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.

The types of high-efficiency thin-film polycrystalline silicon solar cells are classified by structure, and are roughly divided into natural surface texture and enhanced absorption with back reflector (NSTAR) solar cells and P-i-N tandem solar cells The following two categories will be introduced and discussed in detail.

1. Surface texture/back reflection enhanced absorption (N STAR) solar cell
The N STAR battery will be mainly published by the Japanese solar cell factory Kanaka Corporation. The company has many years of rich experience and excellent technology in this battery structure. The back reflection layer of the first generation NSTAR battery is not textured, and then the back reflection layer of the second generation battery is textured, and then to the third generation battery The cell incorporates light trapping technology to increase its efficiency from 10.7 % to 14.7 %, significantly improving the conversion efficiency.

1) NSTAR solar cell structure
The main structure of the NSTAR cell is glass/back reflector/NiP polysilicon/indium tin oxide (glass/back reflector/NiP poly-Si/ITO), in which the active i layer is a low temperature plasma chemical vapor Deposition method (plasma-enhanced chemical vapor deposition, PECVD).

Figure 1 shows the structure of the first-generation NSTAR thin-film polycrystalline silicon solar cell. One of the characteristics of the cell is that the surface presents a natural textured structure. The uppermost surface structure has a leaf-like shape. The cell with a thickness of 4µm has a roughness of 0.12µm. It is found by XRD measurement that the thin film polysilicon has a columnar structure and a preferred orientation of (110); its crystalline volume percentage is nearly 90% determined by ellipsometry analysis. Figure 2 ( a ) shows the structure of the first generation NSTAR, which is mainly characterized by natural surface texture and a flat back reflection layer; Figure 2 ( b ) shows the structure of the second generation NSTAR, the back reflection layer After texture treatment, it can improve its light absorption efficiency.

The third-generation NSTAR structure is to add an interlayer to increase the light trapping effect. As shown in Figure 3, its structure is an amorphous silicon/microcrystalline silicon (a-Si/μc-Si) PiN stack with an interlayer. The battery has an intermediate layer between the upper a-Si and the lower μc-Si.

2) Manufacturing steps of NSTAR solar cells
The experimental steps for the first-generation NSTAR structure (Figure 1) are described below. Typical NSTAR cell structure is ITO(800nm)/P-μe-Si:H(20nm)/i-poly-Si(4.7μm)/N–poly-Si(300nm)/P”-poly-Si(300nm) )/glass, the production steps are as follows.

The PECVD conditions for making P–poly-Si are RF power density=40mW/cm², H₂/SiH₄=40, B₂H₆/SiH₄=10^-6, pressure is 1 Torr, temperature is 200°C, and brick concentration is 10¹⁶cm^-3. The PECVD conditions of N⁺-poly-Si are RF power density=200mW/cm², .H₂/SiH₄=20, PH₃/SiH₄=10^-2 and pressure 1Torri, then a back reflection layer is formed on the glass substrate, and then PECVD is used to deposit The N-type Si thin film is deposited on the back reflection layer. Next, the i-poly-Si film is also deposited on the N-type Si film by PECVD, and then the P-type Si film is deposited to form a P-i-N junction. Indium tin oxide (ITO) is deposited on top of the solar cell as a transparent conductive electrode. The Ag grid electrode is made at the top. The maximum temperature for all manufacturing processes is 550°C.
The third-generation NSTAR structure is shown in Figure 3.

3) Efficiency of NSTAR solar cells
Japan’s Kaneka company has developed a stable 8% amorphous silicon single-junction large-area solar cell module through advanced process equipment, the size of which is 910mm × 55mm. Since the fall of 1999, the company has been capable of mass production of about 20MW of solar energy per year. Battery. When the company developed its next-generation thin-film silicon solar cells, the company focused on thin-film polysilicon and amorphous silicon tandem solar cells. In 1996, Meier of the University of Neuchatel invented a-Si/mc-Si stack cells with 7% microcrystalline silicon (c-Si) cells and an initial efficiency of 13%. In 1997, Kaneka used PECVD to manufacture low-temperature thin-film polycrystalline silicon solar cells on glass substrates with a cell thickness of 2.0 μm and a conversion efficiency of 10%. The company’s current focus is on improving the efficiency and mass production of a-Si/poly-Si stacked modules. Figure 4 shows the development timeline of Kaneka’s silicon thin-film-based solar cells and modules. It can be seen from the figure that the company is working on hybrid (HYBRID) solar cells (i.e. a-Si/poly-Si stack cells). In terms of mixed use), it has reached a state of stable production for many years.

Figure 5 shows the photovoltage characteristics of a 2.0 μm thick NSTAR cell [with Japan Quality Assurance (JQA) as the measurement standard], its intrinsic efficiency is 10.7%±.5%, and its aperture efficiency is 10.1% ±.5%, open circuit voltage (Voc) is 0.539V±0.005V, short circuit current, current density (Jsc) (essential) is 25.8±0.5mA/cm², short circuit current density (Jsc) (pore size) is 24.35±0.5mA /cm², the difference between the intrinsic efficiency and the aperture efficiency is that the aperture has a silver electrode on the ITO.

Figure 6 shows an a-Si/interlayer/poly-Si hybrid cell with an area of ​​1 cm², which can achieve an initial efficiency of 14.7% under optimized deposition conditions.

As shown in Figure 7, the large-area 910mm×455mm hybrid solar cell module mass-produced by Kaneka in 2004, its initial efficiency can reach 13.5% [Voc=137V, Isc=0.536A (Jsc=14.0mA/cm²), F_F =0.706].

2. P-i-N tandem solar cells
1) Introduction of P-i-N tandem solar cells
Recently, hydrogenated amorphous silicon (a-Si:H) single-junction solar cells have achieved efficiencies of up to 13% through continuous optimization of materials, interface fabrication, and device geometry. But in any case, because the band gap of a-Si devices is 1.7~1.8 eV, the average efficiency of a-Si can only reach 14%~15% according to theoretical calculations, and in practical applications, a-Si Si has obvious photo-induced degradation, and it has not been completely solved. To address the a-Si efficiency barrier, a stacked structure can be used in combination with narrow-gap materials to make the most of the solar radiation spectrum.

The advantages of a tandem solar cell integrating a Si and poly Si are as follows:
(1) Combining small energy gap poly-Si with high energy gap a-Si.
(2) The mature hydrogen passivation poly-Si thin film growth technology can be applied.
(3) There is no Steabler-Wronski effect at the underlying poly-Si junction.
(4) Low cost.
The efficiency of a-Si/poly-Si quadruple tandem solar cells can be as high as 20%.

2) Upper layer a-Si unijunction cell
The structure of a-Si/poly-Si four terminal tandem solar cell is an upper a-Si cell and a lower poly-Si cell. 5.23 shows an a-Si single heterojunction solar cell with the structure Glass/T CO/ P µc-SiC/ P a-SiC/a-SiC/ ia-Si/N µc-Si/ ITO/ Ag, in which the textured Glass/TCO structure has an optical confinement effect.

In this solar electric tree planting, the gas source is used as the plasma excitation gas, and the ECR (electron cyclotron resonance) plasma-enhanced CVD method is used for deposition at a low temperature of 180°C and a microwave power of 200 W fl£ A P 11.c-SiC electrode layer with an energy gap of 2.7 eV and a high dark conductivity of 0.1 S/cm was fabricated; then PaSiC/a-SiC/ia was formed by means of RF PECVD -Si/N PC-Si heterojunction structure; then ITO with a thickness of about 80 nm was fabricated by electron beam evaporation; finally, a silver backside electrode was used to provide high photon reflectivity. The device processes are all carried out at an average temperature of 200°C (except for the C P µc-SiC electrode layer). Because the Pµc-SiC layer is grown by ECR PECVD, the TCO layer is bombarded by dense hydrogen plasma in ECR plasma. So there are serious flaws. To eliminate this disadvantage, the TCO layer is overlaid on a plasma-resistive ZnO layer.

Figure 9 shows the optimized light output characteristics of a-Si single heterojunction solar cell, its efficiency is 12.3%, Voc=0.916 V, j SC = 19. am A/cm² and F_F = 70.6 % .

3) Lower poly Si battery
The underlying cell structure is ITO/P u c-SiC/P a -SiC/N poly-Si/N u c-Si/Al. Among them, the poly-Si substrate is a cast-wafer with a thickness of 250-300 μm and a resistivity of 0.5-5 Ω/cm. The fabrication steps of this cell are as follows: first, an N μc-Si layer is deposited on the backside of the acid-etched poly-Si wafer substrate by conventional methods to provide BSF effect between N-type poly-Si and Al electrodes and good Ohmic contact; a p-type a-SiC buffer layer is deposited on a clean poly-Si surface. The fabrication temperature is about 100 °C, and the microwave power is 200 W; then, at a higher temperature of 250 °C and 320 A P-type μc-SiC layer was deposited with a microwave power of W; finally, an ITO film with a thickness of 800 Å was deposited on the substrate by electron beam evaporation as the anti-reflection layer and front electrode.

4) a-Si and poly Si four-terminal stack cells
Figure 10 shows the structure of a-Si and poly-Si four-terminal stacked cells; Figure 11 shows the light output characteristics of a-Si and poly-Si four-terminal stacked cells. This four-terminal stack cell uses a-Si as its upper cell, and its intrinsic layer (i-layer) thickness is 100 nm; another P µc-SiC/N poly-Si heterojunction device is used as the lower cell. Among them, the upper cell efficiency is 7.25% ( Voc= 0.917 V , Jsc = 10.4m A/cm² , F_F = 76.0% ), while the lower cell efficiency is 13.75% (Voc = 0.575 V , Jsc = 30.2 mA /cm² , F_F = 79. 2 % ), so the total conversion efficiency of the entire stacked cell is as high as 21. 0 %.

Polycrystalline silicon solar cells can be divided into two types: bulk polycrystalline silicon (bulk multicrystalline) and thin-film polycrystalline silicon (thin-film polycrystalline). This section first introduces bulk polycrystalline silicon solar cells.

Monocrystalline silicon solar cells have disadvantages such as high cost and small wafer size. Polycrystalline silicon solar cells are another choice for solar cells due to their advantages of reducing cell cost and increasing the use area. However, polysilicon defects and potential barriers at the grain boundaries cause solar cell short-circuit current and conversion efficiency to decrease in order to reduce these negative effects. In fact, some methods need to be developed to tune these grain boundaries and to use ITO (indium-tin-oxide) films [32] as the top electrode, which have high conductivity and high visible light transmittance.

Generally speaking, the fabrication steps of bulk polycrystalline silicon solar cells are as follows: substrate cleaning (such as organic cleaning), surface polishing (particle removal), preferential grain etching, doping of PN interface (such as POCI, doping Miscellaneous), back metallization treatment (such as Al back electrode), ITO front electrode treatment, etc. The main production process and conditions are roughly as follows: use a 10 cm × 10 cm polysilicon substrate. The thickness is about 350um, the resistivity is in the range of 1~50/cm, the life of the minority current is more than 5,us, and the grain size varies from 5μm to 50um. The average size is 16.9um; the polysilicon substrate uses various etching methods. Preferential grain etching is carried out, then phosphorous (phosphorous) doping is used to form the interface, and X-type emitter interface is formed by diffusion. The ITO film is prepared by magnetron sputtering. Figure 1 shows the structure of a bulk polycrystalline silicon solar cell.

1. Surface texture of bulk polycrystalline silicon solar cells
For solar cells, silicon surface texturing is a very important key technology. Especially polycrystalline silicon solar cells. Its main purpose is to reduce surface reflection and increase cell efficiency, and surface texture can reduce light reflection from 35% to 50% to 20% to 25%. The traditional monocrystalline silicon solar cell etching method cannot be directly applied to the polycrystalline silicon surface because it has grains with different crystal orientations. Therefore, it is important to study the surface texture of polysilicon. There are different methods such as dry phase wet etching for the surface texture process of polysilicon crystal clusters, and the etched effects are also different.

Generally speaking, the wet method of silicon texture is to use HF-H NO3-based solvent, or use alkaline water-based solvent machine containing inorganic or organic salt (salt). A hemispherical structure is formed on the silicon surface by means of alkaline solvent etching. The anisotropic texture (anisotropic texturing) etched on the silicon surface by means of an acidic solvent is usually a pyramid or a tilted pyramid. For example, Park et al. use the spray method. The etching solution is a combination of HF-HNOx (1:20) based solvent, sulfuric acid (H2SO4), NaNO2 and other additives. Another method of silicon texturing is dry etching, such as reactive ion etching (RIE) or electron discharge etching.

1) Wet etching method
The wet etching method introduced here is a negative potential (NPD) method, in which the dissolution of silicon occurs only when the potential is lower than -10 V, while the surface texture of the silicon and the current time and potential are very important. Clear relationship, for single crystal silicon, increasing KOH concentration and negative potential can reduce etch time and roughness.

Figure 2 shows the current-time relationship curve of the NP D method in the electrolyte with a low alkaline concentration of 24% (mass fraction) KOH, and the potential range is -30~10 V. The greater the negative potential, the current will increase: 0.75 A at -10 VF, 2 A at -20 V, and 3 A at -30 V. In addition, Figure 2 shows that the current recordings at a 10 V and a 20 V were stable, but there was a significant reduction in cathodic current, probably due to the elimination of defect areas. The etch rate of polysilicon increases with the increase of the negative NPD potential, which is about 15.5 µ.m/h at 10 V, and can reach 60 µ.m/h at 30 V. However, at low potential and low etching rate, the defect area cannot be completely removed, resulting in a stable etching state within 600 s.

Figure 3 shows the SEM micrograph of polysilicon textured at -30V for 600 s by NP D method at a low alkaline concentration of 24% (mass fraction) KOH. Figure 3(a) shows the two main crystallographic orientations (100) and (110) of the polysilicon substrate resulting in two textured surface metallographic phases; steps.

Figure 4 shows the current-time curves of NPD at different alkaline concentrations (20%~50% KOH). The figure shows that 20% KOH can get the best cathode current of about 3.75 A at a 30 V; when the electrolyte concentration is slowly reduced to 24% KOH, the current is almost constant. However, when the electrolyte concentrations were 32%, 38% and 50% KOH, the maximum current values ​​were 3.4~3.6 A, and the final currents dropped significantly to 1.5 A, 1.3 A and 1.5 A. 1 A, so increasing the KOH concentration will cause the current to decay rapidly.

Figure 5 shows the effect of alkaline electrolyte concentration on polysilicon surface; the conditions are – 30 V, 120 so Figures 5 Ca), C b) surface roughness under the condition of 24 % KOH It is obvious that only some areas have jagged damage Figures 5(c) and (d) are the anisotropic textured surfaces obtained at a concentration of 32 % KOH, and the jagged damage area is completely removed; Figures 5 (e) and (f) are 38 % and 50 %, respectively. % KOH. Its textured surface is not a general metallographic phase, but shows grain boundaries.

Figure 6 shows the light reflection spectrum of the polysilicon substrate after etching at -30 V and 120 s under the conditions of KOH electrolysis concentration of 24%, 32% and 38% by NPD method. The top curve in the figure is the polished polysilicon. The graph shows that the reflectance has a maximum value at a wavelength of 0.4µm and a minimum value at a wavelength of 0.72µm. The minimum reflectance of the obtained polysilicon textured surface at 32 % electrolyte concentration is 25.7 % (at 0.72 μm wavelength), while at 24 % and 38 % KOH concentration, the minimum reflectance is 28.3 % and 28.3 %, respectively. 34.7%. The above results show that the best surface metallography and the smallest reflectance can be obtained when the electrolyte concentration is 32 %. In addition, the main advantages of NPD texturing are the use of non-toxic electrolytes and a fast texturing process.

2) Dry etching method
Wet etching can etch a uniform surface on monocrystalline silicon, but on polycrystalline silicon wafers, due to the changeable crystal orientation, the uneven surface is caused, so that the polycrystalline silicon conversion efficiency cannot achieve the best effect. Inomata et al mentioned M. Takayama et al used NaOH solution to etch the surface in the fabrication process of 15 cm × 15 cm large-scale solar cells, and the maximum conversion efficiency obtained was 16. 4 % (Is c = 7. 96 A , VOC = 0. 611 V , FF = 0.759 ) , while Y. lnomata can obtain 17.09 % Usc = 8.136 A , V cx: = 0.621V , A = o.7615) conversion efficiency. The reason is that wet etching cannot effectively reduce surface reflection, because different crystal orientations of polysilicon surface have different etching rates, and when photolithography technology is used, wet etching is not suitable for mass production. Therefore, it is more suitable to use the RIE method to form a low-reflection polysilicon surface as a texture technology for large-area high-efficiency polysilicon solar cells.

The gas, gas flow, reaction pressure and RF power introduced in the RIE fabrication process will affect the results of polysilicon etching. The following is a brief introduction to the RIE method proposed by Y. Inomata, the result of which is to effectively form a uniform pyramid-like structure on the polysilicon surface, and this method can easily control the surface by controlling the flow rate of chlorine gas (Cl2). aspect ratio. The results shown in Figure 7 are a comparison of RIE dry etched and previously NaOH wet etched surfaces, with a clear reduction in reflectivity. The research team found that the maximum short-circuit current and the maximum open-circuit voltage can be obtained under the condition of a chlorine flow rate of 4.5 sccm.

2. Battery manufacturing and characteristics
Figure 8 shows the structure of a high-efficiency bulk polysilicon cell. The substrate of the battery is P-type polysilicon 15 cm×15 cm provided by the company, the thickness of the substrate is 270 µm, the surface texture is formed by passing 4.5 chlorine gas through RIE, and the front surface emitter is formed by diffusing PO Cl3 doping source. B SF is formed by screen printing and firing method of aluminum bonding, and Si N film is deposited by PE CVD as bulk passivation and anti-reflection layer, under N2/H 2 600℃ Bottom annealing, the top contact electrode (iron/silver) is patterned by vapor deposition and lift-off method, and then copper plating layer is used as the uppermost metal. Table 1 shows the efficiency performance of this solar cell, under two different emitter sheet resistances (52.0./□ and 89.0./□), it is clear that the fabrication using RIE yields a higher wettability than the previously mentioned NaOH Etch better short-circuit current and open-circuit voltage, and its highest efficiency can reach 17.09%, as shown in Figure 9.

Generally speaking, solar cell structure design mainly considers two directions: one is to improve efficiency; the other is to reduce manufacturing cost. In order to improve efficiency, factors such as material, light absorption, junction depth, anti-reflection layer, surface passivation, texture, thin film stacking, etc. need to be considered. Through various considerations and analysis, the optimized photovoltaic (photovoltaic, PV) characteristics can be obtained. . The following mainly discusses the light storage, thin film stacking, hydrogen passivation and impurity adsorption of thin film polycrystalline silicon solar cells.

1. Light trapping technology
Light trapping technology is one of the main methods to increase the efficiency of solar cells. Because thin-film solar cells are only a few microns thick, they cannot absorb enough incident light and thus cannot obtain sufficient photocurrent. The trapped light can prolong the optical path of the incident light, so that the incident light can increase the multiple reflections of the light in the solar cell and the degree of absorption of the light by the active layer.

Light trapping technology often uses the following four methods:
(1) Surface texture reduces frontal reflection.
(2) Use a flat high reflectivity material as the bottom reflective layer.
(3) Internal light trapping.
(4) Add anti-reflection layer (AR coating).

Usually, the light-receiving surface of a solar cell is a flat mirror surface. If light is formed through surface texture, there will be multiple reflections on the active layer to reduce the reflection of light on the surface, so that the travel distance of the light is lengthened and the absorption of light by the battery is increased.

Figure 1(a) shows the bottom layer with a flat and high reflectivity reflective layer, and Figure 1(b) shows the bottom layer as a reflective layer through texture to increase its reflective optical path. In addition, the reflective layer can also be used as the back electrode of the solar cell. If the thickness of the reflective layer is less than 4µm, the pyramid structure cannot be formed, and the light trapping effect cannot be achieved. Therefore, the thickness of the reflective layer has an inseparable relationship with the trapping of light.

Another light trapping technology is called internal light trapping. This structure is to sandwich a transparent layer in a stacked solar cell, so that the incident light can be reused in the upper cell (amorphous silicon) to reduce the light attenuation of amorphous silicon. effect, thereby improving the efficiency of solar cells. Figure 2 shows the technical structure of an interlayer with internal light trapping. Figure 3 is a conceptual diagram showing the internal light trapping.

Adding anti-reflection film is another option for light trapping technology. For example, Si has a refractive index of 3.50 to 6.00 in the wavelength range of 400 to 1100 nm, so the reflection loss is 54% in the short wavelength range and 34% in the long wavelength range. In order to reduce the reflection loss, an anti-reflection coating (anti-reflection coating) is made of transparent materials with different refractive indices. The relationship between the optimal refractive index n and thickness d of the anti-reflection film and the wavelength λ of the incident light is:
λ=4nd·n²=n_sin_o
In the formula, n_si is the refractive index of Si; n_o is the refractive index of the environment. In the air environment, no=1, so the optimal refractive index of the anti-reflection film is n=(n_si) 1/2. Table 1 shows the refractive index of common antireflection coating materials.

2. Stack structure
Although the efficiency of solar cells made of thin-film polysilicon can reach 10%, the device efficiency is still lower than that of polysilicon solar cells made of bulk materials, so a breakthrough must be made in the structure of solar cells. Therefore, a two-layer or three-layer (hybrid) solar cell structure is used to achieve the desired efficiency. In order to make all the fabrications can be made at low temperature, the fabrication process of multilayer films from Player, i-layer to N layer are all made by CVD, forming a PiN stacked multilayer film structure. Reduce costs and simplify production steps. Because the solar cells of the multilayer thin film structure can absorb different incident light, and can use the existing materials and process methods to achieve better device characteristics, the use of stacked multi-layer solar cells has the following advantages:

(1) It can absorb a wider spectrum and use incident light as efficiently as possible.
( 2 ) A higher open circuit voltage (Voc) can be obtained.
(3) It can suppress the photo-degradation of solar cells.

In addition, the bottom layer of the stacked structure can use polysilicon (poly-Si) to absorb infrared wavelengths, and the upper layer can use amorphous silicon (Ca-Si) to reduce the surface recombination rate of polysilicon, so the current leaking from the rough surface of polysilicon will Reduced due to amorphous silicon layer.

Using the solar cell with this stacked structure can certainly improve the efficiency of the solar cell. If a transparent interlayer can be added to the structure, and applied and improved to achieve the internal light trapping effect, the efficiency of the solar cell can be further improved. effectiveness. Therefore, thin-film solar cells with stacked structure will be the focus of future development.

3. Hydrogen passivation of polysilicon
In order to reduce the intrinsic cost of solar cells, most low-cost polysilicon or thin-film polysilicon use substrates with lower intrinsic cost, resulting in poor crystalline quality and high impurity and defect content, which will seriously affect the diffusion length of charge carriers. Thus, the efficiency of the battery is greatly limited. An effective method to change this poor electrical quality of polysilicon is hydrogen passivation. This method uses hydrogen to remove impurities and defects in silicon and passivate grain boundaries. There are several methods for introducing hydrogen into silicon. The most common methods are diffusing hydrogen in plasma or depositing layers such as silicon nitride. Since these deposited layers are usually formed by plasma-enhanced chemical vapor deposition (PECVD), they may cause plasma-induced damage; recently, the use of microwave remote plasma oxygen passivation has been developed. (microwave-induced remote plasma hydrogen passivation, RPHP) method to avoid this problem. Another technique is the implantation of hydrogen ions, which involves implanting high concentrations of hydrogen near the surface of the silicon.

Commonly used polysilicon hydrogen passivation methods are as follows:
(1) SiN:H by PECVD, hydrogen diffusion from the silicon nitride deposition layer.
(2) Low-energy hydrogen ion implantation (HID) is performed with Kaufman ion swimming.
(3) Remote plasma hydrogen passivation (CRPHP).

In the high temperature fabrication step, using Si H4 as the supply gas for the PECVD deposition of silicon nitride, hydrogen can be diffused into the silicon layer at the same time, and the formed silicon amide layer (SiN layer) can be used as the front anti-reflection layer. . The main disadvantage of this method is that it requires high temperatures in excess of 600°C. The low-energy hydrogen ion implantation method can control the high concentration of hydrogen in the silicon towel, however, the backside of the silicon chip will cause defects due to ion bombardment. As for the remote plasma hydrogen passivation method, microwaves are used to generate low-temperature plasma remotely, and the low-temperature plasma is diffused to the test piece, so there is no ion bombardment phenomenon. The equipment structure is shown in Figure 4.

Remote plasma hydrogen passivation method equipment The remote plasma hydrogen passivation method is a plasma-enhanced annealing technology. The preferred production process parameters are to pass 40 sccm of hydrogen (H2) and 50 sccm of argon at 400 °C. (Ar) and 10 sccm of oxygen (O2) for 1 h at a pressure of 1 mbar and a microwave power of 200 W. As shown in Figure 5, the R P HP method has the best improvement effect on the effective diffusion length Leff of the minority carriers in the silicon crystal. Especially on grain boundaries.

4. Impurity adsorption of polysilicon
To improve the efficiency of polycrystalline silicon solar cells, effective impurity adsorption is widely used in solar cell processes. For polysilicon materials, phosphorus (P) adsorption and aluminum adsorption are the two most commonly used and effective warm-adsorption techniques. It can greatly improve the electrical properties of polysilicon. Phosphorus adsorption can use POCl3 diffusion to diffuse phosphorus atoms into polysilicon, while aluminum adsorption can be performed by electron beam evaporation of a 0.5 µm aluminum film on the silicon surface, followed by a process of high temperature annealing for several hours.

5. Polysilicon Film Deposition Technology
One of the main purposes of using thin-film polysilicon in solar cells is to improve efficiency and reduce costs. At the same time, the main process used in the solar cell can follow the existing mature technology of semiconductor technology, so it is highly compatible with general semiconductor technology in equipment and mass production. The two main thin film growth techniques required for polycrystalline silicon solar cells are described below:

(1) Vapor phase growth method.
( 2 ) Solid phase crystallization method.

Generally speaking, the low temperature polysilicon thin film process is mainly two kinds of vapor phase growth method and solid phase crystallization method. For the cost-effectiveness and performance improvement of solar cells, the following three items are very important:
(1) Manufactured at low temperature so that low-cost substrates can be used.
(2) Develop large-area technology, such as the application of amorphous silicon (a-Si: H) production technology.
(3) Reduce film thickness and develop optical enhancement structures. Vapor deposition is the most widely used technique for making polysilicon thin films, usually using plasma-enhanced or catalytic (hot-wire CVD) methods.

1). Vapor phase growth method
The vapor phase growth method is a more expensive and complex method for depositing silicon because of the large amount of precursor and diluent gases used in the vapor phase growth process. The main reason for using the vapor phase growth method is that a high-quality silicon thin film can be obtained.

The process methods of the vapor phase growth method are roughly as follows:
(1) Atmospheric pressure CVD (atmospheric pressure CVD, APCVD).
(2) Low pressure CVD (low pressure CVD, LPCVD).
(3) Rapid thermal CVD (rapid thermal CVD, RTCVD).
(4) plasma enhanced CVD (plasma enhanced CVD, PECVD).
( 5 ) Electron cyclotron resonance CVD (electron cyclotron resonance CVD, ECRCVD) o
(6) Hot wire CVD (hot-wire CVD, HWCVD).

2). Solid phase crystallization
It will be introduced that Japan Sanyo Company uses solid phase crystallization (SPC) to grow polycrystalline silicon thin films and use them on solar cells to improve conversion efficiency and reduce costs. The experimental results show that the solar cell has high light sensitivity in the long wavelength region, and there is no light-induced degradation phenomenon after exposure to light.

The SPC method uses a PECVD a-Si film as a pre-step for forming a polysilicon film. Figure 6 is a schematic diagram of the SPC method, which is mainly composed of two steps. The first step is to deposit a phosphorus-doped a-Si film on the substrate by PECVD; the second step is to recrystallize the phosphorus-doped a-Si film into an N-type polysilicon film using a low-temperature annealing method at about 600 °C.

This SPC method has the following three main advantages:
(1) The production process is very simple.
(2) The production process temperature is low.
(3) Suitable for making large-area solar cells.

At present, more than 95% of commercial solar cells are made of silicon. The advantages of silicon are mainly due to its abundant raw materials, mature process technology and no toxicity. The cost of silicon wafers accounts for 40% to 60% of the entire process, so material cost is an important issue. Both single-crystalline silicon (sc-Si) and multi-crystalline silicon (mc-Si) wafers are widely used, especially polycrystalline silicon wafers have great application potential due to their low cost advantages. Gradually increasing trend yang. The conversion efficiency of commercial polycrystalline silicon solar cells is generally 12% to 15%, and can be as high as 17% with more sophisticated solar cell designs. The potential of polysilicon is very high, and its efficiency has been increased to about 20% in the laboratory recently, which greatly increases its commercial viability.

The performance of polycrystalline silicon solar cells is mainly limited by the minority carrier recombination rate. During the crystallization process, the material will produce different defect structures, which determine and limit the efficiency of the cell. Generally speaking, dislocations and intra-grain defects, such as internal impurities, atomic clusters or precipitates, are the main reasons for the recombination of carriers; for relatively large grains on the centimeter scale, The grain boundaries become irrelevant.

Most of the cost of solar cells comes from the cost of substrates and manufacturing. Early solar cells are dominated by monocrystalline silicon. Because it can provide good conversion efficiency and use mature semiconductor manufacturing technology, it is generally used under non-cost considerations. In non-electric applications or artificial satellites or scientific experiments that require small area and high power generation, such as automobiles, etc. If it is to be commercialized and popularized, the cost of the product is an urgent problem that we need to solve, so the solar cell technology of polysilicon and amorphous silicon came into being.

There are two types of polycrystalline silicon solar cells: bulk polycrystalline silicon (bulk silicon) and thin-film polycrystalline silicon (thin-film pol e silicon). Because thin-film polycrystalline silicon has the advantages of reducing the dependence on wafers and reducing cost, so polycrystalline silicon thin-film solar cells are used. is an important trend yang. Because the thickness of the light absorbing layer of the solar cell is 2~3 times the thickness that the sunlight can absorb, and most of the electron-hole pairs act at the interface, it is only necessary to ensure that the grain size of the polysilicon film is larger than that of the film. thick, so that there are more minority carriers for effective power generation at the interface than short-lived carriers that flow into the grain boundaries, thereby suppressing the influence of grain boundaries, and then using an inexpensive substrate to make tandem silicon thin films (tandem thin films) -film) structure to form thin-film solar cells, and large-area solar cell modules can be fabricated.

Since the energy crisis occurred in the 1970s, people have begun to apply solar cells to general livelihood purposes. At present, in countries such as the United States, Japan and Israel, solar energy installations have been widely used and are moving towards the goal of commercialization. Among these countries, the United States established the world’s largest solar power plant in California in 1983, which can generate up to 16 MW of electricity. South Africa, Botswana, Namibia and other countries in southern Africa have also set up programs to encourage the installation of low-cost solar cell power systems in remote rural areas. The most active country in the implementation of solar power generation is Japan. In 1994, Japan implemented the subsidy and incentive method to promote the “mains parallel solar photovoltaic power system” of 3000 W per household. In the first year, the government subsidizes 49% of the funds, and the subsidy thereafter decreases year by year. “Mains parallel type solar photovoltaic power system” is when the sunshine is sufficient, the solar cell provides electricity to the load of the home, and if there is excess electricity, it will be stored separately. When the power generation is insufficient or does not generate electricity, the required power is provided by the power company. By 1996, 2,600 households in Japan had installed solar power generation systems, with a total installed capacity of 8 MW. A year later, there were 9,400 installations, with a total installed capacity of 32 MW. In recent years, due to the rising awareness of environmental protection and the government’s subsidy system, it is estimated that the demand for household solar cells in Japan will also increase rapidly. In terms of industry, the total output of solar cells in Japan in 1999 was 86 MW, and in 2000 it had increased to 120 MW, ranking first in the world for two consecutive years. Recently, many Japanese solar cell manufacturers, such as Sharp Corporation and Mitsubishi Heavy Industries, have expanded their production plants. In the United States, the “Million Roofs Solar Power” plan proposed by former President Clinton is planned to be completed before 2010. The construction of 1 million solar power generation systems has been completed. In addition to Japan and the United States, Detong also started in 1990. Begin to implement the home-based plan, each household’s installed capacity of solar power is 15 kW, and the government will subsidize 70% of the funds. By 1995. 2,250 households have installed solar energy systems, with a total installed capacity of 5.6 MW. In addition, the Dutch government expects that the total installed capacity of solar energy systems can reach 1,450 MW in 2020. Other countries, such as Switzerland, Norway and Australia, have also implemented plans to install thousands of solar cells each year. in Taiwan, China. At present, the main manufacturers of solar cells include Guanghua, Motech and Shilin Electric. Guanghua Development Technology Co., Ltd. has been mainly producing amorphous silicon solar cells since 1988. Mainly used in super-eliminating electronic products, such as sub-tables, calculators, etc.

In 1999, Moody Corporation began to set up a factory in the same district of Tainan Science and Industry. It mainly produces solar cells of polycrystalline silicon and monocrystalline silicon. Shihlin Electric has also sent a research and development team to the United States for training to learn the manufacturing and packaging technology of solar panels used by satellites. At the same time, after the successful launch of China Satellite 1 in 1999, it further invested in the research and development of solar cells for people’s livelihood. In addition, the Institute of Materials of Industrial Technology Research Institute has also successfully developed the manufacturing and packaging technology of solar cells, and transferred the technology to Moody Corporation and Shihlin Electric Company to promote the solar power generation business. In recent years, Taiwanese manufacturers have gradually become interested in investing in the solar cell business. The main reason is that in addition to the shortage of supply in the international market, another factor is that Taiwan has vigorously promoted solar cell power generation since 1999, and started to promote various incentives. As a result, the number of companies investing in this business has also increased significantly. At present, there are still some difficulties in the implementation of solar power generation in Taiwan. The main reason is that it is obviously much more convenient to apply for utility power compared to the application procedures for general utility power and solar power generation, and the installation of solar power generation must first be invested in a funds. Based on economic considerations, it is indeed difficult for the general public to accept. even so. Look at it from another angle. Taiwan has favorable conditions such as abundant sunshine, sound development of semiconductor and power electronics industries, and strong official promotion, plus possible tourism, crisis, and popularization of environmental awareness. The solar power generation business indeed has a very large space for development in Taiwan. I believe that as long as the manufacturing cost can be greatly reduced. It can occupy a place in the field of solar power generation in the world.

In addition, the development and use of solar energy. The inability to generate electricity at night is a major disadvantage of solar cells. But for this shortcoming, scientists use two ways to overcome. The first way is to convert sunlight energy during the day into other forms of energy for storage, such as batteries, flywheel devices, pumped storage power plants, etc., and release the stored energy at night. Another way is the “satellite solar power station” (SPSS) that the United States and Japan are working on Places, such as near the equator, launch satellites with solar cells or thermal power generation systems, and use artificial satellites to absorb solar energy in space to generate electricity. Due to the avoidance of factors such as day and night, temperature difference and climate, artificial satellites can continuously and stably receive solar energy, convert it into electrical energy, and then transmit it back to the earth in the form of microwaves. After being received by the earth’s microwave receiving station, it is converted back into electrical energy. delivered to various places. Currently. Because scientists continue to study. Coupled with the advancement of semiconductor industry technology, the efficiency of solar cells is gradually increasing, and the unit cost of power generation systems is also decreasing year by year. Therefore, as the efficiency of solar cells increases and the cost decreases, the use of solar cells will become more and more common.

The solar cell technology has been transferred from the space technology application in the 1950s to the general commercial use of people’s livelihood. With the reduction of cost and environmental protection considerations, the use of monocrystalline silicon solar cells has become more and more common. The main applications are as follows.

(1) Household power generation system 5 from 20W to tens of thousands of watts. Depends on demand.

Located in California, USA, it was built in 1983 and completed in 1986. It is a 6 MW PV power plant; there are more than 20 relatively small-scale PV systems. It has also been successively adopted by many power companies in the past 10 years, as an experimental auxiliary device or installed on residential roofs to provide household electricity. A 6-year PV experiment program of the New England Electric Power Company (NEES) selected some residential buildings to install 2.2 kW C 10 pieces of 220 W) PV photovoltaic panels. The result is an average saving of about 50% of summer electricity bills. And users respond well. The Sacramento Electric Power Company in California) installed and tried two 1000 kW PV systems from 1984 to 1986 in accordance with the requirements of local residents to pay attention to ambient air quality; since 1993, a large number of medium-scale PV systems have been installed, and the total installed capacity has now over 3.7 MW .

The application of solar photovoltaic power generation system is quite extensive, with different application occasions. The system architecture is also different. For example, systems used in remote areas without electricity are stand-alone systems. In areas where there is electricity, the utility power parallel system can be used. When the power generated by the solar power generation system is greater than the load power, the excess power can be sent back.

The independent solar power generation system mainly includes solar cell modules, charge controllers, batteries, converters and lighting loads. The solar cell first converts light energy into direct current, then charges the battery through the charge controller, and finally converts the direct current into alternating current through the converter to supply the lighting load. The solar power independent system has three possible operation modes: ① When the output power of the solar cell is greater than the load power, the excess power will be stored in the storage battery; otherwise. When the output power of the solar cell is less than the load power, the insufficient power will be provided by the battery. ② Add ATS between the converter and the load. There are two power sources for the load, one is the solar photovoltaic power generation system. One is the power system. When the power of the photovoltaic power generation system is sufficient to supply the load, the photovoltaic power generation system supplies the load power; when the power of the photovoltaic photovoltaic power generation system is insufficient, the ATS switches to the power system instead of the power system. This ensures that the load has an adequate source of power. ③ Integrate the charge controller with the converter.

(2) Traffic: electric vehicles, charging systems, road lighting systems and traffic signals.

(3) High-power solar power generation system.

(4) Agriculture: Power systems such as irrigation and pumping

(5) Telecommunications and communications: wireless power, wireless communications.

(6) Backup power: disaster recovery.

(7) Power supply for low-power commodities.

(8) Outdoor positioning monitoring system: electronic bus stop signs, billboards, etc.

The traditional energy sources that people mainly rely on today are limited. It is estimated that the remaining oil reserves are 1,033.8 billion barrels, which can be used for 43 years; the natural gas reserves are 146M, which can be used for 62 years; the coal reserves are 9,841.2 billion tons, which can be used for 230 years; the uranium reserves are 395 million. 10,000 tons can be used for 64 years. In addition, in recent years, as the issue of global warming has been paid attention to by countries all over the world, major countries in the world have actively developed clean renewable energy such as solar photovoltaic energy to replace fuel power generation in recent years, in order to alleviate the pollution problems caused by traditional power generation methods. Therefore, increasing the development and use of solar photovoltaic energy is an important direction for human life and survival. The solar photovoltaic industry is one of the most important energy technologies in the 21st century. Many companies have vigorously developed and promoted them. Many companies have also expanded their production capacity. In the past five years, the global solar cell production has grown at an average annual growth rate of more than 30%. , showing its unlimited development potential in the field of renewable clean energy. Taiwan has a complete semiconductor industry base and excellent conditions for the development of solar cells. Due to the promotion of policies and the vigorous development of the world market. At present, the solar cell industry has gradually emerged. Integrate related capital, technology, equipment and other manufacturers and research units, and cooperate with each other to develop competitive product technology. in order to enhance international competitiveness. To build a sustainable energy business.

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.

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.

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.

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%.

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.

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.

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.

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.

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%.

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.

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.

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.

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.

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.

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%.

1. Consideration of semiconductor materials
Generally speaking, the chips used in solar cells can be divided into P-type and N-type, but because the diffusion length of excess minority carriers (electrons) in P-type is longer than the diffusion of excess minority carriers (holes) in N-type The length is too long. In order to obtain a larger photocurrent, a P-type chip is generally selected as the substrate.

2. Considerations of spectral response
Figure 1 is the solar radiation spectrum of AMO and AMI. In solar radiation, the most energy is in the short wavelength region close to UV. in solar cells. Since only some photons with energy greater (E_g) than the energy gap of the solar cell material can be absorbed by the material, other photons with energy less than the energy gap of the material cannot be absorbed and penetrated; in addition, if a material with a small energy gap is selected. Although most of the spectrum can be absorbed, the intrinsic concentration n_i of the material is large, the dark current of the cell will increase and the efficiency of the solar cell will decrease. Generally speaking, the material energy gap of solar cells is between 1~2 eV, and the corresponding light wavelength is 0.6~1.3µm, so the material energy gap is between silicon (Si), gallium arsenide (GaAs), Indium phosphide (lnP) is a very good solar cell material.

3. Considerations for Shallow Interfaces
Based on the above considerations, we chose the material as monocrystalline silicon (Si) to consider the design of the shallow interface. When the light wavelength is less than the short wavelength of 400nm, the light absorption coefficient of silicon will become very large (>10⁵cm¹), so most of the incident photons will be absorbed near the surface depth (0.1~0.3µm) to generate electron-hole pairs. For the light absorption coefficient of a material, the larger the light absorption coefficient, the stronger the absorption of photons near very shallow surfaces. To improve the conversion efficiency of the cell, the key is to reduce the interface depth of the silicon solar cell to less than 0.3µm, so as to enhance the absorption of short wavelengths and improve the efficiency.

4. Considerations for Anti-Reflection Layers
For single crystal silicon, the light reflectivity on the silicon surface under normal illumination is 30%~35%. In order to reduce the surface reflectivity, layers of materials with different refractive indices can be used to stack up anti-reflection layers. As shown in Figure 2, the interface reflection coefficient R between air and medium can be calculated according to the following formula

In the formula, n₀ is the refractive index of air; n₁ is the refractive index of the anti-reflection layer; n_s is the refractive index of the semiconductor. Since n₀=1 (air), when n₁=(n_s) ^1/2, the reflection coefficient R=0, the refractive index of single crystal silicon at the incident light wavelength of 400~1100nm is 6.0~3.5, so in order to reduce the surface reflectivity, The refractive index of the antireflection layer is preferably between (3.5) ^1/2 = 1.87 and (6) ^1/2 = 2.5. Commonly used anti-reflection layer materials are SiO₂ (n=1.5), Si₃N₄ (n=2.0), Ta₂O₅ (n=2.25), TiO₂ (n=2.3) and so on.

5. Considerations for Surface Passivation
The surface passivation (surface passivation) treatment is generally to grow a layer of oxide layer to reduce the surface carrier recombination speed at the Si-SiO₂ interface.
The total carrier recombination rate per unit area of ​​the semiconductor surface is

where n_th is the electron thermal speed; σ_n is the electron-capture cross-section; σ_p is the hole capture cross-section; N_st is the surface recombination rate (#/cm³); p_s is the hole concentration on the surface; n_s is the electron concentration on the surface; E is the energy level of the defect center. Under low injection, n_s≈N_D>>p_s, n_s﹥﹥n_i, exp(Et-EFi/kT), so the above formula can be rewritten as

Here S_p=v_stσ_pN_st, S_p is the surface recombination velocity of holes. The surface recombination velocity has a great influence on the reverse saturation current. The faster the surface recombination speed, the greater the reverse saturation current, resulting in a decrease in the photocurrent. , which affects the efficiency of solar cells.

6. Considerations for textured structures
Anisotropic chemical etching on the single crystal silicon surface in the (100) lattice direction will form a tiny inverted pyramid structure in the (111) lattice direction. As shown in Figure 3, this textured structure can cause multiple reflections of incident light, which can increase the travel path of light in addition to reducing the reflection of incident light. For long-wavelength incident light, since the light absorption coefficient α of single crystal silicon material is very low, the incident light will be reflected at the bottom and then refracted by the surface textured structure to increase the light absorption.
incident light

The spectrum of solar radiation is mainly centered on visible light, and its main distribution range is 0.3 ultraviolet to several microns of infrared. If converted into the energy of photon, it is 0.4~4eV. When the energy of the photon is less than the energy gap of the semiconductor, the photon is not absorbed by the semiconductor, and the semiconductor is transparent to the photon. When the energy of the photon is greater than the energy gap of the semiconductor, the energy equivalent to the energy gap of the semiconductor will be absorbed by the semiconductor to generate electron-hole pairs, while the rest of the energy is consumed in the form of heat. Therefore, the energy gap of solar cell materials must be carefully selected to efficiently generate electron-hole pairs. In general, ideal solar cell materials must have the following properties:
(1) The energy gap is 1.1~1.7 eV.
(2) Direct energy gap semiconductors.
(3) The composition of the material is non-toxic.
(4) The technology of thin film deposition can be used, and it can be manufactured in a large area.
(5) Good photoelectric conversion efficiency.
(6) It has long-term stability.

The energy gap of silicon is 1.12 eV. And silicon is an indirect energy gap semiconductor, it does not absorb light well, so silicon is not the most ideal material in this regard; but on the other hand, silicon is in the earth’s crust The second most abundant element, and silicon itself is non-toxic, its oxide is stable and not water-soluble. Therefore, the development of silicon in the semiconductor industry has a solid foundation, and currently solar cells still use silicon as the main material.

There are three types of silicon solar cells: monocrystalline silicon, polycrystalline silicon, and amorphous silicon. The appearance is shown in Figure 1, and most of the current market applications are monocrystalline silicon and polycrystalline silicon. The reasons are: ① the highest efficiency of single crystal; ② more Crystal technology tends to mature, the price is cheaper. The efficiency is close to that of single crystal silicon; ③ Amorphous has the lowest efficiency and can only be applied to low-end products, and the above two are easier to recut and process. The characteristics of various classes are compared in Table 1.

Type	Efficient/%	Advantage	Shortcoming
Monocrystalline silicon	24	High conversion efficiency Long service life	Higher production cost Long manufacturing time
Polysilicon	18.6	Simple steps to make Lower cost	Lower efficiency than monocrystalline silicon
Amorphous silicon	15	Cheapest Fastest production	Reduced output power after outdoor setup Photodegradation

(1) Monocrystalline silicon solar cells.
Monocrystalline silicon cells are the most common. It is mostly used in power plants, charging systems, road lighting systems and traffic signs, etc. It generates a wide range of power and voltage, has high conversion efficiency, and has a long service life. The world’s major manufacturers, such as Germany’s Siemens, BP and Japan’s Sharp, are all The production of such monocrystalline silicon solar cells is the main one, and the market share is about 50%. The efficiency of monocrystalline silicon cells ranges from 11% to 24%. Of course, the higher the efficiency, the more expensive the price.

(2) Polycrystalline silicon solar cells.
The efficiency of polycrystalline silicon cells is lower than that of monocrystalline silicon, but because of its simpler fabrication steps and lower cost, which is 20% cheaper than monocrystalline silicon cells, some low-power power application systems use polycrystalline silicon solar cells.

(3) Amorphous silicon solar cells.
Amorphous silicon power is also the commercial solar cell with the lowest cost at present, and it does not require packaging, and the production is also the fastest. There are many types of products, which are widely used, and are mostly used in consumer electronic products, and new application products are constantly being developed.

What is Edge-defined film fed growth method？

The preparation methods of polysilicon include crucible descending method, casting method, heat exchange method and edge-defined film fed growth method.

Although the growth technology of monocrystalline and polycrystalline silicon, including the Czochralski method and the floating zone method, is quite mature, the complex growth system, the growth furnace price is too high, the chip is too thick, and the silicon block material is in the cutting and polishing process. , The material loss of more than 50% is its disadvantage. In addition, the diced silicon chip requires proper chemical corrosion to remove surface scratches, and a large amount of chemical solution will cause environmental pollution.

Compared with the traditional crystal growth method, the edge-defined film fed growth method has the following characteristics:

① The edge-defined film fed growth method uses the capillary principle to directly separate the silicon chip from the molten silicon, so the silicon chip does not need to be cut and polished, and the loss of raw materials is less than 20%.

② The crystal shape can be controlled by the geometry of the top of the mold.

③ Fast growth rate.

④ Easy to fill continuously.

In 1972, TFCiszek first proposed to grow solar silicon chips using the edge-defined film fed growth method. At that time, only one silicon chip could be grown at a time, with an area of ​​about 2.5cm × 2.5cm. The growth rate was 6 cm²/min. 1971 In 1999, HELaBelle used the edge-defined film fed growth method to grow a hollow column with a diameter of 0.95 cm, a wall thickness of 0.005 to 0.1 cm, and a length of 140 cm. The growth rate was 12cm/min. In 1990, JP Kalejs and others could grow 15 cm in diameter. Hollow column. After years of hard work, the current industrial edge-defined film fed growth method can grow 5-6m octahedral hollow silicon pillars, which are then cut into 12.5cm × 12.5cm wafers by laser. The chip thickness is about 300µm, and the growth rate is 150cm²/min. D. Garcia et al. proposed an improved edge-defined film fed growth method in 2001 to grow cylindrical hollow silicon crystals. The purpose is to reduce the defect density and excessive thermal stress; at the same time, the cylindrical edge-defined film fed growth method also allows the mold to rotate. Therefore, silicon chips can become thinner. The current silicon full-core column has a diameter of up to 50cm, a chip thickness of 100 to 150µm, and a growth rate of 1.5 to 3cm/min. The current research direction focuses on how to increase the size of the chip, reduce the thickness of the chip and improve the quality of the crystal.

Figure is a cross-sectional schematic diagram of the edge-limited flake crystal growth method, in which the mold is made of a material infiltrated with liquid silicon (such as graphite), and the bottom has a porous structure that leads to the top. The meniscus is a thin layer of liquid connected between the upper surface of the mold and the crystal interface. The mold is the core component of the entire system, which controls the shape of the crystal, the crystal interface and the transfer of heat and silicon from the crucible to the meniscus. The crystal growth process is briefly described as follows: heating makes the silicon raw material in the graphite crucible melt, and the liquid silicon enters the mold from the pores at the bottom of the mold under capillary action and rises to the top. Move the seed crystal down so that the bottom edge of the seed crystal is in contact with the molten silicon at the top of the mold. Under the action of surface tension, the crystalline silicon and the molten silicon will be connected together to form a curved surface. Lower the temperature of the system until the seed crystal can move freely, then increase the pulling speed while reducing the system temperature. Pull the seed crystal upward to pull out the molten silicon, and the pulled molten silicon will solidify at the solid-liquid interface to form silicon crystals. . At the same time, the molten silicon in the crucible will continuously rise to the top of the mold under capillary action to replenish the pulled molten silicon. In order to enable the continuous growth of silicon crystals, molten silicon can be continuously added to the crucible. During the crystal growth process, the system is placed in an inert gas or vacuum atmosphere to prevent silicon oxidation.

The silicon chip of the edge-defined film fed growth method is usually composed of many large columnar crystal grains that are parallel to the growth direction and penetrate the entire chip thickness, and the chip surface is in a (110) preferred orientation. The formation of this orientation has nothing to do with the seed crystal. Even if the single crystal silicon seed crystal starts to grow, during the growth process, external crystal grains will be produced at the contact point and quickly grow across the surface to form a multi-product structure. Since the size of the component particles is greater than the thickness of the silicon chip and the diffusion length of minority carriers, the efficiency of solar cells made of edge-limited flaky crystalline silicon chips is basically not affected by the grain boundaries.

The shape of the crystal grown by the edge-defined film fed growth method is determined by the meniscus. The pulling speed and system temperature can be changed to control the shape and position of the meniscus, so that it can continue to grow from simple columnar or filamentary to almost arbitrarily complex geometry Shape crystals. For the growth of silicon crystals by the edge-defined film fed growth method, it is mainly to control the thickness of the growth of the silicon chip, which is determined by the height and shape of the meniscus. The height and shape of the meniscus can be changed by controlling the temperature of the mold top, the pulling speed, the temperature gradient in the vertical direction of the system, and the height of the mold from the molten silicon, so that the thickness of the silicon chip can be controlled. The thickness of the silicon chip can be adjusted in the following ways.

(1) Increase the pulling speed when the mold top temperature is constant, the height of the meniscus is getting higher and higher, and the thickness of the silicon chip will become thinner and thinner, until a cavity appears, and then the growth is stopped.

(2) Reduce the pulling speed when the mold top temperature is low, the height of the meniscus will become lower and lower, and the thickness of the silicon chip will become thicker and thicker until the solid-liquid interface moves down to the top position of the mold. Growth is interrupted.

(3) When the pulling speed is constant, lower the mold top temperature, the meniscus height becomes lower and lower, and the thickness of the silicon chip will become thicker and thicker.

(4) Different heights from the top of the mold to the molten silicon will affect the curvature of the meniscus, which in turn affects the thickness of the growing silicon chip. When the pulling speed is relatively low, the height of the top of the mold from the molten silicon has a significant effect on the thickness of the silicon chip. When the pulling speed is constant, the higher the height of the top of the mold from the molten silicon, the greater the curvature of the meniscus, and the thinner the grown silicon chip.

(5) With the temperature gradient in the vertical direction of the system, the lower the height of the meniscus, the thicker the grown silicon chip. During crystal growth, choosing a suitable mold can prevent silicon from solidifying at the top of the mold. And when the pulling speed is very high, it is necessary to let the top temperature of the mold pass the ridge to absorb the heat that cannot be completely taken away by the crystal. The thickness of silicon chips is becoming thinner and thinner. This is because the reduction in the thickness of the silicon chip can improve the efficiency of the solar cell and can save the loss of silicon raw materials. At present, the main problem of reducing the thickness of EFG silicon chips is that as the growth of silicon chips becomes thinner, the thickness uniformity along the width direction will become worse, and bending deformation is likely to occur.

The growth of edge-limited flake crystals is a non-equilibrium process, which contains a variety of complex physical phenomena, such as the formation mechanism of the curved surface, the movement of the crystal interface and the interaction of crystal-liquid-gas. The geometry of the mold, the growth rate, the stability of the solid-liquid interface and the thermal field are important factors that affect the growth and crystal quality. Common defects are micro twins, grain boundaries, inclusions, dislocations, and stacking faults parallel to the growth direction. These defects can be observed by chemical corrosion and transmission electron microscopy. Different growth rates will have different defects. Too high a growth rate will not affect the crystal surface, but it is easy to cause high-angle grain boundaries.

The corrosion of the mold material is another main cause of crystal defects. The mold material is mainly porous carbon fiber. When the silicon chip grows, the mold is immersed in the molten silicon, and the silicon melt will penetrate into the black through the pores of the carbon fiber; At the same time, the carbon yuan ping will diffuse into the silicon melt in the opposite direction, and the graphite crucible and the molten silicon are infiltrated and corroded, resulting in the carbon content in the edge-limited flaky crystalline silicon chip almost at a saturation level. These diffusion processes will stop when the mold surface is completely covered by silicon carbide and the broken fiber holes are filled with silicon melt. When the silicon carbide particles adhere to the growth gate of the edge-limited flake crystals, it will cause defects such as double products and large-angle grain boundaries.

When growing silicon chips with the edge-defined film fed growth method, the segregation of impurities at the solid-liquid interface causes the concentration of impurities in the ficus liquid to accumulate, which in turn affects the quality of senior silicon chips.