Organic passivation layer on flexible Surlyn substrate for encapsulating organic photovoltaics

Article abstract of DOI:10.1063/1.4895719

Barrier materials are required for encapsulating organic devices. A simple methodology based on organic passivation layer on a flexible substrate has been developed in this work. Stearyl stearate (SS) was directly coated over the flexible Surlyn film. The barrier films with SS passivation layer exhibited much lower water vapor transmission rates compared to the neat Surlyn films. Moreover, the effect of the process of deposition of organic passivation layer on the resultant water vapor properties of the barrier films was evaluated. The accelerated lifetime studies conducted on encapsulated organic photovoltaics showed that the passivation layer improved the device performance by several fold compared to the non-passivated barrier films.

Full text of DOI:10.1063/1.4895719

Organic passivation layer on flexible Surlyn substrate for encapsulating organic
photovoltaics
Sindhu Seethamraju, Praveen C. Ramamurthy, and Giridhar Madras
Citation: Applied Physics Letters 105, 104102 (2014); doi: 10.1063/1.4895719
View online: http://dx.doi.org/10.1063/1.4895719
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APPLIED PHYSICS LETTERS 105, 104102 (2014)
Organic passivation layer on flexible Surlyn substrate for encapsulating
organic photovoltaics
1
Sindhu Seethamraju, Praveen C. Ramamurthy, and Giridhar Madras
1,2
3
1
Centre for Nanoscience and Engineering, Indian Institute of Science, Bangalore 560012, India
Department of Materials Engineering, Indian Institute of Science, Bangalore 560012, India
Department of Chemical Engineering, Indian Institute of Science, Bangalore 560012, India
2
3
(Received 11 July 2014; accepted 2 September 2014; published online 11 September 2014)
Barrier materials are required for encapsulating organic devices. A simple methodology based on
organic passivation layer on a flexible substrate has been developed in this work. Stearyl stearate (SS)
was directly coated over the flexible Surlyn film. The barrier films with SS passivation layer exhibited
much lower water vapor transmission rates compared to the neat Surlyn films. Moreover, the effect of
the process of deposition of organic passivation layer on the resultant water vapor properties of the
barrier films was evaluated. The accelerated lifetime studies conducted on encapsulated organic
photovoltaics showed that the passivation layer improved the device performance by several fold
compared to the non-passivated barrier films. VC 2014 AIP Publishing LLC.
[http://dx.doi.org/10.1063/1.4895719]
The use of barrier materials spans a wide range of pack-
aging industries such as food, pharmaceuticals, optics, and
electronics. Ultra low water vapor and oxygen permeable,
protective barrier coatings or passivation layers enable the
commercialization of flexible organic electronic devices
such as organic photovoltaics (OPVs), organic light emitting
diodes (OLEDs), and organic field effect transistors
deposition of organic molecules along with inorganic oxides
22,23
to decrease the extent of formation of defective films.
However, this modification further complicates the process
of deposition and requires to be optimized for the formation
of defect free films. The presence of structural sub-
nanometer defects and pinholes in the deposited layers
affects the performance of the barrier and controls the resid-
ual WVTR and OTRs. Further, the detection of the defect
1–3
(OFETs). These devices require flexible, viable, thermally
stable, and roll processable encapsulants with ultra high gas
21
sites at nano scale has been another challenging aspect.
4
barrier. Gases such as H O and O are detrimental to organic
devices due to the degradation of active components used in
The vacuum based processes, such as ALD and PVD, used
for the deposition of these passivation layers are also com-
plicated, expensive, and not suitable for roll processing.
Therefore, much simpler and economically viable ultra low
permeable barrier coatings need to be developed. Dense thin
film hydrophobic coatings of organic molecules over flexi-
ble substrates could offer a better solution for achieving
lower WVTRs.
2
2
5,6
the devices and formation of dark spots. The permeation of
polar H O molecules further enhances the degradation activ-
2
7,8
ity due to oxygen and light.
Though polymeric substrates offer flexible and an eco-
nomic solution for encapsulating organic devices, they do not
meet the requirement of ultra low water vapor transmission
ꢀ
6
ꢀ2
ꢀ1
rates (WVTRs) of <10 g m day and oxygen transmis-
ꢀ1
The process of permeation through the polymer matrix
occurs by dissolution of permeant and diffusion through the
matrix. In the case of inorganic passivation layers, the per-
meation is through the defect sites or pinholes. When hydro-
phobic compounds are deposited on polymer substrates,
lower WVTRs can be achieved due to the high energy barrier
ꢀ
5
sion rates (OTRs) of <10 cc m day . Therefore, various
ꢀ2
inorganic passivation layers, such as Al O , SiO , SiC, TiO ,
2
3
2
2
and SiN , based on atomic layer deposition (ALD), physical
x
vapor deposition (PVD), and plasma enhanced chemical vapor
deposition (CVD) have been being developed to achieve ultra
9–13
low WVTRs.
These single layered inorganic passivation
offered for the penetration of polar molecules such as H O.
2
layers deposited on flexible polymeric substrates, such as
polyethylene terephthalate (PET) and poly ethylene naphtha-
Further, the formation of pinholes and defect sites can be
avoided in organic coatings. Therefore, in this letter, we
show the ability of organic passivation layers on polymers in
improving moisture barrier.
ꢀ
2
ꢀ2
late (PEN), offer permeation rates only up to ꢁ10 g m
ꢀ
1 14,15
day . In the case of multiple, thicker and multilayered
coatings (with 5–6 bilayers of inorganic/organic layers) much
Octadecyl octadecanoate (Stearyl stearate, SS) was syn-
thesized by esterification reaction from stearic acid and stea-
lower WVTR (ꢁ10ꢀ g m day ) have been achieved.
5
ꢀ2
ꢀ1
16–18
24
However, the presence of defect sites/pinholes on the passiva-
19–21
tion layers affects the resultant barrier properties.
ryl alcohol and used for fabricating the barrier material.
Stearic acid and stearyl alcohol are taken in 1:1 molar ratio
The defects and pinholes in the thin films are formed
during their growth on polymer substrates due to the pres-
ence of dust particles, surface inhomogeneities on sub-
strates, and generated stresses while deposition. Thus,
defect sites are inevitable on such thin films coated on poly-
mers. Therefore, few researchers have worked on the
along with the catalyst, SnCl (10 mmol/mol) into a round
2
bottomed flask. The reaction was carried under inert condi-
ꢂ
tions at 85 C for 24 h. The formation of SS was verified by
Fourier transformed infra red spectroscopy (FTIR) spectros-
copy. The melting temperatures for SS and neat Surlyn were
ꢂ
ꢂ
found to be at ꢁ59 C and 98 C, respectively, from
0003-6951/2014/105(10)/104102/4/$30.00
105, 104102-1
VC 2014 AIP Publishing LLC
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1

104102-2
Seethamraju, Ramamurthy, and Madras
Appl. Phys. Lett. 105, 104102 (2014)
differential scanning calorimetry (DSC), shown in Figure S1
2
in the supplementary material.
5
The barrier films were fabricated by following three dif-
ferent methodologies. The first set of samples was prepared
by spin coating of dissolved SS in hexane, at 1500 rpm over
the Surlyn film. The second set of samples was fabricated
ꢂ
by direct spin coating (4000 rpm) of melted SS at 80 C. The
ꢂ
Surlyn substrate was maintained at 100 C during spin coat-
ing. Third set of samples was obtained by organic thermal
ꢀ
1
ꢂ
˚
evaporation of SS on to Surlyn at ꢁ10 A s at ꢁ 275 C.
ꢂ
The Surlyn substrate was maintained at 75 C during the
deposition in order to improve the adhesion of SS. The sam-
ples were designated as SSC, MSC, and OTE for solvent
spin coating, melt spin coating, and organic thermal evapo-
ration, respectively. SS was coated in three weight percen-
tages, 5%, 10%, and 15%, for each method of deposition
and numbered as 1, 2, and 3, respectively. Deposition of
higher weight content resulted in flaking of SS due to poor
adhesion and, therefore, was not studied further. Neat
Surlyn film was designated as S0.
The depression in melting point of Surlyn was observed
with increasing the weight content of SS from 5% to 15%
over Surlyn film from DSC (Figure S1). The melting point
ꢂ
ꢂ
decreased by 9 C for S0 to MSC1 and a further 9 C from
MSC1 to MSC2. This reduction in melting point is due to
the reduction in enthalpic contributions to the melting of
Surlyn with the addition of SS resulting from the positive
interactions between the two components.
The WVTRs through the barrier films were measured
using the Ca-test method. In Ca-test, the degradation of cal-
cium thin film due to oxidation of metallic calcium is deter-
mined from the increase of resistance with time due to the
FIG. 1. Reduction in normalized conductance from calcium degradation test
for the films by (a) solvent spin coating, (b) melt spin coating, and (c) ther-
mal evaporation of SS.
2
6
formation of nonmetallic Ca(OH)2. It is further correlated
with the number of H O molecules permeating through the
2
barrier film used for sealing the calcium thin film to calculate
WVTR using the following equation:
MSC3 films, respectively, with ꢁ2 order decrease in water
vapor permeabilities (Figure 1(b)). The films with thermally
evaporated SS, OTE1, OTE2, and OTE3 exhibited lowest
ꢀ
ꢁ
values for WVTR ꢁ0.006, 0.001, and 0.0001 g mꢀ day
2
ꢀ1
,
1
ꢀ
ꢁ
d
MH2O
l
R
respectively (Figure 1(c)). This shows that the simple coating
of SS by organic thermal evaporation over Surlyn decreased
the water vapor permeation by ꢁ4 orders. These results
show that the thermally evaporated SS showed better per-
formance for moisture barrier compared to spin coated SS.
With increasing the weight content of SS over Surlyn, the
WVTRs decreased for all the processes of deposition, result-
ing in lower values for samples with 15 wt. % of SS.
WVTR ¼ ꢀ2 MCa @q
:
(1)
b
dt
2
Calcium thin films of 200 lm thick (1 ꢃ 1 cm ) were
thermally evaporated on to cleaned glass substrates at the
centre. Aluminium electrodes were deposited on either side
to take electrical contacts for resistance measurements (sche-
2
5,27
matic shown in Figure S2).
These devices were further
sealed with neat Surlyn and SS coated barrier films, using
epoxy sealant on the edges. Thus, sealed calcium devices
Water contact angle measurements were carried out to
determine the effect of coating on the hydrophobicity of the
neat Surlyn film. It can be observed that the hydrophobicity
ꢂ
were exposed to humid environment at ꢁ95% RH and 35 C.
ꢂ ꢂ ꢂ
The calcium thin film resistances were monitored with time
(Figures 1(a)–1(c)) and the WVTRs through the barrier films
were calculated using Eq. (1) and given in Figures
of the Surlyn film increased from 78 to 105 (65 ) for pas-
sivated Surlyn films due to the coating of SS (Figure S4).
25
The contact angle did not vary significantly for different
processes of deposition of SS. This shows that the SS coating
increased the hydrophobicity of the Surlyn films.
25
S3(a)–S3(c).
The WVTR through the neat Surlyn film was observed
ꢀ
2
ꢀ1
to be ꢁ4 g m day . The barrier films with spin coated SS
To investigate the effect of organic thin film passivation
layer of SS on organic devices, OPVs were fabricated, encap-
sulated with S0, SSC3, MSC3, and OTE3 films as shown in
by dissolving in solvent exhibited one order reduction in
WVTR with ꢁ0.18, 0.14, and 0.015 g mꢀ day for SSC1,
SSC2, and SSC3 films, respectively (Figure 1(a)). The sam-
ples with melt spin coated SS exhibited lower values of
2
ꢀ1
25
the schematic Figure S5. These were subjected to acceler-
ꢂ
ated weathering studies (at ꢁ65 C and 95% RH). The life-
ꢀ
2
ꢀ1
ꢁ
0.08, 0.06, and 0.01 g m day for MSC1, MSC2, and
times of the devices under these accelerated conditions are
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104102-3
Seethamraju, Ramamurthy, and Madras
Appl. Phys. Lett. 105, 104102 (2014)
showed the best performances among all the encapsulated
OPV devices for water vapor barrier followed by MSC3 and
SSC3 films, which is in agreement with the previously dis-
cussed WVTR results.
Three different processes, namely, SSC, MSC, and OTE
were used for the fabrication of SS coating on Surlyn. It can
be clearly observed that the resulting barrier properties are
dependent on the process of deposition and are influenced by
molecular orientation and arrangement. In the case of SSC,
solvent was used and in MSC, the melt SS was directly
employed. The volatile solvent evaporates leading to the for-
mation of porous film resulting in lower water vapor barrier
when compared to that observed in MSC samples. In thermal
evaporation, SS sublimes and deposits on the substrate
resulting in an ordered assembly compared to that obtained
by spin coating. Further, thermal evaporation is conducted in
FIG. 2. Accelerated aging studies for encapsulated devices with neat Surlyn
and SS coated Surlyn barrier films.
ꢀ
6
vacuum (<4 ꢃ 10 bars) compared to spin coating process,
which is carried out under atmospheric condition. Therefore,
the SS layer on Surlyn is more uniform and ordered in OTE
samples compared to the samples prepared by spin coating.
Thus, OTE samples exhibited the highest barrier to water
vapor.
ꢂ
equivalent to ꢁ1000 times the lifetimes at 25 C and 45%
28,29
RH.
Representative I-V curves for the encapsulated (S0,
SSC3, MSC3, and OTE3) and unencapsulated devices are
2
5
given in Figure S6. From device (I-V) characteristics, it
was observed that the device current decreases with increas-
ing time of exposure to accelerate weathering conditions.
This can be attributed to the increased moisture content in the
device, which causes change of active layer morphologies in
In conclusion, three approaches have been employed in
this work with simple and viable processing to design better
water vapor barrier materials. SS has been coated on to flexi-
ble Surlyn film by spin coating and thermal evaporation to
improve the water vapor barrier. The increase of water con-
tact angles suggests the hydrophobic nature of SS. The
WVTR decreased by four orders for thermally evaporated
SS over Surlyn film. Moreover, the prolonged performances
(under accelerated weather conditions) of the encapsulated
OPV devices with SS on Surlyn when compared to neat
Surlyn indicate the effectiveness of the organic passivation
layer on flexible polymer substrates.
8
the device.
The average decrease in the normalized OPV efficien-
cies for at least 3 encapsulated devices is used to evaluate
the performance of the barrier films used for encapsulation
(Figure 2). The performance of the unencapsulated device
dropped to ꢁ1% in 30 min. This shows that the OPV devices
are unstable in humid environment. The OPV devices encap-
sulated with the Surlyn films deposited with thermally
evaporated SS (OTE3) retained ꢁ97%, while the device
encapsulated with neat Surlyn could retain only 10% of its
initial performance after 30 min of accelerated aging (Figure
The authors thank SERB, Department of Science and
Technology (1362/2014) for funding this work.
2). The Surlyn films with spin coated SS by melting and so-
lution could retain ꢁ75% (MSC3) and 55% (SSC3) of their
initial performances, respectively. After 90 min of acceler-
ated weathering, the encapsulated OPV devices by thermal
evaporation, melt and spin coatings, exhibited 54%, 34%,
and 11% of their initial performances, respectively. This
shows that the barrier films with the SS passivation layers
are effective in protecting the devices from moisture.
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