Two key issues in developing solar cells are: improving conversion efficiency and reducing cost. Due to the low cost and easy mass production of amorphous silicon thin film solar cells, they have been widely paid attention to and developed rapidly. As early as the early 1970s, Carlson et al. started the research and development of amorphous silicon solar cells. In recent years, its research and development work has developed rapidly. At present, many companies in the world are producing such solar cells. product. As a solar material, amorphous silicon is a good cell material, but its optical band gap is 1.7 eV, which makes the material itself insensitive to the long-wavelength region of the solar radiation spectrum, which limits amorphous silicon solar cells. conversion efficiency. In addition, its photoelectric efficiency will decay with the continuation of illumination time, the so-called photodegradation effect, which makes the performance of solar cells unstable. The way to solve these problems is to prepare tandem solar cells, which are prepared by depositing one or more PiN layer cells on the prepared PiN layer single junction solar cells. The tandem solar cell improves the conversion efficiency, and the key problems to solve the instability of the single-junction cell are:
(1) It combines materials with different forbidden band widths to improve the spectral response range.
(2) The i-layer of the top cell is relatively thin, and the electric field intensity generated by illumination does not change much, which ensures the extraction of photogenerated carriers in the i-layer.
(3) The carriers generated by the bottom cell are about half of that of the single cell, and the light-induced degradation effect is reduced.
(4) The sub-cells of the tandem solar cells are connected in series.
Due to its unique physical properties and the advantages of easy processing in manufacturing technology, amorphous silicon has become the focus of research on the production of large-area high-efficiency solar cells. Amorphous silicon has a high absorption coefficient for sunlight and produces the best photoconductivity value, and is a good photoconductor; it is easy to achieve high-concentration doping and obtain excellent PN junctions; it can be used in a wide range of It can control its energy gap change in the sub-range.
Amorphous silicon has many defects due to the lack of regularity in the atomic arrangement of crystalline silicon. Therefore, in a pure amorphous silicon PN junction, the tunnel current is often dominant, making it exhibit tunnel current characteristics without rectification characteristics. In order to obtain good diode rectification characteristics, a thick intrinsic layer i must be added between the P layer and the N layer to suppress its tunnel current, so amorphous silicon solar cells generally have a PiN structure. In order to increase efficiency and improve stability, tandem cells with multi-layer PiN structures are sometimes fabricated, or some transition layers are inserted.
Amorphous silicon solar cells are the most complete thin-film solar cells. Their structure is usually PiN (or NiP) shape. The P and N layers are mainly used to establish an internal electric field, and the i layer is composed of amorphous silicon. Due to the high light absorption ability of amorphous silicon, the thickness of the i-layer is usually only 0.2~0.5μm. Its forbidden band width ranges from 1.1 to 1.7 eV, which is different from 1.1 eV of wafer silicon. Amorphous substances are different from crystalline substances, and their structure uniformity is low. Therefore, electrons and holes are conducted inside the material. If the distance is too long, the two The coincidence probability is extremely high. In order to avoid this phenomenon, the i-layer should not be too thick, but if it is too thin, it is easy to cause insufficient light absorption. To overcome this problem, this type of solar cell is designed with a multi-layer structure stacking to take into account both light absorption and photoelectric efficiency.
There are many preparation methods for amorphous silicon thin film solar cells, including reactive sputtering, PECVD, LPCVD, etc. The reactive raw material gas is SiH4 diluted with H2, and the substrate is mainly glass and stainless steel sheet. Single-junction solar cells and tandem solar cells can be obtained from thin films through different solar cell processes.
Amorphous silicon solar cells are generally formed by decomposing and depositing silane (SiH4) gas by high-frequency glow discharge and other methods. Due to the low decomposition and deposition temperature (about 200°C), the energy consumption during production is low and the cost is relatively low, and this method Suitable for mass production, monolithic solar cells can be made large (such as 0.5mx 1.0m), neat and beautiful. At present, two major advances have been made in the research of amorphous silicon solar cells:
(1) The conversion efficiency of three-layer structure amorphous silicon solar cells reaches 13%
(2) The annual production capacity of three-layer solar cells reaches 5Mw.
Amorphous silicon solar cells have great potential for development due to their high conversion efficiency, low cost and small weight. But at the same time, because of its low stability, it directly affects its practical application. If the stability problem can be further solved and the conversion rate improved, amorphous silicon solar cells are undoubtedly 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. Usually, the thickness of the silicon film is only 1~2μm, which is 1% of the thickness of monocrystalline silicon or polycrystalline silicon solar cells (about 0.5mm). /500, so the resource consumption of making amorphous silicon solar cells is low.
Amorphous silicon has a light fatigue effect due to the instability of its internal structure and a large number of hydrogen atoms, which is aimed at the long-term operation stability of amorphous silicon solar cells. In the past 10 years, after diligent research, although some improvements have been made, the problem has not been completely solved, so it has not been widely used.
At present, the research of amorphous silicon solar cells mainly focuses on improving the properties of the amorphous silicon film itself to reduce the defect density, accurately design the cell structure and control the thickness of each layer, and improve the interface state between the layers to obtain high efficiency. and high stability.
At present, the highest efficiency of amorphous silicon single junction cells has reached about 14.6%, the industrial production can reach 8% to 10%, and the highest efficiency of stacked amorphous silicon solar cells can reach 21.0%.
Due to the limitation of material properties, there is limited room for further improvement of the efficiency of crystalline silicon solar cells. Currently, the multi-junction tandem solar cells with more potential for growth are. The market share of solar cells using silicon as raw material is 96%, of which monocrystalline silicon is 39%, polycrystalline silicon is 44%, and amorphous silicon is 13%.