Silicon solar cells are solar cells that use silicon as the substrate material. The earliest silicon solar cells arose from interest in using silicon for point-contact rectifiers. The rectifying properties of sharp metal contacts for various crystals were discovered as early as 1874.
In the early days of radio technology, this type of crystal rectifier was widely used as a detector in radio receiving equipment. However, with the development of thermionic tubes, this crystal rectifier has been largely replaced, except in the ultra-high frequency field. The most typical example of this type of rectifier is the point contact of tungsten on a silicon surface. This technology facilitated improvements in silicon purity and fueled the desire to further understand the properties of silicon.
Silicon solar cells have unique design and material requirements compared to most other silicon electronic devices. To achieve high energy conversion efficiency, silicon solar cells not only require near-ideal silicon surface passivation, but also must possess uniform and high-quality bulk material properties. This is because some wavelengths of light must propagate through silicon for hundreds of micrometers before being absorbed, and the resulting charge carriers must still be able to be collected by the cell.
Classification of silicon solar cells
Silicon solar cells are solar cells that use silicon as the substrate material. Based on the thickness of the silicon wafer, they can be divided into crystalline silicon solar cells and thin-film silicon solar cells. Based on the crystal morphology of the material, crystalline silicon solar cells are further divided into monocrystalline silicon (c-Si) and polycrystalline silicon (p-Si) solar cells; thin-film silicon solar cells are divided into amorphous silicon (a-Si) thin-film solar cells, microcrystalline silicon (c-Si) solar cells, and polycrystalline silicon (p-Si) thin-film solar cells.
1. Monocrystalline silicon solar cells
Monocrystalline silicon solar cells have the highest conversion efficiency and the most mature technology. The highest conversion efficiency in the laboratory is 24.7% (theoretically, the highest photoelectric conversion efficiency is 25%), while the efficiency in mass production is 18% (as of 2011). It still dominates in large-scale applications and industrial production. However, due to the high cost of monocrystalline silicon, significantly reducing its cost is difficult. To save silicon materials, polycrystalline silicon thin films and amorphous silicon thin films have been developed as alternatives to monocrystalline silicon solar cells.
2. Polycrystalline silicon solar cells
Polycrystalline silicon solar cells generally use low-grade semiconductor polycrystalline silicon or cast polycrystalline silicon specifically manufactured for solar cell applications. Compared to monocrystalline silicon solar cells, polycrystalline silicon solar cells have lower costs and their conversion efficiency is close to that of monocrystalline silicon solar cells, making them one of the main types of solar cells. Polycrystalline silicon solar cell wafers have low manufacturing costs, and the modules have high efficiency, reaching approximately 18% in mass production. The dominance of polycrystalline silicon solar cells is due not only to their superior performance but also to the abundant, inexpensive, non-toxic, and pollution-free availability of silicon raw materials. Furthermore, the recent reduction in the cost of polycrystalline silicon will further solidify their widespread adoption.
3. Amorphous silicon thin-film solar cells
Amorphous silicon thin-film solar cells are low-cost, lightweight, and easy to mass-produce, showing great potential. Amorphous silicon, unlike crystalline silicon, has an irregularly arranged atomic structure, making it an amorphous crystalline semiconductor. Amorphous silicon belongs to the direct ribbon system and has a high solar absorption coefficient; a film only 1 μm thick can absorb 80% of sunlight. Amorphous silicon thin-film solar cells were first introduced in 1976. The shortage and rising price of silicon raw materials spurred the development of technologies for high-efficiency silicon utilization and amorphous silicon thin-film solar cells. The low cost of amorphous silicon thin-film cells compensates for their lower photoelectric conversion efficiency. However, due to the numerous defects in amorphous silicon, the efficiency of the fabricated solar cells is relatively low, and they are also subject to photoelectric efficiency degradation caused by the material itself, resulting in low stability, directly affecting their practical applications.
Microcrystalline silicon (μc-Si) thin-film solar cells also suffer from performance instability due to photoelectric efficiency degradation, which limits their development.
Polycrystalline silicon thin-film solar cells have become a hot topic in solar cell research in recent years. Although polycrystalline silicon is an indirect bandgap material and not an ideal material for thin-film solar cells, with the continuous development of light trapping, passivation, and carrier confinement technologies, it is entirely possible to fabricate high-efficiency and inexpensive polycrystalline silicon thin-film solar cells.
