General photovoltaic cells rely on the photosensitive properties of semiconductor materials to convert light energy into electrical energy. Therefore, they are all made of semiconductor materials. It can be said that semiconductors are the host materials for the manufacture of photovoltaic cells.
Photovoltaic cells have requirements for semiconductor quality, especially purity, and not any semiconductor material of any purity can be used to make photovoltaic cells. For example, the purity of polycrystalline silicon used in silicon crystalline photovoltaic cells must be above 6N and less than 6N, and it is basically useless. why?
Because practice has proved that impurities are harmful to photovoltaic cells, and very small amounts of impurities can have a decisive impact on the physical and chemical properties of semiconductor materials, resulting in fatal hazards. If silicon is doped with B impurities in a ratio of 105:1, the conductivity increases by a factor of 1000. That is to say, adding one-hundred-thousandth of B impurities into silicon can increase the conductivity of silicon by 1000 times. If the impurities in silicon are a little more, the silicon will undergo qualitative changes, and it may not be a semiconductor, nor will it have photosensitive properties. Therefore, for photovoltaic cells, purity is crucial.
In order to ensure the quality of photovoltaic cell products, such a stringent purity standard was formulated for polysilicon, and it was decided that polysilicon with a purity of more than 6N must be used in the production of photovoltaic cells.
Why must the purity of polysilicon used in the production of photovoltaic cells be above 6N?
This depends on the photoelectric conversion efficiency of the photovoltaic cell. At present, the photoelectric conversion efficiency of monocrystalline silicon photovoltaic cells required internationally is above 17%. According to the current production technology, only polysilicon with a purity higher than 6N is competent, so polysilicon with a purity lower than 6N is not used. But it does not mean that polysilicon with a purity lower than 6N cannot produce photovoltaic cells. Polycrystalline silicon with a purity of less than 6N can also produce photovoltaic cells. At present, many manufacturers have produced photovoltaic cells with 5N polysilicon produced by physical methods, and the initial photoelectric conversion efficiency of the produced cells has reached about 15%.
However, the retention period of the photoelectric conversion efficiency of such photovoltaic cells is too short and the decay is too fast. After one or two days of light experiments, the photoelectric conversion efficiency of these photovoltaic cells decays to 10%~12%.
Such photovoltaic cells, if installed on the roofs of ordinary people, can also generate electricity, but the difference is that the same power generation requires a larger area of photovoltaic cells, more photovoltaic cells, and a short service life. . In a word, this kind of photovoltaic cell is of low quality, and its use value is too low to be suitable for development.
The quality of photovoltaic cells produced with 5N polysilicon is low, then the quality of photovoltaic cells produced with polysilicon below 5N is naturally even lower. Therefore, according to the international requirements for photovoltaic cell photoelectric conversion efficiency and existing production Technology, polysilicon below 5N cannot be used to produce photovoltaic cells. That is to say, in order to ensure the quality of photovoltaic cells, the materials used in photovoltaic cells, especially semiconductor materials, must have purity requirements, which are extremely demanding.
