Читать книгу Green Energy - Группа авторов - Страница 20

Fabrication of solar cells is based on semiconductor fabrication technology through which crystalline silicon solar cells are produced at the industrial scale. The conversion efficiency of industrial-grade silicon solar cells is still around 15%, which is a matter of concern. This is a costly technology and constant efforts are directed towards developing a cheaper method with maximized conversion efficiency of the solar cell. Intense research has been going on worldwide to develop organic solar cells, which is a low-cost process. Semiconductor manufacturing and subsequent device fabrication is a technology which requires certain well-defined procedures and protocols, such as, requirement of a clean room. A clean room is a room with special types of filters attached, for example, HEPA or high energy particulate filter with purifier) for extraction of smaller particles of 0.3 μm or larger in size. In certain cases, ultra-low particulate air (ULPA) filters are required when there is need to extract particles smaller than the aforementioned particles. There are well-defined protocols to work in these clean room fabrication laboratories or manufacturing facilities. A special clean room suite or gown is required before entering the laboratory or manufacturing facility and it is a requirement to pass through the chamber which sucks all smaller particles before entering the actual laboratory or facility. All the staff entering the clean rooms are required to take a written test and undergo extensive training before beginning to work in the laboratory. Moreover, special boots, gloves, teflon-made lab utensils like petri dishes are required in case of engineers performing etching, oxidation, development processes inside the clean room. A strategy of monitoring of contaminants, controlling various process variables and stringent feedback and feed-forward mechanisms ensure defect-free semiconductor manufacturing. Humidity and temperature control are other factors that are specifically required and a laminar flow of such controlled air is maintained. This is done to reduce distribution of contaminants along sideways inside the clean room.

The class of a clean room is determined by the number of particles of a given size enclosed in a particular volume of air. For illustration, a class 100 clean room consists of not more than 100 particles of a minimum size of 0.5 μm contained per cubic meter of the volume of air. Excessive cleanliness is the prerequisite of the industrial-grade fabrication processes due to the fact that introduction of an impurity atom even in parts per million or parts per billion can lead to manufacturing of a defect-infested device. This leads to the problems in overall modular or array structure of operation of solar cells and affects conversion efficiencies. Nowadays, all automated machines are present which perform all semiconductor manufacturing and processing tasks, such as, etching and ion implantation. No human interference or involvement is required physically in the clean room and this also ensures superior manufacturing of devices. However, considering the fact that conversion efficiencies of solar cells are still not that high, the costs involved in manufacturing are a bit higher, which severely limits its applications. Hence, there has been a surge around the world to find alternate technology that can produce highly efficient solar cells at a lower cost of manufacturing. This chapter presents a concise effort to apprise nitty-gritties of semiconductor physics and fabrication technology in relation to solar cells which is practically applied at the industrial level. The fabrication steps discussed in the chapter are actually applicable to various other types of semiconductor-based device manufacturing standards as well.