KR101370126B1 - Method for forming selective emitter of solar cell using annealing by laser of top hat type and Method for manufacturing solar cell using the same - Google Patents

Abstract

The present invention relates to a method of forming a selective emitter of a solar cell using a top hat laser annealing, comprising: (a) preparing a silicon substrate doped with impurities of a first conductivity type; (b) forming an insulating film containing impurities of a second conductivity type opposite to the impurities; (c) performing an annealing process in an oxygen atmosphere to inject impurities in the insulating layer onto the silicon substrate to form an emitter layer; (d) irradiating a laser having an energy distribution in the form of a "top hat" along the point where the finger front electrode is connected while the insulating film remains on the silicon substrate. Forming a selective emitter on top of the substrate at the laser irradiation point by inducing further diffusion of impurities; And (e) removing the insulating film.

Solar Cell, Selective Emitter, Laser Annealing, Top Hat, PSG

Description

Method for forming selective emitter of solar cell using annealing by laser of top hat type and Method for manufacturing solar cell using the same}

The present invention relates to a solar cell, and more particularly, a top hat type laser capable of forming a selective emitter using a top hat type laser at a point where a finger front electrode on a silicon substrate is connected in a solar cell manufacturing process. It relates to a method of forming a selective emitter of a solar cell using annealing and a method of manufacturing a solar cell using the same.

With the recent depletion of existing energy resources such as oil and coal, interest in alternative energy to replace them is increasing. Among them, solar energy is particularly attracting attention because it is rich in energy resources and has no problems with environmental pollution. There are two methods of using solar energy: solar energy that generates steam required to rotate turbines using solar heat, and solar energy that converts photons into electrical energy using the properties of semiconductors. In general, it refers to a solar cell (hereinafter referred to as a "solar cell").

1 showing a basic structure of a solar cell, a solar cell, like a diode, has a junction structure of a p-type semiconductor 101 and an n-type semiconductor 102. When light is incident on the solar cell, Electrons and electrons charged by (-) electrons escape from the interaction with the material constituting the semiconductor, and positive holes with positive charges are generated, and current flows while they move. Electrons among the p-type 101 and the n-type semiconductor 102 constituting the solar cell are referred to as the n-type semiconductor 102 and the holes are referred to as p-type semiconductor (hereinafter, referred to as " p- 101 to the electrodes 103, 104 bonded to the n-type semiconductor 101 and the p-type semiconductor 102. When these electrodes 103, 104 are connected by electric wires, electricity flows, Can be obtained.

The output characteristics of such a solar cell are generally evaluated by measuring the output current voltage curve using a solar simulator, and the maximum output Pm is the point where the product of the output current Ip and the output voltage Vp is maximized. The value obtained by dividing Pm by the total light energy incident on the solar cell (SXI: S is the device area and I is the intensity of light irradiated to the solar cell) is defined as the conversion efficiency η. To increase the conversion efficiency η, increase the short-circuit current Jsc (output current when V = 0 on the current voltage curve) or open voltage Voc (output voltage when I = 0 on the current voltage curve) or the square of the output current voltage curve. You should increase the fill factor (FF), which is close to. The closer the value of FF to 1, the closer the output current voltage curve is to the ideal square and the higher the conversion efficiency η.

FIG. 2 is a view schematically showing the structure of a general solar cell currently being developed. The solar cell shown in the drawing includes a silicon substrate 201 doped with an impurity of a first conductivity type forming a pn junction, an emitter layer 202 doped with an impurity of a second conductivity type opposite to the impurity, and a front surface thereof. In addition to the electrode 203 and the back electrode 204, an anti-reflection film 205 and a back surface field (BSF) 206 are further provided to improve the absorption of the sunlight and the transfer resistance of the carrier to improve the efficiency. Also, in recent years, in order to reduce the contact resistance between the front electrode 203 and the emitter layer 202, the emitter layer 202 in the region in contact with the front electrode 203, such as the solar cell shown in FIG. And areas that are not, are made thinner, thereby improving the life time of the carrier. The emitter layer of this structure is called a 'selective emitter'. This selective emitter contributes to increasing the efficiency of the solar cell by reducing the contact resistance with the front electrode 203. However, in order to form the selective emitter, a patterning process and two impurity diffusion processes must be involved, which leads to a complicated manufacturing process and excessive manufacturing cost.

The present invention was devised to solve the above-mentioned problems of the prior art, and is simple and easy without proceeding a conventional patterning process to form a selective emitter under the finger front electrode in a solar cell manufacturing process. It is an object of the present invention to provide a method of forming a selective emitter of a solar cell using a top hat type laser annealing that can form a selective emitter and a method of manufacturing the solar cell using the same.

According to an aspect of the present invention, there is provided a method of forming an emitter of a solar cell using a top hat type laser annealing, comprising: (a) preparing a silicon substrate doped with impurities of a first conductivity type; (b) forming an insulating film containing impurities of a second conductivity type opposite to the impurities; (c) performing an annealing process in an oxygen atmosphere to inject impurities in the insulating layer onto the silicon substrate to form an emitter layer; (d) irradiating a laser having an energy distribution in the form of a "top hat" along the point where the finger front electrode is connected while the insulating film remains on the silicon substrate. Forming a selective emitter on top of the substrate at the laser irradiation point by inducing further diffusion of impurities; And (e) removing the insulating film.

Preferably, in step (d), the insulating film remaining on the silicon substrate is a PSG (phosphorous silicate glass) film.

Preferably, in the step (d), the beam width of the laser is controlled to be larger than the width of the finger front electrode.

In the present invention, in the step (d), the number of the laser beam is irradiated corresponding to the number of finger front electrodes.

In the present invention, the step (d) is a step of irradiating the laser while moving the silicon substrate in the advancing direction of the finger front electrode while the laser irradiation means is fixed. Alternatively, step (d) is a step of irradiating a laser while moving the laser irradiation means in the advancing direction of the finger front electrode while the silicon substrate is fixed.

Preferably, a laser having a top hat energy distribution passes through a laser having a Gaussian energy distribution through a laser filter lens having a laser filter coating layer whose thickness is adjusted to reduce energy at the center of the laser. It's a laser you get let go.

Preferably, the laser filter coating layer has a thickness of a central portion greater than that of an outer portion.

Preferably, the energy density of the laser is 0.3 to 1.2 J / cm ² , the number of laser irradiation per laser irradiation point is controlled to 2 to 20 circuits.

According to an aspect of the present invention, there is provided a method of manufacturing a solar cell, including: (a) preparing a silicon substrate doped with impurities of a first conductivity type; (b) forming an insulating film containing impurities of a second conductivity type opposite to the impurities; (c) performing an annealing process in an oxygen atmosphere to inject impurities in the insulating layer onto the silicon substrate to form an emitter layer; (d) irradiating a laser having an energy distribution in the form of a "top hat" along the point where the finger front electrode is connected while the insulating film remains on the silicon substrate. Forming a selective emitter on top of the substrate at the laser irradiation point by inducing further diffusion of impurities; And (e) removing the insulating film; (f) forming an anti-reflection film on the entire surface of the silicon substrate; (g) forming a finger front electrode pattern on the anti-reflection film at the point where the selective emitter is formed and screening a rear electrode on the back surface of the silicon substrate using a screen printing method; And (h) performing one heat treatment process to penetrate the anti-reflection film to ohmic contact the finger front electrode pattern to the selective emitter, and to ohmic contact the back electrode to the back side of the silicon substrate and to the back side of the substrate in contact with the back electrode. Forming a back surface field (BSF).

According to the present invention, unlike the conventional selective emitter forming process, it is possible to simply form the selective emitter by the laser annealing process without going through the patterning process. In addition, the number of irradiation of the laser can be reduced by using the top hat type laser as the laser used for laser annealing. Accordingly, manufacturing cost of the solar cell can be reduced and productivity can be improved.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. Prior to this, terms and words used in the present specification and claims should not be construed as limited to ordinary or dictionary terms, and the inventor should appropriately interpret the concepts of the terms appropriately It should be construed in accordance with the meaning and concept consistent with the technical idea of the present invention based on the principle that it can be defined. Therefore, the embodiments described in this specification and the configurations shown in the drawings are merely the most preferred embodiments of the present invention and do not represent all the technical ideas of the present invention. Therefore, It is to be understood that equivalents and modifications are possible.

4 to 9 are process cross-sectional views sequentially illustrating a method of manufacturing a solar cell in which a selective emitter is formed using a top hat laser annealing according to a preferred embodiment of the present invention.

In the method of manufacturing a solar cell having a selective emitter according to the present invention, first, as shown in FIG. 4, a silicon substrate 301 doped with impurities of a first conductivity type is prepared. Preferably, the silicon substrate 301 is a single crystal, a polycrystalline silicon substrate or an amorphous silicon substrate. However, the present invention is not limited thereto. The silicon substrate 301 is a substrate obtained by wet etching saw damage generated on the surface of the silicon substrate 301 during slicing by a pretreatment process.

Then, as shown in FIG. 5, the emitter layer 302 is formed by implanting impurities of the second conductivity type opposite to the first conductivity type on the silicon substrate 301. When the emitter layer 302 is formed, a p-n junction is formed on the silicon substrate 301. Here, the silicon substrate 301 may be used both p-type and n-type, of which the p-type substrate has a long life and mobility of minority carriers (in the case of p-type electron is a minority carrier) most preferably Can be used. The p-type substrate is typically doped with Group 3 elements such as B, Ga, and In. When the substrate is p-type, the n-type emitter layer is formed by diffusing the Group 5 elements such as P, As, and Sb.

When the emitter layer 302 of the second conductivity type is formed, the silicon substrate 301 is first placed in a diffusion furnace, and an oxygen gas and an impurity gas of the second conductivity type are injected to form a second layer on the substrate. An insulating film 303 containing a conductive impurity is formed. Here, when the silicon substrate 301 is p-type, POCL ₃ may be used as the impurity gas. In this case, the insulating film 303 has a composition of P ₂ O ₅ . Thereafter, heat treatment is performed at a high temperature in an oxygen atmosphere to drive-in impurities in the insulating layer 303 to the surface of the silicon substrate 301. Then, an emitter layer 302 having a predetermined thickness is formed on the silicon substrate 301, and the insulating film 303 formed on the surface of the substrate 301 is changed into a PSG (phosphorus silicate glass) film by diffusion of silicon atoms. do. Meanwhile, the process of forming the emitter layer 302 disclosed herein is merely an example, and alternatively, the emitter layer 302 may be formed using a spray method or a paste coating method. It is self-evident to those of ordinary knowledge in Esau.

When the emitter layer 302 is formed through the above-described process, as shown in FIG. 6, the laser is located along the point where the finger front electrode is connected while the insulating layer 303 remains on the emitter layer 302. (L) is irradiated to perform selective annealing using the laser (L) on the emitter layer 302 of the laser (L) irradiation point. Then, the insulating film 303 at the point where the laser L is irradiated is heated to a high temperature by the laser L so that the second conductive type impurities contained in the insulating film 303 further diffuse to the surface of the emitter layer 302. Thus, the emitter layer 302 at the point where the laser L is irradiated is formed with a selective emitter 302 'having a higher impurity concentration than other portions.

As described above, the energy distribution of the energy density of the laser used for laser annealing is not a separate optical system such as a homogenizer, and as shown in FIG. 10, a Gaussian energy distribution is shown. Has

Referring to FIG. 10, when the energy density of the laser does not exceed the melting threshold level, such as A, sufficient melting does not occur at the laser irradiation point, and further diffusion of impurities contained in the insulating film 303 is not performed. As a result, it is difficult to expect a decrease in sheet resistance at the laser irradiation point. On the contrary, when the energy density of the laser is excessive, such as C, to exceed the ablation threshold level, the width W _{C that} is melted at the laser irradiation point is formed wide. However, excessive energy above the ablation threshold level causes excessive melting, resulting in mass transfer due to the difference in density between the liquid phase and the solid phase of the melted portion, resulting in a rough surface of the laser irradiation point. This may cause a problem of increasing the sheet resistance. In order to anticipate the effect of laser annealing, it is preferable to operate the laser so that proper dissolution is performed at the laser irradiation point by using a laser in which the energy density of the laser exceeds the melting threshold level and does not exceed the melting threshold level as shown in B. However, in this case, since the energy distribution with respect to the energy density of the laser to be irradiated has a Gaussian shape, there is a problem that the number of irradiation of the laser needs to be increased because the width W _{B that} is melted at the laser irradiation point is narrow.

In the present invention, in order to solve the problem caused when using the above-described laser having a Gaussian-type energy distribution, as shown in Figure 11, the laser annealing using a laser having a top-hat (energy distribution) energy distribution To be implemented.

Top-hat lasers can be controlled so that the energy distribution over energy density evenly exceeds the melting threshold level at which laser annealing can work effectively. Therefore, the width W _T which is melted at the irradiation point of the laser L can be formed wider than when using a Gaussian type laser, so that the laser annealing can be effectively performed even though the number of irradiation of the laser is reduced.

A process of forming the selective emitter 302 'using the top hat laser annealing will be described in detail with reference to FIGS. 12 to 14. First, as shown in FIG. 12, the silicon substrate 301 is positioned on a stage (not shown) for laser annealing. Then, the plurality of laser irradiation means 401 is positioned so as to correspond to the point 403 where the finger front electrode of the silicon substrate 301 is to be connected. At this time, the laser irradiation means 401 includes a laser filter lens 410 having a laser filter coating layer 420 on the exit side to which the laser is irradiated, as shown in A of FIG. 12. In general, the laser has a reduction rate depending on the thickness and the refractive index of the medium passing through a certain laser wavelength. The laser filter lens 410 according to the present invention has a laser filter coating layer 420 having a refractive index that can reduce the energy of the laser. The laser filter coating layer 420 is made of a metal or a ceramic material, and is bonded to a transparent material substrate. The laser filter coating layer 420 has a function of reducing the center energy of the laser passing through the thickness of the central portion is thicker than the outer portion. The Gaussian type laser irradiated from the laser irradiation means 401 may be converted into a top hat type laser having a flat energy distribution with respect to the maximum energy density of the laser by passing through the laser filter lens 410. This is because the energy reduction rate of the central portion of the laser filter coating layer 420 is relatively larger than that of the outer portion.

Subsequently, while moving the silicon substrate 301 in the X-axis direction, that is, in the direction of travel of the point 403 to which the finger front electrode is connected, the laser front means irradiates the laser L having the top hat state from the laser irradiation means 401. The insulating film 303 formed at the point 403 to be connected is laser annealed. At this time, the beam width W _L of the top hat type laser L irradiated onto the silicon substrate 301 is sufficient to anneal the point 403 to which the finger front electrode is connected. It is preferable to irradiate larger than _F ) (refer FIG. 14). On the other hand, when performing laser annealing, the laser irradiation means 401 may be moved in the X-axis direction in a state where the silicon substrate 301 is fixed, as described above.

In the present embodiment, the top L-shaped laser L is irradiated with a plurality of laser irradiation means 401 corresponding to the number of points 403 to which the finger front electrode of the silicon substrate 301 is connected, and the laser L Since the beam width W _L is irradiated larger than the width W _F of the finger front electrode, a uniform laser annealing is simultaneously performed with respect to the point 403 to which the plurality of finger front electrodes are connected in one laser L scanning process. Can be done. As a result, mass productivity of solar cell manufacture can be improved.

When the silicon substrate 301 is annealed by the top hat laser L, the insulating film 303 remaining on the emitter layer 302 is heated to a high temperature and impurities contained in the insulating film 303 are emitter layer. Further diffusion onto the surface 302 results in the formation of an optional emitter 302 'having an impurity concentration higher than the other portions at the point annealed by the laser L. At this time, the energy density of the laser beam (L) of the top hat type is 0.3 to 1.2 J / cm ² , the number of laser (L) beam irradiation per laser (L) beam irradiation point is preferably controlled 2 to 20 circuits. If the energy density of the laser beam L of the top hat type is less than 0.3 J / cm ² , sufficient melting does not occur at the laser L irradiation point, and further diffusion of impurities contained in the insulating layer 303 may not be easily performed. If the energy density of the laser (L) beam exceeds 1.2 J / cm ² , excessive melting occurs at the irradiation point of the laser (L), resulting in mass transfer of the molten portion, resulting in rough surface of the laser (L) irradiation point. This is not preferable because of the problem that the surface resistance increases rather than this.

Meanwhile, in the above-described embodiment, the selective emitter 302 'is formed at the point 403 to which the finger front electrode is connected by using a plurality of laser irradiation means 401. However, the method using the plurality of laser irradiation means 401 is just one example. As another example, as illustrated in FIG. 13, the excimer laser L _e irradiating a large area at one time is irradiated to the shadow mask 402 on which the laser transmission pattern is formed, thereby exiting the transmission pattern laser L _e ′. ) Can be irradiated simultaneously to the point 403 to which a plurality of finger front electrodes will be connected. In this case, as shown in B of FIG. 13, the transparent pattern of the shadow mask 402 includes the shadow mask filter coating layer 430 and the laser L _e ′ exiting the transparent pattern has a top hat energy distribution. It is desirable to control so that. The method of converting a laser form into a top hat laser has already been described above.

When the selective emitter 302 'is formed through the above-described process, as shown in FIG. 7, the insulating film 303 remaining on the silicon substrate 301 is removed. The insulating film 303 may be removed by a wet etching process using dilute hydrofluoric acid, but the present invention is not limited thereto.

Then, an edge isolation process is performed to remove the emitter layers 302 formed on the side surfaces and the rear surfaces except for the emitter layer 302 formed on the front surface of the silicon substrate 301. The emitter layer 302 formed on the side and the back of the silicon substrate 301 is not preferable because the front electrode and the back electrode are electrically connected directly if not removed. The removal of the emitter layer 302 formed on the side and back of the silicon substrate 301 may include a wet etchant containing HF, HNO ₃ and H ₂ O mixed with the front emitter layer 302 protected from the wet etchant. The silicon substrate 301 may be immersed in a container for a predetermined time, but the present invention is not limited thereto.

Then, as shown in FIG. 8, an antireflection film 304 is formed on the emitter layer 302 formed on the entire surface of the silicon substrate 301. Optionally, before forming the anti-reflection film 304, a passivation film made of a silicon oxide film or a silicon oxynitride film may be further formed. The anti-reflection film 304 is formed to lower the reflectance to sunlight, and typically may include a silicon nitride film, and in the group consisting of plasma chemical vapor deposition (PECVD), chemical vapor deposition (CVD) and sputtering It can be formed by the method selected. Thereafter, a conventional front electrode forming paste including silver and glass frit is printed on the anti-reflection film 304 at the point where the selective emitter 302 'is formed by using a screen printing method. ) And a back electrode 306 is formed by printing a conventional back electrode forming paste including aluminum on the back surface of the silicon substrate 301. The order in which the finger front electrode 305 and the back electrode 306 are formed is not limited, and any electrode may be formed first.

Then, as shown in FIG. 9, a heat treatment process for contacting the finger front electrode 305 and the back electrode 306 is performed. The finger front electrode 305 which has undergone the heat treatment process penetrates through the anti-reflection film 304 and is ohmic contacted with the selective emitter 302 '. The finger front electrode 305 includes silver and thus has excellent electrical conductivity.

The back electrode 306 subjected to the heat treatment process is ohmic contacted to the back surface of the silicon substrate 301. The back surface of the silicon substrate 301 is doped to form a back surface field (307) by doping the electrode forming material (Al) from a surface in contact with the back electrode 306 to a predetermined depth. Since the rear electrode 306 includes aluminum, not only the electrical conductivity is excellent, but also the affinity with silicon is excellent, and the bonding property is excellent. In addition, aluminum forms a P ⁺ layer, that is, BSF 307 at the interface with the silicon substrate 301 as a group 3 element, and serves to collect carriers in the BSF direction without disappearing from the surface. When the finger front electrode 305 and the back electrode 306 are formed on the silicon substrate 301, the solar cell manufacturing method using the laser annealing of the top hat form according to the present invention is completed.

As described above, although the present invention has been described by way of limited embodiments and drawings, the present invention is not limited thereto and is intended by those skilled in the art to which the present invention pertains. Of course, various modifications and variations are possible within the scope of equivalents of the claims to be described.

BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate preferred embodiments of the invention and, together with the description of the invention given above, serve to further the understanding of the technical idea of the invention, And should not be construed as limiting.

1 is a schematic view showing a basic structure of a solar cell.

2 is a view schematically showing the structure of a typical solar cell.

3 is a diagram schematically illustrating a structure of a solar cell having a selective emitter.

4 to 9 are process cross-sectional views sequentially illustrating a method of manufacturing a solar cell in which a selective emitter is formed using a top hat laser annealing according to a preferred embodiment of the present invention.

10 is a diagram illustrating an energy distribution of an energy density of a Gaussian laser.

11 is a view showing the energy distribution of the energy density of the top hat laser according to the present invention.

12 is a schematic diagram showing a laser annealing method of the top hat type according to the first embodiment of the present invention.

13 is a schematic diagram illustrating a laser annealing method of the top hat type according to the second embodiment of the present invention.

FIG. 14 is a view illustrating a relationship between a beam width of a laser and a width of a finger front electrode during laser annealing of a top hat type according to the present invention.

Claims (10)

(a) preparing a silicon substrate doped with impurities of the first conductivity type; (b) forming an insulating film containing impurities of a second conductivity type opposite to the impurities; (c) performing an annealing process in an oxygen atmosphere to inject impurities in the insulating layer onto the silicon substrate to form an emitter layer; (d) irradiating a laser having an energy distribution in the form of a "top hat" along the point where the finger front electrode is connected while the insulating film remains on the silicon substrate. Forming a selective emitter on top of the substrate at the laser irradiation point by inducing further diffusion of impurities; And (e) removing the insulating film; the method of forming an emitter of a solar cell using a top hat type laser annealing, comprising: a. The method of claim 1, wherein in step (d), The insulating film remaining on the silicon substrate is a PSG (phosphorous silicate glass) film, the method of forming a selective emitter of a solar cell using a laser annealing of the top hat type. The method of claim 1, wherein in step (d), The beam width of the laser, the method of forming a selective emitter of a solar cell using a top hat type laser annealing, characterized in that larger than the width of the finger front electrode. The method of claim 1, wherein in step (d), The method of forming an emitter of a solar cell using a top hat laser annealing, characterized in that the number of the laser beam corresponds to the number of finger front electrodes. The method of claim 1, wherein the step (d) A method of forming an emitter of a solar cell using a top hat type laser annealing, wherein the laser is irradiated while the silicon substrate is moved in the advancing direction of the finger front electrode while the laser irradiating means is fixed. The method of claim 1, wherein the step (d) A method of forming an emitter of a solar cell using a top hat type laser annealing, wherein the laser is irradiated while moving the laser irradiation means in the direction of travel of the finger front electrode while the silicon substrate is fixed. The method of claim 1, A laser with an energy distribution in the form of a top hat, A solar cell using a top hat laser annealing, characterized in that the laser is obtained by passing a laser having a Gaussian energy distribution through a laser filter lens having a laser filter coating layer whose thickness is adjusted to reduce the energy of the laser center. Selective emitter formation method. 8. The method of claim 7, The laser filter coating layer is a method of forming a selective emitter of a solar cell using a top hat type laser annealing, characterized in that the thickness of the central portion is larger than the thickness of the outer portion. 8. The method of claim 7, The energy density of the laser is 0.3 to 1.2 J / cm ² , Method for forming a selective emitter of a solar cell using a laser annealing of the top hat type, characterized in that the number of irradiation of the laser per laser irradiation point is 2 to 20 times. (a) preparing a silicon substrate doped with impurities of the first conductivity type; (b) forming an insulating film containing impurities of a second conductivity type opposite to the impurities; (c) performing an annealing process in an oxygen atmosphere to inject impurities in the insulating layer onto the silicon substrate to form an emitter layer; (d) irradiating a laser having an energy distribution in the form of a "top hat" along the point where the finger front electrode is connected while the insulating film remains on the silicon substrate. Forming a selective emitter on top of the substrate at the laser irradiation point by inducing further diffusion of impurities; And (e) removing the insulating film; (f) forming an anti-reflection film on the entire surface of the silicon substrate; (g) forming a finger front electrode pattern on the anti-reflection film at the point where the selective emitter is formed and screening a rear electrode on the back surface of the silicon substrate using a screen printing method; And (h) A single heat treatment process is performed to penetrate the anti-reflection film to ohmic contact the finger front electrode pattern to the selective emitter, to ohmic contact the back electrode to the backside of the silicon substrate, and to contact the back electrode with the BSF forming a back surface field; and manufacturing a solar cell.

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