N-PERT BACK JUNCTION SOLAR CELLS:
AN OPTION FOR THE NEXT INDUSTRIAL TECHNOLOGY GENERATION?
Bianca Lim*, Till Brendemühl, Miriam Berger, Anja Christ, Thorsten Dullweber
Institute for Solar Energy Research Hamelin (ISFH)
Am Ohrberg 1, D-31860 Emmerthal, Germany
*
telephone: +49 5151 999 313, fax: +49 5151 999 400, email address: b.lim@isfh.de
ABSTRACT: In this work, we present back-junction (BJ) n-PERT (Passivated Emitter, Rear Totally Diffused) solar
cells with a processing sequence based on an industrial-type p-PERC (Passivated Emitter and Rear Cell) process,
with the addition of a boron diffusion. Through this, we achieve efficiencies up to 20.5% on n-PERT BJ solar cells,
which do not degrade under subsequent illumination. For comparison, reference p-PERC solar cells fabricated on 1
and 3  cm boron-doped Cz-Si achieve efficiencies up to 20.6% before light-induced degradation (LID) and 20.1%
(3  cm) and 19.7% (1  cm), respectively, after LID. We find that the width of the laser contact opening (LCO) on
the rear strongly influences the n-PERT BJ solar cell performance. Wider LCOs significantly increase the shortcircuit current, open-circuit voltage, and pseudo fill factor, resulting in efficiency increase of up to 1.2% absolute. We
attribute this to an increased thickness and homogeneity of the Al-p+ regions beneath the contacts, which effectively
reduces recombination at the contacts. By varying the metallization fraction on the rear side, we determine the
specific contact resistance of the Al contact to be c = (8 ± 2) m cm2 and the saturation current density to be J0.met of
(320 ± 50) fA/cm2.
Keywords: n-type, Silicon Solar Cell, Recombination
1
INTRODUCTION
The vast majority of crystalline silicon solar cells are
based on boron-doped (B) silicon and feature a full-area
aluminum (Al) contact on the rear side, which results in
strong rear surface recombination. Currently, a number of
solar cell manufacturers are introducing a new solar cell
design: the Passivated Emitter and Rear Cell (PERC),
which features a full-area dielectric rear passivation and
only local Al rear contacts (see Fig. 1(a)). Through this,
recombination losses are strongly reduced and higher
open-circuit voltages can be obtained. In addition, the
presence of the dielectric increases the rear side
reflectance, thus increasing the generated current in the
solar cell [1].
Using B-doped Czochralski-grown silicon (Cz-Si),
PERC solar cell efficiencies between 20.0% and 20.9%
have been obtained by several solar cell manufacturers
[2-5]. Recently, a new record efficiency of 21.2% was
reported for industrial p-PERC solar cells [6], using a
processing sequence very similar to the one used in this
study. However, after light-induced degradation (LID)
the efficiency of PERC solar cells decreases between
0.5% abs. and 1.0%abs. depending on the wafer resistivity
[7,8].
A possibility to circumvent the detrimental effect of
LID is the use of n-type Si wafers and the concept of the
Passivated Emitter, Rear Totally Diffused (PERT) solar
cell, which typically features a boron-doped emitter at the
front and a phosphorus-doped BSF at the rear and applies
screen-printed contacts grids on both the front and the
rear side. Such bifacial PERT solar cells have so far
achieved energy conversion efficiencies up to 20.5% [9].
At the same time, n-PERT back-junction (BJ) solar cells,
which have the boron emitter at the rear as shown in Fig.
1(b) and are very similar to p-PERC solar cells in terms
of cell architecture and processing sequence, have
already achieved efficiencies up to 20.7% [10].
In this work, we process p-PERC and n-PERT BJ
solar cells in parallel and analyze the performance of
both. We determine the saturation current densities of the
passivated diffusions as well as of the contacted areas. In
addition, we determine the specific contact resistance of
the Al contact on the rear side by varying the rear side
metallization fraction from 3% to 30%.
2
Figure 1: Schematic drawing of (a) p-type PERC solar
cells and (b) n-type PERT back-junction solar cells.
SOLAR CELL PROCESS
For the p-PERC reference solar cells, we use 239 cm2
B-doped Cz-Si wafers with resistivities between 1 and 3
 cm. The n-PERT BJ solar cells are fabricated on 6 
cm P-doped Cz-Si (also 239 cm2). After damage etching,
the n-PERT BJ solar cells undergo a BBr3 quartz furnace
diffusion. Subsequently, the rear side of all solar cells (pPERC and n-PERT BJ) is coated with a protection layer,
which acts as etching and diffusion barrier in the
following alkaline texturing and phosphorus diffusion.
After a POCl3 quartz furnace diffusion with a sheet
resistance of 80 /sq., the protection layer and the
phosphorus glass are removed by wet chemistry and the
Table I: Processing steps for p-PERC and n-PERT BJ
solar cells. Blue processing steps are identical for both
solar cells concepts.
p-PERC
n-PERT BJ
Wafer cleaning
Wafer cleaning
Rear protection layer
Rear protection layer
B-diffusion
Texturing
Texturing
P-diffusion
P-diffusion
PSG + dielectric etch
PSG + dielectric etch
Passivation
Passivation
Laser contact opening
Laser contact opening
Screen-printing
Screen-printing
Co-firing
Co-firing
rear side is passivated using a stack of atomic layer
deposited Al2O3 and plasma-enhanced chemical vapor
deposited (PECVD) SiNx. Then, the front side is
passivated with PECVD SiNx. Laser contact openings
(LCO) are formed on the rear side using a picosecond
laser with 532 nm wavelength. For the silver front side
metallization we use print-on-print as a fine line printing
technique. The rear is full-area printed with a
commercially available Al paste which has been
specifically designed for p-PERC cell applications. Both
the front and the rear contacts are fired in a single step.
An overview of the processing steps is given in Tab. I. As
can be seen, the only difference in the processing
sequence of p-PERC and n-PERT BJ cells is the
additional boron diffusion.
3
IMPACT OF REAR CONTACT WIDTH
Despite the strong resemblance of p-PERC and nPERT BJ solar cells, there are some notable differences,
especially in mode of operation. In n-PERT BJ solar
cells, the Al-p+ region underneath the Al rear contacts
acts as part of the rear-side emitter, in particular since the
B-diffusion profile in that region is completely removed
through the Al-Si alloying process during the firing step.
As a consequence, a continuous Al-p+ region of good
quality is crucial for n-PERT BJ solar cells in order to
avoid shunts or enhanced space charge region
recombination. In contrast, in p-PERC solar cells the Alp+ acts as a back surface field (BSF), which repels
minority charge carriers from the rear contacts. The AlBSF is not critical for shunts and enhanced rear contact
recombination would not impact the diode quality factor.
Also, the boron-diffused emitter has a much higher
conductivity than an average p-type wafer, allowing for
wider contact spacing on the rear side for n-PERT BJ
cells in comparison to p-PERC cells.
As a consequence, we investigated the impact of
different LCO widths on the performance of n-PERT BJ
solar cells. We kept the total metallization fraction of the
rear constant at 10%, adjusting the contact spacing
accordingly. After firing, the width of the rear contacts
increases by 20 µm to 30 µm due to the alloying process,
as has been reported before [11].
Figure 2(a) depicts the dependence of the efficiency
 on the relative LCO width, where 1 corresponds to the
LCO width used for the p-PERC baseline process at
Figure 2: (a) Efficiency , (b) open-circuit voltage Voc,
(c) short-circuit current density Jsc, and (d) pseudo fill
factor pFF of n-PERT BJ (red symbols) and p-PERC
solar cells (blue symbols) as a function of relative laser
contact opening (LCO) width.
ISFH. The red symbols correspond to n-PERT BJ solar
cells, whereas the blue symbols refer to p-PERC solar
cells. Each data point represents one solar cell, however,
the same trend is observed for the average value of 5
solar cells. In Fig. 2(b) the dependence of the open-circuit
voltage Voc is displayed, Fig. 2(c) shows the short-circuit
current density Jsc and Fig. 2(d) the pseudo fill factor
pFF.
For the n-PERT BJ solar cells, all parameters show a
strong increase with increasing LCO width: Voc increases
from 650 mV to 667 mV, while Jsc increases from 38.4
mA/cm2 to 38.9 mA/cm2. At the same time, the pseudo
fill factor pFF increases from 81.0% to 83.0%. In total,
these gains result in an efficiency increase from 19.4% to
20.6%. However, beyond a relative LCO width of 2.5 the
efficiency decreases due to an increased series resistance
caused by the wide rear contact spacing. In p-PERC solar
cells, on the other hand, Jsc and pFF are unaffected by the
LCO width while Voc changes by 2 mV. While this is
close to the measurement uncertainty, it is also
comparable to the change observed for n-PERT BJ solar
cells.
For further characterization, we measured JscVoc
curves and fitted them using the two-diode-model, thus
obtaining J01 and J02. Figure 3 depicts the JscVoc curve of
a p-PERC (black circles) and n-PERT BJ (red diamonds)
solar cell. In addition, the slope of a diode with an
ideality factor of 1 (dashed blue line) and 2 (dashed green
line) is shown. The data of the p-PERC solar cell closely
follows a diode with an ideality of 1, resulting in a low
J02 value of 1 nA/cm2. In contrast, the n-PERT BJ solar
cell converges to a diode with ideality factor of 2 at
voltages lower than 550 mV, which corresponds to larger
J02 values of 5 nA/cm2 at optimal LCO width and up to
20 nA/cm2 at narrowest LCO width. The increase of J02
increases the ideality factor at maximum power point and
thus decreases the pFF.
At the same time, we observe that J01 increases from
200 fA/cm2 at optimal LCO width to 300 fA/cm2 at the
narrowest investigated LCO width, which corresponds to
the decrease of Voc with decreasing LCO width.
Further investigations using scanning electron
microscopy (SEM) indicate that narrower LCOs more
often exhibit thinner or even missing Al-p+ regions, in
particular in combination with voids (i.e. missing Al-Si
eutectic beneath the Al). Wider lines, on the other hand,
exhibit continuous and homogeneous Al-p+ regions, even
in the presence of voids, which is similar to the results
reported in Ref. [12].
The increased J01 term could thus be a result of
increased recombination at the Al contact, caused by an
increased minority-carrier gradient in the emitter due to
the thinner Al-p+ region. The increased J02 term, on the
other hand, indicates recombination in the space charge
region between n-type wafer and p+-type Al-emitter, and
thus indicates the presence of defects in the Al-p+ region.
Note that such defects would not induce increased J02
terms in p-PERC solar cells, since the Al-p+ acts as a
BSF.
However, even at optimal LCO width, J02 of the nPERT BJ solar cells is about 3 times higher than for the
p-PERC solar cells, resulting in 0.6% (absolute) lower
pFF. Further improvement of the quality of the Al-p+
region will thus be important to fully realize the potential
of n-PERT BJ solar cells.
The results of the best solar cells are summarized in
Tab. II. Using 1  cm p-Si as well as 3  cm p-Si, the pPERC solar cells achieve energy conversion efficiencies
up to 20.6% (before light-induced degradation), whereas
the n-PERT BJ solar cells yield up to 20.5%
(independently measured at ISE CalLab). While the
Figure 3: JscVoc curves of a p-PERC (black circles) and
n-PERT BJ (red diamonds) solar cell. The dashed blue
line indicates the slope of a diode with an ideality factor
of 1, whereas the green dashed line indicates the slope of
a diode with an ideality factor of 2. The p-PERC solar
cell follows the n = 1 curve closely, whereas the n-PERT
BJ solar cell starts to converge to n = 2 around 0.55 V.
As a consequence, the pseudo fill factor pFF of the nPERT BJ solar cell is lower than for the p-PERC solar
cell.
short-circuit current density Jsc is very similar for both
solar cell types, the open-circuit voltage Voc and fill factor
FF differ slightly. The p-PERC solar cells feature a lower
Voc but a higher FF. As already discussed above, the
higher FF is not exclusively due to a lower series
resistance but is also a result of a higher pseudo fill factor
pFF.
After complete LID (16 h of illumination at room
temperature for 1  cm p-Si and 32 h for 3  cm p-Si),
the p-PERC solar cells fabricated on 1  cm Cz-Si
achieve efficiencies up to 19.7% ( = 0.9%) while the 3
 cm solar cells yield 20.1% ( = 0.5%). The n-PERT
BJ solar cells, on the other hand, are stable under
illumination (efficiencies ±0.1% were measured after 16h
of illumination, which is within the measurement
uncertainty) and thus yield 0.4% higher efficiencies than
the degraded p-PERC solar cells.
Table II: Solar cell parameters of the best p-type PERC
solar cells (before and after LID) and the best n-PERT BJ
solar cell obtained from IV measurements performed at
standard testing conditions (25 °C, AM1.5G spectrum).
Best cell
η
[%]
Jsc
[mA/cm2]
Voc
[mV]
FF
[%]
p-PERC, 1  cm
20.6*
38.8*
658*
80.5*
p-PERC, 1  cm
(degraded)
19.7
38.1
645
80.1
p-PERC, 3  cm
20.6
38.9
661
80.1
p-PERC, 3  cm
(degraded)
20.1
38.8
657
79.0
n-PERT BJ, 6  cm
20.5*
38.7*
665*
79.8*
*independently measured at ISE CalLab
4 CHARACTERIZATION
CONTACTS
OF
LOCAL
AL
After finding an optimal LCO width of 2.5 times the
standard p-PERC LCO width, we further characterized
the local Al contacts and the local Al-p+ emitter on the nPERT BJ solar cells by varying the rear contact pitch and
consequently the metallization fraction f of the rear.
Changing f from 3% to 30%, we observe a strong
dependence of both the open-circuit voltage Voc and the
series resistance Rs. The Voc decreases from 662 mV to
655 mV with increasing metallization fraction f due to
increased recombination.
The overall recombination is the sum of
recombination in the base, the FSF, the emitter, and at the
contacts. By varying the rear side metallization fraction,
we are able to extract the saturation current density at the
rear contacts, assuming all other recombination currents
remain the same. This is true for all contributions except
the passivated emitter, since its fraction decreases with
increasing emitter contact area. However, the change is in
the range of 3 fA/cm2 and can thus be neglected.
Instead of converting the open-circuit voltage of the solar
cells into an overall J0, which relies on the assumption
that the ideality of the solar cells equals 1, we analyze the
J01 term of the 2-diode-model fit of the JscVoc curve. We
plot J01 as a function of f in Fig. 4. The linear fit to the
experimental data (red line) yields (320 ± 50) fA/cm2 for
the saturation current density at the rear contacts. Note
that a similar study using the standard p-PERC LCO
width yielded a higher J0.met value of 650 fA/cm2. The
latter value is in good agreement with experimental data
of the surface recombination velocity at local Al contacts
published in Ref. [12], where Smet = 400 cm/s was
reported, corresponding to a saturation current density of
approximately 750 fA/cm2.
In addition, we have measured the saturation current
densities J0 of the wafer, the passivated emitter, and the
passivated FSF using non-metallized lifetime samples
and assuming an intrinsic carrier concentration ni = 9.65
× 109 cm3. Using published data for the contacted FSF
[13] as well as for the rear surface recombination velocity
Table III: Contributions to the overall saturation current
density J0 of the different solar cell regions.
J0
[fA/cm2]
Area
fraction
Weighted
[fA/cm2]
Contact P-diff (FSF)
500
0.061
31
Pass. P-diff (FSF)
120
0.939
113
Wafer
10
1
10
Pass. B-diff (emitter)
25
0.90
22
Contact B-diff (emitter)
320
0.10
32
Total
208
n-PERT BJ
Corresponding Voc
J0
[fA/cm2]
Area
fraction
Weighted
[fA/cm2]
Contact P-diff (emitter)
500
0.061
31
Pass. P-diff (emitter)
120
0.939
113
Wafer (before LID)
40
1
40
Pass. base
20
0.89
18
Contact base
750
0.11
83
Total
285
p-PERC, 3  cm
Corresponding Voc
659 mV
of p-PERC solar cells [12] and area weighting all
contributions, we obtain J01.total = 285 fA/cm2 for the pPERC solar cells (before LID) and J01.total = 208 fA/cm2
for the n-PERT BJ solar cells (see Tab. III). Assuming a
short-circuit current density Jsc of 39.0 mA/cm2, these
J01.total values imply open-circuit voltages Voc of 667 mV
(n-PERT BJ) and 659 mV (3  cm p-Si), respectively,
which is in good agreement with the open-circuitvoltages
obtained on the finished solar cells (see Tab. II).
The decrease of the series resistance Rs from 2.25 
cm2 to 0.54  cm2 with increasing metallization fraction
is mainly due to a reduced series resistance contribution
of the emitter. However, the larger contact area and thus
a lower overall contact resistance also reduces the total
series resistance. Taking into account the resistance
contribution of the emitter Rem through two-dimensional
simulations using the conductive boundary (CoBo) model
[14] and Quokka software [15] and assuming that all
other resistance contributions remain unchanged, the
contact resistance of the local Al contact can be extracted
by plotting the difference of total series resistance Rs and
emitter resistance contribution Rem as a function of
inverse metallization fraction 1/f, as shown in Fig. 5. The
linear fit (red line) yields
Rs – Rem = 1/f × 8 m cm2 + 0.49  cm2
Figure 4: Total saturation current density J01 of n-PERT
BJ solar cells determined from the 2-diode fit as a
function of rear side metallization fraction f. The red line
corresponds to a linear fit of the data. The gradient yields
the J0 contribution of the Al contact J0.met = (320 ± 50)
fA/cm2.
667 mV
(1)
i.e. c = (8 ± 2) m cm2, which is close to what Urrejola
et al. found (8 to 18 m cm2) [16] but about a factor 5
smaller than what has been published by Gatz et al. (40 to
55 m cm2) [11].
Interestingly, Müller and Lottspeich very recently
PERT BJ solar cell, these defects would lie within the pnjunction, inducing non-ideal recombination, whereas in
p-PERC solar cells, the Al-p+ acts as BSF and defects at
the pp+-junction would contribute to J01.
For an optimal LCO width, we also determined the
saturation current density of the Al contact J0.met of (320
± 50) fA/cm2 and a specific contact resistance of (8 ± 2)
m cm2. Using the former in an analysis of the overall
recombination, we find that for our current n-PERT BJ
solar cells more than 50% of the total recombination is
caused by the passivated phosphorus-diffusion, i.e. the
front surface field. In order to further improve the nPERT BJ solar cells, improvement of the FSF is thus
mandatory.
References
Figure 5: Series resistance Rs of n-PERT BJ solar cells,
determined using the double-light-level method, plotted
as a function of inverse rear side metallization fraction
1/f. The experimental data is fitted by a linear function
(red line): Rs – Rem = 1/f × 8 m cm2 + 0.49  cm2. We
thus determine a contact resistance of the Al to the Al-p+
emitter of (8 ± 2) m cm2.
observed that previous publications might have
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absolute contact resistance per line of 0.46  cm.
Assuming the contact width of 60 µm used in their device
simulations, this corresponds to a specific contact
resistance of 3 m cm2, which is even lower than what
we found.
5
CONCLUSIONS
We have fabricated fully screen-printed n-PERT BJ
solar cells with efficiencies up to 20.5% based on a well
established processing sequence for p-PERC solar cells.
p-PERC reference solar cells made on 1  cm and 3 
cm B-doped Cz-Si, respectively, achieved efficiencies up
to 20.6%. However, during illumination at room
temperature the 1  cm p-PERC solar cell degraded by
0.9% to 19.7% and the 3  cm p-PERC solar cell
degraded by 0.5% to 20.1%, whereas the n-PERT BJ
solar was stable under illumination, thus yielding 0.4%
higher efficiency.
Varying the width of the laser contact openings on
the rear side, we observed a strong increase of opencircuit voltage, short-circuit current density, pseudo fill
factor, and consequently of solar cell efficiency, for the
n-PERT BJ solar cells with increasing LCO width.
Comparing the solar cell characteristics at narrowest
investigated and optimal LCO width, we find that Jsc
increases by 0.4 mA/cm2, Voc increases by about 15 mV,
pFF increases by 2.0%, and  increases by more than 1%
absolute.
The improved solar cell performance with wider
LCO contacts may be due to thicker and more
homogeneous Al-p+ regions underneath the Al contacts,
as indicated by SEM investigations. The increased
thickness of the Al-p+ region reduces the minority charge
carrier gradient in the p+ region, thereby reducing the
contact recombination. In addition, the reduced pFF of nPERT solar cells in comparison to p-PERC solar cells
suggests the presence of defects in the Al-p+ region. In n-
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