1. Introduction

Heterojunction (HJT) solar cells are one of the most promising technologies in photovoltaics due to their remarkable efficiency potential [1–3]. Electrode metallization is critical to the photovoltaic performance and long-term reliability of HJT solar cells. Screen-printed silver paste is conventionally used for metalizing Heterojunction Technology (HJT) solar cells. However, limitations arise from the precision constraints of the screen-printing process, impeding further enhancements in solar cell efficiency. Additionally, HJT cells necessitate low-temperature silver pastes (around 250°C), which, compared to conventional high-temperature pastes (800°C–900°C), exhibit increased resistivity and cost [4]. Approximately 20% of the manufacturing costs for HJT solar cells are attributable to metallization, including silver paste and screens [5]. High metallization costs are currently one of the most pressing challenges preventing the industrial scale-up of HJT solar cells.

Copper (Cu) exhibits high intrinsic electrical and thermal conductivity, comparable to that of silver (Ag) but surpassing gold (Au). Moreover, Cu's cost is approximately 1% of Ag's, and its abundance is 1000 times greater. These attributes establish copper as an exceptionally promising electrode material for future applications. However, a notable challenge with copper is its tendency to form an insulating oxide layer in air, necessitating processing in inert environments or vacuum chambers. For these reasons, direct printing of copper nanoparticles does not have much advantage over traditional printing methods using precious metals [6]. In addition, traditional methods for fabricating metal electrodes, such as photolithography [7], screen printing [8], and electroplating [9], often encounter challenges due to their cumbersome and inflexible processes. The TCO layer in heterojunction cells poses a distinctive challenge for electroplating metallization. Owing to the TCO's conductivity, it becomes fully electroplated during the plating process. Therefore, this method necessitates the consideration of removing the mask and seed layer post-process, which is a cumbersome process. While photolithography-based TCO masks serve as effective tools for copper plating of heterojunction cells, the photolithography process itself is not practically feasible for industrial-scale production of heterojunction solar cells. Hence, an efficient and low-cost method for manufacturing large-scale copper gate electrodes on the surface of HJT solar cells is rare. Laser direct writing is widely used to develop electronic devices due to its low cost, maskless, high flexibility, and direct fabrication under ambient conditions [10,11]. Andreas [12] suggested a technique employing laser-induced forward transfer for metalizing silicon heterojunction solar cells via copper plating. In comparison to solar cells fabricated using low-temperature screen-printing, those produced by this method achieve a maximum efficiency of 22.2%. Dabirian [13] proposed a method that integrates nanosecond laser patterning and Ni-Cu electroplating for metallizing SHJ solar cells. The findings demonstrate that the metallization quality attained through this approach is comparable to that achieved with Ag screen printing and Cu plating based on photolithography. Regrettably, reports on manufacturing copper electrodes on solar cell surfaces via laser-induced selective metallization are scarce.

Hence, we proposed a novel method via LISM of CuO NPs to prevent copper oxidation during processing. This method enabled the successful fabrication of large-scale copper grid electrodes on ITO layers, aligning with design path and conductivity specifications. The effects of different laser processing parameters on the electrical properties and microstructure of the electrodes were investigated. Furthermore, the reaction mechanism of laser selective metallization was proposed.

2. Experimental procedure

2.1 Setup and materials

Figure 1(a) illustrates the schematic of the experimental setup in the metallization process of heterojunction cells. A picosecond laser with a wavelength of 532 nm, a pulse duration of 9 ps, a repetition frequency of 2 MHz, and a maximum output power of 15 W was used as the laser source. The laser beam was focused by SCANLAB's high-speed scanner and an F-theta lens with a focal length of 160 mm to obtain a Gaussian spot with radius ω₀ = 16 µm. Electrode patterns were generated using CAD software embedded in the scanner. The HJT cell structure studied in this research consists of an n-type c-Si wafer base with the front face covered with i-a-Si: H and p-a-Si: H in turn, while the back face covered with i-a-Si: H and n-a-Si: H in turn. Typically, single-crystal Si substrates are wet chemically woven in an alkaline solution that anisotropically etches the wafer surface to produce dimensional inhomogeneous pyramids. Electrodes are usually formed by transparent conductive oxide (TCO) layers such as indium tin oxide (ITO) and metal grids. The ITO layer functions as a lateral conductor, facilitating current transfer to the metal grid in the gate. The cross-section schematic of the cell structure and the surface pyramid structure are shown in Fig. 1(b-c).

 

Fig. 1. (a) Schematic diagram of the experimental setup for LISM. (b) Cross-sectional schematic. (c) Pyramid topography and (d) experimental flowchart

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2.2 Laser-induced reduction selective metallization process

Figure 1(d) illustrates the LISM process utilized for the metallization of heterojunction cells. Firstly, a transparent dispersant solution with certain adhesion was prepared by dissolving polyvinylpyrrolidone (PVP, 13% wt%) into ethylene glycol (EG, 27% wt%) through mechanical stirring, and then CuO NPs (60 wt%) were dispersed into the above solution in a certain proportion, and well-dispersed CuO NPs inks were prepared through the combined effect of water bath heating, magnetic stirring and ultrasonic vibration. Here, EG is used as a solvent and PVP is used as a reducing and dispersing agent. After that, the prepared CuO nano-ink was spin-coated on the substrate at 3500 rpm for 30 s, and the prepared samples were heated at 60° for 30 min to obtain a dry CuO nano-coating. Finally, laser selective metallization was carried out via an ultrafast laser along the predetermined path. Subsequently, the sample underwent rinsing with ethanol to remove the CuO NPs from the unprocessed areas. Thus, high-performance copper grid electrodes were obtained.

2.3 Characterization

The surface morphology of the copper gate electrode is measured by using a laser-scanning confocal microscope (OLS4000 series from Olympus). The microstructure of the copper electrode is observed with an optical microscope and a scanning electron microscope (SEM, SU8010 from Hitachi). In addition, energy dispersive X-ray spectroscopy (Hitachi's EDS, SU8010) was used in elemental analysis to quantitatively study the major compositional changes in the copper electrodes. The width and thickness of the Cu are measured by an optical microscope. A digital four-point probe instrument (Jandel 4-probe system)) is used to measure the conductivity.

3. Results and discussion

Laser pulse energy is a key parameter in the laser irradiation process, which determines the continuity, reduction products, width and resistivity of the Cu electrode. The surface morphology and composition of the ultrafast laser-induced reduction products were first examined, as well as the electrical properties and linewidths obtained at different laser pulse energies (Fig. 2). Figure 2(a-c) demonstrates how the pulse energy affects the surface morphology and porosity of the Cu electrodes. At lower pulse energies, the Cu electrode exhibits discontinuities as in Fig. 2(a). High magnification SEM images [Fig. 2(a2) and (a3)] reveal numerous large holes and gaps between the Cu NPs, and the incompletely reacted CuO nanoparticles and organics can be clearly seen. Correspondingly, the electrode exhibits a significant porosity of 28.78%, as illustrated in Fig. 2(a4). As pulse energy increases, the CuO NPs are fully reduced, the phenomenon of sintering and connection among Cu nanoparticles becomes evident, with a gradual decrease in porosity, as demonstrated in Fig. 2(b). However, excessively high energy can result in ablation and reoxidation, detrimental to the electrical properties of the electrodes [Fig. 2(c2) and (c4)]. The high-resolution SEM images further show that the reduced Cu electrodes have the porous structure. Moreover, the linewidth of the electrodes increases linearly with the increase of laser energy, as depicted in Fig. 2(d). This is due to the violent photo-thermal-chemical reduction reaction caused by high laser energy, which occurs during the sintering process of nanoparticles. The width of the electrodes was slightly larger than the laser beam diameter, which was attributed to the heat diffusion on the substrate and across the thin film during the LISM process [14,15].

 

Fig. 2. Comparison of (a-c) surface morphology, (d) line width, (e) resistivity and (f) chemical composition before and after laser irradiation at different laser energy densities

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Figure 2(e) delineates the resistivity of the Cu electrode as a function of varying laser pulse energy at different speeds. As the laser energy increases, the resistance initially decreases and then increases. The Cu electrode has a minimum resistivity of 1.98 × 10⁻⁵ Ω·cm when the pulsed laser energy is 135 nJ at a speed of 6 mm/s, which is similar to the typical resistivity of metallic copper (1.75 × 10⁻⁶ Ω·cm), representing an order of magnitude improvement in conductivity compared to previous studies [16]. To further explain this phenomenon, the chemical composition of the copper electrode was analyzed as shown in Fig. 2(f). At the lower energy of 79 nJ, unreacted CuO and organic matter are observed to remain in the electrode. This resulted in a relatively high C and O content, as the laser pulse energy is insufficient to fully reduce the CuO nanoparticles to Cu nanoparticles. As the laser pulse energy increases, the Cu/O ratio continues to increase, and the CuO nanoparticles are fully reduced to Cu nanoparticles. Under the action of van der Waals forces, a neck-like structure is formed between the Cu particles, which provides a uniform and continuous electron flow path, thereby improving the electrical performance of the electrode. With further escalation of laser energy, an increase in oxygen content was observed, indicating that excessive laser pulse energy leads to reoxidation of the electrode. In extreme cases even destroyed the electrode surface or damaged the substrate. These findings suggest that the Cu content, continuity and size of the electrode are critical determinants of its conductivity.

Another key parameter to control the temperature during laser irradiation is the scanning velocity, which also determines the morphology (Fig. 3(a)), linewidth (Fig. 3(b)), resistivity (Fig. 3(c)) and reduction products (Fig. 3(d)) of the Cu gate electrode. Figure 3 demonstrates how the scanning velocity affects the morphology and porosity of the Cu electrode. With a certain beam radius and repetition frequency, the effective pulse number $N = \frac{{2{\omega _0}}}{\textrm{v}}f\; $ (where ω₀ is the laser beam radius, v is the scanning velocity) is directly determined by the scanning velocity. At the scanning velocity of 2 mm/s, the larger effective number of pulses leads to a severe thermal build-up. Consequently, clear ablation defects can be observed along the laser scanning trajectory (as indicated by the arrows in Fig. 3(a)). High magnification SEM images revealed that the Cu nanoparticles are completely remelted and sintered into larger Cu particles due to the laser thermal effect. As the scanning velocity was increased to 4 mm/s, a decrease in the effective pulse number and the thermal effect was observed. Despite this, the reduced Cu particles still underwent re-oxidation, leading the Cu electrode to exhibit colorful oxidation colors, as shown in the dashed box in Fig. 3(a). Further increase of the scanning velocity to 6 mm/s resulted in the circuit exhibiting good morphology. High-magnification SEM images indicated that the Cu NPs are well connected, exhibiting low porosity and resistivity at this stage. Upon further increasing the velocity, the reduced effective number of pulses led to insufficient thermal build-up for the complete reaction of CuO NPs to Cu NPs, and a discontinuity appears at the Cu electrode, which matches well with the results of the variation of the line widths in Fig. 3(b).

 

Fig. 3. Comparison of (a) surface morphology, (b) line width, (c) resistivity and (d) chemical composition before and after laser irradiation at different velocity

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Laser pulse energy and scanning velocity are two important parameters that affect the quality of Cu electrodes. Depending on the surface morphology of Cu electrodes and substrate morphology under various combinations of laser pulse energy and scanning velocity, all samples can be classified into three categories: well-formed Cu, discontinuous Cu and damaged substrate. The typical morphology of well-formed Cu and damaged substrates is shown in Fig. 4(a). Figure 4(b) illustrates a plot of the acceptable relationship between laser parameters and electrode quality. Well-formed Cu electrodes were obtained in the range of 79nJ-225nJ, which was matched with a decreasing velocity range as the laser energy increased.

 

Fig. 4. (a) Schematic diagram of the well-formed Cu and damaged substrate and (b) Process window for the preparation of copper electrodes through LISM

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On the basis of the above experiments, the optimum selective metallization conditions were determined to be a pulsed laser energy of 135nJ and a scanning velocity of 6 mm/s by measuring its resistivity. Figure 5(a) illustrates the successful fabrication of a large-scale copper electrode (180 mm x 180 mm) on a silicon HJT solar cell substrate, achieved under optimized conditions. The confocal microscopy image and its cross-sectional profile (Fig. 5(b)) showed a flat surface with a nominal thickness of 4 µm and width of 50 µm. The elemental mapping of Cu and O obtained using energy dispersive x-ray spectroscopy (EDS) confirms the reduction of CuO to Cu after laser irradiation, as shown in the inset of Fig. 5(c). The images show a clear contrast between the Cu and O elements in the electrode, indicating that laser irradiation effectively reduced the CuO nanoparticles to form a Cu electrode. The Cu/O ratio in the electrode reaches 11.53:1. It is confirmed that the proposed method can effectively achieve the goal of replacing copper for silver in silicon HJT cells.

 

Fig. 5. (a) Large area copper grid wire electrode on silicon heterojunction solar cell; (b) Microscopic image; (c) EDS diagram. (Inset) Cu circuit element distribution.

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The reduction mechanism of CuO nanoparticles by laser irradiation in a solution consisting of PVP and EG has not been fully revealed, and it can be determined that a photo-thermal-chemical reduction reaction occurs in this process. Kim [17] explained that intense pulsed light irradiation decomposes PVP to produce carboxylic acid, which acts as a reducing agent to reduce CuO nanoparticles to Cu nanoparticles. Huang [16] discussed the mechanism of femtosecond laser-induced reduction, and they concluded that the solvent of the ink is closely related to the reduction reaction. When the ink temperature reaches 160-200°C (close to the boiling point of EG), the organic solvent EG begins to dehydrate and produce acetaldehyde, which can be used to reduce CuO NPs to Cu NPS. To verify whether the solvent is necessary for the reduction reaction in our experiments, we performed the spin-coated samples on completely dried CuO films baked at 60° for 30 min, when the solvent had already evaporated, confirming that the solvent EG is not a necessary ingredient for inducing the reduction. We also tried that laser irradiation could not induce reduction of CuO nanoparticles in the absence of PVP addition to the ink. Considering these results, we propose the mechanism for laser-selective metallization of Cu electrodes as follows: (1) Under laser irradiation, the CuO NPs coating absorbs the laser energy and converts the photon energy into thermal energy. The hydroxide first reacts with the lactam ring of PVP, the OH radical attacks and destroys the N-C bond, and the reactive HN-(CH2)₃-CO cleaves at the C-C bond to form methylamine and propionic acid. Methylamine undergoes rapid degradation to produce NH₄ ions and CO₂ gas. Propionic acid is degraded by continued photodegradation to acetic acid and formic acid, and eventually formic acid reduces copper oxide to copper. The specific reaction process is shown in Fig. 6. (2) Under the thermal action of the high repetition rate laser, the Cu nanoparticles are observed to undergo remelting and re-solidification. This process forms a neck-like structure between the Cu particles, which provides a homogeneous and continuous path for the electron flow, leading to the formation of a homogeneous and well-conducting electrode.

 

Fig. 6. Laser selective metallization reaction mechanism

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4. Conclusion

In conclusion, we have demonstrated that copper electrodes for silicon HJT solar cells based on transparent oxide conductive layers can be fabricated via a laser selective metallization process. The photothermal chemical reduction reaction under laser irradiation resulted in the complete reduction of CuO nanoparticles to Cu nanoparticles and sintering of Cu electrodes under laser irradiation. The Cu electrodes exhibited a low resistivity of 1.98 × 10⁻⁵Ω.cm, indicating its enormous potential for practical applications.