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What are the characteristics of II-VI and I-III-VI compound semiconductor materials?

What are the characteristics of II-VI and I-III-VI compound semiconductor materials?

one II-VI compounds

As shown in the periodic table of elements in Fig. 1, group E elements (including Zn, CD, Hg) and group H elements (i.e. s, Se, Te, etc.) constitute group II and VI compounds with semiconductor properties, Its basic properties are shown in Fig. 1[5l. Except that CdTe and ZnSe have n-type and p-type conductive properties, other II-VI compounds can only obtain a single conductive form, such as p-type ZnTe and n-type CDs, CdSe, ZnS, etc. Therefore, there are some restrictions on the matching of materials. According to theoretical calculation, PN homojunction (Homo J function) The energy conversion efficiency of solar cell is the highest, and its energy gap is 1 About 5 EV, so CDT has naturally become the most suitable material for solar cells in h-group compounds.

periodic table of ele ments
periodic table of ele ments

II-VI compounds have stronger ionic bonds than V group. In addition, the vapor pressure of group II and VI elements is similar and their value is not low. Therefore, in the preparation of materials, it is easy to form compounds by self interactive coordination. For the growth of thin films, if the substrate temperature is set so that the same atoms (ii-ii or vi-vi) cannot form valence bonds, the elements of group H and group m only have interactive valence bonds to form group II-VI compounds; Under normal coating conditions, the temperature is not high, about 300 ℃.

Some of the crystalline structures of II-VI compounds are cubic sphalerite structures, such as ZnSe, ZnTe, CdTe, etc., while others are hexagonal structures, such as CdSe; CDs and ZnS have one or both of the above two structures according to the material preparation conditions. Different structures have different properties.

In semiconductor materials, the strength of valence bond is directly related to the energy gap. From the periodic table in Fig. 1, those with small atomic numbers of constituent elements in II-VI compounds have strong ionic bonds. For example, Zn Se is stronger than Zn te, and CD-s is also stronger than CD te; Their energy gap also reflects the strength of bidding bond. Compared with CD te, Zn te has stronger valence bond and larger energy gap.

  1. I-ill-vi group 2 compounds

Group I VI 2 compounds can be regarded as three element compounds derived from group II VI, that is, the elements of group H are replaced by group I (Cu, Ag) and group I (al, GA, in) and combined with group VI (CS, Se, TE). Because there are two kinds of atoms arranged alternately and orderly in the lattice position occupied by cations, the unit cell seems to be formed by stacking the unit cells of two amphibolites. However, the unit length ratio of c-axis and a-axis is different due to the valence bond strength of two different cations and anions, which is not equal to 2. Such a crystalline structure is called chalcopyrite structure, as shown in Figure 2.

crystal structure of I-III-VI compounds
crystal structure of I-III-VI compounds

As can be seen from the periodic table of Fig. 1, this series of compounds include 18 compounds such as group I elements Cu and Ag, group IV elements Al, in and GA and group M elements s, Se and te. Like other compound semiconductors, their energy gaps can form quaternary or even quinary compounds through the mutual substitution of homologous elements in a certain range. The biggest difference between it and tianv or HVI compounds is that there is no composition very close to the chemical ratio. The composition stability range of single phase can reach 3% ~ 5%, and the degree of deviation from the fixed ratio composition of 1:1:2 is considerable. The above situation can be seen from the pseudo binary phase diagram of Fig. 2. The situation of deviating from the fixed specific composition will produce intrinsic point defects in the material. There are 12 kinds of ternary compounds of III-VI group, and the distribution of the number of various point defects is closely related to the chemical composition and the formation energy of defects.

Read more: how to make hydrogenated amorphous silicon solar cells?

How to make hydrogenated amorphous silicon solar cells

How to make hydrogenated amorphous silicon solar cells?

Typical single junction a-Si: H solar power grounding, (a) is in the form, and (b) is in the form of substrate. The substrate is the window of light incidence, generally glass or transparent polymer is used, and the front electrode is a layer of transparent conductive oxide (TCO) film. The commonly used materials are sn02: F, LN20,: Sn (ITO) or ZnO (AL) [28]. The quality requirements of TCO film are that the transmittance within the working spectrum range of layer I is greater than 85%, the block resistance value is less than 10 Ω / port, the contact resistance with layer P is small, the surface roughness increases the light scattering into layer I, and has sufficient chemical stability to resist the hydrogen atom etching during chemical vapor deposition. Tin oxide (fluorine) (SN 02: F) TCO films are usually prepared by atmospheric pressure CVD (a PCVD) at 500 ~ 600 ℃, and the deposited films have rough surfaces. Tin oxide (ITO) films are generally prepared by sputtering at a temperature of 200 ~ 250 ℃, and the deposited films have a smooth surface. The oxide thump is also generally produced by sputtering. The temperature ranges from room temperature to 300 ℃. The deposited film has a smooth surface and can be etched into texture by HCl. Table 6 5 shows the characteristics of these three TCO films.

P-i-n three-layer hydrogenated amorphous silicon semiconductor films are generally deposited by PECVD at a temperature of 200 ~ 250 ℃. The p layer is a window layer incident by sunlight, with a thickness of 10 ~ 30 nm. In order to improve the proportion of light incident on layer I, the effective method is to add carbon atoms into this layer to improve the energy gap value [25.26], improve the absorption efficiency of short, medium and long wavelengths, and increase the open circuit voltage of solar cells.

Adding a buffer layer [30.31] with asymptotic energy gap change at the P / I interface and adjusting the doping distribution of p layer can reduce the defects caused by the lattice stress of discontinuous energy gap, avoid the diffusion of tottering atoms from p layer to layer, reduce the back diffusion of photogenerated electrons, and distribute the internal electric field to the layer to separate photogenerated electrons and holes and reduce recombination. These effects can improve the efficiency of solar cells. The layer is the active layer of solar cell, which absorbs the incident light and generates electron hole pairs. It is separated and drifted to the front and rear electrodes by the built-in electric field, resulting in photocurrent. The thickness is 200 ~ 500 N M. the quality of the layer must meet the above device quality requirements. The general deposition rate of PECVD deposition layer is 0.1 ~ 0.5% 3 N M / s, which is an important “bottleneck” o

The N layer is the contact layer of the back electrode, with a thickness of 20 ~ 30 n M. the penetration of sunlight into the N layer is mainly medium and long wavelength. The photogenerated carriers near the I / N interface and the N layer are mainly contributed by photons with longer wavelength. In order to improve the built-in electric field strength and reduce the contact resistance with the back electrode, the N layer can be made into a microcrystalline structure. The rough interface between the N layer and the back electrode can increase the trapping of light reflected into the i layer. Adding a TCO film between the N layer and the back electrode can enhance the land capture of light [32], especially increase the absorption of 600 ~ 800 nm long wavelength light, and the short-circuit current density of solar cells can be increased. Generally, ZnO (AL) is used as the TCO film to increase reflection.

The substrate of substrate type solar cell is the back of the solar cell. Generally, stainless steel or polymer film coated with metal film is used as the back electrode in addition to the substrate. These two kinds of thin sheets have the characteristics of soft and local bending, which makes it easy to install, can be applied to different environments, and their weight is light, which is suitable for portable mobile power. Making rough Ag / ZnO film on stainless steel sheet substrate can improve the reflectivity of light and increase the short-circuit current. The deposition order of hydrogenated amorphous silicon semiconductor film is n-i-p. the N layer can use microcrystalline structure to improve the built-in electric field and reduce the contact resistance, and the N / I interface needs to add a buffer layer to alleviate the change of energy gap between I layer and N layer. The deposition of p layer is after I layer. Therefore, there is no problem of plasma etching TCO film. The TCO film of the front electrode is deposited above the p layer, and the temperature for making TCO film is limited to be no higher than the deposition temperature of n-i-p a-Si: H film. The resistance of the TCO film must be maintained at a low value to reduce the contact resistance. In order to achieve the effect of anti reflection, the thickness of the TCO film needs to be controlled at 70 ~ 80 nm. Taking ITO film as an example, the block resistance under this thickness condition will be greater than 500 / port. In order to reduce the series resistance, a metal grid must be made on TCO film to improve the filling factor and conversion efficiency of solar cells.

For a single cavity PECVD system, p-i-n three-layer a-Si: H films are deposited in this cavity in sequence. The system is simple and convenient. However, due to the cross pollution of shed and phosphorus atoms left in the cavity after deposition, the quality of solar cells is not easy to control, and the cross pollution limits the improvement of efficiency.

In order to improve the problem of cross contamination, Sanyo company of Japan has developed a continuous separation cavity system. For this purpose, the multi reaction cavity PECVD system deposits p-i-n three-layer a-Si: H films in isolated cavities. Each cavity is separated by a gate valve, and the substrate can be transported between adjacent cavities by a conveyor belt. Each cavity is deposited at the same time, and the substrate is placed and removed from the insertion cavity and extraction cavity in order to achieve the purpose of continuous production. P. The thickness of I and N layers is determined by the deposition rate and the length of the cavity. Since the thickness of layer 1 is relatively thick, the cavity length of layer I needs to be longer to maintain the continuity of production.

This system can solve the cross pollution problem of shed and phosphorus residue. Due to the design of insertion and extraction cavity, the three deposition cavities P, I and N are isolated from the atmosphere. Therefore, the cavity is not polluted by water vapor and other pollution sources in the atmosphere, so that the purity of the film can be well controlled. P. The method of depositing I and N layers by separators has been adopted by most manufacturers in the world.

The production of Si: H solar cell template adopts integrated series structure. TCO film, p-i-na-si: H film and Al film are cut by laser to form up-down series connection.

Q-switched Nd: YAG laser is usually used, and the cutting of TCO film is 1 At the laser wavelength of 06 µ m, 0.5 μ m is used for the segmentation of a Si: H film and Al film The laser wavelength is 53 µ m, and the laser cutting rate is 20 ~ 50 cm / s.

The production process of integrated structural a-Si: H solar cell template [35] has 12 procedures from broken glass cleaning to packaging, which are: ① cleaning glass; ② A thin SiO2 film of about 50 nm was plated by APCVD, and then a SnO2 film of 600 ~ lOOO nm was plated; ③ Silver glue is coated on the edge of sn02 film as anode sink line; ④ Hardening silver glue with heating furnace; ⑤ Laser cutting of sn02 film; ⑥ Clean the substrate; ⑦ P-i-n a-Si: H film was deposited by PECVD; ⑧ Then, aluminum or aluminum and oxide back reflection electrode are deposited; ⑨ By two-stage laser cutting, the back reflection electrode is cut into blocks, and then the back reflection electrode is heated by laser through the pi-na-si: H film layer and fused with the front electrode TCO film to form a series; ⑩ Electrical test and fabrication of cathode current collector Formwork performance measurement; @ Template encapsulation.

Through the above production procedures, the fabrication of large-area solar cell template can be completed. The main key affecting the production lies in the fabrication of p-i-n three-layer a-Si: H film and the integrated series connection of laser cutting. At present, there is no standardized production equipment. Manufacturers must design and construct their own production equipment to meet the requirements of production. The cost of plant construction is much higher than that of standard production equipment for single polysilicon solar cells. The quality control of p-i-na-si: H film requires certain technology and knowledge of process equipment and film process conditions.

The advantages and disadvantages of hydrogenated amorphous silicon solar cells are introduced below. Compared with silicon wafer solar cells, hydrogenated amorphous silicon thin film (a-Si: H) solar cells have the following advantages.

(1) a-Si: H thin film solar cells have better average energy yield (kW • H / kW). Figure 6 12. NEDO (new energy and Industrial Technology Development Organization) of Japan measured the one-year power generation efficiency of monocrystalline silicon and polycrystalline silicon wafer solar cells and amorphous silicon thin film solar cells of Kaneka company of Japan from August 1998 to July 1999. The results show that a-Si: H thin film solar cells are under high temperature conditions, especially in the afternoon of summer, Table 6.6 shows the field test of solar cells with different materials on the market for half a year by ECN (Netherlands Energy Research Foundation) in the Netherlands in 2000. The results show that although the conversion efficiency of amorphous silicon thin film solar cells is the lowest among these commercial solar panels, the energy yield for a long time is the highest, The reasons include better temperature coefficient under high temperature and higher photoelectric conversion performance under low illumination. Nankai University conducted a one-year field test on the same 2kwp monocrystalline silicon and a Si: H solar cell power generation system from 2004 to 2005. The results also show that the amorphous silicon solar cell module has a high total power generation. The above three independent research reports point out that amorphous silicon thin films and solar cells have excellent power generation performance.

C 2) a-Si: H thin film solar cells consume less materials. Because a-Si: H thin film solar cells have high light absorption coefficient, the required thickness of light absorption layer can be reduced by 600 times compared with silicon wafer materials.

(3) the energy PA Y-Back time of a-Si: H thin film solar cell is short. Generally, the temperature required for the production of a-Si: H solar cells is lower than 300 ℃, while the production of bulk silicon wafer solar cells requires 1000 ~ 1500 ℃. Taking the annual power generation of 30 MW as an example, the recovery time of a-Si: H is about 1 In 6 years, poly Si needs about 2.5% 2 years.

These research reports clearly point out that silicon thin-film solar cells have low temperature coefficient and can have stable power output in high temperature environment. Because silicon thin-film solar cells have high light absorption characteristics, long-term field tests show that under the condition that the sunlight energy changes continuously in different time (morning to evening) and different seasons (spring to winter), The annual average power generation efficiency is better than that of silicon wafer solar cells.

A Si: H thin film solar cells have many of the above advantages, and a-Si: H thin film solar cells can be made on low-cost glass, stainless steel or soft substrates. They can be made in a large area in production, save material consumption, and the production is not affected by wafer supply. Therefore, they have the economic advantage of reducing cost.

Hydrogenated amorphous silicon solar cells have two main disadvantages: one is that their electrical properties will deteriorate after sunlight irradiation; Second, the conversion efficiency is low.

In 1977, staebler and wronski first pointed out that the conductivity of hydrogenated amorphous silicon films will degrade under sunlight, and the conductivity can be restored after annealing. This effect is called photodegra ­ Dation effect, also known as staebler wronski effect (SWE). After more than 20 years of research, wronski pointed out in his review article in 1997 that the cause of swe effect has not been fully confirmed. It is generally accepted that the energy released when the electron hole pair recombines will break the H bond, so it will cause the light degradation effect. Hydrogen atoms play a role in compensating hanging bonds in amorphous silicon structure, but excessive hydrogen atoms will form a large number of long-chain or clustered silicon hydrogen bonds, which will loosen the structure of hydrogenated amorphous silicon film. These long-chain or clustered silicon hydrogen bonds are easy to break under sunlight, causing a large number of defects and reducing the conductivity.

At present, the efforts to improve swe are mainly to use hydrogen dilution method or plasma gas-phase reaction control to search the silicon hydrogen structure of the film from amorphous to amorphous structure, while in the device structure, it is mainly to reduce the layer thickness, improve the built-in electric field strength and reduce the carrier recombination. This method is applied to the top cell of multi junction structure. These practices can reduce the degree of light degradation to less than 20%.

The stable efficiency of single junction a-Si: H solar cells is recorded. The data show that the stable power generation efficiency is less than 10%. The power generation efficiency is 2 / 3 lower than that of monocrystalline polysilicon solar cells. The application of large power supply is limited due to the large area required.

The method to improve the power generation efficiency can be realized by improving the film quality and increasing the application of solar spectrum. The method to improve the film quality is mainly to gradually change the deposited film from amorphous structure to microcrystalline silicon (µ c-Si) structure by hydrogen dilution method. The increase of crystalline composition in the film changes the structure, electrical and optical properties, and the optical energy gap can gradually change from L. 7 ev to 1 1 E V adjustment.

At present, many efforts are focused on changing the structure of a-Si: H films, especially transforming the amorphous structure of a-Si: H films into hydrogenated polystructured Si (PM Si: H) [43 ~ 45], hydrogenated protocrystalline Si (PC Si: H) [46.47] and hydrogenated microcrystalline Si (µ C Si: h) [48 ~ 51] films with high order and different crystalline phases through plasma process control. Due to the increase of the order of the film structure, these films have better resistance to light degradation. Generally, the basic method of making the above three films is hydrogen dilution, which controls different dilution ratio and plasma reaction to obtain different crystallization phases. PMSI • H thin film is a kind of stick phase thin film with sparse distribution of silicon nanocrystals with the size of 2 ~ 4 nm and doped with amorphous silicon network (the crystallization ratio is about 2%), and its composition is not affected by the film thickness. PC Si: H film is a kind of film whose structure gradually changes from the incubation phase containing crystal nucleus to the mixed phase of amorphous and microcrystalline [(a + µ C) Si: H phase] with the increase of thickness, and finally reaches the microcrystalline phase (µ c-Si: H phase). Its structure changes with the increase of thin film thickness. µ c-Si: H film is a kind of film that directly changes from very thin incubation phase to microcrystalline phase (MC Si: H phase). PM Si: H thin films generally form macromolecular groups in the gas phase reaction of plasma close to generating dust at high gas pressure, and the reaction groups are deposited into the thin films to form silicon nanocrystals. In order to make the macromolecular reactant reach the substrate smoothly, the temperature gradient between the plasma and the substrate must be controlled. PC Si: H films are generally controlled by adjusting the dilution ratio of hydrogen, and the thickness of incubation layer corresponding to the transformation from amorphous phase to amorphous and microcrystalline mixed phase is used as the basis for process selection. The fabrication of µ c-si:h thin films is carried out at a very high hydrogen dilution ratio. The high hydrogen atom concentration in the plasma is conducive to the formation of µ C: H thin films. The most important control factor of the above three film manufacturing methods with different crystalline phases is how to control the gas-phase reaction in the plasma to form different reaction groups, especially the control of the size of silicon clusters and the content of hydrogen atoms.

Generally, the crystal structure of single junction solar cells with high stability and efficiency is the transition zone from amorphous silicon to microcrystalline silicon. The grains with crystal size of 20 ~ 30 nm are surrounded by amorphous structure. Maintaining appropriate hydrogen content is the key point of making good electrical solar cells. The efficiency of solar cells made of microcrystalline silicon is 10.52% [1.5]

Effectively improving the conversion efficiency can be achieved by multi junction solar cells or tandem cells.

The i layer of the upper battery is a Si: H film, and its energy gap value is about 1 8 E V, used to capture blue light photons. The i layer of medium electric ground is doped by 10%,..:, The energy gap of a-Si Ge: H film with 15% wrong atoms is about 1 6 e v, used to capture green photons; The i layer of the lower battery is a-Si Ge: H thin film doped with 40% and 50% wrong atoms, and its energy gap is about 1 4 E V to capture red photons. Due to the expansion of the application range of the solar spectrum, the stability efficiency is effectively improved. The experimental data show that the area of O. 25 C and FF are 2.5 respectively 30 V 、7. 56 MA / cm2 and 0 70. The stable conversion efficiency can reach about 13%. San yo’s use of a-Si: H / a-Si Ge: H double junction solar cells can also effectively improve the conversion efficiency. The experimental data show that in the area of lcm2, FF is 1.5% respectively 54 V 、10. 8 MA / cm2 and O. 73, the conversion efficiency is 11 7 % 。

In recent years, the international research on multi junction solar cells mainly focuses on the research and development of a-Si: H / µ c-Si: H double junction solar cells. Compared with the three junction structure, the manufacturing process of double junction solar cells is less, the required equipment is cheaper, the process parameters are easy to control, and the current matching is easy to achieve. A-Si: H / µ c-Si: H is an all silicon process without adding additional wrong atoms, which can save the cost of wrong Wan and avoid the problem of wrong pollution. The energy gap of µ C — Si: H thin film can be reduced to about 1 1 eV, so it can effectively absorb long wavelength light and replace the function of a-sige: H film. However, the thickness of P.C Si: H film which can effectively absorb red light photons needs to be about 2 p.m When PECVD l~3 A / S deposition rate of 56 MHz is used to make this film, the production rate will be too slow. Therefore, the current focus is on the production of c-Si: H films by VHF-PECVD at 30 ~ 130 MHz. Due to the high ion density in UHF plasma, the deposition rate can be improved, the ion energy is low and the damage to the films is low, so µ c-Si: H films with good quality can be deposited quickly. Internationally, the research and development achievements of Kaneka and MHI in Japan are the most representative. Kaneka company adds TCO interlayer to a-Si: H / TCO interlayer / µ c-Si: H double junction structure, and the FF is 1.5% respectively under the area of 1 cm2 41 V 、14. 4 mA / cm2 and 72 8. The conversion efficiency is 14 7% 0 in the study of double junction structure, another noteworthy is the a-Si: H / a-sige: H structure of Sanyo company as mentioned above, which is characterized by using a Si: H and a SiGe: Hi layers with a thickness of about 100 nm respectively, reducing the film thickness, saving consumption and improving the production rate, and maintaining the use of the original 13 56 m Hz PECVD system, relative to V H F ­ PECVD system makes it easier to quantify production in a large area.