METHOD OF PATTERNING A THIN-FILM PHOTOVOLTAIC LAYER STACK

Abstract

The present disclosure relates to a method of patterning a thin-film photovoltaic layer stack (20), the method comprising the steps of:—providing of a continuous layer stack (20), the layer stack (20) comprising a planar substrate (21), a first electrode layer (22) on the substrate (21) and a photovoltaic layer (24) on the electrode layer (22),—immersing the layer stack (20) into an electrically conductive solution (40),—applying a bias voltage between the electrolyte solution (40) and the first electrode layer (22) and—converting of a first material (51, 53) or a first material composition provided in at least a first portion (50, 52, 54) of the layer stack (20) into a first reaction product (56) by an electrochemical reaction, wherein the first reaction product (56) has an electrical conductivity that is lower than an electrical conductivity of the first material (51, 53) or first material composition, or—removing a first material (51, 53) or a first material composition provided in at least a first portion (50, 52, 54) of the layer stack (20) by an electrochemical reaction.

Claims

1-15. (canceled)

16. A method of patterning a thin-film photovoltaic layer stack comprising: providing a continuous layer stack, the layer stack comprising a substrate, a first electrode layer on the substrate and a photovoltaic layer on the first electrode layer; immersing the layer stack into an electrically conductive solution; applying a bias voltage between the electrically conductive solution and the first electrode layer; and converting a first material or a first material composition provided in at least a first portion of the layer stack into a first reaction product by an electrochemical reaction, wherein the first reaction product has an electrical conductivity that is lower than an electrical conductivity of the first material or the first material composition; or removing a first material or a first material composition provided in at least a first portion of the layer stack by an electrochemical reaction.

17. The method according to claim 16, further comprising selectively removing the photovoltaic layer along a first line and forming a first trench in the photovoltaic layer before or after immersing the layer stack into the electrically conductive solution.

18. The method according to claim 17, wherein selectively removing the photovoltaic layer and forming the first trench comprises: applying a laser beam onto the layer stack and moving the laser beam along the first line; or moving a mechanical scribing tool along the first line.

19. The method according to claim 17, wherein the first portion of the layer stack is a side edge of the photovoltaic layer extending along the first trench.

20. The method according to claim 16, wherein the first portion of the layer stack is an impurity in the photovoltaic layer.

21. The method according to claim 16, wherein the first material or the first material composition of the first portion of the layer stack comprises a metal and wherein the first reaction product comprises one of a metal oxide, a metal selenide and a metal sulfide.

22. The method according to claim 21, wherein the first material or the first material composition of the first portion of the layer stack comprises Molybdenum (Mo) and wherein the first reaction product comprises MoO.sub.x, with 2.5<x≤3.

23. The method according to claim 16, wherein the layer stack is immersed into a salt solution containing at least one of Zn, Cd and Ca and wherein at least one of the first material, the first material composition and the first reaction product is transformed into one of ZnMoO.sub.4, CaMoO.sub.4, and CdMoO.sub.4.

24. The method according to claim 16, wherein the first portion of the layer stack is a portion of the first electrode layer.

25. The method according to claim 16, wherein the first material or the first material composition of the first portion of the layer stack comprises a CIGS semiconductor material with a composition of Cu.sub.x(In,Ga).sub.ySe.sub.z, with x+3y>2z and wherein the first reaction product comprises a CIGS semiconductor material with a composition of Cu.sub.x(In,Ga).sub.ySe.sub.z,O.sub.w, with x+3y2(w+z) and w>0.

26. The method according to claim 16, wherein the electrically conductive solution is an electrolyte solution comprising at least one of NH.sub.3.H.sub.2O and (NH.sub.4).sub.2SO.sub.4 or mixtures thereof.

27. The method according to claim 16, wherein immersing the layer stack in the electrically conductive solution includes immersing the layer stack in a basic solution and thereafter immersing the layer stack in a neutral pH or acidic conductive solution.

28. The method according to claim 17, wherein a window layer is deposited onto the layer stack and wherein the photovoltaic layer is selectively removed along a second line to form a second trench parallel to the first trench and adjacent to the first trench.

29. The method according to claim 28, wherein a second electrode layer is deposited onto the layer stack and wherein the photovoltaic layer is selectively removed along a third line to form a third trench parallel to the second trench and adjacent to the second trench.

30. A thin-film photovoltaic layer stack comprising: a substrate; a first electrode layer on the substrate; and a photovoltaic layer on the first electrode layer; wherein at least one of the first electrode layer and the photovoltaic layer comprises a first reaction product obtained through an electrochemical reaction of a first material or a first material composition of a first portion of the first electrode layer or photovoltaic layer; and wherein the first reaction product has an electrical conductivity that is lower than an electrical conductivity of the first material or the first material composition.

Description

DETAILED DESCRIPTION

[0099] In the following numerous embodiments of the method and of a thin-film photovoltaic layer stack are described in greater detail by making reference to the drawings, in which:

[0100] FIG. 1 schematically illustrates a photovoltaic layer stack after scribing of a first trench,

[0101] FIG. 2 shows the layer stack of FIG. 1 of the conversion of at least a first portion of the bottom electrode layer into a first reaction product,

[0102] FIG. 3 shows the deposition of a window layer on the layer stack of FIG. 2,

[0103] FIG. 4 shows the layer stack of FIG. 3 after scribing of a second trench,

[0104] FIG. 5 shows the layer stack of FIG. 4 after deposition of the top electrode layer, and

[0105] FIG. 6 shows the layer stack of FIG. 5 after scribing the third trench,

[0106] FIG. 7 shows a current/voltage diagram of a thin-film solar module produced in accordance with the method described herein,

[0107] FIG. 8 schematically illustrates the immersion of the layer stack into an electrolyte solution and an enlarged section of the layer stack,

[0108] FIG. 9 schematically illustrates the formation and/or conversion of a portion of the layer stack adjoining the first trench,

[0109] FIG. 10 is illustrative of an embodiment, wherein an impurity embedded in the layer stack is subject to an electrochemical conversion or transformation,

[0110] FIG. 11 is another example of an impurity inside the layer stack,

[0111] FIG. 12 shows another example of a thin-film layer stack comprising a defect or a pinhole,

[0112] FIG. 13 illustrates the layer stack of FIG. 12 after deposition of the window layer on the photovoltaic layer,

[0113] FIG. 14 shows the conversion of at least a portion of the bottom electrode layer through the electrochemical reaction even with the window layer deposited on top of the photovoltaic layer and

[0114] FIG. 15 is illustrative of a flowchart of conducting the method of patterning a thin-film photovoltaic layer stack.

DETAILED DESCRIPTION

[0115] In FIG. 1 an example of a thin-film photovoltaic layer stack 20 is illustrated. The layer stack 20 comprises a planar shaped substrate 21. On a top surface of the substrate 21 there is applied a bottom electrode layer 22. The bottom electrode layer 22 is continues over the entire substrate 21. The bottom electrode layer 22 typically comprises an electrically conducting metal, such as molybdenum. On top of the bottom electrode layer 22 there is provided a photovoltaic layer 24 or semiconductor layer. The photovoltaic layer 24 may be also denoted as an absorber layer or photoelectric conversion layer. With some examples the photovoltaic layer 24 comprises a CIGS layer. Optionally and on top of the photovoltaic layer 24 there is provided a buffer layer 25.

[0116] There is further provided a first trench 31 extending along a straight line 30. The first trench 31 forms a gap in the layer stack 20. The gap is only or exclusively present in the photovoltaic layer 24 and in the optional buffer layer 25. The scribing of the first trench 31 may leave the bottom electrode layer 22 substantially unaffected. For scribing of the first trench 31 there is typically used a laser beam moving along the first line 30.

[0117] In the region of the first trench 31 there is provided a first portion 50 of the bottom electrode layer. The layer stack 20 as illustrated in FIG. 1 is immersed into an electrolyte solution 40 as illustrated in FIG. 8. Moreover, the bottom electrode layer 22 is connected to the anode of a DC voltage supply 44. There is further provided and another electrode 42. The further electrode 42 is connected to the cathode of the DC voltage or current supply. Also, the second electrode 42 is immersed in the electrolyte solution 40. By immersing the layer stack 20 in the electrolyte solution 40 and by applying a DC voltage between the electrode 42 and the bottom electrode layer 22 an electrochemical reaction of the first portion 50 with the electrolyte solution 40 takes place. Hence, the material 51 or material composition located in the first portion 50 of the bottom electrode layer 22 is converted into at least a first reaction product 56. It may be further converted into a second or third reaction product depending on the specific electrochemical reaction process.

[0118] The first reaction product 56 obtained in the first portion 50 exhibits an electrical conductivity that is lower than the electrical conductivity of the first material of the bottom electrode layer 22 in regions outside of and/or adjacent to the first portion 50. In this way, the first portion 50 may be entirely or at least partially converted or transferred into the first reaction product 56. The first portion 50 of the bottom electrode layer 22 may be then transformed or converted from an electrically conducting material into an electrically insulating material and/or into an electrically conducting material exhibiting a reduced electrical conductivity compared to the original material of the first portion 50.

[0119] In the process of patterning the thin-film photovoltaic layer stack 20 there is then applied an intrinsic window or window layer on top of the layer stack 20 as illustrated in FIG. 3. The window layer 26 typically comprises intrinsic ZnO or i-ZnO. Thereafter and as illustrated in FIG. 4 a second trench 33 is scribed along a second line 32. In the region of the second trench 33 the layer stack 20 is at least partially removed. Removal of the layer stack particularly applies to the photovoltaic layer 24, to the optional buffer layer 25 and the window layer 26. Removal of the material may exclude removal of the bottom electrode layer 22 and the first portion 50 of the electrode layer that comprises the first reaction product 56.

[0120] In a direction perpendicular to the elongation of the first trench 31 or first line 30 the first portion 50 comprises a certain width. It extends from a first longitudinal edge 57 towards an opposite edge 58. The region of the bottom electrode layer 22 confined by the first and second edges 57, 58 defines the first portion 50 that is subject to the electrochemical reaction. As it is apparent from FIG. 2, the width between the first and second edges 57, 58 can be even larger than the corresponding width of the first trench 31.

[0121] As illustrated in FIG. 4, the second trench 33 is scribed across or over the second edge 58. In this way, the second trench uncovers or reveals a portion of the original bottom electrode layer 22 at an adjacent part of the first portion 50 that has been converted into the first reaction product 56.

[0122] Thereafter and as illustrated in FIG. 5 the top electrode layer 27, e.g. in form of a transparent front electrode is deposited on the layer stack 20 as illustrated in FIG. 5. The top electrode layer 27 may comprise at least one or a composition of the following materials: ZAO (zinc aluminum oxide or doped zinc aluminum oxide) and TCO (transparent conducting oxide).

[0123] Thereafter and as illustrated in FIG. 6 a third trench 35 is scribed along a third line 34. The third trench 35 is scribed adjacent to the second trench 33. The third trench 35 forms a gap in the layer stack 20 above the bottom electrode layer 22. Here, the photovoltaic layer 24, the optional buffer layer 25, the window layer 26 and the top electrode layer 27 are removed while the bottom electrode layer 22 may remain on the substrate 21. In this way, a first solar cell 12 and a second solar cell 14 can be formed on the substrate 21. Of course, the entire substrate 21 is provided with a repetitive structure of first, second and third trenches to form a multitude of adjacently located solar cells 12, 14.

[0124] A bottom layer portion 22 of the first solar cell 12 is electrically connected to the top electrode layer portion 27 of the second solar cell 14. The bottom electrode layer portion 22 of the second solar cell 14 is electrically insulated from the bottom electrode layer portion 22 of the first solar cell 12 by the reaction product 56 in the first portion 50 of the bottom electrode layer 22. In FIG. 7 a diagram of an applied current versus voltage for a 30 cm×30 cm module produced according to the present method. The module comprises 67 cells of 4 mm width and 280 mm length connected in serial. The diagram and hence the module shows high shunt resistance as we see from the current/voltage diagram.

[0125] In FIG. 8, the apparatus for conducting the electrochemical reaction is illustrated in greater detail. Typically, the electrolyte solution 40 comprises unionized water and numerous electrolytes. The pH and the concentration of the electrolyte can be controlled during the electrochemical reaction. As described above the bottom electrode layer 22 serves as an anode and a further electrode 42 serves as a cathode for the electrolytic conversion process. With some examples the DC voltage should be less than 3V so that the photovoltaic area will not be oxidized as the p-n junction built-in field blocks the current under low forward bias.

[0126] Nonetheless, the bottom electrode layer 22 is directly exposed to the electrolyte solution 40 and may quickly undergo the electrochemical reaction thereby converting or transforming the first portion 50 of the bottom electrode layer 22 into the first reaction product 56. The level of electrochemical reaction, e.g. the level of oxidation, selenization or sulfurization can be controlled by the following parameters: type of electrolyte, concentration of electrolyte, applied voltage, e.g. applied DC potential, and duration of the electrochemical reaction.

[0127] As illustrated in the enlarged portion of FIG. 8 there may evolve numerous reaction products in a sequential manner. Typically, the first portion 50 of the bottom electrode layer 22, hence a first material 51 thereof may be entirely converted into at least an intermediate product 55 before the intermediate product 55 is converted into the final reaction product, e.g. the first reaction product 56. From another perspective the intermediate product 55 may be also regarded as a first reaction product, which during the ongoing electrochemical reaction is converted or transformed further into a second reaction product 56.

[0128] With molybdenum as a first material 51 of the bottom electrode layer 22 an intermediate product 55 may comprise Mo.sub.2O.sub.5, which is further converted into MoO.sub.3 as the first reaction product 56. The electrochemical conversion of the initial material of the bottom electrode layer into the intermediate product 55 may be comparatively fast compared to the conversion of the intermediate product 55 into the first reaction product 56

[0129] By way of example, the metal electrode inside the trench will be oxidized rather quickly. The molybdenum could be “over oxidized” to convert even the material under the absorber layer into Mo oxide as shown in the enlarged section of FIG. 8. In this way the latter applied front electrode layer, e.g. a TCO layer will not short to the bottom electrode layer 122. The cross section structure of the oxidation interface shows the oxidation processes. The molybdenum quickly oxidizes into the intermediate reaction product 55 Mo.sub.2O.sub.5 in the Mo/MoO.sub.x interface. Then the intermediate product 55 Mo.sub.2O.sub.5 slowly oxidizes to MoO.sub.3. During this second phase of oxidation, the concentration of oxygen increases inside the MoO.sub.x part until the oxide is well oxidized to MoO.sub.3. Thick oxide layer impedes the transfer of reactants so the oxidation slows down. As a part of the MoO.sub.3 may even dissolve into the solution, the oxidation may move forward.

[0130] The MoO.sub.x may comprise a combination of MoO.sub.2, Mo.sub.2O.sub.5 and MoO.sub.3. The higher the x value, the higher will be the resistivity. The resistivity can be controlled by the oxidation process. The over oxidation is to achieve high shunt resistance for solar cells and solar modules. When the width between the boundaries 57, 58 is at least about 10 μm or at least about 15 μm or at least about 20 μm larger than the width of the trench 31 a high quality insulation can be obtained in principle.

[0131] In FIG. 9, another example of a thin-film photovoltaic layer stack 20 is illustrated, there, side edges 61, 62 of the photovoltaic layer 24 extending along the first trench 31 have been subject to a melting during the scribing of the first trench 31. Hence, these side edges 61, 62 can be equally regarded as a first portion 52 of the layer stack comprising a first material 53 or a first material composition that is convertible or transformable into a first reaction product 56 by the above-described electrochemical reaction. The first portion 52 of the layer stack 20 as illustrated in FIG. 9 may comprise Cu.sub.x(In,Ga).sub.ySe.sub.z, with x+3y>2z. In view of the above-described electrochemical reaction the respective first portions 52 of the layer stack can be transferred or converted into a reaction product, such as, Cu.sub.x(In,Ga).sub.ySe.sub.z,O.sub.w, with x+3y≤2(w+z) and w>0.

[0132] Here, the damaging of the semiconductor material of the photovoltaic layer 24 that is due to the scribing process of the first trench 31 can be at least partially compensated by e.g. an anodic oxidation process incorporating oxygen in the previously damaged semiconductor, thereby converting the damaged semiconductor material into a material composition exhibiting a lower conductivity compared to the damaged semiconductor material.

[0133] In the examples of FIGS. 10 and 11 it is even shown, that also an impurity 60 entirely or at least partially embedded in the thin-film layer stack 20 can become equally subject to the electrochemical reaction when immersed in the electrolyte solution 40 as described above in connection with FIG. 8. In the example of FIG. 10 the impurity 60 is covered by e.g. the buffer layer 25. The impurity 60 comprises the first material 53 or material composition. The buffer layer 25 may be permeable at least to a certain degree for the electrolyte solution so that the intended electrochemical reaction of the first material 53 may take place. Hence, the entirety or part of the first portion 54, hence of the impurity 60, spatially enclosing or coinciding with the first material 53, may take place in a similar way as described above.

[0134] Depending on whether the droplets are more indium or gallium rich or more copper rich, they are covered by the buffer layer (FIG. 9) or the buffer layer deposition is locally suppressed (FIG. 10). It doesn't matter what the case is. In both cases the particle shunts the cell and in both cases the shunt can be removed by the anodic oxidation process. Typically the droplets are metallic and partially selenized during the absorber deposition process (composition with selenium deficiency). Their initial composition can be Cu.sub.x(In,Ga).sub.ySe.sub.z with x+3y>2z. In both cases of FIGS. 10 and 11 the conducting impurity 60 can be converted by the anodic process into high resistive selenized/oxidized material. Their new composition is Cu.sub.x(In,Ga).sub.ySe.sub.z,O.sub.w, with x+3y≤2(w+z) and w>0. This material is less conducting and does no longer behave like a shunt.

[0135] In FIG. 11, an example is illustrated, wherein the buffer layer 25 only partially covers the impurity 60. Here, the electrochemical reaction may take place at an increased rate.

[0136] In the sequence of FIGS. 12-14 the negative impact of a pinhole in the photovoltaic layer 24 at its compensation through the electrochemical reaction as described herein is described in greater detail. A pinhole 70, e.g. in form of a gap or recess in the photovoltaic layer 24 may be due to a perturbation in the deposition process. In practice such pinholes 70 may be due to the presence of dust particles on top of the bottom electrode layer 22. Even if such a pinhole 70 is overgrown or covered by the buffer layer 25 as illustrated in FIG. 13 it may lead to an electrical shunt between the bottom electrode layer 22 at the top electrode layer 27 that is applied later on.

[0137] During the above-described electrochemical reaction the portion 50 of the bottom electrode layer 22 located in direct vicinity or located underneath the area of the pinhole 70 can be subject to a conversion into the above-described reaction product 56 so that an eventual shunt between the bottom electrode 22 and the top electrode 27 is effectively avoided and prevented. Even though the buffer layer 25 may have a certain damping effect on the electrochemical reaction the electrochemical reaction with the electrolyte solution 40 can still take place because the buffer layer 25 is comparatively thin. It is hence permeable for the electrolyte solution 40 at least to a certain extent.

[0138] In FIG. 15, a flowchart of the method of patterning the thin-film photovoltaic layer stack 20 is provided. In a first step 100 the layer stack comprising a continuous bottom electrode layer 22 and at least a continuous photovoltaic layer 24 is provided. In a further step 102 and to the photovoltaic layer is at least partially removed along a first line 30, thus forming a first trench 31. Thereafter, in step 104 the layer stack 20 is subject to the electrochemical reaction with the electrolyte solution 40 as described in connection with FIG. 8. Thereafter, in particular after termination of the electrochemical reaction the layer stack 20 is subject to the deposition of a window layer in step 106. Application of such a window layer is indicated in FIG. 3. Subsequently, in step 108 and as illustrated in FIG. 4, a second trench 33 is scribed along a second line 32. Thereafter, in step 110 the top electrode layer 27 is deposited on the layer stack 20 as illustrated in FIG. 5. Thereafter and in a final process step 112 a third trench line 35 is described along a third line 34 thus electrical he disconnecting and shaping numerous solar cells 12, 14 on the substrate 21.

REFERENCE NUMBERS

[0139] 10 thin-film solar module

[0140] 12 solar cell

[0141] 14 solar cell

[0142] 20 layer stack

[0143] 21 substrate

[0144] 22 bottom electrode layer

[0145] 24 photovoltaic layer

[0146] 25 buffer layer

[0147] 26 window layer

[0148] 27 top electrode layer

[0149] 30 line

[0150] 31 trench

[0151] 32 line

[0152] 33 trench

[0153] 34 line

[0154] 35 trench

[0155] 40 electrolyte solution

[0156] 42 electrode

[0157] 44 DC voltage

[0158] 46 electrode

[0159] 50 portion

[0160] 51 material

[0161] 52 portion

[0162] 53 material

[0163] 54 portion

[0164] 55 intermediate product

[0165] 56 reaction product

[0166] 57 boundary

[0167] 58 boundary

[0168] 60 impurity

[0169] 61 side edge

[0170] 62 side edge

[0171] 70 pinhole