The following contribution was presented at the
28. European PV Solar Energy Conference and Exhibition
in Paris, France.
30.9.2013 - 4.10.2013
2BV.2.8 Presented at the EU-PVSEC Paris, September – October 2013
RECOMBINATION AND CONTACT PROPERTIES IN METALLISED REAR SIDE REGIONS OF PERC
CELLS
R. Ferré1,*, T. Pernau1,a), F. Schwarz1, T. Baier1, S. Nölker1, B. Huth1, S. Rommel1, M. Okanovic1, G. Gläser1,b),
T. Neubert2, T. Dullweber2
1
Manz AG, Steigaeckerstrasse 5, 72768 Reutlingen, Germany
2
Institute for Solar Energy Research Hamelin (ISFH), Am Ohrberg 1, 31860 Emmerthal, Germany
*)
Corresponding author: rferre@manz.com
a)
Now with Rehm Thermal Systems, Leinenstrasse 7, 89143 Blaubeuren
b)
Now with Robert Bosch GmbH, Tuebingerstrasse 123, 72766 Reutlingen
ABSTRACT: We study the laser contact opening as used in industrial PERC solar cells. We open Al2O3/SiN
multilayer with a nanosecond Flat-Top laser and apply Al screen printing and firing. As already published, we
confirm that the surface recombination velocity decreases with increasing line width. Accordingly, with the ns laser
we achieve Scont values as low as 400 cm/s for line widths above 100 µm, and Scont values of 600 cm/s for 40 µm wide
lines. On the contrary, the contact resistance tends to increase slightly with the line width to values around 10 Ω cm2.
Low values of these two parameters are due to an Al-BSF layer, either grown after Al-Si re-solidification after firing
or by laser doping before firing. We show evidences of laser doping for ns laser, whereas this effect for ps laser
seems to play a very minor role, if any.
Keywords: Back-Surface-Field, c-Si, High Deposition Rate, Laser Processing, PECVD, Recombination, PERC
1
INTRODUCTION
The PERC solar cell technology demonstrated its
potential for industrial applications when several
independent solar cell producers and research institutes
reached efficiency values above 20% for large areas
[1-3]. PERC record efficiencies up to 21.0% [4] have
been reported when applying screen-printed front and
rear contacts. However, the gap between industrial and
lab PERC/PERL technologies achieved in the 90’s [5] is
still around 4% absolute. The advantages of lab cells lay
mainly on a) higher light trapping, b) lower bulk
resistivity for better carrier collection with simultaneous
higher bulk carrier lifetime enabling open circuit voltages
above 700 mV, c) use of a (local) high-low junction at
the rear side enabling lower contact resistance and better
carrier collection, therefore and additionally allowing d)
lower rear metal fraction (<1%) with evaporated Al.
When applying Local Contact Opening (LCO) as
contact solution laser ablation is usually preferred. Ultrashort pulse times in picosecond (ps) and femtosecond (fs)
range benefit from negligible electron-lattice interaction
in comparison to nanosecond (ns) pulses, thus avoiding
thermalisation and therefore damage of the Si crystal
[6-8]. Du et al. obtained similar cell results with
nanosecond (ns) and picosecond (ps) laser when opening
Al2O3/SiN, although for ns laser damage recovering was
necessary [9]. Ideal ablation conditions were also found
for sub-silicon bandgap (800 nm) and 50 ps laser [10].
The previous literature data refer to Gauss profiles. In
this work we propose ns laser with rectangular shaped
Flat-Top spots for the local contact opening of rear side
Al2O3/SiN passivation layers. We characterize the quality
of our contacts by determining both surface
recombination and resistance at the contacts.
2
EXPERIMENTAL
2.1 Sample preparation
We use Cz silicon wafers, 1.5 to 2 Ù cm p-type
doped, with surface exposed to the (100) orientation. Due
to wafer availability we use two types of material with
similar bulk resistivity but different electronic quality.
The wafers are submitted to a 65 Ù/sq emitter diffusion
and emitter etch back for gettering purposes and to match
the thermal budget of standard industrial cell processing.
Isotropic alkaline etching produces a planar surface with
truncated pyramids. We prepare symmetrical test
structures consisting of passivation, line-shaped LCO,
and screen-printed aluminium paste with defined area.
The Al paste is commercially available for PERC cell
applications and contains less than 2.5 wt. % of glass frit.
Structures without LCO and/or without printing are used
to support the posterior characterisation. After firing, we
measure the total dark resistance between both sides of
the wafer. Subsequently, we perform chemical removal
of the aluminium paste and measure the effective lifetime
ôeff by means of Quasi Steady State Photoconductance
(QSS-PC) technique [11].
For surface passivation we use Manz PECVD
Vertical Coating System, VCS 1200 tool. This equipment
uses inductively coupled High Pressure Plasma (HPP)
source and processes the silicon wafers vertically without
the aid of any pins as holders, thus allowing a shadow
free and homogeneous coating. The deposition of
Al2O3/SiN multilayer with typical layer thicknesses takes
place without breaking the vacuum in less than 29 s.
Additional handling, loading, and unloading steps cause a
cycle time of less than 32 s for eight simultaneously
coated wafers, ensuring a throughput of at least 900
wafers per hour (wph). The coating of SiN for the front
side passivation purposes is designed for 1200 wph. For
more details about the VCS 1200 tool see for example
references [12, 13].
The local contact opening takes places in Manz Laser
Ablation System LAS 2400, equipped with 40 ns laser
working at 532 nm. A Flat-Top rectangular shaped spot
ensures homogeneous ablation of the dielectric
passivation layer. The width and the length of the spot
can be tuned from 10 to 100 µm by modifying
accordingly the optical path of the laser beam. Fig. 1
shows exemplary the simulation, measurement and result
on a real silicon wafer coated with Al2O3/SiN of an
opened contact with the LAS 2400 tool.
As reference for the Manz ns laser with Flat-Top
profile we use ps laser also at 532 nm and Gauss beam
profile.
2BV.2.8 Presented at the EU-PVSEC Paris, September – October 2013
velocities at the contact regions, Scont, and at the
passivated regions, Spass, is provided by Fischer [14]:
a)
 R  W
1
S eff   s


D
f
S
cont

RS ,lines
c)
 

 2 cosh a  1 
W



4
W
p
  W 1  e  p

ln 


2 
a

cosh
1 


4W






(3)
with:
 a
tanh
 4W
Figure 1: a) Simulation for the production of
40 x 120 µm² Flat-Top laser beam and b) its measured
intensity profile for the 40 µm side. c) Example of a
75 x 75 µm² Flat-Top spot with ns laser applied on a
shiny etch c-Si wafer coated with Al2O3/SiN.
Table 1. Firing profiles used in this work.
(1)
(2)
(3)
(2)
where f refers to the metallisation fraction and ñ to
the wafer resistivity. Rs is the contribution of the bulk to
the series resistance, accounting for lateral transport. For
line pattern this parameter is described by Plagwitz in
reference [15]:
b)
Profile
1
S

  pass
1
f

Belt
Speed
[cm/s]
500
500
700
Set Peak
Temperature
[°C]
870
800 (with Plateau)
800
Meas. Peak
Temperature
[°C]
750
715
700
2.2 Determination of Spass and Scont
We measure lifetimes at an injection level Än = 1014
cm-3 to extract the effective surface recombination
velocity, Seff, by using:
1
 eff

1
 bulk

W
W2
 2
2 Seff  Dn
(1)
where W is the wafer thickness, D the diffusion
coefficient, ôeff the measured effective lifetime, and ôbulk
the bulk lifetime. The latter is determined for the Cz
wafers as follows. We apply Al2O3/SiN annealed 430°C
for 5 min, obtaining surface recombination velocities
below 7 and 2.5 cm/s on Float Zone wafers with
resistivity of 1 and 2 Ù cm, respectively. Extracting this
contribution to the total recombination we estimate ôbulk
to be 100 ± 10 µs for the first set of Cz samples and
200 ± 20 µs for the second set of Cz samples. A typical
approach to decouple the surface recombination
 1
 
2

(3.1)
Condition (3.1) implies that the contact line width, a,
has to be smaller than the wafer thickness W for equation
(3) to be valid. The line distance is denoted by pitch p.
Since our samples are symmetrically contacted we should
apply equation (2) and (3) two times with wafer widths
equal to W/2. At this wafer position we assume an
equipotential surface. A more simplified model is given
by the small scale case [16], which assumes spatially
homogeneous minority carrier concentration at the
surface:
S eff , small case  f S cont  1  f  S pass
(4)
The first two methods (applying equation 3.1 one
time with W or two times with W/2) are consistent within
each other and deliver slightly higher Scont values than the
small scales case method. However for those data in
which Spass approaches Scont the equation (2) is invalid and
produces even negative values of Scont. Since these
discrepancies vanish with equation (4), and since the
differences between equation (2) and (4) are within the
dispersion of our data we plot values of Scont in Fig. 5
using equation (4) only.
2.3 Specific contact resistance
We measure the total contact resistance as sketched
in Fig. 2. Assuming no resistive losses within the Al
paste bulk (therefore assuming electrostatic equipotential
surface at the contacts) and neglecting the contribution to
the bulk series resistance it follows:
Rcont 
1
Rmeasured  Rbulk  Acont
2
(5)
where Rmeasured is the measured resistance (in Ù), Rbulk is
the contribution of the series resistance of the bulk
according to equation (3), and Acont is the metallised area
per side. Proper accounting for Rbulk results in negative
values of Rcont. Therefore, we neglect this term in
equation (5). In order to understand this phenomenon
more investigations are required.
2BV.2.8 Presented at the EU-PVSEC Paris, September – October 2013
width ratio tends to a value equal to 1. For the coldest
profile (3) we do not observe neither broadening of the
line width nor any other surface or edge modification in
the whole line width range investigated. Moreover, noncontact 3D laser profiling measurements confirm a
deepening of the contacts for profiles (1) and (2), but not
for profile (3). This makes us assume that no appreciable
Al-Si liquid phase formation takes place during the firing
with profile number (3). Therefore, the formation of a
BSF layer for this third profile is unlikely.
Figure 2: Schematic of the measurement system for the
determination of the specific contact resistance, Rcont.
3
RESULTS
3.1 Broadening of the contacts after firing
During the firing a melting pool with Al-Si liquid
phase grows laterally and vertically. The extension, i.e.,
width and length of this melting pool depends on the
temperature, duration, opening geometry, and material
properties, like the Al paste solubility [17]. During the
cooling phase the silicon solidifies epitaxially with
incorporated doping Al atoms, thus forming a Si:Al(p+)
Back Surface Field (BSF). Consequently, a condition for
the formation of BSF layer implies observing a
modification of the local contact opening, e.g. by
broadening the contacts, as it has been reported in Ref.
[18].
3.2 Results on Spass and Scont
Figure 4 shows the surface recombination velocity
Spass obtained by Al2O3/SiN passivation after firing with
and without Al paste for the three firing profiles
described above. The surface passivation degrades clearly
with the thermal budget applied during the firing. Lowest
surface recombination velocity at a minority carrier
density of 1014 cm-3 is 15 cm/s for profile (3).
Additionally we observe lower recombination velocities
for samples that are Al printed than for those samples
fired without printing. This behaviour contradicts our
expectations that the glass frit contained in the paste
would etch part of the dielectric layer. Continuous
development of our PECVD tool lead us to improved
Spass values for new firing conditions suitable for
producing a melting pool. In that case we obtain after
firing, without printing, 5 and 8 cm/s for FZ 1 Ù cm and
Cz 1.5 Ù cm, respectively. The values of those newer
layers are not implemented for the experiments of this
work.
250
14
-3
Cz-Si 1.5 Ù cm at Än = 10 cm
(1) 750°C peak
200
S pass [cm/s]
Ratio of line width
(after firing/ after laser ablation)
4
3
(2) Plateau + 715 °C peak
2
Not Printed
150
Printed
100
50
1
(3) 700°C peak
0
(1)
750°C
0
0
20
40
60
80
100
120
Line width after laser ablation [µm]
Figure 3: Broadening of the contacts for three different
firing profiles (1, 2, and 3). Labelled are the measured
peak temperatures. Profile (2) includes a previous extra
long Plateau at T > 577 °C.
Figure 3 shows the fired to ablated line width ratio,
determined from averaged measurements in light
microscope images. Depending on the firing profiles
(refer to Table 1) we identify two different regimes. For
those with sufficient thermal budget broadening of the
contacts takes place due to the formation of Al-Si liquid
phase. Refer to profiles (1) and (2) in Fig. 3. The fired to
ablated line width ratio decreases drastically with the line
width up to around 35 µm, and settles at a level of 1.15
and 1.3 for profiles (1) and (2), respectively. For
sufficient wide lines the microscope images still show
broadening and softening of the line edge, but at a much
slower growth rate. In this case the fired to ablated line
(2)
715°C
(3)
700°C
Figure 4: Surface passivation provided by the Al2O3/SiN
multilayer after firing at different conditions: 1, 2, and 3,
as specified in Table 1.
Figure 5 shows decreasing surface recombination
velocity at the contacts, Scont, with the opened line width.
Values as low as 400 cm/s for line widths above 100 µm
are obtained using the ns Flat-Top laser with firing
profile (1). Similar contact recombination velocities have
already been reported with ps laser contact opening [18].
The low thermal budget profile number (3), however,
results in very high contact recombination velocity above
2000 cm/s for both ns laser and ps reference laser. Profile
(2) with ns laser performs the best Scont values for line
widths below 40 µm. In that case the lowest values are
around 600 cm/s. It is known that the geometry of the
local contact opening strongly influences the thickness of
the built-in BSF layer during the firing, while thicker
BSF layers are preferred to reduce the surface
recombination at the contacts [17-19].
2BV.2.8 Presented at the EU-PVSEC Paris, September – October 2013
Figure 6 shows the microscope images of the contacts
for both ns and ps laser ablated films subjected to profile
(3) once the Al paste is etched. Apart from small rests of
paste the dielectric layer is intact. The openings appear to
be unmodified after the firing except for Al spikes
distributed along the laser lines. For ns laser ablation with
Flat-Top those spikes appear sporadically and are
irregularly distributed, whereas for ps laser with Gauss
profiles they are clearly located at the overlapped regions
of two consecutive spots. This suggests that defect-rich
regions produce either release of silicon or faster
propagation of aluminium resulting in the formation of
spikes.
100000
(1) 750°C - ns
(2) 715°C + Plateau - ns
(3) 700°C - ns
(3) 700°C - ps
Scont [cm/s]
10000
1000
As for the ps laser ablated samples fired with the
weak profile (3) in Fig. 3, the contact resistances reach
values of about one order of magnitude higher than those
performed with the ns laser under the same firing
condition.
3.4 Al-Doping during laser ablation
We have analysed the effect of the low temperature
firing profile (3) on the contact broadening, Scont, and
Rcont, and compare it for ns and ps laser. Since this profile
does not change the opening widths we can assume that
there is almost no Al-Si alloy that forms an Al+-BSF
layer coming from the screen printed paste.
Consequently, both recombination velocity and contact
resistance should be higher than for those profiles
producing broadening of the contacts. This is observed in
Rcont for ps laser, but not for ns laser and may be
explained by laser doping of aluminium from the
Al2O3/SiN layer as reported in [22]. Irradiation of Al2O3
using a 1064 nm laser with pulse energy around 4 to
5 J cm-2 results in a doping level of 5 x 1019 cm-3 and
2 µm BSF [23]. Other authors report sheet resistance
values as low as 180 Ù/sq with ns laser at 355 nm, while
ps laser at 532 nm produces only very shallow doping
profiles with more than 6000 Ù/sq [24].
0
20
40
60
80
100
120
140
Line width after firing [µm]
Figure 5: Surface recombination velocities at the
contacts for different firing profiles. Nanosecond laser is
equipped with Flat-top generator while picosecond laser
uses a Gauss profile.
a)
b)
Specific contact resistance [Ù cm²]
1.000
100
(1) 750°C - ns
(2) 715°C + Plateau - ns
(3) 700°C - ns
(3) 700°C - ps
0.100
0.010
0.001
0
20
40
60
80
100
120
140
Line width after firing [µm]
Figure 7: Contact resistance determined using equation
(5).
Figure 6: Microscope images for the laser ablated
dielectrics fired with Al paste, firing profile (3). Then the
paste is removed. a) ns laser with Flat-Top, average width
38 µm. b) ps laser with Gauss profile, average width 70
µm.
3.3 Results on specific contact resistance, Rcont
Figure 7 presents the contact resistance at the Al-Si
interface for the same samples described in Fig. 5. We first
analyse the samples processed with ns laser. In general the
contact resistance values range from 5 to 20 mÙ cm2 and
increase slightly and almost monotonically with the line
width. The observed values agree quite well with the one
published by Rohatgi et al., reporting 10 mÙ cm2 [20].
However, Gatz et al. reported 55 mÙ cm2 determined by
pitch variations of PERC solar cells [21]. The root cause
for the large deviations in reported contact resistances for
local Al contacts is unclear, so far.
The weak firing profile (3) applied to ps laser opened
regions produces slightly higher surface recombination
and about 10 times higher contact resistance. We support
the idea that the observed Al thermal diffusion for ns
laser pulses does not take place in ps laser pulses [25].
In order to confirm the doping we treated fields of
15 x 150 mm² overlapped pulses and measure the sheet
resistance with a for point probe. Fig. 7 plots the
measured values for different Al2O3/SiN layers. Those
are coated on n-type Cz, KOH etched, wafers in order to
avoid measuring the parallel sheet resistance of the
substrate and the diffusion. The term “overlap fraction” is
defined by (1- p/d ), where p is the pitch of the laser and
d the laser beam diameter. The sheet resistance is reduced
with the Al2O3 thickness. For layers with thickness
equivalent to those used in Fig. 5 for the determination
of Scont we would then obtain a sheet resistance around
800 Ù/sq, whereby typical high-low junctions used to
reduce contact recombination are well below 200 Ù/sq.
Electrically active doping profiles from Electro
Capacitance Voltage (ECV) on the same samples show
2BV.2.8 Presented at the EU-PVSEC Paris, September – October 2013
junction depths of around 1 µm and surface doping
densities up to 1020 cm-3. Those profiles correspond to
sheet resistance values that are about one order of
magnitude lower than the ones obtained by the four point
probe. The source of these strong discrepancies between
both methods could be a) bad contact during the four
point probe due to a possible oxidation of the silicon
wafer after laser irradiance, or b) interrupted or
inhomogeneous laser diffusion due to ablation of regions
that are already diffused.
Ideally, the most accurate determination of the sheet
resistance under the laser ablation opening should take
into account the same geometry used for the opening
purposes, i.e. one should measure the sheet resistance
under one ablated line.
850
Sheet resistance [Ù/sq]
12 nm Al2O3
750
18 nm
650
550
450
37 nm
350
Laser 532 nm, 40 ns
250
0.0
0.1
0.2
0.3
0.4
0.5
0.6
Overlap fraction
Figure 7: Sheet resistance after laser irradiation on
different Al2O3/SiN dielectric layers. SiN thickness is
105 nm.
5
CONCLUSIONS
We opened Al2O3/SiN multilayer with nanosecond
laser and Flat-Top profile and applied Al printing and
firing. We confirmed an already published trend of
decreasing surface recombination with the line width,
achieving Scont values as low as 400 cm/s for sufficiently
wide line widths, and as low as 600 cm/s for widths of
only 40 µm (after firing). On the contrary, the contact
resistance tends to grow slightly with the line with. Low
values of these two parameters are due to an Al-BSF
layer, either grown after Al-Si re-solidification after
firing or by laser doping before firing. We show
evidences of laser doping for ns laser, whereas this effect
was not observed with ps laser ablation.
ACKNOWLEDGEMENTS
This work was funded by the German federal
ministry of education and research (BMBF) within the
project “FeinPass” under the contract no. 13N11621.We
kindly thank Fraunhofer ISE for wafer preparation and
interesting discussions, especially thanks to Pierre SaintCast and Marc Hofmann. We also acknowledge
Christopher Jahn and Thomas Wahl for the design of
Flat-Top profiles.
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