ION-IMPLANTED POLY-SI / C-SI JUNCTIONS AS A BACK-SURFACE FIELD IN BACK-JUNCTION
BACK-CONTACTED SOLAR CELLS
Udo Römer1, Agnes Merkle1, Robby Peibst1, T. Ohrdes1, B. Lim1, J. Krügener2, and R. Brendel1,3
1
Institute for Solar Energy Research Hamelin (ISFH), Am Ohrberg 1, 31860 Emmerthal, Germany
Institute of Electronic Materials and Devices, Leibniz Universität Hannover, 30176 Hanover, Germany
3
Institute of Solid State Physics, Leibniz Universität Hannover, 30176 Hanover, Germany
*corresponding author: Tel.: +49 (0) 5151 999-643, Fax: +49 (0) 5151 999 400, roemer@isfh.de
co-authors: merkle@isfh.de, peibst@isfh.de, ohrdes@isfh.de, lim@isfh.de, kruegener@mbe.uni-hannover.de,
brendel@isfh.de
2
ABSTRACT: We present back-junction back-contacted n-type silicon solar cells with a carrier selective back-surface
field fabricated by ion-implanted n+ polycrystalline (poly-) Si. In terms of the two-diodes model, we find an enhanced
J02-like recombination compared to reference solar cells fabricated with a conventionally doped back-surface field in
monocrystalline (c-) Si. By comparing the J02-values of solar cells with different geometries we find that the
J02-values are correlated with the index of the back-surface field. Thus, we conclude that this recombination
component is caused by the p+n+ junction between the poly-Si and the BBr3 diffused c-Si emitter, which probably
exhibits a lot of defects in the poly-Si side of the space charge region. Due to the high J02-like recombination, the best
efficiency measured on a cell with poly-Si back-surface field of 21.7 % does not exceed the best efficiency of
23.35 % measured on a reference with a c-Si back-surface field. Approaches to suppress the recombination at the
p+n+ junction between emitter and BSF in poly-Si BJBC cells are proposed.
Keywords: Passivated Contact, Back-Junction Back-Contact, Solar Cell, Recombination
1
INTRODUCTION
Back-junction back-contacted (BJBC) Si solar cells
with carrier selective junctions are a promising cell
structure to achieve highest efficiencies due to the
absence of optical shading and an excellent interface
passivation also in the metallized regions [1, 2, 3].
Polycrystalline (poly-) Si / monocrystalline (c-) Si
junctions are a promising candidate for carrier selective
junctions regarding both, performance and processing
issues. We recently achieved recombination current
densities down to 1 fA/cm2 on phosphorus ion-implanted
poly-Si / c-Si junctions [4]. While our final goal is a
BJBC cell utilizing poly-Si junctions for emitter and
back-surface field (BSF), we evaluate, as a first step, ion
implanted n+ poly-Si as BSF in a hybrid BJBC cell
structure with a “conventional” boron emitter diffused in
c-Si. The hybride approach was also chosen by F.
Feldmann et al., who integrated n+ poly-Si as BSF in
double-side contacted n-type cells with a conventional
boron front-side emitter [5].
The integration of poly-Si / c-Si junctions for one
polarity (n+ BSF in this study) in our baseline RISE (rear-
side single evaporation) BJBC cell process [6, 7] is
straight-forward and does not require modifications of the
laser-ablation based patterning of the doped regions. The
latter will be required when the junctions of both
polarities – emitter and BSF - shall be realized as polySi/c-Si junctions. However, the integration of poly-Si /
c-Si junctions for one polarity (n+ BSF in this study)
should already (i) yield a significant efficiency
improvement over our BJBC baseline with conventional
junctions, and (ii) point out issues - process-related or
related to device physics - relevant for the poly-Si / c-Si
BJBC cell structure. Examples for (ii) are the necessity of
an optimization of the laser parameters for the creation of
local contact openings in a dielectric rear-side reflector
deposited on the poly-Si, and, the influence of the quality
of p+n+-junctions in poly-Si. Those aspects are also
relevant for BJBC cells with poly-Si / c-Si junctions for
both polarities.
Therefore we compare conventional BJBC cells with
cells featuring an n+ poly-Si BSF and compare the
different device geometries enabling an identification of
additional recombination paths.
Figure 1. Illustration of the cell structures investigated: (a) n-RISE BJBC cell with n+ poly-Si BSF, (b) reference nRISE BJBC cell with conventional c-Si BSF. Both groups of cells feature a full-area Al2O3/SiNx passivation on the cell
rear-side.
Table 1: Comparison of IV-quantities for the best cell with poly-Si BSF and with conventional BSF, respectively. Both
cells have a BSF index of 1150 µm and an emitter area fraction of 78 %. All data refer to in-house measurements on
3.96 cm2 designated area.


best cell
conventional
BSF
23.35
best cell polySi BSF
21.7
2
Voc
[mV]
Jsc
[mA/cm2]
FF
[%]
pFF
[%]
677.5
42.74
80.7
82.23
673.6
42.1
77
78.8
SOLAR CELL PROCESS FLOW
In this study we fabricated n-type RISE BJBC [4, 6]
cells from symmetric n+ poly-Si/ n c-Si/ n+ poly-Si test
structures as studied in previous works [7]. The test
structures have been fabricated on 156 mm × 156 mm
large pseudo square n-type Cz wafers with a resistivity of
6 Ωcm for the fabrication of the solar cells. The damageetched and RCA-cleaned wafers receive a 2.4 nm thick
thermal oxide [8] and a 150 nm thick intrinsic amorphous
Si layer. Next we apply a phosphorus ion implantation
step. After high temperature treatment for dopant
activation, which possibly also yields a local break-up of
the interfacial oxide [9], a forming gas anneal was
performed. At this stage, we performed photo
conductance decay (PCD) measurements and extracted
emitter saturation current densities J0 of 1 fA/cm2 and
implied open circuit voltages Voc, impl. of 742 mV [7, 10].
The upper limit for the specific interface resistance of the
poly-Si / c-Si junction was determined to be 10 mΩcm2
by a method described in Ref. [8]. For completing the
BJBC cells, we deposited a SiNx capping layer on the cell
rear-side, which was laser-structured in order to define
the emitter regions. In these regions, the n+ poly-Si (and
further ~7 µm of c-Si) was removed by KOH. We
processed 24 cells with an area of 2 cm × 2 cm on each
wafer. The device geometry, in particular the BSF index,
was varied from 450 µm to 1150 µm. We also varied the
finger widths in order to realize emitter area fractions
Rs (shift light
IV/JscVoc)
[Ωcm2]
J02
[nA/cm2]
Rshunt
(fit JscVoc)
[kΩcm2]
0.37
146
6
317
0.4
112
33
8.5
from 66 % to 82 %. The emitter junction was formed by
BBr3 boron diffusion. One should note that this method
does imply a doping of the edge regions, i.e., a direct
touching of the p+ boron emitter with the n+ poly-Si BSF
(Fig. 1 (a)). After BSG removal, an Al2O3 / SiNx stack
was deposited on the rear-side. This stack served as a
passivation layer on the boron emitter and as a rear-side
reflector on both the c-Si emitter and the poly-Si BSF.
The stack furthermore served as a protection layer in the
subsequent alkaline front-side texturing step. After
cleaning, the front-side was passivated by SiNx. In order
to create contact openings, local laser ablation of the
AlOx / SiNx stack was applied. This step is especially
challenging on the rather thin poly-Si BSF regions, since
damaging of the interfacial oxide has to be avoided. In
Fig. 2 the lifetime ratio measured by means of dynamic
infrared lifetime imaging (dyn. ILM) [11] of a poly-Si
test structure is shown before and after full area laser
ablation of the dielectric stack. The laser pulse power has
been varied between 0.8 µJ and 2.3 µJ. It is found that for
the two highest settings the lifetime is reduced. In the
lower part of Fig. 2 optical microscope images for the
four highest laser power settings are shown, for the
lowest investigated setting no image is shown, because
no ablation of the dielectric has been obtained. For the
second lowest laser power setting, the ablated areas are
very inhomogeneous in size and shape and thus not
appropriate for the stable processing of solar cells.
Therefore the middle setting was chosen for the contact
openings. The metallization of the solar cells was
performed by Al evaporation, followed by sputtering of
SiOx and a subsequent RISE [12] contact separation. The
final cell structure is illustrated in Fig. 1 (a). As
reference, we also processed “conventional” n-type RISE
cells with POCl3 and BBr3 diffused junctions in c-Si
(Fig. 1 (b)). These reference cells also feature a full-area
AlOx / SiNx passivation on the rear and a SiNx
passivation on the front.
3
Figure 2: Upper picture: Lifetime ratio of a poly-Si
test structure with 100 nm PECVD SiNx before
divided by the lifetime after full area laser ablation as
measured by means of dynamic infrared lifetime
mapping. The laser pulse power was varied between
0.8 µJ and 2.3 µJ. Lower pictures: Optical microscope
images of single laser spots on the same test structure.
J01
[fA/cm2]
RESULTS AND DISCUSSION
We expected a significant improvement of the
performance of the cells with n+ poly-Si BSF as
compared to the cells with conventional BSF. Besides the
suppression of recombination at the BSF metal contacts,
the poly-Si BSF does also imply lower recombination in
the passivated regions since AlOx does not provide a
good passivation on n+ doped c-Si regions [13]. On test
structures, we extracted a saturation current density of
1 fA/cm2 for the poly-Si BSF and of 202 fA/cm2 for the
conventional BSF and AlOx passivation. When
Figure 3: Jsc-Voc curves of the best cells with conventional
BSF and poly-Si BSF and respective fits with the twodiodes-model. For the cell with poly-Si BSF, the single
current contributions are shown (dashed red lines).
exclusively considering the passivated BSF regions, we
therefore expected a reduction in the pre-factor J01 of the
total cell recombination current density between
30 fA/cm2 and 67 fA/cm2 for our cells with BSF area
fractions between 15 % and 33 %. This would correspond
to an increase in the open circuit voltage Voc between
7.5 mV and 14 mV.
However, this improvement was not observed yet in
this study. Rather, the efficiency of 21.7 % for the best
cell with poly-Si BSF is lower than the highest efficiency
of 23.35 % measured on a reference cell with
conventional BSF (Tab. 1). While both cell types exhibit
maximum short circuit current densities > 42 mA/cm2
and exhibit comparable series resistance values, both the
open circuit voltage Voc and the pseudo fill factor pFF of
the n+ poly-Si BSF cells are significantly lower than the
values of the cells with conventional BSF. By fitting the
Jsc-Voc curves of the solar cells with the two diodes model
(Fig. 3), we found that the major reason for the worse cell
performance of the poly-Si BSF cells is a strongly
increased J02-like recombination. It compromises both,
Voc and pFF significantly. When changing the J02-value
of the fitted Jsc-Voc curve to 6 nA/cm2 as in the case of the
cell with conventional BSF, the Voc-value increases to
683 mV and the pFF-value to 83.0 %. Based on the
following arguments, we conclude that the J02-like
recombination can be attributed to the p+n+ junction
meander between the n+ poly-Si and the p+ c-Si emitter,
and the other origins, in particular the junction between
the n+ poly-Si and the n-type c-Si base, can be excluded:
(i) The recombination behavior of the junction
between the n+ poly-Si and the n-type c-Si base was
studied previously on full-area test structures [7, 8]
without any indications for a significant J02-like
recombination.
(ii) The J02 component correlates well with the BSF
index, i.e., the areal density of p+n+-junction lines
between the emitter and BSF region (Fig. 4).
(iii) Similar J02 values are obtained for different
emitter (BSF) area fractions (Fig. 4). If the J02-like
recombination would be occurring at the junction
between the n+ poly-Si and the n-type c-Si base, a
different area fraction of the n+ poly-Si BSF would imply
Figure 4: J02 as a function of the BSF index (of the
density of pn-junction lines) for cells with n+ poly-Si BSF
and cells with conventional c-Si BSF. Different symbols
represent different emitter area fractions (circle 66 %,
square 80 %, diamond 82 %, and triangle 85 %). All data
refer to the best cells (in terms of efficiency) per group,
respectively. On poly-Si BSF cells with slightly lower
efficiency, we even obtained J02 values up to 150 nA/cm2.
different J02 values.
(iv) In previous work [10] we observed a strongly
non-ideal recombination behavior on test structures with
intentionally fabricated p+n+ junctions in poly-Si films.
A plausible explanation for the poor recombination
behavior with an ideality factor close to 2 is the
presumably high defect density in the poly-Si, which
mediates strong recombination processes in the space
charge region. Trap-assisted tunneling might also
contribute to a certain amount to the poor recombination
behavior. A similar recombination behavior was reported
for pn-junctions in amorphous Si [14]. By contrast, a
touching of n+ BSF- and p+ emitter regions in
monocrystalline Si, where the defect density is low, does
not result in enhanced J02-like recombination [15].
Unfortunately (at least from this point of view), our n+
poly-Si BSF with a sheet resistance of 50 Ω/□ exhibits
sufficient lateral conductivity to prevent a “decoupling”
of the p+n+ junction from the emitter.
4
OUTLOOK
Experimentally, a suppression of the recombination
at the p+n+ junction between emitter and BSF should be
achievable by the avoidance of a touching of highlydoped poly-Si regions of one polarity with highly doped
regions of the other polarity, regardless whether the latter
is also realized in poly-Si or conventionally in c-Si. Thus,
an undoped or vey lightly doped region separating
emitter and BSF in BJBC would be desirable.
For our n-type RISE cell structure which features a step
on the cell rear-side, this undoped region could be the
edge of the step. For example, when using ion
implantation as doping method for the c-Si boron emitter,
the edge of the step should not be doped, and thus an
undoped gap between emitter and BSF is formed without
adding process complexity.
5
ACKNOWLEDGEMENTS
The project HERCULES has received funding from the
European Union’s Seventh Program for research,
technological development and demonstration under
grant agreement No 608498. Parts of this work were
performed in the framework of HERCULES. Major
contributions to this work were generated in the
SIMPLIHIGH project, which is funded by the German
Federal Ministry for Economic Affairs and Energy under
Grant 0325478. We thank Susanne Mau, Thomas
Friedrich and David Sylla for their help with sample
processing.
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