Using high-efficiency single-crystal semiconductor solar cells on the surface to generate electricity, whether it is a single-crystal Si solar cell or a GaAs solar cell, the cost of the battery is too expensive. An alternative is to use a concentrating solar cell power generation system, using cheap concentrating modules to reduce the usage of single crystal semiconductor solar cells to one percent, or even one thousandth.
Table 1[Z] shows the structural parameters of concentrating GaAs solar cells, including P on-N type and N-on-P type. Compared with non-concentrating GaAs solar cells, the structure of concentrating GaAs solar cells has an extra layer under the base layer to reflect secondary carriers back to improve cell efficiency. The concentrating design can not only reduce the amount of solar cells, but also improve the conversion efficiency of solar cells, which is mainly due to the increase of the open circuit voltage Voe and the increase of the fill factor. At present, the highest conversion efficiency of GaAs solar cells is 27.6% (AM 1.5, 255 suns) under concentrated light conditions.
In concentrated solar cell power generation system. Since a single solar cell produces currents of several amperes or more, series resistance has a very large effect on efficiency. Too high series resistance will reduce the fill factor and severely reduce the cell efficiency. The main sources of series resistance include the resistance Re of the grid electrode line, the contact resistance Rs between the metal and the semiconductor (Fig. 1), and the lateral sheet resistance of the current passing through the emitter layer. There are several ways to reduce the series resistance of the battery itself. click here to open to learn more.
(1) Increase the density of grid electrodes. Increasing the density of the grid electrode can simultaneously reduce the grid electrode line resistance, contact resistance and lateral sheet resistance of the entire solar cell, but increasing the grid electrode density often increases the shading area, thus reducing the efficiency of the cell.
(2) Increase the conductivity of the emitter layer. Increasing the conductivity of the emitter layer can reduce the lateral sheet resistance R day. Since N-type GaAs has better conductivity than P-type GaAs, the N on P structure is a better choice.
(3) Reduce contact resistance For concentrating GaAs solar cells, the contact resistance Re must usually be small. A thin layer of alloy, such as Au/Zn/Au or Au/Ge/Ni/Au, is added between the gate electrode and the semiconductor, and with suitable thermal fusion conditions, the contact resistance can be effectively reduced to the required level scope.
Figure 2 shows the electrode design of a concentrator solar cell. The radial grid electrodes guide the current from the central area of the solar cell to the periphery, although the grid electrode density is actually much higher than shown in the figure. In order to reduce the series resistance and reduce the shading area, the shading ratio of the grid electrode will be controlled at 4%~8% as much as possible.
Figure 3 shows the efficiency comparison of Pon-N type and N-on-P type concentrating GaAs solar cells under different concentrating conditions. The dense grid electrode design is used to reduce the shading area and achieve higher efficiency. However, when the concentrated light intensity exceeds 400 times, the efficiency of N-on-P type GaAs solar cells designed with low m-density grid electrodes begins to saturate, while the efficiency of N-on-P type GaAs solar cells designed with high-density grid electrodes begins to saturate. The efficiency of on-N type GaAs solar cells can continue to increase as the concentration of light increases by a factor of 1000.
This result shows that the design of the grid-like concentrating GaAs solar electric electrode is the key point of the concentrating solar cell, according to the design of the grid-like electrode of the cell. The actual grid concentration magnification optimizes the design of the grid electrode, effectively reducing the density of the grid electrode, which is much higher than the series resistance shown in the figure, and at the same time reducing the shading area as much as possible to increase the photocurrent, in order to improve the efficiency of the battery.
The range of III-V compound semiconductor materials is quite wide, and the properties of various materials, epitaxy technology and device use can become a big topic. This time, I will give a brief introduction to III-V semiconductors, especially the materials and epitaxy technology related to solar cells.
Introduction to III-V semiconductor materials
In simple terms, group III-V compound materials are compounds formed by group III elements and group V elements on the periodic table. The III-V compounds formed by different elements have very different properties, ranging from semi-metals to semiconductors and insulators. In the common field V group semiconductor materials, the H group elements include aluminum CAD, gallium (Ga), indium Cln), etc., while the V group elements include nitrogen (N), phosphorus (P), arsenic (As), bromine (Sb) ) Wait. Binary materials formed by these III-V group elements (referring to a semiconductor formed by a group H element and a group V element, such as GaAs, InP, etc.), with the different elements, the crystal lattice size is related to the semiconductor. The energy band gap is also different. → Generally speaking, the smaller the lattice constant, the larger the energy band gap of the semiconductor. The ternary or quaternary materials formed by mixing various III-V elements can achieve the required lattice size or energy band gap size by adjusting the element composition ratio in the material. III-V compound semiconductors are widely used in the manufacture of various optoelectronic materials, including light-emitting diodes, laser diodes, photodetectors, solar cells, and high-frequency electronic devices.
Figure 1 shows the lattice constant (symbol a, unit A) and energy band gap (symbol Eg, unit eV) of different III-V compound semiconductors. In the figure, binary materials, such as GaAs, lnP, GaSb, etc., are represented by open circle symbols, while lattice constants and GaAs are represented by solid circles. The line between two binary materials shows the change of lattice constant and energy band gap when the composition is changed; such as the line between GaAs and InAs, it indicates that the ternary materials of Ga, ln1 and As are in different Ga and In Lattice constant versus bandgap at scale.
The lattice constant of ternary materials has a linear relationship with the material composition, which is called Vega’s law. This simple law allows us to analyze the composition of ternary materials using high-resolution X-ray diffraction methods. It is generally believed that quaternary materials also obey Vegard’s law.
III-V semiconductor materials related to solar cells
When referring to the semiconductor materials related to solar cells, the most important ones should be GaAs series materials, and the so-called GaAs series actually refers to materials whose lattice constants are consistent with GaAs, including Al, Ga1, As materials (X= O~1) and Ge etc. In addition, in the development of III-V semiconductor solar cells, solar cells such as InP solar cells and GaSb have also appeared.
GaAs series (1) GaAs:GaAs is a material with a direct energy band gap. At room temperature, the lattice constant of GaAs is 5.6532, and the energy band gap is 1.424 eV. The energy band gap is theoretically suitable for making single-junction solar energy. Battery. The application of GaAs wafers is also quite common and is often used to manufacture various optoelectronic devices.
(2) AIAs is an indirect energy band gap material with an energy band gap of 2.14 eV and a lattice constant of 5.660, which is almost identical to GaAs. Since the lattice constant of AlAs is very consistent with that of GaAs, the density of defects such as stacking faults and dislocations grown on GaAs is very low. Table 1 lists the compositional change versus energy bandgap of many ternary III-V semiconductor materials at 300 K. The transition from direct bandgap to indirect bandgap occurs when the Al ratio is about 45%. In single-junction GaAs solar cells, it is mainly used as a window layer to reduce the influence of GaAs surface defects, and the Al content is 80%~90%. In addition, in the application of double junction solar cells, it can also be used to make upper stratum solar cells.
Introduction of epitaxy methods for III-v semiconductor materials
The device structure of a solar cell is mainly a PN junction. At the beginning, the PN junction fabrication method of III-V semiconductor solar cells with industrial materials such as GaAs, InP, etc., mostly adopted the impurity diffusion method. The main mode of structure of compound semiconductor solar cells. The epitaxy methods of III-v semiconductors can be mainly divided into blue types: liquid phase epitaxy (LPE), molecular beam epitaxy (MEE), and organic metal vapor phase epitaxy (OMVPE). At present, high-efficiency III-V solar cells, whether single-junction GaAs solar cells or stacked multi-junction solar cells, all use metal-organic vapor phase epitaxy as the epitaxy method for fabricating solar cell device structures.
3.1 Liquid phase epitaxy
Liquid-phase epitaxy (LPE) was often used in early research on III-V or II-VI semiconductors. The liquid phase epitaxy method melts the elements as components of the epitaxial layer into a liquid state at high temperature, controls the temperature to keep the liquid in a saturated or supersaturated state, and then immerses the epitaxial substrate in the solution or tries to make the surface of the substrate contact the liquid to make the supersaturated liquid Ficus liquid grows semiconductor crystals on the substrate. Figure 2 is a schematic diagram of three methods commonly used in liquid phase epitaxy, where (a) is the dipping method, (b) is the tipping method, and (c) is the sliding boat method.
The advantages of phase epitaxy are its simplicity and the ability to grow high-quality materials with low impurities and low point defect concentrations. Another advantage is that liquid phase epitaxy has the function of removing oxygen when growing materials containing aluminum components (such as AlGaAs), because aluminum will combine with oxygen to form stable, floating on the surface of the liquid. This makes the quality of AlGaAs grown by liquid phase epitaxy far superior to other epitaxy techniques in the early days.
In 1972, IBM’s Hovel and other researchers used liquid phase epitaxy to grow PN GaAs solar cells. The surface of this cell used a P-type doped AlGaAs window layer to reduce the influence of GaAs surface defects. In 1979, researchers such as Yoshida further achieved a conversion efficiency of 19% under AMO conditions.
However, the liquid phase epitaxy method also has many disadvantages: the liquid phase epitaxy method cannot grow too many complex epitaxial structures, the interface between the epitaxial layers cannot be very steep, and the thickness of the epitaxial layer cannot be controlled very thinly and accurately, so many compound semiconductors The epitaxial growth of the device structure is currently carried out by molecular beam epitaxy and organic metal vapor phase epitaxy.
3.2 Molecular beam epitaxy
Basically, molecular beam epitaxy (MBE) is a method of vacuum evaporation of solid-state thin films. The molecular beam epitaxy method heats the epitaxial substrate in an ultra-high vacuum environment, and then shoots the molecular beam or atomic beam of the element onto the surface of the epitaxial substrate to grow the epitaxial layer. Since the epitaxy process is a physical reaction without complex chemical reactions, the growth kinetics of the epitaxy becomes relatively simple and easy to study, and the composition control of the epitaxial film is relatively simple. Molecular beam epitaxy uses elements as raw materials for epitaxy. The elemental material is placed in a special “collapse” called effusion cells, and the emission amount of the molecular beam is adjusted by controlling the temperature of the effusion cells, and the baffle can control whether the molecular beam is emitted to the surface of the epitaxial substrate n Figure 3 is old A diagram of the vacuum chamber setup of a molecular beam epitaxy system.
Since the reaction chamber of the molecular beam epitaxy system is an ultra-high vacuum system, it is convenient to install various analysis equipment on the reaction chamber for various real-time analysis, such as Auger Electron Spectroscopy (AES) and reflective high-energy electric hand. Diffraction Equipment (RHEED) etc. AES is mainly used to monitor the surface state of the epitaxial substrate and confirm the cleanliness of the substrate surface. RHEED provides the reconstructed state, microstructure and surface flatness of the epitaxy process surface, which is helpful for the analysis of the growth mechanism, and can also provide information such as epitaxy rate and thickness.
Molecular beam epitaxy can provide atomic-level thickness control and a very steep epitaxial interface, which is very suitable for making special epitaxial structures such as quantum wells or superlattice. However, molecular beam epitaxy also has many disadvantages, including not being suitable for growing mixed-side and phosphorus (As/concave) compound semiconductors, not easy to grow antimonide semiconductors, and not easy to grow nitride (Nitride) semiconductors, as well as low productivity, Equipment is expensive etc.
Gas source molecular beam epitaxy (gas source MBE) and metal-organic molecular beam epitaxy (metal-organic MBE) are improved versions of molecular beam epitaxy. They manage to use gaseous materials such as P-phosphate, NH, or N2, etc., as well as organometallic materials in molecular beam epitaxy systems. These improvements expand the variety of compound semiconductors that can be grown by molecular beam epitaxy.
Based on the consideration of mass production, in the past, except for a few academic research papers, almost no field V solar cells were epitaxy using molecular beam epitaxy. However, in the future, some new research findings may change this situation. For example, the quantum dot structure solar cells that are currently being studied by the academic community, or the solar cells of ln N materials, etc. At present, molecular beam epitaxy is more stable than organometallic vapor phase epitaxy for the control of lnAs and InGaAs quantum dots.
3.3. Organometallic vapor phase epitaxy
Organometallic vapor phase epitaxy (MVPE) is also known as organometallic chemical vapor deposition method. The English abbreviation of this epitaxy method may appear in various writing methods such as OMVPE, MOVPE, OMCVD, MOCVD, etc., but they all refer to the same epitaxy method. Organometallic vapor phase epitaxy is currently the most commonly used epitaxy method for making III-V compound semiconductor devices, including various light-emitting diodes, light-emitting diodes, photodetectors, solar cells, and microwave devices such as HBT and HEMT.
Organometallic vapor phase epitaxy uses organometallic materials and hydrides as raw materials for epitaxy. Almost most of the III-V group elements and the II-V-VI group elements used for doping have corresponding organometallic raw materials or hydride raw materials, so the range of materials that can be grown by organometallic vapor phase epitaxy is very wide. Whether it is nitride, compound, phosphide, Jin compound, or various ternary or multi-component compound semiconductor materials. All can be epitaxially grown by metal-organic vapor phase epitaxy.
Figure 4 is a system diagram of an organic metal vapor phase epitaxy system. Generally, an organic metal vapor phase epitaxy system can be divided into eight subsystems, and the eight parts will be described below. (1) carrier gas module (carrier gas module). Organometallic vapor phase epitaxy requires high-purity gases to push the reactants into the reaction chamber, and also requires these gases to establish a stable gas flow field in the reaction chamber. The most commonly used carrier gas is hydrogen (H2), which is generally purified using a palladium cell. Nitrogen (N2), ammonia (He) or oxygen (Ar) are sometimes used as the carrier gas, and adsorption purifiers are mainly used to purify the carrier gas.
(2) hydride gas module (hydride gasmodule). Hydride is mainly used as group V raw material and part of N-type doping raw material. The main hydrides include As, PH, and N. These hydrides, especially AsH3 and PH3, are highly toxic and dangerous gases. It needs to be used and handled with caution. Commonly used N-type doping raw materials include SiH, , Si2H6 (providing Si impurity of group N), H2Se (providing Se impurity of group VI), and DETe belonging to organometal (providing Te impurity of group VI), etc. These N-type dopant materials are diluted with hydrogen or nitrogen to reduce the density to the range of 100-1000 ppm for ease of use.
(3) Organometallic raw material module (metal-organic module). Organometallic raw materials mainly provide D group raw materials, as well as some P-type doping raw materials. Commonly used Hui family organometallic raw materials include Zn, etc., while P-type doping raw materials include DMZn, DEZn (providing Zn of H group), Cp2Mg (providing Mg of H group), etc. Most of the organometallic raw materials are liquid at room temperature, and a few are homomorphic. Organometallic raw materials are generally packed in metal cylinders with high cleanliness. When using, the cylinder is immersed in a constant temperature water tank to keep the temperature of the organic metal raw material stable. Thereby fixing the vapor pressure of the organometallic raw material. Introduce high-purity hydrogen or nitrogen as the carrier gas, and take the vapor of the organic metal raw material out of the cylinder and send it into the reaction chamber. When the carrier gas is passed in, bubbles will be generated in the liquid organic metal, so the cylinder of the organic metal raw material is also called bubbler. To stabilize the supply of organometallic raw materials, the temperature of the cylinder must be strictly maintained and the pressure of the cylinder must be controlled. The flow of the organometallic feedstock is then controlled by controlling the flow of the carrier gas.
(4) Dilute module (dilute module). Sometimes very low amounts of reactants are required for epidemiology, and the concentration of the reactants can be reduced further using the dilution module. The dilution module adds the carrier gas to the reactant raw material, and then takes out the required amount and then flows into the reaction chamber. For example, when 0.1sccm of SiH4 is required in the experiment, 10sccm of SiH4 can be mixed with 990sccm of hydrogen. Then the mixed gas is taken 10 sccm into the reaction chamber, and the equivalent flow rate of 0.1 sccm can be achieved.
(5) switching module (switching manifold). In order for the epitaxy system to grow multiple quantum wells or superlattice structures, the reactants must be rapidly switched during the epitaxy process, and during the switching process, the flow rate of the reactants, the flow rate of the carrier gas in the reaction chamber and the A stable balance of pressure in the reaction chamber is required to maintain the steepness of the heteroepitaxial interface. The key point of the design of the switching module is to minimize the dead space on the valve pipeline, so as to avoid the continuous release of a small amount of residual reactant into the reaction chamber after the reactant is closed, and pay attention to make appropriate compensation for changes in flow and pressure during switching.
(6) the design of the reaction chamber (reactor design). The design of the reaction chamber is the core part of the metal-organic vapor phase epitaxy, in order to achieve the uniformity of the epitaxy and improve the utilization efficiency of the reactants. The design of the reaction chamber must take into account the gas flow field, the temperature gradient distribution in the reaction chamber, and the concentration distribution of reactants. Now, due to the improved understanding of the CVD reaction mechanism and the help of numerical simulations using computers, the new organometallic epitaxy reaction chamber has been achieved. Quite a large production volume.
Figure 5 is a typical example of several reaction chamber designs. ① Horizontal reaction chamber. This is one of the commonly used reaction chamber forms in early metal-organic vapor phase epitaxy systems. It is still commonly used in small experiments and research and development. ② Vertical reaction chamber. This is also one of the early commonly used reaction chamber forms, and is still commonly used in small experiments and research and development. ③ Barrel-shaped reaction chamber. The design of this reaction chamber can be said to be a multi-chip reaction chamber design in which the horizontal reaction chamber shown in Figure 5 is rolled up. Compared with the horizontal reaction chamber, the uniformity becomes better because the boundary on both sides disappears, and the susceptor can be managed to rotate. In the past, it was mainly used to grow GaAs, AIGaAs materials.
④ There is a multi-wafer planetary reaction chamber (multi-wafer planetary reaction or) at the current inlet. This epitaxial reaction chamber design is one of the reaction chambers currently used for mass production of III-V semiconductor devices. Through the revolution and rotation of low speed. It can effectively control the epitaxy uniformity of multiple chips.
⑤ High speed rotating-disk reactor. This is also one of the reaction chambers currently mainly used for mass production of III-V semiconductor devices. Using the plug flow mode (plug flow mode) generated by adjusting the rotation speed of the epitaxial base, and adjusting the distribution of reactants at the air inlet, good uniformity control of multi-chip epitaxy is achieved. (7) the exhaust system. The exhaust system consists of a set of air extraction pumps and throttling. to control the pressure in the reaction chamber. At the same time, this part also includes some filtering devices to filter out some products of the epitaxy process as much as possible, so as not to block the pump and the exhaust pipeline at the rear end.
(8) Exhaust gas treatment system (toxic gas scrubbing system). Since the organometallic vapor phase epitaxy system uses toxic gases and produces many toxic compounds at the same time, it is necessary to have a good waste gas treatment system to treat the exhaust gas. At present, waste gas treatment methods are roughly divided into three categories: ① Adsorption type, which uses activated carbon or resin products to adsorb toxic substances; ② Combustion type. Add an appropriate amount of oxygen to burn toxic substances or decompose them at high temperature; ③ wet treatment type. Use chemicals to react toxic substances into other compounds. Many exhaust gas treatment systems may combine two or three of these for best results.
At present, in the epitaxy technology of III-V semiconductors, including III-V semiconductor solar cells, metal organic vapor phase epitaxy has the most technical and mass production advantages. Of course, the disadvantages of expensive equipment and raw materials, and the use of toxic gases in the epitaxy process must also be considered in a detailed evaluation.
To use solar cells in space. Basically, there are three important points to consider. (1) High energy conversion efficiency The solar spectrum (AMO) on the satellite or in space is different from the surface < AM I. 5), and the difference is that the AMO is mainly short-wavelength light. The efficiency of solar cells will be poor for AMO, and can only reach 0.85~0.9 of AM 1.5. In the past, silicon solar cells and GaAs solar cells were mainly used on satellites. Generally, the efficiency of silicon solar cells for satellites is 12.7%~14.8%, and the high-efficiency silicon solar cells can reach 16.6%. The efficiency of single-junction GaAs solar cells is 19%, the efficiency of double-junction III-V solar cells is 22%, and the efficiency of triple-junction → V group solar cells is up to 26.8%.
( 2 ) Good radiation resistance There are radiations of various energies in space, ranging from 50keV to 50 MeV. Space radiation can cause defects inside solar cells and reduce conversion efficiency. Therefore, if the factor of energy conversion efficiency attenuation is not considered into the system design. After the solar cell runs for a period of time. There will be a problem of insufficient power supply. Among single-junction solar cells, InP solar cells have the best radiation resistance, GaAs solar cells are the next, and silicon solar cells are the worst.
(3) Lightweight The cost of launching a satellite or spacecraft is about $10,000 per stand, and in order to reduce the launch cost, the weight of the solar cell must be considered, or the power/weight ratio CW/kg). Silicon solar cells have a good advantage in this regard, and their weight per unit area is 0.13~0.50 kg/m2. The GaAs substrate is too heavy and has weak mechanical strength, so Ge is used as the substrate. 8 ~ 1. Okg/m2 0 For GaAs solar cells with Ge substrate or double junction and blue junction solar cells, the weight per unit area is still 0.8 ~ 1.0kg/m2 0 Due to its high energy conversion efficiency and good radiation resistance, III-V solar cells have gradually replaced silicon solar cells and are used in satellites and spacecraft.
In recent years, GaAs solar cells have replaced silicon solar cells on newly launched satellites. GalnP/GaAs/Ge solar cells are very similar to GaAs solar cells in the assembly and integration of solar cell modules, with higher energy conversion efficiency and as good radiation resistance as GaAs solar cells, while adding the advantages of high voltage and low current, Therefore, it is expected to become the next generation of space solar cells.
Surface power generation
At present, solar power generation systems are widely used on the surface, from the power supply of small consumer products to large power plants. As mentioned in the preface, because the cost of Tian V solar cells is too expensive, Tian V solar cells are rarely used to make flat-panel solar cells except for special purposes such as satellites. According to the calculation, if we want to achieve an acceptable power generation cost, the concentrating conditions must be at least 400 times (400suns) above.
Using GalnP/GaAs/Ge high-efficiency solar cells, under high concentration conditions (such as 1000 suns), the power generation cost has the opportunity to drop to 0.07 US dollars / (kW h). At present, the annual production capacity of solar cells for space is about 0.5 MW. If these production capacities are converted into 1000 × concentrated solar cells, the annual power generation can reach about 0.5 GW. The current highest efficiency record for GalnP/GaAs/Ge solar cells is 34 % (AM 1.5 G, 210 suns), while the conversion efficiency of outdoor modules is 25 % 29 % at low concentration magnification. These data show that III-V solar The potential for batteries to be used for surface power generation is very high. Of course, before the actual large-scale investment in power generation systems, stable and reliable products are still required.