TWI465615B - Multicrystalline silicon wafer, multicrystalline silicon ingot, and method for manufacturing multicrystalline silicon ingot - Google Patents

Description

Polycrystalline germanium wafer, polycrystalline germanium ingot and method for manufacturing polycrystalline germanium ingot

The present invention provides a crystal structure and method for reducing the density of defects in a cast polycrystalline germanium ingot, which is used for a substrate of a germanium wafer of a polycrystalline germanium solar cell.

The distinction between germanium-based solar energy can be divided into two major categories: single crystal germanium wafers and polycrystalline germanium wafers. Among them, polycrystalline germanium wafers play a major role in the market of solar germanium wafers due to high output and low energy consumption. The polycrystalline germanium wafer is prepared by placing a high-purity germanium in a crucible containing a release agent and then placing it in a directional solidification growth furnace for melting, crystal growth, and cooling annealing. The cooled polycrystalline germanium ingot is processed by various processing tools and cut into tantalum wafers having a thickness of about 200 microns. At this point, the germanium wafer can enter the process of solar cell chip, and perform etching, PN interface engineering, anti-reflection layer and conductive electrode on the germanium wafer.

Compared with single crystal germanium wafers, the main reason for the conversion efficiency of polycrystalline germanium wafers in solar cells is that (a) multiple crystal orientations are on one wafer: polycrystalline germanium grows freely in the crucible, and directional solidification crystallization can only be limited. The crystals exhibit a columnar crystal structure and cannot compete with each other in the growth plane. Therefore, in the solar cell process, the polycrystalline germanium can only select the acid etching process with poor anti-reflection; (2) the polycrystalline germanium has a relatively uneven grain size: the polycrystalline germanium ingot has multiple crystals, so there are intergranular grain boundaries. , and the existence of grain boundaries limits the effective diffusion length of photocurrent carriers, And adsorbing impurities, and finally reducing the open circuit voltage of the solar cell; and (3) the defect density of the polycrystalline germanium is high: polycrystalline germanium is mainly used in the casting process, the use of the release agent, the thermal field design of the solidification furnace, and the annealing in the process conditions. Stage to eliminate the thermal stress of the ingot. Such a process has an effect on a wide range of stress management and can avoid cracking of the entire ingot. However, there is no effect on local defects such as elimination of gaps and stacking errors. The presence of defects attracts the adsorption of impurities, thereby reducing the crystal energy gap voltage. At the same time, the existence of defects also reduces the energy level gap voltage.

Although polycrystalline germanium has the above-mentioned disadvantages, since the ingot is solidified from the contact surface of the crucible in the polycrystalline germanium process, a solid ingot is formed on the contact surface with the crucible, thereby isolating the liquid melted crucible for a long time and the quartz crucible. Contacting prevents a large amount of quartz from being dissolved in the molten crucible, thereby suppressing the ingot containing a high concentration of oxygen impurities. Since the interstitial oxygen doped into the germanium wafer is the main cause of the light induced degradation (LID) of the post-production solar cell, the polycrystalline germanium has the advantages of avoiding the generation of oxygen impurities and thereby suppressing photo-efficiency degradation. Therefore, the high unit yield, low cost and low photo-induced degradation of polycrystalline germanium make it the most economical product in the overall solar power generation market.

At present, the biggest problem of polysilicon enthalpy conversion efficiency is the dendritic crystal structure formed by the casting, which can not limit the competition between the crystals in the growth plane, resulting in defect concentration areas caused by uneven grain interlacing.

As mentioned earlier, polycrystalline germanium wafers are inevitable due to their own manufacturing process. It is accompanied by the generation of crystal grain boundaries, defects, impurities, and the like. Accordingly, in view of the above-described deficiencies of the prior art, it is an object of the present invention to provide a method for effectively improving efficiency and to increase the open circuit voltage wherein the increase in open circuit voltage is a goal of the wafer wafer manufacturer to improve.

In order to achieve the above object, the present invention provides a polycrystalline germanium wafer comprising: a plurality of crystal grains having a long diameter and a short diameter, wherein a ratio of the long diameter to the short diameter is not more than 3 or the length of the crystal grain The aspect ratio is no more than 3.

In the polycrystalline silicon wafer of the embodiment of the present invention, the area occupied by the crystal grains satisfying the above ratio is not less than 25% of the area of the polycrystalline silicon wafer.

Furthermore, the roundness of each of the grains is between 0.4 and 1.0.

Further, the present invention further provides a polycrystalline germanium ingot which is diced to form a polycrystalline silicon wafer as described above.

In addition, the present invention further provides a method for manufacturing a polycrystalline germanium ingot, the method comprising: providing a mold having a plurality of recesses on a bottom surface thereof; filling a mold into the mold; heating the mold to make the material Melting to form a molten crucible that fills the pockets; and cooling the mold by a certain solidification process to cause the molten crucible to begin to crystallize from the pockets to form a polycrystalline ingot.

In the method of the embodiment of the invention, the recesses are disposed on the bottom surface of the mold.

In the method of the embodiment of the invention, the pockets are wavy pockets.

In the method of the embodiment of the present invention, the period of the waveform formed by the surfaces of the wavy recesses is 0.05 to 5 cm.

In the method of the embodiment of the present invention, the waveform formed by the surface of the wavy recess has an amplitude of 0.5 to 5 cm.

In addition, the present invention further provides a method for manufacturing a polycrystalline germanium ingot, the method comprising: providing a mold having a plurality of bumps inside the mold; filling a mold into the mold; heating the mold to make the material Melting to form a melting crucible that fills the area between the bumps; and cooling the mold by a certain solidification process to cause the melting crucible to begin to crystallize from the region between the bumps A polycrystalline germanium ingot is formed.

Wherein, the bumps can be disposed on a bottom surface of the mold.

In summary, the focus of the present invention is to utilize the recesses or bumps on the bottom surface of the mold used in casting the polycrystalline germanium ingot, so that the growth of the crystal structure of the dendritic crystal of the cast polycrystalline crucible can be restricted and formed during the crystal growth process. The size and shape of the grains are limited. Through such a crystal structure, it is possible to reduce a defect concentration region caused by the interlacing of large crystals when the dendrite grows. Thereby reducing the defect density in the twin ingot and greatly increasing the open circuit voltage and conversion efficiency of the solar cell.

To fully clarify the objects, features, and advantages of the present invention, those of ordinary skill in the art of the invention can understand the invention and practice the invention. Schematic, a detailed description of the present invention, illustrated as follows:

definition

If the quantity, concentration or other value or parameter is in the range, the preferred range Or a range of upper and lower limits, which are to be construed as being limited to any range of the upper or preferred value of any range and the lower or preferred range of any range, whether or not the ranges are disclosed separately. . In addition, when a range of values is recited herein, unless otherwise stated, the range shall include its endpoints and all integers and fractions within the range.

In the present invention, before the object can be attained, the numerical value should be understood as having the accuracy of the number of significant digits. For example, the number 40 should be understood to cover the range from 35.0 to 44.9, while the number 40.0 should be understood to cover the range from 39.50 to 40.49.

Method for producing polycrystalline germanium ingot

Referring to FIG. 1, the method for producing a polycrystalline germanium ingot of the present invention comprises the steps of providing a mold S11, filling a crucible S12, heating a mold S13, and growing a crystal S14. The details of each step are described below so that those of ordinary skill in the art can understand the contents and implement them accordingly.

Provide mold S11

One of the main objects of the present invention is to produce a low defect density twin ingot by casting, and first provide a container, such as tantalum, as a mold for the ingot during the process. It should be noted that, as shown in FIG. 2, the bottom surface of the mold 10 of the present invention has a plurality of undulating recesses 101, and the top view of the mold 10 is as shown in FIG. 3, wherein the wavy recesses 101 are In one embodiment, the opening of the mold 10 is rectangular, and the wavy recesses 101 are arranged in a checkerboard pattern. Meanwhile, please refer to FIG. 4, each of the wavy recesses 101 The amplitude A of the waveform formed by the surface along the top edge of the section parallel to one side of the mold 10 is 0.5~5cm, and the period P of the waveform is 0.05~5cm. It should be understood that the waveforms disclosed above are only aspects of the preferred embodiment. In other embodiments of the present invention, the waveforms described above may be other types. It is to be noted that the bottom surface structure of the mold 10 of the present invention is described by a wavy recess, but it is not limited thereto, and may be other regular or irregular shaped recesses or bumps.

Fill in the dip S12

Next, a mash is filled in the mold, and as shown in Fig. 2, the mash 20 is a solid block.

Heating mold S13

After filling in the dip, the heating process can be performed. In this step, after the mold is placed in the heating furnace, the mold is heated to melt the material therein, and after melting, as shown in Fig. 2, a melting crucible 30 is formed, at which time, due to the melting crucible The 30 series is liquid, so that the wavy pockets 101 can be filled. If the bottom surface has the embodiment of the bump, the molten germanium fills the area between the bumps.

Changjing S14

After the molten crucible is obtained, the crystal growth is carried out by a directional solidification process. In this step, heat is removed from the bottom surface of the mold to cool the mold, and the crystallization is promoted by lowering the temperature of the molten crucible below its melting point, wherein the melting of the mold is also caused by cooling the bottom surface of the mold. The nucleation starts at the bottom surface. At this time, the nucleation size of the nucleation is restricted by the wavy recesses 101, and the concentration of defects caused by the interlacing between the crystals is avoided, and the temperature gradient direction formed by the melting enthalpy is further Curing from the bottom surface toward the top, after being fully cured, forming the polycrystalline silicon of the present invention Ingot.

Polycrystalline germanium wafer of the present invention

The polycrystalline germanium ingot obtained by the above method is clamped and fixed on the processing stage, and then cut to form the polycrystalline silicon wafer of the present invention. As shown in FIG. 5, the polycrystalline germanium wafer is photoluminescent (PL). , photoluminescence) It is obvious that the size of the crystal grains contained therein is high after imaging. In contrast, the wafer grains of the prior art, as shown in Fig. 6, have a relatively large grain size distribution. .

Furthermore, it can be seen from Fig. 7 which is partially enlarged by Fig. 5 that the grain size is between about 2.6 mm and 18.2 mm. Meanwhile, in order to specifically define the shape of the crystal grain, the present invention defines a long diameter and a short diameter according to the polygon formed by the boundary of the grain (the dark color line in FIG. 7), wherein the long diameter is the polygon The distance from the longest point of the two ends, the short diameter is the distance between the ends of the polygon at the shortest distance, and the long diameter a and the short diameter b are measured by the randomly selected crystal grains (circle around) in the figure. In this embodiment, the long diameter a is 16.6 mm and the short diameter b is 8.8 mm, so that the ratio (a/b) of the two is 1.89, which is not more than 3, and has a long diameter and a short diameter. A grain having a ratio of not more than 3 or an area having an aspect ratio of not more than 3 accounts for not less than 25% of the area of the polycrystalline silicon wafer.

In addition, according to the crystal grains shown in Fig. 7, after measuring the area and the circumference, the roundness of each crystal grain can be obtained, which is calculated by the following formula 1: C = 4 × π ×A'/p ² formula 1

Where C is the roundness of the crystal grains, π is the pi, and A' is the area of the crystal grains. p is the perimeter of the grain.

The crystalness of the crystal grains of the polycrystalline silicon wafer of the present invention is calculated by the above formula 1 to be between 0.4 and 1.0.

In summary, the polycrystalline germanium wafer produced by the present invention has a long diameter/short diameter (a/b) ratio or aspect ratio of not more than 3, and unique uniformity and roundness characteristics of each grain size. The boundary dislocation density between the crystal grains due to the competition in the crystal growth process is greatly reduced, so that the lattice defects in the crystal can be significantly reduced, thereby improving the efficiency performance of the subsequently fabricated solar cell.

Testing and comparison

To further illustrate the advantages of the present invention, the following is a table to illustrate the efficacy of the solar cell made of the polycrystalline silicon wafer of the present invention and to illustrate it.

Table 1 above is the conversion efficiency measured by the solar cells (Examples 1 to 7) made of the polycrystalline germanium wafer of the present invention and the solar cell (Comparative Example) made of the conventional polycrystalline germanium wafer, which can be clearly found from the average value. The efficiency performance of each embodiment of the present invention is higher than 17%, and the efficiency performance of the comparative example is only 16.23%. Therefore, the present invention can be found based on the above measurement results. The embodiment significantly improved the cell efficiency by about 6.41% to 8.44% due to the uniformity of the grain size of the polycrystalline germanium wafer of the present invention.

Further, the open circuit voltages measured by the above respective examples and comparative examples are as shown in Table 2 below:

From the average of the open circuit voltages in Table 2 above, the same trend as the efficiency data shown in Table 1 can be observed, which is more sufficient to demonstrate that the embodiment of the present invention has superior performance compared to the prior art.

The invention has been described above in terms of the preferred embodiments, and it should be understood by those skilled in the art that the present invention is not intended to limit the scope of the invention. It should be noted that variations and permutations equivalent to those of the embodiments are intended to be included within the scope of the present invention. Therefore, the scope of protection of the present invention is defined by the scope of the patent application.

10‧‧‧Mold

101‧‧‧ wavy pocket

20‧‧‧Information

30‧‧‧ 矽

40‧‧‧Polycrystalline ingots

401‧‧‧ grain

S11~S14‧‧‧Steps

a‧‧‧Long Trail

b‧‧‧Short Trail

P‧‧ cycle

A‧‧‧ amplitude

Figure 1 is a flow chart of a method of making a polycrystalline germanium ingot of the present invention.

Fig. 2 is a schematic view showing an embodiment of a method for producing a polycrystalline germanium ingot of the present invention.

Figure 3 is a mold used in the method for producing a polycrystalline germanium ingot of the present invention. Top view.

Fig. 4 is a partially enlarged view showing a section of the mold of Fig. 3 taken along the line C-C.

Figure 5 is a photoluminescence image of a polycrystalline germanium wafer of the present invention.

Figure 6 is a photoluminescence image of a conventional polycrystalline germanium wafer.

Fig. 7 is a partial enlarged view of Fig. 5.

S11~S14‧‧‧Steps

Claims (9)

A polycrystalline germanium wafer comprising: a plurality of crystal grains having a long diameter and a short diameter, wherein a ratio of the long diameter to the short diameter of the plurality of crystal grains is not more than 3, and the crystal grains satisfying the ratio of the ratio The area is not less than 25% of the area of the polycrystalline silicon wafer. The polycrystalline germanium wafer of claim 1, wherein each of the crystal grains has a circularity of between 0.4 and 1.0. A polycrystalline germanium ingot which is diced to form a polycrystalline germanium wafer as described in any one of claims 1 to 2. A method for manufacturing a polycrystalline germanium ingot, the method comprising: providing a mold having a plurality of pockets therein, wherein the pockets are wavy pockets; filling a mold into the mold; heating the mold , the molten material is melted to form a molten crucible, the molten crucible is filled with the recesses; and the mold is cooled by a certain solidification process to cause the molten crucible to start crystal growth from the cavities A polycrystalline germanium ingot is formed. The method of claim 4, wherein the pockets are disposed on a bottom surface of the mold. The method of claim 4, wherein the period of the waveform formed by the surface of the wavy recess is 0.05 to 5 cm. The method of claim 4, wherein the waveform of the surface of the wavy recess has an amplitude of 0.5 to 5 cm. A method of making a polycrystalline germanium ingot, the steps of which comprise: Providing a mold having a plurality of protrusions therein, wherein the protrusions are wavy protrusions; filling a mold into the mold; heating the mold to melt the material to form a melting enthalpy The molten tantalum fills the area between the bumps; and the mold is cooled by a certain solidification process to cause the molten germanium to grow crystallized from the region between the bumps to form a polycrystalline germanium ingot. The method of claim 8, wherein the bumps are disposed on a bottom surface of the mold.