TWI620838B - Crystalline silicon ingot including nucleation promotion particles and method of fabricating the same - Google Patents

Abstract

A twin crystal ingot and a method of manufacturing the same. The manufacturing method utilizes a plurality of nucleation promoting particles to allow a plurality of ruthenium grains from a smelting broth to nucleate on a plurality of nucleation promoting particles and grow in a vertical direction until the bismuth melt is completely solidified. Each of the nucleation promoting particles is composed of a primary particle and a plurality of secondary particles bonded to the surface of the primary particle. The average particle diameter of the plurality of secondary particles is smaller than the average particle diameter of the plurality of primary particles.

Description

Twin crystal ingot containing nucleation promoting particles and manufacturing method thereof

The present invention relates to a crystalline silicon ingot and a method of manufacturing the same, and, in particular, to a twin crystal using a nucleation promoting particle such that the bottom thereof is a small-sized germanium crystal grain and has a low overall defect density. Ingot and its manufacturing method.

Most solar cells absorb sunlight and produce a photovoltaic effect. At present, most of the materials of solar cells are mainly coffins, mainly because coffins are the second most easily available elements on the earth, and they have the advantages of low material cost, no toxicity, and high stability. And it has a solid foundation in the application of semiconductors.

The solar cells based on coffins include three types: single crystal germanium, polycrystalline germanium and amorphous germanium. The use of polycrystalline germanium as a raw material for solar cells is mainly based on cost considerations because it is compared to the conventional CZ method and the floating zone method (FZ method). Crystal germanium, polycrystalline germanium prices are relatively cheaper.

The use of polycrystalline germanium in the manufacture of solar cells has traditionally been produced using conventional casting processes. The use of a casting process to prepare polycrystalline germanium, which is then applied to solar cells, is prior art in the art. Briefly, high purity germanium is melted in a mold (e.g., quartz crucible) into a silicon melt, and the melted soup is cooled under controlled solidification to form a polycrystalline germanium ingot. Next, the polycrystalline germanium ingot is cut into wafers close to the size of the solar cell and used in the manufacture of solar cells. The polycrystalline tantalum ingot produced in this way is an aggregate of ruthenium crystal grains in which crystal orientations of crystal grains with each other are actually random.

In the polycrystalline silicon produced by the conventional casting process, it is difficult to roughen the surface of the wafer to be formed because of the random crystal orientation of the crystal grains. The roughening of the surface can reduce the light reflection and increase the absorption of light energy through the surface of the battery to improve the efficiency of the photovoltaic cell. Further, the "kneading" formed in the grain boundaries between the existing polycrystalline germanium grains tends to form clusters of nucleation difference rows or structural defects in the form of a plurality of line difference rows. These rows and the impurities they tend to attract cause a rapid recombination of charge carriers in photovoltaic cells made from existing polycrystalline germanium. This can result in a decrease in the efficiency of the battery. Photovoltaic cells made from such polycrystalline germanium are generally less efficient than equivalent photovoltaic cells made from single crystal germanium, even considering the radial distribution of defects present in single crystal germanium fabricated by the prior art. However, since the fabrication of existing polysilicon is relatively simple and less costly, as well as defect passivation that is effective in battery processing, polysilicon has become a widely used form of tantalum material for the fabrication of photovoltaic cells.

The prior art discloses the use of a single crystal twin seed layer and a twin crystal ingot based on directional solidification, and generally uses a single crystal germanium cube having a large size and a crystal orientation of (100) as a main seed crystal. It is desirable to use a germanium single crystal solar cell to fabricate a germanium wafer with a crystal orientation of (100) direction because a light-trapping surface is conveniently formed by an etching method. Unfortunately, grains in the (100) crystal orientation during crystallization during the (100) crystal orientation of the grains compete with randomly nucleated grains. In order to maximize the crystal volume of seeding in the ingot, the prior art discloses that the area of the seed crystal of the (100) crystal orientation is surrounded by the boundary of the (111) crystal orientation. This boundary is very successful in suppressing crystals in other crystal orientations. In this way, an ingot having a high performance single crystal germanium and/or a bi-crystal germanium block can be cast, which maximizes the lifetime of minority carriers of the resulting wafer. Used to manufacture high efficiency solar cells. Here, the term "single crystal germanium" means a main body of a single crystal germanium having a uniform crystal crystal orientation over the entire range. The term "bimorph" refers to a body of ruthenium having a uniform crystal orientation in a range of greater than or equal to 50% of the volume of the body, and having another uniform crystal orientation within the remaining volume of the body. . For example, such a twin germanium may comprise a single crystal germanium body having a crystal crystal orientation, which is adjacent to another single crystal germanium body having a different crystal crystal orientation in the immediate vicinity of the remaining volume of the crystalline germanium. Further, the prior art polycrystalline germanium refers to a crystal enthalpy having a fine distribution of a centimeter scale, and having a plurality of crystals having a random crystal orientation in the main body of the crucible. However, the aforementioned prior art is a method of utilizing an expensive single crystal twin seed crystal, which greatly increases the manufacturing cost of the entire twin crystal ingot.

Another prior art does not rely on expensive single crystal twins, which utilizes local undercooling to first fill the lateral growth of the crucible at the bottom of the crucible, and then grow up the columnar structure, the large size of the crucible has a low defect density. . Therefore, according to other prior art tantalum ingots, the sliced silicon wafer is made into a solar cell, and high photoelectric conversion efficiency can be obtained.

However, the techniques proposed in the prior art using local supercooling have only been successfully verified in the laboratory. When extended to industrial grades, polycrystalline tantalum casting is difficult to control the crystal lattice dendritic growth to fill the bottom of the crucible with local subcooling. Industrial grade polycrystalline tantalum casting is affected by the uniformity of enthalpy and overall heat, and the variation of initial supercooling is controlled. It is easy for polycrystalline germanium to grow into large grains at the bottom of the crucible and become a region with high defect density, which increases more rapidly during growth and extension. The defect density causes the overall crystal quality of the twinned ingot to deteriorate, and the photoelectric conversion efficiency of the subsequently produced solar cell is also low.

Therefore, the technical problem to be solved by the present invention is to provide a twin-crystalline ingot which utilizes nucleation-promoting particles to assist in nucleation of ruthenium grains, and which grows into a small-sized ruthenium crystal at the bottom, and has a low overall defect density, and a method for producing the same. The solar cell produced by the twin crystal ingot of the present invention has a lower cost and a higher photoelectric conversion efficiency.

In a preferred embodiment of the invention, a method of making a twine ingot is first to lay a plurality of nucleation promoting particles at the bottom of a mold. The mold itself defines a vertical direction. Each of the nucleation promoting particles is composed of a primary particle and a plurality of secondary particles bonded to the surface of the primary particle. The average particle diameter of the plurality of secondary particles is smaller than the average particle diameter of the plurality of primary particles. Next, the method installs a stack of raw materials into the mold and is placed on a plurality of nucleation promoting particles. Next, the method is to heat the mold until the crucible material is completely melted into a crucible soup. Next, the method controls at least one thermal control parameter regarding the bismuth melt, such that a plurality of ruthenium grains from the bismuth melt are nucleated on the plurality of nucleation promoting particles and grow in the vertical direction. Finally, the method continues to control at least one thermal field parameter such that the plurality of tantalum grains continue to grow in the vertical direction until the tantalum melt is fully solidified to obtain a twinned ingot.

In one embodiment, each of the primary particles may be formed of graphite, tantalum, alumina, tantalum carbide, tantalum nitride, aluminum nitride, or other materials having a melting point above about 1400 °C.

In one embodiment, each particle may be formed from graphite, tantalum, alumina, tantalum carbide, tantalum nitride, aluminum nitride, or other materials having a melting point above about 1400 °C.

In one embodiment, the plurality of primary particles have an average particle size of greater than about 1 mm.

In one embodiment, the plurality of secondary particles have an average particle size of less than about 1 [mu]m.

The twinned ingot of the present invention comprises a plurality of germanium grains grown in a vertical direction of one of itself and a plurality of nucleation promoting particles positioned at the bottom thereof. Each nucleation promoting particle consists of a primary particle and a plurality of secondary particles bonded to the surface of the primary particle. The average particle diameter of the plurality of secondary particles is smaller than the average particle diameter of the plurality of primary particles.

Different from the prior art, the invention does not need to rely on expensive single crystal twins, and does not need to perform the difficult local subcooling to nucleate the germanium grains at the bottom of the crucible, and the present invention uses the lower cost nucleation promoting particles to directly provide the crucible. The dense nucleation point of the melt is used to produce a high-density grain distribution to suppress the rapid growth of crystal orientation, thereby achieving a large reduction in the proportion of large-size tantalum crystals. Because the small-sized 矽-grain pattern has less grain competition in the process of crystal growth, and the small-sized 矽 crystal grain distribution tends to tend to grow upwards in a single direction, reducing the grain size and avoiding the columnar crystal. Unable to grow a complete situation. In addition, the accompanying high-ratio grain boundary can attract the defect concentration or the sliding on the grain boundary to release the thermal stress in the process of crystal growth, and suppress the rapid increase of defects such as poor row, thereby obtaining high-quality twin crystal casting. Ingots, the solar cell produced subsequently has a higher photoelectric conversion efficiency.

The advantages and spirit of the present invention will be further understood from the following detailed description of the invention.

Referring to FIGS. 1A through 1D, a method of manufacturing a twin ingot in a preferred embodiment of the present invention is schematically illustrated in a cross-sectional view.

As shown in Fig. 1A, the manufacturing method of the present invention substantially follows a directional solidification system (DSS) using a DSS crystal growth furnace 1. The structure of the DSS crystal growth furnace 1 comprises a furnace body 10, an insulating cage 12 formed by an upper heat insulating cover 122 and a lower heat insulating plate 124, a directional solidification block 18 disposed in the heat insulating cage 12, and a supporting directional solidification block 18 At least one support post 19, a base 17 disposed on the directional solidification block 18, a mold 16 disposed in the base 17, a heater 14 disposed above the mold, and the furnace body 10 and the insulating cage 12 An inert gas conduit 11.

In practice, the mold 16 can be a quartz crucible. The directional solidification block 18 can be made of graphite. The susceptor 17 can be made of graphite. The inert gas conduit 11 is used to introduce argon into the adiabatic cage 12.

As shown in Fig. 1A, the method of the present invention first lays a plurality of nucleation promoting particles 2 to the bottom of the mold 16. The modulo 16 itself defines a vertical direction V. It is preferred that a plurality of nucleation promoting particles 2 have been laid over the bottom of the mold 16, and the number of layers of the plurality of nucleation promoting particles 2 is not limited.

Please refer to FIG. 2, which is a schematic diagram of the structure of the nucleation promoting particles 2. Specifically, each of the nucleation promoting particles 2 is composed of a primary particle 22 and a plurality of secondary particles 24 bonded to the surface of the primary particle 22. Further, the average particle diameter of the plurality of secondary particles 24 is smaller than the average particle diameter of the plurality of primary particles 22.

Next, the method of the present invention installs a raw material 30 into the mold 16 and is placed on a plurality of nucleation promoting particles 2. A mold 16 in which a plurality of nucleation promoting particles 2 and a crucible raw material 30 are loaded is placed in the susceptor 17, as shown in Fig. 1A.

Next, the method of the present invention heats the mold 16 until the crucible material 30 is completely melted into the crucible soup 32 as shown in Fig. 1B.

Next, the method of the present invention controls at least one thermal field parameter for the crucible soup 32 such that a plurality of niobium grains 34 from the crucible soup 32 nucleate on the plurality of nucleation promoting particles 2 and along the vertical direction V Grow, as shown in Figure 1C. At least one thermal field parameter includes a heat transfer flux. As shown in Fig. 1C, in the process of crystal growth of the DSS crystal growth furnace 1, the upper heat insulating cover 122 is slowly raised, so that a gap is formed in the sealed space which is originally covered by the heat insulating cage 12, and the gap becomes heat inside and outside the heat insulating cage 12. The exchanged pipes produce heat transfer flux.

Finally, the method of the present invention continues to control at least one thermal field parameter, allowing a plurality of germanium grains 34 to continue to grow in the vertical direction V , and until the crucible melt 32 is fully solidified to obtain a twinned ingot 3, as shown in FIG. Show.

In one embodiment, each of the primary particles 22 may be formed of graphite, tantalum, alumina, tantalum carbide, tantalum nitride, aluminum nitride, or other materials having a melting point above about 1400 °C.

In one embodiment, each of the particles 24 may be formed of graphite, tantalum, alumina, tantalum carbide, tantalum nitride, aluminum nitride, or other materials having a melting point above about 1400 °C.

In one embodiment, the plurality of primary particles 22 have an average particle size of greater than about 1 mm.

In one embodiment, the plurality of secondary particles 24 have an average particle size of less than about 1 [mu]m.

In one embodiment, the plurality of nucleation promotes the particles 2 and inhibits the increase in defect density of the plurality of germanium grains 34 during growth. The growth rate of the defect density in the twin crystal ingot from the vertical direction V is 0.01%/mm~10%/mm. The increase in defect density in a twinned ingot is calculated by the following formula:

_{(D x2 -D x1) / (} x2-x1); wherein X2, X1 respectively V silicon ingot at different heights in the vertical direction, D _x2, D _x1 are silicon ingot X2, X1 of the section Defect density.

Small size germanium grains can also effectively suppress the increase in defect density. The twin crystal ingot of the present invention has a high probability of growing small-sized germanium grains (<10 mm) at the center bottom, and the side or corner bottom may have only locally grown small-sized germanium grains (<10 mm). Silicon ingot of the present invention cut perpendicular to the vertical direction V, a ratio of the area occupied by the small size of the silicon particles affects the rate of grain growth and the growth rate defect density.

In one embodiment, the nucleation promoting particles 2 are prepared by first preparing a slurry containing the secondary particles 24, and the slurry containing the secondary particles 24 is coated on the surface of the primary particles 22, and then the primary particles of the coating slurry are applied. 22 is placed in a high temperature furnace to at least dry the moisture, and to form nucleation promoting particles 2. The temperature of the high temperature furnace can be maintained at 1000~1100 °C. The secondary particles 24 may be bonded to the surface of the primary particles 22 by chemical covalent bonds. The temperature of the high temperature furnace can be higher, allowing the secondary particles 24 to be sintered or completely sintered with the surface portion of the primary particles 22.

With the prior art of strontium seed crystals, in the process of melting all of the bismuth raw materials into bismuth melt, it is necessary to control the thermal field parameters to partially melt the strontium seeds, but not all of them. Therefore, the prior art is complicated and difficult in controlling the parameters of the thermal field. The invention utilizes the technology of nucleation-promoting granules, and only needs to melt all the strontium raw materials into bismuth melted soup, and the problem of uncontrolled crystallization of the strontium seeds must be solved. Therefore, the method of the present invention is superior to the prior art in controlling the thermal field parameters. Simple and easy.

Referring again to FIGS. 1A through 1D, the heater 14 is placed over the mold 16. The directional solidification block 18 is placed below the mold 16 in indirect contact with the mold 16. The at least one thermal field parameter may include a first temperature gradient from the heater 14 to the die 16, a second temperature gradient from the bottom of the crucible soup 20 to the top of the directional solidification block 18, or a heat transfer flux, etc. Field parameters. In practice, the first temperature gradient needs to be controlled below 0.4 ° C / cm, by increasing the distance between the heater 14 and the mold 16, or by controlling the heating temperature of the heater 14 below 1410 ° C, etc. The method is to achieve. The second temperature gradient needs to be controlled above 17 ° C / cm, which can be achieved by increasing the thickness of the directional solidification block 18, and the like. The heat transfer flux needs to be controlled to be higher than 37,000 W/m ² , which can be achieved by raising the opening speed of the upper heat insulating cover 122 to 3 cm/hr or more.

Referring to Fig. 3, there is shown a schematic structural view of the twinned ingot 3 of the present invention. The twinned ingot 3 of the present invention comprises a plurality of tantalum grains 34 which grow V in one of their perpendicular directions and a plurality of nucleation promoting particles 2 which are located at the bottom thereof. As shown in Fig. 2, each of the nucleation promoting particles 2 is composed of a primary particle 22 and a plurality of secondary particles 24 bonded to the surface of the primary particle 22. Also, the average grain size of the germanium grains 34 of the twinned ingot 3 adjacent to the plurality of nucleation promoting particles 2 is less than about 10 mm. Further, the increase rate of the defect density in the twin crystal ingot 3 in the vertical direction ranges from 0.01%/mm to 10%/mm.

The materials and dimensions of the primary particles 22 and the secondary particles 24 constituting the nucleation promoting particles 2 have been described in detail above and will not be described herein.

Referring to Figure 4, the A ingot is a twined ingot of the present invention, and the average grain size change along the height of the twin ingot is indicated in Figure 4. In Figure 4 and the average grain size of the B ingot along the height of the twine ingot is indicated as a control. B ingot is a twinned ingot manufactured according to the method proposed in the prior art.

Referring to Figure 5, the defect density of the corner region, the sidewall region, and the central region of the A ingot along the height of the twine ingot is shown in Figure 5. The defect density in Fig. 5 is expressed in terms of the defect area ratio. As a comparison, the ratio of the defect area of the corner region, the side wall region, and the central region of the B ingot along the height of the twine ingot is also shown in FIG.

Referring to Figure 6, the photoelectric conversion efficiency of the solar cell fabricated from the bottom region, the middle region, and the top region of the A ingot (about 250 mm from the bottom of the A ingot) is shown in Fig. 6. As a comparison, the photoelectric conversion efficiency of the solar cell fabricated from the bottom region, the middle region, and the top region of the B ingot (about 250 mm from the bottom of the B ingot) is also shown in Fig. 6. The average photoelectric conversion efficiency of the solar cell fabricated from the A ingot is higher than the average photoelectric conversion efficiency of the solar cell fabricated from the B ingot by about 0.24%. The photoelectric conversion efficiency of the solar cell produced from each region of the B ingot was 16.8%. The photoelectric conversion efficiency of solar cells made from various regions of A ingot ranges from 16.96% to 17.11%. Compared with the solar cells, the photoelectric conversion efficiency of solar cells made in each region is quite close, which is beneficial to battery manufacturers in battery applications. Manufacturing, more commercial application value.

From the data in Fig. 4, Fig. 5 and Fig. 6, it can be clearly seen that the growth process of B ingot grows into large grains at the bottom of the crucible and becomes a region with low defect density, but increases more rapidly during growth and extension. The defect density causes the overall crystal quality of the twinned ingot to deteriorate, and the photoelectric conversion efficiency of the subsequently produced solar cell is low. Compared with the B ingot, the long crystal of the A ingot uses the nucleation promoting layer to directly provide the nucleation point of the bismuth melting furnace, so as to greatly reduce the proportion of the large size 矽 crystal. Because the small-sized 矽-grain pattern has less grain competition in the process of crystal growth, and the small-sized 矽 crystal grain distribution tends to tend to grow upwards in a single direction, reducing the grain size and avoiding the columnar crystal. Can't grow to be complete. In addition, the grain boundary with high distribution density in the A ingot can attract the defects in the stress field or slip on the grain boundary to release the thermal stress, and suppress the rapid increase of defects such as poor row, and then let the twin ingots The overall crystal quality is better, and the photoelectric conversion efficiency of the subsequently produced solar cell is also higher.

The features and spirit of the present invention are intended to be more apparent from the detailed description of the preferred embodiments. On the contrary, the intention is to cover various modifications and equivalents that are within the scope of the invention as claimed. Therefore, the scope of the patent application of the present invention should be construed broadly in the light of the above description, so that it covers all possible changes and arrangements.

1. . . DSS crystal furnace

10. . . Furnace body

11. . . Inert gas conduit

12. . . Insulation cage

122. . . Upper heat shield

124. . . Lower insulation board

14. . . Heater

16. . . mold

17. . . Pedestal

18. . . Directional solidification block

19. . . Support column

2. . . Nucleation promoting particles

twenty two. . . Primary particle

twenty four. . . Secondary particle

3. . . Twin crystal ingot

30. . .矽 raw materials

32. . . Melting soup

34. . .矽 grain

V. . . Vertical direction

1A to 1D are schematic cross-sectional views showing a preferred embodiment of the method of manufacturing a twinned ingot of the present invention.

Figure 2 is a schematic view showing the structure of the nucleation promoting particles of the present invention.

Figure 3 is a schematic view showing the structure of the twin crystal casting of the present invention.

Figure 4 is a graph showing the comparison of the grain size of the tantalum ingots produced by a preferred embodiment of the present invention with the twin crystal ingots of the control.

Figure 5 is a graph showing the comparison of defect densities of twinned ingots produced by a preferred embodiment of the present invention and their control twinned ingots.

Figure 6 is a graph showing the results of comparison of the average photoelectric conversion efficiency of a solar cell produced by a twinned ingot of a preferred embodiment of the present invention and a controlled twin ingot.

Claims (10)

A method of making a twin ingot comprising the steps of: laying a plurality of nucleation promoting particles at the bottom of a mold, wherein the mold itself defines a vertical direction, each of the nucleation promoting particles being composed of a primary particle and Forming a plurality of secondary particles bonded to the surface of the primary particle, the average particle diameter of the plurality of secondary particles being smaller than an average particle diameter of the plurality of primary particles; installing a raw material into the mold, and placing the same a plurality of nucleation promoting particles; heating the mold until the crucible material is completely melted into a crucible; controlling at least one thermal field parameter about the crucible soup, so that a plurality of niobium grains from the crucible soup are in the a plurality of nucleation promoting nucleation on the particles and growing in the vertical direction; and continuing to control the thermal field parameters, allowing the plurality of germanium grains to continue to grow in the vertical direction until the crucible melts all solidify to obtain the twin crystals Ingot casting. The method of claim 1, wherein each of the primary particles is formed of one selected from the group consisting of graphite, ruthenium, aluminum oxide, tantalum carbide, tantalum nitride, and aluminum nitride. The method of claim 2, wherein each of the particles is formed of one selected from the group consisting of graphite, ruthenium, aluminum oxide, tantalum carbide, tantalum nitride, and aluminum nitride. The method of claim 3, wherein the plurality of primary particles have an average particle size of greater than about 1 mm. The method of claim 4, wherein the plurality of secondary particles have an average particle size of less than about 1 [mu]m. A twinned ingot having a bottom and a vertical direction, characterized in that the twinned ingot comprises a plurality of germanium grains grown along the vertical direction and a plurality of nucleation promoting particles at the bottom, wherein Each of the nucleation promoting particles is composed of a primary particle and a plurality of secondary particles bonded to the surface of the primary particle, and the plurality of secondary particles have an average particle diameter smaller than an average particle diameter of the plurality of primary particles. The twinned ingot according to claim 6, wherein each of the primary particles is formed of one selected from the group consisting of graphite, ruthenium, aluminum oxide, tantalum carbide, tantalum nitride, and aluminum nitride. The twinned ingot according to claim 7, wherein each of the particles is formed of one selected from the group consisting of graphite, ruthenium, aluminum oxide, tantalum carbide, tantalum nitride, and aluminum nitride. The twinned ingot of claim 8, wherein the plurality of primary particles have an average particle size greater than about 1 mm. The twinned ingot according to claim 9, wherein the plurality of secondary particles have an average particle size of less than about 1 μm.