As we all know, amorphous silicon and microcrystalline silicon materials grown by plasma enhanced chemical vapor deposition (PECVD) have always been promising materials in the preparation of thin-film solar cells.
Among them, amorphous silicon cells have extremely high light absorption coefficients and are easy to mass produce. They are currently the most common silicon-based thin-film cells. However, amorphous silicon has two major defects: the photodegradation effect (SW effect: after a long period of strong light irradiation or current passing through amorphous silicon thin films, defects will be generated inside the thin films, causing the performance of the thin films to decline, which is called the SW effect. However, after high-temperature annealing in summer, the photodegradation can be partially restored) and insufficient absorption of long-wavelength light, which also affects the conversion efficiency to a certain extent. In order to reduce the impact of photodegradation, amorphous silicon materials are usually made into amorphous silicon/amorphous silicon double junction structures. The first junction amorphous silicon is used to absorb short-wavelength light waves, and the second junction is used to absorb long-wavelength light waves.
Using microcrystalline silicon material to replace amorphous silicon at the second junction will better solve the photoinduced degradation effect, while increasing the absorption of long-wavelength light to improve the conversion efficiency of the device. However, the expensive equipment price limits the large-scale production of microcrystalline silicon.
Another method to improve the efficiency of silicon-based thin-film solar cells is to add an appropriate amount of germanium when depositing the amorphous silicon intrinsic layer (i.e., the undoped area) to make an amorphous silicon/amorphous silicon germanium/amorphous silicon germanium triple junction device. Amorphous silicon germanium not only has the high absorption coefficient of amorphous silicon, but also has the long-wave absorption function of microcrystalline silicon. Therefore, amorphous silicon germanium is a very ideal thin-film solar cell material.
Adding appropriate germanium elements to amorphous silicon can improve the absorption of long-wavelength light. By changing the germanium content, the light absorption efficiency of the three sub-cells of amorphous silicon germanium can be optimized, because each sub-cell of this three-junction stack structure absorbs light waves of the corresponding band and can absorb the light of each band more fully. Due to its good absorption coefficient, each absorption layer can be made very thin, so that the transmission distance of carriers (current carriers) is shorter and more conducive to collection, thereby obtaining a higher fill factor and reducing the light-induced attenuation effect.
Of course, any material that has not been widely used in industry has its own challenges and difficulties to overcome. For amorphous silicon germanium, the difficulty is how to make a large-area uniform amorphous silicon germanium film layer. As the raw material gas of germanium, germanium is easier to decompose than silane in the plasma field. Therefore, when making thin films on a large area, the uneven distribution of germanium content seriously affects the conversion efficiency of the device. In particular, in order to better absorb infrared light waves, the germanium content must be increased on the third junction cell, which will inevitably aggravate the uneven distribution of germanium. This spatial uneven distribution is particularly obvious in the single-chamber multi-chip parallel mode PECVD system. Because germanium is more easily decomposed, the difference in germanium content between the upper and lower parts of the device made in this parallel mode is very large, causing the conversion efficiency of the device to drop to an unacceptable level. In view of the high price of germanium, the single-chamber multi-chip parallel mode PECVD equipment is a very ideal and economical industrial equipment. Depositing high-quality amorphous silicon germanium films on this equipment is very promising. In recent years, PULE has been engaged in research and development and production in this area. At present, it can deposit amorphous silicon germanium films with good uniformity in a single-chamber multi-wafer parallel mode PECVD system.
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