has the highest efficiency with 20.4%.
Another option next to Si solar-based cells is the slender film solar-powered cells manufactured on glass substrates. The primary negative aspects of utilizing glass substrates are the delicate nature of modules, the cost of glass wafer (300-400μm), and low explicit force (kW/kg), and so forth. Explicit force is a significant factor when sunlight-based cells are utilized in space applications. A high explicit force surpassing 2 kW/kg can be accomplished [4] by adaptable sunlight-based cells on polymer films, which is helpful for earthly as well as space applications. Creation cost can be brought down by utilizing adaptable substrates and roll to roll creation (R2R) system. Besides lightweight, adaptability, and less expense of establishing, adaptable cell preparation also includes a low warm spending plan with low material utilization. Other than sun-based cell applications, lesser specific applications are starting to turn out to be progressively feasible autonomous markets, including applications for versatile force and building or item combination, which can profit enormously from adaptable slight film alternatives. Adaptable cells on structures (known as building incorporated photovoltaics or BIPV) can limit the expense of help, shipments, and so on, and establishments can be taken care of without any problem. Be that as it may, adaptable solar cell innovation is less developed when contrasted with the cells created on unbending substrate partners. Because of four principle prerequisites – high effectiveness, minimal effort creation, high throughput, and high explicit force, a significant innovative work center has been moved towards adaptable/flexible PV cells [5].
Figure 2.1 A typical solar cell design [2].
Figure 2.2 Efficiencies of the different substrate when deposited with CIGS [3].
2.2 Silicon Solar Cell Technologies
The most basic form of a solar cell contains a light-absorbing semiconductor having hole and electron selective contacts of fixed energy bandgap for carrying extraction and separation of charges. The absorber materials used in solar cells are nontoxic, abundant, stable and its characteristics are well known. The available technologies, i.e., thin-film and crystalline depend solely on the manufacturing equipment and knowledge of solid forms of Si as well as know-how from the microelectronics industry.
2.2.1 Crystalline Structured Silicon (c-Si)
The part that absorbs c-Si-cells consists of fine wafers of Si in the form of crystalline. This is a well-known fact that the band-gap is 1.12 eV for Si, a value that is very close to spectrum of the sun and the same is closer to the optimal conversion quantity from sunlight to electrical energy while utilizing an absorber of a light source. The silicon, in general, has a very low coefficient of absorption near the infrared region. With the advancement in the design of light management techniques such as coating of antireflection, rear mirrors, use of efficient light absorption and enhancement of surface texture have enabled these techniques to be used in very thin layers, i.e., even up to 100 mm thin layers while presently crystalline silicon is being produced only up to 180 mm thin [6]. On the better side, the indirect bandgap and also inefficiency in the recombination of radiative terms, provides longer life to the generated electrons and holes. The impurities, also known as crystallographic defect have a very adverse effect on the recombination of the carrier in silicon. Recently, the quality of c-Si thin layers has been increased because of improvements in passivation of the surface and the schemes of contact. To minimize the recombined photogenerated carriers at the surface of silicon, the method of passivation is used. By the means of termination of hydrogen to the dangling-bonds of the detrimental side, and by the use of the mechanism of field effect and also by providing the inbuilt electric field helps in repelling charge carriers from the interface. The passivation technique is usually attained by the use of dielectric materials which include silicon nitride, aluminum oxide, silicon oxide, hydrogenated amorphous silicon, and siliconcarbide [7]. If one considers crystalline silicon cells, then one must see that practically to apply the mono and multi-crystalline Si thin wafer, Si blocks made of casting, and ingots are often utilized for manufacture of c-Si-based solar panel. On the basis of market survey, it can be concluded that thin multi-crystalline silicon(mc-si) wafers have a share in the PV market of more than 50% in the manufacturing industry of the silicon solar cell [6]. Czochralski method is the conventional scheme for the enhancement of ingots for single crystals used in the manufacturing solar units. This process consists of putting the Si in the crucible of pure quartz, then simultaneously pulling it upwards and at the same time rotating it. A better, but more expensive available alternative is FZ-Si (Float zone silicon) method. In it, a skin current produced by the electromagnetic field is used to heat the 99% pure polycrystalline rod, and then the molten zone of silicon is allowed to pass through it. The silicon is not allowed to come in contact with the surroundings except for the ambient conditions that are made inert in nature. This process also purifies the impurities during the melt, resulting in very low contamination. The method used for the manufacturing of mc-Simodule sare where Si is provided in a molten form then converted to blocks and finally allowed to cool to get the finished product. It can be concluded that the mc-Si is much cheaper than mono-Si as one can visualize that there will be a larger amount of impurities as well as a defect in mc-Si and as a result of the manufacturing process used it will be more brittle. Due to continuous improvement in the quality of material, now both the form of silica solar cells can get efficiencies as high as 22% [8].
2.2.2 Silicon-Based Thin-Film PV Cell
When comparisons are done on consideration of the price between crystalline wafers and thin-film technology, the latter does not demand the expensive sawing and crystal traction. This technology as well restricts the losses due to kerf, which is normally about 100 mm per cut of wafer and is an added benefit as compared to c-Si technology. In its manufacturing process, layers of silicon are allowed to get deposited on the glass or on other very low cost material at the temperature which should be very low, even below the melting temperature of c-Si. It is quite possible to have other types of silicon films that are electronically very active and are deposited with the help of plasma on the surface of the Si and H and most of them contain silane. The commonly available scheme is plasma-enhanced chemical vapour deposition (PECVD) in which reactants the kept over the electrodes of a structure made of parallel plate and that too within a reactor chamber with the vacuum. Thereafter, precursor gases having controlled flows are allowed to enter into the area where RF power, of frequency 13.56 MHz, is introduced to the electrodes. Sometimes, frequencies of greater discharge are also used in PECVD technique. Generally, thin-film type silicon is amorphous as compared to FCC c-Si lattice and the earlier contains a large amount of H, which gets deposited in due course. The atoms of silicon try to retain its tetrahedron shape due to covalent bonds with four other atoms of silicon. Because of the distortions in the bond angle and also due to the dangling nature of bonds, distortions are normally observed in the long and medium range. Uniformity of structures is often found in tiny volumes and because of the principle of uncertainty, momentum of the charge could not be determined at a fixed spatial location [9]. The family of Si thin-film also involves mc-Si:H, nitrogen and oxygen compounds, and alloys of carbon or germanium. To make it more precise, it should be understood that it’s not only material, yet it forms mixed phase class of materials with the wider scale of very small-scale structures of micron level where phases are mixed and the conditions are mainly dependent on the rate of deposition and the substrate. The process of deposition begins with a phase of amorphous and a minimum thickness is needed before the nucleation process starts. At the occurrence of nucleation phase, the crystal begins to grow around the deposits. When the alloying of silicon is done with germanium, the band gap becomes narrower as the content of
