First, copper electroplating helps reduce costs and increase efficiency in the metallization of photovoltaic solar cells

1.1 Constrained by high-cost silver paste, the metallization process of photovoltaic cells needs to be innovated urgently

Photovoltaic solar cells are the most core devices for photovoltaic systems to achieve photoelectric conversion, and their preparation process is mainly divided into several major process links such as cleaning and texturing, diffusion knotting and knotting, front and back coating, metallization printing and solidification. Among them, the metallization link is mainly used to make the photovoltaic solar cell electrode grid wire, and solidifies the metal electrode by printing silver paste on both sides of the cell, so that the electrode and the cell are tightly combined to form an efficient ohmic contact to achieve current output.

The metallization link mainly includes silver paste screen printing, silver coated copper screen printing, laser transfer, electroplating copper, inkjet printing and other types of processes, the traditional screen printing mature and simple is the current main flow production technology route, other processes have not yet achieved large-scale industrialization.

At present, the core key material in the solar cell metallization process is silver paste, which not only directly determines the conductivity of the cell, but also the core consumables second only to silicon wafers in the cost of the solar cell. With the acceleration of the expansion of N-type cells, the demand for silver for photovoltaic cells will grow rapidly.

According to the statistics of the China Photovoltaic Industry Association, in 2022, the consumption of positive silver and back silver of P-type cells is about 91mg/piece, the average consumption of double-sided silver paste (aluminum) (95% silver) of N-type TOPCon cells is about 115mg/piece, the consumption of double-sided low-temperature silver paste of HJT solar cells is about 127mg/piece, and the consumption of silver paste of N-type solar cells is significantly increased compared with PERC.

At present, TOPCon solar cells generally use high-temperature silver paste or silver aluminum paste, HJT solar cells due to the high hydrogen content of amorphous silicon film, the temperature of the production process is controlled below 250 °C, the use of more expensive low-temperature conductive silver paste, the optimization and cost reduction of slurry is especially important for the economic improvement and industrialization of N-type batteries.

Silver paste uses expensive silver powder as the main substrate, and the current supply of silver powder mainly depends on foreign imports, and the cost of silver paste will be disturbed by silver prices and exchange rate fluctuations. The large consumption and high cost of silver paste are one of the pain points restricting the industrialization of N-type solar cells such as HJT, and the industry urgently needs to innovate metallization processes to achieve cost reduction and efficiency increase.

At present, there are two main directions for the process technology of reducing silver consumption: one is to reduce the amount of high-priced silver paste, such as the application of SMBB or 0BB, laser transfer and other technologies; The second is to use base metal copper instead of silver powder to reduce the amount of silver powder, such as the application of silver-clad copper, electroplated copper and other technologies.

According to the photovoltaic industry association, the metal electrode of heterojunction cells is still dominated by silver electrodes, and the market share of low-temperature silver paste electrodes will reach 98.2% in 2022, and some enterprises and research institutions are actively developing electrode technologies that replace silver such as base metal copper, including silver-clad copper paste combined with silk screen printing technology and electroplating copper technology. Recently, under the background of continuous innovation in solar cell metallization process, the development of copper electroplating process as a completely silver-free technology has accelerated.

1.2 Electroplated copper helps photovoltaic cells reduce costs and increase efficiency, with significant development advantages

Copper plating is a non-contact copper electrode preparation process that is expected to help photovoltaic cells achieve complete silver-free. Copper electroplating technology is mature in printed circuit board PCB and other industries, and can also be used in the metallization process of crystalline silicon cells, the principle of which is to deposit metal copper on the surface of the base metal by electrolysis to make copper grid wires, and then as an electrode to collect the carriers generated by the photovoltaic effect.

Compared with silver paste screen printing, copper plating process has lower silver paste cost, better conductivity, better plasticity and aspect ratio, which is expected to replace high-silver screen printing technology, further improve solar cell efficiency and reduce silver paste cost, and help HJT and XBC solar cells reduce costs, increase efficiency and scale development.

(1) Cost reduction of copper plating advantages: lower silver paste cost than silver paste screen printing. As of September 5, the spot closing price of silver on the Shanghai Gold Exchange was 5814 yuan/kg (Wind), the silver front silver paste was 6391 yuan/kg (Baichuan Yingfu), while the average market price of electrolytic copper 1# was only 69.83 yuan/kg (Baichuan Yingfu), and the copper price was only about 1% of the silver paste.

Copper plating technology eliminates the need for expensive silver raw materials, and uses low-cost pure copper instead of electrodes, greatly reducing the cost of raw materials to achieve silver-free. In addition, electroplated copper equipment can be applied to HJT, XBC, Topcon and other technologies, and can complete double-sided plating on the front and back of the solar cell at the same time, with broad cost reduction space and application potential.

(2) Synergy of copper plating advantages: 

(1) The conductivity of copper electrode is better than that of silver grid wire, and the contact characteristics with the TCO layer are better, which promotes the improvement of cell conversion efficiency. 

A. Metal resistivity affects the power loss and conductivity of the electrode, and pure copper has lower resistivity. Heterojunction low-temperature silver paste is mainly composed of silver powder, organic resin and other materials, and some organic matter is not conductive after the slurry is cured, so that the resistivity of low-temperature silver paste is higher and the power loss of electrode is large.

At the same time, because the sintering temperature of low-temperature silver paste does not exceed 250°C, the bond between Ag particles in the slurry is not tight, and there are more voids, resulting in the increase of its line resistance and the increase of series resistance. The copper plating grid wire uses pure copper, its resistivity is close to pure silver but significantly lower than that of low-temperature silver paste, and its electrode structure is dense and uniform, there is no obvious void, which can achieve lower line resistivity, reduce solar cell electrode ohmic loss, and improve electrical performance. 

B. The contact characteristics of metal and TCO layer affect the carrier collection, adhesion characteristics and electrical properties of heterojunction solar cells, and copper electroplated electrodes have more advantages. There are many holes in the contact between the silver paste and the TCO transparent conductive film, resulting in the increase of metal-semiconductor contact resistance and the decrease of electrode adhesion, which affects the carrier transport.

The copper electrode is easy to adhere closely to the transparent conductive film without obvious holes, so that the contact resistance is small, which can improve the probability of carrier collection.

(2) The copper electrode has ultra-fine line width, better height to width ratio and better plasticity, which promotes the improvement of cell conversion efficiency. The width of silver grid wire for silver paste screen printing is generally about 30-40μm, while the width of copper plated copper grid is about 15-20μm.

In the electroplating process, the deposition of copper metal is limited to the mask, the graphical equipment can design a thinner electrode line width, the electrode has a better aspect ratio after peeling off the mask, and is shaped in a rectangular shape, which can maximize the grid and reduce shading loss, superimposed on the decrease of copper resistivity, can significantly improve the generation and collection of photogenerated carriers, and promote the efficiency of the cell by 0.3%-0.5%.

At present, the copper electroplating process is still facing certain mass production difficulties, and it is expected to be optimized and improved through technology, process, equipment, materials and other measures. Specific mass production difficulties include: 

(1) Compared with traditional screen printing, the electroplating copper process is longer, prone to de-meshing, oxidation, etc., and there are difficulties in equipment process cost and yield control, and large-scale mass production technology needs to be further matured.

(2) The electroplating process will retain various heavy metals and waste liquids, and the cost of environmental protection treatment is high. Subsequently, the industry is expected to optimize and improve technology, processes, equipment, materials, and solar cell components through upstream and downstream collaboration, and accelerate the progress of verification and mass production.

Second, the process focuses on graphics and electroplating, and the technical route has not yet been finalized.

2.1 Copper plating process: seed layer deposition - patterning - electroplating - post-treatment

The photovoltaic electroplating copper process mainly includes seed layer deposition, graphics, electroplating and post-processing four major links, the current technical routes of each link are different, a variety of combination process schemes parallel, need comprehensive performance, cost to choose the appropriate process route.

(1) seed layer deposition: the surface of the heterojunction cell is deposited with a transparent conductive film (TCO) as a conductive layer, anti-reflective layer, due to the poor adhesion of copper on the TCO layer, the physical contact between the two, the adhesion is mainly van der Waals force, the electrode is easy to fall off, generally need to use PVD equipment to deposit the seed layer (100nm) on the TCO layer before electroplating to improve the electrode contact and adhesion problems. 

(2) Graphic: Since copper plating on TCO is non-selective, it is necessary to form a graphical mask to show the line pattern of the copper plating area, and divide the conductive and non-conductive parts.

In the process of patterning, the photosensitive adhesive layer is covered on the surface of the HJT solar cell, and the positional photosensitive material that does not need copper plating is modified by selective illumination, while the positional photosensitive material that needs copper plating remains unchanged.

In the development step, the photosensitive material that has not undergone a modification reaction in the copper plating area will be removed to form a patterned mask, and the copper will be deposited on the exposed seed layer in the pattern area of the line and conductive during subsequent plating, while copper deposition will not occur in other locations, achieving selective plating. 

(3) Electroplating: the solar cell (cathode) to be electroplated is immersed in the copper sulfate (anode) solution of the electroplating equipment, electrolysis is carried out by electricity, and the copper ion (cation Cu2+) is reduced, and copper is deposited into copper in the line area that needs to be plated to form a selective copper electrode. 

(4) Post-treatment: electroplating tin or using anti-copper oxidant to make a protective layer, wet etching, laser etching, plasma etching and other processes can be applied to peel off the mask, etch off the remaining seed layer, and do surface treatment, and finally obtain a copper electrode with excellent shaping and good selectivity.

2.2 Seed layer preparation: mainly involves PVD equipment

The preparation methods of seed layer include physical vapor deposition PVD, chemical vapor deposition CVD, spraying, printing, etc., among which PVD equipment is the current mainstream. Copper metal with good economic efficiency is often used in the preparation of seed layer, and with the increase in demand for circuit stability, some copper alloy materials that can improve the stability of copper wire are also being evaluated.

The technical differences in this link mainly lie in whether to prepare the seed layer, whether to prepare the whole surface or local seed layer, and the choice of metal in the seed layer.

2.3 Graphical link: mainly involving exposure machine, developer, laser equipment

The patterned process of electroplating copper mainly includes several steps: masking, exposure, and development. Among them, the mask link is to coat the anti-etching photosensitive material on the surface of the solar cell to cover and protect the area that does not need to be plated, and the photosensitive materials mainly include wet film ink, dry film materials, etc.

The exposure and development links are to transfer the pattern to the photosensitive material, and the main technologies are LDI laser direct writing lithography (no mask required), conventional mask lithography technology, laser slotting, inkjet printing, etc.; Among them, the application potential of LDI laser direct writing lithography technology without masking is great, and laser slotting has been mass-produced and applied on BC batteries, and the overall graphical technical route is expected to be gradually clarified and finalized.

At present, the layout of photovoltaic electroplating copper graphics process has realized the shipment of electroplated copper LDI direct writing lithography equipment and proximity/contact lithography equipment.

Lithography technology refers to the use of optical-chemical reaction principle and chemical and physical etching methods to transfer the designed micro-graphic structure to the surface of substrates such as wafers, glass substrates, and copper clad laminates covered with photosensitive materials.

Lithography equipment is a key equipment for micro-nano manufacturing, in the field of pan-semiconductors, according to whether to use the mask version, lithography technology is mainly divided into direct writing lithography and mask lithography, of which mask lithography can be further divided into proximity/contact lithography and projection lithography.

Mask lithography is a beam of light emitted by a light source, imaged on a photosensitive material by a mask version, which can be divided into proximity, contact lithography and projection lithography. Among them, projection lithography is more advanced, enabling smaller scale images to be obtained using the same size mask plate, resulting in finer imaging.

Direct writing lithography, also known as maskless lithography, refers to the computer converting the circuit design pattern into machine-recognizable graphic data, and the computer-controlled beam modulator realizes the real-time display of the pattern, and then focuses the graphic beam on the surface of the substrate coated with photosensitive materials through the optical imaging system, and directly scans and exposures.

Direct writing lithography not only has the technical characteristics of projection lithography, such as projection imaging technology, double table technology, step scanning exposure, etc., but also has the technical characteristics of high flexibility, low cost and shortening of process flow that projection lithography does not have.

Micro-mounted direct writing lithography technology is a direct writing technology using high-speed real-time dynamic surface scanning, using high-power ultraviolet laser or LED light source, illuminating on the digital micromirror device, generating dynamic graphics in real time through the data link, and then directly projecting onto the substrate covered with photosensitive materials through the high-precision, low-distortion projection exposure lens to efficiently form exposure patterns in real time.

2.4 Electroplating link: main equipment electroplating machine

The electroplating process mainly includes vertical electroplating, horizontal electroplating, plug-in electroplating, etc., of which vertical electroplating mainly includes vertical lifting electroplating and vertical continuous plating, and the current value of electroplating equipment is about 40-50 million yuan / GW.

At present, a variety of technical routes are parallel, how to improve electroplating capacity and quality performance, reduce fragmentation rate will be the direction of solar cell electroplating machine to improve and improve, with the optimization of electroplating process, electroplating copper technology is expected to gradually clarify the technology selection in 2024. 

Vertical plating: rack plating, the fixture clamps the solar cell to be plated vertically into the electroplating tank for electroplating. The production capacity of vertical continuous plating has increased compared to vertical lift plating.

Plug-in plating: The solar cell to be plated is placed on a cathodic conductive bracket and inserted downward so that a conductive support unit is positioned between two adjacent anode plate assemblies for plating. 

According to the invention patent "A plug-in solar cell copper electrode electroplating device and method thereof" obtained by Robotec, CN115613106A the equipment can achieve double-sided plating, a single line can achieve 14000 whole pieces / hour, the fragmentation rate is <0.02%, which improves the capacity and plating quality of the device, reduces the defective rate, has a reasonable structure and a small footprint.

In addition, in April this year, Robotec applied for a utility model patent (authorization announcement number CN219342367U, authorization announcement date 2023.07.14) for "a multi-channel solar cell copper electrode electrode plating device", by setting the cell on the conductive support unit, moving the cathode conductive basket to make the conductive support unit move between the anode plate components of multiple anode units to achieve electroplating; The electroplating device of this utility model has a capacity of up to 24,000 whole pieces/hour, which has better plating uniformity, improves plating quality, and reduces the risk of fragmentation.

Third, electroplated copper is expected to accelerate the pilot test and gradually introduce mass production, and the market space will be further opened

3.1 Electroplated copper is expected to accelerate pilot testing and gradually introduce mass production, and silver-free technology will accelerate the industrialization of HJT and XBC batteries

At present, TOPCon technology has gradually established the mainstream position of photovoltaic cell module expansion with superior economy and cost performance; It is expected that there will be a large-scale production capacity in the fourth quarter of this year, and the annual shipment is expected to reach 100-150GW, and the subsequent use of bifacial Poly, TBC and other technologies is expected to strengthen the competitive advantage.

HJT solar cell is in the critical period of cost reduction and efficiency improvement and market introduction, with the application and introduction of double-sided microcrystalline, silver-clad copper paste, 0BB technology, UV conversion film and other products, the industrialization process has accelerated, and the bidding scale of central state-owned enterprises HJT has recently expanded; In the future, through electroplating copper silver-free, low indium laminated film and other technologies, it is expected to further promote HJT technology to reduce costs and increase efficiency.

Recently, LONGi has clearly taken BC solar cell as the company's main technical route in the future, of which HPBC, TBC, HBC has its own advantages, some enterprises mainly promote silver-free ABC technology route, a number of head solar cell module companies have laid out BC technology, with the expansion of XBC cell production in the future, silver-free electroplating copper process is expected to accelerate the introduction of production.

At present, it is expected that the non-silicon cost of PERC, TOPCon and HJT solar cells using conventional silver paste screen printing process is about 0.14, 0.17 and 0.28 yuan/W, respectively, and the non-silicon cost of HJT solar cells is significantly higher than that of TOPCon and PERC batteries.

Among them, compared with TOPCon technology, HJT technology has a large gap with it in terms of equipment depreciation, and the cost of silver paste per watt is close to 2 times that of TOPCon. The cost of HJT solar cell silver paste accounts for about 40% of its non-silicon cost, which greatly affects the economy of HJT products, and the demand for silver consumption reduction is more urgent.

Silver reduction through 0BB/NBB, silver-clad copper, electroplated copper and other processes is expected to help solve the problem of high non-silicon cost of N-type solar cells such as HJT.

At present, the metallization cost of HJT solar cells using traditional sterling silver screen printing is about 0.14 yuan/W; Among the various types of silver reduction technologies, the pilot verification and mass production introduction of silver-clad copper and 0BB/NBB technology are fast, and it is estimated that the metallization cost of HJT solar cells in the superimposed application of the two technologies (comprehensive pure silver accounts for about 50%) is about 0.08 yuan/W, of which silver consumption is about 9-10mg/W, which will become a metallization cost reduction path that can be mass-produced in the short term;

It is estimated that the non-silicon cost of HJT solar cells applied in the pilot stage in the short term is about 0.11 yuan/W, which is more economical than silver paste screen printing, and the economy still needs to be improved compared with silver-clad copper +0BB technology. 

In 0BB (no main gate) technology, the main gate is eliminated in the cell link and the current is derived by connecting the welding strip to the secondary gate, which can reduce the consumption of silver paste by about 30.