HIT Solar Cell Development Overview

The energy crisis and environmental pollution issues have promoted the extensive research and application development of clean energy. Solar photovoltaic power generation is a new type of power generation technology that uses photovoltaic effect to directly convert solar radiation energy into electrical energy. It has the advantages of adequate resources, cleanliness, safety, long life, and is considered as one of the most promising renewable energy technologies. Has become the fastest growing and most dynamic research field in renewable energy technologies. Recently, solar cells on the international photovoltaic market mainly include crystalline silicon (including monocrystalline silicon, polycrystalline silicon), amorphous/single-crystal heterojunction (HIT), amorphous silicon thin films, cadmium telluride (CdTe) thin films, and copper indium selenium ( CIS) thin film solar cells. Among them, commercial crystalline silicon solar cells still occupy the mainstream, and their photoelectric conversion efficiency has reached 25%. However, due to the material purity and the limitations of the T-technique, it is difficult to increase the conversion efficiency or reduce the cost; while amorphous silicon solar cells can Large-scale production, low cost, but its conversion efficiency is still relatively low, and poor stability.

In order to reduce costs while maintaining high conversion efficiency, HIT batteries have been rapidly developed in recent years. This heterojunction-structured battery is the best design that combines the advantages of both with the full strength of each. This paper introduces the structure and characteristics of HIT batteries, summarizes the development status of HIT batteries, and looks into the future of HIT batteries.

1, HIT solar cell structure and characteristics

1.1HIT solar cell structure

FIG. 1 shows the basic structure of a HIT and a solar cell, which is characterized by a pi-type a-Si:H film (with a film thickness of 5 to 10 nm) on the light-irradiated side and an in-type a-Si:H film (having a film thickness of 5 to 10 nm) on the back surface side. The crystal silicon wafer was sandwiched, and transparent electrodes and collectors were formed on the top layers on both sides to form a HIT solar cell having a symmetrical structure.

1.2 Characteristics of HIT Solar Cells

(1) Low temperature process

HIT cells combine the advantages of low temperature ("250 °C") fabrication of thin film photovoltaic cells to avoid the use of conventional high temperature ("900 °C") diffusion processes to obtain pn junctions. This technology not only saves energy, but also makes the a-Si:H-based thin film doping, band gap and thickness can be controlled more precisely in a low-temperature environment, and it is easy to optimize the device characteristics in the process; in the low-temperature deposition process, the single-piece silicon wafer bends and deforms. Small, so that its thickness can be used as the minimum required for the background light absorbing material (about 80μm); at the same time, the low temperature process eliminates the performance degradation of the silicon substrate in high temperature processing, allowing the use of "low quality" crystalline silicon or even polycrystalline silicon. For the substrate.

(2) High efficiency

The HIT battery unique heterojunction structure with intrinsic thin film structure completes the surface passivation of r single crystal silicon while p_n forms a junction, which greatly reduces the surface and interface leakage current and improves the cell efficiency. At present, the laboratory efficiency of HIT batteries has reached 23%, and the battery efficiency of commercially available 200W modules has reached 19.5%.

(3) High stability

The illumination stability of HIT cells is good, and studies have shown that there is no Staebler-Wronski effect on amorphous silicon films in non-silicon/crystalline silicon heterojunctions, so no conversion efficiency similar to that of amorphous silicon solar cells will occur due to light. The phenomenon of recession; HIT battery temperature stability, and a single crystal silicon battery temperature coefficient of 0.5% / °C compared to the temperature coefficient of HIT battery can reach a 0.25% / °C, making the battery even in the light of temperature conditions There is still a good output ''.

(4) Low cost

The thin thickness of the HIT battery can save the silicon material; the low temperature process can reduce the energy consumption and allow the use of inexpensive substrates; the high efficiency makes it possible to reduce the area of ​​the battery under the same output power, thereby effectively reducing the cost of the battery.

2, HIT solar cell development status

2.1HIT Solar Cell Technology Development

In 1990, Japan's Sanyo Corporation began researching heterojunction solar cells. In 1992, Tanaka et al. set a record of 18.1% of the photoelectric conversion efficiency of a pa-Si:H/ia-Si:H/n_c-Si solar cell, and called this structure with an intrinsic thin layer a HIT structure. . Since then, China, the United States, Germany, France, Italy, and the Netherlands have also successively invested in HIT solar cell research (Table 1 shows the types of HIT cells studied by various countries, their preparation processes, and the conversion efficiencies they can achieve). In order to further improve the efficiency of the battery, its research mainly focuses on the following aspects.

(1) Optimization of heterojunction band structure

The biggest difference between H1T batteries and traditional batteries is the heterojunction structure composed of amorphous silicon and crystalline silicon. By designing the barrier height of the heterojunction interface, a suitable energy band structure can be obtained to improve the conversion efficiency of the battery. Taking Sanyo's HIT cell as an example, in the (p)a-Si/(i)a-Si/(n)c-Si heterojunction structure, the valence band dislocations between amorphous silicon and monocrystalline silicon are small In order to collect holes, the dislocations in the conduction band should be as large as possible to prevent the passage of I7 electrons. The design of the heterojunction barrier height is mainly achieved by controlling the deposition parameters of the amorphous silicon thin film.

(2) Preparation method of amorphous silicon layer

Amorphous silicon layers of HIT cells are typically prepared using plasma enhanced chemical vapor deposition (PECVD) techniques. In recent years, Zhang Qunfang, a graduate school of the Chinese Academy of Sciences, and TH Wang of the National Renewable Energy Laboratory (NERL) have prepared PIT-based HIT cells using hot wire enhanced chemical vapor deposition (HWCVD) technology. Compared with PECVD, HWCVD produces lower plasma energy, which can effectively avoid the bombardment of ions, and can generate low-energy atomic hydrogen for pretreatment of the surface of silicon wafers. There is less dust in the preparation process and it is not easy to make a-Si:H. Thin layer short circuit. In addition, B. Jagannathan of the State University of New York in the United States also used a DC magnetron sputtering technology to prepare a P-type HIT cell, and obtained an open circuit voltage of 550 mV and a short-circuit current of 30 mA/cm2 in an area of ​​0.3 cm2.

(3) Study of Back Surface Field (BSF)

The back surface field can improve the backside recombination rate and back surface reflection, thereby increasing the open circuit voltage and increasing the short circuit current. The traditional method for preparing the back surface field is pin alloy method, boron diffusion method, phosphorus diffusion method, etc. However, these processes require a high temperature process, and only the back surface field and the amorphous silicon film can be deposited first. The preparation process compatible with the low temperature process of the HIT battery mainly includes depositing a heavily doped amorphous silicon film on the back surface of the single crystal silicon to form a back surface field. Toru Sawada et al. used a PECVD method to prepare a back surface field of an HIT structure (i/na-Si) on an n-type substrate. The back surface field utilizes the properties of a heterojunction and can be formed without the need for heavy doping. The results show that the back surface field of the HIT structure achieves better surface passivation than thermal oxygen passivation. Y. Ves-chetti et al. [u80] also implemented LocalBSF with photolithography and boron ion implantation. Compared with the full-area (Full) aluminum alloy backface, the open-circuit voltage has been greatly improved to 676mV, which is a P type HIT. The highest value of the open circuit voltage of the battery. HDGoldbach et al. used P “μc-Si to fabricate the back surface field of P-type HIT cell. Because μc-Si has higher doping efficiency than a-Si, it can achieve high concentration of doping, thus reducing the activation energy and forming the performance. The excellent back surface field improves the battery conversion efficiency.Numerical simulation results show that the addition of a heavily doped n+ layer on the backside of an n-type substrate HIT cell can serve as a back-surface field, increasing the efficiency of the cell to 24.35%.

(4) Selection of Substrate Material

'The type of substrate is different, the conversion efficiency of the battery is also different. TucciM et al. found that the HIT battery with n-type substrate has the advantages of heterojunction band structure, and its conversion efficiency is slightly higher than that of P-type substrate, but P-type substrate solar battery interface The requirements are lower and therefore easier to prepare. TH Wang et al. made HIT cells using P-type zone melting (FZ) silicon and CZ silicon, respectively, and found that the efficiency of the substrate as FZ silicon solar cell was higher than that of CZ silicon. The efficiency of HIT solar cells fabricated on Fz substrates by HQCVD at WangQi et al. However, the price of FZ silicon is higher than that of CZ silicon. Therefore, a new substrate should be selected based on the combination of efficiency and cost. In addition, in order to reduce the battery's reflection of incident light, the suede substrate was also applied to the HIT battery, and a well-reflected antireflection effect was obtained.

(5) Innovation of Emitter Materials

In order to reduce the absorption of incident light by the amorphous silicon layer, wide bandgap materials such as microcrystalline silicon (μc-Si), nanocrystalline silicon (nc-Si), etc. may be used as emitters to increase the light transmittance. CSummontc et al. used an RF-PECVD technique to prepare an n-type μc-Si emitter on a P-type substrate through a highly hydrogen diluted gas source. The result showed that the short-circuit current of the HIT cell was comparable to that of the a-Si emitter. Conversion efficiency has improved significantly. Xu Ying and others from the Institute of Semiconductors, Chinese Academy of Sciences also used RF-PECVD to fabricate n-type nc-Si emitters on p-type substrates. In addition to the PECVD method, Zhang Qunfang of the Graduate School of the Chinese Academy of Sciences also used the HWCVD method to prepare μc-Si emitters. In addition, J.Danmon-Lacoste et al. used PECVD to fabricate the intrinsic layers of HIT cells under conditions of poly-silicon (pm-Si) formation. The test results show that the current-carrying lifetime of pm-Si is greater than that of a-Si. One order of magnitude higher.

2.2 Industrialization of HRR Solar Cells

HIT battery modules have been developing rapidly since the investment market in 1997. Figure 2 shows the market share of various types of solar cells in 2004. It can be seen from Figure 2 that HIT cells have occupied a market share of 5% of the world's photovoltaic market in a few years. Sanyo made important contributions in the process of research and its large-scale industrialization. Since 1991, the HIT battery has made breakthroughs in research, and has produced a HIT battery with a conversion efficiency of 20.0% in a 1cm2 area. Sanyo has introduced a battery module called HITPower21 in industrial production with a conversion efficiency of 17.39%. It consists of 96 HIT cells. The output power is 180W. At the same time, Sanyo also introduced a cost-effective solar cell module (HIT power roof) that can replace roof tiles. The HIT power double was also available later, especially for installation on the ground and walls.

In April 2003, Sanyo Corporation introduced the HIT battery module with an output power of 200W. The module's battery conversion efficiency reached 19.5%, module efficiency was 17%, and the temperature characteristics were greatly improved. The annual power generation was 43% more than traditional solar cells. %. In 2006, the highest conversion efficiency of HIT battery reached 21.8%, and the 270W HIT battery module was first listed in Europe. The amount of solar battery modules used in the project can be reduced by about 25%.

In May 2009, Sanyo Company increased the conversion efficiency of HIT pool to a world record of 23%. In September of the same year, the company achieved a cell conversion efficiency of 22.8% with a 98μm HIT solar cell having a thickness of only about 1/2 of the previous (open circuit voltage (Voc) of 0.743, short-circuit current (Isc) of 38.8mA/cm 2 ). The fill factor (FF) is 79.1%. The cell area is 100.3 cm2). Although the thickness is halved, the conversion efficiency of the cell is only reduced by 0.2%. The use of silicon, which accounts for 1/2 of the cost, has been reduced. This has opened the way for the cost reduction of HIT batteries. At the same time, Sanyo plans to apply this technology to volume production in the near future and win the largest share of Japan's PV market in FY2013, showing that HIT batteries have great potential for development.

Germany has made great progress in software simulation. It has improved the conversion efficiency to 19.8%; the HIT battery efficiency studied in the United States has also reached 19.1%. However, due to the fact that the industrial production technology of core process technologies and key equipment technologies is not yet very mature, industrialized battery efficiency is not very high. They will vigorously improve the technology in the future research and realize large-scale industrial production.

3、Conclusion

Although the HIT battery has developed rapidly, there are still many problems. Due to the stringent process requirements in each step of the production process, large-scale mass production requires further research under the condition of ensuring high efficiency. Although the efficiency of HIT has been chosen as 23%, and the cost is gradually decreasing, the cost of power generation is still much higher than the cost of power generated by traditional methods.

At present, the most studied HIT cells are amorphous silicon/single-crystal heterojunction cells, in which the amount of cheap amorphous silicon is small, while the expensive monocrystalline silicon still accounts for the majority. Therefore, in order to meet the needs of the national production of solar modules, in the future research, we should vigorously develop new technologies to reduce the thickness of HIT batteries while ensuring battery conversion efficiency; on the other hand, inexpensive materials are used instead of expensive ones. Monocrystalline silicon material to reduce costs, such as polysilicon. It is also possible to reduce the production cost of monocrystalline silicon by developing new technologies.