# Patent

## Title
Solar cell with electroplated metal grid

## Abstract

One embodiment of the present invention provides a method for fabricating solar cells. During operation, an anti-reflection layer is deposited on top of a semiconductor structure to form a photovoltaic structure, and a front-side electrode grid comprising a metal stack is formed on top of the photovoltaic structure. The metal stack comprises a metal-adhesive layer comprising Ti or Ta, and a conducting layer comprising Cu or Ag situated above the metal-adhesive layer.

## Background
RELATED APPLICATION
This application claims the benefit of U.S. Provisional Application No. 61/381,659, entitled “SOLAR CELL WITH METAL GRIDS FABRICATED BY USING ELECTROPLATING,” by inventors Jianming Fu, Zheng Xu, Chentao Yu, and Jiunn Benjamin Heng, filed 10 Sep. 2010.


BACKGROUND
Field
This disclosure is generally related to designing of solar cells. More specifically, this disclosure is related to a solar cell that includes a metal grid fabricated by an electroplating technique.
Related Art
The negative environmental impact caused by the use of fossil fuels and their rising cost have resulted in a dire need for cleaner, cheaper alternative energy sources. Among different forms of alternative energy sources, solar power has been favored for its cleanness and wide availability.
A solar cell converts light into electricity using the photovoltaic effect. There are several basic solar cell structures, including a single p-n junction solar cell, a p-i-n/n-i-p solar cell, and a multi junction solar cell. A typical single p-n junction structure includes a p-type doped layer and an n-type doped layer. Solar cells with a single p-n junction can be homojunction solar cells or heterojunction solar cells. If both the p-doped and n-doped layers are made of similar materials (materials with equal bandgaps), the solar cell is called a homojunction solar cell. In contrast, a heterojunction solar cell includes at least two layers of materials of different bandgaps. A p-i-n/n-i-p structure includes a p-type doped layer, an n-type doped layer, and an intrinsic (undoped) semiconductor layer (the i-layer) sandwiched between the p-layer and the n-layer. A multi junction structure includes multiple single junction structures of different bandgaps stacked on top of one another.
In a solar cell, light is absorbed near the p-n junction, generating carriers. The carriers diffuse into the p-n junction and are separated by the built-in electric field, thus producing an electrical current across the device and external circuitry. An important metric in determining a solar cell's quality is its energy-conversion efficiency, which is defined as the ratio between power converted (from absorbed light to electrical energy) and power collected when the solar cell is connected to an electrical circuit.
FIG. 1 presents a diagram illustrating an exemplary homojunction solar cell based on a crystalline-Si (c-Si) substrate (prior art). Solar cell 100 includes a front-side Ag electrode grid 102, an anti-reflection layer 104, an emitter layer 106, a substrate 108, and an aluminum (Al) back-side electrode 110. Arrows in FIG. 1 indicate incident sunlight.
In conventional c-Si based solar cells, the current is collected by front-side Ag grid 102. To form Ag grid 102, conventional methods involve printing Ag paste (which often includes Ag particle, organic binder, and glass frit) onto the wafers and then firing the Ag paste at a temperature between 700° C. and 800° C. The high-temperature firing of the Ag paste ensures good contact between Ag and Si, and lowers the resistivity of the Ag lines. The resistivity of the fired Ag paste is typically between 5×10−6 and 8×10−6 ohm-cm, which is much higher than the resistivity of bulk silver.
In addition to the high series resistance, the electrode grid obtained by screen-printing Ag paste also has other disadvantages, including higher material cost, wider line width, and limited line height. As the price of silver rises, the material cost of the silver electrode has exceeded half of the processing cost for manufacturing solar cells. With the state-of-the-art printing technology, the Ag lines typically have a line width between 100 and 120 microns, and it is difficult to reduce the line width further. Although inkjet printing can result in narrower lines, inkjet printing suffers other problems, such as low productivity. The height of the Ag lines is also limited by the printing method. One print can produce Ag lines with a height that is less than 25 microns. Although multiple printing can produce lines with increased height, it also increases line width, which is undesirable for high-efficiency solar cells. Similarly, electroplating of Ag or Cu onto the printed Ag lines can increase line height at the expense of increased line width. In addition, the resistance of such Ag lines is still too high to meet the requirement of high-efficiency solar cells.
Another solution is to electroplate a Ni/Cu/Sn metal stack directly on the Si emitter. This method can produce a metal grid with lower resistance (the resistivity of plated Cu is typically between 2×10−6 and 3×10−6 ohm-cm). However, the adhesion of Ni to Si is less than ideal, and stress from the metal stack may result in peeling of the whole metal lines.
SUMMARY
One embodiment of the present invention provides a method for fabricating solar cells. During operation, an anti-reflection layer is deposited on top of a photovoltaic structure, and a front-side electrode grid comprising a metal stack is formed on top of the anti-reflection layer. The metal stack comprises a metal-adhesive layer comprising Ti or Ta, and a conducting layer comprising Cu or Ag situated above the metal-adhesive layer.
In a variation on the embodiment, the metal-adhesive layer further comprises one or more of: TiN, TiW, TiSi, TaN, and Co.
In a variation on the embodiment, the metal-adhesive layer is formed using a physical vapor deposition (PVD) technique including one of: evaporation and sputtering deposition.
In a variation on the embodiment, a soldering layer comprising Sn or Ag is formed on top of the conducting layer.
In a variation on the embodiment, the metal-adhesive layer has a thickness between 1 nm and 1000 nm.
In a further variation, the metal-adhesive layer has a thickness between 5 nm and 50 nm.
In a variation on the embodiment, the conducting layer is formed by depositing a metal seed layer above the metal-adhesive layer, and depositing a bulk-metal layer above the metal seed layer.
In a further variation, the metal seed layer is formed using a physical vapor deposition (PVD) technique including one of: evaporation and sputtering deposition.
In a further variation, the bulk-metal layer is formed by depositing a patterned masking layer on the metal seed layer and plating a layer of metal over the patterned masking layer. Openings of the masking layer correspond to positions of the front-side electrode grid, and the plated metal has a similar material makeup as that of the metal seed layer.
In a further variation, the method comprises removing the masking layer and performing an etching process to remove portions of the metal seed layer and the metal-adhesive layer that are not covered by the plated metal.
In a further variation, the bulk-metal layer is formed by depositing a patterned masking layer on the metal seed layer, performing an etching process to remove portions of the metal seed layer and the metal-adhesive layer that are not covered by the patterned masking layer, removing the patterned masking layer to expose un-etched portions of the metal seed layer, and plating a layer of metal over the un-etched portions of the metal seed layer. Areas covered by the patterned masking layer correspond to positions of the front-side electrode grid, and the plated metal has a similar material makeup as that of the metal seed layer.



BRIEF DESCRIPTION OF THE FIGURES
FIG. 1 presents a diagram illustrating an exemplary solar cell (prior art).
FIG. 2 presents a diagram illustrating an exemplary process of fabricating a solar cell in accordance with an embodiment of the present invention.
FIG. 3 presents a diagram illustrating an exemplary process of fabricating a solar cell in accordance with an embodiment of the present invention.



In the figures, like reference numerals refer to the same figure elements.
DETAILED DESCRIPTION
The following description is presented to enable any person skilled in the art to make and use the embodiments, and is provided in the context of a particular application and its requirements. Various modifications to the disclosed embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the present disclosure. Thus, the present invention is not limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein.
Overview
Embodiments of the present invention provide a solar cell that includes a metal grid formed by electroplating. The solar cell includes a crystalline-Si (c-Si) substrate, an emitter layer, a passivation layer, a metal-adhesion layer, and front- and back-side electrode metal grids. The metal-adhesion layer is formed using a physical vapor deposition (PVD) technique, such as sputtering or evaporation. The front-side metal grid is formed by selectively electroplating a metal stack, which can be a single-layer or a multi-layer structure, on the metal-adhesion layer. The back-side electrode is formed by screen-printing, electroplating, or aerosol-jet printing of a metal grid.
Fabrication Process
FIG. 2 presents a diagram illustrating an exemplary process of fabricating a solar cell in accordance with an embodiment of the present invention.
In operation 2A, a Si substrate 200 is prepared. In one embodiment, Si substrate 200 can be a p-type crystalline-Si (c-Si) wafer. In a further embodiment, preparing Si substrate 200 includes standard saw damage etch (which removes the damaged outer layer of Si) and surface texturing.
In operation 2B, a lightly doped emitter layer 202 is formed on top of Si substrate 200. Depending on the doping type of Si substrate 200, emitter layer 202 can be either n-type doped or p-type doped. In one embodiment, emitter layer 202 is lightly doped with n-type dopant. In a further embodiment, emitter layer 202 is formed by diffusing phosphorous. Note that if phosphorus diffusion is used for forming emitter layer 202, phosphosilicate glass (PSG) etch and edge isolation is needed.
In operation 2C, an anti-reflection layer 204 is formed on top of emitter layer 202. In one embodiment, anti-reflection layer 204 includes, but not limited to: silicon nitride (SiNx), silicon oxide (SiOx), titanium oxide (TiOx), aluminum oxide (Al2O3), and their combinations. In one embodiment, anti-reflection layer 204 includes a layer of a transparent conducting oxide (TCO) material, such as indium tin oxide (ITO), aluminum zinc oxide (AZO), gallium zinc oxide (GZO), tungsten doped indium oxide (IWO), and their combinations.
In operation 2D, back-side electrode 206 is formed on the back side of Si substrate 200. In one embodiment, forming back-side electrode 206 includes printing a full Al layer and subsequent allying through firing. In one embodiment, forming back-side electrode 206 includes printing an Ag/Al grid and subsequent furnace firing.
In operation 2E, a number of contact windows, including windows 208 and 210, are formed in anti-reflection layer 204. In one embodiment, heavily doped regions, such as regions 212 and 214 are formed in emitter layer 202, directly beneath contact windows 208 and 210, respectively. In a further embodiment, contact windows 208 and 210 and heavily doped regions 212 and 214 are formed by spraying phosphorous on anti-reflection layer 204, followed by a laser-groove local-diffusion process. Note that operation 2E is optional, and is needed when anti-reflection layer is electrically insulating. If anti-reflection layer 204 is electrically conducting (e.g., when anti-reflection layer 204 is formed using TCO material), there is no need to form the contact windows.
In operation 2F, an adhesive layer 216 is formed on anti-reflection layer 204. In one embodiment, materials used to form adhesive layer 216 include, but are not limited to: Ti, titanium nitride (TiNx), titanium tungsten (TiWx), titanium silicide (TiSix), titanium silicon nitride (TiSiN), Ta, tantalum nitride (TaNx), tantalum silicon nitride (TaSiNx), nickel vanadium (NiV), tungsten nitride WNx), Co, W, Cr, Mo, Ni, and their combinations. In a further embodiment, adhesive layer 216 is formed using a physical vapor deposition (PVD) technique, such as sputtering or evaporation. The thickness of adhesive layer 216 can range from a few nanometers up to 100 nm. Note that Ti and its alloys tend to form very good adhesion with Si material, and they can form good ohmic contact with heavily doped regions 212 and 214.
In operation 2G, a metal seed layer 218 is formed on adhesive layer 216. Metal seed layer 218 can include Cu or Ag. The thickness of metal seed layer 218 can be between 5 nm and 500 nm. In one embodiment, metal seed layer 218 has a thickness of 100 nm. Similarly to adhesive layer 216, metal seed layer 218 can be formed using a PVD technique.
In operation 2H, a patterned masking layer 220 is deposited on top of metal seed layer 218. The openings of masking layer 220, such as openings 222 and 224, correspond to the locations of contact windows 208 and 210, and thus are located above heavily doped regions 212 and 214. Note that openings 222 and 224 are slightly larger than contact windows 208 and 210. Masking layer 220 can include a patterned photoresist layer, which can be formed using a photolithography technique. In one embodiment, the photoresist layer is formed by screen-printing photoresist on top of the wafer. The photoresist is then baked to remove solvent. A mask is laid on the photoresist, and the wafer is exposed to UV light. After the UV exposure, the mask is removed, and the photoresist is developed in a photoresist developer. Openings 222 and 224 are formed after developing. The photoresist can also be applied by spraying, dip coating, or curtain coating. Dry film photoresist can also be used. Alternatively, masking layer 220 can include a layer of patterned silicon oxide (SiO2). In one embodiment, masking layer 220 is formed by first depositing a layer of SiO2 using a low-temperature plasma-enhanced chemical-vapor-deposition (PECVD) technique. In a further embodiment, masking layer 220 is formed by dip-coating the front surface of the wafer using silica slurry, followed by screen-printing an etchant that includes hydrofluoric acid or fluorides. Other masking materials are also possible, as long as the masking material is electrically insulating.
In operation 2I, one or more layers of metal are deposited at the openings of masking layer 220 to form a front-side metal grid 226. Front-side metal grid 226 can be formed using an electroplating technique, which can include electrodeposition, light-induced plating, and/or electroless deposition. In one embodiment, metal seed layer 218 and/or adhesive layer 216 are coupled to the cathode of the plating power supply, which can be a direct current (DC) power supply, via an electrode. Metal seed layer 218 and masking layer 220, which includes the openings, are submerged in an electrolyte solution which permits the flow of electricity. Note that, because only the openings within masking layer 220 are electrically conductive, metals will be selectively deposited into the openings, thus forming a metal grid with a pattern corresponding to that of the previously formed contact windows on anti-reflection layer 204. Depending on the material forming metal seed layer 218, front-side metal grid 226 can be formed using Cu or Ag. For example, if metal seed layer 218 is formed using Cu, front-side metal grid 226 is also formed using Cu. In addition, front-side metal grid 226 can include a multilayer structure, such as a Cu/Sn bi-layer structure, or a Cu/Ag bi-layer structure. The Sn or Ag top layer is deposited to assist a subsequent soldering process. When depositing Cu, a Cu plate is used at the anode, and the solar cell is submerged in the electrolyte suitable for Cu plating. The current used for Cu plating is between 0.1 ampere and 2 amperes for a wafer with a dimension of 125 mm×125 mm, and the thickness of the Cu layer is approximately tens of microns.
In operation 2J, masking layer 220 is removed.
In operation 2K, portions of adhesive layer 216 and metal seed layer 218 that are originally covered by masking layer 220 are etched away, leaving only the portions that are beneath front-side metal grid 226. In one embodiment, wet chemical etching process is used. Note that, because front-side metal grid 226 is much thicker (by several magnitudes) than adhesive layer 216 and metal seed layer 218, the etching has a negligible effect on front-side metal grid 226.
FIG. 3 presents a diagram illustrating another exemplary process of fabricating a solar cell in accordance with an embodiment of the present invention.
In operation 3A, a Si substrate 300 is prepared using a process similar to the one used in operation 2A.
In operation 3B, a lightly doped emitter layer 302 is formed on top of Si substrate 300, using a process similar to the one used in operation 2B.
In operation 3C, an anti-reflection layer 304 is formed on top of emitter layer 302, using a process similar to the one used in operation 2C.
In operation 3D, back-side electrode 306 is formed on the back side of Si substrate 300, using a process similar to the one used in operation 2D.
In operation 3E, a number of contact windows, including windows 308 and 310, are formed, using a process similar to the one used in operation 2E.
In one embodiment, heavily doped regions 312 and 314 are formed in emitter layer 302.
In operation 3F, an adhesive layer 316 is formed on anti-reflection layer 304, using a process similar to the one used in operation 2F.
In operation 3G, a metal seed layer 318 is formed on adhesive layer 316, using a process similar to the one used in operation 2G.
In operation 3H, a patterned masking layer 320 is deposited on top of metal seed layer 318 using a process similar to the one used in operation 2H. However, unlike patterned masking layer 220, patterned masking layer 320 covers areas that correspond to the locations of contact windows 308 and 310, while still being located above heavily doped regions 312 and 314. Note that the covered areas are slightly larger than contact windows 308 and 310.
In operation 3I, portions of adhesive layer 316 and metal seed layer 318 that are not covered by masking layer 320 are etched away using a process similar to the one used in operation 2K.
In operation 3J, masking layer 320 is removed to expose the leftover portions of metal seed layer 318.
In operation 3K, one or more layers of metal are deposited on the leftover portions of metal seed layer 318 to form a front-side metal grid 326, using a process similar to the one used in operation 2I. Note that, because only the leftover portions of metal seed layer 318 are electrically conductive, a plating process can selectively deposit metal on top of the leftover portions of metal seed layer 318.
The foregoing descriptions of various embodiments have been presented only for purposes of illustration and description. They are not intended to be exhaustive or to limit the present invention to the forms disclosed. Accordingly, many modifications and variations will be apparent to practitioners skilled in the art. Additionally, the above disclosure is not intended to limit the present invention.

## Claims

1. A solar cell, comprising:
a semiconductor photovoltaic structure comprising an emitter layer positioned on a base layer; and
a first metallic grid positioned on a front side of the photovoltaic structure, wherein the first metallic grid includes:
a metal-adhesive layer which comprises Ta and is in direct contact with the emitter layer;
a Cu seed layer positioned on the metal-adhesive layer; and
an electroplated Cu layer positioned on the Cu seed layer.

2. The solar cell of claim 1, wherein the metal-adhesive layer further comprises one or more of: Ti, Co, W, Cr, Mo, Ni, titanium nitride (TiNx), titanium tungsten (TiWx), titanium silicide (TiSix), titanium silicon nitride (TiSiN), tantalum nitride (TaNx), tantalum silicon nitride (TaSiNx), nickel vanadium (NiV), tungsten nitride (WNx), and their combinations.

3. The solar cell of claim 1, wherein the first metallic grid further comprises a soldering layer comprising Sn or Ag positioned on the electroplated Cu layer.

4. The solar cell of claim 1, wherein the metal-adhesive layer has a thickness between 1 nm and 1000 nm.

5. The solar cell of claim 4, wherein the metal-adhesive layer has a thickness between 5 nm and 50 nm.

6. The solar cell of claim 1, wherein the photovoltaic structure further comprises an anti-reflection layer positioned on the emitter layer.

7. The solar cell of claim 6, wherein the anti-reflection layer is electrically insulating, and wherein the anti-reflection layer includes a number of openings, through which the metal-adhesive layer is in direct contact with the emitter layer.

8. The solar cell of claim 7, wherein the insulating anti-reflection layer includes at least one of:
silicon nitride (SiNx);
silicon oxide (SiOx);
titanium oxide (TiOx); and
aluminum oxide.

9. The solar cell of claim 6, wherein the anti-reflection layer includes a transparent conducting oxide (TCO) layer, and wherein the metal adhesive layer is in contact with the TCO layer.

10. The solar cell of claim 9, wherein the TCO layer includes at least one of:
indium tin oxide (ITO);
aluminum zinc oxide (AZO);
gallium zinc oxide (GZO); and
tungsten doped indium oxide (IWO).

11. The solar cell of claim 1, wherein the Cu seed layer has a thickness between 5 nm and 500 nm.

12. The solar cell of claim 1, wherein the metal adhesive layer and the Cu seed layer are formed using a physical vapor deposition technique.

13. The solar cell of claim 12, wherein the physical vapor deposition technique includes one of: evaporation and sputtering deposition.