Читать книгу Green Energy - Группа авторов - Страница 29

Classically, c-Si type PV cells are generally based on the wafers of silicon with doping of p-type. This enables better and efficient transportation of major hole carriers to the destined location; however, it is required to keep lower order to make auger recombination to the minimal. The preference of p-type over n-type Si is because of its hardness to bear space radiation and due to the same; it finds application in satellite technology. Simplicity in manufacturing of wafers is also one of the reasons for it. The role of wafers of silicon within the device enables better absorption of light on a relatively large area, and it allows photo-generated carriers of charge with quite a less amount of recombination. Contacts of carrier selective in the case of a solar cell represent the two surfaces where high doping has been done. Classically, the designs were obtained as a result of doping with phosphorus and aluminium are set of electron and hole in front and back side respectively. This completes the structure of crystalline homo-junction PV cell. For optimization of optics, the surfaces of wafers are textured up to the length of a few micrometers for the reduction in the amount of backlight that has been reflected and at the same time increasing the length of the path for the photons that is located at the inner side the absorber. Further, the reflection in the front is lowered by incorporating a coating of anti-reflection and that too again is dependent on the refractive index obtained by matching with the thickness of a quarter wavelength and a layer of passivation. The sunlight exposed front side is completed using an electrode made of metal and the back side is made fully metallic. This rear part that has been made of metallic plays an important role of reflecting the longer wavelength light not absorbed in the silicon when it is allowed to pass through the wafer. After that, chemically treated results in the same structure on both the faces. The benefit of this process is removal of a few microns of the damaged material with the help of wire sawing. The wafers of mono-Si are etched anisotropically with potassium hydroxide, thus producing random pyramids of square shape. The case of mc-Si is very much different as it needs isotropic etching (grains are differently oriented) by the help of solutions that are acidic in nature. The wafers are allowed to undergo cleaning by the diffusion of phosphorus by exposing it to POCl₃ (phosphorus oxychloride) and using oxygen with nitrogen as a carrier. This is done by the use of a furnace of quartz with very high temperature of about 750-850 °C. This step helps in the creation of emitter region that has been created as a result of unwanted doped regions in the front and the rear partand in the later stages removed because of several issues of selectivity. Diffusion of phosphorus raises the intake of impurities from the wafers in bulk, but making them harmless. The silicate of phosphorus glass produced the wafer surface, while in process of diffusions are removed with the help of etching and the coating of anti-reflection which is to be applied on the front side. The stratum, with the refractive list nearby two in the significant zone and depth of about 75 nm with impact of ideal anti-reflection, likewise assumes a passive job via providing hydrogen (H) end to Si dangling-bondon the edge and by impact of field, its thickness of fixed positively charges are high. The wafer of mc-Si, its surface enables the beneficial effect of H here on abandons within the wafer in the last stage of termination. Metal contacts are then distinguished by the subsequent printing of different Ag and Al adhesives, consisting of metal fragments, glass frits as a seal, in addition to solvents and other natural additives. In face side printlike examples on H comprising 50-100 nm wide finger gridlines associated by more extensive busbars. At the back, cushions as a consistent or an intruded on line are right off the bat printed, trailed by complete Al inclusion. The cushions are used to absorb the current from the metallized area, ensuring a highly conductive welding anode.