KR100838167B1 - Manufacturing Method of Back Contacts for CIGS Solar Cell - Google Patents

Abstract

The present invention relates to a method for manufacturing a back electrode of a solar cell having a cupper-indium-gallium-selenide (CI (S)) light absorbing layer, wherein a pressure of a reaction gas for plasma generation during a sputtering process for forming a back electrode is described. The present invention provides a method of improving the characteristics of an electrode by applying RF bias to a substrate at the time of DC sputtering.

According to the present invention, by changing the pressure of the reaction gas supplied in the sputtering process, it is possible to manufacture the back electrode having excellent adhesion between the electrode and the glass substrate and easy sodium diffusion, and also RF bias to the substrate during DC sputtering Simultaneous application of the back electrode enables the production of a relatively low resistivity electrode characteristics.

Description

Manufacturing Method of Back Contacts for CI (G) S Solar Cell}

1 is a schematic diagram of a sputtering process according to one embodiment of the present invention;

2 is a schematic diagram of the conditions of metal bonding affecting the tensile force of the conductive layer;

3 is a graph showing the change in intrinsic stress of the entire conductive layer according to the amount of RF bias power in the sputtering process according to an embodiment of the present invention;

4 is a graph showing a change in the specific resistance value according to the amount of RF bias power in the sputtering process according to an embodiment of the present invention.

The present invention relates to a method for manufacturing a back contact of a solar cell including a cupper-indium-gallium-selenide (CI (S)) light absorbing layer, and more particularly, a reaction gas of a relatively high pressure. Forming a first conductive layer on the substrate by DC sputtering, and forming a second conductive layer on the first conductive layer by DC sputtering while applying an RF bias on the substrate under a relatively low pressure reaction gas. It provides a method for manufacturing a back electrode of a solar cell comprising the process of.

Recently, with increasing interest in environmental problems and energy depletion, there is a growing interest in solar cells as an alternative energy with abundant energy resources, no problems with environmental pollution, and high energy efficiency.

Solar cells are classified into silicon semiconductor solar cells, compound semiconductor solar cells, stacked solar cells, and the like according to the constituents, and the solar cells including the CI (G) S light absorption layer of the present invention are classified among the compound semiconductor solar cells. Belongs to.

The CI (G) S light absorbing layer-based solar cell is manufactured by sequentially forming a back contact such as molybdenum and a CI (G) S light absorbing layer on a substrate such as glass. The manufacturing method of such a rear electrode is partially disclosed in US Patent No. 6,258,620. The patent discloses a process for producing molybdenum into a bi-layer structure by a sputter deposition process, in which the first step forms a first molybdenum layer exhibiting high adhesion to the substrate under relatively high argon pressure. And in the second step, form a low resistivity second molybdenum layer on the first layer under relatively low argon pressure.

However, the patent only suggests the formation of a conductive layer of a bi-layer structure, and it has been confirmed that the conductive layer produced by the patent does not exhibit a sufficiently low specific resistance to a desired degree. In addition, according to the patent, the high density between the metal atoms is to cause the diffusion of sodium supplied from the substrate to suppress the effective process concentration acting as an important factor for the performance of the solar cell.

Therefore, there is a high need for a technology capable of manufacturing a solar cell of higher efficiency while fundamentally solving these problems.

The present invention aims to solve the problems of the prior art as described above and the technical problems that have been requested from the past.

The present invention has a lower specific resistance than the conventional bi-layered back electrode forming technology, and at the same time the manufacture of a back electrode easy to diffuse sodium in the CI (G) S light absorption layer and It is to provide a high efficiency CI (G) S solar cell comprising the same.

The manufacturing method of the back electrode of the CI (G) S light absorption layer based solar cell according to the present invention for achieving the above object,

(a) forming a first conductive layer on the substrate by DC sputtering under a relatively high pressure reaction gas; And

(b) forming a second conductive layer on the first conductive layer by DC sputtering while applying an RF bias on the substrate under a relatively low pressure reaction gas;

It is configured to include.

Therefore, according to the method of manufacturing the back electrode according to the present invention, first, in the step (a) of forming the first conductive layer, which is a part in direct contact with the substrate, the process is performed under a high pressure reaction gas to improve adhesion to the substrate. Proceed. Then, in the step (b) of forming the second conductive layer, the pressure of the reaction gas is lowered to form a thin film of dense structure, and at the same time, the RF bias is applied to the substrate in the DC sputtering process, thereby having a low resistivity characteristic. Can be deposited more efficiently.

According to the experiments performed by the inventors of the present application, when the RF bias was applied to the substrate portion in the DC sputtering process, it was confirmed that the specific resistance and the intrinsic stress were lowered in proportion to the amount of power of the RF bias. Based on this, when RF bias is applied with an output amount such that the sum of intrinsic stress of the entire conductive layer becomes '0', it is possible to form a thin film having a low specific resistance and no residual stress. It was also confirmed that it can.

In general, as a method of forming the conductive layer, e-beam evaporation, DC sputtering, or RF sputtering is used. However, in the case of RF sputtering, shielding and grounding by a noise filter or an insulator are required because noise may be generated in other digital circuits, and a deposition rate is lower than that of DC sputtering. In addition, the electron beam deposition method has a problem that when the area to be deposited is wide, it is difficult to deposit with a uniform thickness and the productivity is inferior. Therefore, DC sputtering can be preferably used.

More preferably, the DC sputtering is DC magnetron sputtering. Magnetron sputtering is a method in which the generated plasma is collected by a flux generated from a permanent magnet and deposited on a substrate. When deposition is performed using this method, the generated plasma becomes uniform throughout, resulting in a uniform thin film. The permanent magnets used are mainly NbFeB-based, and are generally manufactured in the form of a flat plate and placed at the bottom of a cathode (target).

As the reaction gas, argon (Ar), which is an inert gas, is preferably used. In the case of using argon as a reaction gas, the process is briefly described as follows. First, in the sputtering apparatus, a target to be formed into a film is used as a cathode, and a substrate is used as an anode. When the power is applied, the injected argon collides with the electrons emitted from the cathode side and is excited to become argon ions (Ar ⁺ ), and the argon ions are attracted toward the cathode to collide with a target to be formed. At this time, each argon ion particle has an energy equal to E = h ν , and the energy is transferred to a target to be formed in a collision. When the transferred energy can overcome the binding force of the elements forming the target and the work function of the electrons, the plasma is released, and the released metal ions are deposited on the substrate.

In particular, if the RF bias is applied to the substrate additionally in the process of forming the second conductive layer as in the present invention, it may have a similar effect to that of the negative bias applied to the substrate (target). Since the straightness of the metal ions separated from can be increased, a more dense structure can be formed.

The substrate may be variously used, such as a glass substrate, aluminum foil, carbon film or polyimide, preferably a glass substrate is used.

The first conductive layer and the second conductive layer may be prepared by selecting one or more of molybdenum, aluminum, tungsten, chromium or tantalum, and in particular, molybdenum is preferably used. The reason for using molybdenum in the manufacture of the conductive layer is first, when using a glass of 2 ~ 4 mm thickness as the substrate of the CI (G) S solar cell, it has excellent adhesion to the glass substrate and has a low specific resistance This is because the characteristics are excellent, and secondly, in the process of forming the subsequent CI (G) S light absorbing layer, a high temperature of about 500 ° C. is applied. Third, because the Mo (S) ₂ phase is formed at the interface between the CI (G) S light absorbing layer and the subsequent high temperature process, it has a low contact resistance.

In one preferred embodiment, in addition to the first conductive layer and the second conductive layer, a third conductive layer may be further formed as a third thin film. The third conductive layer may apply the reaction gas at a pressure in a range where the intrinsic stress is maximum. Accordingly, the third conductive layer may have a maximum tensile force, and the spacing between particles is stably maintained, so that sodium, etc. contained in the glass substrate during the heat treatment process of the subsequent CI (G) S light absorbing layer can be easily performed. In order to be diffused, it may have a structure (hereinafter, referred to as an 'open structure') in which any material is easily diffused by interparticle spacing.

As described above, even when the third conductive layer having the maximum tensile force and the open structure is formed, the sum of the total stresses of the entire conductive layers including the third conductive layer is preferably set to approximately '0', thereby preventing external stress. Can be kept in a stable state. In some cases, the specific resistance value of the entire conductive layer may be somewhat higher than that of the two-layer structure that does not form the third layer, but much higher than that of the two-layer structure that does not apply RF bias according to the prior arts. The low specific resistance can be maintained, and the problem of the two-layer structure according to the prior arts, that is, the problem of difficulty of diffusion of sodium ions and the problem of lowering efficiency, has an advantage of very excellent efficiency.

The third conductive layer may be prepared by selecting one or more of molybdenum, aluminum, tungsten, chromium or tantalum, and preferably molybdenum. In addition, the material may be the same as the material used for the first conductive layer and the second conductive layer, or may be a different material.

In general, diffusion of sodium into the CI (G) S light absorbing layer is known to play a role in improving the efficiency of polycrystalline CI (G) S solar cells having grain boundaries. Therefore, in the present invention, in order to maximize the efficiency of the solar cell without causing any deterioration of other electrode properties, it is preferable to manufacture the back electrode in the following process sequence and conditions.

First, in the first step, a thin film (first conductive layer) having excellent adhesion to a substrate in a range of 5 to 15 mTorr (high pressure) with a thickness of about 100 to 300 nm is formed. In the second step, a thin film (second conductive layer) having a low specific resistance (10 ⁻⁴ to 10 ⁻⁵ ohm · cm) in a range of 1 to 5 mTorr (low pressure) with a thickness of about 500 to 1000 nm is formed. Then, in the third step, it has a maximum tensile force in the range of 3 to 8 mTorr with a thickness of about 100 to 300 nm and has a suitable resistance (10 ^-3 to 10 ^-4 ohmcm) while the metal bond is maintained. While forming an open structure thin film (third conductive layer), it is possible to induce the diffusion of sodium can easily occur at the interface between the conductive layer and the CI (G) S light absorbing layer. Moreover, the sodium diffusion effect can also be expanded by using the glass substrate from which sodium content differs. At this time, the sum of the total stresses of the entire conductive layers is preferably configured to be approximately '0', so that it can be maintained in a very stable state against external stresses.

This multilayer structure, that is, a first conductive layer having excellent adhesion to a substrate, a second conductive layer formed on the first conductive layer and having a resistance of 10 ⁻⁴ to 10 ⁻⁵ ohm · cm, the second A solar cell back electrode structure including a third conductive layer formed on the conductive layer and having an open structure having a resistance of 10 ^-3 to 10 ^-4 ohm cm and easy diffusion of any material from the substrate is known. Not a new structure.

As mentioned above, although arbitrary materials mentioned above can mention the sodium contained in a glass substrate as a typical example, it is not limited only to this.

It is preferable that the total thickness of the said 1st conductive layer, the 2nd conductive layer, and a 3rd conductive layer exists in the range of 700-1500 nm.

The present invention also provides a CI (G) S solar cell comprising a back electrode manufactured by the above method, and a solar cell module consisting of a plurality of the solar cells. Since the configuration of the CI (G) S solar cell and the solar cell module and its manufacturing method are known in the art, a detailed description thereof is omitted herein.

Hereinafter, the present invention will be described in more detail with reference to the drawings, but the scope of the present invention is not limited thereto.

First, referring to FIG. 1, a process of forming a second conductive layer by applying an RF bias to a substrate in a DC sputtering process according to an example of the present invention is illustrated. DC power is supplied by using a target to form a cathode as a cathode and a glass substrate as an anode.

At this time, if an RF bias is applied to the substrate side, electrons are attracted to the substrate when positive voltage is applied, and positive ions are attracted to the substrate when negative voltage is applied. In this process, due to the difference in mass between the electrons and the cations, the state of the electrons in the substrate (substrate) as a whole. Therefore, it is possible to create an atmosphere such that a negative bias is applied, so that the linearity of molybdenum ions separated from the target to be deposited can be increased, thereby making a more dense structure.

As a result, in the production of CI (G) S solar cells, it is possible to form a back electrode having a very low specific resistance. This means that the specific resistance of molybdenum having physically high specific resistance is lower than that of aluminum used as the back electrode of the crystalline silicon solar cell.

Figure 2 schematically shows the conditions of the metal bond affecting the tensile strength of the conductive layer. First, in a state in which the metal atoms are concentrated (1), the tensile force of the conductive layer is weakened due to the repulsive force between adjacent atoms, and the diffusion of sodium becomes difficult, thereby decreasing the efficiency of the solar cell. On the contrary, if the bonding distance between atoms becomes too far (3), a relatively more open structure is formed, which facilitates the diffusion of sodium, but it is undesirable because the metal bond is broken and the specific resistance is increased again. Therefore, it is necessary to find a condition having the maximum tensile force and maintain the state (2) which forms a structure that is as open as possible while maintaining a low specific resistance while maintaining a metal bond.

By applying this principle, it is possible to form a two-layer or three-layer conductive layer by measuring the change in tensile strength according to the pressure of the reaction gas (argon) in the sputtering process, and thereby applying the reaction gas at an appropriate pressure. .

More specifically, first, in a state where the reaction gas is at a low pressure (1 to 5 mTorr), metal atoms are deposited in a dense state, and thus, tensile strength is lowered by compressive stress, but low resistivity Indicates. Therefore, according to one embodiment of the present invention, it is possible to form a second conductive layer (see Fig. 2- (1)) in which the low specific resistance is required under the above conditions.

As the pressure of the reaction gas increases, the density of the deposited metal atoms decreases, and the tensile stress increases, so that the tensile force also increases. In particular, since it has a large open structure at an appropriate level of pressure (3 to 8 mTorr) having a maximum tensile force, in the above conditions, a third conductive layer requiring an open structure together for high tensile force and easy diffusion of sodium is required. (See Fig. 2- (2)).

By continuously increasing the pressure of the reaction gas (5 to 15 mTorr), the bonding distance between the deposited metal atoms is further increased, which reduces the tensile force, but facilitates diffusion of sodium and increases adhesion to the glass substrate. Therefore, the first conductive layer (see Fig. 2- (3)) can be formed under the above conditions.

In this case, since the sum of intrinsic stresses of the entire conductive layer is preferably configured to have a value close to '0', not only when the two-layer structure of the first conductive layer and the second conductive layer is formed, Even when the three-layer structure in which the third conductive layer is additionally formed is formed, it is preferable that the sum of the intrinsic stresses has a value close to '0', thereby maintaining a stable state against external stress as a whole.

In the two-layer structure, the intrinsic stress of each conductive layer may be configured such that, for example, the stress of the first conductive layer is +6 as a relative value, and the stress of the second conductive layer is -6 correspondingly. In the three-layer structure, the absolute value of the intrinsic stress of the second conductive layer in the compressive stress range can be adjusted to have a value substantially equal to the sum of the first conductive layer and the third conductive layer in the tensile stress range. For example, it can be comprised so that the stress of each conductive layer may become the stress (+3) of a 1st conductive layer, the stress (-10) of a 2nd conductive layer, and the stress (+7) of a 3rd conductive layer, respectively. To this end, an RF bias may be appropriately applied during deposition of the second conductive layer, and the intrinsic stress of the first conductive layer is intrinsic stress of the second conductive layer and the intrinsic stress of the third conductive layer by adjusting the pressure of the reaction gas. It can also be configured to have a difference value of.

Therefore, the conductive layer according to the present invention has a low resistance value due to the second conductive layer, but has a relatively large interparticle spacing in the first conductive layer and the third conductive layer, thereby making it easy to diffuse sodium, thereby achieving excellent efficiency. Have That is, by changing the pressure of the reaction gas (argon) in three steps as described above, it is possible to manufacture a solar cell back electrode having a CI (G) S light absorption layer that can optimize the electrode characteristics.

3 and 4 are graphs illustrating changes in intrinsic stress and specific resistance of the entire conductive layer according to the amount of RF bias power in the sputtering process when forming a conductive layer having a two-layer structure according to one embodiment of the present invention. Are respectively shown. For reference, in FIG. 3, sig 11 represents a stress value in the x direction, and Sig 22 represents a stress value in the y direction.

In the sputtering process, molybdenum is grown to a thickness of 200 nm in an argon atmosphere of 10 mTorr to form a first conductive layer, and RF bias power is changed to 0 to 100 watts in an argon atmosphere of 2 mTorr, respectively. Molybdenum was grown on the first conductive layer to a thickness of 500 nm to form a second conductive layer.

Referring to these figures, it can be seen that when RF bias is not applied, the intrinsic stress value is 3000 MPa and the specific resistance is 1.5 ㅧ 10 ^-5 Ωcm or more.

On the other hand, in accordance with a preferred embodiment of the present invention, is approaching the 37 ~ 35 W or so, a zero value of intrinsic stress of the entire conductive layer is added an RF bias of the, at this time, the specific resistance is 1.2 ~ 1.0 × 10 ^- It can be seen that the value is very low as ⁵ Ωcm.

Therefore, the conductive layer according to the present invention has excellent adhesion, while having a multilayered structure of a first conductive layer having a relatively high stress and a second conductive layer having a very low specific resistance by applying RF bias. It can be seen that the sum is close to '0' to form a conductive layer having a stable structure and exhibiting a low specific resistance.

Furthermore, as described above, when the third conductive layer is formed on the second conductive layer with a maximum tensile stress value and the sum of the stresses of the entire conductive layer is set to be close to '0' so that there is little residual stress. In addition, since it has an open structure in which sodium ions can be easily diffused and maintains a relatively low specific resistance as compared with the case where no RF bias is applied, a conductive layer having a very high efficiency, that is, a rear electrode can be manufactured.

Although described above with reference to the drawings according to an embodiment of the present invention, those of ordinary skill in the art will be able to perform various applications and modifications within the scope of the present invention based on the above contents.

As described above, the rear electrode of the solar cell having the CI (G) S light absorbing layer according to the present invention, the adhesion between the electrode and the glass substrate by changing the pressure of the reaction gas supplied in the sputtering process for forming the rear electrode It is possible to manufacture a back electrode having excellent properties and also improving diffusion effects such as sodium. In addition, by simultaneously applying the RF bias during DC sputtering it is possible to efficiently manufacture the back electrode having a relatively low resistivity.

Claims (15)

A method of manufacturing a back contact of a solar cell including a cupper-indium-gallium-selenide (CI (S)) light absorption layer, (a) forming a first conductive layer on the substrate by DC sputtering under a reaction gas at a pressure of 5 to 15 mTorr; And (b) forming a second conductive layer on the first conductive layer by DC sputtering while applying an RF bias on the substrate under a reaction gas at a pressure of 1 to 5 mTorr in a range lower than the pressure of step (a). process; Method comprising a. The method of claim 1, wherein the RF bias is applied at an output amount such that the sum of intrinsic stresses of the first conductive layer and the second conductive layer is '0'. The method of claim 1, wherein the DC sputtering is DC magnetron sputtering. The method of claim 1, wherein the substrate is at least one selected from the group consisting of a glass substrate, an aluminum foil, a carbon film and a polyimide substrate, and the first conductive layer and the second conductive layer are molybdenum, aluminum, tungsten, chromium and And at least one selected from the group consisting of tantalum. 5. The method of claim 4 wherein the substrate is a glass substrate and the first conductive layer and the second conductive layer are molybdenum. The method of claim 1 wherein the reaction gas is argon. The method of claim 1, wherein the first conductive layer is formed to a thickness of 100 to 300 nm in an argon atmosphere of 5 to 15 mTorr and exhibits high adhesion to the substrate. The method of claim 1, wherein the second conductive layer is formed to a thickness of 500 to 1000 nm in an argon atmosphere of 1 to 5 mTorr, and has a resistivity of 10 ⁻⁴ to 10 ⁻⁵ Ωcm. The method of claim 1, wherein after the process (b) is performed to form the second conductive layer, a third conductive layer having an open structure with the maximum tensile force between the metal atoms is added on the second conductive layer. Forming step (c) further comprising. The method of claim 9, wherein the third conductive layer is formed in a thickness of 100 to 300 nm in an argon atmosphere of 3 to 8 mTorr and has a specific resistance of 10 ^-3 to 10 ^-4 Ωcm. 10. The method of claim 9, wherein an RF bias is applied at an output amount such that the sum of intrinsic stresses of the first conductive layer, the second conductive layer, and the third conductive layer is '0'. A first conductive layer having excellent adhesion to the substrate; A second conductive layer formed on the first conductive layer and having a resistance of 10 ⁻⁴ to 10 ⁻⁵ Ωcm; And a third conductive layer formed on the second conductive layer, having a resistance of 10 ⁻³ to 10 ⁻⁴ Ωcm, and having an open structure in which any material is easily diffused from the substrate. Solar cell back electrode. The solar cell back electrode of claim 12, wherein the first conductive layer, the second conductive layer, and the third conductive layer have a total thickness of 700 to 1500 nm. A CI (G) S solar cell comprising a back electrode prepared by the method according to any one of claims 1 to 11. The solar cell module consisting of a plurality of solar cells according to claim 14.

Images