Manufacturing Pathways of Crystalline Silicon PV Modules

The manufacture of PV modules involves several stages, from quartz mining to PV module production, as shown in Fig. 2. The system starts with silica sand acquisition, of which only heat and sand are added to the first stage to obtain silica sand⁶¹ . Metallurgical grade silicon, a crucial stepping stone in the refining process of silicon metals, is then yielded by a carbothermic reduction reaction from silica sand with other material inputted, including petroleum coke, wet wood chips, etc., into the second stage^{62 (link)}. After metallurgical grade silicon is obtained, electronics grade silicon is produced through the Siemens process, which involves the deposition of silicon from a mixture of purified silane with an excess of liquid hydrogen onto high purity metallurgical grade silicon. Solar grade silicon is produced through a modified Siemens process, which involves additional processing to separate the toxic and corrosive gas from the reduction process of metallurgical grade silicon^{63 (link),64 (link)}. These procedures to obtain all these types of silicons are homogeneous regardless of c-Si technology type, although the quantities needed to produce the same functional unit of three types of c-Si PV modules are different. After solar grade and electronics grade silicon are obtained, the manufacturing configurations of PV systems start to differ by the type of c-Si selected as the semiconductor material to form cells and modules. When sc-Si is the semiconductor material, the Czochralski crystal growth technique is implemented to form sc-Si crystal blocks in an inert atmosphere, such as argon in this study^{65 (link)}. These crystals then go through the wafer sawing process in that individual silicon chips are mechanically separated from each other for cell manufacturing^{66 (link)}. When r-Si is the semiconductor material, solar grade silicon and electronics grade silicon are used directly for r-Si wafer production, of which carbon-based strings are pulled upward through holes with molten silicon, and sawing loss is avoided^{67 (link)}, leading to relatively low energy required to manufacture r-Si PV module compared with sc-Si and mc-Si technologies. When mc-Si is picked as the semiconductor material, solar grade silicon and electronics grade silicon are melted and cast into quartz crucibles to form mc-Si ingots^{68 (link)}. Similar to sc-Si crystals, mc-Si ingots then go through the process of wafer sawing^{69 (link)}. Processing of silicon wafers into solar cells involves texturing, acid cleaning, diffusion, etching, etc., while electrical contacts are placed between the cells and then wired and arrayed to form modules. Despite the differences in wafer types, the cell manufacturing and module assembly processes are similar for all three types of c-Si technologies^{70 (link)}.