What are GaAs Solar Cells for Concentrator Modules?

What are GaAs Solar Cells for Concentrator Modules?

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.

Table 1 - Structural Parameters of Concentrating GaAs Solar Cells
Table 1 – Structural Parameters of Concentrating GaAs Solar Cells

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[2]), 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.

Figure 1 - Tandem cell analysis of concentrating GaAs solar cells
Figure 1 – Tandem cell analysis of concentrating GaAs solar cells

(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.

Figure 3 - Efficiency comparison of Pon-N type and N-on-P type concentrating GaAs solar cells under different concentrating conditions
Figure 3 – Efficiency comparison of Pon-N type and N-on-P type concentrating GaAs solar cells under different concentrating conditions

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.

Read more: CdTe thin film process and the development trend of CdTe thin film solar cells

III-V semiconductor materials and epitaxy technology related to solar cells

III-V semiconductor materials and epitaxy technology related to solar cells

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.

  1. 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.

Figure 1 Relationship between the quality constant and the energy band gap of common semiconductors
Figure 1 Relationship between the quality constant and the energy band gap of common semiconductors

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.

  1. 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.

  1. 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.

Table 1-Influence of the composition ratio of III-V ternary compound semiconductors on the energy bandgap of semiconductors at 300 K
Table 1-Influence of the composition ratio of III-V ternary compound semiconductors on the energy bandgap of semiconductors at 300 K
  1. 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.

Figure 2 - Schematic diagram of three methods commonly used in liquid phase epitaxy
Figure 2 – Schematic diagram of three methods commonly used in liquid phase epitaxy

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.

Figure 3 - The vacuum chamber device diagram of the molecular beam epitaxy system
Figure 3 – The vacuum chamber device diagram of the 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.

Figure 4 - System Diagram of Metal Organo Vapor Phase Epitaxy System
Figure 4 – System Diagram of Metal Organo Vapor Phase Epitaxy System

(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.

Figure 5 - Typical examples of several reaction chamber designs
Figure 5 – Typical examples of several reaction chamber designs

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.

Read more: What properties do the materials of II-VI and I-III-VI compound semiconductors have?

Two major applications of III-V solar cells

Two major applications of III-V solar cells

  1. Use on satellites or in space

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.

III-V solar cells
III-V solar cells

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.

  1. 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.

Read more: What is the casting method?

原文 I - Ⅲ -Ⅵ 2 族半导体太阳能电池/CuInSe2 的薄膜工艺 自 20 世纪 50 年代以来 ,I - Ⅲ -Ⅵ 2 族化合物开始被研究以了解其结晶相和材 料性质 ,Shay 和Wernick 在 1975 年将这些研究成果结集出版。以 CulnSe2 搭配 CdS 所制作的 PN 结太阳能电池的器件特性也随后在 1976 年发表 。之后 CIS 太阳能电地效率随着持续地改良工艺与搭配的材料而逐步提高,目前最高效率的I - Ⅲ -Ⅵ 2 薄膜太阳能电池 ,其主吸收层,其 Ga/ In 的比值必须维 持在0.3以内。若高 于 0.3 ,则导致 太阳能电池效率 大幅降低 。以三元化合 物 CulnSe2 制成的太阳能电池效率稍低 ,这是因为虽然该材料拥有相当高的光吸收 系数,但是 CulnSe2 的能隙仅约 1.O eV 。Ga 的加入使材料成为四元化合物 ,可 增 大CulnSe2 的能隙 ,也使得电池的开路电压( open circuit voltage ) 得以提升并降低 电流减少电阻损失 ,进而获得更高的电池效率 。之前我无意中浏览到一篇文章,作者在对电池效率影响的各个因素写的很细,很全面。如果您感兴趣,请访问tycorun.com。 CulnSe2是所有的 I - Ⅲ -Ⅵ 2 族化合物当中被研究得最多的 ,当然也获得很好 的成果。然而即使加入 Ga 成为来增大能隙以提升电池效率 ,但如前所述,当 x 值超过 0. 3 时就无法得 到 预期的效果 ,而 :r = 0. 3 时,其能隙仅 达 1. 15 eV 。曾经有一些努力 试图借助四元化合物,甚至五元化合物的组成调配来改变能隙 ,以尝试提升电池效率 ,但结果并非 如此 ,反而使得效率降低 L 叫 。似乎长时间以来 ,在CulnSe2 和Se2 所获 得的材料匹配和改良的成功经验无法直接被应用到其他I - Ⅲ -Ⅵ 2 系列的化合物 上,尤其是CulnSe2 ,它是除了CulnSe2 之外 ,另外通过组成调配可得到N 型和 P 型 的 I - Ⅲ -Ⅵ 2 族化合物 ,见表 1 ,它的能隙大约为 1. 5 eV ,如前所述 ,它是一种具有适当能隙值的太阳能电池材料 。往往这些材料特性的变化包括界面和内在缺陷等。 加总之后的影响足以导致可观的负面结果,显然针对这些 I - Ⅲ -Ⅵ 2 系列的化合 物仍需要投入足够的研究力量,以呈现其该有的器件表现 。因此以下即针对高效率太阳能电池的器件结构以及工艺等进行介绍 。 1. CuInSe2 的薄膜工艺 高效率 CIS 太阳能电池都是使用共蒸发 ( co-evaporation ) 或晒化 ( selenization) 反应法镀制 CIS 薄膜。其他如共溅射方式则存在因较高能量的薄膜 表面撞击而产生缺陷 ,以及In 排斥现象造成对薄膜组成的控制有所不足等缺憾 ,目前尚无法以此工艺产出高效率太阳能 电池;另外,如低成本的电化学沉积法 (electro­ deposition ) 所镀制的薄膜则因质量不佳也不适用于高效率太阳能电池的制作。 三源( CIS) 或四源 ( CIGS) 的共蒸发工艺是使用元素态蒸发源 ,在450 ~600℃的基板温 度下蒸发 。因为 Se 元素的蒸 气压高 ,所以常 在 Se 维持过量 ( over pressure) 的情况下来调整 Cu 与第三族元素 1) 时,晶粒约有几百甚至上千纳米的大小 ,薄膜呈现粗糙的 表面 ;而富 In 薄膜的晶粒小于几十纳米 ,故薄膜表面平滑光亮。CIS 薄膜蒸镀的 过程中 ,先形成 Cu2Se 和 In2Se3 两个二元相 ( binary phase ) ,再进一步反应生成 CuInSe2 三元相 ( ternary phase) 。 在 20 世纪 80 年代 ,美国波音公司实验室所制作的 CIS 太阳能电池突破 10 % 的能量转换效率 ,达到12% ,其 CIS 薄膜是先后分别在富 Cu 和富 In 的蒸发条 件下制备的 。在富 Cu 的条件下可得到大晶粒的 CJS 薄膜但却含有 Cu2Se 二次 相 ,在多晶薄膜的生 长过程中 ,Cu2Se 为 液 态并 呈 现 在 表面 和晶界 面 ( grain boundary ) ,此有助于晶粒长大 ;紧接着富In 的生长条件将可让多余的 In2Se3 二 元相跟富 Cu CIS 膜中的 Cu2Se 二次相完全反应而被消除 ,最后得到的是大晶粒且单一相的 CIS 薄膜 ,当然薄膜中先形成的组分含稍多的铜而呈现 P 型导电特 性 ,而稍后形成的部分则为In 偏多的组分,因而具高电阻率N 型接近本征 ( in­ trinsic) 半导体的导电特性 。 同时期 ,ARCO solar 公司亦发展 CIS 的晒化工艺 ,以该工艺制作的 CIS 太 阳能电池,其效率虽略逊于以蒸发工艺所制作的太阳能电池,但紧随其后。晒化 工艺是先分别镀制特定厚度的 Cu 和 In 金属膜以达成特定的原子数配比 ,再置于 H2Se 气体或 Se 蒸气之中 ,以 400 ℃以上的温度让其反应成 Cu2Se 。以晒化法 制备的薄膜 ,其组成均匀度略逊于蒸发者,但仍能满足高效率太阳能电池的要 求 ,其好处在于可使用大面积溅射工艺,甚至低成本的墨印( ink printing ) 工 艺 ,适合量产的规划 。另外 ,新式的晒化法采用快速退火 ( rapid thermal ling ,升温速率至少10℃/s 以上),将萃板上 Cu/ In/Se 三元素预镀层 ( precursor film ) 以 400~500℃的温度在极短时间 (1~5 min ) 内完成晒化反应显然此工艺更具产量大与戚本低的优势 。 晒化法若以逐渐升温的方式为之其成膜过程是一系列的反应先后在进行 ,亦 即先有铜化耐和锢化晒等多神二元中间相的形成 ,最后合成三元的 CIS 单一相。 如果使用快速升温即可跳过中间相的形成 .直接合成CIS 化合物 ,以此方式 所制备的材料并不会造成电池效率减损 ,至于薄膜形成的机制则因合成速度太快 而难以得知 ,但仍可利用间接的方式寻求更好的材质 。 目前以 CIGS 为主要吸收层的太阳能电池 ,其最高转换效率已达19. 2 % ,是 由美国 NREL 于 2003 年提出 。NREL 使用改良式的蒸发 法 称 为 三 阶段工艺 ( three-stage process ) ,顾名思义,此法是将整个工艺分为三个阶段来调变基板温 度与控制不同的元素源及其蒸发温度。 第一阶段是将已 镀上 铝 ( Mo ) 金属的铀玻璃加热至 260℃,井同时提供元素 In 、Ga 和 Se 生长 On ,Ga)先驱物 ;第二阶段则关闭元素 In 和 Ga ,改提供元素Cu 和 缸,井加热 基板至 560°c ,此时四元化合物Cu On , Ga)开始形成 ,并伴随产生二次相于薄膜表面 ,该二次相呈液态,有助于产生大晶粒且致密的柱状结晶;第三阶段即关闭元素 Cu 并保持基板温度在 560℃,再继续提供In 、Ga 和 Se 一小段时间后也关闭 In 和 Ga ,使足够的元素 In 和 Ga 与二次相再次反应 ,在表面形成Cu2(In,Ga)4 Se7或Cu1(In,Ga)3 Se5 薄膜 ,故最后会 形成铜稍微不足 ( Cu-poor ) 的 Cu(In,Ga)4 Se2薄膜 ,其组成比值为o. 93< Cu/ ( In十Ga)≤O. 97 ,最后在元素 Se 气氛下 ,做 5~10 min 的高温退火进行再结晶 。 以三阶段蒸镀工艺生长的 CIGS 薄膜的剖面品粒结构 以此法制备的 CIGS 薄膜,其组成元素纵深分布如图1 所示 0 借着加 入 Ga 形成四元化合物 ,可增大主 吸收层的能隙值 ,如此能使开路电压增加 20~30 mV 。至于 Ga 的浓度分布梯度则运用了 a-SiGe 太阳能电池的 能带设计概念 ,使导带形成V 形双斜率,此设计带有背向电场用来推动电子向 PN 结移动 ,有助于减少在背面金属接触界面处复合 ( recombination ) 的机会并 加强电荷的收集 ,而光入射方向则得益于能隙渐减的斜率设计,扩大光吸收波段 的涵盖植围 。整体而言,可增加短路电流 。 上述的材料 和器件设计理念的成功运用是 将 CIGS 太阳能电池效率推向15 % 以上 的主要原因 。事实上 ,以硫取代部分晒成 为四元化合物也可 以使能隙增加而得到像 CIGS 一样的效果 ,只是硫的蒸气压比晒高许多,若使用蒸发法在控制上需使用特殊的蒸发 源设计以特制阀门调节硫蒸气的输出。目前在 CIG 晒化量产工艺中 ,也可以同时置入晒 和硫合成 CIGSS 五元化合物。 阅读更多:你对多晶硅太阳能电池了解多少? I - Ⅲ -Ⅵ 2 zú bàndǎotǐ tàiyángnéng diànchí/CuInSe2 de bómó gōngyì zì 20 shìjì 50 niándài yǐlái,I - Ⅲ -Ⅵ 2 zú huàhéwù kāishǐ bèi yánjiū yǐ liǎojiě qí jiéjīng xiāng hé cáiliào xìngzhì,Shay hé Wernick zài 1975 nián jiāng zhèxiē yánjiū chéngguǒ jiéjí chūbǎn. Yǐ CulnSe2 dāpèi CdS suǒ zhìzuò de PN jié tàiyángnéng diànchí de qìjiàn tèxìng yě suíhòu zài 1976 nián fābiǎo. Zhīhòu CIS tàiyángnéng diàn dì xiàolǜ suízhe chíxù dì gǎiliáng gōngyì yǔ dāpèi de cáiliào ér zhúbù tígāo, mùqián zuìgāo xiàolǜ de I - Ⅲ -Ⅵ 2 bómó tàiyángnéng diànchí, qí zhǔ xīshōu céng, qí Ga/ In de bǐzhí bìxū wéichí zài 0.3 Yǐnèi. Ruò gāo yú 0.3, Zé dǎozhì tàiyángnéng diànchí xiàolǜ dàfú jiàngdī. Yǐ sān yuán huàhéwù CulnSe2 zhì chéng de tàiyángnéng diànchí xiàolǜ shāo dī, zhè shì yīnwèi suīrán gāi cáiliào yǒngyǒu xiāngdāng gāo de guāng xīshōu xìshù, dànshì CulnSe2 de néng xì jǐn yuē 1.O eV.Ga de jiārù shǐ cáiliào chéngwéi sì yuán huàhéwù, kě zēng dà CulnSe2 de néng xì, yě shǐdé diànchí de kāilù diànyā (open circuit voltage) déyǐ tíshēng bìng jiàngdī diànliú jiǎnshǎo diànzǔ sǔnshī, jìn'ér huòdé gèng gāo de diànchí xiàolǜ. Zhīqián wǒ wúyì zhōng liúlǎn dào yī piān wénzhāng, zuòzhě zài duì diànchí xiàolǜ yǐngxiǎng de gège yīnsù xiě de hěn xì, hěn quánmiàn. Rúguǒ nín gǎn xìngqù, qǐng fǎngwèn tycorun.Com. CulnSe2 shì suǒyǒu de I - Ⅲ -Ⅵ 2 zú huàhéwù dāngzhōng bèi yánjiū dé zuìduō de, dāngrán yě huòdé hěn hǎo de chéngguǒ. Rán'ér jíshǐ jiārù Ga chéngwéi lái zēng dà néng xì yǐ tíshēng diànchí xiàolǜ, dàn rú qián suǒ shù, dāng x zhí chāoguò 0. 3 Shí jiù wúfǎ dédào yùqí de xiàoguǒ, ér:R = 0. 3 Shí, qí néng xì jǐn dá 1. 15 EV. Céngjīng yǒu yīxiē nǔlì shìtú jièzhù sì yuán huàhéwù, shènzhì wǔ yuán huàhéwù de zǔchéng diàopèi lái gǎibiàn néng xì, yǐ chángshì tíshēng diànchí xiàolǜ, dàn jiéguǒ bìngfēi rúcǐ, fǎn'ér shǐdé xiàolǜ jiàngdī L jiào. Sìhū cháng shíjiān yǐlái, zài CulnSe2 hé Se2 suǒ huòdé de cáiliào pǐpèi hé gǎiliáng de chénggōng jīngyàn wúfǎ zhíjiē bèi yìngyòng dào qítā I - Ⅲ -Ⅵ 2 xìliè de huàhéwù shàng, yóuqí shì CulnSe2, tā shì chúle CulnSe2 zhī wài, lìngwài tōngguò zǔchéng diàopèi kě dédào N xíng hé P xíng de I - Ⅲ -Ⅵ 2 zú huàhéwù, jiàn biǎo 1, tā de néng xì dàyuē wèi 1. 5 EV, rú qián suǒ shù, tā shì yī zhǒng jùyǒu shìdàng néng xì zhí de tàiyángnéng diànchí cáiliào. Wǎngwǎng zhèxiē cáiliào tèxìng de biànhuà bāokuò jièmiàn hé nèizài quēxiàn děng. Jiā zǒng zhīhòu de yǐngxiǎng zúyǐ dǎozhì kěguān de fùmiàn jiéguǒ, xiǎnrán zhēnduì zhèxiē I - Ⅲ -Ⅵ 2 xìliè de huàhéwù réng xūyào tóurù zúgòu de yánjiū lìliàng, yǐ chéngxiàn qí gāi yǒu de qìjiàn biǎoxiàn. Yīncǐ yǐxià jí zhēnduì gāo xiàolǜ tàiyángnéng diànchí de qìjiàn jiégòu yǐjí gōngyì děng jìnxíng jièshào. 1. CuInSe2 de bómó gōngyì gāo xiàolǜ CIS tàiyángnéng diànchí dōu shì shǐyòng gòng zhēngfā (co-evaporation) huò shài huà (selenization) fǎnyìng fǎ dù zhì CIS bómó. Qítā rú gòng jiàn shè fāngshì zé cúnzài yīn jiào gāo néngliàng de bómó biǎomiàn zhuàngjí ér chǎnshēng quēxiàn, yǐjí In páichì xiànxiàng zàochéng duì bómó zǔchéng de kòngzhì yǒu suǒ bùzú děng quēhàn, mùqián shàng wúfǎ yǐ cǐ gōngyì chǎn chū gāo xiàolǜ tàiyángnéng diànchí; lìngwài, rú dī chéngběn de diàn huàxué chénjī fǎ (electro­ deposition) suǒ dù zhì de bómó zé yīn zhìliàng bù jiā yě bù shìyòng yú gāo xiàolǜ tàiyángnéng diànchí de zhìzuò. Sān yuán (CIS) huò sì yuán (CIGS) de gòng zhēngfā gōngyì shì shǐyòng yuánsù tài zhēngfā yuán, zài 450 ~600℃de jībǎn wēndù xià zhēngfā. Yīnwèi Se yuánsù de zhēngqì yā gāo, suǒyǐ cháng zài Se wéichí guòliàng (over pressure) de qíngkuàng xiàlái tiáozhěng Cu yǔ dì sān zú yuánsù 1) shí, jīng lì yuē yǒu jǐ bǎi shènzhì shàng qiān nàmǐ de dàxiǎo, bómó chéngxiàn cūcāo de biǎomiàn; ér fù In bómó de jīng lì xiǎoyú jǐ shí nàmǐ, gù bómó biǎomiàn pínghuá guāngliàng.CIS bómó zhēng dù de guòchéng zhōng, xiān xíngchéng Cu2Se hé In2Se3 liǎng gè èr yuán xiāng (binary phase), zài jìnyībù fǎnyìng shēngchéng CuInSe2 sān yuán xiāng (ternary phase). Zài 20 shìjì 80 niándài, měiguó bōyīn gōngsī shíyàn shì suǒ zhìzuò de CIS tàiyángnéng diànchí túpò 10% de néngliàng zhuǎnhuàn xiàolǜ, dádào 12% , qí CIS bómó shì xiānhòu fēnbié zài fù Cu hé fù In de zhēngfā tiáojiàn xià zhìbèi de. Zài fù Cu de tiáojiàn xià kě dédào dà jīng lì de CJS bómó dàn què hányǒu Cu2Se èr cì xiāng, zài duō jīng bómó de shēngzhǎng guòchéng zhōng,Cu2Se wèi yètài bìng chéngxiàn zài biǎomiàn hé jīng jièmiàn (grain boundary), cǐ yǒu zhù yú jīng lì zhǎng dà; jǐn jiēzhe fù In de shēng cháng tiáojiàn jiāng kě ràng duōyú de In2Se3 èr yuán xiāng gēn fù Cu CIS mó zhōng de Cu2Se èr cì xiāng wánquán fǎnyìng ér bèi xiāochú, zuìhòu dédào de shì dà jīng lì qiě dānyī xiàng de CIS bómó, dāngrán bómó zhōng xiān xíngchéng de zǔ fèn hán shāo duō de tóng ér chéngxiàn P xíng dǎodiàn tèxìng, ér shāo hòu xíngchéng de bùfèn zé wèi In piān duō de zǔ fèn, yīn'ér jù gāo diànzǔ lǜ N xíng jiējìn běn zhēng (in­ trinsic) bàndǎotǐ de dǎodiàn tèxìng. Tóngshí qí,ARCO solar gōngsī yì fāzhǎn CIS de shài huà gōngyì, yǐ gāi gōngyì zhìzuò de CIS tàiyángnéng diànchí, qí xiàolǜ suī lüè xùn yú yǐ zhēngfā gōngyì suǒ zhìzuò de tàiyángnéng diànchí, dàn jǐn suí qí hòu. Shài huà gōngyì shì xiān fēnbié dù zhì tèdìng hòudù de Cu hé In jīnshǔ mó yǐ dáchéng tèdìng de yuánzǐ shù pèi bǐ, zài zhìyú H2Se qìtǐ huò Se zhēngqì zhī zhōng, yǐ 400 ℃yǐshàng de wēndù ràng qí fǎnyìng chéng Cu2Se. Yǐ shài huà fǎ zhìbèi de bómó, qí zǔchéng jūnyún dù lüè xùn yú zhēngfā zhě, dàn réng néng mǎnzú gāo xiàolǜ tàiyángnéng diànchí de yāoqiú, qí hǎochù zàiyú kě shǐyòng dà miànjī jiàn shè gōngyì, shènzhì dī chéngběn de mò yìn (ink printing) gōngyì, shìhé liàng chǎn de guīhuà. Lìngwài, xīnshì de shài huà fǎ cǎiyòng kuàisù tuìhuǒ (rapid thermal ling, shēngwēn sùlǜ zhìshǎo 10℃/s yǐshàng), jiāng cuì bǎn shàng Cu/ In/Se sān yuánsù yù dùcéng (precursor film) yǐ 400~500℃de wēndù zài jí duǎn shíjiān (1~5 min) nèi wánchéng shài huà fǎnyìng xiǎnrán cǐ gōngyì gèng jù chǎnliàng dà yǔ qī běn dī de yōushì. Shài huà fǎ ruò yǐ zhújiàn shēngwēn de fāngshì wéi zhī qí chéng mó guòchéng shì yī xìliè de fǎnyìng xiānhòu zài jìnxíng, yì jí xiān yǒu tóng huà nài hé gù huà shài děng duō shén èr yuán zhōngjiān xiàng de xíngchéng, zuìhòu héchéng sān yuán de CIS dānyī xiāng. Rúguǒ shǐyòng kuàisù shēngwēn jí kě tiàoguò zhōngjiān xiàng de xíngchéng. Zhíjiē héchéng CIS huàhéwù, yǐ cǐ fāngshì suǒ zhìbèi de cáiliào bìng bù huì zàochéng diànchí xiàolǜ jiǎnsǔn, zhìyú bómó xíngchéng de jīzhì zé yīn héchéng sùdù tài kuài ér nányǐ dé zhī, dàn réng kě lìyòng jiànjiē de fāngshì xúnqiú gèng hǎo de cáizhì. Mùqián yǐ CIGS wéi zhǔyào xīshōu céng de tàiyángnéng diànchí, qí zuìgāo zhuǎnhuàn xiàolǜ yǐ dá 19. 2% , Shì yóu měiguó NREL yú 2003 nián tíchū.NREL shǐyòng gǎiliáng shì de zhēngfā fǎ chēng wèi sān jiēduàn gōngyì (three-stage process), gùmíngsīyì, cǐ fǎ shì jiāng zhěnggè gōngyì fēn wéi sān gè jiēduàn lái tiáo biàn jībǎn wēndù yǔ kòngzhì bùtóng de yuánsù yuán jí qí zhēngfā wēndù. Dì yī jiēduàn shì jiāng yǐ dù shàng lǚ (Mo) jīnshǔ de yóu bōlí jiārè zhì 260℃, jǐng tóngshí tígōng yuánsù In,Ga hé Se shēngzhǎng On,Ga) xiānqū wù; dì èr jiēduàn zé guānbì yuánsù In hé Ga, gǎi tígōng yuánsù Cu hé gāng, jǐng jiārè jībǎn zhì 560°c, cǐ shí sì yuán huàhéwù Cu On, Ga) kāishǐ xíngchéng, bìng bànsuí chǎnshēng èr cì xiāng yú bómó biǎomiàn, gāi èr cì xiāng chéng yètài, yǒu zhù yú chǎnshēng dà jīng lì qiě zhìmì de zhùzhuàng jiéjīng; dì sān jiēduàn jí guānbì yuánsù Cu bìng bǎochí jībǎn wēndù zài 560℃, zài jìxù tígōng In,Ga hé Se yī xiǎoduàn shíjiān hòu yě guānbì In hé Ga, shǐ zúgòu de yuánsù In hé Ga yǔ èr cì xiāng zàicì fǎnyìng, zài biǎomiàn xíngchéng Cu2(In,Ga)4 Se7 huò Cu1(In,Ga)3 Se5 bómó, gù zuìhòu huì xíngchéng tóng shāowéi bùzú (Cu-poor) de Cu(In,Ga)4 Se2 bómó, qí zǔchéng bǐzhí wèi o. 93< Cu/ (In shí Ga)≤O. 97, Zuìhòu zài yuánsù Se qìfēn xià, zuò 5~10 min de gāowēn tuìhuǒ jìnxíng zài jiéjīng . Yǐ sān jiēduàn zhēng dù gōngyì shēngzhǎng de CIGS bómó de pōumiàn pǐn lì jiégòu yǐ cǐ fǎ zhìbèi de CIGS bómó, qí zǔchéng yuánsù zòngshēn fēnbù rú tú 1 suǒ shì 0 jièzhe jiārù Ga xíngchéng sì yuán huàhéwù, kě zēng dà zhǔ xīshōu céng de néng xì zhí, rúcǐ néng shǐ kāilù diànyā zēngjiā 20~30 mV. Zhì yú Ga de nóngdù fēnbù tīdù zé yùnyòngle a-SiGe tàiyángnéng diànchí de néng dài shèjì gàiniàn, shǐ dǎo dài xíngchéng V xíng shuāng xiélǜ, cǐ shèjì dài yǒu bèi xiàng diànchǎng yòng lái tuīdòng diànzǐ xiàng PN jié yídòng, yǒu zhù yú jiǎnshǎo zài bèimiàn jīnshǔ jiēchù jièmiàn chù fùhé (recombination) de jīhuì bìng jiāqiáng diànhè de shōují, ér guāng rùshè fāngxiàng zé dé yì yú néng xì jiàn jiǎn de xiélǜ shèjì, kuòdà guāng xīshōu bōduàn de hángài zhí wéi. Zhěngtǐ ér yán, kě zēngjiā duǎnlù diànliú. Shàngshù de cáiliào hé qìjiàn shèjì lǐniàn de chénggōng yùnyòng shì jiāng CIGS tàiyángnéng diànchí xiàolǜ tuī xiàng 15% yǐshàng de zhǔyào yuányīn. Shìshí shàng, yǐ liú qǔdài bùfèn shài chéng wèi sì yuán huàhéwù yě kěyǐ shǐ néng xì zēngjiā ér dédào xiàng CIGS yīyàng de xiàoguǒ, zhǐshì liú de zhēngqì yā bǐ shài gāo xǔduō, ruò shǐyòng zhēngfā fǎ zài kòngzhì shàng xū shǐyòng tèshū de zhēngfā yuán shèjì yǐ tèzhì fámén tiáojié liú zhēngqì de shūchū. Mùqián zài CIG shài huà liàng chǎn gōngyì zhōng, yě kěyǐ tóngshí zhì rù shài hé liú héchéng CIGSS wǔ yuán huàhéwù. Yuèdú gèng duō: Nǐ duì duōjīngguī tàiyángnéng diànchí liǎojiě duōshǎo? 展开 3,754 / 5,000 翻译结果 I-III-VI Group 2 Semiconductor Solar Cell/CuInSe2 Thin Film Process

I-III-VI Group 2 Semiconductor Solar Cell/CuInSe2 Thin Film Process

Since the 1950s, I-III-VI 2 compounds have been studied to understand their crystalline phases and material properties, and Shay and Wernick published the results of these studies in 1975. The device characteristics of PN junction solar cells fabricated with CulnSe2 and CdS were also published in 1976. After that, the electrical efficiency of CIS solar energy has been gradually improved with the continuous improvement of the process and matching materials. The current highest efficiency I-III-VI 2 thin film solar cell, its main absorber layer, its Ga/In ratio must be maintained within 0.3. . If it is higher than 0.3, the solar cell efficiency will be greatly reduced. The efficiency of solar cells made with the ternary compound CulnSe2 is slightly lower because although the material has a rather high light absorption coefficient, the energy gap of CulnSe2 is only about 1.0 eV. The addition of Ga makes the material a quaternary compound, which can increase the energy gap of CulnSe2, and also increase the open circuit voltage of the battery and reduce the current to reduce the resistance loss, thereby achieving higher battery efficiency. Before I accidentally browsed an article, the author wrote very detailed and comprehensive on the various factors affecting battery efficiency. If you are interested, please visit tycorun.com.

CulnSe2 is the most studied of all I-III-VI 2 compounds, and of course good results have been obtained. However, even if Ga is added to increase the energy gap to improve the cell efficiency, as mentioned above, when the value of x exceeds 0.3, the expected effect cannot be obtained, and when r = 0.3, the energy gap can only reach 1.15 eV. There have been some attempts to change the energy gap by means of the composition of quaternary compounds, or even pentads, in an attempt to improve cell efficiency, but this has not been the case, but has reduced the efficiency. It seems that for a long time, the successful experience of material matching and improvement obtained in CulnSe2 and Se2 cannot be directly applied to other I-III-VI 2 series compounds, especially CulnSe2, which is in addition to CulnSe2, additionally by composition. The I-III-VI 2 compounds of N-type and P-type can be obtained by formulating, as shown in Table 1, its energy gap is about 1.5 eV, as mentioned above, it is a kind of solar cell with appropriate energy gap value Material. Often these changes in material properties include interfacial and intrinsic defects.

Table 1-Energy gap and electrical properties after annealing of I-III-VI group 2 compounds
Table 1-Energy gap and electrical properties after annealing of I-III-VI group 2 compounds

The combined effect is enough to lead to considerable negative results, and it is clear that enough research efforts are still needed for these I-III-VI 2 series compounds to show their proper device performance. Therefore, the following is an introduction to the device structure and process of high-efficiency solar cells.

  1. Thin film process of CuInSe2

High-efficiency CIS solar cells use co-evaporation (co-evaporation) or solarization (selenization) reaction method to coat CIS thin films. Other methods, such as co-sputtering, have defects such as defects caused by high-energy film surface impact, and insufficient control of film composition due to In repulsion. At present, high-efficiency solar cells cannot be produced by this process; For example, the thin films plated by low-cost electrochemical deposition (electro deposition) are not suitable for the fabrication of high-efficiency solar cells due to their poor quality.

The three-source (CIS) or four-source (CIGS) co-evaporation process uses an elemental evaporation source to evaporate at a substrate temperature of 450 to 600 °C. Because the vapor pressure of the Se element is high, the ratio of Cu to Group III elements 1), the grain size is about several hundreds or even thousands of nanometers, and the film has a rough surface; while the grain size of the In-rich film is less than tens of nanometers, so the surface of the film is smooth bright. In the process of CIS thin film evaporation, two binary phases (Cu2Se and In2Se3) are formed first, and then the CuInSe2 ternary phase is formed by further reaction.

In the 1980s, the CIS solar cell produced by the Boeing laboratory in the United States broke through 10% of the energy conversion efficiency, reaching 12%, and its CIS thin film was successively prepared under Cu-rich and In-rich evaporation conditions. CJS films with large grains can be obtained under Cu-rich conditions but contain Cu2Se secondary phases. During the growth of polycrystalline films, Cu2Se is liquid and appears on the surface and grain boundaries, which helps Grain growth; followed by In-rich growth conditions, the excess In2Se3 binary phase will completely react with the Cu2Se secondary phase in the Cu-rich CIS film and be eliminated, resulting in a large-grain and single-phase CIS The film, of course, the component formed first in the film contains a little more copper and exhibits P-type conductivity, while the part formed later is a component with more In, so it has a high resistivity N-type close to intrinsic (in trinsic) ) Conductivity properties of semiconductors.

At the same time, ARCO solar company also developed the CIS solarization process. Although the efficiency of CIS solar cells produced by this process is slightly lower than that of solar cells produced by evaporation process, it is close behind. The tanning process is to first coat Cu and In metal films with specific thicknesses to achieve a specific atomic number ratio, and then place them in H2Se gas or Se vapor, and react them into Cu2Se at a temperature above 400 °C. The composition uniformity of the thin film prepared by the solarization method is slightly inferior to that of the vaporizer, but it can still meet the requirements of high-efficiency solar cells. Process, suitable for mass production planning. In addition, the new tanning method adopts rapid thermal annealing (rapid thermal ling, the heating rate is at least 10°C/s or more), and the Cu/In/Se three-element pre-plating layer (precursor film) on the extraction plate is heated at a temperature of 400~500°C. The drying reaction is completed in a very short time (1~5 min), obviously this process has the advantages of large output and low cost.

If the solarization method is gradually heated, the film formation process is a series of reactions that are carried out successively, that is, the formation of polythematic binary mesophases such as copper resistance and induration, and finally the synthesis of ternary. CIS single phase.

The formation of intermediate phases can be skipped if rapid heating is used. Directly synthesizing CIS compounds, the materials prepared in this way will not cause loss of cell efficiency. As for the mechanism of film formation, it is difficult to know because the synthesis speed is too fast, but indirect methods can still be used to find better materials.

At present, the highest conversion efficiency of solar cells with CIGS as the main absorber layer has reached 19.2%, which was proposed by NREL in the United States in 2003. NREL uses an improved evaporation method called a three-stage process. As the name suggests, this method divides the entire process into three stages to modulate the substrate temperature and control different element sources and their evaporation temperatures.

The first stage is to heat the uranium glass plated with aluminum (Mo) metal to 260°C, and to provide the elements In, Ga and Se to grow On, Ga) precursors at the same time; in the second stage, the elements In and Ga are turned off and replaced to provide Elemental Cu and vat, well heat the substrate to 560°C, at this time the quaternary compound Cu On, Ga) begins to form, accompanied by the formation of a secondary phase on the film surface, which is liquid and helps to generate large grains And dense columnar crystals; the third stage is to turn off the element Cu and keep the substrate temperature at 560 ° C, and then continue to provide In, Ga and Se for a short period of time and also turn off In and Ga, so that enough elements In and Ga are combined with the secondary phase. Reaction again to form Cu2(In,Ga)4Se7 or Cu1(In,Ga)3Se5 film on the surface, so Cu(In,Ga)4Se2 film with slightly insufficient copper (Cu-poor) will eventually be formed. Its composition The ratio is 0.93<Cu/(In+Ga)≤0.97. Finally, in the elemental Se atmosphere, do high temperature annealing for 5~10 min for recrystallization.

Cross-sectional grain structure of CIGS thin films grown by three-stage evaporation process
The depth distribution of the constituent elements of the CIGS thin film prepared by this method is shown in Figure 1. By adding Ga to form a quaternary compound, the energy gap value of the main absorber layer can be increased, so that the open circuit voltage can be increased by 20-30 mV. As for the concentration distribution gradient of Ga, the energy band design concept of a-SiGe solar cells is used to make the conduction band form a V-shaped double slope. The opportunity for recombination at the back metal contact interface enhances the collection of charges, and the light incident direction benefits from the design of the slope of the energy gap, which expands the coverage of the light absorption band. Overall, the short-circuit current can be increased.

Figure 1-Depth distribution of constituent elements of CIGS thin films
Figure 1-Depth distribution of constituent elements of CIGS thin films

The successful application of the above-mentioned material and device design concepts is the main reason for pushing the efficiency of CIGS solar cells above 15%. In fact, replacing part of the tanned with sulfur into a quaternary compound can also increase the energy gap to obtain the same effect as CIGS, but the vapor pressure of sulfur is much higher than that of tanning. If the evaporation method is used, a special evaporation source design is required for control. The output of sulfur vapour is regulated by a special valve. At present, in the mass production process of CIG tanning, it is also possible to simultaneously insert tanning and sulfur to synthesize CIGSS five-membered compounds.

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