Читать книгу Green Energy - Группа авторов - Страница 26

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].