Low-cost solar cell metallization over TCO and methods of their fabrication

Abstract

Methods for fabricating busbar and finger metallization over TCO are disclosed. Rather than using expensive and relatively resistive silver paste, a high conductivity and relatively low cost copper is used. Methods for enabling the use of copper as busbar and fingers over a TCO are disclosed, providing good adhesion while preventing migration of the copper into the TCO. Also, provisions are made for easy soldering contacts to the copper busbars.

Claims

1. A method for fabricating a solar cell, comprising: texturing a front surface of a silicon wafer; fabricating a photovoltaic structure over the front surface of the silicon wafer; fabricating a transparent conductive oxide (TCO) layer over the photovoltaic structure; and, fabricating front metallization layer in electrical contact with the TCO, by the steps of: forming a mask to delineate the metallization design; sputtering a copper seed layer over the mask; electroplating a copper layer or an alloy with 50% or more of copper over the coper seed layer; and, removing the mask; wherein the method further comprises: treating the TCO to become hydrophilic prior to fabricating the metallization layer by exposing the TCO to plasma having one of CH4/Ar, Ar/H2, or H2 after the step of forming the mask.

2. The method of claim 1, wherein forming the mask comprises forming a mask using wax.

3. The method of claim 1, wherein fabricating front metallization layer further comprises fabricating a cap layer over the copper layer.

4. The method of claim 3, wherein fabricating a cap layer comprises sputtering a metal layer.

5. The method of claim 1, wherein removing the mask comprises using solvent of diluted KOH.

6. The method of claim 1, wherein fabricating front metallization layer further comprises depositing an adhesion layer prior to sputtering the seed layer.

7. The method of claim 6, wherein depositing the barrier layer comprises depositing one of chromium paste, titanium paste or silver paste; and, annealing the barrier layer.

8. The method of claim 1, further comprising electroplating a cap layer over the copper.

9. The method of claim 1, further comprising depositing a layer of at least one of nickel, chromium, and tin over the metallization copper layer.

10. A method of fabrication a solar cell, comprising: texturing a front surface of a silicon wafer: fabricating a photovoltaic structure over the front surface of the silicon wafer; fabricating a transparent conductive oxide (TCO) layer over the photovoltaic structure; and, fabricating front metallization layer in electrical contact with the TCO, by the steps of: forming a mask to delineate the metallization design; sputtering a cooper seed layer over the mask; electroplating a copper layer or an alloy with 50% or more of copper over the coper seed layers; and, removing the mask: and, further comprising treating the TCO to become hydrophilic prior to fabricating the metallization layer, wherein treating the TCO comprises rinsing the TCO with soap solution.

11. A method of fabricating a solar cell, comprising: texturing a front surface of a silicon wafer: fabricating a photovoltaic structure over the front surface of the silicon wafer; fabricating a transparent conductive oxide (TCO) layer over the photovoltaic structure: and, fabricating front metallization layer in electrical contact with the TCO, by the steps of: forming a mask to delineate the metallization design; sputtering, a copper seed layer over the mask electroplating a copper layer or an alloy with 50% or more of copper over the coper seed layer; and removing the mask; and, further comprising treating the TCO to become hydrophilic prior to fabricating the metallization layer wherein treating the TCO comprises treating the front surface of the TCO with a surfactant and wherein treating the front surface of the TCO with a surfactant comprises rinsing the TCO with a solution of sodium alkyl sulfates.

12. A method of fabricating a solar cell, comprising: texturing a front surface of a silicon wafer; fabricating a photovoltaic structure over the front surface of the silicon wafer; fabricating a transparent conductive oxide (TCO) layer over the photovoltaic structure; and, fabricating front metallization layer in electrical contact with the TCO, by the steps of: forming a mask to delineate the metallization design sputtering a copper seed layer over the mask; electroplating a copper layer or an alloy with 50% or more of copper over the coper seed layer; and, removing the mask; and, further comprising treating the TCO to become hydrophilic prior to fabricating the metallization layer, wherein treating the TCO comprises treating the TCO with UV light in an ozone atmosphere.

13. The method of claim 12, wherein treating the TCO is performed at temperature of 100-200 C.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Other aspects and features of the invention would be apparent from the detailed description, which is made with reference to the following drawings. It should be appreciated that the detailed description and the drawings provides various non-limiting examples of various embodiments of the invention, which is defined by the appended claims.

(2) The accompanying drawings, which are incorporated in and constitute a part of this specification, exemplify the embodiments of the present invention and, together with the description, serve to explain and illustrate principles of the invention. The drawings are intended to illustrate features of the exemplary embodiments in a diagrammatic manner. The drawings are not intended to depict every feature of actual embodiments nor relative dimensions of the depicted elements, and are not drawn to scale.

(3) FIG. 1 illustrates a HIT solar cell according to the prior art.

(4) FIG. 2 illustrates a SmartSilicon solar cell according to the prior art.

(5) FIG. 3 illustrates a process according to an embodiment of the invention.

(6) FIG. 4 illustrates the resulting structure of the process of FIG. 3.

(7) FIG. 5 illustrates another process according to an embodiment of the invention.

(8) FIG. 6 is a flow chart of another process according to an embodiment of the invention, while FIG. 7 illustrates a cross section of the progression of the resulting structure.

(9) FIG. 8 is a flow chart of another process according to an embodiment of the invention, while FIG. 9 illustrates a cross section of the progression of the resulting structure.

(10) FIG. 10 illustrates another embodiment of the invention, while FIG. 11 illustrates a cross section of the progression of the resulting structure.

(11) FIG. 12 illustrates an embodiment of the invention with metallization before TCO process, while FIG. 13 illustrates a cross section of the progression of the resulting structure.

(12) FIG. 14 illustrates another embodiment of the invention with metallization before TCO process, while FIG. 15 illustrates a cross section of the progression of the resulting structure.

(13) FIG. 16 illustrates yet another embodiment of the invention with metallization before TCO process, while FIG. 17 illustrates a cross section of the progression of the resulting structure.

DETAILED DESCRIPTION

(14) Embodiments of the subject invention provide methods for manufacturing solar cells at reduced costs. Embodiments of the invention provide a lower cost alternative to silver metallization, which provide low resistivitynearly ten times lower resistivity than silver paste based metallizationthereby enhancing current collection from the photovoltaic cell. Also, lower deposition temperatures of essentially room temperature in order to achieve the lower resistivities. Various methods are provided to increase adhesion and enable soldering to the busbars.

(15) FIG. 3 is a flowchart illustrating a process according to an embodiment of the invention, while FIG. 4 illustrates a cross-section of the resulting structure, exemplified by the broken-line arrow in FIGS. 1 and 2. In FIG. 3, the process starts after the photovoltaic (PV) cell 400 has been fabricated on a substrate and a TCO layer 405, e.g., an ITO has been deposited over the PV cell 400. The PV cell 400 may be any PV cell requiring a TCO as its top layer, for example, the HIT structure from Sanyo or the SmartSilicon structure from Sunpreme. In step 300, a mask 410 is formed over the TCO to delineate the design of the metallization. The mask may be, for example, a hot wax deposited using technique such as inkjet printing, or a resist material that can be cured via exposure to heat or certain illumination, e.g., UV curing, and being deposited using, e.g., the silkscreen technique. According to one embodiment, an inkjet system is used to deposit hot wax mask. Thereafter, the wax mask is reflowed by annealing the wafer, so as to provide enhanced coverage, especially when the surface of the wafer is textured.

(16) Then, in step 305 a barrier/adhesion layer 415 is deposited. This layer is needed for two reasons. First, it is difficult to have copper adhere to TCO, especially to ITO. Second, copper tends to migrate and a study already showed that ITO is not a very good diffusion barrier to copper. The adhesion/barrier layer may be of a transition metal such as, e.g., chromium, nickel, titanium, etc. It may be deposited by, e.g., electroplating, electroless plating, PVD sputtering, etc. In step 310 copper layer 420 is plated over the barrier/adhesion layer 415. In this example the copper is plated using electroplating. In step 315 a cap layer 425 is formed over the copper 420. The cap layer 425 may be electroplated tin layer, which enables easy soldering onto the metallization layer, so as to connect a plurality of solar cells together, normally in a series connection. Alternatively, the barrier layer 415 may be sensitized by dipping in a liquid solution containing Pd++ (e.g. PdCl2) and then electrolessly plated with Copper. The final Ni layer is also electroless plated on top of the electroless Copper. Both electroless plating steps do not require an external field to be applied during the plating process.

(17) FIG. 5 illustrates another process according to an embodiment of the invention. The process of FIG. 5 is similar to that of FIG. 3, except that a plasma treatment step has been added to the process flow. According to one embodiment, as shown in FIG. 5, the plasma treatment is performed after forming the mask 410. On the other hand, the plasma treatment may also be performed prior to forming the mask 410. In one embodiment the plasma treatment comprises a CH4 plasma formed in situ or using remote plasma source. The plasma treatment may be performed at elevated temperature, e.g. 100 C.-200 C. for, say, 5-20 minutes. The plasma treatment helps forming good adhesion to the TCO, especially when using electroplating to form the barrier/adhesion layer 415. The plasma treatment is used to reduce the TCO to enable better electroplating. In other embodiments, CH4/Ar or Ar/H2 plasma is used. According to yet another embodiment, an H2 plasma treatment is performed before the mask is formed.

(18) FIG. 6 is a flow chart of another process according to an embodiment of the invention, while FIG. 7 illustrates a cross section of the progression of the resulting structure. As before, a PV solar cell 700 having a top TCO layer 705 is prepared for front surface metallization, i.e., fingers and busbars. The PV cell 700 is placed inside a PVD chamber having a target for a barrier/adhesion layer, e.g., nickel or titanium. For an improved adhesion, it is suggested that for this step a PVD chamber having a sputtering shutter be used. In step 600, the shutter is closed and plasma is ignited so as to treat the target by sputtering the target with plasma while the shutter is closed, so that no sputtered material reaches the PV cell. This can be a very short process, e.g., 2-10 minutes. At step 605 the shutter is opened and the plasma is maintained, so that an adhesion/barrier layer 710 is deposited on the TCO layer 705. The barrier layer can be 250-750 thick.

(19) Step 610 is optional, but is shown in FIG. 6 in broken-line box and in FIG. 7 as part of the device for the completeness of illustration. In step 610 a seed copper layer 715 is deposited over the barrier/adhesion layer 605. In this embodiment the seed layer 715 is also PVD sputtered and can be very thin, e.g., 100-500 .

(20) In step 615 a mask 720 is formed, e.g., using wax inkjet printing or photoresist silkscreen, so as to delineate the metallization design. The PV cell is then transferred to an electroplating system to electroplate copper layer 725. In step 625 a cap layer 730 is also electroplated over the copper layer 725. Here, the thickness of the cap layer is deposited thicker than the final desired thickness since, as will be shown later, part of this cap layer 730 will be removed during further processing. As before, the cap layer may be nickel, chromium, tin, etc. In step 630 the mask 720 is removed using proper solvent, depending on the type of mask material used. For example, a diluted mixture of KOH (less than 10%) can be used to remove wax or resist mask at room or elevated temperature (e.g., 50 C.). Then, a mixture of sodium persulfate or ammonium persulfate is used to remove the copper that was exposed when the mask was removed. Thereafter, a mixture of potassium permanganate is used to remove the part of the barrier/adhesion layer that was exposed by the removal of the copper. In this step, part of the cap layer may also be removed, which is why it is suggested to make the cap layer thicker than the desired final thickness. Also, in this step the potassium permanganate does not etch the TCO, so that in effect there is a natural etch stop when the barrier/adhesion layer is fully removed. In order to prevent any lateral etching of the barrier and seed layers, especially undercutting of the barrier layer underneath the copper fingers, the permanganate etching may be done with a jet spray to impart directionality to the etch, minimize isotropic etching resulting from immersing the wafer in a stationary liquid bath.

(21) FIG. 8 is a flow chart of another process according to an embodiment of the invention, while FIG. 9 illustrates a cross section of the progression of the resulting structure. This embodiment utilizes conventional silkscreen technology together with plating technology. In step 800, a silkscreen system is used to deposit a barrier/adhesion layer 910 using, e.g., chromium paste, titanium paste, or standard silver paste. The deposited barrier/adhesion layer 910 is annealed at relatively low temperatures, such as 150-250 C. In optional step 805 a seed layer 915 made of copper paste is deposited, also using silkscreen technique. Then, a mask 920 is formed in step 810, to delineate the design of the busbar and fingers. In this example, since the barrier/adhesion and seed layers were formed using silkscreen technology, it may be simpler to use silkscreen to also deposit a photoresist mask. At step 815 the wafer is taken to an electroplating system and electroplated with copper so as to form copper metallization layer 925. Thereafter in step 820 a cap layer 930 is deposited, also using electroplating. The mask 920 is then removed in step 825, so as to leave the metallization stack according to the mask design.

(22) FIG. 10 illustrates another embodiment of the invention, while FIG. 11 illustrates a cross section of the progression of the resulting structure. In the embodiment of FIGS. 10 and 11, the process begins before the deposition of the TCO. As shown in FIGS. 10 and 11, in step 1000 a barrier/adhesion layer is first deposited over the top layer of the PV device. For example, in the HIT structure it will be deposited over the top p-type amorphous silicon layer 110, while in the SmartSilicon device it will be deposited over the top p-type amorphous silicon layer 210. This is illustrated in FIG. 11 as adhesion layer 1110 deposited over device layer 1100. In this embodiment, the adhesion layer is PVD sputtered metal, such as, e.g., titanium, tantalum, etc. In optional step 1005 a copper seed layer 1115 is PVD sputtered over the adhesion layer 1110. In step 1010 a mask 1120 is formed to delineate the design of the metallization. The mask may be inkjet wax, silkscreen photoresist, etc. In step 1015 the wafer is electroplated with copper, to form copper metallization 1125. In optional step 1020 a cap layer 1130 is electroplated over the copper layer 1125. In step 1025 the mask is removed and in step 1030 the seed and adhesion layers are removed as well. Note that in this step since the adhesion and seed layers are much thinner than the copper metallization layer 1125, it is very easy to etch them without harming the metallization layer 1125. Thus, the seed and adhesion layer can be removed in a diluted HF solution. Again, the use of directional wet etching using a jet spray will help prevent any undercutting. In step 1135 TCO is deposited over the entire substrate, thereby providing TCO over the top device layer 110, and also covering, and thereby protecting, the sidewall and the top of metallization 1125. Since the TCO provides protection over the metallization layer, step 1020 of depositing a cap layer may be dispensed with. Still, it is recommended to deposit a cap layer of easily solderable material, such as tin. This will enable easy soldering of the PV cell array. In this respect, it is noted that the TCO will need to be partially removed from the top of the busbars for soldering. This can be easily done with diluted HF.

(23) For all of the above embodiments, when using electroplating, it is beneficial to prepare the surface of the TCO so that it is hydrophilic. This can be done by any of the following exemplary methods, or any combination thereof. According to one embodiment, the wafer with the ITO is rinsed in a soap-like solution to clean the surface of the TCO from any organic material. An example of such a solution may be the Micro-90, commercially available from Cole-Parmer of Vernon Hills, Ill. According to another embodiment, the surface of the TCO is treated with a surfactant. The surfactant treatment may be instead or in addition to the cleaning step. An example of surfactant may be a solution of sodium alkyl sulfates, mainly the lauryl, such as sodium dodecyl sulfate. According to another embodiment, the TCO is treated with UV light in an ozone atmosphere. This can be done at room or elevated temperature, e.g., 100-200 C.

(24) FIG. 12 illustrates an embodiment of the invention with metallization before TCO process, while FIG. 13 illustrates a cross section of the progression of the resulting structure. According to this embodiment, the metallization is fabricated first directly on the top layer 1300 of the photovoltaic device, and the TCO is fabricated later. In step 1200 a mask 1320 (FIG. 13) is formed over the top layer of the photovoltaic device to delineate the fingers and busbars. The top layer may be, e.g., the amorphous n-type or p-type silicon layer of the photovoltaic device junction. In step 1205 a barrier/adhesion layer 1310 is fabricated directly over the top device layer 1300, also somewhat covering the mask 1320. Optionally, in step 1210 a seed layer 1315 is fabricated over the barrier/adhesion layer. Then, in step 1215 the mask is replaced by first removing the mask 1320 so as to leave only metallization trace 1325, and then forming a new mask 1322. Then is step 1220 copper layer 1330 is formed by, e.g., electroplating. It should be noted that if electroplating is used, copper will be formed only where electrical potential is exposed, such that no copper will be formed over the mask. In step 1225 a cap layer 1335 is formed over the copper. Again, if electroplating is used, the cap layer will be formed only over the copper and not over the mask. Then is step 1230 the mask is removed, so as to leave only the metallization structure 1345. In step 1235 TCO is deposited over the entire wafer, thereby forming somewhat of an interdigit structure of the metallization a TCO over the top layer of the photovoltaic device. In this respect, it is possible to select the material of the cap layer such that TCO will not adhere to it. Regardless, the coverage of the stack 1345 by TCO is not detrimental and, in fact can be used as a good protection layer against oxidation of the stack.

(25) FIG. 14 illustrates another embodiment of the invention with metallization before TCO process, while FIG. 15 illustrates a cross section of the progression of the resulting structure. According to this embodiment, the metallization is fabricated first directly on the top layer 1500 of the photovoltaic device, and the TCO is fabricated later. In step 1400 a mask 1520 (FIG. 15) is formed over the top layer 1500 of the photovoltaic device to delineate the fingers and busbars. The top layer 1500 may be, e.g., the amorphous n-type or p-type silicon layer of the photovoltaic device junction. In step 1405 a barrier/adhesion layer 1505 is fabricated directly over the top device layer 1500, also somewhat covering the mask 1520. Optionally, in step 1410 a seed layer 1510 is fabricated over the barrier/adhesion layer. Then, in step 1420 copper layer 1515 is formed by, e.g., electroplating. In step 1425 a cap layer 1520 is formed over the copper. Then is step 1430 the mask is removed, so as to leave only the metallization structure 1525. In step 1435 TCO is deposited over the entire device. In this respect, it is possible to select the material of the cap layer such that TCO will not adhere to it. Regardless, the coverage of the stack 1525 by TCO is not detrimental and, in fact can be used as a good protection layer against oxidation of the stack.

(26) FIG. 16 illustrates yet another embodiment of the invention with metallization before TCO process, while FIG. 17 illustrates a cross section of the progression of the resulting structure. This embodiment utilizes conventional silkscreen technology together with plating technology, whereby the metallization is fabricated first directly on the top layer 1700 of the photovoltaic device, and the TCO is fabricated later. In step 1700, a silkscreen system is used to deposit a barrier/adhesion layer 1705 directly on the top device layer 1700, using, e.g., chromium paste, titanium paste, or standard silver paste. The deposited barrier/adhesion layer 1705 is annealed at relatively low temperatures, such as 150-250 C. In optional step 1605 a seed layer 1710 made of copper paste is deposited, also using silkscreen technique. Then, a mask 1720 is formed in step 1610, to delineate the design of the busbar and fingers. In this example, since the barrier/adhesion and seed layers were formed using silkscreen technology, it may be simpler to use silkscreen to also deposit a photoresist mask. At step 1615 the wafer is taken to an electroplating system and electroplated with copper so as to form copper metallization layer 1725. Thereafter in step 1620 a cap layer 1730 is deposited, also using electroplating. The mask 1720 is then removed in step 1625, so as to leave the metallization stack 1745 according to the mask design. Then, in step 1630 a TCO layer 1705 is deposited over the entire device.

(27) It should be understood that processes and techniques described herein are not inherently related to any particular apparatus and may be implemented by any suitable combination of components. Further, various types of general purpose devices may be used in accordance with the teachings described herein. It may also prove advantageous to construct specialized apparatus to perform the method steps described herein.

(28) The present invention has been described in relation to particular examples, which are intended in all respects to be illustrative rather than restrictive. Those skilled in the art will appreciate that many different combinations of hardware, software, and firmware will be suitable for practicing the present invention. Moreover, other implementations of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims. For example, the disclosure relates to using copper; however, it should be appreciated that an alloy comprising copper and other materials, such as, e.g., nickel and/or tin can be substituted.