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4 types of photovoltaic energy storage systems. Do you know them all

4 types of photovoltaic energy storage systems. Do you know them all? 1 article amazing makes it clear!

4 types of photovoltaic energy storage systems. Do you know them all? According to different application scenarios, solar photovoltaic energy storage power generation systems are divided into four types: photovoltaic off-grid power generation systems, grid-connected off-grid energy storage systems, photovoltaic grid-connected energy storage systems, and microgrid energy storage systems.

Since the Energy Bureau issued a new policy on May 31, distributed photovoltaics have only arranged a subsidy scale of about 10G. Before June 1, the installation scale of distributed photovoltaics nationwide had exceeded 10GW. Therefore, after June 2018, distributed photovoltaics may no longer have national subsidies.

If there is no subsidy, the income of projects fully connected to the grid, projects with a small proportion of self-use, and areas with low electricity prices will drop significantly and have no investment value. The investment income of pure photovoltaic projects has declined, so everyone has turned their attention to photovoltaic energy storage, hoping to make breakthroughs in this field and increase new income.

Photovoltaic energy storage is different from grid-connected power generation. It is necessary to add batteries and battery charging and discharging devices. Although the initial cost will increase by 20-40%, the scope of the application is much wider.

The difference between photovoltaics and solar energy is also very big, photovoltaics refers to the technology of directly converting light energy into electrical energy through the photoelectric effect, while solar energy refers to all technologies that use solar energy in a broad sense, including photovoltaics, solar thermal, etc. According to different application scenarios, solar photovoltaic energy storage power generation systems are divided into four types: off-grid power generation systems, off-grid energy storage systems, grid-connected energy storage systems, and multi-energy hybrid microgrid systems.

Photovoltaic off-grid power generation system

The photovoltaic off-grid power generation system operates independently without relying on the power grid and is used in remote mountainous areas, powerless areas, islands, communication base stations, street lights, and other application places. The system consists of a photovoltaic array, a solar controller, an inverter, a battery pack, a load, etc.

The photovoltaic array converts solar energy into electrical energy in the presence of light, and supplies power to the load through a solar control inverter, while charging the battery pack; when there is no light, the battery supplies power to the AC load through the inverter.

Photovoltaic off-grid power generation system is specially used for places in areas without power grids or areas with frequent power outages. It is a rigid demand. The off-grid system does not rely on the power grid. It relies on the working mode of “storing while using” or “storing first and using later”. It is a “helping in need”.

For families in areas without power grid or areas with frequent power outages, the off-grid system is very practical. The cost of photovoltaic off-grid electricity is about 1.0-1.5 yuan, which is much higher than the grid-connected system, but it is more economical and environmentally friendly than the cost of fuel generator electricity of 1.5-2.0 yuan.

Off-grid energy storage system

Off-grid photovoltaic power generation system is widely used in places where there are frequent power outages, or photovoltaic self-generation and self-use cannot be connected to the grid, the self-use electricity price is much more expensive than the grid electricity price, and the peak electricity price is much more expensive than the trough electricity price.

The system consists of a photovoltaic array composed of solar cell modules, solar off-grid integrated machine, battery group, load, etc. The photovoltaic array converts solar energy into electrical energy when there is light, supplies power to the load through the solar control inverter, and charges the battery pack at the same time; when there is no light, the battery supplies power to the solar control inverter and then supplies power to the AC load.

Advances in photovoltaic energy storage systems are making renewable energy more accessible
Advances in photovoltaic energy storage systems are making renewable energy more accessible

Compared with the grid-connected power generation system, the off-grid system adds a charge and discharge controller and a battery, and the system cost increases by about 30%, but the application range is wider. First, it can be set to output at rated power when the electricity price is peak, reducing electricity expenses. The application of solar cells not only plays a key role in the photovoltaic array but also reflects its importance in the charging process of the battery, ensuring the stable operation of the system when there is insufficient light.

Second, it can charge in the valley section of electricity price and discharge in the peak section, making money by using the peak-valley price difference; third, when the power grid is out of power, the photovoltaic system continues to work as a backup power source, the inverter can be switched to off-grid working mode, and the photovoltaic and battery can supply power to the load through the inverter.

Photovoltaic grid-connected energy storage system

The grid-connected energy storage photovoltaic power generation system can store excess power generation and increase the proportion of self-generation and self-use. It is used in places where the surplus of photovoltaic self-generation and self-use cannot be connected to the grid, the self-use electricity price is much more expensive than the grid electricity price, and the peak electricity price is much more expensive than the flat electricity price.

The system consists of a photovoltaic array composed of solar cell components, a solar controller, a battery pack, a grid-connected inverter, a current detection device, and a load. When the solar power is less than the load power, the system is powered by solar energy and the grid together. When the solar power is greater than the load power, part of the solar energy is used to power the load, and part is stored through the controller.

In some countries and regions, a photovoltaic system was installed before, and then the photovoltaic subsidy was canceled. A grid-connected energy storage system can be installed to make photovoltaic power generation completely self-generated and self-used. The grid-connected energy storage machine is compatible with inverters from various manufacturers, and the original system can be left unchanged.

When the current sensor detects that there is current flowing to the grid, the grid-connected energy storage machine starts working and stores the excess electricity in the battery. If the battery is also full, the electric water heater can be turned on. When the household load increases at night, the battery can be controlled to supply power to the load through the inverter.

Microgrid energy storage system

The microgrid system consists of a solar cell array, a grid-connected inverter, a PCS bidirectional converter, an intelligent switching switch, a battery pack, a generator, and a load.

The photovoltaic array composed of monocrystalline silicon solar cells converts solar energy into electrical energy when there is light, supplies power to the load through the inverter, and charges the battery pack through the PCS bidirectional converter; when there is no light, the battery supplies power to the load through the PCS bidirectional converter.

Microgrids can fully and effectively tap the potential of distributed clean energy, reduce unfavorable factors such as small capacity, unstable power generation, and low reliability of independent power supply, ensure the safe operation of the power grid, and are a beneficial supplement to large power grids.

Microgrids can promote the upgrading of traditional industries and play a huge role from the perspective of economy and environmental protection. Experts say that microgrids are flexible in application and can range from several kilowatts to tens of megawatts. They can be developed in factories, mines, hospitals, schools, or even in buildings.

The main advantages of microgrid systems over pure off-grid systems

  1. Wider application range. Off-grid systems can only be used without the large grid, while microgrid systems include all applications of off-grid systems and grid-connected systems, with multiple working modes
  2. Flexible system configuration. Grid-connected inverters can be selected as single or multiple units for free combination according to the actual situation of customers. String inverters or centralized inverters can be chosen, and even inverters from different manufacturers can be selected. The power of grid-connected inverters and PCS converters can be equal or different.
  3. High system efficiency. The photovoltaic power generation of the microgrid system can be directly used for the load nearby through the grid-connected inverter. The actual efficiency is as high as 96%. The bidirectional converter mainly plays a role in voltage stabilization.
  4. Strong load-carrying capacity. The grid-connected inverter and bidirectional converter of the microgrid system can supply power to the load at the same time. When the lighting conditions are good, the load-carrying capacity can be greatly increased.
Photovoltaic energy storage systems convert sunlight into electricity and store it for later use
Photovoltaic energy storage systems convert sunlight into electricity and store it for later use
The difference between photovoltaics and solar energy. 1 accurate article make it clear

The difference between photovoltaics and solar energy. 1 accurate article make it clear!

As two important renewable energy technologies, solar energy and photovoltaics are playing an increasingly important role in our daily lives. Well, do you know the difference between photovoltaic and solar energy?

Solar energy and photovoltaics

The main differences between photovoltaic and solar energy are working principle, application fields and energy conversion efficiency. Photovoltaic technology directly converts sunlight into electrical energy through the photoelectric effect of semiconductor materials. It is mainly used in fields such as solar cells. The conversion efficiency is generally between 15% and 20%. Solar technology more broadly covers the process of using solar thermal energy to generate electricity, such as solar water heaters and solar thermal power stations, and its conversion efficiency depends on the specific application and technology. Overall, photovoltaics are an efficient and direct way to utilize solar energy, while solar technology is more diverse and widely used.

Let’s take a closer look at the differences between them.

Differences in working principles

Photovoltaic technology, also known as PV technology, works mainly based on the photoelectric effect. Simply put, when sunlight strikes a photovoltaic cell made of a semiconductor material, the photons interact with the atoms in the material, causing electrons to be released from the atoms, forming an electric current. Due to the properties of solar cell materials, this process of directly converting sunlight into electrical energy without any intermediate process makes photovoltaic technology an efficient and direct way to utilize solar energy.

Solar technology covers a wider range of ways to use sunlight for energy conversion. In addition to the photovoltaic effect, solar technology also includes the application of solar thermal energy. Solar water heaters and solar thermal power generation are two important applications of solar technology. Solar water heaters heat water by absorbing heat from sunlight to meet the hot water needs of homes or public places. Solar thermal power generation collects solar heat through a heat collection device, and then uses this heat to drive a steam turbine to generate electricity. This method requires converting solar energy into thermal energy, and then converting thermal energy into electrical energy, so the energy conversion process is relatively complex.

Characteristics of application areas

Photovoltaic technology has been widely used in many fields because it can directly convert sunlight into electrical energy. Solar panels are a typical application of photovoltaic technology. They are installed on rooftops, open fields and even oceans to convert sunlight into electricity, providing clean, renewable energy for homes, businesses or public facilities. In addition, photovoltaic technology is also used in solar street lights, aerospace, railway transportation and other fields to provide convenient and efficient energy solutions for these fields.

Solar thermal power generation is mainly used in fields such as solar water heaters, solar heat pumps and large-scale solar thermal power stations. Solar water heaters are commonly used water heating equipment in homes and businesses. They use solar heat to heat water and are energy-saving and environmentally friendly. Solar heat pumps use solar heat to provide cooling or heating functions, providing a comfortable indoor environment for buildings. Large-scale solar thermal power stations use heat collection devices to collect a large amount of solar energy heat, and convert the heat energy into electrical energy through heat exchangers, steam turbines and other equipment to provide stable power output for the power grid.

The utilization of solar energy reduces carbon emissions and promotes sustainability
The utilization of solar energy reduces carbon emissions and promotes sustainability

Differences in equipment structure

Photovoltaic equipment usually consists of solar panels, brackets, inverters and batteries. Solar panels are the core component of photovoltaic equipment, responsible for collecting light energy and converting it into electrical energy. The bracket is used to support and fix the solar panel to ensure that it receives sunlight stably. An inverter converts the direct current generated by the solar panels into alternating current for use in a home or business. The battery is used to store excess electrical energy to provide power during periods of no sunlight such as at night or on cloudy days.

The solar thermal power generation system includes heat collectors, heat exchangers, heat storage equipment and generators. The collector collects solar heat and transfers it to the heat exchanger. The heat exchanger transfers heat energy to the working medium, such as water or steam, so that it reaches the temperature and pressure required for power generation. Thermal storage equipment is used to store excess heat to provide stable heat output when needed. The generator converts the thermal energy of the working medium into electrical energy to provide power output for the power grid.

Comparison of environmental protection levels

Photovoltaic technology produces no pollution and does not emit greenhouse gases during the power generation process. It is a green and environmentally friendly way of utilizing energy. Thanks to powerful light trapping technology, the installation and operation of photovoltaic equipment does not require the burning of any fossil fuels, so its environmental impact is small. In addition, photovoltaic equipment will not produce noise, radiation and other pollution during operation, and will have less impact on the surrounding environment and residents’ lives.

In contrast, solar thermal power systems may require the use of fossil fuels as an intermediate medium in some applications, such as during the start-up phase or when supplementing heat. This may bring some environmental pollution, although this pollution is far less than traditional fossil fuel power generation methods. However, with the advancement of technology and the improvement of environmental protection requirements, more and more solar thermal power generation systems are beginning to use clean and renewable energy as auxiliary heat sources to reduce environmental pollution.

Differences in energy conversion efficiency

The conversion efficiency of photovoltaic technology mainly depends on the performance of photovoltaic materials and manufacturing processes. At present, the conversion efficiency of commercial photovoltaic panels is generally between 15% and 20%. With the continuous advancement of technology and the development of new materials, this efficiency is expected to be further improved. At the same time, photovoltaic technology has less energy loss, making it highly efficient in the energy conversion process.

The efficiency of solar thermal power generation depends on the efficiency of the heat collection device, the performance of the heat exchanger and the heat loss of the entire system. Since solar thermal power generation involves the transfer and conversion of thermal energy, its efficiency may be affected by a variety of factors, such as the type and area of ​​the heat collection device, the nature and state of the working medium, etc. Therefore, the efficiency of solar thermal power generation will vary in specific applications. In order to improve the efficiency of solar thermal power generation, researchers are constantly improving the design of heat collectors and heat exchangers, as well as optimizing the operation of the entire system.

write at the end

In one sentence, although photovoltaics and solar energy are both related to sunlight, they have obvious differences in working principles, application fields, equipment structures, environmental protection and energy conversion efficiency. As renewable energy sources become increasingly important, we should gain a deeper understanding of these technologies in order to better utilize the endless energy treasure trove of solar energy.

Harnessing solar energy through photovoltaic panels converts sunlight into electricity
Harnessing solar energy through photovoltaic panels converts sunlight into electricity
Theory of Multijunction Solar Cells

Theory of Multijunction Solar Cells

Before discussing multi-junction solar cells, let’s review the basic reasons for the energy conversion limitations of single-junction solar cells. Consider an ideal solar cell made of a material with a semiconductor bandgap. When the photon energy is incident inside the cell, the photon will be absorbed and converted into electrical energy, but the electrical energy converted at this time has only excess energy, which will become heat dissipation. And when h v < E, the photons will not be absorbed into electricity. And only in the case of hν>E, the efficiency of converting light energy into electrical energy can reach the highest. It should be noted that even in the case of hv = E, the maximum energy conversion efficiency is still below 100%.

The range of the solar spectrum is very wide, ranging from 0 to 4 eV, so the conversion efficiency of single-junction solar cells to sunlight is naturally much lower than that of single-frequency light. A simple idea to solve this problem is to divide the solar spectrum into several bands according to its energy, and each band uses a solar cell with a suitable energy bandgap to convert light energy into electrical energy, so as to improve the efficiency of converting light energy into electrical energy. E.g. Divide the solar spectrum into blue bands hv1 , hv2 , hv3 , hv4 , hv4 , hv5 , where hv1 <hv2 <hv3 and the photons in these spectral ranges will use three different band gaps of the solar energy Eg1 = hv1 , Eg2 =hv2, Eg3 =hv3 to convert light energy. If the number of spectral bands can be divided into more, higher energy conversion efficiency can theoretically be achieved.

The scholar Henry has calculated under the conditions of A Ml. 5 and 1 sun. Energy conversion efficiency limit of multijunction solar cells. When the number of junctions of the solar cell is 1, 2, 3 and 36, the highest energy conversion efficiencies are 37%, 50%, 56% and 72%, respectively. When the energy conversion efficiency is changed from a single-junction solar cell to a 2-junction solar cell, the increase is most obvious, and the improvement of the energy conversion efficiency of more junction pairs becomes limited. This calculation is good news in some ways, because it is very difficult to actually make a solar cell with 4 or 5 junctions, and if a multi-junction solar cell is to really improve the energy conversion efficiency, each It is very important whether the energy band gap of a sub cell material is selected correctly.

What are the common types of high-efficiency monocrystalline silicon solar cells?

What are the common types of high-efficiency monocrystalline silicon solar cells?

At present, the commercialized monocrystalline silicon solar cells have an efficiency of 15%~20%, and the service life of the module is about 20 years; by 2010, it is hoped that the efficiency will be increased to 25%, and the chip thickness will be reduced to 50µm, which will reduce the cost to half of the current one. The module service life is expected to exceed 30 years; by 2030, the efficiency is expected to increase to more than 30%. Due to the current market expansion and high product competitiveness, some large companies are actively investing in the development of new technologies. The main development directions are: ① the improvement of the quality and thickness of silicon chips; ② the improvement of battery efficiency and cost reduction. As for how to improve the conversion efficiency of solar cells (greater than 25%), it has always been the direction of the industry and academia.

Monocrystalline silicon solar cells are currently more than 20% efficient and have been commercialized, as shown in Figure 1. Figure 1(a) is the solar cell Sun Power A-300 developed by Sun Power Company, which is characterized by designing the electrode parts on the same side and the back side>, so that the front side of the cell does not have any shielding area, and its highest efficiency has reached 21.5 %; Figure 1(b) is a solar cell developed by BP Solar, which uses laser to embed the front electrode into the cell to increase the carrier collection effect and improve the efficiency, and the highest efficiency can reach 20.5%; Figure 1 ( c) HIT high-efficiency solar cells developed for Sanyo Company, the highest efficiency can reach 20.1%. In addition, there are several other high-efficiency solar cell structures, including emitter passivated and rear locally diff used (PERL) solar cells, grating solar cells, point-contact solar cells , obliquely evaporated contact solar cells, metal insulating layer semiconductors, solar cells, screen printing (screen printing) solar cells, etc., the monocrystalline silicon solar cells with different structures will be introduced and discussed below.

What are the common types of high-efficiency monocrystalline silicon solar cells?
Figure 1 Commercialized monocrystalline silicon solar cell structure

1. Emitter Passivated Back Local Diffusion Solar Cell
In recent decades. The high-efficiency monocrystalline silicon solar cell is most famous for the emitter-passivated backside local diffusion (emitter and rearlocally diffused, PERL) device developed by the University of New South Wales, Australia, and its efficiency is as high as 24.7%, as shown in Figure 2 Show. The structure is surface textured with inverted pyramids, and is also coated with double anti-reflection layers of MgF2 ( n = 1. 38 ) and ZnS ( n = 2. 4 ) to increase the Light absorption to increase photocurrent generation: Passivation of silicon surface with thermal oxide layer. To avoid photocarrier recombination at the boundary; the design of local diffusion on the back forms a back surface field (BSF), which can bounce minority carriers, and due to the design of local diffusion, the majority of carriers are avoided on the boundary. The composite mountain increases the collection of majority carriers; BBr3 and PBr3 liquid sources are used for doping at the metal contact position to reduce the contact resistance.

What are the common types of high-efficiency monocrystalline silicon solar cells?
Figure 2 PERL solar cell structure

2. Buried Contact Solar Cells
In the past 15 years, the efficiency of solar cells has been improved a lot. The most striking structure is the buried-contact solar cell (BCSC), which was developed by the University of New South Wales in Australia, and was developed by BP Solar in the United States. commercialized, and its structure is shown in Figure 3. The BCSC solar cell structure combines the advantages of the early PE SC structure and the recent PERL structure. Part of the cell is etched and surface textured, and then the cell surface is diffused and passivated to achieve the best resistance through oxide layers and nitrides. Reflection and surface passivation, and then use YAG-Laser to carve grooves (grooving) on ​​the surface of the battery. The depth of the groove should not exceed 60µm, otherwise it will affect the open circuit voltage of the battery. In addition, in order to increase its mass production speed, The laser grooving process can also be changed to mechanical grooving. Although the use of the mechanical process may result in poor cell uniformity, the subsequent etching can be used to smooth the groove; The secondary diffusion process and the deposition of the back aluminum electrode, and then use the electroplating technology to deposit three metal alloys of inlay, copper and silver on the groove and the back of the battery.

What are the common types of high-efficiency monocrystalline silicon solar cells?
Figure 3 Buried Contact Solar Cell (BCSC) Structure

The solar cell efficiency of the BCSC structure is higher than that of the general commercial screen-printed solar cells, which not only improves the current and voltage output, but also improves the series resistance effect. Since it is easier to absorb the incident light of the blue-ray technology, the current output is improved; in addition, the carrier combination rate of the electrode is reduced, so the voltage output is improved. The fill factor is also increased due to the improved open circuit voltage and lower series resistance. Therefore, the overall efficiency of the battery has been improved a lot, reaching 19.9%.

In addition, the general buried structure can be slightly changed into a double-sided contact (double-sided contact, DSBC) solar cell structure, as shown in Figure 4. It uses lower temperature and lithography process, spin coating liquid diffusion source to reduce the cost of solar cells, which has been proved by experiments. Its conversion efficiency is 17%, which is not as efficient as the general buried structure. The reason is that the liquid diffusion source tends to drive in the trenches. If the manufacturing process is improved, it is expected to reach 20%.

What are the common types of high-efficiency monocrystalline silicon solar cells?
Figure 4 Double-sided buried contact solar cell structure

3. Grating solar cells
The grating structure solar cell is one of the solar cells designed in recent years. Its main concept is to use various etching techniques to make the cell surface structure into a grating shape to increase the utilization of the incident light source. A research team used the reactive ion etching (RIE) process to etch the cell surface into gratings with depths ranging from 10 to 30 µm, as shown in Figure 5, and found that the grating structure can better absorb incident light (visible light band). As shown in Figure 6, it is found that the effect of using a 2-dimensional grating structure is better than that of a 1-dimensional structure. If passivation treatment is added, the recombination rate of electron-hole pairs can be reduced, and the short-circuit current density and internal quantum efficiency are also improved. Can be improved a lot.

What are the common types of high-efficiency monocrystalline silicon solar cells?
Figure 5 Grating structure
What are the common types of high-efficiency monocrystalline silicon solar cells?
Figure 6 Comparison of light absorption by 1D and 2D grating structures

Later, other research teams used ZnO as the main material of the solar cell grating structure, used photolithography to define the pattern, and etched the grating depth of about several hundreds of nanometers, as shown in Figure 7 using SEM scanning. The fill factor of the battery can reach 68%, and it is found that it has a better response to the red and blue wavelengths, that is, the grating structure increases the utilization of the incident red and blue wavelengths.

What are the common types of high-efficiency monocrystalline silicon solar cells?
Figure 7 Grating structure using ZnO thin film

4. Thin intrinsic layer heterojunction HIT solar cells
The thin intrinsic layer heterojunction HIT (heterojunction with intrinsic thin layer) solar cell was developed by Japan Sanyo Company and has been commercialized. It uses an N-type silicon chip, which is different from the general battery using a P-type. The thickness of the overall HIT cell does not exceed 200µm, and a thin amorphous layer (i/P, i/N layer) is deposited on the upper and lower layers of the N-type silicon chip, and the front and back sides of the cell are both transparent conductive oxide layers (TCO) , which also acts as an anti-reflection layer, as shown in Figure 8. The fabrication of this cell emphasizes that no high temperature diffusion is required to form the PIN interface, and the fabrication temperature is lower than 200 °C, so it is easier to use thinner silicon chips and reduce costs.

What are the common types of high-efficiency monocrystalline silicon solar cells?
Figure 8 Structure of Sanyo’s HIT solar cell

Sanyo started mass production of HIT solar cells in 1999, and in April 2003 published a commercial record of 21.5% high-efficiency HIT solar cells, which achieved the efficiency improvement goal through process improvement, mainly by improving the silicon thin film. quality, making it more efficient. Later, Sanyo company developed a new conductive adhesive with higher conductivity, which can obtain higher filling factor and short-circuit current value, and its conversion efficiency also reached 21.5% (Voc = O. 712 V, lsc = 3. 837 A , FF = 78. 7 % ), and the battery area is 100. 3 cm². In addition, San yo also mentioned that the conversion efficiency of general solar cells will decrease with the increase of temperature, but the efficiency of HIT solar cells has the slowest decrease rate, which is 0.25%/℃, as shown in Figure 9, showing that the Battery performance is good.

What are the common types of high-efficiency monocrystalline silicon solar cells?
Figure 9 Conversion efficiency versus temperature of HIT solar cells

5. Backside Contact Solar Cells
In order to reduce the cost of the solar cell process, Sun Power Corporation of the United States has developed a high-efficiency, low-cost back-contact solar cell, as shown in Figure 10. The FZ chip with a resistivity of 2.0Ω/cm and a thickness of 200 µ.m is mainly used to texture the front side of the battery, passivate the front and back sides of the battery with an oxide layer, and reduce the surface reflection with a double-layer anti-reflection layer. The N+/P contact is formed on the backside by diffusing from the side. The aluminum electrode is designed to be finger-shaped and all distributed on the backside to increase the utilization of incident light. The overall cell area is 22 cm², and the highest cell efficiency reaches 23%.

What are the common types of high-efficiency monocrystalline silicon solar cells?
Figure 10 The structure of the back contact solar cell developed by SunPower

In addition, the thickness of the backside contact cell affects the conversion efficiency of the cell. If the thickness of the battery is reduced, the electron-hole pairs will be reduced, because a large number of photons cannot be absorbed; ② The collection efficiency of minority carriers can be increased, and the reduction of the thickness will shorten the moving distance of the carriers; ③ The dark current can be reduced; ④ Reduces the effect of edge current carrying on recombination. The resistivity of the battery is also closely related to the battery efficiency. In addition to affecting the carrier mobility, it also affects the bulk recombination current (bulk recombination current), as well as the parallel resistance and the edge carrier recombination rate. In order to reduce costs, the following chips such as high-quality CZ chips can also be used. These two chips also have the effect of carrier lifetime greater than 1ms, and the battery conversion efficiency can reach more than 19%.

6. Point contact solar cell
In 1986, the Stanford University research team developed a point contact solar cell, as shown in Figure 11. The maximum conversion efficiency of the cell can reach 28.3% under the condition of collecting light of 050 suns.

What are the common types of high-efficiency monocrystalline silicon solar cells?
Figure 11 Point contact solar cell structure

The main feature of this structure is to reduce the emitter area on the back of the battery, which is similar to the back contact structure developed by Sun Power. Both positive and negative electrodes are designed on the back. It can be used more effectively; in addition, since the back electrode is composed of several layers of metal, the series resistance of the structure is quite low, so that the output power loss will not be too much, about several percentages. However, the area of ​​the solar cell that was originally designed with this structure is quite small, about 1.21 cm², which makes modularization very difficult.

The main process of point contact solar cells is as follows 2. Using < 100 > FZ grade N-type chip, the thickness is 130 233µm, the resistivity is 100 200Ω/cm, and the diffusion of P-type and N-type is about 1000 Ω/cm. The SiO2 oxide layer is deposited by TCA process in the environment of ℃, the thickness is about 1000Å, and the sheet resistance value is 5~6Ω/port, which also reduces the surface bonding rate. At the same time, the oxide layer can also be used as an anti-reflection layer. The electrode materials of N-type and N-type are all aluminum, and the resistivity of the electrode is (1~2)×10-6Ω/cm.

7. OECO solar cells
Oblique plating contact (obliquely evaporated contact, OECO) solar cell is developed by German research institute ISF H (Institute Fur Solar) in recent years, its main feature is the use of special aluminum metal film oblique plating (obliquely evaporated) equipment. Different from the general vertical method, the evaporation process adopts the inclined method, so that the electrode can be plated on the side of the trench without any mask and alignment, and the width of the metal electrode can be easily adjusted. As shown in Figure 12, the front electrode of the battery is placed on the side of the parallel groove. Therefore, the shielding area of ​​the metal electrode is very small. This cell uses an MIS structure, so the thickness of the thin oxide layer must be carefully controlled. At present, the conversion efficiency of such solar cells is 18%~21%. Figure 13 shows the main structure of OECO solar cells.

What are the common types of high-efficiency monocrystalline silicon solar cells?
Figure 12 Evaporation system of OECO solar cells
What are the common types of high-efficiency monocrystalline silicon solar cells?
Figure 13 Main structure of OECO solar cell

8. Metal insulating layer semiconductor solar cells
As early as the 1970s, MIS structured solar cells attracted everyone’s attention. Whether in theory or in practical preparation, MIS structured solar cells can make up for the shortcomings of Schottky barrier solar cells. MIS solar cells are also known as MIS solar cells. for low open circuit voltage solar cells. The MIS insulating layer is quite thin, and in addition to controlling the huge dark current, it can also control the type of majority or minority carriers. In the 1990s, the MIS-IL (metal-insulator-semiconductor inversion-layer) solar cell was developed by the German IS FH organization, as shown in Figure 14. The conversion efficiency is 15.7% on a 2 cm × 2 cm FZ chip; on a 10 cm × 10 cm CZ chip. Its conversion efficiency is 15.3%. In order to achieve higher conversion efficiency, the battery conversion efficiency can reach 18.5% by improving the following three parameters:
(1) Reduce the loss of surrounding carrier recombination.
(2) Use the grid electrode on the front of the battery to improve the electrode impedance.
(3) Reduce the carrier recombination loss on the back of the battery.

What are the common types of high-efficiency monocrystalline silicon solar cells?
Figure 14 Solar cell structure of MIS-IL

The following are the main fabrication steps of solar cells with MIS-IL structure:
(1) Chemical etching (texturing the surface of the battery, such as a pyramid).
(2) The backside of the battery is deposited with an aluminum electrode by an evaporation method.
(3) A tunnel oxide layer is grown at 500°C.
(4) Use the metal mask to define the pattern and deposit the aluminum electrode on the front side of the battery.
(5) The excess aluminum electrodes are removed by etching.
(6) Immersion in calcium to increase the positive charge density on the silicon surface.
(7) Deposit Si Nx on the entire front side of the cell using PE CVD.

In 1997, the ISFH institute developed a more efficient MIS-N+P solar cell, as shown in Figure 15, mainly by changing the following manufacturing processes:
(1) Place the MIS electrode on the N+ diffused emitter.
(2) The electrodes on the front and back of the battery are made of aluminum.
(3) Double-layer SiNx is deposited by PECVD, which is used as a double-layer anti-reflection layer (DLAR) as a passivation layer. Compared with the MIS-IL structure solar cell, the open circuit voltage and fill factor of the MIS-N+P structure are not improved. The open circuit voltage is increased from 595mV to 656mV, the fill factor is also increased from 74.4% to 80.6%, and the overall battery efficiency is increased from 18.5% to 20.9%. Generally speaking, the production process and structure design of MIS structure solar cells are not difficult, the cost is not high, and the efficiency has reached more than 20%. If it can be improved and researched and developed, it is expected to become the mainstream of monocrystalline silicon solar cells.

What are the common types of high-efficiency monocrystalline silicon solar cells?
Figure 15 Structure of MIS-N+P solar cell

9. Screen-printed solar cells
Since the advent of screen printing technology, it has been used in quite a few occasions. In addition to the printing of circuit boards, it is also used in the electrode manufacturing process of solar cells. The process is quite fast, simple and low-cost. At present, many manufacturers of solar cells, in order to increase the speed of mass production, mostly use screen printing technology to print the electrode part on the emitter (30~55Ω/□) instead of the shallow emitter (shallow emitter, 90~ 100Ω/□) to avoid high electrode impedance, the structure of the screen-printed solar cell is shown in Figure 16.

What are the common types of high-efficiency monocrystalline silicon solar cells?
Figure 16 Structure of screen-printed solar cell

Heavy doping of the emitter will reduce the short-wavelength response, and will make the emitter saturation current higher, which will make the solar cell less efficient. Therefore, in order to increase the efficiency of solar cells, a high sheet resistance value of the emitter can be used to provide an effective emitter surface passivation. Usually screen printing is a part of the solar cell manufacturing process, which belongs to the latter part of the solar cell manufacturing process. Aluminum-containing glue is often used. The aluminum glue is printed on the back of the battery by screen printing, and is placed in a 200 ℃ environment first, and then removed. After removing excess water, the silver-containing jelly is printed on the anti-reflection layer of the battery, and then the battery is put into the furnace tube (CIR or RTP furnace tube) for sintering, thus completing the production of screen-printed solar cells. Using screen-printed solar cells, the efficiency can exceed 18%.

What aspects need to be considered in battery structure design?

What aspects need to be considered in battery structure design?

1. Consideration of semiconductor materials
Generally speaking, the chips used in solar cells can be divided into P-type and N-type, but because the diffusion length of excess minority carriers (electrons) in P-type is longer than the diffusion of excess minority carriers (holes) in N-type The length is too long. In order to obtain a larger photocurrent, a P-type chip is generally selected as the substrate.

2. Considerations of spectral response
Figure 1 is the solar radiation spectrum of AMO and AMI. In solar radiation, the most energy is in the short wavelength region close to UV. in solar cells. Since only some photons with energy greater (Eg) than the energy gap of the solar cell material can be absorbed by the material, other photons with energy less than the energy gap of the material cannot be absorbed and penetrated; in addition, if a material with a small energy gap is selected. Although most of the spectrum can be absorbed, the intrinsic concentration ni of the material is large, the dark current of the cell will increase and the efficiency of the solar cell will decrease. Generally speaking, the material energy gap of solar cells is between 1~2 eV, and the corresponding light wavelength is 0.6~1.3µm, so the material energy gap is between silicon (Si), gallium arsenide (GaAs), Indium phosphide (lnP) is a very good solar cell material.

What aspects need to be considered in battery structure design?
Figure 1 Solar radiation spectrum

3. Considerations for Shallow Interfaces
Based on the above considerations, we chose the material as monocrystalline silicon (Si) to consider the design of the shallow interface. When the light wavelength is less than the short wavelength of 400nm, the light absorption coefficient of silicon will become very large (>105cm1), so most of the incident photons will be absorbed near the surface depth (0.1~0.3µm) to generate electron-hole pairs. For the light absorption coefficient of a material, the larger the light absorption coefficient, the stronger the absorption of photons near very shallow surfaces. To improve the conversion efficiency of the cell, the key is to reduce the interface depth of the silicon solar cell to less than 0.3µm, so as to enhance the absorption of short wavelengths and improve the efficiency.

4. Considerations for Anti-Reflection Layers
For single crystal silicon, the light reflectivity on the silicon surface under normal illumination is 30%~35%. In order to reduce the surface reflectivity, layers of materials with different refractive indices can be used to stack up anti-reflection layers. As shown in Figure 2, the interface reflection coefficient R between air and medium can be calculated according to the following formula

What aspects need to be considered in battery structure design?
1
What aspects need to be considered in battery structure design?
Figure 2 The structure of the anti-reflection layer of the solar cell

In the formula, n0 is the refractive index of air; n1 is the refractive index of the anti-reflection layer; ns is the refractive index of the semiconductor. Since n0=1 (air), when n1=(ns) 1/2, the reflection coefficient R=0, the refractive index of single crystal silicon at the incident light wavelength of 400~1100nm is 6.0~3.5, so in order to reduce the surface reflectivity, The refractive index of the antireflection layer is preferably between (3.5) 1/2 = 1.87 and (6) 1/2 = 2.5. Commonly used anti-reflection layer materials are SiO2 (n=1.5), Si3N4 (n=2.0), Ta2O5 (n=2.25), TiO2 (n=2.3) and so on.

5. Considerations for Surface Passivation
The surface passivation (surface passivation) treatment is generally to grow a layer of oxide layer to reduce the surface carrier recombination speed at the Si-SiO2 interface.
The total carrier recombination rate per unit area of ​​the semiconductor surface is

What aspects need to be considered in battery structure design?
2

where nth is the electron thermal speed; σn is the electron-capture cross-section; σp is the hole capture cross-section; Nst is the surface recombination rate (#/cm3); ps is the hole concentration on the surface; ns is the electron concentration on the surface; E is the energy level of the defect center. Under low injection, ns≈ND>>ps, ns﹥﹥ni, exp(Et-EFi/kT), so the above formula can be rewritten as

What aspects need to be considered in battery structure design?
3

Here Sp=vstσpNst, Sp is the surface recombination velocity of holes. The surface recombination velocity has a great influence on the reverse saturation current. The faster the surface recombination speed, the greater the reverse saturation current, resulting in a decrease in the photocurrent. , which affects the efficiency of solar cells.

6. Considerations for textured structures
Anisotropic chemical etching on the single crystal silicon surface in the (100) lattice direction will form a tiny inverted pyramid structure in the (111) lattice direction. As shown in Figure 3, this textured structure can cause multiple reflections of incident light, which can increase the travel path of light in addition to reducing the reflection of incident light. For long-wavelength incident light, since the light absorption coefficient α of single crystal silicon material is very low, the incident light will be reflected at the bottom and then refracted by the surface textured structure to increase the light absorption.
incident light

What aspects need to be considered in battery structure design?
Figure 3 Surface textured structure
What are the characteristics of PN junction diodes

What are the characteristics of PN junction diodes

The basic structure of a semiconductor solar cell device is a PN junction diode. When the P-type and N-type semiconductors contact to form a PN junction, there is a huge difference in the carrier concentration at both ends of the PN junction. The neutral property is destroyed, and a space charge zone (depletion zone) is formed at the junction; a built-in electric field is generated, and the minority carriers are affected by the built-in electric field to move, forming a drift current. When the drift current of the carrier reaches equilibrium, the net carrier current is zero and the system returns to the thermal equilibrium state. What happens when a photon with an energy greater than the energy gap is injected from one end of the PN junction structure?

First, if the two ends of the PN junction are connected together, the electron-hole pairs generated by the light in the depletion zone will be affected by the built-in electric field. The electrons will drift to the N-type semiconductor region, and the holes will go to the P-type semiconductor region. Drift, resulting in a drift current flowing from the N-type to the P-type. As for the electron-hole pairs generated by illumination in the N-type and P-type semiconductor regions outside the depletion region, due to the lack of a built-in electric field, and the majority carrier concentration is basically not affected by the effect of light, it is obvious. Change (under the hypothesis of a low injection of the solar spectrum), so only a minority carrier diffusion current will be generated. Taking the P-type semiconductor region as an example, since the electrons in the depletion region near the P-type end region continue to flow to the N-type semiconductor region, the electron concentration at the edge of the depletion region is low, so the P-type
The electrons generated by light in the semiconductor region will diffuse into the depletion region, and then flow into the N-type semiconductor region; that is, the illumination effect will generate minority carrier diffusion currents in the N-type and P-type semiconductor regions outside the depletion region, and the electrons are caused by The P-type semiconductor region flows to the N-type semiconductor region, and the holes flow from the N-type semiconductor region to the P-type semiconductor region. Therefore, the sum of the drift current in the depletion region, the electron diffusion current generated by the P-type semiconductor region, and the hole diffusion current generated by the N-type semiconductor region is the so-called photocurrent, that is, the short-circuit current, which flows to the PN junction. The current of the tube under forward bias is opposite.

When a load resistor is connected at the two ends of the PN junction, the photocurrent generated by the illumination effect flows out of the P pole and flows through the load resistance, resulting in a potential difference between the two ends of the load resistor. The direction of this potential difference is like a forward bias, resulting in a PN junction. The built-up potential in the depletion region decreases, so the majority carrier diffusion current increases, which cancels part of the photocurrent.

If the two ends of the PN junction are open (not connected), it means that when the photocurrent generated by the illumination effect flows to the surface of the two ends of the PN junction, it cannot be discharged, and negative charges (electrons) will accumulate on the end surface of the N-type semiconductor region at the same time. Positive charges (holes) are on the surface of the end of the P-type semiconductor region, causing a parallel plate capacitance effect. When the voltage generated by the accumulated charge suppresses the built-in voltage in the depletion region, the majority of carriers are easily diffused into the depletion region, and the light is minor. The carrier diffusion current and the drift current in the depletion region recombine, and the net current will approach zero. The voltage at this time is the so-called open circuit voltage. The terminal potential of the P-type semiconductor region is higher than the terminal potential of the N-type semiconductor region, which is the so-called forward bias.

PN junction diode

The so-called PN junction is the junction formed by contacting the N-type semiconductor and the P-type semiconductor. The most important characteristic of the PN junction is that it has rectify properties, that is, when a positive bias is applied to the P-type semiconductor terminal ( Called forward bias), current can easily flow from the P-type semiconductor terminal to the N-type semiconductor terminal; on the contrary, if a positive bias is applied to the N-type semiconductor terminal (called reverse bias), the current cannot Flow from the N-type semiconductor terminal to the P-type semiconductor terminal. Figure 1.1 is the current-voltage characteristics of a typical silicon semiconductor PN junction. The abscissa represents the voltage applied to the P-type semiconductor terminal (in V), and the ordinate represents the current that flows from the P-type to the N-type semiconductor terminal (in mA). It can be found from the figure that when the operation is forward biased (the voltage is positive), the current-starts to be almost zero, and as the voltage continues to increase to about 0.TV, the current starts to increase rapidly, that is, the forward conduction starts. . When operating in reverse bias (the voltage is negative), the current is almost zero, and does not change with the increase in voltage, until it reaches a maximum critical voltage (VB), the current suddenly increases rapidly, this phenomenon It is called junction breakdown, and its critical voltage depends on the semiconductor material, doping concentration and the structure of the junction and other parameters, which can range from several volts to several thousand volts.

Current and voltage characteristics of a typical silicon semiconductor PN junction


Figure 1.1 Current and voltage characteristics of a typical silicon semiconductor PN junction

To understand the reasons for the above-mentioned current-voltage characteristics, we must start with the discussion of the combination of two different doping types of semiconductors. Figure 1.2(a) shows uniformly doped and separated P-type and N-type semiconductor materials and their corresponding energy band diagrams. The majority carriers in the P-type semiconductor are holes, and the minority carriers are electrons, and the Fermi level is close to the top of the valence band; on the contrary, the majority carriers in the N-type semiconductor are electrons, and the minority carriers are electrons. It is empty six, and its Fermi level is close to the bottom of the conduction band.

When the P-type and N-type semiconductors are tightly combined together [Figure 1.2(b)], a carrier concentration gradient will immediately form at the junction, causing the majority of the carrier holes at the P-type semiconductor end to diffuse into the N-type semiconductor, and at the same time, The majority carrier electrons of N-ming semiconductors also diffuse into P-type semiconductors. Therefore, the holes in the P-type semiconductor near the junction region either diffuse into the N-type semiconductor, or recombine with the electrons from the N-type semiconductor and disappear, resulting in the negatively charged acceptor impurity ions (N); while the N-type semiconductor is near the junction The electrons in the region either diffuse into the P-type semiconductor, or recombine with the holes from the P-type semiconductor and disappear, leaving positively charged donor impurity ions. Therefore, a negative space charge is formed at the P-type semiconductor terminal near the junction, and a positive space charge is formed at the N-type semiconductor terminal near the junction, and a N-type semiconductor is directed to the P-type semiconductor at the junction. This electric field will drive the minority carrier electrons of the P-type semiconductor terminal to drift to the N-type semiconductor terminal, and at the same time, it will also drive the minority carrier holes of the N-type semiconductor terminal to drift to the P-type semiconductor terminal.

When the PN junction reaches a state of thermal equilibrium, a fixed-width carrier-depleted region is formed at the junction, which is called a depletion region, also called a space charge region. At this time, the diffusion current caused by the concentration gradient and the drift current caused by the built-in electric field of the space charge will completely cancel out [Figure 1.2(c)].

Features of PN junction diode

Figure 1.2 (a) Uniformly doped and separated P-type and N-type semiconductors and their corresponding energy band diagrams; (b) When the P-type and N-type semiconductors are connected together, the majority carriers at both ends begin to diffuse to the junction , Recombination occurs; (c) When the thermal equilibrium state is reached, a depletion zone and a built-in electric field will be formed at the junction, and a drift electron hole flow will be generated to counteract the diffusion electron hole flow.

What are the donors and acceptors of semiconductors

What are the donors and acceptors of semiconductors

Donor and Receiver
The conductivity of semiconductors is not strong, basically equivalent to insulating materials, not much use. However, if appropriate impurities are added, it will be found that the conductivity of semiconductors can be greatly adjusted. This kind of semiconductor doped with impurities is called extrinsic semiconductor. The concept of doping impurities to change the conductivity of semiconductor materials can be Learn from Figure 1.1. Take the silicon semiconductor material as an example. When the silicon material is doped with arsenic (As) element of group V [Figure 1.1(a)], a silicon atom in the lattice is replaced by an arsenic atom with 5 valence electrons. Arsenic atoms tend to form covalent bonds with 4 adjacent silicon atoms. Although the fifth valence electron is still bound, the arsenic atom forms a covalent bond with the surrounding silicon atoms, resulting in the binding energy of the fifth valence electron of the arsenic atom. It is greatly weakened and can be ionized into conduction band electrons near room temperature. The arsenic atom seems to play the role of providing conduction band electrons, therefore, the arsenic atom is called the “donor”. At this time, the number of conductive electrons in the semiconductor material is determined by the concentration of doped impurities, and is generally much greater than the intrinsic carrier concentration. At this time, the transport in the semiconductor is dominated by electrons (negative charges), so it is called It is “N-type semiconductor”.

Doping impurities to change the conductivity of semiconductor materials

Figure 1.1 Doping (a) V-valent arsenic atoms and (b) Ⅲ-valent boron atoms in silicon semiconductor materials; (c) arsenic atoms will generate additional confined energy in the forbidden band between the conduction band and the intrinsic Fermi level The ED is called the donor level. The electrons on the donor level are easily heated and vibrated to transition to the conduction band and become conductive electrons; (d) the boron atom will be in the valence band and the intrinsic Fermi An additional limited energy level (EA) is generated in the forbidden band in the middle of the energy level, which is called the acceptor level. There is a lack of an electron on the acceptor level, so the electrons in the valence band are easily subjected to thermal vibration and jump to the acceptor level. The main energy level leaves a vacancy in the valence band and becomes a conductive hole.

In the same way, when silicon material is doped with boron (B) element of group III [Figure 2.17(b)], a silicon atom in the lattice is replaced by a boron atom with 3 valence electrons, and the boron atom will tend to Yu forms a covalent bond with four adjacent silicon atoms, but lacks a valence electron, just like there is a vacancy on the covalent bond. At close to room temperature, the valence electrons in the covalent bond formed by the surrounding silicon atoms are extremely It may be ionized to replace the insufficient valence electrons of the boron atom and generate conductive holes in the valence band. Boron atoms seem to play the role of accepting valence electrons, therefore, boron atoms are called “acceptors”. At this time, the number of conductive holes in the semiconductor material is determined by the concentration of doped impurities. At this time, the transport in the semiconductor is dominated by holes (positive charges), so it is called “P-type semiconductor”.

From the perspective of energy and energy level, the valence electrons of arsenic atoms are ionized to make them conductive electrons, or valence band electrons are excited to boron atoms, which are bound by boron atoms. The required energy is called ionization energy, so arsenic The atom will generate an additional limited energy level (ED) in the forbidden band between the conduction band and the intrinsic Fermi level, called the donor level (Figure 1.1(c)), the electrons on the donor level , It is easy to be heated and vibrated and ionized to transition to the conduction band and become conductive electrons. The arsenic atom that loses one valence electron becomes arsenic positive ion (As+). The boron atom will generate an additional limited energy state (EA) in the forbidden band between the valence band and the intrinsic Fermi level, called the acceptor level (Figure 1.1(d)), the acceptor energy There is a lack of an electron in the valence band, so the electrons in the valence band are easily subjected to thermal vibration and ionization transition to the acceptor energy level, leaving a hole in the valence band and becoming a conductive hole. Therefore, the boron atom has one more electron and becomes a boron anion (B).

Figure 1.2 shows the donor or acceptor energy levels corresponding to semiconductors such as germanium, silicon and gallium arsenide doped with different impurities. It must be noted that a single atom impurity may form several energy levels. Taking carbon into a silicon semiconductor as an example, it will form a donor level and an acceptor level.

donor or acceptor energy levels corresponding to semiconductors such as germanium


Figure 1.2 Donor or acceptor energy levels and ionization energies (in eV) corresponding to different impurities in semiconductors such as germanium, silicon and gallium arsenide. The energy level is higher than the energy gap center, except for the energy level marked A as the acceptor energy level, it is the donor energy level. The energy level is lower than the energy gap center, except for the energy level marked D as the donor energy level, it is the acceptor energy level. The ionization energy of all donor impurities is measured from the bottom of the conduction band. The ionization energy of all acceptor impurities is measured from the top of the valence band.

What is the electronic band structure

What is the electronic band structure

In the crystal lattice of diamond or sphalerite structure, each atom is surrounded by 4 nearest atoms, each atom provides 4 valence electrons, and forms a covalent bond with the valence electrons of the nearest atom, so each covalent bond contains one For electrons [Figure 1.1(a)]. At low temperatures, valence electrons are trapped between atoms and cannot move freely, so they cannot conduct electricity. But at high temperature, thermal vibration will excite the bound valence electrons into free electrons, which can participate in the conduction of electric current. While the valence electrons are excited to become free electrons, the original bonding electron pair loses one valence electron, forming a vacancy [Figure 1.1(b)] This vacancy may be filled by neighboring valence electrons, resulting in a movement equivalent to the vacancy. Phenomenon, here we must pay special attention to fill the vacancy must be bound valence electrons to form a vacancy movement, if it is filled with free electrons, the vacancy will disappear instead of moving, called recombination (recombintion). Therefore, the vacancy can be recombined. Imagine an electron-like particle with a positive charge (because the vacancy moves in the electric field in the opposite direction to that of the electron), which is called a hole (Figure 1.1(c)).

The movement of hole

Figure 1.1 (a) In a semiconductor material, each atom is surrounded by 4 nearest atoms, each of which provides 4 valence electrons to form a covalent bond with the valence electrons of the nearest atom;

(b) The valence electron in the A valence bond absorbs enough energy, and the excitation becomes a free electron. Before it returns to the vacant valence bond position, it can move between the lattices, so it is called a conductive electron. The bond electron pair loses a valence electron, forming a vacancy, called a hole;

(c) The valence electron of the B valence bond runs to fill the vacancy of the A valence bond, causing the vacancy to move from the A valence bond to the B valence bond position, which can be regarded as the movement of hole.

From the Bohr hydrogen atom model, we know that in an isolated atomic system, the electron energy can only allow discontinuous energy levels to exist. The energy level state can be represented by the principal quantum number (n) and angular momentum quantum number (l). When two atoms are far enough apart, their electronic system does not undergo any quantum interaction (Pauli exclusion principle), so the electrons in the two atomic systems have the same energy state, which is called double simple The doubly degenerate state. But when two atoms are close enough to interact, the originally degenerate electron energy level will be divided into two. The energy level with lower energy is called bonding orbital, and the energy is higher. The energy level of is called antibonding orbital, as shown in Figure 1.2. When N atoms gather to form a solid, the principle of the relationship between the inner electrons (core electrons) of the atoms and the inner electrons of the surrounding atoms The above will not interact with each other, and still maintain the same discrete energy state as the isolated atomic system. The outer electrons of the atom will overlap and interact. At this time, the energy levels are split into N separate but very close energy levels. When N is large, a continuous energy band will be formed. Therefore, when discussing the electronic properties of solid materials, valence electrons play a major role.

(a) Hydrogen molecular orbital; (b) Energy band formation; (c) Semiconductor electric band and valence band formation

Figure 1.2 (a) Hydrogen molecular orbital; (b) Energy band formation; (c) Semiconductor electric band and valence band formation

Diamond structure and (b) corresponding insulator characteristic molecular orbital energy state diagram; (c) Graphite structure and (d) corresponding metal property molecular orbital energy state diagram

Figure 1.3 (a) Diamond structure and (b) corresponding insulator characteristic molecular orbital energy state diagram; (c) Graphite structure and (d) corresponding metal property molecular orbital energy state diagram

Figure 1.3 takes a carbon atom as an example to illustrate that when the valence electrons of the carbon atom are bonded by sp3 hybrid orbitals, because the sp3 hybrid orbital bonding configuration has a tetrahedral structure, it will finally be stacked to form a diamond structure solid. Material, the formed band structure shows that there is a forbidden gap with an energy difference of about 6 eV (electron volts) between the molecular orbital that is completely filled with electrons (molecular orbital) and the molecular orbital that is completely unoccupied by electrons, which is Band gap (or energy gap, energy gap), showing the properties of an insulator. But when the valence electrons of carbon atoms are bonded by sp2 hybrid orbitals, because the valence structure of sp2 hybrid orbitals has a planar triangular configuration, it will finally be stacked to form a graphite layered structure solid material, between the atomic layer and the layer. The valence bond is relatively weak, and the formed energy band structure shows the nature of the molecular orbital completely filled with electrons and the molecular orbital completely occupied by no electrons, showing the characteristics of metal.

The actual energy band splitting in semiconductors is more complicated. When the distance between atoms is shortened, the quantum energy states (such as s and p) will interact and overlap. In the equilibrium state, the atomic distance and energy band will split again (figure) 1.2(c)]. The lower energy has 4N quantum states, and the higher energy band has 4N quantum states. Because each atom has 4 valence electrons, there are 4N valence electrons in total. At absolute zero, electrons will start to occupy the lowest energy state, so the lower energy band (namely the valence band) is just completely filled, while the energy state of the higher energy band (namely the conduction band) is not occupied by any electrons. There is no energy state between the top of the valence band and the bottom of the conduction band. Naturally, there will be no electrons distributed in this energy range, which belongs to the forbidden energy region. Therefore, the energy difference between the top of the valence band and the bottom of the conduction band is called Energy gap. Physically, the energy gap value represents the minimum energy required to ionize the valence electrons of the semiconductor material from the valence bonds into free electrons.

Schematic diagram of the relationship between energy and momentum of direct-gap semiconductor materials

Note: The electrons at the top of the valence band are excited by heat to transition to the empty energy level at the bottom of the conduction band, leaving holes. The electrons excited to the conduction band and the holes remaining in the valence band are respectively negatively charged and positively charged free charges, resulting in the unique conduction properties of semiconductor materials.

Figure 1.4 Schematic diagram of the relationship between energy and momentum of direct-gap semiconductor materials

It should be noted that the effective mass of each energy band is not constant. In addition, at the top of the valence band, its effective mass is actually negative. The electrons originally filled the entire energy band from bottom to top, but some electrons at the top were affected by thermal vibration and jumped to the conduction band, making the energy state near the top of the valence band empty. These empty energy states show conduction currents as positive charge carriers participate in conduction. The carriers whose effective mass is positive are called hole. Conceptually, it is easier to deal with hole with positive effective mass, which behave like classic positively charged particles. The top of the valence band and the bottom of the conduction band tend to be parabolic in appearance, and the effective mass of the electron near the bottom of the conduction band is constant, just like the effective mass of the hole near the top of the valence band. When the minimum value of the conduction band and the maximum value of the valence band occur at the same crystal momentum at the same time, as shown in Figure 1.4, this semiconductor is a semiconductor with a direct band gap. When they are not on a straight line, the semiconductor is called an indirect band gap semiconductor.

Relative to the momentum of electrons, the momentum of photons is extremely small. Therefore, when the photon interacts with the electrons in the semiconductor material, the momentum that can be exchanged is extremely limited, which leads to the transfer of electrons between energy bands without the participation of other particles. The change is minimal. In other words, valence electrons in indirect band gap semiconductor materials are less likely to transition from the top of the valence band to the bottom of the conduction band by absorbing the energy of a photon; it is also not easy for conductive electrons to radiate a photon energy to transition from the bottom of the conduction band to the valence. With top. In contrast, direct energy band semiconductor materials can more effectively absorb or emit photons through the transition of electrons between energy bands.

Even amorphous solid materials will exhibit a similar band structure (Figure 1.5). The reason is that the amorphous material still has a certain degree of short range order, and in the extremely short range, the atoms still have a certain degree of regular arrangement. Taking amorphous silicon material as an example, its atomic structure still maintains a tetrahedral valence bond configuration, that is, the valence bond between the atoms is still sp3 hybrid orbital covalent bonding, and the average distance between atoms and the average valence bond angle are still similar to Crystalline silicon material. Therefore, the local electronic wave function still needs to comply with the periodic potential energy established by the regular arrangement of atoms, and the entire bulk electronic wave function can be regarded as the superposition of the local wave functions. Simply put, amorphous material can be regarded as a material made up of many tiny molecules connected in a network. The atomic structure of each tiny molecule is not exactly the same, but there are still corresponding energy band characteristics. There are many defects between tiny molecules. Therefore, the energy band characteristics of the whole amorphous material include the superposition of the band structures of many tiny molecules and the combination of local defect states. Because the wave function of electrons in extremely tiny molecules is affected by the periodic potential energy of atoms, their band structure is still similar to that of crystal materials, with valence band and conduction band characteristics. However, unlike crystalline materials, due to the disordered structure, the extended state (extended state) in some crystalline materials will be transformed into a localized state. The extended energy band of the entire bulk material also includes the local state where the electron wave function is limited to a local area; among them, because electrons or hole occupy the local energy state, they cannot move effectively, and the electrons and hole are only in the material. It can move through overlapping and expanding energy bands, so in amorphous materials, the definition of “mobility gap” (mobility gap) is defined to replace the concept of energy gap of crystalline materials, and the valence band is divided into two parts: valence band mobility boundary energy The following energy bands and valence band tail states (va-lence band tail). The conduction band also includes above the conduction band mobility boundary and conduction bandtail. Amorphous materials contain high-density dangling bonds. These dangling bonds have only one electron, which may capture an electron or release an electron, thus forming a defect, which will form a defect state near the Fermi energy and reduce the carrier. Longevity makes material properties worse. Actually, in the application of amorphous materials, it is often necessary to dope a large amount of hydrogen atoms to passivation the dangling bonds, reduce the defect state, and improve the conductivity.

Schematic diagram of electronic energy states of amorphous semiconductor materials

Figure 1.5 Schematic diagram of electronic energy states of amorphous semiconductor materials

What is the crystal structure of solid materials 3

What is the crystal structure of solid materials

Solid materials can be divided into conductors, semiconductors and insulators according to their electrical conductivity. Among them, the conductivity of semiconductor materials is between that of conductors and insulators, and is easily affected by temperature, light, magnetic fields and impurity atoms. In fact, the conductivity of semiconductor materials can be adjusted by doping different concentrations of impurity atoms, and the adjustment range can be as high as 1010. In addition, the effect of temperature on the conductivity of semiconductor materials is also very different with the difference of metal materials. Generally speaking, the conductivity of metal materials is not greatly affected by temperature changes, and basically the higher the temperature, the smaller the conductivity; while the conductivity of semiconductor materials is closely related to temperature, and as the temperature increases, its conductivity increases. However, due to its high conductivity and sensitivity, semiconductors have become the most important materials in electronic device applications.

Most semiconductor solar cell products are mainly made of silicon semiconductors, and some are made of semiconductor materials such as gallium arsenide (GaAs), indium gallium phosphide (GalnP), copper indium gallium selenium (CuInGase) and cadmium telluride (CdTe). Among them, semiconductors such as silicon and germanium are composed of one element, called element semiconductor. Other semiconductors composed of two elements (group III and group V, group II and group VI), three elements or even four elements are called compound semiconductors. Compared with elemental semiconductors, the synthesis of compound semiconductors often requires more complicated processes. When selecting materials, they usually choose the absorption characteristics that conform to the solar spectrum, and consider the material and preparation costs.

1.1 Crystal structure

Solid materials can also be classified according to their atomic arrangement, valence bond type, and crystal geometric structure (Figure 1.1). One type of solid material lacks long-range order or obvious short-range order. Order structure (short range order), called amorphous (amorphous) material, another type of solid material, atoms or groups of atoms arranged in a regular and orderly manner to form a periodic three-dimensional space array, called crystalline (crystalline) material. Crystal materials can be further divided into single-crystalline solids and polycrystalline solids. As the name implies, the single crystal structure is that the atoms inside the material are regularly arranged throughout the crystal, while the polycrystalline structure is that there are many hundreds of angtrom to hundreds of micrometers (micrometer) in the entire bulk. Grain, although the atoms inside each grain are arranged in a regular and orderly manner like a single crystal, there is no regular orientation and spacing between the grains, so there are grain boundaries.

Three types of structures of solid materials

Figure 1.1 Three types of structures of solid materials

There are often many defects, dangling bonds and impurities (impurity) in the grain boundary, which adversely affect the physical and chemical properties of the material, especially for the carrier transport characteristics, the trapping caused by the grain boundary (trap) and scattering (scattering) effects seriously affect the mobility of carriers in the material.

Through X-ray and electron beam diffraction (diffraction) technology, single crystal, polycrystalline or amorphous structures can be accurately distinguished. However, because the analysis of the behavior of electrons in amorphous materials is much more complicated than the behavior of electrons in single crystal materials, the single crystal structure becomes the basis for solving the physical properties of solid-state materials. Although the analysis of the properties of amorphous and polycrystalline semiconductor materials is complicated and difficult to understand, in fact, the concept extended from the solid-state theory of crystal structure combined with defect theory can still be applied to these materials. Therefore, the basic concepts of semiconductors introduced in this chapter also focus on single crystal semiconductor materials.

Single crystal structure has periodic three-dimensional spatial atomic arrangement, and some building blocks can be found according to its arrangement rules and symmetry. If these constituent units are repeatedly stacked together and extend continuously in all directions, the entire crystal structure can be produced. Therefore, this constituent unit is called a unit cell. For a specific crystal structure, there are many possibilities for the choice of a unit cell. Bravais found from symmetry analysis that crystals can be divided into 14 structures, including triclinic, monoclinic, orthorhombic, tetragonal, cubic, and triangular. There are seven types of crystals (trigonal) or rhombohedral and hexagonal crystals. Of which cubic

The body can be divided into simple cubic (simple cubic) body-centered cubic (body-centered cubic) and face-centered cubic (ace-centered cubic.) three structures. For semiconductor materials, face-centered cubic crystals are one of the most important structures, and both diamond structure and sphalerite structure belong to face-centered cubic crystals.

Figure 1.2 is the periodic table of the elements. The elements marked with color are the main members of the semiconductor material. Among them, silicon belongs to the group IV element, which means that it has 4 valence electrons, and these 4 valence electrons can form covalent bonds with neighboring atoms. . In single crystal silicon, atoms are arranged in a diamond lattice with tetrahedral valence bonds (diamond lattice Figure 1.3(a)), and the angle between each valence bond is 109.5°. In particular, this arrangement can use two The unit cells of a face-centered cube are stacked through each other. The lattice constant a is the side length of the unit cell. A complete lattice can be formed by stacking several unit cells. The other is similar to the diamond structure is the zinc blende lattice (zincblende lattice) Figure 1.3(b)], it is different from the diamond structure in that the constituent atoms in the two interpenetrating face-centered cubic sub-lattices are different, one is group III or group II, and the other is group V or group VI .Most of III-V group semiconductors

And II-VI group semiconductors have this structure, such as gallium arsenide (III-V group) and cadmium telluride (II-VI group).

Periodic Table of Elements

Figure 1.2 Periodic Table of Elements

Two common structures of semiconductor materials

(a) Diamond lattice: elemental semiconductor (such as silicon, germanium, carbon)

(b) Sphalerite lattice: III-V compound semiconductors (such as gallium arsenide, gallium phosphide, indium phosphide…)

Figure 1.3 Two common structures of semiconductor materials

For a single crystal material, the arrangement of atoms is a three-dimensional space, arranged along each plane or direction, the arrangement period of the atoms and the distribution of the valence bond electron cloud are different. It is conceivable that the physical properties will also be different. Therefore, in crystals, the so-called Miller indices (Miller indices) are used to define different planes in a crystal. The Miller index determination law is the intercept of the plane on the three orthogonal axes of the orthogonal coordinate system (using the lattice The constant is the unit of measurement) take the reciprocal and reduce it to the simplest ratio of integers, and finally express the ratio of integers in (hkl), which is the Miller index of a single plane. Figure 1.4 shows the method of determining the Miller index and the cube The Miller index of important planes in the crystal.

Miller index determination method and Miller index of important plane in cubic crystal

Figure 1.4 Miller index determination method and Miller index of important plane in cubic crystal

What is semiconductor solar cell and black body radiation

What is semiconductor solar cell and black body radiation

The semiconductor solar cell is a semiconductor photodetector that has been optimized to absorb part of the sunlight and convert the voltage and current. However, it is different from general battery applications: the output voltage and current of the semiconductor solar cell will be affected by the load and change, unlike the general battery that can output a fixed voltage; when there is appropriate light, the semiconductor solar cell can output electrical energy, that is, Said that semiconductor solar cells do not have the ability to store electrical energy.

Semiconductor materials can absorb photons to generate electrons and holes. Through appropriate design, the semiconductor materials of different doping types are combined to form a diode. A semiconductor diode has a built-in electric field, which separates carriers (electrons and holes are collectively referred to as carriers) and forms a current in a specific direction. Therefore, a semiconductor solar cell is basically a designed semiconductor diode that can absorb light waves with energy greater than the energy gap of the semiconductor in the solar spectrum, and convert the energy of sunlight into electrical energy. Figure 1.1 is a schematic diagram of the semiconductor solar cell structure. Sunlight enters from the front of the battery, most of the light waves penetrate the anti-reflection layer and enter the semiconductor layer, and a small part of the light waves will be reflected back to the atmosphere by the metal mesh grid and the anti-reflection layer. The upper electrode contact of the diode is composed of a metal mesh grid. The design considers that the light wave is injected into the semiconductor by reducing the shading area, and the semiconductor absorbs the light energy and converts it into electrical energy. The anti-reflection layer between the mesh grids will increase the amount of light absorbed by the semiconductor. A semiconductor diode is composed of N-type semiconductor and P-type semiconductor. To make such a device, impurities need to be doped through diffusion, ion implantation or deposition to form a PN junction. The other electrode contacts of the diode are behind the solar cell. It is made by plating with a metal layer.

Schematic diagram of semiconductor solar cell structure

      Figure 1.1 Schematic diagram of semiconductor solar cell structure

All electromagnetic radiation, including sunlight, is composed of photons, and they all carry a specific amount. Photons also have the properties of waves, so they have wavelength properties. The corresponding relationship between photon energy and light wave wavelength is

         Eλ=hc/λ

In the formula, h is Planck’s constant; c is the speed of light. Only photons with sufficient energy (greater than the energy gap of semiconductor materials) can generate electron-hole pairs, which is helpful for the generation of electric energy. Therefore, the solar spectrum is an important factor when designing effective solar cells.

The surface temperature of the sun is about 5762K, and its radiation energy spectrum is very close to blackbody radiation, covering the spectrum from ultraviolet to infrared (0.2~3um). Solar radiation is isotropic like all black body radiation. However, the distance between the sun and the earth is very far (approximately 1.5×108km), so only part of the photons can directly hit the earth. In practical applications, the sunlight incident on the surface of the earth is often regarded as a parallel beam. Outside the earth’s atmosphere, at the average distance of its orbit around the sun, the solar radiation intensity is defined as the solar constant, which is about 1366W/m2. When sunlight enters the atmosphere from outside the atmosphere, it will be scattered and absorbed by the clouds and the atmosphere. Its energy intensity decreases with the path length of the light through the atmosphere (or the air quality through which the light passes), so it is defined as “Air Mass” (Air Mass). ) To indicate how much of the solar radiation reaches the surface of the earth after the solar radiation passes through the atmosphere. Because the air quality through which sunlight passes is basically related to the angle between the sun’s azimuth and the vertical line of the earth’s surface, the air quality value is defined as

Air Mass=1/cosθ

In the formula, θ is the angle of incidence (when the sun is directly above the head, θ=0). The air quality can be easily deduced from the height H of the object and its shadow length S

Air Mass

Since sunlight is scattered and reflected in the atmosphere, it absorbs the diffused part of the sunlight (indirect incidence) on the surface. This part of the light is about 20% of the direct incident light. Due to the diffuse part, for the sake of clarity, g (global) or d (direct) is often added to the air quality value to further define it. For example, the AM1.5g spectrum includes diffuse light, and the AM1.5d spectrum does not include diffuse light.

Black body radiation

Figure 1.2 Black body radiation

Figure 1.2 shows the black body radiation (T=5762K), AM0 and AM1.5g solar radiation spectra, where the AM0 curve represents the zero air quality situation and represents the solar spectrum outside the earth’s atmosphere. The AMO spectrum is related to satellite and space exploration applications. Generally, on the surface of the earth, the solar radiation spectrum is represented by air quality 1.5 (AM1.5). This spectrum represents the spectrum of sunlight falling on the surface of the earth when the sunlight is at an angle of 48° to the vertical, and its total incidence The power density is about 963W/m2.