21st European Photovoltaic Solar Energy Conference, 4-8 September 2006, Dresden, Germany
DAMP HEAT DEGRADATION OF CIGS-BASED PV MODULES
P-O. Westin, P. Neretnieks and M. Edoff
Ångström Solar Center, University of Uppsala, P.O.Box 534, SE-75121 Uppsala, Sweden
Corresponding author: per-oskar.westin@angstrom.uu.se, Phone +46-18-4713149, Fax +46-18-555095
ABSTRACT: Modules from the micro-pilot line at Ångström Solar Center were tested under the IEC 61646
conditions for damp heat testing, i.e. 85°C and 85% relative humidity for 1000h. Testing was made on both
unencapsulated and encapsulated modules, applying conventional glass/EVA/glass encapsulation. Alternative
encapsulation, such as replacing EVA with PVB, or adding an edge seal to the packaging was also evaluated in damp
heat testing. Degradation of the modules was monitored and it was shown how conventional encapsulation leads to a
reduced degradation that concentrates on FF losses suspected to be related to the window/CIGS interface. PVB
lamination was not successful, which may be related to unrefined lamination process. Adding an edge seal to the
module packaging effectively reduced the moisture ingress into the modules, showing efficiency losses within the 5%
limit for 1000h.
Keywords: Cu(InGa)Se2, Encapsulation, Degradation
1
INTRODUCTION
Commercial solar cell module manufacturers offer
long-term product warranties in the range of 20-30 yrs.
To ascertain long-term stability without actual field data,
several types of accelerated tests are employed. An
important tool in the qualification of solar cell modules is
the Damp Heat test, where modules are placed in a
humidity and temperature controlled environment, for the
duration of 1000h. According to the IEC61646 standard
modules must degrade no more than 5% in relative output
efficiency during 1000h at 85°C and 85% Relative
Humidity to pass the DH test.
The CIGS technology is one of the most promising
new technologies for achieving low-cost, high-efficiency
solar cell modules. A challenge for CIGS modules is to
prove long-term stability comparable to existing
technologies. Several investigators have shown how
conventional ethylene vinyl acetate, or EVA
encapsulation schemes fall short, and successful results
of damp heat testing have only been achieved with
“alternative materials” or “modified encapsulation
schemes” [1,2]. Such modified encapsulations are now
used, and CIGS or CIS modules that pass IEC 61646 are
commercially available [3].
Previous ÅSC work has primarily regarded
degradation of single cells [4,5,6,7] and modules without
encapsulation [8]. Results have shown how cells degrade
mainly by Voc and FF losses [4], and that modules suffer
additional FF losses, associated with series resistance
increase in the front contact and interconnect zones [8].
In the present work the damp heat behavior of CIGS
modules is presented, evaluating stability of modules
from our micro pilot line. The CIGS deposition was made
using an evaporation system that approaches an in-line
industrial process but works on a small scale using 5”
glass substrates (for more info see [9]). The impact of
encapsulation on module stability is discussed. In order to
achieve successful results of the qualification tests,
experimentation is underway with regards to materials
selection and design of the module packaging. As an
alternative material to EVA for encapsulation, poly-vinyl
butyral, PVB, has been suggested [10,11]. PVB has been
used for a long time in the automotive industry and
therefore has a proven UV-stability in the field, with
comparable transmission properties and good adhesion to
glass [10]. Its main disadvantage is long lamination cycle
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times, although this can be improved [11]. An advantage
of using PVB would be that it already complies with
certain building safety regulations, facilitating market
deployment [12].
2
EXPERIMENTAL
Modules were prepared in the ÅSC lab-line,
described elsewhere [9]. Edge deletion was performed
with a bead-blast setup using SiO2 beads (44-74 m),
isolating a ~1.5 cm perimeter around the cells. Modules
consist of 16 cells with a 75 cm2 active area, contacted by
tab-wires that were soldered or attached using a
conductive adhesive.
Lamination was performed in a PANAMAC®
vacuum laminator. A “standard” module is laminated
using an ETIMEX® fast-cure EVA, with a cycle time of
approx 20 min and Tcure of 150°C. Along with standard
modules, several experimental encapsulations have been
tested using alternative materials, e.g. PVB from
TROSIFOL®, or by using an edge sealant.
Furthermore, interconnect test structures (ITS) were
prepared, consisting of front and back contact sections
with interconnects that circumvented the CIGS p-n
junction (see ref [13]). The purpose of these structures
was to isolate, evaluate and monitor the degradation of
series resistance in modules. In all aspects except the
position of interconnect zones, the ITS were therefore
processed as modules, including buffers, edge deletion
and wiring. Individual front and back contact films were
also prepared by sputtering onto glass substrates.
Damp heat testing of modules and ITSs was
performed in a Vötsch V2020 climate chamber. The
modules were removed at different intervals and cooled
in ambient temperature (for at least 15 min) before
testing. No light soaking was performed prior to testing.
Module I/V-characteristics were examined using a
Quicksun® flash solar simulator setup. ITS structures
were examined for series resistance, and the contact films
for sheet resistance. Unencapsulated modules were also
tested at a lower relative humidity, at 85°C and 65% RH,
and both encapsulated and unencapsulated modules were
tested at 85°C in a standard oven as a “dry” reference.
21st European Photovoltaic Solar Energy Conference, 4-8 September 2006, Dresden, Germany
RESULTS
3.1 Unencapsulated modules
As in previous investigations, unencapsulated
modules (E and F) exhibit a sharp drop in Voc, losing
15% during the initial 60 hrs of the test (see Figure 1).
The module voltage then remains stable for the duration
of the test. FF losses show a similar behavior, rapidly
dropping 20-30% depending on the humidity conditions,
the 85%RH conditions imposing more severe losses than
65% RH. After these initial losses a steady but much
slower degradation is observed throughout the test (see
Figure 2). Higher humidity in the test chamber induces
slightly more severe FF degradation. Short circuit current
shows no degradation during the initial 100 hrs of testing
followed by a slow and steady decrease throughout the
testing.
As expected [1,7], dry heat testing shows steady
performance of the modules (D), underlining the
importance of humidity in the degradative processes of
CIGS based modules.
3.2 Encapsulated modules
Encapsulating the modules (A and C) improves the
resistance to damp heat induced degradation. Losses are
observed only after 500 hrs of testing, and essentially
only as FF losses (see Figure 2). Hence, encapsulation
hinders degradation for a period of time, after which a
point is reached when the encapsulation no longer
withstands the environmental attack. The degradation that
follows is not manifested by the same losses as that of
bare modules. This is discussed further in the following
section. Dry heat testing showed stable performance for
over 1000h.
Using a modified encapsulation scheme (B), with an
edge seal around the perimeter of the module, CIGS
modules pass the 1000h limit with less than 5% loss in
efficiency at 85°C and 85% RH (Sample B, maintains all
I/V-characteristics throughout the test. See Figure 1 and
3, and note that current was maintained for all
encapsulated samples).
Normalised Voc
1,2
1,1
A
1
B
0,9
C
0,8
D
0,7
E
0,6
F
0,5
0,4
0
200
400
600
Time [h]
800
1000
Figure 1. Voc development during 1000h of exposure to
different conditions: A, D – Dry 85°C; B, C, F – 85°C
and 85% RH; E – 85°C and 65% RH. Filled symbols are
standard encapsulated modules, outlined symbols are
unencapsulated, and X are encapsulated with an edge
seal.
1,2
1,1
Normalised FF
3
A
1
B
0,9
C
0,8
D
0,7
0,6
E
0,5
F
0,4
0
200
400
600
Time [h]
800
1000
Figure 2. FF development during 1000h of exposure to
different conditions: A, D – Dry 85°C; B, C, F – 85°C
and 85% RH; E – 85°C and 65% RH. Filled symbols are
standard encapsulated modules, outlined symbols are
unencapsulated, and X are encapsulated with an edge
seal.
3.3 Contacts and ITS
The front contact ZnO film showed a moderate but
slow increase in sheet resistance, reaching a 50% increase
at the end of 1000 hrs, while the Mo back contact sheet
remained conductive, even with strong visible evidence
of corrosive attack on the Mo surface, for between 100200 hrs after which corrosion had penetrated through the
450nm film and resistance increased catastrophically.
Similarly, the ITSs displayed a catastrophic increase
in series resistance after 400 hrs of damp heat exposure
(see Figure 3). This coincided with P3 isolation scribes
becoming completely transparent. Loss of connection in
the P3 scribe area occurred also for several of the
modules. Investigation by optical microscope of the P3
scribe during the damp heat treatment showed how the
surface attack is most pronounced along the center of the
scribe line where the mechanical tip has damaged the Mo
surface (see Figure 4). Adjacent Mo which has been
exposed through chipping of the CIGS/ZnO layers does
not exhibit the same level of corrosion.
By correlating the observed values for ITS series
resistance and contact film sheet resistance, an
approximation of the P2 interconnect contact resistance
(Rc) can be calculated. These results give an
approximation of interconnect resistance degradation
during damp heat testing. Plotting these results show how
the interconnect contribution to series resistance starts out
as modest but goes on to be the dominant factor after a
few hundred hours of damp heat exposure (see Figure 3).
Even when correcting for increased window resistance
caused by underlying buffer layer [4] (dashed lines), the
contact resistance dominates after a few hundred hours.
During this investigation, ITSs have been subject
only to unencapsulated damp heat testing. Testing of
encapsulated ITSs is underway.
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21st European Photovoltaic Solar Energy Conference, 4-8 September 2006, Dresden, Germany
4
40
Rtot
Rc
Rc (CdS)
ZnO R (CdS)
ZnO R
Resistances [Ohms]
35
30
25
20
15
10
5
0
0
100
200
300
Time [h
400
500
Figure 3. The development of resistances during damp
heat treatment of contact films and ITSs. Rtot is the total
resistance, R ZnO is the contribution by the front contact,
calculated from sheet resistance measurements of ZnO
films. RC is the derived ZnO/Mo connection resistance.
Correction for increased ZnO resistance in the presence
of underlying buffer layer is shown as dashed lines.
Figure 4. Corrosion of the Mo back contact in the P3
isolation scribe is mainly concentrated to the area where
the mechanical tip has damaged the surface.
3.4 PVB results
I/V-measurements do not show any conclusive
difference between the EVA and PVB laminated modules
regarding module performance and efficiency. Evidence
of FF degradation appears after 500 hrs of DH testing for
all modules. However, series resistance measurements
show how the PVB laminated modules reach
considerably higher values for series resistance towards
the end of the test. Visual inspection reveals how the
PVB modules exhibit a discoloration phenomenon, with
clouding of the films in a circular pattern from the edges
of the module toward the middle. PVB modules also
exhibit brown-colored substance around the tab wires
indicative of corrosion. On the other hand, modules
laminated with EVA remained visually clear but
exhibited corrosion along the P3 scribe for approximately
5 mm into the module. These differences are discussed
briefly below.
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DISCUSSION
4.1 Unencapsulated module degradation
It has been shown previously that the degradation of
unencapsulated CIGS PV modules is more severe than
that of single cells [8]. Concerning cells, degradation of
the CIGS absorber layer and/or the CIGS/CdS interface
has been put forward as the cause of the observed rapid
losses in Voc and FF [14,15]. In particular, Schmidt et al.
suggested that losses in FF would be principally caused
by changes in the front window and CIGS/window
interface, while Voc losses are related mainly to
degradation in the bulk of the absorber layer [14].
Additional FF losses in the modules are suggested to be
caused by increased series resistance in the front contact
layer and interconnects [8].
In this work, the modules that were placed in 65%
RH and 85°C showed slightly less degradation than those
in 85%RH. This difference was shown in the FF losses
(see Figure 2) whereas Voc behavior was similar for both
conditions. Modules in dry heat did not degrade to a
significant degree, and this, yet again, underlines the
importance of humidity in CIGS module degradation.
4.2 Encapsulation
Performance degradation is observed in all modules
that were encapsulated using only standard glass/EVA/
substrate configurations. This degradation set on after
around 400-500 hrs of DH testing suggesting either a
breakdown of the encapsulant, such as loss of adhesive
strength which can follow exposure to damp heat [16], or
a saturation phenomenon with humidity ingress reaching
a critical level.
Encapsulated module degradation was limited to FF
losses and increased series resistance. The pronounced
drop in Voc that is characteristic for unencapsulated
modules and cells is not present which would suggest that
the degradation that occurs of encapsulated modules is by
a different mechanism. Following the reasoning of
Schmidt et al.[14] it is possible that this degradation is
limited to the front window layer, causing not only an
increased series resistance but a shift in the electric field,
severely deteriorating the FF. The relative stability of the
Voc would be explained by stable absorber bulk.
Doubtlessly, there is also a considerable contribution to
FF degradation from increased series resistance in the
interconnects.
Three different reasons why degradation could differ
between encapsulated and unencapsulated modules seem
possible: 1) Difference in RH at the module surface, 2)
difference in oxygen-to-water ratio, and 3) encapsulant
material interactions, for example the degradation of
EVA into acetic acid. If difference in humidity is the only
factor, then it must be more pronounced than that of
lowering RH to 65%, as this investigation has shown.
Further investigation into the behavior of CIGS cells and
modules in differently composed environmental
conditions would help in understanding this. The
similarity in the degradation of modules laminated using
EVA and PVB would suggest that encapsulant chemistry
is not a primary cause for the losses, but may come into
play for secondary effects, namely the corrosion of tab
wires and Mo surfaces.
In any case, this behavior looks promising in that
module stability may be improved by alterations in the
front contact and interconnects, without manipulating the
absorber layer. It should also be mentioned that no
21st European Photovoltaic Solar Energy Conference, 4-8 September 2006, Dresden, Germany
particular efforts were made in cleaning or priming the
edge deleted regions to improve adhesion of encapsulant.
Further investigation will include adhesive properties
of the encapsulant in relation to processing and damp
heat.
4.3 Alternative encapsulation
The initial tests using PVB as a module encapsulant
show discouraging results. No particular gain was seen
with regards to the electronic performance of the
modules, and visual inspection revealed both clouding of
the PVB in the aperture area and brown corrosion
products precipitated along the tab wires. This in spite of
strong opinions that these types of problems were a thing
of the past with up-to-date PVB [12]. However, there are
PV module manufacturers that use PVB as an
encapsulant for commercial modules (e.g. ref [17]) and it
is not unlikely that the problems encountered in this
investigation will be helped with improved processing
and handling of the PVB films prior to lamination.
The observed P3 corrosion in EVA laminated
modules that was not present in the PVB modules suggest
a negative interaction between the EVA and the Mo film
that does not affect the performance of the module, or is
overshadowed by other degradation mechanisms.
Possibly acetic acid from the EVA could be the cause for
this.
Adding an edge seal to the conventional EVA
encapsulation we were able to produce modules that pass
the criteria of the EN 61646 standard for damp heat
degradation (see Figure 2). This will not hinder our
efforts to further investigate the stability of our modules
and the influence of the encapsulant on long-term
stability. For example improved interconnect stability and
optimization of lamination conditions for different
laminates.
4.4 Contacts and ITS
The molybdenum layer, and consequently the P3
interconnect, retains its low resistivity although severe
corrosion is observed visually. Only when the film is
completely penetrated by corrosion does the resistivity
increase, but then it is a catastrophic rise, and connection
between adjacent cells is broken. During the mechanical
patterning step the Mo surface was damaged by
scratching, increasing its sensitivity to corrosion. This has
previously been observed [2,18] and at present the
mechanical scribing step has been improved to cause less
damage to the underlying Mo contact.
The results of series resistance investigation using
ITSs show how the interconnect resistance in its initial
state contributes only modestly to the resistance of the
entire structure. During damp heat treatment this relation
shifts and after a few hundred hours the contact resistance
ZnO:Al/Mo becomes the dominant factor.
5
characteristic Voc drop when encapsulated modules
degrade. This could be explained by encapsulant material
interaction or changes in the humidity and oxygen levels.
The interconnect contact resistance was shown to start
out as inferior to the contribution of the front contact, but
increased during damp heat treatment to become
dominating after a few hundred hours.
6 ACKNOWLEDGEMENTS
This work is funded by the Swedish Energy Agency.
Many thanks to M. Ruth for providing the CIGS for the
modules and ITSs investigated. The authors would also
like to thank U.Malm and Dr. U. Zimmermann for
valuable insights, K. Ottoson for test collaboration and
the entire ÅSC and Solibro teams for cooperation and
support.
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CONCLUSION
In this investigation encapsulation and damp heat testing
of CIGS-based PV modules has been performed. It was
shown how encapsulation improves the stability of the
modules but that addition of an edge seal was necessary
in order to pass 1000h with less than 5% performance
loss.
Encapsulating modules appears to change the conditions
for degradation, reflected by the absence of the
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