PROGRESS IN PHOTOVOLTAICS: RESEARCH AND APPLICATIONS
Prog. Photovolt: Res. Appl. 2004; 12:529–538
Published online 29 July 2004 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/pip.544
Shunt Types in Crystalline
Silicon Solar Cells
O. Breitenstein*
,
y
, J. P. Rakotoniaina, M. H. Al Rifai and M. Werner
Max Planck Institute of Microstructure Physics, Weinberg 2, D-06120 Halle, Germany
Nine different types of shunt have been found in state-of-the-art mono- and multi-
crystalline solar cells by lock-in thermography and identified by SEM investigation
(including EBIC), TEM and EDX. These shunts differ by the type of their I–V char-
acteristics (linear or no nlinear) and by their physical origin. Six shunt types are pro-
cess-induced, and three are caused by grown-in defects of the material. The most
important process-induced shunts are residues of the emitter at the edge of the cells,
cracks, recombination sites at the cell edge, Schottky-type shunts below grid lines,
scratches, and aluminum particles at the surface. The material -induced shunts are
strong recombination sites at grown-in defects (e.g., metal-decorated sma ll-angle
grain boundaries), grown-in macroscopic Si
3
N
4
inclusions, and inversion layers
caused by microscopic SiC precipitates on grain boundaries crossing the wafer.
Copyright # 2004 John Wiley & Sons, Ltd.
key words: shunts; thermography; lock-in; silicon; monocrystalline; multicrystalline
INTRODUCTION
A
solar cell, as simulated by essentially one-dimensional models, is assumed to show a homogeneous
current flow across the whole area, both under illumination and in the dark. In the traditional inter-
pretation of I–V characteristics of solar cells all nonlinear currents belonged to the cell, and only
ohmic current paths across the pn junction have been attributed to ‘shunts’. With the availibility of precision
lock-in thermography techniques these shunts can be made visible, so in the following all bright features visible
in thermography have been called ‘shunts’. However, by later investigations it has turned out that there are
not only ohmic shunts, but also diode-like ones, e.g., caused by local recombination sites. So the question, what
is a shunt and what belongs to the undisturbed cell, has a philosophical dimension: can, e.g., a region of
lower crystal quality be called a shunt? This question is still under discussion, but throughout this work we will
use the term ‘shunt’ for any position in a solar cell showing under forward or reverse-bias a dark-current
contribution additional to the diffusion current. In this sense edge leakage currents are shunting currents, but
a region of lower crystal quality, where only the saturation current density of the diffusion current is increased,
is not. Future discussions will show whether this definition will survive or has to be replaced by a more
precise one.
Received 20 August 2003
Copyright # 2004 John Wiley & Sons, Ltd. Revised 3 December 2003
*Correspondence to: O. Breitenstein, Max Planck Institute of Microstructure Physics, Weinberg 2, D-06120 Halle, Germany.
y
E-mail: breiten@mpi-halle.de
Contract/grant sponsor: BMWi; contract/grant number: 0329846 D (ASIS).
Contract/grant sponsor: EU; contract/grant number: ENK6-CT-2001-00573.
Research
The technique of infrared (IR) lock-in thermography, which has been commercially available for solar cell
investigations since 2000,
1
allows one to perform an efficient and systematic investigation of shunts in solar
cells.
2–5
This technique detects the periodic local surface temperature modulation in the positions of local
shunts with a sensitivity below 100 mK by applying a pulsed bias to the cell in the dark. Lock-in thermography
may not only image the position of shunts up to an accuracy of 5 mm, but it also allows one to easily check
whether shunts have a linear or a nonlinear I–V characteristic, as well as to measure local I–V characteristics
quantitatively in a non-destructive way.
3–6
However, knowing the positions of shunts and estimating their influ-
ence on the dark I–V characteristic of the cell is only the first step in understanding shunt phenomena. The next
step is to find out the physical nature of the shunts in order to avoid them in future. This can be done most
successfully by using microscopic and microanalytic techniques such as EBIC (electron-beam-induced current)
and EDX (energy-dispersive X-ray analysis) performed in the scanning electron microscope, TEM (transmis-
sion electron microscopy), and light microscopy. This contribution summarizes our efforts from the last 8 years
to identify the physical origin of different shunt types in monocrystalline and multicrystalline silicon solar cells
made by different producers.
7
EXPERIMENTAL
The lock-in thermography investigations have been carried out partly with a home-made IR lock-in thermogra-
phy system based on a 128 128 pixel InSb focal plane array thermocamera
2,4
Amber AE 4128, and partly with
the commercial TDL 384 M ‘Lock-in’ thermography system made by Thermosensorik GmbH Erlangen,
1,4,5
having a resolution of 384 288 pixel. For most IR investigations the sample was covered with a 20-mm-thin
black-painted plastic film, which is sucked on to the cell by a vacuum and acts as an efficient IR emitter. For
identifying the shunts, EBIC and EDX investigations were made on JEOL 6300/6400 scanning electron micro-
scopes at acceleration voltages between 5 and 20 kV, which are equipped with an EBIC amplifier based on a
DLPCA-200 variable gain current amplifier made by FEMTO and an EDX system IDFix made by SAMx. TEM
investigations were made with a 1 MV acceleration voltage on a JEOL 1000 high-voltage transmission electron
microscope.
RESULTS
Before lock-in thermography allowed one to perform systematic shunt investigations, it was suspected that most
shunts in multicrystalline silicon solar cells originated at crystal defects such as grain boundaries, precipitates,
or dislocations. However, our investigations on hundreds of cells have revealed that by far most of the shunts are
process-induced, whereas material-induced shunts are rather an exception than the rule. Moreover, in the past it
was often assumed that shunts are generally characterized by a linear (ohmic) I–V characteristic. However, we
have found that in state-of-the-art silicon solar cells most shunts show a nonlinear (diode-like) I–V character-
istic. The linearity of the shunt characteristic can easily be checked by comparing lock-in thermograms taken at
05 V forward bias with thermograms taken at 05 V reverse bias. If a shunt shows the same thermal signal
under both conditions, its I–V characteristic is linear. Otherwise, the characteristic is nonlinear. In silicon solar
cells we have never found a case where the thermal signal was larger under low reverse bias than under the same
forward bias. However, under large reverse bias of 5to20 V new types of field-induced shunts may appear,
especially in multicrystalline cells, even in positions, where no remarkable shunts have been detected under
low-bias conditions.
8
Therefore, shunt investigations performed under large reverse bias may be misleading
for the shunting behavior at the working point of the cell. These field-induced shunts are beyond the scope
of this contribution. Generally, it can be stated that linear shunts are more dangerous for degrading the perfor-
mance of solar cells than nonlinear ones. In the following we will discuss the appearance of different process-
induced and material-induced shunt types in lock-in thermography and present microscopic results supporting
their physical interpretation.
530 O. BREITENSTEIN ET AL.
Copyright # 2004 John Wiley & Sons, Ltd. Prog. Photovolt: Res. Appl. 2004; 12:529–538
Process-induced shunts
Linear edge shunts
This was the dominant shunt type in earlier solar cell technologies, but with improving edge isolation technol-
ogies it appears less frequently now. Figure 1 shows a lock-in thermogram of a cell measured at 05 V forward
bias compared with one measured at 05 V reverse bias, both scaled from 0 mK T-modulation amplitude
(black) to 1 mK (white). The comparison of the brightness of the shunts measured under both conditions shows
that only some of the edge shunts marked by arrows in Figure 1 are ohmic.
In order to investigate their nature, EBIC investigations have been performed in the edge region in shunt
positions. Figure 2 shows such an EBIC image, together with the corresponding secondary electron (SE)
Figure 1. Lock-in thermogram of a cell containing edge shunts, measured under: (a) þ05 V; (b) 05 V bias
Figure 2. (a) SE image; (b) EBIC image at the position of a linear edge shunt. Image taken at an angle of 45
from the
rear side
SHUNT TYPES IN CRYSTALLINE SILICON SOLAR CELLS 531
Copyright # 2004 John Wiley & Sons, Ltd. Prog. Photovolt: Res. Appl. 2004; 12:529–538
image.
6
We are looking at an angle of approximately 45
from the rear to the edge of the cell. Here the pn
junction was opened by grinding a 45
bevel at the rear edge, which actually should show no EBIC signal.
In this bevel we see a horizontal stripe where an EBIC signal linearly decreases from a maximum value at
the emitter (left) to zero at the back contact. Such a behavior is expected for a pn junction, which is short-
circuited at one side. The stripe is a remainder of the emitter, which was not ‘opened’ by the edge insulation
procedure. So linear edge shunts are usually due to an incompletely opened emitter at the edge.
Nonlinear edge shunts
Those edge shunts which appear in Figure 1 predominantly under forward bias (þ05 V), show a nonlinear
(diode-like) I–V characteristic. These shunts can be interpreted to be recombination sites, which are acting
at the edge where the pn junction crosses the surface. If the recombination activity of these surface states
remains low, their recombination current has an exponential characteristic with an ideality factor of 2, as
expected from Shockley diode theory.
9
However, many of these shunts show an exponential characteristic with
an ideality factor of 3 and above.
3
These large ideality factors seem to be a general property of nonlinear local
shunts, since we have measured them very often. The physical mechanism leading to large ideality factors is not
clear yet.
Cracks and holes
It was shown that cracks in readily processed solar cells lead to a weak nonlinear edge recombination current,
similar to nonlinear edge shunts.
2
However, if a crack is already present in the wafer before processing, or if it
appears during processing before finally screen-printing the contact metallization, cracks may lead to severe
ohmic shunts. If the crack is already present in the raw wafer, an emitter layer may be established across the
crack. This layer shorts the emitter against the base contact, hence a linear (ohmic) shunt appears along the
crack, acting like a linear edge shunt. Figure 3 shows an example of such a crack-induced shunt at the lower
edge of a cell. The maximum T-modulation amplitude measured in Figure 3, which is a measure of the current
flowing in this individual shunt, was in the order of 10 mK. However, since this contribution deals rather with
the qualitative description of different shunt types than with their quantitative description,
5
the T-modulation
amplitudes are not given for all thermograms of this contribution. Note that the weak vertical bright stripe above
the crack is the electrical connection from the major grid to the crack shunt, which becomes thermographically
visible by Joule heating. If during screen-printing some metal paste penetrates a crack, after firing this may
Figure 3. Lock-in thermogram of a shunt caused by a crack at the lower edge of a cell
532 O. BREITENSTEIN ET AL.
Copyright # 2004 John Wiley & Sons, Ltd. Prog. Photovolt: Res. Appl. 2004; 12:529–538
produce especially strong linear shunts. The shunts described above may also emerge if there are any holes
present in a cell, e.g., coming from laser cutting.
Schottky-type shunts
In order to have a good blue response, the emitter of silicon solar cells has a thickness in the order of only
03 mm. If the emitter metallization is sintered, it consumes some silicon material. If the sintering parameters
are not optimized to the emitter doping profile, the emitter metallization may punch through the emitter, finally
leading to a Schottky-type direct contact between the metal and the p-type base material. As a rule, this contact
has a more or less rectifying characteristic, but it is far away from an ideal Schottky diode. Just as for Schottky
diodes containing some interface layer, its ideality factor is not unity, but may be even larger than two.
Schottky-type shunts may also appear if the emitter metallization is printed at a position having no emitter layer
at all. Such ‘holes’ in the emitter may be due to mechanical violation of texturization pyramide tips after emitter
doping or due to dust particles or insufficient contact of the phosphorus glass to the silicon in some positions.
Figure 4 shows two EBIC and one SEM image of a region below a grid finger (region between the two lines:
Kontakt) in the position of a nonlinear shunt after dissolving the metal with HF.
6
The dark spot in EBIC is a
mechanically violated tip of a large texturization pyramid, where the emitter is missing. In rare cases ohmic
shunts were also found below grid lines, the nature of which is still unclear.
Scratches
A scratch at the surface of a solar cell across the emitter layer brings the pn junction to a surface with a high
density of recombination centers. Thus, scratches act like nonlinear edge shunts, leading typically to an expo-
nential characteristic with a large ideality factor, like nonlinear edge shunts. Figure 5 shows a lock-in thermo-
gram of a cell containing two scratches, which were only visible under forward bias. If scratches are made
before emitter diffusion, or if they do not penetrate the pn junction, they generate local recombination centers,
also acting as nonlinear shunts, similar to strongly recombination-active grown-in crystal defects (see below).
Aluminum particles
Whenever aluminum reaches the front surface of a cell before firing, which may happen, e.g., by cross-contam-
ination during stacking of the cells, it will create shunts. During firing Al will alloy in, leading to a p
þ
-doped
Figure 4. EBIC and SE images of a shunt below a grid line
SHUNT TYPES IN CRYSTALLINE SILICON SOLAR CELLS 533
Copyright # 2004 John Wiley & Sons, Ltd. Prog. Photovolt: Res. Appl. 2004; 12:529–538
