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j.matlet.2018.07.127

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Materials Letters 231 (2018) 146–149
Contents lists available at ScienceDirect
Materials Letters
journal homepage: www.elsevier.com/locate/mlblue
Direct soldering of screen-printed Al-paste layer on back-side of silicon
solar cell using SnAg solder
Weibing Guo a,b, Xinran Ma a, Mingze Gao a, Jiuchun Yan a,⇑
a
b
State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, China
School of Material Science and Engineering, Hebei University of Technology, Tianjin 300130, China
a r t i c l e
i n f o
Article history:
Received 15 August 2017
Received in revised form 9 July 2018
Accepted 28 July 2018
Available online 30 July 2018
Keywords:
Solar energy materials
Sn-Ag solders
Ultrasonic-assisted soldering
Microstructure
Electrical properties
a b s t r a c t
Direct joining of Al back surface field (Al BSF) in a polycrystalline silicon solar cell using a green Sn-3.5Ag
solder by the assistance of ultrasound was investigated. SEM, peel force, electrical resistance and
open-circuit voltage (Voc) tests were used to study the effect of ultrasonic action time on the performance
of the solar cell. The results show that with the increasing of ultrasonic action time, more Al particles in
the paste residual layer dissolved into the solder layer. The dissolved Al existed in the bond metal as
forms of a-Al and Ag-Al compound phases. The solder bonded directly with the Al-Si eutectic layer under
ultrasonic action time of 6 s. The resistance of the joints was 1.27 mX and the peel force could reach as
high as 1.44 N/mm. The Voc of solar cell was 524 mV, which was higher than the 467 mV of solar cell
soldered with Ag electrode.
Ó 2018 Elsevier B.V. All rights reserved.
1. Introduction
Crystalline silicon solar cell is trending towards high efficiency,
low cost [1] and environmental friendliness. In the packaging
process, the rear contact of cell requires to be bonded with the
top contact of another cell to conduct current in a solar power
module [2]. There will be three layers after sintering aluminum
powder suspension on the rear side of Si: the Al-doped p+-layer,
the eutectic layer and the layer of paste residuals. The sintering
process realizes the doping on the rear side of solar cells and
produces Al back surface field [3,4]. However, the Al BSF is very
difficult to solder. In order to make soldering easier, silver busbars
are printed on the back surface, and Cu interconnector ribbons
coated with SnPb solder are used to join the silver busbars [5].
The metallization processes are complicated and the noble metals
of silver and copper are used. Moreover, the Sn-Pb solders are not
environment-friendly. In addition, the silver busbars prevent the
formation of BSF, which would raise the recombination rate [6]
and lead to a lower open-circuit voltage [7]. If the solder can form
direct bond with the Al BSF underneath the metallization, the
metallization processes can be simplified, costly Ag can be avoided
and the performance and reliability of solar power modules can be
improved. However, the oxide film on the surface of Al is very
stable, resulting in poor solderability. In recent years, Al and its
⇑ Corresponding author.
E-mail address: jcyan@hit.edu.cn (J. Yan).
https://doi.org/10.1016/j.matlet.2018.07.127
0167-577X/Ó 2018 Elsevier B.V. All rights reserved.
alloys were successfully soldered with Sn-based alloys. Pure Sn
was used to solder Al 2024 directly assisted by ultrasound [8,9].
Sn-3.5Ag solder was also used to solder Al 1070 [10]. Recently,
green solders such as Sn-Zn, Sn-Ag and Sn-Ag-Cu have been developed to join the Ag electrodes of solar cells [11,12] and Al ribbon
was developed for lower cost than the tin-coated Cu ribbon [13].
Here, we soldered the Al BSF and Al ribbon directly using a
Sn-3.5Ag solder with the assistance of ultrasonic waves. The effect
of ultrasonic action time on the microstructure, mechanical
properties, electric conductivity and photoelectric properties of
joints was investigated in details. The work is helpful to realize
the packaging of solar cells with low cost, high performance and
less pollution.
2. Experimental procedure
The commercial polycrystalline solar cells were provided by
Shanghai Suiying Photovoltaic Co. Ltd. The solar cell was 200 lm
in thickness and the nominal Voc was 0.5 V. The rear side was
printed with Al pastes. Commercial Sn-3.5Ag (wt%) alloys were
used as filler metals, whose melting point is 221 °C. Pure Al 1060
alloy was used as ribbons, which were 100 lm in thickness and
2 mm in width.
In the process of soldering, 30 mg solders were placed on the
surface of Al BSF. Then the ultrasonic waves were applied into
the melting solders with a ultrasonic iron. The ultrasonic generator
in the iron was equipped with a power supply, a vibrator, a
W. Guo et al. / Materials Letters 231 (2018) 146–149
transducer, a booster and a sonotrode. The frequency of the ultrasonic vibration was 60 kHz with an amplitude of 3 lm and the
power of induction heating was 60 W. After the application of
ultrasound for 2 s, 4 s and 6 s during soldering, the solder layer
had dimensions of 6 4 mm2. Then the interconnector ribbon
147
was joined to the solder layer on Al BSF surface with the ultrasonic
iron.
The microstructures of the interfaces were observed by scanning electron microscopy (FEI-Quanta 200F) equipped with an
energy dispersive X-ray spectroscopy (EDS). X-ray diffraction
(XRD) tests were carried out by Bruker D8 Advance multi crystal
diffractometer. A four-point resistance measurement was used to
evaluate the electrical conductivity of the joint, as shown in
Fig. 1. The values of resistance were provided by KEITHLEY-2420
multifunctional digital meter. Conventional 90° peel tests were
performed to evaluate the mechanical properties of the joints using
an adapted method of DIN EN 50 461 [14]. The tests were conducted using an microforce tester (Instron-5948). The Voc of solar
cell was determined with Oriel Sol1A sunlight simulator
(p = 23.5 mW/cm2) and Versa STAT 3 electrochemical workstation.
3. Results and discussion
Fig. 1. (a) and (b) Schematic of 4-point resistance measurement.
Fig. 2a–c show the microstructure of the interfaces and the element distribution along the corresponding lines under the ultrasonic action for 2 s, 4 s, and 6 s. The thickness of the original Al
paste residual layer was about 35 lm and this layer was composed
of loosely sintered Al particles. When the ultrasonic action time
was 2 s, the solder could wet and bond with the Al layer. The solder
layer was mainly composed of b-Sn (Fig. 2a). The EDS results show
the phase at point G had compositions of 64% Ag, 32% Al and 4%Sn
(at.%), which can be identified as Ag2Al phase. The Al paste residual
layer was still complete, as the element distribution along line AB
indicates that the existence of the Al particle layer, with the thickness of about 35 lm. As the ultrasonic action time increased to 4 s,
more Al particles dissolved into the solder layer and the solder had
Fig. 2. Microstructure of the interfaces and the element distribution along the corresponding lines under the ultrasonic action for (a) 2 s, (b) 4 s and (c) 6 s; EDS Color maps for
(b) 4 s of (d) Al, (e) Si, (f) Ag, and (g) Sn; (h) XRD patterns of joints for different ultrasonic action time.
148
W. Guo et al. / Materials Letters 231 (2018) 146–149
penetrated into the Al paste residual layer, as shown in Fig. 2b.
According to the element distribution along line CD, the Al-Si
eutectic layer and Al particle layer can be identified and the
penetration depth of the solder was about 20 lm. Fig. 2d–g present
the distribution of Sn, Ag, Si, Al around the joint in Fig. 2b. It could
be learnt from Fig. 2d that Al diffusion layer in the Si substrate
existed and it formed Al BSF. Sn penetrated through the Al particles
(Fig. 2f) and Ag concentrated on the interface of Al particle layer
and solder alloy (Fig. 2g). Thus, Ag-Al phases existed near this
interface, which could also be seen in Fig. 2b by distinct contrast.
After further increasing the ultrasonic action time to 6 s, the large
amount of Al particles could no longer be observed. Only few
incomplete Al particles existed near the interface, as shown in
Fig. 2c. Almost the entire Al paste residual layer was dissolved into
the solder layer. The phase at point I had compositions of 60% Ag,
37%Al, and 3%Sn (at.%), which can be identified as Ag3Al2 phase.
The chemical compositions of phase at point H were: 92% Al, 2%
Ag, and 6%Sn (at.%), indicating that the phase was a-Al phase.
The EDS scanning result along line EF in Fig. 2c shows the dissolution and diffusion layer of Al and Si at the interface, indicating that
the Al-Si eutectic layer and p+-layer were not broken during the
soldering process. XRD tests were carried out to provide evidence
of Ag-Al phases in the solder layer, as shown in Fig. 2h. It could
be seen that Sn phase (JCPDS 04-0673, I41/amd space group,
a = b = 0.5831 nm, c = 0.3182 nm) existed, and as ultrasonic time
was prolonged and more Al (JCPDS 65-2869, Fm-3 m space group,
a = b = c = 0.40497 nm) particles dissolved into the solder alloys,
Ag2Al (JCPDS 14-0647, P63/mmc space group, a = b = 0.2885 nm
and c = 0.4624 nm) reduced gradually and disappeared. Small
amount of Ag3Al2 [15] was found when the ultrasonic action time
was 6 s. The formation of Ag-Al compound is complicated for different atomic ratios of Ag and Al. As soon as Al dissolved into the
solder, Al would react with Ag and Ag-Al compound formed. The
compound was Ag-rich firstly [15,16], including Ag2Al and Ag3Al
(JCPDS 28-0034, P4132 space group, a = b = c = 0.693400 nm). As
Ag-Al continued to dissolve into the solder, Ag-rich compounds
transformed into Al-rich Ag3Al2 gradually.
The ultrasonic action was a key factor to realize the soldering of
screen-printed Al-paste layer. It contributed to the wetting, oxide
film breaking and dissolution. The oxide film on the surface of Al
particles was wetted by the liquid solder with the assistance of
ultrasound. The physical mechanism is such as: the continuous
propagation of ultrasonic wave in liquid alloys induced cavitation
bubbles. When the bubbles collapsed in a very short time, a
micro-jet with very high shock pressure could be generated near
the interface to break the oxide film of the aluminum alloys. The
solder filled the gaps between the Al particles for the ultrasonicinduced capillary effect [17]. For a small Al particle, the liquid solder could surround it very quickly and the notches appeared on the
oxide film of Al particles. More notches on oxide film accelerated
the dissolution of Al particles.
As shown in Fig. 2a–c, the effect of ultrasonic waves on the
joints was not uniform. The inhomogeneity of the microstructure
could be attributed to two main factors. Firstly, the diffusion of
these spheres started as soon as ultrasonic cavitation caused
notches on the oxide film [18]. However, the cavitation occurred
Fig. 3. (a) Experimental setup for peel force test; (b) Peel force of the joints by different ultrasonic action time; Fracture surfaces of joints under ultrasonic action for (c) 2 s
and (d) 6 s.
W. Guo et al. / Materials Letters 231 (2018) 146–149
149
interface directly. For the sound interfacial bonding and good conductibility of the SnAg alloy, the resistance dropped to 1.27 mX.
The Voc of solar cells soldered with Al BSF by ultrasonic action time
of 2 s, 4 s and 6 s were 483 mV, 535 mV and 524 mV, while Voc of
solar cells soldered with Ag electrode was only 467 mV. The Ag
busbars on the rear side prevented the formation of BSF, which
would raise the recombination rate, leading to a lower opencircuit voltage. Therefore, direct soldering Al BSF can lower costs
and raise Voc of the solar cells.
4. Conclusions
Fig. 4. Resistance of the joints and VOC of solar cells under different ultrasonic
action time.
randomly and the diffusion process might vary among different
spheres in different sizes. Secondly, the ultrasonic field in the liquid is not uniform [19]; the microstructure of joints may show
inhomogeneity with different distances away from the ultrasonic
iron.
Peel force tests were carried out with a constant speed of
1 mm/min, as shown in Fig. 3a. Fig. 3b shows the relationship of
the ultrasonic action time and the peel force of the joints. By
short ultrasonic action time of 2 s, the peel force of the joint was
0.97 N/mm. The solder bonded with the Al paste residual layer
indeed. The bonding between Al particles was extremely weak,
resulting in low peel force of the joint. The joints fractured inside
the Al paste residual layer, and the fracture surface shows the
spherical Al paste residual particles (Fig. 3c). When the ultrasonic
action time was 4 s, more Al particles were dissolved into the solder layer. The Sn-Ag solder had penetrated into the Al paste residual layer for the ultrasonic-induced capillary effect. The peel force
of the joint was about 1.17 N/mm, which was slightly higher than
that of the joint under ultrasonic action for 2 s. After further
increasing the ultrasonic action time to 6 s, the solder bonded with
the Al-Si eutectic layer directly. The peel force of the joints
increased to about 1.44 N/mm, which reflects the metallurgical
bonding strength of SnAgAl/AlSi alloy interface. The fracture
occurred at the interface of the solder layer and the solar cell.
The chemical compositions of phase at point J were: 87% Al and
13%Si (at.%), indicating that the fracture surface shows the Al-Si
eutectic layer (Fig. 3d).
Fig. 4 demonstrates the trends of joint resistance and VOC under
different ultrasonic action time. The resistance of joint was
1.79 mX under ultrasonic action for 2 s. The resistance was relatively large for most of the Al paste particles still existed beneath
the solder layer. The oxide film on the surface of Al particles is
rather thick and stable. The conductivity of the oxide film is poor
and the bonding between the oxide films is very weak, resulting
in high resistance. When the ultrasonic action time was 4 s, the
electrons needed to go through less Al particles and the resistance
of joint decreased to 1.66 mX. Further increasing the ultrasonic
action time to 6 s could lead to the direct bond between the solder
and the Al-Si eutectic layer. The electrons could pass through the
The Al back surface field was soldered using Sn-3.5Ag solder by
the assistance of ultrasound. The ultrasonic action time had great
influence on the interfacial microstructure and properties of the
joints. When the ultrasonic action time was 2 s, the solder bonded
with the Al paste residual layer for the little dissolution. The joints
had poor mechanical properties and electrical conductivity. The Al
paste residual layer could be dissolved completely under ultrasonic
action for 6 s and the solder bonded with the Al-Si eutectic layer
directly. The resistance of the joints was 1.27 mX and the peel
force could reach 1.44 N/mm. The Voc of solar cell was 524 mV,
which was higher than the 467 mV of solar cell soldered with Ag
electrode, for the existence of Al BSF.
Acknowledgment
This research was sponsored by the National Natural Science
Foundation of China (Grant No. 51435004).
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