SCREEN PRINTED N-TYPE SILICON SOLAR CELLS FOR INDUSTRIAL APPLICATION
Valentin D. Mihailetchi1, Johann Jourdan1, Alexander Edler1, Radovan Kopecek1, Rudolf Harney1,
Daniel Stichtenoth2, Jan Lossen2, Tim S. Böscke2, Hans-Joachim Krokoszinski2
1
International Solar Energy Research Center (ISC), Rudolf-Diesel-Str. 15, D-78467 Konstanz, Germany,
Phone: +49 7531 3618348; Fax: +49 7531 3618311; Email: valentin.mihailetchi@isc-konstanz.de.
2
Bosch Solar Energy AG, Wilhelm-Wolff-Str. 23, D-99099 Erfurt, Germany
ABSTRACT: The rapid fall in module prices demands researchers to come up with substantial efficiency
improvement and cost reductions in the solar cell and module processes. This paper presents a solar cell
development on n-type Cz-Si substrates with homogeneous diffusion of boron emitter and phosphorous backsurface-field. The resulting solar cell is bifacial and it is fabricated using only industrial compatible techniques.
The best achieved solar cell efficiency, using a screen printed and firing through metallization, on 241 cm2 (total
area) wafers is 18.6%. Moreover, we present a new method to passivate boron emitters which substantially
reduces surface recombination resulting in improved VOC of the cells. We show that internal quantum efficiencies
of these n-type cells when illuminated from the front- and from the back side are identical when good quality CzSi substrates are used in the fabrication.
1
INTRODUCTION
The rapid fall in module prices leads researchers to
intensify their research on efficiency improvement of
solar cells and to further reduce the costs of the
fabrication process. Additionally, new cell concepts are
currently preparing to enter into production. One of these
cell concepts that recently receive much attention is
based on n-type silicon substrates that feature a front
boron emitter, a rear phosphorous back-surface-field
(BSF), as well as front- and rear screen printed
metallization grid. Efficiencies up to 18.5% have been
reported on large area for such n-type solar cells
fabricated using industrial applicable techniques [1], thus
exceeding the typical efficiencies currently achieved by a
similar process based on p-type substrates.
N-type silicon has long been proven to have higher
tolerance to common transition metal impurities,
potentially resulting in higher minority carrier diffusion
lengths compared to p-type substrates [2,3]. Additionally,
the minority carrier lifetime does not suffer from light
induced degradation due to the Boron-Oxygen related
defect which is commonly present in p-type Cz-Si [4]. In
spite of these fundamental material advantages, n-type
cells comprising homogeneous front emitter and rear BSF
are not readily produced industrially due to insufficient
understanding and the lack of a cost-effective production
process allowing for high conversion efficiencies. It is
crucial to improve the boron diffusion to form the
emitter, passivation of boron emitters, and metallization.
This paper presents a solar cell process based on ntype monocrystalline wafers with boron front emitter, and
low cost fabrication processes ready for industrial use
(such as screen printing and microwave plasma enhanced
chemical vapour deposition (PECVD) of hydrogenated
silicon nitride (SiNx)). Using this process we demonstrate
efficiencies of 18.6% on 156×156 mm2 monocrystalline
Cz-Si wafers. Additionally a new method to passivate
boron emitters is presented which results in an excellent
passivation quality. This new method is simpler and more
cost-effective compared to well-established methods,
such as those based on atomic layer deposition of Al2O3
[5] and thermally or chemically grown SiO2 layers [6-8].
We also show that internal quantum efficiencies of these
n-type cells when illuminated from the front- and from
the back side are identical, if the process conditions are
adjusted with the aim to obtain bifacial cells.
2
EXPERIMENTAL DESIGN
We developed our n-type process on 156×156 mm2
phosphorous-doped monocrystalline Czochralski (Cz) Si
wafers with base resistivity ranging between 1 and 10
Ωcm. The n-type Cz crystals are grown and sliced into
wafers at Bosch Solar Energy AG in Arnstadt, Germany.
The crystal growth and wafering is performed on
industrial equipment. Figure 1 shows a schematic crosssection of our complete fabricated n-type solar cell. The
solar cell process comprises an alkaline texture on the
front side and a polished back side. On the front side
(light receiving side), a p+ emitter with a sheet resistance
of typical 60 Ω/square was diffused from a boron
tribromide (BBr3) source in a quartz tube furnace of an
industrial scale. The BSF on the back side is formed in a
separate diffusion step by diffusing phosphorous in a
similar quartz-tube furnace using a POCl3 source. The
passivation of the BSF diffusion region is achieved by a
PECVD SiNx deposition while for the front boron emitter
the passivation is achieved using an in-house developed
method consisting in a passivating layer and a SiNx antireflection coating stack. The fabrication process includes
also the necessary cleaning steps using baths containing
HCl, HF, H2O2/H2SO4, and rising water. The
metallization on the front and back side was applied by
screen printing
Figure 1: Schematic cross-section of the fabricated ntype solar cells.
and firing through of metal pastes using an H-pattern
screen design. The cell area is 241 cm2 which equals to
the area of our pseudo squared 156×156 mm2
monocrystalline wafer with a diagonal of 205 mm. Hence
no edge isolation by snapping the edges or similar tricks
was applied.
3
RESULTS AND DISCUSSION
3.1 Passivation of boron emitters
One of the drawbacks in the development of an
industrially compatible n-type cell process is the
passivation of the boron emitter. As PECVD-SiNx, which
is commonly used to passivate phosphorous emitters in
the standard p-type process, does not passivate boron
emitters, a considerable amount of effort has been put to
find new methods to passivate boron emitters. Currently
the most successful methods used are stacks in which the
top layer is SiNx used as antireflection coating layer and
as hydrogen source for the passivating layer(s)
underneath. Among the passivation layers used in
combination with SiNx are: a thin atomic layer deposition
of Al2O3 [5], amorphous silicon deposition [9], thermally
grown SiO2 [6,7], chemically grown SiO2 [8]. However,
some of these methods are quite costly, other yield only
layers of limited quality. In our quest to continuously
improve the passivation quality of boron emitter and to
reduce the costs, we have developed a new method for
passivation. The method consists of a thin passivating
layer and a SiNx anti-reflection coating stack. Details
about the properties of the passivating layer and its
application method will be the subject of a later
publication. Herein we have designed an experiment to
test the passivation quality of our new method compared
with other known methods used to passivate industrialtype boron diffused emitters. The test structure consists
of a polished n-type monocrystalline Cz substrate
symmetrically diffused with 60 Ω/square boron emitter
on both sides. After the necessary cleaning steps the
passivation layer(s) are applied on p+ diffused regions.
The samples were subsequently subjected to a firing step
followed by a QSSPC measurement. From this the
effective recombination lifetime τeff , the implied VOC at 1
sun intensity, and the emitter saturation current density
JoE at high injection level (∆n=2×1016 cm-3) were
extracted. A schematic cross-section of such a test
structure is shown in figure 2.
Figure 3 compares the τeff and JoE of our passivation
method against several other passivation methods that
includes a wet thermal SiO2/SiNx stack, a chemical
SiO2/SiNx stack (NAOS method), and just a SiNx layer.
The results in figure 3 clearly demonstrate the excellent
passivation potential of our developed method, with τeff
=517 µs, implied VOC=683 mV, and JoE = 14 fA/cm2 per
side.
Figure 2: Schematic cross-section of the fabricated test
structures to investigate the passivation of boron emitters.
Figure 3: Effective minority carrier lifetime τeff measured
on a p+np+ test structure at 1 sun illumination and emitter
saturation current density JoE measured at an injection
level of 2×1016 cm-3 as a function of different methods to
passivate the p+ (boron emitter) region. The passivation
methods tested in this study are: a silicon nitride layer
(SiNx), a stack of chemically grown SiO2 in a nitric acid
solution (NAOS) and SiNx, a stack of wet thermally
grown SiO2 (20 nm) in a quartz tube furnace and SiNx,
and our new developed method consisting in a stack of a
passivating layer and SiNx.
These results are comparable to the best achieved
passivation of p+ surfaces up to now, namely the atomic
layer deposition of Al2O3 and SiNx stack [5]. Although
thermal SiO2/SiNx stack produce very good passivation
as well, as can be seen from the results of figure 3, it
requires, however, an extra high temperature step in the
solar cell process.
3.1 Solar cell results and discussion
The excellent passivation results obtained on boron
emitters were then applied to fabricate n-type cells using
the process described above and schematically shown in
figure 1. On the front boron emitter we have applied our
passivation method while on the phosphorous BSF we
deposited the commonly used SiNx passivating layer. The
metallization was applied on both sides of the cells by
screen printing metal pastes. The cells went through a
firing step to complete the contact formation. Table 1
shows the solar cell parameters of our best fabricated cell
and a solar cell with the best bifacial efficiencies ratio.
The best achieved efficiency is 18.6%, and it is certified
by Fraunhofer ISE CalLab in Germany.
It should be noted that all solar cells that we have
processed so far have a polished rear side and a random
pyramid texture on the front side. This results in different
short-circuit current densities (JSC) under front and back
side illumination. However, this effect could be
eliminated by comparing the internal quantum
efficiencies (IQE) measured under front and back side
illumination
conditions.
Table I: Comparison of the best solar cell parameters measured under standard test conditions (AM1.5G, 100 mW/cm2, 25
o
C). The solar cells were fabricated on 241 cm2 monocrystalline n-type Cz Si with base resistivity of 8 Ωcm. The result for
the best efficiency of a bifacial fabricated cell is also shown. The implied VOC value was measured using QSSPC on a similar
cell but without the metal contacts on front and back side.
Substrate,
Surface
Jsc
implied Voc
FF
pseudo-FF
Voc
η
illumination
texture
[mA/cm2]
[mV]
[mV]
[%]
[%]
[%]
Cz, best, front
Rand. pyramids
38.3*
637*
664
76.5*
82.5
18.6*
Cz, front
Rand. pyramids
38.5
634
75.2
18.3
Cz, back
alkaline polished
34.2
631
75.9
16.4
*values measured by ISE CalLab.
Figure 4 shows the IQE measurement of our best cell
(black circles) illuminated from the front side. The
response observed at short wavelengths (below 0.5 µm)
confirms the excellent passivation of boron emitter (60
Ω/square) demonstrated in figure 3. Moreover, figure 4
also shows the IQE data of an optimized bifacial cell,
which have different BSF sheet resistance and
metallization grid as compared with cell optimized only
for the front side. The IQE results of the bifacial cell
show that independent from which side the cell is
illuminated the response of the cell is the same. This
demonstrates on one hand the outstanding passivation of
the diffused surfaces and on the other hand the very high
lifetime of the material, which obviously does not suffer
substantially during the process. A symmetrical IQE from
both sides is an excellent characteristic because it allows
us to fabricate bifacial cells with the same power
conversion efficiencies from the front- or back side
illumination, providing that the back surface is also
textured. Based on the experimental data shown in table
1 and figure 4 the major difference between front side
and back side performance of the cells is in the JSC.
Therefore work is under way to adapt our cell process to
allow for both sides to be textured in order to match the
JSC.
An advantage of realizing cells with equal
performance from the front- or back side (fully bifacial)
can be taken in the module interconnection. If such a
performance match is desired for certain applications,
then equal performance match will allow for the
introduction and fabrication of planar interconnected ntype cells in a module. A schematic representation of our
proposed Planar Interconnected N-type Cells (PINC) in a
module is shown in figure 5.
Figure 4: Internal quantum efficiency (IQE) of our best
fabricated cell (confirmed by ISE CalLab), and of a
bifacial cell illuminated from the front- and back side.
Figure 5: Schematic representation of our proposed
Planar Interconnected N-type Cells (PINC) in a module
of our fully bifacial cells. In the PINC module the boron
emitter side of a cell is planar connected with the BSF
side of an adjacent cell.
4
CONCLUSSION
We have presented a simple industrial process to
fabricate n-type solar cells with boron front emitter and
phosphorous back-surface-field which leads to an
efficiency of 18.6% for large area (241 cm2)
monocrystalline Cz-Si substrate. We demonstrated that
this cell process results in identical internal quantum
efficiencies when a cell is illuminated from the emitter
side or from the back-surface-field side. This feature
allows for the fabrication of fully bifacial cells, with
identical efficiencies from both sides, and to introduce an
innovative cells interconnection in the module.
Additionally, we introduce a new method to passivate
boron emitters for industrial application with excellent
passivation quality.
ACKNOWLEDGEMENTS
This work is financially supported by German
government (BMU) within EnSol project, contract
number 0325120A.
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