Solar cell and method fabricating the same

A solar cell according to an embodiment includes a pattern layer arranged on a substrate and including a uneven pattern; a back electrode arranged on the pattern layer; a light absorption layer arranged on the back electrode; a buffer layer on the light absorption layer; and a front layer arranged on the buffer layer.The method fabricating a solar cell according to an embodiment includes forming a pattern layer including a uneven pattern on a substrate; forming a back electrode on the pattern layer; forming a light absorption layer on the back electrode; forming a buffer layer on the light absorption layer; and forming a front electrode on the buffer layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the U.S. national stage application of International Patent Application No. PCT/KR2010/007493, filed Oct. 28, 2010, which claims priority to Korean Application No. 10-2009-0103076, filed Oct. 28, 2009, the disclosures of each of which are incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to a solar cell and a method fabricating the same.

2. Description of the Related Art

In recent, as the demand of the energy increases, developments for the solar cell converting solar energy into electrical energy are proceeding.

Particularly, a CIGS-base solar cell, that is, p-n hetero junction device having a substrate structure including a substrate, a metal back electrode layer, p-type CIGS-base light absorption layer, a high-resistant buffer layer, n-type transparent electrode layer and the like is widely used.

Various types of substrates may be used as the substrate, but when the substrate is flexible, in the case that the substrate is curved, there is problem in that the crack occurs in the metal back electrode layer formed on the substrate.

SUMMARY OF THE INVENTION

An advantage of some aspects of the invention is that it provides a solar cell and a method fabricating the same capable of increasing coupling force between the substrate and the back electrode.

A solar cell according to the embodiment includes a pattern layer arranged on a substrate and including an uneven pattern; a back electrode arranged on the pattern layer; a light absorption layer arranged on the back electrode; a buffer layer arranged on the light absorption layer; and a front layer arranged on the buffer layer.

The method fabricating a solar cell according to an embodiment includes forming a pattern layer including an uneven pattern on a substrate; forming a back electrode on the pattern layer; forming a light absorption layer on the back electrode; forming a buffer layer on the light absorption layer; and forming a front electrode on the buffer layer.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the description of the embodiment, in a case where each substrate, layer, a film or a electrode and the like is described to be formed “on” or “under” thereof, “on” or “under” also means one to be formed “directly” or “indirectly (through other component)” to component. Also, the criteria regarding “on” or “under” of each component will be described based on the drawings. In the drawing, the size of each component may be exaggerated to describe, and does not mean the size that is in fact applied.

FIG. 11is a section view of a solar cell according to an embodiment of the present invention.

As shown inFIG. 11, a solar cell of the embodiment includes a substrate100, a pattern layer170, a back electrode200, a light absorption layer300, a buffer layer400and a front electrode500.

In this case, the pattern layer170includes an uneven pattern150, curves having a quadrangular pyramid or sine wave shape may be periodically formed in the uneven pattern150.

Further, as shown inFIG. 3, the uneven pattern150includes grooves110and protrusions120, the width of the grooves is 100˜300 nm, the width of the protrusions is 100˜200 nm, and the height of the grooves and protrusions may be 100˜300 nm.

The grooves110and the protrusions120are formed by an uneven structure, so the grooves120have the shape protruded from the substrate100.

Further, the grooves110and the protrusions120allow a contact area to widen, it is possible to increase the combination between the substrate100and the back electrode formed hereafter.

Particularly, when the substrate100is flexible, although the substrate100is curved, it is possible to prevent generation of the crack in the back electrode by the pattern layer170.

Further, the back electrode is formed even in the inside of the grooves110of the uneven pattern150, so it is possible to increase the combination force the substrate100and the back electrode.

The pattern layer170may be formed by the material containing resin of single or mixture type such as epoxy, epoxy melanin, acrylic and urethane resin.

Hereinafter, the solar cell will be described in more detail according to the process of fabricating the solar cell.

FIGS. 1 to 11are sectional views showing the method of fabricating the solar cell according to an embodiment of the present invention.

First, as shown inFIG. 1, the pattern layer170including the uneven pattern150is formed on the substrate100.

The substrate100uses glass and also uses ceramic substrate such as alumina, stainless steel, titanium substrate or polymer substrate and the like, as the material thereof.

The glass substrate may use sodalime glass, and the polymer substrate may use PET (polyethylen terephthalate), and polyimide.

Further, the substrate100may be rigid or flexible.

After forming the resin layer on the surface of the substrate100, the uneven pattern150may form the uneven pattern in the resin layer.

At this moment, as shown inFIG. 2, a method forming the pattern forms the resin layer on the substrate100, and applies a molding process using a mold230while simultaneously applying UV hardening process.

When applying the resin layer on the substrate100, it proceeds to a spin coating process.

The resin layer may be formed by the material containing resin of single or mixture type such as epoxy, epoxy melanin, acrylic and urethane resin.

However, the method forming the pattern is not limited thereto, after forming the resin layer on the substrate100, it may be formed using laser light source.

FIGS. 3 and 4in detail show ‘A’ region ofFIG. 1, the uneven pattern150, the uneven pattern150includes the grooves110and the protrusions120, and the curve of the uneven pattern150having a square pillar shape is periodically formed.

The grooves110and protrusions120are formed by an uneven structure, so the grooves120have the shape protruded from the substrate100.

Further, the grooves110and the protrusions120allows a contact area to widen, so it is possible to increase coupling force between the substrate100and the back electrode formed hereafter.

Particularly, when the substrate100is flexible, although the substrate100is curved, tensile stress is transferred into the back electrode by the pattern layer170, thereby to prevent the generation of the crack.

In this case, the width f of the grooves110is 100˜300 nm, the width g of the protrusions120is 100˜200 nm, and the height b of the grooves110and the height c of protrusions120may be 100˜300 nm.

In the present embodiment, the uneven pattern150includes the grooves110and the protrusions120, but is not limited thereto, and may be formed by the structure formed with the pattern capable of improving the coupling force with the back electrode to be formed later.

Although not shown in drawings, the uneven pattern150having a square pillar shape may be formed longer in one direction.

In this case, the uneven pattern150is not limited to the square pillar, as shown inFIG. 4, the curve of the uneven pattern160having a curved sine wave shape may be periodically formed.

The pattern layer170may be formed by the material containing resin of single or mixture type such as epoxy, epoxy melanin, acrylic and urethane resin.

When the substrate100is formed by the polymer substrate, that is, PET and polyimid, since the coupling force between the pattern layer170and the substrate100is strong, the coupling force between the substrate100and the back electrode to be formed later may be also strengthened.

Further, as shown inFIGS. 5 and 6, the back electrode201is formed on the pattern layer170.

The back electrode201becomes a conductive layer. The back electrode layer201allows charges produced from the light absorption layer300of the solar cell to move, such that current may flow outside the solar cell. The back electrode layer201should be have high electrical conductivity and small specific resistance to perform above function.

Further, The back electrode layer201should be maintained to have high temperature stability when heat-treating under the atmosphere of sulfur(S) or selenium (Se) accompanied in forming CIGS compound.

Such a back electrode201may be formed by any one of molybdenum (Mo), gold (Au), aluminum (Al), chromium (Cr), tungsten (W) and copper (Cu) Among them, particularly, the molybdenum (Mo) may allow the characteristic required for the back electrode layer201to generally satisfy.

The back electrode layer201may include at least two layers. In this case, each layer may be formed by same metal or metals different from each other.

At this moment, the back electrode201is also inserted into the inside of the grooves110of the uneven pattern150to increase the coupling force between the back electrode201and the substrate100.

The side in which the back electrode201contacts the pattern layer170may be formed to have a concave-convex corresponding to the uneven pattern of the pattern layer170, and a top surface of the back electrode201may be formed to have the side parallel to the substrate100.

Particularly, when the substrate100is flexible, although the substrate100is curved by the difference in the thermal expansivity between the substrate and the back electrode, it is possible to prevent the generation of the crack between the substrate100and the back electrode, by the uneven pattern150formed on the substrate.

In this case, the thickness of the substrate100is thicker than that of the uneven pattern150and the back electrode201, and the thickness of the back electrode201is thicker than that of the uneven pattern150.

That is, the relationship about the thickness and the size of the substrate100, the uneven pattern150and the back electrode201may be expressed as follows with reference toFIG. 6
(a+b)=W(c+d)  (1)
(c)=X(d)  (2)
(d)=Y(e)  (3)
(f)=Z(g)  (4)

Where, W has a value of 0.17˜0.43, X has a value of 0.03˜0.15, Y has a value of 0.04˜0.12, and Z has a value of 1˜2.

In the conditional expression, a is a distance from the top surface of the uneven pattern150, that is, the top surface of the protrusions120to the top surface of the back electrode pattern201, b is a height of the grooves110, c is a height of the grooves120, and d is a thickness from the bottom surface of the grooves110to the substrate100in the pattern layer170.

Further, e is a thickness of the substrate100, f is a width of the grooves110, and g is a width of the protrusions120.

The conditional expression (1) shows the relationship between the back electrode201and the pattern layer170.

As shown in the conditional expression (1), (a+b), that is, the entire thickness of the back electrode201may become 0.17˜0.43(W) times (c+d), that is, the entire thickness of the pattern layer170.

In this case, when the value of the W is smaller than 0.17, the buffer layer, that is, the d region of the pattern layer170become thicker, thereby to reduce the adhesion between the substrate100and the pattern170.

Further, when the value of the W is larger than 0.43, the difference in the thickness between the entire back electrode201and the pattern170is decreased, and therefore, d has not enough thickness in the pattern170, such that the buffer function preventing the generation of the crack may be reduced.

The conditional (2) means the percentage of the protrusions120or the grooves11in the entire thickness of the pattern layer170.

That is, the height c of the protrusions120may be 0.03˜0.15(X) times d, that is, the thickness from the bottom surface of the grooves110to the substrate100in the pattern layer170.

At this moment, when the value of X is smaller than 0.03, the height of the protrusions120, that is, the value of c is too small, thereby reducing adhesion area with the back electrode201and simultaneously, the uneven pattern150is too small, thereby reducing the buffer function preventing the generation of the crack.

Further, when the value of X is larger than 0.15, the height of the protrusions120, that is, the value of c becomes larger, and therefore, it is hard to manufacture the uneven pattern150. Further, it is not deposited up to the bottom surface of the grooves110when depositing the back electrode201, thereby to reduce the buffer function preventing the generation of the crack.

The conditional expression (3) shows the relationship between the substrate100, and the region of d, that is, the thickness from the bottom surface of the grooves110to the substrate100in the pattern layer170.

That is, the value of d, that is, the thickness of the pattern layer170from the bottom surface of the grooves110to the substrate100may become 0.04˜0.12(Y) times the substrate100.

At this moment, when the value of the Y is smaller than 0.04, the value of d is small, thereby to reduce the buffer function preventing the generation of the crack by the substrate100.

Further, when the value of the Y is larger than 0.12, the thickness of the substrate100is relatively decreased, such that bending phenomenon easily occurs in the substrate, thereby to easily produce the crack.

The conditional expression (4) shows the relationship about the percentage between the widths f of the grooves110and the widths g of the protrusions120.

That is, the width f of the grooves110may be 1˜2(Z) times the width g of the protrusions120.

Further, the period h of the uneven pattern150may be formed regularly or irregularly, and may be formed by the period of 200˜500 nm.

Further, looking into the hardness of the substrate100, and the uneven pattern150and the back electrode201, the back electrode201is harder than the substrate100and the uneven pattern150, and the hardness of the substrate100is harder than or equal to that of the uneven pattern150.

Subsequently, as shown inFIG. 7, the back electrode pattern200is formed by applying a patterning process to the back electrode201.

The back electrode pattern200may be formed by applying a photolithography process to the back electrode201.

Further, the back electrode pattern200may be arranged in a stripe type or a matrix type to correspond to each cell.

However, the back electrode pattern200is not limited to above type, and may be formed in various types.

At this moment, the portion of the uneven pattern150formed on the substrate100may be exposed through the back electrode pattern200.

Next, as shown inFIG. 8, the light absorption layer300and the buffer layer400are formed on the back electrode pattern200.

The light absorption layer300includes p-type semiconductor compound. In more detail, the light absorption layer300includes group I-III-VI-base compound. For example, The light absorption layer300may has copper-indium-gallium-selenide-base (Cu(In,Ga)Se2; CIGS-base) or copper-gallium-selenide-base crystal structure.

For example, to form the light absorption layer300, a CIG-base metal precursor film is formed on the back electrode pattern200by using copper target, indium target and gallium target.

Hereinafter, the metal precursor film reacts with selenium by a selenization process to form a CIG-base light absorption layer300.

Further, during the process forming the metal precursor film and the selenization process, alkali ingredients contained in the substrate100are diffused into the metal precursor film and the light absorption layer300through the back electrode pattern200.

The alkali component improves a grain size of the light absorption layer300to improve crystallizability.

Further, the light absorption layer300may be formed by copper, indium, gallium and selenium (Cu, In, Ga and Se) by co-evaporation.

The light absorption layer300receives light incident from the outside, and converts the received light into electrical energy. The light absorption layer300produces light electromotive force generated by photoelectric effect.

At this moment, the portion of the light absorption layer300contacting the substrate100is also formed on the uneven pattern150.

That is, the portion of the light absorption layer300is also coupled with the grooves110and the protrusions120of the uneven pattern150, such that coupling force of the light absorption layer300and the substrate100may be increased.

The buffer layer400may be formed by at least one layer, may be formed by any one of cadmium sulfide (CdS), ITO, ZnO and i-ZnO or laminating of them on the substrate100formed with the light absorption layer300, and may obtain low resistance by doping indium (In), gallium (Ga) and aluminum (Al).

At this moment, the buffer layer400is an n-type semiconductor layer, and the light absorption layer300is a p-type semiconductor layer. As a result, the light absorption layer300and the buffer layer400form pn-junction.

The buffer layer400is arranged between the light absorption layer300, and the front electrode to be formed later.

That is, since the light absorption layer300and the front electrode have large difference in a lattice constant and band gap energy, to form good junction, a buffer layer to be positioned in the middle of the two materials is necessary due to the difference in the band gap.

In the present embodiment, although a buffer layer is formed on the light absorption layer300, is not limited thereto, and the buffer layer400may be formed by a plurality of layers.

Subsequently, as shown inFIG. 9, contact patterns are formed through the light absorption layer300and the buffer layer400.

The contact patterns310may be formed by applying a mechanical method or a process using laser, and the portion of the back electrode pattern200is exposed.

Further, as shown inFIG. 10, the front electrode500and a connecting interconnection700are formed by laminating transparent conductive material on the buffer layer400.

When laminating the transparent material on the buffer layer400, the transparent conductive material is also inserted into the inside of the contact pattern310to form the connecting interconnection700.

The back electrode pattern200and the front electrode500are electrically connected to each other by the connecting interconnection700.

The front electrode500is formed by zinc oxide doped with the aluminum by applying the sputtering process on the substrate100.

The front electrode500, which is a window layer forming the pn-junction with the light absorption layer300, functions as the transparent electrode of the front of the solar cell, and therefore, is formed by the zinc oxide (ZnO) having high light transmittance and good electrical conductivity.

At this moment, the electrode having a low resistance may be formed by doping the aluminum to the zinc oxide.

The front electrode500, that is, the zinc oxide thin film may be formed by a deposition method using ZnO target by RF sputtering method, reactive sputtering using Zn target, and metal organic chemical vapor deposition.

Further, it is also possible to form double structure by depositing ITO (Indium tin Oxide) thin film having excellent electro-optical characteristics on the zinc oxide thin film.

Subsequently, as shown inFIG. 11, separate patterns320are formed through the light absorption layer300, the buffer layer400, and the front electrode500.

The separate patterns320may be formed by applying a mechanical method or a process using laser, and the portion of the back electrode pattern200is exposed.

The buffer layer400, and the front electrode500may be divided by the separate pattern320, and each of cells C1, C2may be separated to each other by the separate pattern320.

The light absorption layer300, the buffer layer400, and the front electrode500may be arranged in a stripe type or a matrix type by the separate pattern320.

The separate pattern320is not limited to above type, and may be formed in various types.

The cells C1, C2including the back electrode pattern200, the light absorption layer300, the buffer layer400, and the front electrode500are formed by the separate pattern320.

At this moment, each of the cells C1, C2are connected to each other by the connecting interconnection700. The back electrode pattern200of the second cell C2and the front electrode500of the first cell C1contacting the second cell C2are connected to each other by the connecting interconnection700.

The solar cell and method fabricating the same according to the embodiments described above may form the uneven pattern having a nano size on the substrate, thereby to increase the coupling force with the back electrode formed on the substrate.

Particularly, when the substrate is flexible, although the substrate is curved, the crack does not occur between the substrate and the back electrode.

That is, the back electrode is formed even in the inside of the grooves of the uneven structure pattern to increase the coupling force between the substrate and the back electrode.

The light absorption layer, in which the portion thereof contacts the substrate, also contacts the uneven structure pattern to increase the coupling force between the light absorption layer and the substrate.

The solar cell and method fabricating the same according to the embodiment may form the uneven pattern having a nano size on the substrate, thereby to increase the coupling force with the back electrode formed on the substrate.

Particularly, when the substrate is flexible, although the substrate is curved, the crack does not occur between the substrate and the back electrode.

That is, the back electrode is formed even in the inside of the grooves of the uneven structure pattern to increase the coupling force between the substrate and the back electrode.

Further, the light absorption layer, in which the portion thereof contacts the substrate, also contacts the uneven structure pattern to increase the coupling force between the light absorption layer and the substrate.

It is appreciated that the present invention can be carried out in other specific forms without changing a technical idea or essential characteristics by one having ordinary skilled in the art to which the present invention pertains to. Therefore, embodiments described above are for illustration purpose in all respect but not limited to them. The scope of the present invention is represented by claims described below rather than the detailed description, and any change and variations derived from the meaning, the scope and the concept of equality of claims should be interpreted to be included to the scope of the present invention.

In addition, although the preferred embodiments of the present invention are shown and described above, the present invention is not limited to above-described specific embodiment and is variously modified by one skilled in the art without the gist of the present invention claimed in the claim, such that the modified embodiment is not to be understood separately from technical ideas or views of the present invention.