KR20050087248A - Solar cell using double anti-reflection coatings and fabrication method thereof - Google Patents

Abstract

To reduce the reflectance at the light-receiving surface of the solar cell and at the same time to produce a two-layer anti-reflection film to perform a passivation function in a simpler method. To this end, in the present invention, the first conductive semiconductor layer; A second conductive semiconductor layer formed on one surface of the first conductive semiconductor layer and having an opposite conductivity type; A front electrode in contact with at least a portion of the second conductive semiconductor layer; A back electrode in contact with at least a portion of the first conductive semiconductor layer; A first anti-reflection layer formed on the light receiving surface of the second conductive semiconductor layer; And a second antireflection layer formed on the first antireflection layer and having a lower refractive index than the first antireflection layer. At this time, it is preferable that the first antireflection layer and the second antireflection layer are continuously deposited in the same chamber.

Description

Solar cell using double layer anti-reflective coating and manufacturing method {Solar cell using double anti-reflection coatings and fabrication method

The present invention relates to a solar cell and a method of manufacturing the same, and more particularly, to a solar cell and a method for manufacturing the passivation of the surface and the bulk as well as the anti-reflection function by using two layers of anti-reflection film of different refractive index. .

In general, a solar cell has a pair of electrons and holes generated inside the semiconductor of the solar cell by light from outside, and electrons move to an n-type semiconductor by an electric field generated at a pn junction in the pair of electrons and holes. Moving to p-type semiconductors produces power.

In order to increase the efficiency of such a solar cell, it is very important to maximize the number of photons reaching the active layer in the solar cell and to minimize the loss due to reflection of the cell surface.

In the case of silicon (Si), since the refractive index is 3.88 at a wavelength of 600 nm (wavelength region where the incident sunlight is maximum), the refractive index of the ideal antireflection film is 2.1. Titanium oxide (TiOx) is a representative material that satisfies these conditions, and has been used continuously as an anti-reflection film of silicon solar cells. Plasma silicon nitride (SiNx) has a refractive index similar to that of titanium oxide, and even has a passivation effect of the substrate that cannot be expected when titanium oxide is used.

The use of a double layer anti-reflective coating to reduce reflectance is more effective than using a single layer anti-reflective coating. However, in most cases, the silicon nitride film is used as a single layer antireflection film, and when used as a double layer antireflection film, a heterogeneous material having a low refractive index, for example, a magnesium fluoride (MgF ₂ ) film or a silicon oxide (SiO ₂ ) film is used.

However, in this case, there is a disadvantage in that a process is newly added in manufacturing a solar cell, and a mismatch between grids of heterogeneous films having different properties occurs. In addition, the synergy regarding the passivation by hydrogen contained in the silicon nitride film is also not clear.

Therefore, there is an urgent need for a technique for implementing a double layer anti-reflection film that can passivate while reducing reflectance in a simpler method.

The present invention is to solve the above problems, to reduce the reflectance on the light receiving surface of the solar cell and at the same time to produce a two-layer anti-reflection film to perform a passivation function in a simpler method.

In order to achieve the above object, in the present invention, the first conductive semiconductor layer; A second conductive semiconductor layer formed on one surface of the first conductive semiconductor layer and having an opposite conductivity type; A front electrode in contact with at least a portion of the second conductive semiconductor layer; A back electrode in contact with at least a portion of the first conductive semiconductor layer; A first anti-reflection layer formed on the light receiving surface of the second conductive semiconductor layer; And a second antireflection layer formed on the first antireflection layer and having a lower refractive index than the first antireflection layer.

On the second anti-reflection layer, a sequentially stacked structure of the first anti-reflection layer and the second anti-reflection layer may be repeatedly formed.

In this case, it is preferable that the semiconductor layers of the first conductive type and the second conductive type are silicon, and the first antireflection layer and the second antireflection layer are silicon nitride films (SiN _x ).

 The second antireflective layer may have a refractive index of 0.3 to 0.7 lower than that of the first antireflective layer, more specifically, the refractive index of the first antireflective layer may be 2.2 to 2.4, and the refractive index of the second antireflective layer may be 1.7 to 1.9. .

As the difference in refractive index between the first anti-reflective layer and the second anti-reflective layer is larger, the number of photons incident on the solar cell may be increased by decreasing the reflectance on the light-receiving surface, and thus the efficiency of the solar cell may be improved.

The total thickness of the thickness of the first antireflection layer and the thickness of the second antireflection layer may be 70-90 nm.

The first antireflection layer and the second antireflection layer are preferably deposited in the same chamber. For example, the first antireflection layer and the second antireflection layer may be continuously deposited in a plasma chemical vapor deposition (PECVD) chamber.

In addition, the present invention comprises the steps of preparing a semiconductor layer having a first conductivity type; Forming a second conductive semiconductor layer having an opposite conductivity type on one surface of the first conductive semiconductor layer; Forming a first antireflection layer and a second antireflection layer sequentially on the light receiving surface of the second conductive semiconductor layer, wherein the second antireflection layer has a lower refractive index than the first antireflection layer; Forming a front electrode to be connected to at least a portion of the second conductive semiconductor layer; It provides a method of manufacturing a solar cell comprising the step of forming a back electrode to be connected to at least a portion of the first conductive semiconductor layer.

After forming the second anti-reflection layer, the method may further include repeatedly forming a sequentially stacked structure of the first anti-reflection layer and the second anti-reflection layer on the second anti-reflection layer.

In the preparing of the first conductive semiconductor layer, a silicon substrate may be prepared or a silicon thin film may be formed on the substrate.

Hereinafter, with reference to the accompanying drawings for the present invention will be described in detail.

1 is a cross-sectional view showing the structure of a solar cell according to a first embodiment of the present invention. In the solar cell according to the first embodiment of the present invention, as shown in FIG. 1, an n layer 11 is formed on the front surface of the p-type silicon substrate 10, and is electrically connected to the n layer 11. The front electrode 12 is formed on a portion of the n layer, and the rear electrode 13 is formed on the rear surface of the p-type silicon substrate 10.

Here, the first antireflection film layer 100 and the second antireflection film layer 200 having different refractive indices are sequentially on the light receiving surface of the n layer 11, that is, on the entire surface of the n layer 11 except the front electrode 12. Formed. In this case, a second anti-reflection film layer 200 having a lower refractive index is formed on the first anti-reflection film layer 100 having a higher refractive index.

2 is a cross-sectional view showing the structure of a solar cell according to a second embodiment of the present invention. As shown in FIG. 2, the second embodiment of the present invention is a back junction solar cell in which both the n-type emitter 21 and the p-type base 22 are formed on the back of the silicon substrate 20.

The first anti-reflection film layer 100 and the second anti-reflection film layer 200 having different refractive indices are formed on the front surface of the silicon substrate 20, which is the light receiving surface of the back-junction solar cell, and the refractive index is relatively higher. The second anti-reflection film layer 200 having a lower refractive index is formed on the high first anti-reflection film layer 100.

Such a back junction solar cell is particularly advantageous for a condensing solar cell because no electrode is formed on the front surface thereof. In the back junction solar cell, since the passivation of the front surface of the silicon substrate 20 is more important, the effect of the present invention is expected to be more remarkable.

In the present invention, the first material and the second antireflection film layer having different refractive indices are achieved by successive deposition of the same material in the same process chamber. For example, a silicon nitride film (SiN _x ) having different refractive indices may be deposited by a plasma chemical vapor deposition (PECVD) method to be used as a two-layer antireflection film.

When the silicon nitride film is deposited by the PECVD method, the refractive index of the silicon nitride film can be relatively freely changed from 1.6 to 2.7 depending on the deposition conditions.

One method of changing the refractive index is a method of changing the relative ratio of Si and N. 3 is a graph showing a change in refractive index according to the N / Si ratio (x) in the silicon nitride film (SiN _x ). If the silicon nitride film (SiN _x ) is a PECVD method, the content of Si and N, that is, x, may be changed by changing the ratio of SiH ₄ and NH ₃ gas used as source gas, thereby changing the refractive index.

Thus, one of the important factors to be considered when changing the refractive index of the silicon nitride film (SiN _x ) is the absorption coefficient of the film.

When the refractive index of the silicon nitride film is increased, the bandgap is lowered and the absorption rate is increased. This can be confirmed from the graph of FIG. 4 showing the relationship between the refractive index n and the water absorption α of the silicon nitride film.

Strong absorption of the silicon nitride film reduces the probability that photoelectrons reach the active layer of the solar cell. Therefore, in the case of using a silicon nitride film as a single layer anti-reflection film, it is selected to have a refractive index with a low absorption. In the conventional single layer antireflection film, a silicon nitride film having a refractive index of about 1.9 to 2.1 has been used as a single layer antireflection film.

Table 1 shows an example of deposition conditions for continuously depositing a first silicon nitride film and a second silicon nitride film having a refractive index changed by PECVD according to the present invention.

First Silicon Nitride Second Silicon Nitride Substrate Temperature (℃) 450 450 SiH ₄ flow rate (sccm) 800 100 NH ₃ flow rate (sccm) 3400 4000 Chamber Pressure (Torr) 1.7 1.7 HF power (W) 1100 1100

When forming the first and second silicon nitride antireflection layers under the conditions as shown in Table 1, by changing the SiH ₄ flow rate, NH ₃ flow rate, HF power, chamber pressure, etc., the refractive index and thickness uniformity of the silicon nitride film can be controlled. have. That is, the manufacturing conditions of the silicon nitride film should be optimized to obtain the desired refractive index and thickness uniformity.

When continuously depositing the first antireflection layer 100 and the second antireflection layer 200 by the PECVD method, the refractive index and the thickness of each layer should be optimized to satisfy the condition that the reflectance is minimized.

The refractive index of the first anti-reflection layer 100 formed on the light-receiving surface of the n layer is 2.2 to 2.4, the refractive index of the second anti-reflection layer 200 formed on the first anti-reflection layer 100 is 1.7 to 1.9, It is preferable to make the difference in refractive index between the two by 0.3 to 0.7. At this time, as the difference in refractive index between the first antireflection layer 100 and the second antireflection layer 200 increases, the reflectance on the light receiving surface is lowered, thereby increasing the number of photons absorbed by the solar cell, thus improving the efficiency of the solar cell.

The total thickness of the sum of the thickness of the first anti-reflection layer 100 and the thickness of the second anti-reflection layer 200 is preferably 70-90 nm, and each of the first anti-reflection layer 100 and the second anti-reflection layer 200 It is preferable that the layer thickness is uniform.

As an example, the thickness ratio of the second antireflection layer 200 to the thickness of the first antireflection layer 100 may be 0.5 to 0.8.

The thickness of each of the first and second anti-reflection layers may be calculated under the condition of reducing the reflectance by non-linear regression in the wavelength band of 300 to 1000 nm.

FIG. 5 is a graph showing reflectance according to wavelength, and FIG. 5 shows a case of Comparative Example using single layer silicon nitride as an antireflection film, and Examples 3 and 4 using double layer silicon nitride.

In Example 3, the thickness ratio of the second anti-reflective layer 200 to the thickness of the first anti-reflective layer 100 was 0.53. In Example 4, the second anti-reflective layer 200 relative to the thickness of the first anti-reflective layer 100. ), The thickness ratio was 0.72.

In addition, the refractive index according to the wavelength of the first and second antireflection layers used at this time is described by five variables by Tauc-Lorentz parametrization.

On the other hand, when the silicon nitride film deposited by PECVD on the polycrystalline silicon solar cell is used as an anti-reflection film, the efficiency is increased by 1.5 to 2.0% compared with the titanium oxide (TiOx) film. As mentioned above, this is because the silicon nitride film plays a role of passivation for the substrate as well as the anti-radiation film of the polycrystalline silicon substrate.

The passivation effect can be classified into surface passivation related to the silicon interface and bulk passivation effect related to defects in the substrate. In particular, the bulk passivation effect occurs during the contact-firing process after electrode formation of the polycrystalline solar cell.

As the hydrogen (H) atoms contained in the silicon nitride film are diffused into the Si substrate during the heat treatment, various types of defects present in the substrate are passivated, and the charges generated from sunlight are transferred to these defects. By suppressing the recombination (by recombination) it leads to a substantial increase in battery efficiency.

FIG. 6 is a graph illustrating relative lifetimes according to refractive indices. FIG. 6 illustrates relative effective carrier lifetimes of a silicon substrate by silicon nitride films having different refractive indices. At this time, the life was normalized to a case where the refractive index was 1.8.

This shows that the thin film having a high refractive index is more favorable for the passivation effect than the low thin film. Especially, when the refractive index is higher than 2.2, the passivation effect is significantly improved. Assuming the bulk life is the same, this graph shows that the surface passivation effect is advantageous for thin films with high refractive index.

In the present invention, it is intended to increase the synergy effect of bulk and surface passivation by the first anti-reflection layer having a high refractive index positioned close to the light receiving surface of the solar cell. That is, by using only two layers of anti-reflection layers that are continuously deposited in the same process chamber, the surface is not only acting as an anti-reflection layer but also simultaneously implementing surface and bulk passivation.

As described above, the present invention has an effect of reducing the reflectance from 12% to 8% by using a silicon nitride film as a double layer anti-reflection film in a silicon solar cell.

In addition, there is an effect that the carrier life of the silicon substrate by the first silicon nitride film having a relatively high refractive index is increased by 4 to 5 times than that of silicon nitride used as a single layer antireflection film, and thus the first and second silicon nitride films are Due to the bilayer anti-reflection film structure used, the surface passivation effect can be achieved simultaneously with the bulk passivation effect.

In the case of using a conventional anti-reflection film, a heterogeneous material having a large difference in refractive index is used, in which case a new process is introduced. However, since the silicon nitride film having only a difference in refractive index is continuously processed in the same deposition chamber, additional new process chambers are not required, thereby reducing the cost.

1 is a cross-sectional view showing the structure of a solar cell according to a first embodiment of the present invention,

2 is a cross-sectional view showing the structure of a solar cell according to a second embodiment of the present invention,

3 is a graph showing a change in refractive index according to the N / Si ratio (x) in the silicon nitride film (SiN _x ),

4 is a graph showing the relationship between the refractive index n and the water absorption α of the silicon nitride film.

5 is a graph showing the reflectance according to the wavelength,

6 is a graph showing the relative carrier life according to the refractive index.

Claims (18)

A first conductive semiconductor layer; A second conductive semiconductor layer formed on one surface of the first conductive semiconductor layer and having an opposite conductivity type; A front electrode in contact with at least a portion of the second conductive semiconductor layer; A back electrode in contact with at least a portion of the first conductive semiconductor layer; A first anti-reflection layer formed on the light receiving surface of the second conductive semiconductor layer; And A second antireflection layer formed on the first antireflection layer and having a lower refractive index than the first antireflection layer; Solar cell comprising a. The method of claim 1, The solar cell of the first anti-reflective layer and the second anti-reflective layer is repeatedly formed on the second anti-reflection layer. The method of claim 1, The solar cell of the first conductive type and the second conductive type is silicon. The method of claim 1, The first antireflection layer and the second antireflection layer are silicon nitride films (SiN _x ). The method of claim 1, The second anti-reflection layer has a refractive index of 0.3 to 0.7 lower than that of the first anti-reflection layer. The method of claim 1, The refractive index of the first antireflection layer is 2.2 to 2.4, the refractive index of the second antireflection layer is 1.7 to 1.9 solar cell. The method of claim 1, The solar cell of which the efficiency of the solar cell is improved as the difference in refractive index between the first antireflection layer and the second antireflection layer is larger. The method of claim 1, The total thickness of the sum of the thickness of the first anti-reflection layer and the thickness of the second anti-reflection layer is 70-90 nm. The method of claim 1, The first antireflection layer and the second antireflection layer are solar cells deposited in the same chamber. The method of claim 9, Wherein the first antireflection layer and the second antireflection layer are continuously deposited in a plasma chemical vapor deposition (PECVD) chamber. Preparing a semiconductor layer having a first conductivity type; Forming a second conductive semiconductor layer having an opposite conductivity type on one surface of the first conductive semiconductor layer; Forming a first antireflection layer and a second antireflection layer sequentially on the light receiving surface of the second conductive semiconductor layer, and forming the second antireflection layer to have a lower refractive index than the first antireflection layer ; Forming a front electrode to be connected to at least a portion of the second conductive semiconductor layer; Forming a rear electrode to be connected to at least a portion of the first conductive semiconductor layer Method for manufacturing a solar cell comprising a. The method of claim 11, After the formation of the second antireflection layer, repeatedly forming a sequentially stacked structure of the first antireflection layer and the second antireflection layer on the second antireflection layer. The method of claim 11, In the preparing of the first conductive semiconductor layer, a method of manufacturing a solar cell or preparing a silicon substrate, or to form a silicon thin film on the substrate. The method of claim 11, Silicon is formed as the second antireflection layer, The first anti-reflection layer and the second anti-reflection layer to form a silicon nitride film. The method of claim 11, In the forming of the first anti-reflection layer and the second anti-reflection layer, the second anti-reflection layer is formed to have a refractive index of 0.3 to 0.7 as compared to the first anti-reflection layer has a lower refractive index. The method of claim 11, In the forming of the first anti-reflection layer and the second anti-reflection layer, the refractive index of the first anti-reflection layer is 2.2 to 2.4, the refractive index of the second anti-reflection layer is formed so that the 1.7 to 1.9. The method of claim 11, In the forming of the first anti-reflective layer and the second anti-reflective layer, the total thickness of the thickness of the first anti-reflective layer and the thickness of the second anti-reflective layer is formed so that the total thickness is 70-90 nm. The method of claim 11, In the forming of the first anti-reflection layer and the second anti-reflection layer, a method of manufacturing a solar cell that is continuously deposited in a plasma chemical vapor deposition (PECVD) chamber.