Degradation of Transparent Conductive Oxides, and the Beneficial Role of Interfacial
Layers
Heather M. Lemire, Kelly A. Peterson, Mona S. Breslau, Kenneth D. Singer, Ina T. Martin,
Roger H. French
Materials Science and Engineering Department, SDLE Center and MORE Center, Case
Western Reserve University, Cleveland OH 44106
ABSTRACT
The lifetime performance and reliability of photovoltaic (PV) modules are critical factors
in their successful deployment. Interfaces in thin film PV, such as that between the
transparent conductive oxide (TCO) electrode and the absorber layer, are frequently an
avenue for degradation; this degradation is promoted by exposure to environmental stressors
such as irradiance, heat and humidity. Understanding and suppressing TCO degradation is
critical to improving stability and extending the lifetime. Commercially available indium tin
oxide (ITO), fluorine doped tin oxide (FTO) and aluminum doped zinc oxide (AZO) were
exposed to damp heat (DH), ASTM G154 cycle 4, and modified ASTM G154 for up to 1000
hours. The TCOs’ electrical and optical properties and surface energies were determined
before and after each exposure and their relative degradation classified. Data demonstrate
that AZO degraded most rapidly of all the TCOs, whereas ITO and FTO degraded at lower to
non-quantifiable rates. One approach to suppress degradation could be to use interfacial
layers (IFLs), including organofunctional silane layers, to modify the TCO. We modified the
TCO surfaces using a variety of organofunctional silanes, and determined a range of surface
energies could be obtained without affecting the electrical and optical properties of the TCO.
Degradation studies of TCOs with a silane layer were also conducted. We found that an
inhomogeneous silane layer was able to delay the resistivity increase for ITO in DH.
INTRODUCTION
In the movement toward renewable energy sources, the cost of solar power is often cited
as a primary limiting factor for its widespread adoption. One effective way to reduce the
levelized cost of energy (LCOE, the total cost of the installation levelized over its’ lifetime) is
to increase the lifetime of the installation. Many components in PV systems require lifetime
extension; however, TCOs (transparent conductive oxides) are an identified failure mode in
many PV technologies that hold the potential to be cost-effective.
Transparent conductive oxides (TCOs) have widespread utility in PV and other
optoelectronic devices such as display screens, and organic light emitting diodes (OLEDs).
The combination of conductivity and optical transparency makes them ideal electrical
contacts in these systems. However, when deployed in the field, additional requirements for
lifetime and durability arise. Delamination at the TCO-absorber interface has been reported
for both thin film silicon solar modules 1 and OPV technologies. 2 In the bulk TCO, an
increase in resistivity and structural changes are observed. 3 In CIGS photovoltaics, replacing
the TCO of a fully degraded cell restores its properties to near peak performance. 4
TCO surfaces can be modified by the addition of a thin polymer layer, silanization
(resulting in covalent bond formation), and the chemisorption of small molecules. 5,6
PEDOT:PSS (poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate)) is a polymer
commonly used as an electron blocking layer in OLED and OPV devices. 7,8 PEDOT:PSS is
highly acidic and therefore corrosive to the TCO layer. 9,10,11 Allen et al. demonstrated that
placing a monolayer of silane between the PEDOT:PSS and the TCO blocked this interaction
without affecting the performance of the OLED.9 Silane and small molecule modifications
of TCOs have also been used to optimize rates of charged carrier transfer, and increase the
compatibility of the polar TCO with nonpolar materials used in OPVs and OLEDs.7
In this study, three commercially available TCO materials on glass were exposed to three
types of accelerated environmental exposures. Degradation was characterized by monitoring
changes in their optical and electrical properties and surface free energies. Silanes with a
range of functionalities were chosen to modify ITO: amine-terminated APDMES (3aminopropyldimethylethoxysilane) and APTES (3-aminopropyltriethoxysilane), allylterminated ATES (allyltriethoxysilane) and methyl-terminated OTS
(octadecyltrichlorosilane). ATES and OTS hold particular interest because of their
previously demonstrated utility as interlayers in organic electronics. 12 Finally,
allyltriethoxysilane (ATES), used by Allen et al., was applied to TCO samples and subjected
to two accelerated exposures, characterizing the degradation in comparison to the bare TCO.
EXPERIMENTAL METHODS
ITO (Colorado Concept Coatings LLC, Xin Yan Technologies), AZO (Zhuhai Kaivo
Electronic Components Co., Ltd.), and FTO (Hartford Glass Company Inc.) on glass were
purchased commercially. Samples were cleaned before collecting contact angle data (before
and after the accelerated exposure). Sample cleaning involves a series of 10 min sonications
in 30 °C detergent solutions, DI water and/or solvents (acetone, methanol and isopropanol),
drying under nitrogen gas, and a 15 min UV ozone clean at 60 °C (Novascan PSDP-UV8T).
Note that gentle scrubbing with an optical wipe was used after the IPA sonication to remove
strongly adhered surface species after damp heat exposure.
After cleaning, silane films were deposited by exposure of the TCO to a
silane/anhydrous toluene solution; see Table I for the deposition condition summary. For all
samples, the deposition was followed by a series of solvent rinses (anhydrous toluene,
chloroform/dichloromethane, and methanol/ethanol), drying under nitrogen, and a 10 min
bake at 120 °C. Silanized TCOs post environmental exposure were cleaned using the
procedure described above, omitting the UV-ozone exposure, as this removes the silane layer.
Table I. Silanes and their deposition conditions used in this study.
v/v (silane/
Solution
Silane
Time (min)
anhydrous toluene)
temperature (°C)
APDMES
0.2%
2-60
60
APTES
1%
2-60
60
OTS
0.2%
5-60
90
ATES (1)
1%
10
RT
ATES (2)
1%
10
RT
Agitation
None
None
None
None
Periodic
Three types of accelerated exposure were used in this study; modified ASTM G154 (“hot
irradiance”, 70 °C, 1.55 W/m2 @ 340 nm, comparable to 5X Suns UV irradiance), ASTM
G154 (“cyclic”, 8 h @ 70 °C, 1.55 W/m2 @ 340 nm, 4 h @ 50 °C with spray in dark), and
IEC 61626 Damp Heat (“DH”, 85 °C, 85% relative humidity). Samples were subjected to
exposures for up to 1000 h in ~168 h (1 week) increments; samples were arranged in the
environmental test chambers such that the TCO/TCO-silane surface directly exposed. After
each exposure step, 2-3 samples were removed, cleaned, characterized and stored.
To quantify the properties of the TCO, optical, electrical and surface measurement
techniques were used. The surface free energy (SFE) was calculated from contact angles
with water and diiodomethane using the Kruss EasyDrop, DSA4 system and the Wu method.
The unexposed TCO SFE can be seen in Table II. Reported contact angle values are the
average and standard deviation of five 50 frame video measurements on 1-2 samples; SFEs
are calculated from these contact angles. Note 1 mJ/m2 = 1 mN/m = 1 dyne/cm.
Table II. Baseline surface free energies and contact angles of TCOs using the Wu method.
SFE
ITO
AZO
FTO
2
2
78.8 ± 0.3 mJ/m
80.5 ± 2 mJ/m
82.2 ± 0.1 mJ/m2
Total
37.9 ± 0.3 mJ/m2
40.9 ± 1 mJ/m2
42.8 ± 0.1 mJ/m2
Disperse
41 ± 0.1 mJ/m2
39.6 ± 0.5 mJ/m2
39.4 ± 0.03 mJ/m2
Polar
4.5° ± 0.2°
7.2° ± 0.5°
2.3° ± 0.2°
Water C.A.
28.5° ± 0.3°
17° ± 2°
5.7° ± 0.4°
Diiodomethane C.A.
Transmission spectra were obtained using a Varian Cary 6000i or an Ocean Optics USB
4000 Fiberoptic UV-Vis Spectrophotometer. Percent haze, yellowness index, L*, a*, and b*
values were obtained using a Hunterlab UltraScan Pro Colorimeter. The resistivity of the
ITO films was measured using an EDTM R-chek four point probe, and the resistivity of the
AZO and FTO films was measured using a Keithley 2400 Source and Lucas Labs four point
probe. Elemental analysis was performed using a PHI Versaprobe 5000 Scanning X-Ray
Photoelectron Spectrometer (XPS). Spectra were collected using a monochromatic Al Kα Xray source (1486.6 eV, 75 W), hemispherical analyzer, and multichannel detector, with dual
beam charge compensation (10 eV electron flood gun and compensating 10 eV argon beam
gun). Data were collected over 1.1 x 0.270 mm areas. Reported values are averages and
standard deviations of 3 high-resolution measurements, collected using an analyzer pass
energy of 23.50 eV and a step size of 0.2 eV/step.
RESULTS AND DISCUSSION
Surface cleaning of the TCO after exposure is critical to tracking changes in the SFE.
Fig. 1 shows the water contact angle as a function of exposure time for ITO exposed to damp
heat and hot irradiance. Both treatments increase the water contact angle due to random
surface species accumulation
Figure 1. Water contact angles are sensitive to surface free
on the highly reactive TCO
energy changes of cleaned TCOs.
surface, demonstrated by the
plateau. However, if cleaned
before contact angle
measurements are taken, using
the same procedure as for the
TCO baseline, the contact
angle is discovered to trend
with exposure time, making it
an excellent degradation
indicator.
TCO Degradation
AZO was the least durable of the TCOs studied in this work: all properties degraded in
all exposure types with exposure time. For example, in the cyclic exposure, the yellowness
index, percent haze, and resistivity all increased steadily with exposure time (Fig. 2a.). The
transmission spectra also changed with exposure time (Fig. 2b.); the interference fringes seen
in the AZO film, whose period is a function of film thickness, shift with time, indicating a
change in film thickness. Additionally, the peak-trough height of the fringes decreased,
indicating a decrease in reflectivity of the surface, suggestive of surface roughening. These
observations are consistent with the modes of AZO degradation found by Pern et al., who
reported that their AZO film degraded rapidly and became porous after DH exposure.3
Fig. 2a. Electrical and optical properties of
Fig. 2b. Transmission spectra of AZO reveals
AZO increase with exposure time.
structural film changes.
In contrast to AZO, the resistivity, yellowness, and transmission spectra of the FTO did
not change under any of the exposure conditions studied here (data not shown). FTO was
also found to be most stable by Pern et al.3 The ITO films in this study were found to be
stable except for a small increase in resistivity with DH exposure.
TCO material properties were least affected by the hot irradiance exposure. In principle,
the hot irradiance exposure resembles the UV-ozone exposure used as the final cleaning step.
It was found that the hot irradiance exposure maintained a lower water contact angle than
either the DH exposure of an un-cleaned, unexposed sample (~75°), as seen in Fig. 1,
indicating that the exposure acts as a gentle UV ozone clean. This also explains the almost
complete lack of degradation seen in this exposure type.
Silane IFL customization
ITO was modified with APDMES, APTES, OTS and ATES. Water contact angles
measured on silane-modified ITO films are displayed in Fig. 3 as a function of deposition
time, tdep. Water contact angles range from 57 - 94°, and are consistent with literature values
for these silanes on glass or silica surfaces.10,13,14,15,16 While APDMES and APTES both have
amino functional group terminations, the differing number of reactive ethoxy groups results
in distinct film morphologies, apparent in the water contact angles. APDMES has one
hydrolyzable ethoxy group, which allows for a single bond to be made to the TCO surface,
resulting in a self-limiting monolayer 13; thus, increasing tdep does not affect the contact
angle, which is consistent with reported values for APDMES monolayers on glass (62.5°). 14
APTES has three hydrolyzable ethoxy groups, allowing for multiple points of surface
Figure 3. Water contact angles of several silanes on ITO.
attachment and
polymerization with other
APTES molecules, which
can result in formation of a
multilayer. Thus,
increasing the tdep typically
results in a thicker, rougher
film.10, 15
Due to vertical and
horizontal branching, the
film structure is complex
and can be
quite inhomogeneous. The
increased contact angle
compared to the APDMES
monolayer is attributed to
surface methyl groups from
the branched structure. These methyl groups are known to increase the measured contact
angle compared to a pure amine-terminated surface.15,16 As expected, OTS films have among
the highest contact angles, due to the methyl-terminated long chain.
ATES is a triethoxysilane, like APTES, and similarly can deposit multilayer films. The
film structure is sensitive to the deposition conditions, including agitation of the deposition
solution; agitation increases the water contact angle by ~ 20°. ATES was also deposited
(with agitation) on FTO and AZO surfaces. The SFE of ATES on the three TCOs are
presented in Table III. All ATES treated TCO surfaces are hydrophobic, as opposed to the
Table III. SFE and contact angles of ATES on TCOs using Wu method.
SFE
ITO
AZO
FTO
2
2
37.4 ± 3 mJ/m
35.6 ± 0.7 mJ/m
34.8 ± 2 mJ/m2
Total
29.8 ± 0.4 mJ/m2
31.1 ± 0.2 mJ/m2
31.8 ± 0.8 mJ/m2
Disperse
7.7 ± 2.4 mJ/m2
4.5 ± 0.5 mJ/m2
3 ± 1.4 mJ/m2
Polar
88° ± 5°
96° ± 1°
99° ± 3°
Water C.A.
52° ± 1°
51° ± 3°
51° ± 1°
Diiodomethane C.A.
bare clean TCO surface; the silanized TCO SFEs are nearly half of the bare, clean TCO value
(see Table 2), and these surfaces are more hydrophobic than an air-exposed TCO, indicating
the presence of the silane. XPS was conducted on these samples to confirm and semiquantify the deposition of the silane layer. Si, which is not present in the bare TCOs, was
detected by XPS on all three silanized TCOs. The %Si (of the total atomic percent) was 2 ±
0.4%, 1 ± 0.4%, and 0.6 ± 0.2% for AZO, ITO and FTO respectively, indicating that the
surface coverage could depend on the original TCO material.
Measurements of the optical and electrical properties of the silanized TCOs indicate that
the modification procedures did not significantly affect the conductivity or the transmission
of the TCO. All resistivity values were 48 ± 1 ohm/sq, before and after silane deposition.
Transmission in the visible range was defined as the average %T between 400 and 800 nm;
%T is 93 ± 1 for clean ITO. Treatment with ATES and APDMES did not affect the
measured %T; however, treatment with OTS and APTES lowered %T to 89 ± 1, comparable
to the PEDOT:PSS-modified ITO (%T = 89 ± 2).
TCO+Silane Degradation
The effects of silane modification (ATES, without agitation) on the critical properties of
the TCOs were examined after DH and hot irradiance exposures. The AZO was still severely
affected by the DH exposure, with no observed reduction in degradation rate. The FTO
showed no degradation with or without silane in DH. However, the ITO, which had a small
increase in resistivity over the course of a 4 week DH exposure, demonstrated a delayed,
slightly smaller resistivity increase when protected with a silane layer, as seen in Table IV.
Table IV. Resistivity increase of ITO in DH with and without silane.
Resistivity (Ω/sq) of ITO
Time (h)
Damp Heat
Damp Heat with ATES
0
48.9 ± 0.2
48.9 ± 0.2
168
52.0 ± 0.5
48.8 ± 0.6
336
52.3 ± 0.7
50.0 ± 0.4
Note that the ATES modified TCOs used in the exposure studies (deposited without
agitation) had lower contact angles and a larger deviation than those deposited with agitation.
The contact angles indicate the presence of a silane layer, but the large deviation within
samples and between deposition batches suggest inhomogeneous coverage. A more
homogeneous or thicker film may better preserve the TCOs electrical properties.
In the hot irradiance exposure, all TCOs’ SFEs after exposure demonstrated that the
silane layer had been removed from the TCO. The water contact angles after the exposure
have nearly the same hydrophilicity seen in bare, clean TCO surfaces (see Table V). This is
Table V. Water contact angles of bare, ATES silanized, and hot irradiance exposed TCOs.
Bare TCO,
TCO with ATES,
TCO with ATES,
Unexposed
Unexposed
Post Hot Irradiance
4.4° ± 0.5°
74° ± 2°
20° ± 2°
ITO
7°
±
1°
78°
±
2°
9° ± 1°
AZO
2.3° ± 0.5°
75° ± 4°
3.5° ± 0.3°
FTO
reasonable and expected, given that UV ozone clean would remove the ATES and that the hot
irradiance exposure simulates a low intensity UV ozone clean. A silane layer in an
encapsulated configuration would be affected differently by the hot irradiance exposure.
CONCLUSIONS
In this study, AZO, ITO and FTO were exposed to three types of accelerated
environmental exposures, and their optical and electrical properties and SFE were
characterized. It was found that AZO degraded most rapidly of all the TCOs in all the
exposure types, whereas ITO and FTO degraded at lower to non-quantifiable rates. A variety
of silane layers (APDMES, APTES, ATES and OTS) were applied to the TCOs; varying the
functional groups of a silane is an effective way of modifying the surface energy of the TCO
without affecting other critical properties such as %T and resistivity. The ATES molecule
was then selected for use in a degradation study. It was found that the inhomogeneous silane
layer was able to delay the increase in resistivity for ITO in DH exposure. It was also found
that the hot irradiance exposure removed the silane from the TCO surface, seen in the
decrease in water contact angles to be comparable with a bare, clean TCO.
ACKNOWLEDGEMENTS
The authors would like to thank Underwriters Laboratories for their support of this research.
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