtain electronic grade silicon, the impurities (Fe, Al, O, Ca, Cu, and others) must be
removed. There are several processes that can be used to produce silicon with a purity of
greater than 99.9999%. The two most important processes are the Siemens method and
the fluidized bed reactor (FBr) method.

9.2.1.1 The Siemens Method
The main features of this technology are described in more details in refs. [2,3]. In the first
stage, pulverized metallurgic silicon is exposed in a FBr (at temperature approximately
350°C) to hydrochloric gas to prepare trichlorsilane hSiCl 3 :