2.2.1 Single-Crystalline PV Cells
The single-crystalline PV cells are started to use in power production since 1970. It is also called first-generation photovoltaic cell, and this type of technology is more reliable, durable, and good for long time power generation.
Figure 2.2 displays schematic diagram of single-crystalline photovoltaic cells. PV solar manufacturing industry produces multi-crystalline photovoltaic solar cells from highly purified form of silicon. Chemical purification process has been employed for producing multi-crystalline solar cells, metallurgical silicon is used as raw material, and this type of process is also called as Siemens process. This process also involves refinement of unstable silicon compounds into stable silicon at higher temperature. It produces higher grade metallurgical silicon by using metallurgical technique as an alternative of chemical purification process. Multi-silicon contains less than one part of one billion impurities, which is known for less purity silicon grade. Multi-crystalline silicon comprises of small crystals, which gives materials in the form of wafer effects. The size of multi-crystalline solar PV cells is less than 1 mm. It is common type of PV cells, required nearly 5 tons of multi-crystalline silicon for producing 1-megawatt conventional solar unit. These multi-crystalline solar cells are different from single and amorphous crystals.
Figure 2.2 Single-crystalline PV cells.
2.3 Second-Generation Solar PV Technology
Table 2.2 listed manufacturers of second-generation photovoltaic solar cells. It is single connection device, which uses lesser materials and continuing efficiency as first-generation PV cells. Second-generation PV cells, namely, cadmium sulfide/cadmium telluride, amorphous, and multi-crystalline silicon, can be deposited on low expensive substrate called glass. These materials have higher efficiency compared to single-crystalline and multi-crystalline solar cells, and 1- to 10-micrometer materials are only utilized for solar cell fabrication. Light trapping technology has been used to increase efficiency of polycrystalline materials upto 10%.
2.3.1 Amorphous Silicon PV Cell
One or more PV solar cell materials are deposited on the substrate, which is called amorphous silicon. Figure 2.3 shows amorphous silicon photovoltaic solar cell. Vapor depositing technology is used to deposit 1-micrometer silicon materials on substrate (glass or metal). Depositing temperature is upto 75°C for plastic material as substrate. One of major drawbacks is, when exposed to sunlight, degradation has been happened in their output power. Electric field strength is to be increased when thickness of material reduced [3, 26]. Unisolar is a company which inventing solar PV cells using silicon thin film. The three-layer system can be used to optimize absorbing sun light from solar spectrum.
Table 2.2 Second-generation PV cells.
| Manufacturer | Country | Technology |
| Amysolar | France | Amorphous silicon |
| 3sun | Italy | Amorphous silicon |
| Anwell Precision | Hong Kong | Amorphous silicon |
| EcoMedia | Italy | Cadmium sulfide/cadmium telluride |
| Dupont | United States of America | Copper indium gallium diselenide |
| Nexcis | France | Copper indium gallium diselenide |
| First Solar | United States of America | Copper indium gallium diselenide |
Figure 2.3 Amorphous silicon PV cell.
2.3.2 Cadmium Telluride PV Cell
Cadmium telluride is a thin layered film semiconductor, which is used to convert receiving solar light into electrical energy [25]. The cadmium PV solar cell consists of two layers, which are top and bottom layers; the bottom layer is made up of carbon paste and top layer is made up of cadmium built stannous oxide or tin oxide, which is shown in Figure 2.4. Cadmium telluride is located between the top layer and semiconductor. The cadmium telluride-based solar PV cells are the second highest technology available in the market
