1. Photovoltaic modules
Photovoltaic modules are multiple single solar cells interconnected and packaged into modules. It is the smallest indivisible solar cell module device that has external packaging and internal connections, and can independently provide direct current output. The output voltage of a single solar cell is often too low, the output current is not appropriate, and the crystalline silicon cell itself is relatively brittle, and it is difficult to independently withstand the harsh external conditions. Therefore, in actual use, single solar cells need to be connected in series and parallel, and packaged, contacting external wires, to become solar cell modules that can be used independently as photovoltaic power sources, also called photovoltaic modules.
2. Classification of silicon solar cells
Silicon solar cells can be divided into three categories: monocrystalline silicon solar cells, polycrystalline silicon thin film solar cells, and amorphous silicon thin film solar cells. The performance of the three types of silicon-based solar cells are listed below.
Monocrystalline silicon solar cells: The conversion efficiency is the highest, and the technology is the most mature; but the silicon consumption is large, the cost is high, and the process is complicated; the conversion efficiency is 16%-20%.
Polycrystalline silicon thin film solar cells: high conversion efficiency; but polycrystalline silicon production process is complicated and supply is limited; conversion efficiency is 14% to 17%.
Amorphous silicon thin-film solar cells: low cost, large-scale production; but the conversion efficiency is not high, the efficiency of light-induced degradation; the conversion efficiency is 9% to 13%.
(1) Monocrystalline silicon solar cells
This is a solar cell using high-purity monocrystalline silicon rods as raw materials, with the highest conversion efficiency and the most mature technology. High-performance monocrystalline silicon cells are based on high-quality monocrystalline silicon materials and related thermal processing techniques, as shown in Figure 1.
(2) Polycrystalline silicon thin film solar cells
The polycrystalline silicon solar cell is a polycrystalline silicon thin film laid on a low-cost substrate material, and a relatively thin crystalline silicon layer is used as the active layer of the solar cell, which not only has the high performance and stability of the crystalline silicon solar cell been maintained, but the amount of materials has been drastically reduced, significantly reducing the cost of the battery. The working principle of polycrystalline silicon thin-film solar cells is the same as other solar cells. The photovoltaic effect is formed based on the action of sunlight and semiconductor materials.
At present, the photoelectric efficiency of polycrystalline silicon thin-film solar cells can reach 16.9%. If a good method can be found to prepare polycrystalline silicon solar cells with good performance on a cheap substrate, the cells can enter commercial production. Polycrystalline silicon thin-film solar cells are a promising ground-use cheap solar cells due to their good stability and abundant material sources.
(3) Amorphous silicon thin film solar cells
The silicon used in amorphous silicon thin-film solar cells is a-Si, and its basic structure is not a PN junction but a pin junction. Boron doping forms the P region, phosphorus doping forms the N region, and i is an intrinsic layer that is not impurity or lightly doped. Figure 2 is a schematic diagram of the internal structure of the inline double junction amorphous silicon thin-film solar cell represented by the technology of EPV in the United States.
Outstanding features: low cost of materials and manufacturing process; low-temperature manufacturing process (100°C~300°C), low energy consumption; easy to form large-scale production capacity, production can be automated in the whole process; variety and wide use. The preparation process of the amorphous silicon thin-film solar cell is shown in Figure 3.
Problems of amorphous silicon thin-film solar cells: one is that the optical band gap is 1.7 eV, so amorphous silicon thin-film solar cells are not sensitive to the long-wave region, resulting in low conversion efficiency of the cell; the second is the photo-degradation effect-the photoelectric efficiency decays with the continuation of the illumination time.
The way to solve the light-induced degradation effect is to prepare a tandem solar cell, that is, to sink one or more p-i-n sub-cells on the prepared pin single-junction solar cell; production method: reactive sputtering method, PECVD method, LPCVD method; reaction gas: SiH4 diluted with H2; substrate material: glass, stainless steel, etc.