Physical properties and manufacturing process advantages of amorphous silicon solar cells

Due to its unique physical properties and advantages in fabrication technology, amorphous silicon has become one of the research focuses and cores of large area high-efficiency Solar Energy battery. Amorphous silicon is a good photoconductor because of its high absorption coefficient of sunlight and the best photoconductivity; It is easy to achieve high concentration doping and obtain excellent PN junction; Its energy gap can be controlled in a wide range of components. As early as the early 1970s, Carlson et al. had begun the research and development of amorphous silicon solar cells. In recent years, the research and development work has been rapidly developed. At present, many companies in the world are producing amorphous silicon solar cells. Although amorphous silicon is a good solar cell material, its optical band gap is 1.7eV, which makes the material itself insensitive to the long wave region of the solar radiation spectrum, thus limiting the photoelectric conversion efficiency of amorphous silicon solar cells. In addition, its photoelectric conversion efficiency will decline with the duration of illumination, which is the so-called light induced decay effect, making the performance of solar cells unstable. In amorphous silicon, there are many defects due to the lack of regularity in the arrangement of atoms in crystalline silicon. Therefore, in the pure amorphous silicon PN junction, the tunnel current is often dominant, which makes it present the tunnel current characteristics without rectification characteristics. In order to obtain good diode rectification characteristics, a thicker intrinsic layer i must be added between the P layer and the N layer to curb its tunnel current. Therefore, amorphous silicon solar cells generally have a PiN structure. In order to improve the photoelectric conversion efficiency and stability, sometimes a multilayer PiN stack cell is made, or some transition layers are inserted

The laminated solar cell is made by depositing one or more PiN layer cells on the prepared PiN layer single junction solar cell.

Laminated solar cell

The key problems to improve the photoelectric conversion efficiency and solve the instability of single junction solar cells are:

(1) It combines materials with different bandgap widths to improve the spectral response range.

(2) The i-layer of the top cell is thin, and the electric field intensity generated by the light changes little, so as to ensure that the photogenerated carriers in the i-layer are pulled out.

(3) The carrier produced by the bottom battery is about half of that of the single battery, which leads to the reduction of the decay effect.

(4) The sub cells of the stacked solar cell are connected in series.

Amorphous silicon solar cells are the most completely developed thin-film solar cells. Their structures are usually in the form of PiN (or NiP). The main role of P and N layers is to establish the internal electric field, while layer i is composed of amorphous silicon. The thickness of layer i is usually only 0.2~0.5 μ m。 Its band gap width is about 1.1~1.7eV, which is different from the 1.1eV of silicon wafers. Amorphous materials are different from crystalline materials and have low structural uniformity. Therefore, electrons and holes conduct inside the materials. If the distance is too long, the coincidence probability of the two is very high. To avoid this phenomenon, layer i should not be too thick, but if it is too thin, it is easy to cause insufficient light absorption. In order to overcome this problem, this type of solar cell is designed with multi-layer structure stack to give consideration to light absorption and photoelectric conversion efficiency.

There are many methods to prepare amorphous silicon thin film solar cells, including reactive sputtering, PECVD, LPCVD, etc. The reaction raw gas is SiH4 diluted with H2, and the substrate is mainly glass and stainless steel sheets. The amorphous silicon thin film can be prepared into single junction cells and stacked solar cells through different cell processes.

Preparation methods of amorphous silicon thin film solar cells

Amorphous silicon solar cells are generally formed by decomposing and depositing silane (SiH4) gas by high-frequency glow discharge and other methods. Because the decomposition and deposition temperature is low (about 200 ℃), the energy consumption is low and the cost is relatively low during production. This method is suitable for large-scale production. The area of a single solar cell can be large (such as 0.5m × 1.0m), neat and beautiful.

At present, research on amorphous silicon solar cells has made two major advances:

(1) The photoelectric conversion efficiency of the three-layer amorphous silicon solar cell reaches 13%.

(2) The annual production capacity of triple layer solar cells reaches 5MW.

Amorphous silicon solar cells have great potential because of their high photoelectric conversion efficiency, low cost and light weight. But its low stability directly affects its practical application. If we can further solve the problem of stability and improve the photoelectric conversion efficiency, then amorphous silicon solar cells will undoubtedly be one of the main development products of solar cells.

Due to the large absorption coefficient of amorphous silicon to sunlight, amorphous silicon solar cells can be made very thin. Generally, the thickness of silicon film is only 1-2 μ m. It is 1/500 of the thickness of monocrystalline silicon or polycrystalline silicon battery (about 0.5mm), so the resource consumption for making amorphous silicon battery is small.

Amorphous silicon has photofatigue effect due to its instability of internal structure and a large number of hydrogen atoms. In view of the long-term operation stability of amorphous silicon solar cells, after nearly 10 years of hard research, although it has improved, it has not been widely applied because the problem has not been completely solved.

Now, the research of amorphous silicon solar cells mainly focuses on improving the properties of amorphous silicon films to reduce the defect density, accurately designing the cell structure and controlling the thickness of each layer, and improving the interface state between each layer to achieve high efficiency and stability. At present, the highest photoelectric conversion efficiency of amorphous silicon single junction cells can reach 14.6%, that of industrial production can reach 8%~10%, and that of laminated amorphous silicon solar cells can reach 21.0%.

Due to the limitation of material characteristics, there is limited room for further improvement of the photoelectric conversion efficiency of crystalline silicon solar cells. At present, the multi junction tandem solar cells have the potential for growth. The market share of solar cells made of silicon is 96%, including 39% of monocrystalline silicon, 44% of polycrystalline silicon and 13% of amorphous silicon