LBIC investigations of the lifetime degradation by extended defects in
multicrystalline solar silicon
Markus Rinio1, Hans Joachim Möller1 and Martina Werner2,
1
Institute for Experimental Physics, TU Freiberg, Silbermannstr. 1, 09596 Freiberg, Germany,
Tel. +49-3731-39-2896, email: rinio@physik.tu-freiberg.de, moeller@physik.tu-freiberg.de
2
Max Planck Institute for Microstructure Physics, Weinberg 2, 06120 Halle, Germany
Keywords: LBIC, Reflection measurement, Solar cell, EFG, RGS, Recombination, Dislocation
Abstract. A calibrated measurement of the short circuit current and the surface
reflection coefficient can be directly converted into the internal quantum efficiency
(IQE) of a solar cell. The IQE at a wavelength of 833 nm were measured on ingot, EFG
and RGS silicon solar cells with a spatial resolution of 6 µm. Ingot solar cells were
found to be predominantly influenced by a homogeneous distribution of recombination
centers. However, if the dislocation densities exceeded a certain limit the IQE was
reduced by recombination at dislocations. This limit varied in different parts of the
wafer. EFG solar cells only showed a lifetime reduction by dislocations whereas the
investigated solar cells made of RGS silicon were dominated by recombination at grain
boundaries. The RGS silicon was further investigated by TEM- measurements, which
showed that the extended defects were highly decorated with SiO2- and SiCprecipitates.
1 Introduction
Topographical measurement techniques are
suitable to detect areas of efficiency losses in
multicrystalline solar cells. Combining
different mapping methods it is possible to find
correlations with material parameters which
can lead to an understanding of the processes
responsible for the reduction in efficiency. The
efficiency of a multicrystalline solar cell is
mainly limited by defects at which
recombination of minority carriers takes place.
The aim of the work presented here is to
identify the regions within multicrystalline solar
cells where the dominating recombination
processes occur. Secondly we are interested in
the quantitative determination of current losses
due to different types of defects.
Areas with low minority carrier lifetime are
made visible by the light beam induced current
(LBIC) mapping technique. Most of the
published LBIC investigations are without
calibration. In addition, they are not corrected
for reflectivity losses. This is not sufficient for
quantitative investigations.
In this paper we present an advanced LBIC
technique
consisting
of
a
calibrated
measurement of current, light intensity and
reflection coefficient. To correlate the LBIC
maps with small defect structures such as
dislocation clusters or grains with sizes of
about 100 µm the spatial resolution of our
LBIC system was improved up to 6 µm.
Solar Cells made of ingot, EFG and RGS
silicon were investigated. Ingot silicon is
produced by directional solidification in a
quartz crucible. The grains have sizes of a few
millimeters. Oxygen concentrations between
4⋅1017 and 10⋅1017 were measured [1-6]. The
edge-defined film-fed growth (EFG) method is
based on pulling a thin ribbon out of the molt in
vertical direction. EFG ribbons contain a high
fraction of narrow twins, which extend across
the whole wafer. Dislocation densities are in
the range of 105 - 107 cm-2. As grown EFG
wafers have oxygen concentrations of about
1⋅1016 and contain 5 to above 10⋅1017 carbon
atoms/cm3 [7,8].
In the RGS technique a thin film of liquid
silicon is spread on a substrate using a shaping
die. RGS films are produced much faster due to
the fact that the directions of pulling and
solidification are perpendicular to each other.
Typical grain sizes are in the range of 200500 µm. The dislocation densities are ranging
from 105 to 107 cm-2. RGS silicon contains
about 2⋅1018 oxygen atoms per cm3 [1-6].
2 Experimental procedure
In our setup the sample is illuminated by a
focussed diode laser beam with a minimum
spot size of about 6 µm.
The reflection coefficient of the sample is
measured using the upper and lower reflection
detectors, which consist of two calibrated solar
cells. The incident light beam travels through a
hole in the lower detector and then hits the
sample. At the surface of the sample some light
is reflected into side directions where it is
absorbed by the lower detector. Another part of
reflected light travels back through the hole.
This light component is measured by the upper
reflection detector. Both reflection detectors
together are able to detect reflected light within
the complete solid angle from 0° to about 80°
against the vertical direction. A third solar cell
is used to measure the power of the light
illuminating the sample.
beam splitter
lock-inamplifier 3
light
intensity
detector
upper reflection
detektor
lower reflection detektor
solar cell
sample
XYZ stage
The measured short circuit current ISC, the
power PL of the incident light and the reflection
coefficient R are converted into the local
internal quantum efficiency (IQE):
IQE :=
I SC / e
1
⋅
1 − R PL / (h ⋅c / λ)
(1)
(e: elementary charge, h: Planck's constant,
c: velocity of the light, λ: wavelength of the
incident light)
ISC strongly depends on the amount of absorbed
light. This influence is corrected by the first
factor in Eq. 1. The second factor is called
external quantum efficiency (EQE). The system
described here measures the IQE only at the
wavelength of 833 nm, which corresponds to
an absorption length of 15 µm in silicon. This
means that mainly the bulk recombination is
detected, but almost no effects due to the
emitter layer. The measured IQE is also not
influenced by the optical reflection coefficient
of the back contact.
With the calibrated LBIC measurement one can
also identify uniformly distributed defects
which are not individually resolved in the map,
because the IQE would be below 100 % in this
case. In addition it is possible to compare
different types of solar cells quantitatively.
laser
833 nm
objective
The spot size of the laser beam can be adjusted
by measuring LBIC line scans over a sharp
edge which partly covers the sample. The spot
diameters are determined by fitting the
theoretically calculated line scans to the
measured ones.
lock-inamplifier 2
lock-inamplifier 1
Fig. 1 Schematic drawing of the LBIC system
After taking LBIC pictures of the solar cells,
the samples were polished and etched for 4
minutes with Secco etch, a mixture of 9 g
K2Cr2O7, 200 ml H2O and 400 ml HF (40 %).
Spatial resolved dislocation densities were
measured using an image processing system,
which is able to distinguish dislocation etch
pits from other objects, such as grain
boundaries.
Fig. 2 External quantum efficiency map of a section of an ingot solar cell. The contact fingers are visible as
horizontal lines. The large variations between different grains are mainly due to the different light absorption
which can be seen in Figures 3 and 4.
Fig. 3 Reflection coefficient map. The different reflectivity and absorption of the grains are due to their
different texturization which depend on the surface orientation of the crystal. The light vertical strip on the
right side is caused by the antireflection layer which changes in the vicinity of the contact bus.
Fig. 4 Internal quantum efficiency map (scaled with the contrast of Fig. 2). The large variations vanished.
Fig. 5 LBIC map from a multicrystalline solar cell made from ingot silicon. The dark curved lines
correspond to electrically active grain boundaries. The streaks are caused by dislocation clusters.
3.1 Reflection correction
Figures 2, 3 and 4 show the results of an LBIC
measurement of a multicrystalline solar cell
made of ingot silicon. The strongly absorbing
grain in the upper middle part produces higher
currents than the neighboring grains. After
correction by 1/(1-R) this difference vanishes.
Since the reflection coefficient ranges from
3 % to 10 % it is clear that it has a strong
influence on the short circuit current and
cannot be neglected.
3.2 Ingot silicon
Fig. 5 shows an LBIC map taken from a part of
a multicrystalline solar cell. In the regions
marked A and B spatial resolved dislocation
densities were determined after polishing and
etching of the cell. The results are presented in
Fig. 6.
Both regions show a correlation between the
internal quantum efficiency and the dislocation
density. In region B one can conclude that
dislocations play the dominant role as long as
their density exceeds 105 cm-2. The fact, that
the internal quantum efficiency remains
constant at a level of 88 % for dislocation
densities below 7⋅104 cm-2 indicates that there
exist other homogeneously distributed recombination centers within this cell.
The dislocations in region A exhibit a
significantly different behavior to those in part
B. Dislocation densities up to 106 cm-2 seem to
have no considerable effect on the minority
carrier lifetime, which means that again
homogeneously distributed recombination
centers play the dominant role below a certain
density limit of dislocations.
92
internal quantum efficiency [ %]
3 Experimental results and discussion
region A
88
84
region B
80
76
72
10
4
10
5
10
6
-2
etch pit density [cm ]
Fig. 6 Internal quantum efficiency at λ=833nm
versus dislocation etch pit density in two
neighbored regions of the solar cell. Etch pit
densities above 2⋅106 cm-2 are a little underestimated because overlapping etch pits could not
be counted correctly in this case.
Fig. 7 Internal quantum efficiency of an EFG solar cell. The eight areas of reduced current under the contact
busses result from the back contacts without back surface field.
Fig. 8 Internal quantum efficiency measured with a spatial resolution of 12.5 µm in section 5 marked in the
map shown in Fig. 7. The left part is replaced by the optical image of the polished and subsequently etched
surface. In the left picture the light regions result from high etch pit densities. The dark line at the bottom of
the IQE map is due to a random grain boundary.
Fig. 9 IQE map of a section of an RGS solar cell measured with a spatial resolution of 6 µm. The vertical
line is due to a contact finger.
3.3 EFG silicon
The results of LBIC measurements on a EFG
solar cell are shown in Fig. 7. The solar cell
contains grains with uniformly reduced
quantum efficiency, i. e. in the dark stripe
which crosses the marked sections 1 and 2. In
the magnified image in Fig. 8 one can see that
the regions with reduced IQE correlate with
high densities of etch pits. The removal of a
thin surface layer by polishing and subsequent
etching showed that these etch pits appear at
almost the same positions again. Therefore we
conclude that they belong to dislocations. The
dark line in the lower part of the IQE map was
caused by a random grain boundary. The twin
grain boundaries visible in Fig. 8 have no
visible recombination activity. This IQE map
also emphasizes that local internal quantum
efficiencies of 97 % at 833nm wavelength are
possible even in multicrystalline solar cells.
3.4 RGS silicon
Large internal quantum efficiencies were also
found locally on RGS solar cells. Between the
dark lines in Fig. 9 where the recombination is
high the IQE reached values of about 96 %.
Investigations carried out on another sample
indicated that the dark lines in the IQE map
mainly correspond to grain boundaries.
Fig. 10 TEM image of a RGS sample which was not processed into a solar cell
RGS samples were also examined by TEM.
Spherical amorphous SiO2-precepitates with
sizes up to 50 nm and densities between about
1010 and 1013 cm-3 and also SiC-precepitates
were found in the solar cells. Fig. 10 shows that
in RGS silicon dislocations and grain boundaries are highly decorated with precepitates.
3.5 Comparison of ingot, EFG and RGS
solar cells
Tab. 1 shows IQE-values that were obtained by
averaging over different areas excluding the
contact grid. These values indicate, that the
short circuit current of the ingot solar cell is
mainly reduced by homogeneously distributed
recombination centers whereas the EFG and
RGS solar cells are mainly influenced by local
enrichments of extended defects. The
recombination within the grains of the
multicrystalline ingot cell may occur at finely
distributed
microdefects
like
oxygen
precipitates together with metallic impurities.
Also back surface recombination may
contribute to the reduction of the IQE. In EFG
silicon the localized defects were shown to be
dislocations. For RGS silicon the results
indicate that grain boundaries are more
dominant than dislocations.
Tab. 1 Comparison of internal quantum efficiencies
measured on three different solar cells at a
wavelength of 833 nm.
average IQE in the
whole cell
average IQE in „good
grains“
IQE loss from recombination at local
enrichments of
extended defects
total IQE loss
ingot EFG RGS
cell
cell
cell
89 % 93 % 85 %
92 % 98 % 96 %
3%
5 % 11 %
11 %
7 % 15 %
We have to emphasize that the internal
quantum efficiencies measured at a wavelength
of 833 nm are connected to recombination of
minority carriers within the p-doped layer of
the solar cell. There exist, however, other
causes of a reduction of the efficiency. For
instance shunting paths through the pn junction
due to dislocations and grain boundaries [9-11]
as well as recombination within the emitter or
light absorption at the back contact are not
detected by the LBIC system. On the other
hand, it is known that a lower short circuit
current, reduced by recombination processes,
also affects the open circuit voltage. Taking
this into account one can expect that the
efficiency loss due to recombination within the
bulk is larger than the loss of IQE measured
here.
4 Conclusions
A calibrated LBIC technique was used to
investigate regions of enhanced recombination
in three different types of multicrystalline solar
cells. The results show that the short circuit
current of the ingot solar cells are mainly
reduced by homogeneously distributed
recombination centers that may consist of
finely distributed precipitates in association
with metallic impurities. The limit of
dislocation
densities,
at
which
this
recombination rate is exceeded by the
dislocations, depends on their electrical
activity. This density limit was found to be
different in different regions of the cell. In EFG
solar cells the short circuit current is mainly
reduced by dislocations. LBIC measurements
on RGS solar cells indicate that highly
decorated grain boundaries play the dominant
role in this samples. The TEM investigations
show that this material is extremely enriched
with SiO2-precepitates, which partly decorate
extended defects and may act as recombination
centers. SiO2-precepitates also act as gettering
centers for metallic impurities.
5 Acknowledgements
This work was supported by the BMBF,
Federal Republic of Germany, under Contact
Numbers 0329743D7 and 0328661D.
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