The Zebra Cell Concept - ISC Konstanz

THE ZEBRA CELL CONCEPT - LARGE AREA N-TYPE INTERDIGITATED BACK CONTACT SOLAR
CELLS AND ONE-CELL MODULES FABRICATED USING STANDARD INDUSTRIAL PROCESSING
EQUIPMENT
Andreas Halm 1 , Valentin D. Mihailetchi 1 , Giuseppe Galbiati 1 , Lejo J. Koduvelikulathu 1 , Razvan Roescu 1 , Corrado
Comparotto 1 , Radovan Kopecek 1 , Kristian Peter 1 , Joris Libal 2
1. International Solar Energy Research Center (ISC), Konstanz, GERMANY
2. Silfab S.p.A., Padova, ITALY
ABSTRACT: In this paper we present relevant characteristics of our Zebra cells and one-cell modules. The Zebra cell
is an interdigitated back contact (IBC) solar cell produced at ISC Konstanz using only industrially proven
technologies and standard industrial size 156x156mm² n-type Cz wafers. Wafers of different suppliers have been
evaluated. The best efficiencies achieved so far on a batch of 21 wafers are 21% average (champion cell: 21.3%).
Reverse bias electroluminescence (ReBEL) measurements reveal the unique breakdown behavior of the Zebra cell.
Due to its alternating p + n + regions on the rear, a homogenous breakdown along the parallel junctions is observed
which does not lead to local shunting. The Zebra cell can be stringed using standard soldering or bonding with
conductive adhesives. Cell to module losses are comparable to standard p-type Al BSF cells. Due to its open rear side
structure, the Zebra cell is bifacial. When testing one-cell modules with transparent and black backsheet outdoors our
measurements show an average rise in power output of 13% relative for the bifacial module compared to its
monofacial reference.
Keywords: IBC solar cell, n-type, bifacial, industrial process, Zebra cell
1 INTRODUCTION
The first silicon solar cell was produced in 1954 with
IBC architecture. It was realized on an n-type substrate
and had an efficiency of 6.35 % [1]. The introduction of
this concept to industrial mass production followed
roughly 30 years later by Sunpower Corp.. They fabricate
on 125×125 mm² high quality n-type monocrystalline
wafers using special equipment and processes stemming
from microelectronics [2].
Our bottom up approach to the IBC cell concept is
uniquely different. We use industrial processing
machines which are state of the art in standard cell
production. Our processing sequence can be implemented
in a standard p-type production line by simply adding
further equipment to perform five more processing steps
as compared to a standard Al BSF cell [3, 4]. The only
specialized equipment required for our process is a BBR 3
diffusion furnace which is already used for advanced
solar cell production. Following this road we reached
21.3 % cell efficiency on the full area of a 156x156 mm²
n-type Cz substrate. This is – to our knowledge – a world
record and also the only type of IBC solar cell fabricated
on such a large substrate. Additionally, the Zebra cell can
be operated in bifacial mode which enhances its effective
conversion efficiency even further.
In the following we present latest efficiency results on
wafers of different suppliers and base resistivity. The
reverse characteristic of the Zebra cell is discussed since
it is an important factor in module assembly. Being aware
that not only the cell but also the module performance is
relevant in application, we address important points in
respect to module assembly. We show cell to module
losses for different interconnection schemes and results
of the bifacial versus monofacial outdoor performance of
our one-cell Zebra modules.
2 EXPERIMENTAL
Figure 1 shows a cross section image of our Zebra
cell. The production sequence only contains steps already
proven in industrial mass production of solar cells.
Further detail on cell architecture, the process steps and
optimization can be found elsewhere [3, 5].
Figure 1: schematic cross section of the Zebra cell
showing diffused regions, passivation layers and metal
fingers.
2.1 IV results
In order to test process reliability, tolerance against
material quality and base resistivity different batches of
equal wafers are processed at ISC Konstanz. Table 1
shows average IV results of our two latest runs.
Table 1: average IV results and pseudo fill factors with
standard deviations for Zebra cells made from different
materials.
supplier /
cells
resistivity
( Ωcm )
J SC
(mA/cm² )
V OC ( mV )
FF ( % )
pseudo-FF
( % )
η ( % )
A:
21 cells
B:
10 cells
C:
28 cells
A:
21 cells
2.2 3.5 3.7 5.8
40.4
± 0.1
646.3
± 0.9
78.8
± 0.9
82.1
± 0.3
20.6
± 0.2
41.3
± 0.1
647.8
± 1.0
78.6
± 0.3
82.0
± 0.3
21.0
± 0.1
41.0
± 0.1
647.4
± 0.7
78.5
± 0.3
81.9
± 0.2
20.8
± 0.1
41.8
± 0.1
646.7
± 0.8
77.9
± 0.9
82.9
± 0.4
21.0
± 0.2

Average cell results range between 21.0% and 20.6% for
the different materials. Wafers of material A and B with
5.8 Ωcm and 3.5 Ωcm base resistivity reach the highest
average efficiency with 21% and a record cell with
21.3% efficiency which is not yet independently
confirmed. Table 2 shows the IV parameters of our
champion cell.
Table 2: IV parameters champion cell.
J SC
(mA/cm 2 )
V OC
( mV )
FF
( % )
The lowest performing batch from supplier A still shows
20.6% average efficiency. This leads to the conclusion
that the material, not the process was the limiting factor.
It is notable that for all batches the standard deviation of
efficiency is very low. This proves the stability of our
process. SunsV OC measurements reveal high pseudo fill
factors (PFF) proving that our device does not suffer
from shunting. V OC values are nearly identical for all
batches independent of the material. The limiting factor is
the metal contact with the p + emitter. During the sintering
process, metal stemming from the screen printed paste
penetrates into the emitter region and thus limiting V OC
[6]. The dependence of fill factor and J SC on the base
resistivity can be observed clearly comparing the two
batches of supplier A. The 2.2 Ωcm batch features the
highest fill factors but also the lowest J SC while the 5.8
Ωcm batch behaves vice versa. This behavior can be
easily explained regarding the lateral carrier transport
over long distances in IBC solar cells which was already
discussed earlier [5].
2.2 Reverse characteristics
Due to its alternating p + n + regions on the rear, the
reverse behavior of the Zebra cell differs from a standard
cell with a pn + junction. In the following, reverse dark IV
measurements and ReBEL give more insights.
2.2.1 Reverse dark IV measurement
pseudo FF
( % )
η ( % )
41.9 647.9 78.5 82.8 21.3
Figure 2: reverse dark IV characteristics of a Zebra cell.
Figure 2 shows the reverse dark IV curve of a Zebra cell.
The onset of the breakdown begins early, already at -5.2
V the reverse current exceeds 3 A. Common restrictions
of module producers are that the reverse current at -12 V
should be lower than 3 A. Our cell would not pass this
restriction but due to the special architecture of the Zebra
cell its validation is not needed. In the case of a
breakdown, the vertically parallel p + n + regions on the rear
side of the Zebra cell behave differently. The contact of
the two highly doped regions enables the carriers to
tunnel directly from band to band [7] leading to
homogenous heating of the cell in contrary to local
hotspot formation and irreversible shunting occurring in
standard cells. ReBEL images (Figure 3) of the same cell
prove the homogenous breakdown.
2.2.2 ReBEL images
Figure 3: big picture: reverse EL image of a Zebra cell
biased with -6.6 V and -9.9A current flowing, the parallel
line structure shows the breakdown along the junctions;
small picture: magnification of the green bordered region;
overlay of the ReBEL image (red) and a camera image
(cyan) depicting the metal fingers on the rear of the cell.
ReBEL pictures are recorded with our EL/Pl system
developed in house. The big picture in figure 3 shows a
Zebra cell operated in reverse (bias: -6.6V current flow:
9.9 A). The ReBEL signal is structured mostly in parallel
lines along the p + n + junctions. The small picture in the top
left corner of figure 3 shows a magnification of the signal
in the green bordered area. Here, the ReBEL signal is
depicted in red, a photo of the same region of the cell is
overlaid in cyan showing the metallization pattern. It can
be observed that the ReBEL signal shows a homogenous
breakdown along the individual junctions located parallel
to the finger grid. The different spacing between
breakdown sites and the metallization fingers rises from
the cell geometry. The base (n + ) contact fingers are
located nearer to the breakdown sites since the base width
is smaller than the emitter width. It is clearly visible
though that none of the metal fingers are the source of a
breakdown site. This unique characteristic could abolish
the need of mounting bypass diodes in a Zebra module
since the individual cell´s reverse behavior leaves it
unharmed when operating with high reverse currents in
the region of the modules I MPP .

Table 3: IV results before and after 9 hours of
continuous reverse current flowing (-9.9A, -6V).
cell
cell 1,
t = 0 h
cell 1,
t = 9 h
cell 2,
t = 0 h
cell 2,
t = 9 h
J SC
(mA/cm 2 )
V OC
(mV)
FF (%) η ( % )
40,4 641 78,6 20,3
40,3 639 78,6 20,2
40,2 642 79,0 20,4
40,1 638 78,1 20,0
Table 3 shows IV measurements of two Zebra cells under
1 sun illumination taken before and after 9 hours of
continuous reverse current flow of 9.9A. During this time
the cells heated up to a maximum of 100 °C. Cell 1 does
not deteriorate at all for the whole period of time; its FF
stays constant while J SC and V OC change in the range of
the measurement error. Cell 2 shows a 0.9 % absolute
reduction of the fill factor. EL control pictures taken for
this cell before and after the 9 hours reverse bias period
though are identical.
2.3 Cell to module loss
Interconnection of the Zebra cells and assembling
them into modules is important to address since the rear
contacted cell structure poses new challenges. The
current rear side of the Zebra cell contains eight busbars;
four for each polarity. We evaluated different
interconnection schemes. In the following, cell to module
losses for soldering and bonding of conductive adhesives
are regarded in respect to the resulting module
performance. One-cell modules are assembled using the
standard module sandwich structure; the layer stack is
glass – EVA – cell – EVA – backsheet. IV measurements
before and after module assembly are evaluated in
respect to fill factor loss. The fill factor is the most
critical loss when evaluating different interconnection
schemes since it is stemming from the contact of the tabs
with the busbars.
Table 4: relative FF loss for different interconnection
schemes.
interconnection type Δ FF (%)
bonding A 4.12
bonding B 2.58
bonding C 4.72
soldering 5.38
reference: soldered Al BSF cell 3.32
Table 4 shows the relative fill factor loss for Zebra cells
using different conductive adhesive bonding and
soldering of the interconnection tabs. Since the contacts
for measuring our one-cell modules are far away from the
cell itself, module fill factors are measured lower than on
cell level due to the series resistance of the connectors.
That is why table 4 includes the data of a soldered Al
BSF reference. The data shows that bonding with
conductive adhesive B results in the lowest fill factor
loss; even lower than the one for the soldered reference.
Soldering of Zebra cells still leaves room for minimizing
losses but was not yet investigated in detail. The data
reveals though that the Zebra cell is fit for module
assembly using new conductive adhesive gluing but also
classical soldering of the interconnection tabs.
2.4 Outdoor measurements of one-cell modules
After EVA encapsulation with glass on the front and
transparent or black backsheet at the rear, modules were
assembled on the rooftop of ISC Konstanz to test their
outdoor performance. One module was mounted face
down recording the current generated by the albedo light.
A pyranometer located next to the modules with an
inclination of 30°, heading south – same as the modules -
is recording light intensity. The floor underneath the
module array is covered with white backsheet (R 70 %)
to simulate a white painted rooftop. An IV curve of each
module is recorded during 10 s in a 5 minute interval.
Measurements are performed for 21 full days in may
2012. Figure 4 shows a comparison of the normalized
power output of a monofacial versus a bifacial one-cell
module for an overcast day. Normalization is done using
the P MPP measured indoors under 1 sun illumination and
standard conditions. For the bifacial module, front and
back side are characterized separately; in both cases
measured on a black background. The power ratio
(P FRONT /P BACK ) of front- and backside for the bifacial
module is 0.75.
Figure 4: comparison of the normalized power output of
a monofacial versus a bifacial one-cell Zebra module for
an overcast day; blue: P MPP /P MPP@1sun for the bifacial
module; black: P MPP /P MPP@1sun for the monofacial
module; green: outdoor light intensity measured by
pyranometer; orange: albedo contribution quantified by
I SC /I SC@1sun of the face-down mounted module.
Figure 4 shows the normalized power output over time of
both modules for an overcast day. Integrating the power
output versus the light intensity measured by the
pyranometer, the energy harvest is retrieved. For the
overcast day, the bifacial module shows 17.6 % more
power output than the monofacial one with the black
backsheet.

optimized technique in the fabrication process”, Energy Procedia
8, 2011, p. 421
[6]: L. J. Koduvelikulathu, V. M. Mihailetchi, A. Edler, C.
Comparotto, A. Halm, R. Kopecek, K. Peter, “Metallization and
firing process impact on the V OC – a simulation study”,
proceedings of this conference
[7]: K. Mangersnes “Back-contacted back-junction silicon solar
cells”, PhD. thesis, University of Oslo, October 2010
Figure 5: comparison of the normalized power output of
a monofacial versus a bifacial one-cell Zebra module for
a clear day; blue: P MPP /P MPP@1sun for the bifacial module;
black: P MPP /P MPP@1sun for the monofacial module; green:
outdoor light intensity measured by pyranometer; orange:
albedo contribution quantified by I SC /I SC@1sun of the facedown
mounted module.
Figure 5 shows the same measurement done on a clear
day. Here, bifacial module is generating 12.2 % more
power. Over the whole period 21 days the power gain for
the bifacial module with transparent backsheet comparing
to the monofacial module with black backsheet is 13 %.
This is a significant increase of the energy harvest
favoring bifacial operation of the Zebra cell. Indoor
measurements of one-cell Zebra modules using a bifacial
measurement setup as shown in [3, 4] yield in similar
results.
3. SUMMARY
We fabricated highly efficient IBC solar cells, named
“Zebra” cells, on 156 x 156 mm² n-type Cz wafers using
only industrial relevant processing equipment. We prove
stable efficiencies on batches of maximum 28 wafers
testing substrates of different suppliers and base
resistivity. So far, we reached 21.3 % maximum
efficiency with our cell concept. We showed that the
Zebra cell is ready for module assembly and presented
first results of the outdoor performance of bifacial versus
monofacial one-cell modules.
4. REFERENCES
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Silicon p-n Junction Photocell for Converting Solar Radiation
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[2]: D. De Ceuster et al., Proceedings of 22 nd EU-PVSEC, Milan
(2007), p. 816
[3]: V.D. Mihailetchi et al. “high efficiency ibc solar cells
fabricated on large area n-type silicon using industrial available
techniques”, technical digest, 21 International Photovoltaic
Science and Engeneering Conference 28.11.2011-2.12.2011,
Fukuoka, Japan
[4]:G. Galbiati, V. D. Mihailetchi, A. Halm, L. J.
Koduvelikulathu, Razvan Roescu, R. Kopecek, K. Peter, J Libal
“Large Area Back-Contact Back-Junction Solar Cells with
Efficiency Exceeding 21%”, 38th IEEE Photovoltaic Specialist
Conference 2012, Austin, USA
[5]: G. Galbiati, V.D. Mihailetchi, A. Halm, R. Roescu, R.
Kopecek, “Results on n-type IBC solar cells using industrial