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).
Figure 1 (a) High-efficiency CIS solar cell structure, (b) high-efficiency GIGS solar cell structure
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%.
Figure 2 – The electrical improvement of the completed CIGS solar cell after plating a very thin NaF film on top of the Mo layer
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.
Figure 3 – Discontinuous CdS films prevent CIGS solar cells from reaching efficiencies higher than 18%
Figure 4 – Comparison of CIGS cell efficiency using various Cd-free materials versus CdS for the buffer layer
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.
At present, the commercialized monocrystalline silicon solar cells have an efficiency of 15%~20%, and the service life of the module is about 20 years; by 2010, it is hoped that the efficiency will be increased to 25%, and the chip thickness will be reduced to 50µm, which will reduce the cost to half of the current one. The module service life is expected to exceed 30 years; by 2030, the efficiency is expected to increase to more than 30%. Due to the current market expansion and high product competitiveness, some large companies are actively investing in the development of new technologies. The main development directions are: ① the improvement of the quality and thickness of silicon chips; ② the improvement of battery efficiency and cost reduction. As for how to improve the conversion efficiency of solar cells (greater than 25%), it has always been the direction of the industry and academia.
Monocrystalline silicon solar cells are currently more than 20% efficient and have been commercialized, as shown in Figure 1. Figure 1(a) is the solar cell Sun Power A-300 developed by Sun Power Company, which is characterized by designing the electrode parts on the same side and the back side>, so that the front side of the cell does not have any shielding area, and its highest efficiency has reached 21.5 %; Figure 1(b) is a solar cell developed by BP Solar, which uses laser to embed the front electrode into the cell to increase the carrier collection effect and improve the efficiency, and the highest efficiency can reach 20.5%; Figure 1 ( c) HIT high-efficiency solar cells developed for Sanyo Company, the highest efficiency can reach 20.1%. In addition, there are several other high-efficiency solar cell structures, including emitter passivated and rear locally diff used (PERL) solar cells, grating solar cells, point-contact solar cells , obliquely evaporated contact solar cells, metal insulating layer semiconductors, solar cells, screen printing (screen printing) solar cells, etc., the monocrystalline silicon solar cells with different structures will be introduced and discussed below.
Figure 1 Commercialized monocrystalline silicon solar cell structure
1. Emitter Passivated Back Local Diffusion Solar Cell In recent decades. The high-efficiency monocrystalline silicon solar cell is most famous for the emitter-passivated backside local diffusion (emitter and rearlocally diffused, PERL) device developed by the University of New South Wales, Australia, and its efficiency is as high as 24.7%, as shown in Figure 2 Show. The structure is surface textured with inverted pyramids, and is also coated with double anti-reflection layers of MgF2 ( n = 1. 38 ) and ZnS ( n = 2. 4 ) to increase the Light absorption to increase photocurrent generation: Passivation of silicon surface with thermal oxide layer. To avoid photocarrier recombination at the boundary; the design of local diffusion on the back forms a back surface field (BSF), which can bounce minority carriers, and due to the design of local diffusion, the majority of carriers are avoided on the boundary. The composite mountain increases the collection of majority carriers; BBr3 and PBr3 liquid sources are used for doping at the metal contact position to reduce the contact resistance.
Figure 2 PERL solar cell structure
2. Buried Contact Solar Cells In the past 15 years, the efficiency of solar cells has been improved a lot. The most striking structure is the buried-contact solar cell (BCSC), which was developed by the University of New South Wales in Australia, and was developed by BP Solar in the United States. commercialized, and its structure is shown in Figure 3. The BCSC solar cell structure combines the advantages of the early PE SC structure and the recent PERL structure. Part of the cell is etched and surface textured, and then the cell surface is diffused and passivated to achieve the best resistance through oxide layers and nitrides. Reflection and surface passivation, and then use YAG-Laser to carve grooves (grooving) on the surface of the battery. The depth of the groove should not exceed 60µm, otherwise it will affect the open circuit voltage of the battery. In addition, in order to increase its mass production speed, The laser grooving process can also be changed to mechanical grooving. Although the use of the mechanical process may result in poor cell uniformity, the subsequent etching can be used to smooth the groove; The secondary diffusion process and the deposition of the back aluminum electrode, and then use the electroplating technology to deposit three metal alloys of inlay, copper and silver on the groove and the back of the battery.
Figure 3 Buried Contact Solar Cell (BCSC) Structure
The solar cell efficiency of the BCSC structure is higher than that of the general commercial screen-printed solar cells, which not only improves the current and voltage output, but also improves the series resistance effect. Since it is easier to absorb the incident light of the blue-ray technology, the current output is improved; in addition, the carrier combination rate of the electrode is reduced, so the voltage output is improved. The fill factor is also increased due to the improved open circuit voltage and lower series resistance. Therefore, the overall efficiency of the battery has been improved a lot, reaching 19.9%.
In addition, the general buried structure can be slightly changed into a double-sided contact (double-sided contact, DSBC) solar cell structure, as shown in Figure 4. It uses lower temperature and lithography process, spin coating liquid diffusion source to reduce the cost of solar cells, which has been proved by experiments. Its conversion efficiency is 17%, which is not as efficient as the general buried structure. The reason is that the liquid diffusion source tends to drive in the trenches. If the manufacturing process is improved, it is expected to reach 20%.
Figure 4 Double-sided buried contact solar cell structure
3. Grating solar cells The grating structure solar cell is one of the solar cells designed in recent years. Its main concept is to use various etching techniques to make the cell surface structure into a grating shape to increase the utilization of the incident light source. A research team used the reactive ion etching (RIE) process to etch the cell surface into gratings with depths ranging from 10 to 30 µm, as shown in Figure 5, and found that the grating structure can better absorb incident light (visible light band). As shown in Figure 6, it is found that the effect of using a 2-dimensional grating structure is better than that of a 1-dimensional structure. If passivation treatment is added, the recombination rate of electron-hole pairs can be reduced, and the short-circuit current density and internal quantum efficiency are also improved. Can be improved a lot.
Figure 5 Grating structure
Figure 6 Comparison of light absorption by 1D and 2D grating structures
Later, other research teams used ZnO as the main material of the solar cell grating structure, used photolithography to define the pattern, and etched the grating depth of about several hundreds of nanometers, as shown in Figure 7 using SEM scanning. The fill factor of the battery can reach 68%, and it is found that it has a better response to the red and blue wavelengths, that is, the grating structure increases the utilization of the incident red and blue wavelengths.
Figure 7 Grating structure using ZnO thin film
4. Thin intrinsic layer heterojunction HIT solar cells The thin intrinsic layer heterojunction HIT (heterojunction with intrinsic thin layer) solar cell was developed by Japan Sanyo Company and has been commercialized. It uses an N-type silicon chip, which is different from the general battery using a P-type. The thickness of the overall HIT cell does not exceed 200µm, and a thin amorphous layer (i/P, i/N layer) is deposited on the upper and lower layers of the N-type silicon chip, and the front and back sides of the cell are both transparent conductive oxide layers (TCO) , which also acts as an anti-reflection layer, as shown in Figure 8. The fabrication of this cell emphasizes that no high temperature diffusion is required to form the PIN interface, and the fabrication temperature is lower than 200 °C, so it is easier to use thinner silicon chips and reduce costs.
Figure 8 Structure of Sanyo’s HIT solar cell
Sanyo started mass production of HIT solar cells in 1999, and in April 2003 published a commercial record of 21.5% high-efficiency HIT solar cells, which achieved the efficiency improvement goal through process improvement, mainly by improving the silicon thin film. quality, making it more efficient. Later, Sanyo company developed a new conductive adhesive with higher conductivity, which can obtain higher filling factor and short-circuit current value, and its conversion efficiency also reached 21.5% (Voc = O. 712 V, lsc = 3. 837 A , FF = 78. 7 % ), and the battery area is 100. 3 cm². In addition, San yo also mentioned that the conversion efficiency of general solar cells will decrease with the increase of temperature, but the efficiency of HIT solar cells has the slowest decrease rate, which is 0.25%/℃, as shown in Figure 9, showing that the Battery performance is good.
Figure 9 Conversion efficiency versus temperature of HIT solar cells
5. Backside Contact Solar Cells In order to reduce the cost of the solar cell process, Sun Power Corporation of the United States has developed a high-efficiency, low-cost back-contact solar cell, as shown in Figure 10. The FZ chip with a resistivity of 2.0Ω/cm and a thickness of 200 µ.m is mainly used to texture the front side of the battery, passivate the front and back sides of the battery with an oxide layer, and reduce the surface reflection with a double-layer anti-reflection layer. The N+/P contact is formed on the backside by diffusing from the side. The aluminum electrode is designed to be finger-shaped and all distributed on the backside to increase the utilization of incident light. The overall cell area is 22 cm², and the highest cell efficiency reaches 23%.
Figure 10 The structure of the back contact solar cell developed by SunPower
In addition, the thickness of the backside contact cell affects the conversion efficiency of the cell. If the thickness of the battery is reduced, the electron-hole pairs will be reduced, because a large number of photons cannot be absorbed; ② The collection efficiency of minority carriers can be increased, and the reduction of the thickness will shorten the moving distance of the carriers; ③ The dark current can be reduced; ④ Reduces the effect of edge current carrying on recombination. The resistivity of the battery is also closely related to the battery efficiency. In addition to affecting the carrier mobility, it also affects the bulk recombination current (bulk recombination current), as well as the parallel resistance and the edge carrier recombination rate. In order to reduce costs, the following chips such as high-quality CZ chips can also be used. These two chips also have the effect of carrier lifetime greater than 1ms, and the battery conversion efficiency can reach more than 19%.
6. Point contact solar cell In 1986, the Stanford University research team developed a point contact solar cell, as shown in Figure 11. The maximum conversion efficiency of the cell can reach 28.3% under the condition of collecting light of 050 suns.
Figure 11 Point contact solar cell structure
The main feature of this structure is to reduce the emitter area on the back of the battery, which is similar to the back contact structure developed by Sun Power. Both positive and negative electrodes are designed on the back. It can be used more effectively; in addition, since the back electrode is composed of several layers of metal, the series resistance of the structure is quite low, so that the output power loss will not be too much, about several percentages. However, the area of the solar cell that was originally designed with this structure is quite small, about 1.21 cm², which makes modularization very difficult.
The main process of point contact solar cells is as follows 2. Using < 100 > FZ grade N-type chip, the thickness is 130 233µm, the resistivity is 100 200Ω/cm, and the diffusion of P-type and N-type is about 1000 Ω/cm. The SiO2 oxide layer is deposited by TCA process in the environment of ℃, the thickness is about 1000Å, and the sheet resistance value is 5~6Ω/port, which also reduces the surface bonding rate. At the same time, the oxide layer can also be used as an anti-reflection layer. The electrode materials of N-type and N-type are all aluminum, and the resistivity of the electrode is (1~2)×10-6Ω/cm.
7. OECO solar cells Oblique plating contact (obliquely evaporated contact, OECO) solar cell is developed by German research institute ISF H (Institute Fur Solar) in recent years, its main feature is the use of special aluminum metal film oblique plating (obliquely evaporated) equipment. Different from the general vertical method, the evaporation process adopts the inclined method, so that the electrode can be plated on the side of the trench without any mask and alignment, and the width of the metal electrode can be easily adjusted. As shown in Figure 12, the front electrode of the battery is placed on the side of the parallel groove. Therefore, the shielding area of the metal electrode is very small. This cell uses an MIS structure, so the thickness of the thin oxide layer must be carefully controlled. At present, the conversion efficiency of such solar cells is 18%~21%. Figure 13 shows the main structure of OECO solar cells.
Figure 12 Evaporation system of OECO solar cells
Figure 13 Main structure of OECO solar cell
8. Metal insulating layer semiconductor solar cells As early as the 1970s, MIS structured solar cells attracted everyone’s attention. Whether in theory or in practical preparation, MIS structured solar cells can make up for the shortcomings of Schottky barrier solar cells. MIS solar cells are also known as MIS solar cells. for low open circuit voltage solar cells. The MIS insulating layer is quite thin, and in addition to controlling the huge dark current, it can also control the type of majority or minority carriers. In the 1990s, the MIS-IL (metal-insulator-semiconductor inversion-layer) solar cell was developed by the German IS FH organization, as shown in Figure 14. The conversion efficiency is 15.7% on a 2 cm × 2 cm FZ chip; on a 10 cm × 10 cm CZ chip. Its conversion efficiency is 15.3%. In order to achieve higher conversion efficiency, the battery conversion efficiency can reach 18.5% by improving the following three parameters: (1) Reduce the loss of surrounding carrier recombination. (2) Use the grid electrode on the front of the battery to improve the electrode impedance. (3) Reduce the carrier recombination loss on the back of the battery.
Figure 14 Solar cell structure of MIS-IL
The following are the main fabrication steps of solar cells with MIS-IL structure: (1) Chemical etching (texturing the surface of the battery, such as a pyramid). (2) The backside of the battery is deposited with an aluminum electrode by an evaporation method. (3) A tunnel oxide layer is grown at 500°C. (4) Use the metal mask to define the pattern and deposit the aluminum electrode on the front side of the battery. (5) The excess aluminum electrodes are removed by etching. (6) Immersion in calcium to increase the positive charge density on the silicon surface. (7) Deposit Si Nx on the entire front side of the cell using PE CVD.
In 1997, the ISFH institute developed a more efficient MIS-N+P solar cell, as shown in Figure 15, mainly by changing the following manufacturing processes: (1) Place the MIS electrode on the N+ diffused emitter. (2) The electrodes on the front and back of the battery are made of aluminum. (3) Double-layer SiNx is deposited by PECVD, which is used as a double-layer anti-reflection layer (DLAR) as a passivation layer. Compared with the MIS-IL structure solar cell, the open circuit voltage and fill factor of the MIS-N+P structure are not improved. The open circuit voltage is increased from 595mV to 656mV, the fill factor is also increased from 74.4% to 80.6%, and the overall battery efficiency is increased from 18.5% to 20.9%. Generally speaking, the production process and structure design of MIS structure solar cells are not difficult, the cost is not high, and the efficiency has reached more than 20%. If it can be improved and researched and developed, it is expected to become the mainstream of monocrystalline silicon solar cells.
Figure 15 Structure of MIS-N+P solar cell
9. Screen-printed solar cells Since the advent of screen printing technology, it has been used in quite a few occasions. In addition to the printing of circuit boards, it is also used in the electrode manufacturing process of solar cells. The process is quite fast, simple and low-cost. At present, many manufacturers of solar cells, in order to increase the speed of mass production, mostly use screen printing technology to print the electrode part on the emitter (30~55Ω/□) instead of the shallow emitter (shallow emitter, 90~ 100Ω/□) to avoid high electrode impedance, the structure of the screen-printed solar cell is shown in Figure 16.
Figure 16 Structure of screen-printed solar cell
Heavy doping of the emitter will reduce the short-wavelength response, and will make the emitter saturation current higher, which will make the solar cell less efficient. Therefore, in order to increase the efficiency of solar cells, a high sheet resistance value of the emitter can be used to provide an effective emitter surface passivation. Usually screen printing is a part of the solar cell manufacturing process, which belongs to the latter part of the solar cell manufacturing process. Aluminum-containing glue is often used. The aluminum glue is printed on the back of the battery by screen printing, and is placed in a 200 ℃ environment first, and then removed. After removing excess water, the silver-containing jelly is printed on the anti-reflection layer of the battery, and then the battery is put into the furnace tube (CIR or RTP furnace tube) for sintering, thus completing the production of screen-printed solar cells. Using screen-printed solar cells, the efficiency can exceed 18%.
