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The device structure commonly used in high-efficiency CIS solar cells is shown in Figure 1(a). The energy gap values ​​of each layer material are Zn(): 3.30eV, CdS: 2.42eV, CulnSe2: 1.02eV, decreasing from top to bottom , which can cover a broad absorption range of the solar spectrum. Usually, the P-type Cu-rich CIS film is firstly plated on the indium-coated glass substrate to form a good ohmic contact, and then the In evaporation temperature is increased to continue plating the Intrin sic In-rich CIS film. At this time, its grain structure inherits the underlying CuCIS-rich film, which is also a large grain and a rough surface. The N-type CdS buffer layer on the CIS is designed for the best matching of the heterojunction electrical properties, and a chemical bath deposition method is used to cover the slightly rough surface intact. Finally, ZnO and Al/Ni beer films were grown successively by sputtering to complete the entire device structure. The energy conversion efficiency of solar cells with CIS as the main absorber layer is currently up to 15.4%; while the efficiency of the recent CIGS solar cells is close to 20%, and its device structure is shown in Figure 1(b).

The combination of materials in the device structure of the above-mentioned high-efficiency CIS solar cells has been in the same vein since the publication of the first CIS solar cell in 1976, but it is the same as the 10% breakthrough of the CIS solar cell structure by Piyin in the 1980s. compared to the material. However, there have been some important changes, including the use of uranium glass for the Jin glass substrate, the replacement of CIS with CIGS, and the slight changes in the anti-reflection layer and light-transmitting layer. The selection and characteristics of materials for each layer are briefly described below.

In the selection of glass substrates, borosilicate glass was initially used, and later, lower-cost uranium glass was used. Nano-atoms have strong diffusion ability in solid-state materials, and can pass through the aluminum layer into the CIS coating during the CIS evaporation process. The temperature at that time was about 500 °C and the time was about 60 min. Uranium has some unexpected but very positive effects in the growth of CTS films, and some preliminary understandings have been obtained in related studies: ① it inhibits the formation of crystalline defects; ② it contributes to the P-type conductivity. Although clear experimental evidence has not yet been obtained, indirect experimental results indicate that these effects and cell efficiency have been significantly improved. In this experiment, a thin NaF-containing film was deliberately plated on the indium layer to regenerate the CIS and complete the above device structure. , Table 2 shows that the cell efficiency value is improved by about 3%.

In terms of the ohmic contact of the back electrode, it has been experimentally confirmed that there is only a few atomic layers of M0Se2 between CIS and Mo when the CIS film is grown, which improves the CIS/Mo interface with Schottky contact characteristics. quality, while showing good ohmic contact characteristics.

For a long time, CdS is considered to be the best match to form a PN junction with CIS, and the energy band structures formed by the two have been discussed in many papers. Some believe that Cd will diffuse into CIS and replace Cu to become N-type dopant, causing the position of the PN junction to move inward to the CIS, so that the internal electric field formed by the PN junction is separated from the original CdS/CIS heterojunction due to the lattice. Defects due to lattice mismatch create regions of carrier recombination.

Originally in the 1980s. The role of N-CdS is mainly a light-transmitting layer (window layer). But because Cd is a heavy metal element, the thickness of the layer is greatly reduced, and now it is often called a buffer layer (buffer layer). Because the surface of the CIGS thin film is slightly uneven, the preparation of the CdS thin layer is often carried out by chemical bath deposition (CBD) to ensure that the thin film can cover the CIGS surface continuously and completely. Recent studies have found that discontinuous CdS The thin film will make the efficiency of CIGS solar cells not higher than 18% [1:. The relevant data is shown in Figure 3. This buffer layer also has the effect of blocking the atomic impact of ZnO and reducing the generation of defects. The current research also tends to use Cd-free buffer layer materials. Based on the current research results, a direct comparison under the same conditions is shown in Figure 4. Some materials such as ZnS and other buffer layers are matched with CIGS. For solar cells, their efficiency is already on par.

In the previous CIS solar cell structure, ZnO is the anti-reflection layer (anti-reflection layer), but in the high-efficiency CIGS solar cell structure, it has multiple roles, so the i-ZnO, which is an electrical near-intrinsic semiconductor, is Light-transmitting layer, doped with Al, ZnO:AlC with good conductivity, also known as AZO) is the outermost layer with multiple functions such as light-transmitting conductivity and anti-reflection.

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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.

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.

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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1. CdTe thin film process

In order to reduce the cost of CdTe films, polycrystalline structures are often the mainstay, which can be prepared by various methods, including evaporation, close-spaced sublimation (CSS), and vapor transport deposition (vapor transport deposition). , VTD ) , chemical vapor deposition ( chemical vapor deposition , CVD ) , electrochemical deposition ( electro deposition . ED ) , screen printing ( screen printing ) , thermal plate spraying ( spray pyrolysis ) , sputtering ( sputtering ), etc. has been used [7]. Figure 7.4 organizes various CdTe coating processes and related process conditions for reference and comparison [8]. Because CdTe has ionic bonds. It is easy to form Cd and Te bonds during the film formation process, and different coating methods are used. It is not difficult to control the stoichiometric composition. Even elemental films of Cd and Te can be plated separately at room temperature and then heated up. to form compounds.

It is worth mentioning that CdTe solar cells often require an additional heat treatment procedure. Also CdCl2 is usually mixed into the air with a partial pressure of 0.04 and heat treatment at a temperature of 425 ℃ for 20 min. This promotes the growth of CdTe grains and the passivation of the crystal interface, so that the high resistance at the crystal interface disappears, and at the same time, it promotes the mutual diffusion of materials between PN heterojunctions, forming ternary compounds (such as CdS/CdTe near the interface. Produces CdTe1, S,) with a graded composition, thereby reducing the interface defect density caused by the lattice mismatch between CdTe and CdS. This procedure helps to greatly improve the efficiency of solar cells; while 0 in the air has The effect of increasing the P-type conductivity of CdTe and promoting CdS/CdTe interdiffusion. There is also a saying: Since this is a necessary follow-up process. It does not matter which method is used for the CdTe coating, as much as possible using the low-cost method.

2. Electrical properties and device technology of CdTe

The combination of N-CdS/P-CdTe in II-VI solar cells can reach 16.5% energy conversion efficiency. The large-area CdTe solar cell module has entered the mass production stage in recent years, and its energy conversion efficiency has exceeded 10%. Different companies use different preparation methods for the P-CdTe film in the device structure, which are coating technologies such as close-range volatilization, electrochemistry, and vapor transport.

The P-type doping of CdTe is dominated by P or As, and it is usually doped under Cd overpressure, that is, Cd-rich conditions. The hole concentration of 2 × 101; cm – 3 can be obtained for the CdTe epitaxial film prepared by CVD or MOCVD, while the polycrystalline film is only 6 × 101 cm . As doped CdTe prepared by photo-assisted molecular beam epitaxy (PAMBE) has the highest hole concentration of 6. 2 × 1018 cm – 3, which is conducive to the activity of atoms on the surface under illumination , and also makes Te more easily desorbed, and the effect on the doping amount is not obvious. In the MBE process, some people have also used low-energy ion beam doping, and its hole concentration is only 2 × 1017 cm3, but the material defects caused by the ion beam impact make its electrical properties worse, and it often needs to be regrown on it. A layer of undoped CdTe to avoid reducing the short-circuit current of the solar cell.

In terms of ohmic contact, because the work function of P-CdTe is as high as 5.7 eV, no metal can form an ideal ohmic contact with it, so the metal alloy containing Cuo.12Au0.88 is rich in Te after treatment. The surface of CdTe is diffused from Cu into the lattice position where CdTe replaces Cd, which promotes the formation of high content of P-type doping on the surface of CdTe, and holes are conducted through strong electric field tunneling. In the module mass production process, the 1-contact resistance contact of the P-CdTe polycrystalline film is often fabricated by electron beam evaporation on the surface of CdTe after CdCl2 annealing treatment → Cu thin layer, and then heated to promote the diffusion of Cu into CdTe, achieve surface doping.

In summary, the fabrication of high-efficiency N-CdS/P-CdTe solar cells should be based on the following designs and steps: ① Coating the CdTe film on the transparent CdS/TCO/Glass substrate; ② The thinner the CdS light-transmitting layer, the more It can improve the spectral response of short wavelength; ③ After the CdTe coating, a heat treatment procedure is required in the atmosphere of CdCl2 and O2 to improve the grain structure and P-type conductivity of CdTe, and also promote the interaction between CdTe and CdS. The interdiffusion of CdTeS is formed to reduce the number of interface defects; ④ The CdTe layer is chemically treated to make its surface rich in Te, then plated with Cu and heated to diffuse into CdTe to increase the amount of P-type doping, and finally plated on Conductive materials such as Ni or graphite.

3. Future development of CdTe thin film solar cells

CdTe solar cells are mass-produced by one or two companies in the United States and Germany. The energy conversion efficiency of large-area CdTe modules can currently reach 10.5% and the service life is as long as 25 years. Its unit cost has the opportunity to be $1 per watt. The following are the thin-film solar cells with the lowest cost at present. Furthermore, Cd and Te elements can be obtained from the smelting process of basic metals such as copper and iron ore. However, a negative factor in the application of this product is that the Cd contained in the material is a heavy metal element, and the environmental protection problems caused by its process and disposal have attracted much attention. Although the industry emphasizes that the pollution of Cd compounds is very different from that of elemental Cd, and that the process and waste recycling treatment procedures can meet strict requirements, a company in the United States even takes long-term samples of its employees in the production line. Examination of the Cd content in the body has not detected higher than usual results, but it is still worrying. Under the competition of other materials, although the EU’s restrictions on the Cd content of most electronic products have not yet included solar cells, the manufacturers of CdTe solar cells mass-produced in the short term have fewer competitors to produce the same products, and the products are more expensive. This is lower than solar cells made of other materials, thus making profits. Manufacturers even give a plan to pay the cost of recycling solar panels in the future, but in the long run, under the stricter environmental protection requirements in the future, it is not conducive to CdTe solar cells. development of.

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