Schmid and Viechnicki proposed a method of growing sapphire in 1970, called the Schmid-Viechnicki method, compared with other crystal growth methods. The biggest difference in this method is the addition of a heat exchanger. Therefore, in 1972, this method was renamed the heat exchange method. In 1974, C.P.Khattak and F.Schmid applied for the first time to grow silicon crystals.
The growth method of the heat exchange method is to control the heating power so that the solid-liquid interface gradually moves upward from the bottom of the crucible. The crystallization process is as follows.
Place the aluminum crucible filled with silicon raw material above a small diameter heat exchanger, and the seed crystal is placed between the bottom of the crucible and the heat exchanger. During the process of melting and growing, the ammonia gas is continuously passed through the heat exchanger Inside to ensure that the seed crystal will not be melted. After the silicon raw material is melted into a liquid, it needs to stand still for a period of time to reach a stable heat balance. After that, the temperature of the heat exchanger is gradually reduced to start crystal growth, and the temperature of the heat exchanger and the furnace body needs to be reduced at the same time during the final crystallization process. During the growth process, the heat exchanger always plays the role of controlling the temperature gradient. The growth atmosphere of the furnace body needs to be low oxygen and low carbon to prevent silicon crystals from being contaminated by both, and the solidification method is directional solidification. After the crystallization is completed, the silicon crystal is still placed in the thermal field. At this time, the furnace temperature is adjusted to slightly lower than the solidification temperature and annealed to eliminate residual stress, reduce defect density and make the silicon crystal have better uniformity.
Figure 1 Silicon crystallization process in the heat exchange method
TM is the melting point; Ti, T2, and T3 are the temperatures at different positions on the crucible wall; TDF is the temperature of directional solidification
Figure 1 is a schematic diagram of the silicon crystallization process in the heat exchange method. The average temperature of molten silicon is 5~10°C higher than the melting point of silicon. Figure 1(a) is the crucible, silicon raw material and seed crystal; increase the temperature to melt the silicon raw material It becomes liquid, as shown in Figure 1(b); part of the seed crystal is melted and crystallized from there, as shown in Figure 1(b)~(d); The crystal gradually covers the bottom of the crucible, as shown in Figure 1(e) As shown; the solid-coated interface expands to the liquid surface in a nearly ellipsoidal manner and completes the crystallization, as shown in Figure 1(f)~(h). Figure 1(c) is the most critical part of the entire crystallization process. The temperature of the molten silicon and the seed crystal must be measured carefully to ensure that the seed crystal does not melt.
The ammonia flow in the heat exchanger is related to the following factors. ①The size and shape of the furnace body; ②The size and shape of the crucible and the wall thickness of the crucible; ③The relative position of the heat exchanger and the crucible; ④The temperature of molten silicon, etc., and the appropriate ammonia flow rate must be obtained by repeated experiments.
The growth environment of the heat exchange method is close to isothermal. During the process of heating and melting, the temperature gradient of the silicon raw material does not change significantly; during the growth process, the slight temperature gradient change is controlled by the inflow heat exchange The nitrogen flow rate of the reactor, the crystallization process starts from the seed crystal at the bottom of the crucible,
The solid-liquid interface gradually moves to the crucible wall and the top of the crucible. The hot spot is located at the top of the crucible and the cold spot is located at the bottom of the crucible. The stable temperature gradient and small natural convection in the molten silicon are the characteristics of HEM. Since most of the time the silicon crystals are located below the liquid surface, the crystals can be prevented from being affected by mechanical and temperature fluctuations. The stability of the solid-liquid interface is extremely high, so neither the crystal nor the Yutong need to be rotated.
Compared with the general crystal growth method, during the heat exchange method growth process, the positions of the crucible, the crystal and the thermal field remain fixed, and there is no need for specially designed temperature gradients or different heating zones to drive the crystal growth. Its growth is mainly driven by fine-tuning the ammonia flow of the heat exchanger and the temperature of the furnace itself. Most of the heat energy generated by the crucible and crystals can be taken away by the heat exchanger.
Generally speaking, the cycle from feeding to completion of growth is about 50h. Polysilicon grown by the heat exchange method has the following characteristics: better uniformity, small grain boundaries (only up to centimeters), low oxygen pollution, vertical columnar grain boundary growth, and narrower resistance value range, etc. . At present, the solar polysilicon with the highest conversion efficiency (18.6 %, 1 cm 2 area) developed in the laboratory is grown by the heat exchange method. It can grow silicon cubes up to 200kg in length, width and height of about 60 cm each.
Figure 2 shows the heat exchange equipment and furnace structure used by Crystal System Inc. in the United States. It can grow silicon cubes weighing 200kg and each having a length, width, and height of about 60 cm. Swiss Wafer AG in Switzerland and GT Solar Technologies in the United States also use similar growth methods.
The main difference between the crucible descending method and the casting method is that the melting and crystallization of the raw materials of the former are carried out in the same crucible; the melting process of the latter is carried out in the first crucible, and the crystallization is carried out in the second crucible, as shown in Figure 1. Show. The silicon raw material is melted in the first uncoated quartz crucible, and then the molten silicon is poured into the second quartz crucible coated with silicon nitride (Si 3 N 4) film on the inner wall to crystallize the molten silicon. In addition, the crucible descending method is to pass the crucible containing molten silicon through the hot zone of the heating coil to crystallize; while the casting method is to control the temperature by adjusting the power of the heating wire. During the crystallization process, the crucible itself does not move. The liquid interface is buried under the melt, so the influence of temperature fluctuations and mechanical instability can be minimized. These two solidification methods from liquid to solid are called directional solidification (directional solidification), which is easy to cause columnar crystal growth. Therefore, chips cut from the same polysilicon crystal block will Have the same defect structure, such as grain boundaries and dislocations.
Figure 1 Polysilicon is manufactured by casting method. The silicon raw material is melted in the first quartz crucible, and then the molten silicon is poured into the second quartz crucible coated with silicon nitride (Si3N4) film on the inner wall. Compared with the crucible descending method, the casting method has shorter crystallization and cooling time.
Figure 2 is a schematic diagram of the front and cross-section of the molten silicon crystallization process in the casting method. Adjusting the power of the heating coil can change the temperature gradient of the molten silicon. The bottom gradually solidifies upwards; then the grain boundaries will grow laterally in a dendritic crystalline manner (lateral growth), forming a polysilicon layer (see Figure 2(b)). Eventually, the product grain boundaries will gradually grow from the polysilicon film to the liquid surface, forming polysilicon bulk materials, and the dendritic crystals will also become a wide range of grain boundaries, as shown in Figure 2(c).
At present, Deutsche Solar GmbH in Germany and Куocerа in Japan both use the casting method to grow polycrystalline silicon bulk materials, the mass can reach 250~400 kg, and the length, width and height are about 70 cm × 70 cm × 30 cm. German Deutsche Solar GmbH uses the casting method to grow Polycrystalline silicon bulk material, from 1997 to 2004, the mass of the bulk material has increased from 180 kg to 330 kg. Compared with the traditional method, the new type of casting method developed by Deutsche Solar GmbH can save about 30% of the growth time, and the mass of polysilicon bulk material can reach 400kg.
What are the characteristics of the crucible descending method?
The advantage of polysilicon is that it is cheap, and the bulk shape is mostly cube or cuboid, while monocrystalline silicon is mainly round or nearly square. Therefore, for general manufacturing, polycrystalline silicon has a better material usage rate than monocrystalline silicon. The disadvantage is that the conversion efficiency is slightly lower than that of monocrystalline silicon. The commonly used polycrystalline silicon bulk growth methods include: crucible descending method, casting method, heat exchange method and edge-limited flake crystal growth method. The following mainly introduces the characteristics of the crucible descending method.
The crucible descending method is also called the Bridgman-Stockbarger method. The characteristic of this method is to let the melt cool and solidify in the crucible. The solidification process starts from one end of the crucible and gradually expands to the entire melt. The crucible can be placed vertically or horizontally. The solidification process is completed through the solid-liquid interface. The interface can be moved by moving the crucible or moving the heating coil. The crucible descending method can be used to grow optical crystals (such as LiF, MgF2, CaF2, etc.), scintillation crystals (such as NaI(TI), Bi4Ge3O12, BaF2, etc.), laser crystals (such as Ni 2- :MgF2 , V2+: MgF2 etc.).
Since 2004, companies have begun to use the crucible descending method to grow polycrystalline silicon bulk materials, with a growth rate of up to 10 kg/h. The growth method is as follows: the silicon raw material is placed in a quartz crucible, and the inner wall of the crucible is coated with a layer of nitride For the silicon (Si3N4) film, the melting and crystallization of the raw materials are carried out in the crucible. The main purpose of plating silicon nitride film is to prevent polysilicon from sticking to the quartz crucible during the crystallization process, causing the crucible or silicon crystal to crack. Increase the temperature to melt the silicon raw material, and gradually move the crucible down so that the bottom of the crucible passes through the area of higher temperature gradient. The whole crystallization process will begin to crystallize from the bottom of the crucible and gradually extend upward. The solid-liquid interface will gradually move upward as the crucible position drops to complete crystallization. Graphite resistance heating is generally used, and the insulation system is graphite tube and molybdenum tube.
Figure 1.1 The crucible descending method, the melting and crystallization process simultaneously exist in a quartz crucible coated with silicon nitride (Si3N4) film on the inner wall. The crystallization process is to slowly move the molten silicon and crucible below the heating coil to leave the coil. The crystallization process is The report is complete.
Figure 1.2 The relationship between the temperature of the crucible descending method and the crucible moving position, the crystal crystallization process occurs in the region of the temperature gradient in the figure
The silicon crystal grown by the crucible descending method needs to have an appropriate thermal field, including the resistance heater, insulation material, the size of the crucible and the relative position of the heater, adjust these factors to make the temperature gradient of the melting zone smaller, and the crystallization zone The temperature gradient becomes larger. The shape and position of the solid-liquid interface of the crucible descending method are closely related to the integrity and defects of the crystal. Generally speaking, the solid-liquid interface can be divided into three shapes: convex, flat, and concave. From the perspective of reducing dislocations and other defects and avoiding internal stress, the flat interface is the most ideal situation, but in the actual crystal growth process Middle; The concave interface will cause crystals to grow from the edge of the crucible to the center, easy to form polycrystalline, and impurities and bubbles will form inclusions. Therefore, a convex solid-liquid interface is usually maintained, but the convex interface is likely to cause uneven radial temperature distribution and generate internal stress. During the crystal growth process, as the crucible gradually drops, the part in the high temperature zone decreases, and the part in the low temperature zone gradually increases, which will cause the solid-liquid interface to move to the high temperature zone, and the temperature gradient of the interface will become smaller. At this time, it is easy to appear that the crystallization speed is greater than the crucible falling speed, causing bubbles or inclusions inside the crystal. Generally, it can be solved by increasing the control temperature or reducing the crucible falling speed.
Overall, the crucible descending method has these characteristics.
Advantages of the crucible descending method :
(1) The shape of the crystal can be controlled by the shape of the crucible.
(2) The growth direction of the crystal can be determined by the seed crystal, if there is no seed crystal. The crystal will grow along the optimal direction (preference).
(3) Because the growth environment is closed or semi-closed, the volatilization of melt and dopants can be avoided.
(4) The molten silicon starts to crystallize from the crucible wall, which can prevent the molten silicon from being further contaminated by the quartz crucible.
(5) The operation is simple, and large-size crystals can be grown. A single growth furnace can grow several crystals at the same time, which is suitable for industrial mass production.
Disadvantages of the crucible descending method:
(l) The process of crystal growth takes place inside the crucible, so it cannot be observed.
(2) The high growth rate easily makes the temperature gradient of the crystal too large, causing the crystal to break.
(3) The thermal stress of silicon crystal rods is relatively large, which leads to high dislocation density and uneven grain boundary distribution.
(4) The inner wall of the crucible must be specially coated to prevent the crystal from sticking to the crucible, causing the crucible or crystal to break.
(5) During the crystallization process, internal stress is easily introduced into the crystal from the crucible, so the thermal expansion coefficient of the crucible material should be smaller than that of the crystal, and the inner wall of the crucible must be very smooth to prevent stress.
(6) During the growth process, the crystal does not rotate, so the uniformity of the crystal, especially the doped silicon crystal, is worse than that of the Czochralski method.
Silicon is currently the most widely used solar cell material, including single crystal silicon (sc-Si) and polycrystal silicon (polycrystal silicon), with a total solar market share of more than 95%. In the early days, solar cells mainly used Czochralski pulling technique (CZochralski pulling technique, CZ) to grow silicon crystals, but due to market price factors, more and more companies invested in the growth of large-scale polysiliconingot (polysiliconingot).
1.1 Growth of single crystal silicon
The growth methods of monocrystalline silicon mainly include the Czochralski method and the floating zone technique (FZ). In 1950, Teal and Little first applied the Czochralski method to the growth of silicon (Si) and germanium (Ge) single crystals. At present, about 80% of silicon single crystals are grown by the Czochralski method. The single crystal size can reach 12in. In the Czochralski method, because the molten silicon is in direct contact with the crucible and produces a chemical reaction, the silicon single crystal has serious oxygen and carbon pollution problems. Keck and Golay proposed the floating zone method in 1953 to grow silicon single crystals without oxygen and low metal pollution, which are mainly used in high-power transistor devices. However, due to the high cost of the floating zone method, the Czochralski method is still the main method for growing solar-grade silicon single crystals.
1.1.1 Chai-style lifting method
The Czochralski pulling technique was accidentally invented by Professor Jan Czochralski in 1916, originally to study the crystallization rate of metals such as tin, zinc and lead in solid-liquid contact. After the Czochralski method was invented, it was gradually forgotten. Until the end of the Second World War, the semiconductor industry was booming, which made the importance of semiconductor materials such as silicon and germanium increase. In 1950, Teal and Little of Bell Laboratories first applied the Czochralski method to grow silicon and germanium crystals, and obtained high-quality single crystals. In 1958, Dash proposed the use of necking technique to grow silicon single crystals with low dislocation density. At present, the size of silicon ingots has increased from 1in in the 1950s to 12in today. Dr. Lin Mingxian from Taiwan Sino-German Electronics Co., Ltd. once edited the book “Silicon Wafer Semiconductor Material Technology”, which is about the technology of silicon single crystal growth. Including the Czochralski method and the floating zone method, the growth defects of silicon crystals and the processing technology are all introduced in detail. It is a good reference book for those who are engaged in semiconductor devices and solar cell materials. In addition, the book “Crystal Growth Science and Technology” edited by Zhang Kecong and Zhang Leping has a very detailed explanation of the theory of melt growth and the thermodynamics involved.
Figure 1.1 Czochralski pulling technique
Figure 1.1 is a schematic diagram of the Czochralski method. The growth process is briefly described as follows: First, the polysilicon raw material is placed in a quartz crucible, and the crucible is placed in a graphite thermal insulation field; vacuum is drawn from the crystal growth furnace and a certain pressure range is maintained Use graphite resistance heater to melt silicon raw material into liquid (melting point is 1420℃), adjust the temperature so that the center of molten silicon becomes the cooling point in the whole thermal field. When the temperature of the molten silicon stabilizes, the positioned (100) or (111) direction seed crystal (seed) is gradually lowered until it contacts the surface of the molten silicon. Due to the seed crystal itself and the thermal stress when the seed crystal contacts the molten silicon Dislocation (dislocation), at this time, the temperature must be slightly increased to melt part of the seed crystal. At the same time, the seed crystal is rotated on one side and pulled up quickly at the same time, using the crystal neck technology (Dashing technique or necking technique), to pull out the seed crystal with a smaller diameter (3 ~ 6mm) than the original seed crystal and low defect density. As long as the crystal neck is long enough, dislocations can be smoothly discharged from the crystal surface. After the crystal neck process is over, the pulling speed and temperature need to be reduced to gradually increase the crystal diameter to the required diameter. This step is called shoulder growth or crown growth. After the shoulders are placed, the cylindrical body of equal diameter is grown by adjusting the pulling speed and temperature. The most important work in this part is the diameter control. Finally, when the crystal grows to an appropriate length, the temperature must be increased or the pulling speed must be increased to gradually reduce the diameter of the crystal rod until it is completely separated from the liquid surface. Then the ingot is cooled for a period of time and then taken out to complete a life cycle.
The preparation equipment of the Czochralski method can be roughly divided into four parts. (1) Furnace body. The water-cooled stainless steel furnace body can generally be divided into an upper chamber and a lower chamber.The upper furnace chamber is where the silicon crystal rods are cooled, and the lower furnace chamber is the place where crystals grow.
(2) Hot field. Including quartz crucible, graphite crucible (supporting quartz crucible), graphite resistance heater and thermal insulation materials. The problem with the quartz crucible is that it will react with molten silicon at high temperatures, causing oxygen contamination of the silicon crystal rod; the graphite crucible is used to fix the quartz crucible to prevent its softening and deformation. The thermal field, also known as the thermal gradient, can generally be divided into external thermal gradient and internal thermal gradient, the after-heater in the furnace or the radiation shield ) Belongs to the external temperature gradient, while the temperature distribution in the crystal and molten soup belongs to the internal temperature gradient. Because the heating method is resistance heating, heat energy is provided to the quartz crucible and the surrounding insulation materials at the same time, the heating effect on the crucible is more uniform, and it is easy to produce a small temperature gradient, and the position of the crucible has little effect on the temperature gradient.
(3) Ar atmosphere and pressure control system. Because the quartz crucible reacts with molten silicon to produce silicon monoxide (SiO), the reaction of SiO and graphite devices will produce carbon monoxide (CO), and Ar is to take away both SiO and CO gases. (4) Growth control system. The control parameters can include the diameter of the crystal rod, the pulling speed, the rotation speed and the temperature. Generally, the change in the shape of the crystal aperture (meniscus) is used to adjust the temperature or the pulling speed to control the diameter of the crystal rod. The crucible or silicon crystal rotates at the same time, and its purpose is to cause forced convection in the molten silicon, make the dopant concentration uniform, and make the temperature distribution in the furnace more symmetrical. Generally speaking, when making a crystal growth furnace, the furnace body itself will always have slight asymmetries. These asymmetries will cause excessive facet growth, striation, and difficult crystal growth. Controlling equal diameter growth and other shortcomings, rotating crystals and crucibles can effectively reduce these effects.
It is necessary to rely on the matching of the above four parts to grow silicon single crystals with low defect density.
In addition to defects such as dislocation, vacancy and stacking fault in silicon crystals grown by the Czochralski method, the most important defects are non-metallic impurities such as oxygen and carbon. Pollution. The quartz crucible reacts with molten silicon to form SiO2, which will affect the resistance value, conversion efficiency and carrier lifetime of silicon. But when the oxygen concentration reaches a certain level, it will enhance the mechanical strength of the silicon crystal. Because the Si-O bond in SiO2 vibrates and has a strong absorption at the infrared wavelength of 900nm, the oxygen content in the silicon crystal can be measured with an infrared spectrometer. The carbon in the silicon crystal is reacted by SiO and graphite heat insulation material to generate CO and merge into the molten silicon. The carbon content accelerates the deposition of oxygen, which in turn causes microscopic defects. In order to reduce carbon pollution, the gap between the quartz crucible and the graphite can be minimized to make the CO concentration at the contact point higher and prevent the quartz crucible and graphite from continuing to react. In addition, the flow rate of Ar gas can be adjusted to take away the generated CO gas.
The advantage of the Czochralski method is that it is easier to grow large-size and high-doped silicon single crystals, but for the application of solar cells, the most important factor is the price. The price of a single chip can be reduced to the minimum by increasing the crystal size, continuous feeding, and improving the cutting, grinding, and polishing process to minimize the loss of silicon raw materials, so that it is possible to reduce the price of silicon chips.
1.1.2 Floating zone method The floating zone method was proposed by Keck and Golay in 1953, and Theuerer applied it to the growth of high-purity silicon single crystals. The biggest advantage of the FZ method is that no crucible is needed. In the CZ pulling method, molten silicon inevitably comes into contact with the crucible and reacts. Therefore, only a few substances can be used as crucible materials, such as quartz (SiO2), Si3N4, silicon carbide (SiC), graphite (graphite), and so on. Even so, the carbon, oxygen and other metal impurities in the crucible will still flow into the molten silicon, causing pollution to the silicon single crystal.
Figure 1.2 Floating zone method
The growth device of the floating zone method is shown in Figure 1.2. The polycrystalline silicon raw material rod is fixed above the high frequency coil (RFcoils), and the silicon single crystal seed crystal is placed under the polycrystalline silicon raw material rod. When the polysilicon raw material rod is heated by the coil, the bottom will begin to melt. At this time, the raw material rod is lowered, so that the molten area is attached to the seed crystal, forming a solid-liquid phase equilibrium, and the molten area is supported by surface tension. Then, the seed crystal and the raw material rod are rotated in opposite directions to make the distribution of impurities in the molten zone uniform. The melting zone moves from top to bottom, or from bottom to top, so that the polysilicon raw material rod can completely pass through the heating coil to complete the crystallization process. In the floating zone method, the stability of the melt zone is maintained by the balance of surface tension and gravity. The advantage of the floating zone method is that it can grow high-purity and defect-free silicon single crystals. The content of oxygen, carbon and other transition metals can be less than 1011cm-3, and its resistance can easily reach 300Ω·m, which is suitable for high-efficiency solar materials. Although the defect density of the silicon crystal grown by the floating zone method is low, its lifetime is only 0.5ms, which is still far below the theoretical value. The main reason is that the high-purity silicon crystal has many microscopic defects caused by the growth and cooling process. Another disadvantage of the floating zone method is that because the melting zone is only at the top of the crystal rod, only crystals with a smaller diameter can be grown.
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.
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)].
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.
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”.
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.
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.
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)).
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.
Figure 1.2 (a) Hydrogen molecular orbital; (b) Energy band formation; (c) Semiconductor electric band and valence band formation
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.
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.
Figure 1.5 Schematic diagram of electronic energy states of amorphous semiconductor 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.
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).
Figure 1.2 Periodic Table of Elements
(a) Diamond lattice: elemental semiconductor (such as silicon, germanium, carbon)
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.
Figure 1.4 Miller index determination method and Miller index of important plane in cubic crystal
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.
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
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
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
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.
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.