Plated metal adhesion to picosecond laser-ablated silicon solar cells: Influence of surface chemistry and wettability

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Plated metal adhesion to picosecond laser-ablated silicon solar cells: Influence of surface chemistry and wettability Xiaowei Shen a, *, Pei-Chieh Hsiao a, Benjamin Phua a, Alex Stokes b, Vinicius R. Gonçales c, Alison Lennon a a b c

School of Photovoltaic and Renewable Energy Engineering, University of New South Wales, Sydney, NSW, 2052, Australia Department of Physics, Macquarie University, Sydney, NSW, 2109, Australia School of Chemistry, University of New South Wales, Sydney, NSW, 2052, Australia

A R T I C L E I N F O

A B S T R A C T

Keywords: Silicon solar cells Metal plating ps laser ablation Adhesion Nickel silicide

This study investigated the influence of UV picosecond laser fluence, used to ablate the SiNx antireflection coating for Ni/Cu/Ag plated p-type Si solar cells, on busbar and finger adhesion and cell electrical performance. Surface chemistry was characterised post-ablation and post-pre-treatment in 7:1 buffered oxide etch (BOE) using a combination of X-ray photoelectron spectroscopy and contact angle measurements. Although growth of laserinduced Si oxides increases with increasing laser fluence, these oxides are effectively removed in the BOE pretreatment and therefore do not impact plated metal adhesion with busbar pull forces of 1.9 � 0.7 N/mm being achieved when a laser fluence of 0.63 J/cm2 was used to ablate the busbar openings. It is also revealed that the use of high laser fluence leads to a more hydrophobic surface due to reduced residual SiNx, however the complete wetting of the ablated Si surface can be ensured by the use of surfactants in Ni plating electrolytes. Residual SiNx impacts Ni silicidation and reduces the busbar pull force to values of only 0.8 � 0.5 N/mm when average laser fluences �0.45 J/cm2 are used. Finger dislodgement forces are interestingly shown not to be affected by laser fluence and presence of residual SiNx, providing an opportunity to optimise the laser ablation process separately for finger and busbar openings. Finally, it is demonstrated that the use of higher laser fluences does not impact the electrical performance of the Al back surface field cells, with open-circuit voltages of �637 mV and fill factors �80.4% being demonstrated for cells where the average laser fluence used was varied between 0.35 and 0.63 J/ cm2.

1. Introduction Front-surface metallisation of Si p-type solar cells using lightinduced plating (LIP) of Ni and Cu can present an alternative to the current screen-printed Ag in the event of high or volatile Ag prices [1–4]. It can potentially offer the technical advantages of thinner fingers with higher aspect ratio which can lead to reduced front-surface shading and contact-free metallisation. Ultrashort pulsed (ps/fs) laser ablation can be used to form very narrow < 15 μm finger openings for cells [5–11] which can be aligned to pre-formed doped regions of selective-emitter (SE) passivated emitter and rear contact (PERC) cells for higher cell efficiency [9,11]. The laser-induced periodic surface structures (LIPSS) that result on the Si surface from ablation of the SiNx antireflection coating (ARC) [12–19] has been reported to improve finger and busbar adhesion of the plated metal stacks [6–8,10,20–25]. The often-reported

reason for this improved adhesion is the increased surface roughness which can lead to more numerous Ni nucleation sites, thus providing mechanical anchors and improving contact adhesion [26–28]. Interfacial oxides induced by laser ablation on Si surfaces have been shown to influence cell electrical performance [3,29]. Du et al. reported laser-induced Si oxidation at the local rear contact regions for PERC cells. Presence of the oxide was shown to impact cell performance with more severe oxide growth being induced with ns pulse durations compared to that induced by laser pulses having ps durations [29]. Laser-induced interfacial Si oxide growth was also reported by Büchler et al. when using a fs laser to ablate contact grids in the SiNx ARC of p-type cells for Ni/Cu/Ag plated solar cells [3]. They observed enhanced induced oxide growth with decreased laser pulse pitch, however although the resulting oxide inhibited Ni silicide formation and increased contact resistance, busbar adhesion was not affected [3]. This

* Corresponding author. E-mail address: [email protected] (X. Shen). https://doi.org/10.1016/j.solmat.2019.110285 Received 5 April 2019; Received in revised form 14 October 2019; Accepted 7 November 2019 0927-0248/© 2019 Elsevier B.V. All rights reserved.

Please cite this article as: Xiaowei Shen, Solar Energy Materials & Solar Cells, https://doi.org/10.1016/j.solmat.2019.110285

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finding was contrary to previous reports that the formation of a Ni silicide is a critical contributor to sufficiently adhesive metal contacts on textured Si surfaces ablated by ns and ps lasers [11,23,30,31]. With the UV ps laser-ablation process that has been frequently used to form contact grids in SiNx ARCs for p-type Ni/Cu/Ag plated cells [5,6, 9,32–34], some SiNx can remain in the contact openings depending on the laser power and the overlap between the Gaussian pulses. The presence of residual SiNx between ablated, essentially circular, regions does not prevent the formation of continuous conductors as the plated metal will grow laterally from discontinuous contact regions to connect into a connected finger and, in fact, this process has been proposed as early as 1986 to be an effective way of reduce contact recombination and thereby increase open-circuit voltage, VOC [35]. However, if the residual SiNx is partially ablated or its barrier properties have been impacted by the absorption of laser energy, then it may be preferable to completely remove it as it may: (i) provide a path by which Cu from the contact can enter the cell [36]; and (ii) also reduce the interfacial con­ tact area for the metal fingers and busbars and in doing so reduce contact adhesion. In some cases, the area covered by residual SiNx in the contact openings following ps laser ablation may be sufficiently large that it overwhelms the effects of any laser-induced oxides. Furthermore, the etch rate of SiNx (deposited by plasma-enhanced chemical vapour deposition (PECVD)) by hydrofluoric acid (HF)-based solutions can be much slower than for interfacial oxides [37,38], making residual SiNx more difficult to remove than the residual oxides in standard pre-treatment processes before LIP. Although high HF concentrations or longer pre-treatment durations can be used, this is generally not desir­ able due to concomitant etching of the SiNx ARC which can result in overplating. The selection of the ps laser power and pulse overlap directly contribute to the area of residual SiNx in the contact openings. However, the effects of differing area fractions of residual SiNx resulting from different laser ablation conditions is often overlooked in compar­ isons between reports of the electrical performance, reliability and durability of Ni/Cu/Ag plated cells. It is hypothesised here that residual SiNx remaining in the finger and busbar regions after laser ablation and wet chemical pre-treatments may impact Ni silicide formation, electrical performance, contact adhesion, and possibly also module durability. This paper reports how UV ps laser fluence, determined by the power and pulse overlap used to ablate a finger/busbar contact grid within a SiNx ARC for Ni/Cu/Ag plated p-type cells, affects surface chemistry, wettability, finger and busbar contact adhesion, Ni silicide formation and electrical cell performance. Surface chemistry (characterised using X-ray photoelectron spectroscopy (XPS)) and wettability (measured using water contact angle measurements) were assessed before and after wet chemical pre-treatments to ascertain the contributions of laser ablation and chemical etching on laser-induced Si oxides and residual SiNx. Finger and busbar adhesion, Ni silicide formation and electrical performance were measured on full-area Ni/Cu/Ag plated p-type cells with full-area back surface fields (BSFs).

2.5] enabling the measurement of finger/busbar adhesion and Ni silicide formation [see Section 2.6] and electrical performance.

2. Experimental

Contact angle measurements were performed in ambient atmosphere on duplicated samples using a Ram�e-Hart Model 200 (p/n 200-U1) goniometer employing ~ 8 μL of distilled water for contact angle (CA). The Dropimage™ Advanced Software (Ram�e-Hart Instrument co.) was used to quantify the value of contact angles. Comparisons were also made with measurements using the Ni plating electrolyte to identify differences in wettability between water and the electrolyte.

2.2. Laser ablation All laser openings were formed using a 266-nm ps Lumera Super Rapid Nd:YAG laser with β-BaB2O4 (BBO) crystal for the fourth har­ monic with optical scanner for contact angle measurements and stage scanner for XPS analysis and Ni/Cu/Ag plating. Laser speed and power were used to vary the pulse overlap and laser fluence. The latter parameter was calculated as the average laser energy delivered per unit area on the alkaline-textured Si surface rather than the fluence of one single laser pulse (i.e., it depended on the arrangement of pulses on the surface). Single pulse fluence was calculated using online calculator from Ophir Optronics Solutions Ltd based on measured average laser power and pulse diameter [39]. The different UV ps laser settings used in this study with calculated average laser fluence is shown in Table 1. Square openings (1 cm � 1 cm) in the SiNx ARC were ablated on the smaller precursor fragments for the XPS and contact angle measure­ ments. For the cells, finger openings were formed using a single laser pass [see Fig. 1b and Fig. 1d] with different one-dimensional pulse overlaps, the actual opening width varying between 8.0 and 11.5 μm depending on the laser parameters used. The spacing between two adjacent finger laser passes for finger openings was set to 1.2 mm. The Busbar regions were fully-opened in the SiNx ARC with a width of 0.8 mm using a two-dimensional pulse overlap pattern (the overlap in the x and y dimensions was set to be close) with an example shown in Fig. 1c. 2.3. X-ray photoelectron spectroscopy X-ray photoelectron spectroscopy measurements were performed by a Thermo ESCALAB 250Xi spectrometer with a monochromatic Al Kα source at 1486.68 eV. The XPS spectra were obtained in normal emission with a photoelectron take-off angle of 90� in an analyzing chamber operating below 2 � 10 9 mbar. The spot diameter was set to 500 μm and thus XPS measurements were taken on regions with a set of laser scans (busbar regions). The resolution of the spectrometer was 0.6 eV as measured from the Ag 3d5/2 signal (FWHM) with 20 eV pass energy. Survey scans were performed using a step size of 1.0 eV, dwell time of 100 ms, and analyser pass energy of 100 eV. Region scans were per­ formed using a step size of 0.1 eV, dwell time of 100 ms, and analyzer pass energy of 20 eV. The obtained XPS spectra were fitted by Thermo Scientific™ Avantage Software using Lorentzian and Gaussian profiles after background subtraction. All binding energies (in eV) were cor­ rected to C 1s signal at 284.8 eV. The error in the O/Si and N/Si ratios was estimated from a peak-shape analysis as reported by Hesse et al. [40]. 2.4. Contact angle measurements

2.1. Cell precursor fabrication All samples used industrially-produced alkaline-textured mono­ crystalline Al BSF cell precursors without front-side metallisation. The emitter of precursors was formed by POCI3 diffusion with a sheet re­ sistivity of 110–120 Ω/□, a surface concentration of 2 � 1019 cm 3 and junction depth of ~0.4 μm. The SiNx ARC was deposited by direct PECVD and was 72.7 � 0.7 nm thick with a refractive index of 2.09 after rear etching. The rear surfaces were screen-printed with full-area Al and then fired to form a BSF and rear metal contact. After laser ablation [see Section 2.2], some precursors were cleaved into smaller fragments (31.2 mm � 31.2 mm) for XPS [see Section 2.3] and contact angle [see Section 2.4] measurements. Full area cells were then plated [see Section

2.5. Metal plating, post-plating annealing and cell I–V measurements The SiNx-coated surfaces of the laser-ablated cells were immersed in 7:1 BOE for 7 s before LIP, which is the standard pre-treatment process used throughout this study to remove surface oxides either induced by laser ablation or natively grown. The duration of the pre-treatment was selected so as to minimise undesirable etching of the ARC and is 2

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Table 1 UV ps laser settings used for busbar and finger openings. Laser Average Power (mW) 4 6 4 6

Laser Process Speed (mm/s) Busbar

Finger

220 240 135 145

240 260 150 165

Busbar Scan Pitch (μm) 9 11 7 9

Laser Repetition Frequency (kHz)

Average Laser Fluence (J/cm2)

Busbar

Finger

Busbar

Finger

20 20 20 20

25 25 25 25

0.35 0.45 0.56 0.63

0.35 0.45 0.47 0.54

Fig. 1. Diagram showing the laser pulse configuration for busbar ablation with 0% (a) and 35% (c) two-dimensional overlap and finger ablation with 0% (b) and 35% (d) one-dimensional overlap in the ideal case.

consistent with that used by other studies [3,41]. The immersed surface was then rinsed using deionized water for ~15 s in order to remove residual BOE and thus prevent contamination of the Ni plating electro­ lyte. A metal stack of Ni (~1 μm), Cu (~12 μm) and Ag (~0.5 μm) was then deposited using LIP with current densities of 25, 40 and 30 mA/cm2 in plating electrolytes of Barrett Nickel SN1 (MacDermid Inc.), Helios Copper EP2 (MacDermid Inc.), and Helios Silver EPF 400 (MacDermid Inc.), respectively. Rinsing was performed after each step of metal plating to avoid cross-contamination. After deposition of Ag, cells were then annealed at 350 � C for 1 min in N2 ambient in a rapid thermal process furnace. The I–V responses of plated cells were measured directly after post-plating annealing using a WAVELABS SINUS-220 cell tester under standard test conditions. The spectrum was calibrated to one sun using a reference Si solar cell which had been independently measured by Solar Energy Research Institute of Singapore. Suns - VOC measurements were performed using a Sinton Instruments illumination voltage tester, to obtain estimates of the recombination current den­ sities, J01 and J02, and the pseudo fill factor, pFF.

a laser wavelength of 532 nm and average power of 3.4 mW. Finger adhesion was measured using a custom-built stylus-based adhesion tester based on [24,42,43] shown in Fig. 6a with stylus diameter of 0.7 mm and speed at 30 mm/min. Adhesive taps were applied on each side of the test region prior to scanning to prevent measurement interference from previously-tested fingers and hence obtain accurate force measurements as reported by Young et al. [42]. The recorded dislodgment forces were analysed. All finger impacts which were not classified as finger dislodgement [44] were eliminated from the analysis. 3. Results and discussion 3.1. Surface analysis of laser-ablated regions 3.1.1. XPS analysis Fig. 2a shows the Si 2p XPS spectra, normalised to the intensity of the Si 2p3/2 peak at ~99.3 eV, for ps laser-ablated alkaline textured surfaces before pre-treatment. As the laser fluence increased, the intensity of the broad peak of oxidized Si increased relative to the Si peak (at 99.3 eV) and shifted to higher binding energy from 101.4 eV to 103.3 eV after laser ablation, the former and latter peaks corresponding to Si–N [45–48] and Si–O bonds [49,50], respectively. This trend of a reducing density of Si–N bonds with an accompanied increasing density of Si–O bonds with increased laser fluence is confirmed in Fig. 2c and d [black curve data], showing the integrated atomic intensity ratio of O/Si and N/Si. The atomic intensity ratio of O/Si from the samples ablated with a laser fluence of 0.63 J/cm2 is almost twice that for the sample where the

2.6. Adhesion test and Raman mapping Interconnection ribbons, 0.9 mm wide and 250 μm thick, were manually soldered to the plated busbars after post-plating annealing. Busbar adhesion was then measured using a custom-built pull tester at a pull angle of 180� with a speed at 50 mm/min. Measured busbar pull forces were normalised to the plated busbar width of 0.8 mm. Raman mapping measurements were conducted on the busbar regions exposed by the busbar pull tests using Renishaw inVia 2 Raman Microscope with 3

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Fig. 2. XPS analysis of normalised Si 2p spectra for ps-laser ablated alkaline textured surfaces before (a) and after pre-treatment in BOE for 7 s (b) with different laser fluences. The inset in (b) shows an expanded view of binding energy range corresponding to the Si–N bond. The XPS intensity ratio of O (c) and N (d) to Si atomic intensity graphed against the laser fluence before and after pre-treatment.

ARC was ablated using a laser fluence of 0.35 J/cm2. This confirms the conclusions reported by Büchler et al., that higher laser fluences result in increased Si oxide formation. The high atomic intensity ratio N/Si of the ablated surface at � 0.45 J/cm2 laser fluence suggests that the SiNx layer was not completely removed under these ablation conditions. After pre-treatment, Si–O peaks were not evident for all laser flu­ ences [see Fig. 2b], suggesting that surface oxides were effectively removed by the 7:1 BOE pre-treatment for 7 s. This is consistent with the results shown in Fig. 2c, where the atomic intensity ratio of O/Si was decreased to a constantly low value of 0.074–0.17 in the laser fluence range of 0.35 J/cm2 to 0.63 J/cm2. On the other hand, the N/Si intensity ratio follows the same trend of that before pre-treatment [see Fig. 2d]. This is attributed to the slow etching rate of SiNx by BOE [37,38]. Furthermore, the higher N/Si ratio after pre-treatment could be due to the removal of partially oxidized SiNx which is expected to result in relatively more N atoms being detected. A small peak remains at ~101.5 eV for surfaces ablated with a laser fluence of 0.56 J/cm2 [see Fig. 2b], suggesting the presence of some residual Si–N bonds after pre-treatment. Increasing the laser fluence from 0.56 to 0.63 J/cm2 is expected to remove additional SiNx, however the resulting difference in terms of the N/Si ratio is small due to the much larger and dominant Si signal intensity. It was concluded from the XPS results that, although higher laser fluences result in enhanced laser-induced Si oxide growth, the induced oxides can be effectively removed by pre-treatment in 7:1 BOE for 7 s before Ni/Cu/Ag plating. Consequently, they are not expected to impact the plated metal adhesion. However residual SiNx, which increases with lower laser fluences, is not effectively removed by pre-treatment in 7:1 BOE for 7 s. Nickel will therefore not nucleate and grow on the residual SiNx due to its dielectric properties and this can leave gaps in the Ni coverage for layers ~1 μm thick as shown in Fig. 3. The subsequently

plated Cu (~12 μm thick) will grow laterally from the plated Ni and can in doing so cover the gaps in the Ni layer where SiNx remained and form contiguous conductors (e.g., fingers) as observed for all the plated cells. 3.1.2. Contact angle measurements Fig. 4a shows the CA increased with laser fluence with a value of 49.5 � 3.6� being measured when the laser fluence was �0.45 J/cm2 indicating that the Si surface was hydrophilic after BOE pre-treatment (see the image of a water drop in Fig. 4b). The CA increased to

Fig. 3. Scanning electron microscope image showing surface topography in a laser ablated opening after LIP of Ni at 25 mA/cm2 for 2 min. The average thickness of the plated Ni was ~1 μm. 4

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Fig. 4. (a) XPS intensity ratio of O (grey) and N (blue) to Si using atomic percentages and CA (red) as a function of different laser fluences after pretreatment in 7:1 BOE for 7 s. (b) and (c) Images of a water droplet on example alkaline-textured Si surfaces where the SiNx had been ablated with laser fluences of 0.35 J/cm2 and 0.63 J/cm2, respectively, after pre-treatment. (d) Image of a droplet of Ni plating electrolyte on an alkalinetextured Si surface where the SiNx had been abla­ ted with a fluence of 0.63 J/cm2. (For interpreta­ tion of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

74.1 � 3.3� and 76.3 � 4.0� when the laser fluence was increased to 0.56 and 0.63 J/cm2, respectively, indicating that the surface became less hydrophilic. Both the O/Si and N/Si intensity ratios from XPS analysis are shown in column bars in Fig. 4. Presence of both the Si–N and Si–O bonds on the surface can contribute to a more hydrophilic surface due to their higher polar contribution as well as lower surface energy [51–53]. Since the changes in O/Si ratio are relatively small and most surface oxides would have been removed by BOE pre-treatment [see Fig. 2b], the increased hydrophilicity with lower laser fluences should be attributed to the N/Si ratio, further confirming the presence of residual SiNx after laser ablation and pre-treatment. Since the higher CA after BOE pre-treatment may result in poor surface wetting by the plating electrolyte, the contact angle was also imaged with a drop of Ni plating electrolyte. Fig. 4d shows the image when ~8 μL of Ni plating electrolyte was ‘dropped’ onto a pre-treated alkaline-textured Si surface after the SiNx had been ablated using a laser fluence of 0.63 J/cm2. Unlike the water droplet shown in Fig. 4c, the Ni electrolyte spread across the ablated Si surface and the measured CA of Ni plating electrolyte was ~5� , which is significantly lower than the water CA. This improved wettability over water is attributed to the use of surfactants (wetting agents) in the Ni plating electrolyte and suggests the importance of the chemical formulation when depositing Ni directly on Si [54,55].

3.2. Metal adhesion 3.2.1. Busbar adhesion The box plot in Fig. 5a shows that the normalised busbar pull force (represented as the average of the measured forces over each 1-mm long segment of plated busbar) increases as a function of laser fluence used for busbar ablation. Although for all laser fluences there are some spurious very high pull force measurements with a force >2 N/mm, it was concluded that in general a laser fluence �0.56 J/cm2 was required to achieve an average pull force >1 N/mm. Fig. 5b shows the histogram distribution of the busbar pull force measurements (all measured values). The measured pull force of samples ablated with �0.45 J/cm2 laser fluence was 0.8 � 0.5 N/mm. The average value and range of measured pull forces increased with laser ablation fluence, to a value of 1.9 � 0.7 N/mm with 0.63 J/cm2 laser fluence, coupled with increased standard deviation. However, the measured pull forces were more closely clustered with less uniform surface chemistry (more residual SiNx) resulted from low fluence used during ablation, suggesting homogenously weaker busbar adhesion. 3.2.2. Finger adhesion Fig. 6b shows a box plot of finger dislodgement forces measured by the stylus-based adhesion tester [see Fig. 6a]. Unlike the trend observed

Fig. 5. (a) Box plot of average normalised BB pull force for a length of 1 mm as a function of laser fluence used to ablate the SiNx ARC (i.e., each data point represents the average force of 1-mm long plated busbar); and (b) frequency histograms of normalised BB pull force (raw data) for the four different laser fluences used. The data is accumulated over measurements made on five independent cells for each laser fluence. 5

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for busbar pull force in Fig, 3, there was no significant difference in finger dislodgement force between the four laser fluences used for finger opening with an average finger dislodgement force of 0.41 � 0.12 N being measured for each laser fluence, which is different from results on busbar pull forces shown in Subsection 3.2.1. While the different mea­ surement method for busbar and finger adhesion may be a major concern. However, the average finger opening width increased from 8.0 to 11.5 μm when the laser fluence increased from 0.35 to 0.54 J/cm2, due to the increased laser power and/or pulse overlap. This suggests that higher laser fluence for finger ablation did not contribute to stronger finger adhesion even when a wider finger opening was formed. Instead, the wider finger opening may be detrimental to cell performance by: (i) increasing shading; (ii) increasing metal contact recombination; and (iii) potentially also degrading pseudo fill factor with increased laser damage and/or Ni silicide penetrating close to junction during post-plating annealing [23]. Furthermore, lower laser fluences may also be prefer­ able for finger ablation due to the increased laser processing speed with high productivity. This result highlights that laser ablation processes for plated cells should be optimized independently for busbars and fingers on the basis of their respective adhesion force.

Fig. 7. Raman spectra for representative alkaline-textured Si surfaces with and without formation of Ni silicide. The measurement was performed in the busbar regions after a pull test had been performed.

ratio at randomly-selected busbar regions after the pull test are shown in Fig. 8. On most of the Si surface ablated with 0.35 and 0.45 J/cm2 the Ni2Si/Si ratio was lower than 0.04, suggesting that there was limited Ni silicide formed. Consequently, the adhesion between the Si and plated Ni is expected to be determined by only the Ni–Si contact area which is similar for surfaces ablated with 0.35 and 0.45 J/cm2 fluences. How­ ever, for laser fluences �0.56 J/cm2, increasing fractions of Ni2Si are evident on the exposed Si surface, with the Ni2Si/Si intensity ratio being more uniform with the laser fluence of 0.63 J/cm2. This may suggest that even a small amount of residual SiNx can influence the uniformity of silicidation during post-plating annealing. A box plot of the Ni2Si/Si ratio from larger randomly-selected re­ gions is combined with average busbar pull force and N/Si ratio in Fig. 9. This composite view reinforces that more complete ablation of the SiNx through use of a higher laser fluence results in stronger busbar adhesion. This improved adhesion can be due to a combination of increased Ni–Si contact area and the extent and uniformity of the formed Ni silicide. Moreover, it is possible that residual surface SiNx hinders the silicidation between a-Si and plated Ni during post-plating annealing. While it is difficult to detect SiNx after annealing and adhesion test.

3.3. Raman spectroscopy Fig. 7 shows the Raman spectra measured in the busbar regions after removing the plated metal from a cell. When Ni silicide was detected (black curve), the two peaks at 100 and 140 cm 1 represent the for­ mation of crystalline Ni2Si and the low peaks at ~ 189 and 215 cm 1 indicate the presence of NiSi [56–58]. For all cells tested, the phase transition from Ni2Si to NiSi was incomplete with the post-plating annealing at 350 � C. While it has been reported that the formation of NiSi starts at ~290 � C [56,59], the short annealing duration of 1 min used in this study was supposed to result in an incomplete phase tran­ sition to the NiSi form. In the regions where no Ni silicide was detected, the Raman spectrum (red curve) reveals the presence of amorphous Si (a-Si) identified by the broad peak in 100–200 cm 1 and at ~ 470 cm 1. Amorphous Si is known to form with UV ps laser ablation [11,60,61] and Raman scattering of a-Si has been previously reported in Refs. [62–64]. However, it has been reported that Ni silicide can form between Ni and a-Si layers [65,66], consequently laser induced a-Si would not be ex­ pected to be responsible for the poor adhesion with no Ni silicidation. As the Ni2Si peak at 100 cm 1 was most intense and more readily quantified of all the Ni silicide peaks, the intensity ratio of Ni2Si at 100 cm 1 to the crystalline Si peak at 520.5 cm 1 was used for the subsequent analysis. Two-dimensional (2D) Raman maps of the Ni2Si/Si

Fig. 6. (a) Image of the stylus-based finger adhesion tester showing the stylus dislodging the plated fingers. A strip of tape is placed adjacent to the measurement path of the style to ensure that dislodged fingers do not interfere with subsequent measurements. (b) Box plot of finger dislodgement force and average finger width as a function laser fluence used for finger ablation. 6

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Fig. 8. 2D Raman mapping of Ni2Si/Si ratio measured from exemplar plated busbars after pull test for the four different laser fluences used for busbar ablation.

reduced VOC and pFF values. This may be due to the relatively deep junction (~0.4 μm) used for the cells as previous studies have reported that pFF losses can be reduced through the use of a deeper junction [9, 11]. The series resistance, RS was ~0.26 Ω cm2 for all cells resulting in FF of >80%. This was expected to demonstrate the ability of plated metal stacks to form low resistance contact to moderately doped n-type Si surface for all laser fluences used. It may also indicate that presence of SiNx and Ni silicidation, respectively, did not significantly impact con­ tact resistance and hence RS. Moreover, the result highlighted that no negative impact on electrical performance was introduced by higher laser fluence used in this study. 4. Conclusion In this work, the influence of applied laser fluence during ablation on the front surface chemistry of Si solar cells was investigated. Although, laser-induced Si oxide growth is increased with higher laser fluence, the formed oxides are effectively removed using the 7:1 BOE pre-treatment used and consequently do not impact either the contact adhesion or electrical performance. However, it is shown that residual SiNx, result­ ing from incomplete laser ablation, remains on the Si surface after the 7:1 BOE pre-treatment where it can impact the uniformity of the Ni plating coverage over the contact region. It was also demonstrated that high laser fluence can lead to a more hydrophobic surface after 7:1 BOE pre-treatment presumably due to reduced residual SiNx on the surface. Whereas incomplete surface wetting was observed for water, the sur­ factants or ‘wetting agents’ in the Ni plating electrolyte allow that electrolyte to uniformly wet the surface thereby: (i) eliminating concern about the possible role that incomplete wetting may play in poor contact adhesion; and (ii) highlighting the need to use Ni plating electrolytes with wetting agents for direct plating to Si.

Fig. 9. Box plot of Ni2Si/Si (in black) and N/Si (in blue) ratio and line plot of average BB pull force (in red) of plated busbars that were ablated with different laser fluence. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

3.4. Cell electrical performance The electrical performance of the full-area Ni/Cu/Ag plated cells ablated with different laser fluence is summarized in Table 2. Although ps laser ablation has been shown to impact VOC and pFF through increased J01 and J02 recombination currents, respectively [67–69], in this study there was no significant difference in J01 or J02 among cells ablated with different laser fluence, at least in the range that was investigated. Consequently, increased laser fluence did not result in 7

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Table 2 Average I–V parameters as well as Suns-VOC parameters (5 cells) of cells ablated with different laser fluence after Ni/Cu/Ag plating and annealing. Laser Fluence (J/cm2) Busbar

Finger

0.35 0.45 0.56 0.63

0.35 0.45 0.47 0.54

VOC (mV)

JSC (mA/cm2)

FF (%)

RS (Ω∙cm2)

Efficiency (%)

pFF (%)

J01 (pA/cm2)

J02 (nA/cm2)

637.1 � 2.1 638.2 � 0.4 638.0 � 0.4 637.1 � 0.4

38.0 � 0.2 37.9 � 0.1 38.0 � 0.1 37.8 � 0.1

80.5 � 0.3 80.6 � 0.6 80.5 � 0.3 80.4 � 0.4

0.25 � 0.01 0.26 � 0.01 0.26 � 0.01 0.27 � 0.01

19.5 � 0.1 19.5 � 0.2 19.5 � 0.1 19.4 � 0.1

82.1 � 0.3 82.2 � 0.6 82.2 � 0.3 82.0 � 0.4

0.60 � 0.06 0.55 � 0.05 0.57 � 0.03 0.60 � 0.04

10 � 4 15 � 3 10 � 5 10 � 6

Presence of residual SiNx on the surface was shown to reduce busbar adhesion, with average busbar pull forces of 1.9 N/mm being obtained using a laser fluence of 0.63 J/cm2 and reducing to only 0.73 N/mm when the laser fluence was decreased to 0.35 J/cm2. However, it was not possible to determine whether this reduction in busbar adhesion was primarily due to a reduced Ni–Si contact area or perhaps also to inhi­ bition of Ni silicide growth by the residual SiNx. It is concluded that Ni silicidation requires that residual SiNx should be minimised through a use of a higher laser fluence. Interestingly, finger dislodgement force was not increased by the use of higher laser fluences. Although it is not clear why the presence of residual SiNx does not negatively impact finger adhesion, this result demonstrates that laser ablation processes for plated cells should be optimized independently for busbars and fingers on the basis of their respective adhesion force and that less overlap can be tolerated between laser pulses in the finger regions. Furthermore, this finding may have greater significance for PERC cells which are expected to be more sensitive to laser damage due to their longer carrier lifetimes. Importantly for practical outcomes, the findings of this study show that the improved busbar adhesion that can be achieved with the higher laser fluences without negatively impacting the VOC or pFF of Al BSF cells providing that a sufficiently deep junction is engineered. Future work will determine whether the same outcomes can be achieved for higher efficiency PERC cells which may use higher band gap materials such as SiO2 and AlOx in their ARCs, as the latter oxides can be more challenging to be completely removed.

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Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgment This research is supported by the Australian Research Council (Future Fellowship FT170100447) and the Australian Renewable En­ ergy Agency (ARENA). The Australian Government, through ARENA, is supporting Australian research and development in solar photovoltaic and solar thermal technologies to help solar power become cost competitive with other energy sources. The views expressed herein are not necessarily the views of the Australian Government, and the Australian Government does not accept responsibility for either any information or advice contained herein. Laser ablation was performed with the assistance of Dr. Benjamin Johnston at the OptoFab node of the Australian National Fabrication Facility, utilising National Collaborative Research Infrastructure Strategy funding to provide nano and microfabrication facilities for Australia’s researchers. The authors also acknowledge the use of the facilities and the assistance of Dr. Bin Gong at Surface Analysis Laboratory of Mark Wainwright Analytical Centre, at the University of New South Wales. Xiaowei Shen also would like to appreciate assistant in soldering from Mr. Yuanfang Zhang. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.solmat.2019.110285. 8

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