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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