Introduction of solar cells and materials

Introduction

Solar energy is an inexhaustible source of renewable energy for human beings. It is also clean energy and does not cause any environmental pollution. In the effective use of solar energy, Solar Photoelectric Utilization is one of the most popular projects in recent years, which is the fastest growing and most dynamic research field. To this end, solar cells have been developed and developed. The production of solar cells is mainly based on semiconductor materials. Its working principle is to use photoelectric materials to absorb light energy and then undergo photoelectric conversion reaction. According to the materials used, solar cells can be divided into: 1. Silicon solar cells; 2. Inorganic Salts such as gallium arsenide III-V compound, cadmium sulfide, copper indium selenium and other multi-component compounds as materials; 3. Functional macromolecular materials prepared by solar cells; 4. Nanocrystalline solar cells, etc. No matter what kind of material is used to make the battery, the general requirements for solar cell materials are: 1. The forbidden band of the semiconductor material should not be too wide; 2. There must be a high photoelectric conversion efficiency; 3. The material itself does not cause pollution to the environment; 4. The material is easy to industrialize and the material performance is stable. Based on the above considerations, silicon is the most ideal solar cell material, which is the main reason for solar cells to be mainly silicon materials. However, with the continuous development of new materials and the development of related technologies, solar cells based on other village materials are increasingly showing attractive prospects. This paper briefly reviews the types of solar cells and their research status, and discusses the development and trends of solar cells.

1 silicon solar cell

1.1 monocrystalline silicon solar cell

Among the silicon series solar cells, single crystal silicon solar cells have 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 processes. Nowadays, the electrical grounding process of monocrystalline silicon is almost mature. Generally, surface texture, passivation of emission region, zonal doping and other technologies are adopted. The developed batteries mainly include planar single-crystal silicon batteries and slot-embedded grid electrode single-crystal silicon batteries. The improvement of conversion efficiency mainly depends on the surface microstructure treatment and the partition doping process of single crystal silicon. In this respect, the German Franois Freiburg Solar System Institute maintains a world-leading level. The institute used lithography to texture the surface of the cell to create an inverted pyramid structure. And put a 13nm on the surface. The thick oxide passivation layer is combined with two anti-reflective coatings. The ratio of the width and height of the gate is increased by the improved electroplating process: the conversion efficiency of the battery produced by the above is over 23%, which is a large value of 23.3%. The conversion efficiency of large-area (225cm2) single-electroform solar cells prepared by Kyocera is 19.44%. The domestic Beijing Solar Energy Research Institute is also actively researching and developing high-efficiency crystalline silicon solar cells, and developing planar high-efficiency monocrystalline silicon cells (2cmX2cm). The conversion efficiency is 19.79%, and the conversion efficiency of the grooved buried gate electrode crystalline silicon battery (5cmX5cm) is 8.6%.

The conversion efficiency of monocrystalline silicon solar cells is undoubtedly the highest, and still dominates in large-scale applications and industrial production, but due to the price of single crystal silicon materials and the corresponding cumbersome battery process, the cost of single crystal silicon is high. No, it is very difficult to significantly reduce its cost. In order to save high-quality materials and find alternatives to monocrystalline silicon cells, thin-film solar cells have been developed, among which polycrystalline silicon thin film solar cells and amorphous silicon thin film solar cells are typical representatives.

1.2 polysilicon thin film solar cell

A typical crystalline silicon solar cell is fabricated on a high quality silicon wafer having a thickness of 350 to 450μm which is sawed from a lifted or cast silicon ingot. Therefore, the actual consumption of silicon material is more. In order to save materials, polycrystalline silicon films have been deposited on inexpensive substrates since the mid-1970s, but due to the size of the grown silicon films, valuable solar cells have not been fabricated. In order to obtain a film of a large-sized grain, people have not stopped research and proposed many methods. At present, polycrystalline silicon thin film batteries are mostly prepared by chemical vapor deposition, including low pressure chemical vapor deposition (LPCVD) and plasma enhanced chemical vapor deposition (PECVD). In addition, liquid phase epitaxy (LPPE) and sputter deposition methods can also be used to prepare polycrystalline silicon thin film batteries.

The chemical vapor deposition mainly uses SiH2Cl2, SiHCl3, Sicl4 or SiH4 as a reaction gas to react with a certain protective atmosphere to form silicon atoms and deposit them on a heated substrate. The substrate material is generally selected from Si, SiO2, Si3N4 and the like. However, it has been found that it is difficult to form large crystal grains on a non-silicon substrate, and it is easy to form voids between the crystal grains. To solve this problem, a thin layer of amorphous silicon is deposited on the substrate by LPCVD, and then the amorphous silicon layer is annealed to obtain larger crystal grains, and then on the seed crystal. Deposition of thick polysilicon film, therefore, recrystallization technology is undoubtedly an important link. The current technology mainly includes solid phase crystallization and medium melting recrystallization. In addition to the recrystallization process, the polycrystalline silicon thin film battery employs almost all technologies for preparing single crystal silicon solar cells, and the solar cell conversion efficiency thus obtained is remarkably improved. The conversion efficiency of polycrystalline silicon cells produced on the FZSi substrate by the Freiburg Solar Energy Research Institute in Germany was 19%. The Mitsubishi Corporation of Japan used this method to prepare batteries with an efficiency of 16.42%.

The principle of the liquid phase epitaxy (LPE) method is to reduce the temperature of the silicon film by melting the silicon in the matrix. The American Astropower company uses LPE to prepare a battery with an efficiency of 12.2%. Chen Zheliang of China Optoelectronics Development Technology Center used liquid phase epitaxy to grow silicon grains on metallurgical grade silicon wafers, and designed a new type of solar cell similar to crystalline silicon thin film solar cells, called "silicon grain" solar energy. Battery, but reports on performance have not been seen.

Polycrystalline silicon thin film batteries are far less efficient than single crystal silicon, and have no efficiency degradation problem, and may be prepared on inexpensive substrate materials, which is much lower in cost than single crystal silicon cells, and higher in efficiency than amorphous silicon films. Batteries, therefore, polysilicon thin film batteries will soon dominate the solar power market.

1.3 amorphous silicon thin film solar cell

Two key issues in developing solar cells are: improving conversion efficiency and reducing costs. Due to the low cost and convenient mass production of amorphous silicon thin film solar cells, it has been widely recognized and rapidly developed. In fact, as early as the early 1970s, Carlson et al. began to develop amorphous silicon cells. In the year, its research and development work has developed rapidly. At present, many companies in the world are producing such battery products.

Amorphous silicon, as a solar material, is a good battery material, but because its optical band gap is 1.7 eV, the material itself is insensitive to the long-wavelength region of the solar radiation spectrum, thus limiting the amorphous silicon solar cell conversion efficiency. In addition, its photoelectric efficiency will attenuate with the continuation of the illumination time, the so-called photo-degradation S-W effect, which makes the battery performance unstable. The solution to these problems is to prepare a tandem solar cell which is prepared by depositing one or more Pin subcells on a prepared p, i, n layer single junction solar cell. The key problems of the laminated solar cell to improve the conversion efficiency and solve the instability of the single-junction battery are as follows: 1. it puts together the materials of different forbidden band widths to improve the spectral response range; 2 the top layer of the battery is thinner. The intensity of the electric field generated by the illumination does not change much, ensuring that the photo-generated carriers in the i-layer are extracted; 3 the carrier generated by the bottom cell is about half of the single cell, and the photo-induced decay effect is reduced; The batteries are connected in series.

There are many preparation methods for amorphous silicon thin film solar cells, including reactive sputtering, PECVD, LPCVD, etc. The reaction raw material gas is H2 diluted SiH4, the substrate is mainly glass and stainless steel, and amorphous silicon is prepared. The single-cell battery and the tandem solar cell can be separately produced by the film through different battery processes. At present, research on amorphous silicon solar cells has made two major advances: the conversion efficiency of the first and third stacked-structure amorphous silicon solar cells reached 13%, setting a new record; the second-three-layer solar cell has an annual production capacity of 5MW. The single-junction solar cell made by United Solar Corporation (VSSC) has a maximum conversion efficiency of 9.3%, and the maximum conversion efficiency of the three-bandgap triple-layer battery is 13%.

The above maximum conversion efficiency was obtained on a small area (0.25 cm2) battery. It has been reported in the literature that the conversion efficiency of single-junction amorphous silicon solar cells exceeds 12.5%. The Academia Sinica has adopted a series of new measures to achieve a conversion efficiency of 13.2% for amorphous silicon cells. There are not many researches on amorphous silicon thin-film batteries, especially laminated solar cells in China. Yan Xinhua of Nankai University uses industrial materials, and a-back electrode with an area of  20×20 cm2 and a conversion efficiency of 8.28% is prepared. Si/a-Si laminated solar cells.

Amorphous silicon solar cells have great potential due to their high conversion efficiency, low cost and light 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 problem is improved, then the amorphous silicon solar cell is undoubtedly one of the main development products of the solar cell.

2 multi-compound thin-film solar cells

In order to find alternatives to monocrystalline silicon cells, in addition to the development of polycrystalline silicon, amorphous silicon thin film solar cells, and continue to develop solar cells of other materials. Among them, it mainly includes gallium arsenide III-V compound, cadmium sulfide and copper bismuth selenide thin film batteries. Among the above batteries, although the efficiency of cadmium sulfide and cadmium telluride polycrystalline thin film cells is higher than that of amorphous silicon thin film solar cells, the cost is lower than that of single crystal silicon cells, and it is also easy to mass produce, but since cadmium is highly toxic, Serious pollution to the environment, therefore, not the ideal replacement for crystalline silicon solar cells

Gallium arsenide III-V compounds and copper indium selenide thin films have received widespread attention due to their high conversion efficiency. GaAs is a III-V compound semiconductor material with an energy gap of 1.4eV, which is a value of high absorptivity sunlight, and is therefore an ideal battery material. The preparation of III-V compound thin film batteries such as GaAs mainly adopts MOVPE and LPE technology, and the GaAs thin film battery prepared by MOVPE method is affected by many parameters such as substrate dislocation, reaction pressure, III-V ratio and total flow rate.

In addition to GaAs, other III-V compounds such as Gasb, GaInP and other battery materials have also been developed. In 1998, the conversion efficiency of GaAs solar cells produced by the Freiburg Solar System Research Institute in Germany was 24.2%, which was recorded in Europe. The conversion efficiency of the first prepared GaInP battery was 14.7%. See Table 2. In addition, the institute also uses a stacked structure to fabricate GaAs, Gasb batteries, which are stacked with two separate cells. GaAs is used as the upper battery and Gasb is used as the lower battery. The battery efficiency is 31.1%. .

Copper indium selenium CuInSe2 is abbreviated as CIC. The energy of the CIS material is reduced to 1. leV is suitable for photoelectric conversion of sunlight. In addition, there is no photo-induced degradation of CIS thin film solar cells. Therefore, the use of CIS as a material for high conversion efficiency thin film solar cells has also attracted attention.

The preparation of CIS battery film mainly includes vacuum evaporation method and selenization method. In the vacuum evaporation method, copper, indium, and selenium are vapor-deposited using respective evaporation sources, and the selenization method is selenization using a H2Se laminated film, but it is difficult to obtain a uniform CIS by this method. The CIS thin film battery has grown from the initial 8% conversion efficiency in the 1980s to the current 15%. The gallium-doped CIS battery developed by Matsushita Electric Industrial Co., Ltd. has a photoelectric conversion efficiency of 15.3% (area 1 cm²). In 1995, the US Renewable Energy Research Laboratory developed a conversion efficiency of 17.1% of CIS solar cells, which is by far the highest conversion efficiency of the battery in the world. It is expected that the conversion efficiency of CIS batteries will reach 20% by 2000, which is equivalent to polycrystalline silicon solar cells.

As a semiconductor material for solar cells, CIS has the advantages of low price, good performance and simple process, and will become an important direction for the development of solar cells in the future. The only problem is the source of the material. Since both indium and selenium are relatively rare elements, the development of such batteries is bound to be limited.

3 Polymer multilayer modified electrode type solar cell

The replacement of inorganic materials with polymers in solar cells is just the beginning of a solar cell system research direction. The principle is to use a plurality of redox potentials of different redox polymers to perform multi-layer recombination on the surface of the conductive material (electrode) to form a unidirectional conductive device similar to an inorganic P-N junction. The inner layer of one of the electrodes is modified by a polymer having a lower reduction potential, the reduction potential of the outer layer polymer is higher, the electron transfer direction can only be transferred from the inner layer to the outer layer; the other electrode is modified to the opposite, and the first The reduction potentials of the two polymers on the electrodes are higher than the reduction potentials of the latter two polymers. When two modified electrodes are placed in an electrolytic wave containing a photosensitizer, the electrons generated by the photosensitizer are transferred to the electrode with lower reduction potential, and the electrons accumulated on the electrode with lower reduction potential cannot be transferred to the outer layer polymer, and can only be returned to the electrolysis through the external circuit through the electrode with higher reduction potential. Liquid, so there is photocurrent generated in the external circuit.

Due to the flexibility of organic materials, easy fabrication, wide source of materials, and low cost, it is of great significance for the large-scale use of solar energy to provide cheap electrical energy. However, research on the preparation of solar cells from organic materials has only just begun, and neither the service life nor the battery efficiency can be compared with inorganic materials, especially silicon batteries. Whether it can be developed into a product of practical significance remains to be further explored.

4 nanocrystalline chemical solar cells

Silicon-based solar cells are undoubtedly the most mature in solar cells, but because of the high cost, they are far from meeting the requirements of large-scale promotion and application. To this end, people continue to explore processes, new materials, thin film and other aspects, and the newly developed nano-TiO2 crystal chemical solar cells have received the attention of scientists at home and abroad.

Since the development of nano-TiO2 chemical solar cells by Professor Gratzel of Switzerland, some domestic units are also conducting research in this area. Nanocrystalline chemical solar cells (NPCs for short) are formed by modifying and assembling a forbidden semiconductor material onto another large-gap semiconductor material. The narrow bandgap semiconductor material uses transition metals such as Ru and Os. The sensitizing dye, the large-gap semiconductor material is nano-polycrystalline TiO2 and made into an electrode, and the NPC battery also uses a suitable oxidation-reduction electrolyte. The working principle of nanocrystalline TiO2: the dye molecules absorb the solar energy to transition to the excited state, the excited state is unstable, and the electrons are quickly injected into the adjacent TiO2 conduction band. The electrons lost in the dye are quickly compensated from the electrolyte and enter the TiO2 conduction band. The electricity in the final enters the conductive film and then generates a photocurrent through the outer loop.

The advantages of nanocrystalline TiO2 solar cells are their low cost and simple process and stable performance. Its photoelectric efficiency is stable at more than 10%, and the production cost is only 1/5 to 1/10 of the silicon solar cell. Life expectancy can reach more than 20 years. However, due to the research and development of such batteries, it is estimated that they will gradually enter the market in the near future.

5 solar cell development trend

As can be seen from the discussion of the above aspects, as a material of a solar cell, a III-V compound and a CIS are prepared from rare elements, although the solar cells produced by them have high conversion efficiency, but from the material source, this is Solar-like batteries are unlikely to dominate in the future. The other two types of battery nanocrystalline solar cells and polymer modified electrodes exist in solar power. Their research has just started, the technology is not very mature, and the conversion efficiency is still relatively low. These two types of batteries are still in the exploration stage, in a short time. It is impossible to replace the solar cells. Therefore, from the perspective of conversion efficiency and source of materials, the future development focus is still on silicon solar cells, especially polysilicon and amorphous silicon thin film cells. Due to the high conversion efficiency and relatively low cost of polycrystalline silicon and amorphous silicon thin film batteries, the monocrystalline silicon battery will eventually be replaced and become the leading product in the market.

Improving the conversion efficiency and reducing the cost are the two main factors to be considered in the preparation of solar cells. For current silicon solar cells, it is difficult to further improve the conversion efficiency. Therefore, in addition to continuing to develop new battery materials, future research should focus on how to reduce costs. Existing solar cells with high conversion efficiency are made on high-quality silicon wafers, which is the most expensive part of manufacturing silicon solar cells. Therefore, it is very important to reduce the cost of the substrate when the conversion efficiency is still high. It is also an urgent problem for the development of solar cells in the future. Recently, some technologies have been used to produce silicon strips as the substrates of polycrystalline silicon thin film solar cells in foreign countries.

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