Review on Metallization Approaches for High-Efficiency Silicon Heterojunction Solar Cells

Crystalline silicon (c-Si) heterojunction (HJT) solar cells are one of the promising technologies for next-generation industrial high-efficiency silicon solar cells, and many efforts in transferring this technology to high-volume manufacturing in the photovoltaic (PV) industry are currently ongoing. Metallization is of vital importance to the PV performance and long-term reliability of HJT solar cells. In this review, we summarize the development status of metallization approaches for high-efficiency HJT solar cells. For conventional screen printing technology, to avoid the degradation of the passivation properties of the amorphous silicon layer, a low-temperature-cured (< 250 ℃) paste and process are needed. This process, in turn, leads to high line/contact resistances and high paste costs. To improve the conductivity of electrodes and reduce the metallization cost, multi-busbar, fine-line printing, and low-temperature-cured silver-coated copper pastes have been developed. In addition, several potential metallization technologies for HJT solar cells, such as the Smart Wire Contacting Technology, pattern transfer printing, inkjet/FlexTrailprinting, and copper electroplating, are discussed in detail. Based on the summary, the potential and challenges of these metallization technologies for HJT solar cells are analyzed.


Introduction
In recent years, passivating-contact solar cells have become the focus of the photovoltaic (PV) industry due to their remarkable efficiency potential [1]. According to the prediction of the latest International Technology Roadmap for Photovoltaic (13th edition, 2022), passivating-contact silicon heterojunction (HJT, sometimes referred to as SHJ) solar cells and other passivating-contact solar cells are rapidly expanding their market share, occupying more than 75% by 2032 [2]. The tunneling-oxide passivating-contact (TOP-Con) solar cell is a powerful competitor of the HJT solar cell because its fabrication process can be upgraded on the existing PERC production line, minimizing equipment and facility costs. However, new factories usually choose the HJT solar cell technology mainly because of the following reasons: (1) HJT solar cells have a high open-circuit voltage and conversion efficiency and excellent bifaciality factor [3].
(2) HJT solar cell fabrication is a simple (few process steps) and low-temperature process, which is very beneficial for large and thin silicon wafers. (3) HJT solar cells have a lowtemperature coefficient, resulting in higher energy yields compared to other silicon solar cells. (4) The symmetrical structure of HJT solar cells makes them highly adaptable to thin silicon wafers, which will be a potential cost advantage for the industrialization of HJT solar cells [3][4][5]. For example, HJT solar cells with a thickness of ~ 90 μm have been reported by Meyer Burger and Panasonic [6,7].
Traditionally, HJT solar cells use a-Si:H (i/p) and a-Si:H (i/n) stacks to passivate dangling bonds on two surfaces of a wafer and form a passivating-contact, which is the most successful passivating-contact structure [8,9]. The HJT solar cell technology involves two different cell structures: front emitter and rear-emitter. Between the two, rear-emitter HJT solar cells have become the mainstream HJT technology. A highly transparent conductive oxide (TCO) with relatively low conductivity requirements can be applied on the front side, resulting in a high current density [3]. Different from c-Si homojunction solar cells, HJT solar cells require a lowtemperature process. This is because the hydrogen evolution from a-Si:H thin films will significantly occur when the temperature exceeds 200 ℃, which could dramatically deteriorate the passivation quality of dangling bonds on c-Si surfaces [8]. Consequently, a low-temperature metallization process is necessary for HJT solar cells.
As a well-established metallization technique, the screen printing technology is still the most commonly used metallization approach for HJT solar cell fabrication. In this technique, silver (Ag) pastes play a crucial role in contact formation and are also a core factor affecting the cost of HJT solar cells. For HJT solar cells, low-temperature-cured Ag pastes are necessary to form contact electrodes on the TCO layer, and their thermal curing temperature is usually below 250 ℃. In general, the line resistivity of a low-temperaturecured electrode is a factor of 2-3 higher than that of a hightemperature-sintered electrode [9]. To reduce the line resistance (R line ), the electrodes of HJT solar cells require more Ag content and a higher aspect ratio than those of other c-Si solar cells. In addition, the price of low-temperature-cured Ag pastes is > 10% higher than that of the traditional hightemperature sintered Ag pastes [10]. The high metallization cost is currently one of the most pressing challenges hindering the industrial scale expansion of HJT solar cells. In this review, we summarize the development status of the screenprinting technology for HJT solar cells, mainly including screenprinting low-temperature-cured Ag paste and lowtemperature-cured Ag-coated copper (Cu) paste technologies. In addition, we review several potential metallization technologies, such as Smart Wire Contacting Technology (SWCT), pattern transfer printing (PTP), inkjet/FlexTrail printing, and Cu electroplating, to summarize their progress.

HJT Solar Cells
As one of the most promising passivating-contact technologies, silicon HJT solar cells have attracted significant attention due to their high conversion efficiency and lean process sequence. Recently, averaged HJT solar cell efficiencies of more than 25% have been reported by many research institutions and companies [4,11,12]. A milestone with a certified efficiency of 26.5% based on a full M6 area (274.3 cm 2 ) n-type wafer has been reported by LONGi [13]. HJT solar cells have an excellent open-circuit voltage (V oc ) of up to 750 mV and a very high fill factor (FF) of more than 85%, which is due to the superior interface passivation and perfect passivating-contact architecture. However, the short-circuit current density (J sc ) of HJT solar cells is lower than that of other silicon solar cells and even that of conventional aluminum back surface field (Al-BSF) solar cells. This is not only because of the wide bandgap of the amorphous-silicon/monocrystallinesilicon HJT structure but also due to the parasitic optical absorption losses in intrinsic and doped a-Si:H layers, especially in the uppermost front-surface field layer. To mitigate the effects of low J sc , the HJ-IBC (HJT interdigitated back contact) architecture has long been known and developed [14][15][16][17] and has attracted significant attention due to the record-high efficiency of 26.7% achieved by Kaneka [18]. Figure 1a shows the typical process steps of rear-emitter HJT solar cells and structural sketches of monofacial HJT, bifacial HJT, and HJ-IBC solar cells [3]. N-type c-Si substrates are usually used as absorbers for HJT solar cells due to their higher carrier life time and lower sensitivity toward light-induced degradation compared to p-type c-Si substrates [19]. A 5-10 nm intrinsic amorphous silicon (i-a-Si:H) layer was deposited on both sides of a textured and cleaned c-Si substrate as a transition layer to passivate dangling bonds on the c-Si surface [5,20]. Then, 5-10 nm p-doped and 8-15 nm n-doped a-Si:H layers were deposited on the front and rear sides, respectively, to provide carrier selectivity. Afterward, TCOs were deposited on both sides for the lateral charge transport to electrodes. A band diagram of the standard HJT solar cell is sketched in Fig. 1b [21].The i-a-Si:H film, as a buffer layer, enables a low c-Si surface recombination via excellent chemical passivation [22][23][24]. The n-and p-type-doped a-Si:H films were deposited on top of the buffer layers to form electron and hole selective contact with the c-Si absorber and TCOs, which is due to the proper work function difference between the doped a-Si:H and c-Si absorbers [25,26]. Recently, to reduce parasitic absorption, doped microcrystalline/nanocrystalline silicon (μ/nc-Si:H) has been proven to be a promising alternative to the commonly used doped a-Si:H in the PV industry [27,28]. Compared to a-Si:H, μ/nc-Si:H has a higher doping efficiency and optical band gap, which is beneficial for improving the FF and J sc of HJT solar cells [28,29]. In addition, μ/nc-Si:H can suppress the Schottky-barrier effect between p-doped a-Si:H and TCO(n) via the percolation path (Fig. 1c) [27,30]. A schematic diagram of a recent state-of-the-art rear-emitter HJT solar cell featuring μ/ncc-Si:H layers as carrier-selective contacts is shown in Fig. 1d [30].
As the last step in the manufacturing process, electrode metallization has been a major efficiency-limiting and costdetermining step of HJT solar cells. The power conversion efficiency (η) of solar cells is calculated according to Eq. 1: Fig. 1 a Typical process steps of rear-emitter HJT solar cells and structural sketches of monofacial HJT, bifacial HJT, and HJ-IBC solar cells. Reproduced with permission from Ref. [3]. Copyright © 2019 Springer Nature. b Band diagram of the standard HJT solar cell. Reproduced with permission from Ref. [21]. Copyright © AIP where P max is the maximum power output, P in is the incident light power density. Hence, η is mainly determined by three factors: FF, J sc and V oc .
The FF is influenced by series resistance (R s ) and shunt resistance (R sh ), described by Eq. 2 [31]: where FF 0 is the ideal FF without parasitic resistances, which is free from losses owing to the series resistance and recombination in the space charge region [32]. R s consists of gridline, contact, emitter, and base resistances and has a significant impact on the FF [32]. Such an FF value of HJT solar cells is mainly limited by the low conductivity of silver grids produced by screen printing of pastes necessarily designed to cure at low temperatures [33].
The characteristic between the current density and voltage is expressed as Eq. 3 [34]: where J 01 is the saturation current density caused by the recombination of electron-hole pairs in the emitter and base regions; J 02 is the recombination current density in the space charge region, n is the ideality factor of the solar cell; J L is the photocurrent, and q is the elemental charge. Considering the superior passivation performance of the HJT architecture, the saturation current density due to the recombination is much lower than that of other homojunction solar cells. Therefore, in the metallization step, the shading area of electrode grids, which is determined by the linewidth and aspect ratio, is an important factor affecting the J sc of HJT solar cells. V oc is mainly affected by the temperature and damage to the TCO layer during the metallization process [8,35]. In addition, approximately 20% of the manufacturing costs of HJT solar cells currently originate from metallization, including the Ag paste and screen [36]. To improve photoelectric performance and reduce the manufacturing cost, various metallization technologies have been developed in the PV industry. Table 1 summarizes the key parameters of published representative HJT solar cells using various metallization technologies. In the next section, we will review the development status of several major metallization approaches for HJT solar cells.

Screen Printing Technology
Screen printing is the most widely used contact formation technique for industrial c-Si solar cells due to its high productivity, high reliability, easy handling, and cost efficiency. The evolution of the linewidth (finger width, W f ) over the last 15 years obtained with screen printing in the PV industry is shown in Fig. 2a [37]. As can be seen, the speed of the line width improvement has been spectacular. At present, the average finger width W f has been constantly reduced down to approximately 25 μm in industrial production lines and below 20 μm in R&D laboratories. In screen printing, pressure is applied with a squeegee to transfer metal-based pastes through a screen with a pattern to the solar cell surface to form a patterned contact electrode, as shown in Fig. 2b [38]. However, a further reduction in the screenprinting linewidth to approximately or less than 20 μm poses new challenges with respect to the screen, paste, and printing processes and quality assurance [37]. The most common case for screenprinting ultra narrow fingers leads to disruptions in the finger geometry (Fig. 2c, d) [37], finger interruptions, and alignment deviations. In the development of screen technology, the knotless screen technology has recently attracted attention due to its ability to effectively reduce mesh marks and increase the paste transfer capability, resulting in a lower mean lateral finger resistance than conventional fine-line mesh screens (Fig. 2e) and enabling an ultrafine contact [37]. As shown in Fig. 2f, a knotless screen has a mesh angle of 0° with respect to grid fingers [37]. Metal-based pastes are another important aspect that enables a high-precision printing of fine-line grids and are also an important factor that affects the electrical performance and reliability of c-Si solar cells. The following section focuses on low-temperature-cured Ag pastes and Ag-coated Cu pastes for HJT solar cells.

Low-Temperature-Cured Silver Paste
The cross-sectional SEM images of electrodes fabricated using high-temperature sintered Ag pastes and low-temperature-cured Ag pastes are shown in Fig. 3a, b, respectively Fig. 2 a Evolution of the linewidth obtained with screen printing over the last 15 years [37]. b Schematic of a typical screen printer setup [38]. c Top view and d side view of mesh marks along screen-printed fingers due to the regular wire pattern within the finger openings in the screen [37]. e Conventional fine-line mesh screen with a mesh angle of 22.5° [37]. f Knotless screen with a mesh angle of 0° [37]. a, c, d, e, and f were reproduced with permission from Ref. [37]. Copyright © 2022 Elsevier. b was reproduced with permission from Ref. [38]. Copyright © 2019 The Author(s) [39,40]. The high-temperature sintered electrode is very dense and almost becomes a whole bulk. The conductive powders of the low-temperature-cured electrode are relatively dispersed, and many voids exist in the electrode bulk. A low-temperature-cured Ag paste consists of metal powders, binder resins, solvents, curing agents, and other additives [41]. Metal powders are the conductive phase, accounting for more than 90% of the paste. The resins act as the binder of metal powders and provide the adhesion of the electrode to the TCO. The solvents usually consist of shortchain hydrocarbons with low boiling points to adjust the rheological and wettability properties of pastes and are easy to burn off. The curing agents react with resin to prompt the resin polymerization [42]. Low-temperature-cured electrodes usually have a relatively high bulk resistivity, so high consumption is required to ensure an acceptable FF, resulting in high metallization costs. For low-temperature-cured Ag pastes consisting of Ag nanoparticles with high surface energy, although they are conducive to achieving curing at low temperatures, Ag nanoparticles are usually in point-to-point contact (Fig. 3c), which increases the bulk resistivity of the Ag electrode [43,44]. It has been proven that the combination of Ag micronflake powers and Ag nanoparticle powders can significantly improve the conductivity of Ag electrodes cured at low temperatures [44,45]. As shown in Fig. 3d, adding an appropriate proportion of Ag micronflakes into a paste can increase the contact area, and the rearrangement of nanoparticles between micronflakes increases the density of the Ag electrode [44,46]. The schematic diagrams of contact mechanisms corresponding to the above two cases (Fig. 3c, d) are depicted in Fig. 3e, f. During the curing process of low-temperature silver pastes, low-boiling-point solvents gradually burn off along with the shrinkage and cross-linking of the resin, and the diffusion of atoms on the surface of Ag nanoparticles will facilitate the connection of Ag microparticles [42,47]. Finally, a dense conductive mesh was formed by a large thermodynamic driving force provided by the resin and molten silver nanoparticles [41,47]. Therefore, the effect of the organic composition in the paste on the conductivity of the low-temperature-cured electrode is also critical. To increase the conductivity of electrodes, Fig. 3 Cross-sectional SEM images of electrodes fabricated using a high-temperature sintered Ag paste. Reproduced with permission from Ref. [39]. Copyright © 2015 AIP Publishing LLC., and b lowtemperature-cured Ag paste. Reproduced with permission from Ref. [40]. Copyright © 2013 WIP • Sylvensteinstr. Cross-sectional SEM images of the low-temperature-cured electrodes obtained with pastes: c Ag nanoparticle powers. Reproduced with permission from Ref. [43]. Copyright © 2020 The Author(s), and d mixing powers of micronflakes and Ag nanoparticles [44]. e Schematic diagram of the Ag nanoparticle contact. f Schematic diagram of the Ag nanoparticle and micronflakes contact [44]. e, f was reproduced with permission from Ref. [44]. Copyright © 2020 The Author(s) the amount of resin should be appropriately reduced under the condition of ensuring adhesion. An optimal combination of resins, solvents, curing agents, and other additives are desired [42], and organic components need to be burned off as low as possible below the curing temperature [41,47]. According to a report from Fusion New Material [48], a bulk resistivity of ~ 4.5 Ω cm could be achieved in 2022.

Low-Temperature-Cured Silver-Coated Copper Paste
Ag powders account for approximately 98% of the cost of low-temperature-cured Ag pastes, which restricts the decrease in the paste cost. Copper has an electrical conductivity similar to that of Ag and is much less expensive and more abundant than Ag, making it an ideal alternative metal for Ag. However, Cu is less resistant to oxidation [49,50], making it difficult to maintain high electrical conductivity for long periods of time [51]. To overcome this problem, Agcoated Cu paste technology has recently attracted extensive attention in the field of HJT technology.
The coating quality of Ag-coated Cu powders is very important to the performance of Ag-coated Cu pastes. Electroplating [52], electroless plating [53,54], and vacuum processes [55] are commonly used for powder coating processes. The electroless plating process to fabricate Ag-coated Cu powders is widely used due to its simplicity, low cost, and high quality. Shin et al. [56] synthesized thiocyanate-modified Ag-coated Cu particles with excellent oxidation resistance via electroless plating. As shown in Fig. 4a, tiny agglomerates of the β-CuSCN layer Cross-sectional BSE image of Ag-coated Cu particles. c TGA graphs of the Cu particles Cu@Ag particles and thiocyanate-coated Cu@Ag particles under air conditions. a-c were reproduced with permission from Ref. [56]. Copyright © 2015 The Chemical Society of Japan. d Schematic diagram of the Cu@Sn@Ag sandwich structure. e Cross-sectional SEM image of the Cu@Sn@Ag particle and f corresponding EDX line-scan analysis. d-f were reproduced with permission from Ref. [57]. Copyright © 2019, Springer Science Business Media. g I-V characteristics of HJT solar cells metallized separately by Agcoated Cu paste and Ag paste screenprinting. Reproduced with permission from Ref. [59]. Copyright © 2019The Author(s) plugged the pores on the surface of Ag-coated Cu particles, and the Cu cores were uniformly wrapped by an Ag shell of approximately 300 nm (Fig. 4b). Compared with unmodified Ag-coated Cu particles, the oxidation temperature of thiocyanate-modified Ag-coated Cu particles increased by 150 ℃ (Fig. 4c). To address the transient liquid phase bonding of low-temperature pastes, Liu et al. [57] proposed a Cu@Sn@Ag sandwich structure, whose schematic diagram is shown in Fig. 4d. The cross-sectional SEM image and EDX line-scan analysis of the Cu@Sn@ Ag particle (Fig. 4e, f) illustrate that the thicknesses of the Sn and Ag coatings on the Cu particles are 1.2 and 0.4 μm, respectively. Sun et al. [58] used ethylenediamine as the Ag complexing agent and adjusted the pH value of the plating solution to prepare Ag-coated Cu micronflakes with smooth surfaces. The prepared Ag-coated Cu micronflake powders exhibited a resistivity of 3.9 × 10 −4 Ω·cm. Schube et al. [59] compared the electrical characteristics of HJT solar cells metallized separately using Agcoated Cu paste and Ag paste screenprinting. As shown in Fig. 4g, the Ag-coated Cu paste group shows a conversion efficiency similar to that of the Ag paste group, and the high FF caused by a low grid resistance is the main advantage of the Ag-coated Cu paste with a high amount of Ag fill material. Hong [60] reported that approximately 0.09% efficiency loss was obtained on SHJ cells using the Ag-coated Cu paste (HAC539-T3) instead of the front and rear finger Ag pastes of the baseline. According to the report from Toyo Aluminum K.K., the HJT mini-module using the Ag-coated Cu paste (THA-F08C4) has a power degradation of less than 5% after TC 200 [61]. In particular, Ag-coated Cu pastes are not suitable for silicon homojunction solar cells due to the dewetting behavior of the Ag shell at high temperatures [62].
Some problems still need to be solved or improved for screen printing low-temperature-cured metal paste technology. First, the electrodes of HJT solar cells fabricated by screen printing low-temperature-cured metal pastes show relatively poor adhesion on TCO films. Hence, it is an important optimization direction to improve the organic composition in the paste without increasing the contact resistance. Second, due to the high resin content of the lowtemperature-cured paste and the large diameter of the metal microflakes, slow printing and flooding velocities are essential to maintain the printing quality but affect the throughput. In addition, a curing time of 5-8 min is essential even with the rapid low-temperature curing process for HJT solar cells, compared to traditional PERC solar cells with a hightemperature sintering time of approximately 1 min. The long curing time usually leads to low throughput and high capital expenditures, and reliability is still a concern for rapid curing technologies.

SWCT
SWCT, as an innovative HJT solar cell interconnection process for module manufacturing, was initially proposed by Day4 Energy [63] and further developed by Meyer Burger for automated production lines [64]. The SWCT consists of Cu wires coated with a thin low-melting-point alloy (InSn/ Fig. 5 a Picture of a SmartWire foil on a busbar-less cell. b Schematic diagram of automated SmartWire cell interconnection. c and d Cross-sectional BSE images of the contact point between the InSn-coated Cu wire and Ag finger. Reproduced with permission from Ref. [64]. Copyright © 2014 WIP • Sylvensteinstr BiSn) layer and supported by a polymer foil (Fig. 5a, b) [64]. In Fig. 5a, the diameter and gap of the wires can be as small as 70 μm and 4 mm, respectively [64]. The cross-sectional BSE images of the contact point between the coated Cu wire and Ag finger are shown in Fig. 5c, d [64]. Due to the abundant electrical contact points of every cell, the requirements for line conductance can be relaxed, and fine-line printing can be allowed while ensuring high module reliability. Busbarless technology shows prospects for ultralow Ag content with 30 mg laydown [64]. Narrow wires and fingers reduce shadow, thus improving J sc . Moreover, the power loss is positively correlated with the line resistance of fingers, which increases with increasing finger length, and it is negatively related to the number of busbars [65]. The finger length is defined as half the distance from one busbar to an adjacent busbar in a standard H-pattern. The multiwire SWCT technology can significantly increase the module output power by reducing R line and emitter resistivity (R emitter ). Zhao et al. [66] demonstrated a champion cell with 24.02% conversion efficiency in a pilot production line and a 60-M2 cell SWCT monofacial module with 335 W.

PTP
Fine finger technology is beneficial in reducing shading, increase I sc and V oc , and reduce emitter resistivity. The FF loss caused by the increased series resistance can be compensated by SWCT. As mentioned above, due to the limitation of the wire size, mesh, and emulsion, it is difficult for screen printing to steadily print fingers with linewidths less than 20 μm. PTP, a contactless printing technology commercialized by Utilight, has recently received much attention due to its potential for fine and high aspect ratio metallization fingers. As shown in Fig. 6a, b, fingers smaller than 20 μm can be deposited through PTP technology [67]. Its working principle is illustrated in Fig. 6c [67]. First, a transparent polymer tape with trenches is filled with the paste using blades. The polymer tape is then placed approximately 200 μm away from the wafer, and the paste is detached from the trenches by evaporating the solvent with an infrared laser [68]. Maybury et al. [69] compared the electrical characteristics of c-Si PERC solar cells metallized via PTP and screen printing. As shown in Fig. 6d, the PTP technology shows an efficiency gain of 0.14% abs , which is mainly contributed by the I sc and V oc gains. According to a report submitted by Adrian et al. [68], PTP technology could reduce the paste lay down to approximately 30 mg. Compared with its application in PERC solar cells, PTP technology is more suitable for HJT solar cells without SiN x , and the noncontact printing manner is more conducive to the thinning wafer tendency for HJT solar cells. Nevertheless, some problems still need to be solved for the large-scale industrial application of PTP technology. As presented in Fig. 6e, f, a large number of finger interruptions occurred on single PTP-printed cells [67]. In addition, the paste tends to remain in the polymer tape, which means extra paste waste.

Inkjet Printing and FlexTrail Printing
Inkjet printing and FlexTrail printing are ultralow Ag consumption technologies utilizing a commercially available Ag nanoparticle ink. In the inkjet printing process, the ink is ejected from a pulsed voltage-controlled nozzle without touching the wafer, as shown in Fig. 7a [43]. In the Flex-Trail printing process, the ink is pressed out from the lower end of a hollow glass capillary with an inner diameter of several microns, whose end is in a flexible contact with the wafer (Fig. 7b) [43].
The fingers fabricated by inkjet printing and Flex-Trail printing utilizing the same Ag nanoparticle ink are shown in Fig. 7c, d [43]. As can be seen, the width of the inkjet printed finger (75 ± 1 μm) is a factor of 4.7 higher than that of the FlexTrail printed finger (16 ± 1 μm) [59]. The laydown of the latter only consumed 0.3 ± 0.1 mg on a large-area HJT solar cell with 80 front fingers, but it still achieved a conversion efficiency of 23.7%. The two ultralow Ag consumption printing technologies are expected to be breakthrough metallization technologies for HJT solar cells, but their reliability still needs further research.

Copper Electroplating
Although various low-consumption Ag technologies have been introduced above, a certain amount of Ag consumption is unavoidable. Ag-free metallization has received extensive attention from PV academic and industrial circles. As mentioned above, Cu is dozens of times cheaper than Ag, but they have similar conductivity. Cu electroplating, as an Ag-free metallization technique, plays an important role in the new metallization technologies of HJT solar cells [70]. This technology has been industrially proven for silicon homojunction solar cells [71]. Recently, Cu electroplating metallization has attracted much attention for HJT solar cell processing because it is expected to solve the current dilemma of the high cost caused by the high consumption and high price of low-temperature Ag pastes. In addition, the resistivity of the Cu-plated electrode is much lower than that of the screen-printed Ag electrode, and fine fingers can be achieved, thus further improving the conversion efficiency of HJT solar cells. Figure 8a shows the in-line electroplating equipment produced by JBAO Technology Ltd. [72]. The appearance of a Cu-electroplated HJT solar cell is shown in Fig. 8b [72]. In 2015, Kaneka announced that a 159 cm 2 HJT solar cell with a Cu grid electrode achieved 25.1% efficiency [24,65]. In 2021, Maxwell and SunDrive [12] reported an M6 HJT solar cell metallized by seed-free Cu electroplating that achieved 25.54% conversion efficiency. The Cu electroplating technology can achieve such satisfactory efficiencies mainly due to the acquisition of a narrow linewidth and low series resistance. CSEM [73] reported that an HJT solar cell with plated fingers with a linewidth of 15 μm and an aspect ratio of 1:1 can obtain a gain of J sc 1.1 mA/cm 2 compared to a reference cell with Ag paste screenprinting. Moreover, the electroplated Cu electrode is close to bulk Cu [74], and its resistivity (2 × 10 −8 Ω m) is similar to that of pure Cu (1.75 × 10 −8 Ω m), which is several times lower than that of the screen-printed Ag electrode [73,75], thus achieving a lower line resistance and contact resistance.  [67]. d Electrical performance gain of solar cells metallized with PTP compared to reference cells with screen printing [69]. EL images from typical c-Si solar cells metalized by e screen printing and f single PTP printing [67]. a, b, c, e, and f were reproduced with permission from Ref. [67]. Copyright © 2015 Elsevier. d was reproduced with permission from Ref. [69]. Copyright © 2019 The Author(s) Although TCO films can effectively solve the carrier lateral transport problem for HJT solar cells, they bring some difficulties to the Cu electroplating metallization technique. First, a patterned mask process is required to define electrode openings. Due to the high cost of photoresists, various alternative organic and inorganic masks have been developed. Among them, dry films and resin resists exhibit a high resolution and low cost in organic-material masks [74,76]. Yu et al. [76] reported that dry films have a good resolution of 30-50 μm. As shown in Fig. 8c, the opening trench of the dry film is nearly rectangular, and the thickness is close to 40 μm after development. Figure 8d, e shows that the cross section of the electroplated Cu finger is rectangular with a width of approximately 56 μm and a thickness of approximately 25 μm [76]. Li et al. [74] reported a resin solution (resist) obtained from a modified AZ nLOF 2035 photoresist to obtain openings without lithographic development. The development was performed in a tetramethyl ammonium hydroxide solution after baking at 110 ℃, and an approximately 20 μm line opening was obtained, as shown in Fig. 8f. Plated fingers with a width as narrow as approximately 20 μm and a height of approximately 10 μm were achieved, as shown in Fig. 8g, h. In particular, UV exposure is no longer required to use this resin. However, the removal of the organic mask will produce a large amount of wastewater, and the position of Cu fingers in contact with the organic template cannot be easily protected by the Ag/ Sn layer to prevent Cu oxidation [77]. Some people have suggested that inorganic masks are highly suitable for mass production owing to their high throughput and low cost [75]. Native oxides of metals [78,79], SiO x [80], Al 2 O 3 /a-Si stacks [81], and SiO x /SiN x stacks [82] have been reported as favorable inorganic masking materials. For example, Hatt et al. [78] deposited Cu/Al stacks onto a TCO film via physical vapor deposition (PVD), and pattern openings were formed via etching in a sodium hydroxide solution, as shown in Fig. 8i. The finger electrodes plated by a forward/ reverse pulsed current are presented in Fig. 8j, k. Second, the adhesion of an electroplated Cu electrode on TCO should be considered, which will affect the long-term reliability of the corresponding modules [78]. Li et al. [83] used an electrochemical method to reduce indium particles from tungsten-doped indium oxide films as a seed layer, and the maximum peel force of the electroplated Cu busbar reached 4.23 N [83]. Although this method provides a method to solve the low peel force of electroplated Cu electrodes, it is not economical. The research on improving the peel force of electroplated Cu electrodes is still in progress. In addition, the cumbersome process steps bring additional costs, and the long-term reliability still needs to be explored [12].

Summary and Outlook
Herein, we review the development status of metallization approaches for c-Si HJT solar cells. Although much effort has been currently performed in transferring the HJT solar cell technology to high-volume manufacturing, metallization is still one of the major factors slowing down its industrialization progress. Metallization not only affects the electrical performance and cost of HJT solar cells but also affects the long-term reliability of HJT module products.  [43]. c Cross-sectional SEM image of an inkjet printed finger. Inset: corresponding top view SEM image. Microscopy images at a magnification of 50 of the fingers printed by d inkjet printing and e FlexTrailprinting [59]. a, and d were reproduced with permission from Ref. [43]. Copyright © 2020 The Author(s). c, e, and d were reproduced with permission from Ref. [59]. Copyright © 2019 WILEY-VCH Fig. 8 a Electroplating equipment and b Cu-electroplated HJT solar cell from JBAO Technology Ltd. [72]. Microscopic images of c a dry film after development and d electroplated finger. e Cross-sectional SEM image of the electroplated Cu finger [76]. Microscopic images of f a resin obtained by modifying a photoresist after development and g electroplated finger, h FIB cross-sectional image of the electroplated Cu finger [74]. Microscopic images of i a finger opening on the Cu/Al stacks by inkjetprinting NaOH solution, j electroplated Cu finger with Ag capping, and k the finger after etching-back PVD layers [78]. a, and b were reproduced with permission from Ref. [72]. Copyright © JBAO Technology Ltd. c, d, and e were reproduced with permission from Ref. [76]. Copyright © 2017 Elsevier. f, g, and h were reproduced with permission from Ref. [74]. Copyright © 2015 Elsevier. i, j, and k were reproduced with permission from Ref. [78]. Copyright © 2019 WILEY-VCH Among the many metallization technologies, screen printing is the most widely used in HJT solar cell manufacturing. Screens and pastes are two key materials in screen printing and have played very important roles in the impressive progress of c-Si solar cells in the past two decades. With the development of screen technology, screen-printed fingers with a width of 20-25 μm have been achieved. In addition, knotless screens have good development prospects because they support effective paste usage and fine metallization fingers. Low-temperature-cured Ag pastes have become one of the major factors restricting the cost reduction of HJT solar cells due to their high usage and price. Recently, a low-temperature-cured Ag-coated Cu paste was developed to reduce the material cost of low-temperature-cured pastes. Essentially, screenprinting technology based on low-temperature-cured pastes still needs improvement in terms of printing speed, curing time, and electrode adhesion.
To further reduce metallization costs, the industry also focuses on new metallization technologies, such as SWCT, PTP, inkjet/FlexTrailprinting, and Cu electroplating. The SWCT is an innovative interconnection technology for HJT solar cells and modules and has attracted attention for its ability to significantly reduce Ag consumption. The novel PTP technique is capable of depositing very fine metallization fingers (≤ 20 µm) with a high aspect ratio, thus significantly improving the efficiency of HJT solar cells. Nonetheless, some problems still need to be solved before large-scale applications, such as finger interruptions and extra paste waste. Inkjet printing and FlexTrail printing offer significant Ag reduction potential. Compared to the inkjet printing process, the FlexTrail printing process can achieve a finer finger width and consume less Ag ink, showing a great costsaving potential. As an Ag-free metallization technique, Cu electroplating technology has been proven feasible in the industry. However, additional masking steps and subsequent mask-removal steps increase the manufacturing costs and complexity of HJT solar cells. Simple and low-cost mask technology has recently become a topic of increasing interest for the Cu electroplating process, which is also the key to its large-scale application in HJT solar cell manufacturing.
Although many constraints and challenges still need to be addressed, new metallization technologies should also be highlighted to disburden the current limitations of screenprinting metallization and pave the way for further efficiency increases and cost reductions in HJT solar cells and modules. Finally, the large-scale application of new metallization technologies needs to consider the long-term reliability of corresponding module products and patent risks.