Thin Solid Films 568 (2014) 111–116
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Thin Solid Films
journal homepage: www.elsevier.com/locate/tsf
Insulating characteristics of polyvinyl alcohol for integrated electronics
Eliana A. Van Etten a,⁎, Eder S. Ximenes b, Lucas T. Tarasconi c, Irene T.S. Garcia b,d,
Maria M.C. Forte a, Henri Boudinov b,c
a
Lapol, Escola de Engenharia, UFRGS, Porto Alegre, RS 91501-970, Brazil
PGMicro, UFRGS, Porto Alegre, RS 91501-970, Brazil
Laboratório de Microeletrônica, Instituto de Física, UFRGS, Porto Alegre, RS 91501-970, Brazil
d
Instituto de Química, UFRGS, Porto Alegre, RS 91501-970, Brazil
b
c
a r t i c l e
i n f o
Article history:
Received 14 November 2013
Received in revised form 23 July 2014
Accepted 30 July 2014
Available online 6 August 2014
Keywords:
Organic electronics
Gate dielectrics
Poly(vinyl alcohol)
Thermogravimetry analysis
Differential scanning calorimetry
Capacitance–voltage
Dielectric spectroscopy
a b s t r a c t
The aim of this work is to evaluate the effects of molecular weight, hydrolysis degree, and cross-link on the performance of Polyvinyl Alcohol (PVA) when applied as dielectric material in organic field effect transistors. For this
purpose, metal–insulator-structures and polymeric films were characterized. The polymer structure was analyzed by thermogravimetry and calorimetry, and the electrical characterization of the films was performed
through current–voltage and capacitance–voltage curves; and dielectric spectrometry. Cross-linkage, followed
by hydrolysis degree, presented the major impact on polymer properties, due to the strong influence on chain
mobility. The chain mobility increases the dielectric response and decreases the insulation capacity, generating
the need to compromise between these two properties. The largest drawback encountered was the high sensitivity of the films to ambient humidity. The best performance of the organic insulator was obtained from crosslinked films made of an incompletely hydrolyzed PVA.
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
Organic field effect transistors (OFETs) have been a hot topic in the
last 20 years due to their potential to produce low cost, high volume
and flexible electronics. These structures offer a good basis to develop
materials that will be applied in tomorrow's electronics [1–3].
Extensive studies have been done on semiconductor materials to understand and improve charge transport, but much less attention has
been paid to dielectric materials [4]. The path to development of an
all-organic transistor and flexible electronics requires the definition of
an organic gate insulator. The structure of the gate dielectric imparts
various surface and bulk characteristics that strongly influence a
device's performance [5]. A large variety of polymers have been tested
as dielectric layers; among them the most studied are poly(4vinylphenol) [6,4], polyimide [7], poly(methyl methacrylate) [8], poly(vinyl pyrrolidone) [9], poly(vinyl alcohol)(PVA) [10,11] and
photoalignment layers [12]. Reports have been made on the use of
self-assembly monolayer [13,14], multilayer [15], high-k inorganic/
smooth organic bilayer structures [16,17], polymer electrolytes and
ion gels [18,19]. However the integration of these layers would add
complexity,raising fabrication cost and sacrificing flexibility. Therefore
⁎ Corresponding author.
E-mail address: [email protected] (E.A. Van Etten).
http://dx.doi.org/10.1016/j.tsf.2014.07.051
0040-6090/© 2014 Elsevier B.V. All rights reserved.
a single organic dielectric layer is desired to allow a simple, cost effective
solution at low temperatures [20].
PVA is probably the most studied polymer to be applied as a dielectric material in organic thin-film transistors due to its versatility, high
solubility in water, low cost, non-toxicity, flexible hydrophilic network
and, above all, due to its electronic characteristics such as great insulating properties and notably high dielectric permittivity [21]. PVA is a
semi-crystalline polymer, which is predominately produced and used
for non-electronic applications. PVA is produced by the hydrolysis of
polyvinyl acetate (PVAc) obtained by the polymerization of vinyl
acetate monomer. The hydrolysis reaction is incomplete resulting in
polymers with different degrees of hydrolysis [22]. Considering that hydroxyl groups act as cross-linking sites and form hydrogen bonds between adjacent chains, PVA with different hydrolysis degrees and
chain sizes gives rise to different polymer structures. While there is a
good amount of work applying PVA as a dielectric material in OFETs, detailed work exploring the influence of its forms on electrical properties
is much less prevalent.
The electrical nature of many insulating materials, including PVA, is
only partially understood. Some important parameters have been
extensively studied. The effect of impurities has been discussed by
Egginger et al., which recommends extensive purifications to minimize
hysteresis [21,23]. A comparison between PVA and PVP polymers indicated that devices' characteristics are significantly affected by the concentration OH group in the gate dielectric [4]. PVA chain cross-linking
112
E.A. Van Etten et al. / Thin Solid Films 568 (2014) 111–116
2. Experimental
Table 1
PVA characteristics.
Grade
MW (g/mol)
Hydrolysis degree (%)
PVA1
PVA2
PVA3
PVA4
31–50 k
87–89
99
87–89
99
146–186 k
was evaluated by Bettinger et al., which states that devices fabricated on
non-cross-linked PVA films exhibited generally enhanced performance
compared to similar devices fabricated on cross-linked PVA dielectrics
[24]. However, the insulating properties of PVA can be significantly improved using a cross-linking process, enhancing immunity against
moisture and solvent [25]. The composition ratio of cross-linking
agent was evaluated previously [25,26]. Results for PVA cross-linked
with glutaraldehyde were presented by Wang et al., demonstrating
the structural flexibility of the film [27]. Additionally a detailed study
of the PVA molecular dynamics by dielectric spectroscopy and dynamic
mechanical analysis in a wider temperature range was conducted to
shed light into PVA molecular motions [28].
In this paper, we report a study concerning the effect of different molecular weight, hydrolysis degree, and reticulation on the performance
of the PVA for use as a dielectric material in OFETs. The aim of the
work is to compare the different types of PVA structures and seeking
for maximizing dielectric properties that would be useful in organic
electronics applications, such as low leakage current, high dielectric
constant and low hysteresis. For this purpose, MIS structures and polymeric films were prepared and characterized. We have focused on the
dielectric alone, excluding the influence of the organic semiconductor
and therefore no transistor was manufactured.
Four grades of PVA obtained from Sigma-Aldrich, 99.7% purity, described in Table 1, were evaluated. Ammonium dichromate (ADC),
99.9% purity, supplied by B. Herzog (São Paulo/Brazil), was used as a
cross-linking agent for PVA. All materials were used as received.
PVA solutions were prepared by dissolving 5% w/w of PVA in hot distilled and deionized water (60–80 °C). The solutions were then filtrated to
eliminate any particulates. The PVA solutions were prepared according to
PVA grade. The solutions without cross-linking agent were named PVA1,
PVA2, PVA3, PVA4, and those with ADC at a concentration of 0.8% w/w
were named PVA1–ADC, PVA2–ADC, PVA3–ADC and PVA4–ADC.
PVA films were prepared by casting the PVA solutions (5 ml) into a
polystyrene Petri dish, drying it at 60 °C for 36 h and annealing the obtained films at 90 °C in air for 24 h.
Thermal degradation behavior of the PVA films was investigated
using a thermogravimetric analyzer (TGA 2050, TA Instruments) within
a temperature range of 25–900 °C, at a heating rate of 20 °C/min under
nitrogen atmosphere. The degradation temperature, wherein percentage weight loss was determined as a function of temperature, and ash
content were recorded using 20 mg samples. The PVA characteristic
thermal properties such as glass transition and melting point were determined on a Differential scanning calorimeter (DSC 2910, TA Instruments). 5 mg samples at a heating/cooling rate of 10 °C/min, under
nitrogen atmosphere, were heated and hold at 110 °C for 7 min (first
run), cooled down to 20 °C, and then heated up to 250 °C (second
run). Temperature calibration was performed using Indium as a reference (Tm = 156.6 °C and Heat flow = 28.5 J/g) and the baseline was
calibrated without pans.
MIS structures were prepared by spin coating of the PVA solutions
on top of p-type(100) silicon wafers simulating the gate dielectric in
Weight Loss [%]
Deriv. Weight [%/°C]
Weight Loss [%]
100
PVA1
PVA2
PVA3
PVA4
80
60
40
20
a)
0
2.5
2.0
1.5
1.0
0.5
b)
0.0
100
PVA1-ADC
PVA2-ADC
PVA3-ADC
PVA4-ADC
80
60
40
20
c)
Deriv. Weight [%/°C]
0
0.6
0.4
0.2
d)
0.0
0
100
200
300
400
500
Temperature [°C]
600
Fig. 1. TGA and DTA curves of the PVA films.
700
800
900
E.A. Van Etten et al. / Thin Solid Films 568 (2014) 111–116
113
Table 2
Thermal decomposition data of the PVA films analyzed by TGA.
Samples
PVA1
PVA2
PVA3
PVA4
PVA1–ADC
PVA2–ADC
PVA3–ADC
PVA4–ADC
1° Event
2° Event
3° Event
Tmax (°C)
Weight loss (%)
Range (°C)
Tmax (°C)
Weight loss (%)
Range (°C)
Tmax (°C)
Weight loss (%)
60–165
50–175
60–130
40–175
30–135
50–165
40–170
40–160
125
113
96
108
98
102
92
105
6
8
7
7
7
8
8
9
260–395
240–335
260–425
240–330
150–410
165–400
170–400
160–405
330
280
338
269
249
239
256
232
77
70
70
70
37
30
30
34
400–520
385–535
425–525
370–525
410–530
400–525
400–525
405–530
460
457
458
430
453
431
450
430
9
12
15
14
19
22
20
19
organic field effect transistors. These films were prepared with constant
rotational speed of 2000 rpm for 50 s and then annealed at 90 °C for 2 h
on a hot plate in air. The thickness of these films was measured by
ellipsometry and optical reflectance techniques.
Aluminum circular contacts with diameters of 200 μm and thicknesses of 0.5 μm were obtained by resistive evaporation through a mechanical mask. Indium gallium alloy was manually applied to the rear
surface to obtain ohmic contacts. For characterization of the MIS capacitors, a HP4155A Semiconductor Parameter Analyzer was used to measure current–voltage (I–V) characteristics and a HP4284A LCR Meter
was used for capacitance–voltage (C–V) measurements with variable
frequency.
3. Results and discussions
Fig. 1 presents the TGA (weight loss versus temperature) and
DTA(derivative) curves of the samples (weight loss and its derivatives
with respect to the temperature) and Table 2 summarizes the thermal
events observed. The thermal degradation of the non-cross-linked
PVAs follows the pattern described in the literature for these polymers
[24]. TGA thermograms of PVA show three weight loss events. The
first is associated to water evaporation (Tmax: 90–125 °C), since it is a
hydrophilic polymer, the second to chain decomposition (Tmax: 270–
330 °C) and the third to sub-products degradation (Tmax: 430–460 °C)
due to cyclic conjugated compounds. A detectable difference can be observed between thermal degradation curves of the PVA with different
hydrolysis degrees (Fig. 1a and b). PVAs with a higher hydrolysis degree
degrade under lower temperatures, which is consistent with the thermal degradation mechanism described in the literature [29]. In degradation process, the earliest step is the removal of hydroxyl groups from the
chain. Polymers with a higher concentration of hydroxyl groups are
prone to start this process earlier, and therefore are more heat sensitive.
0
Heat Flow [J/g]
Residue at 900 °C (%)
Range (°C)
2
3
4
2
25
23
35
26
The same effect is noticed on the cross-linked PVA films (Fig. 1c and d),
however in a more subtle way.
The TGA curves of the cross-linked PVAs show different thermal profile due to the chain reticulation process involving the hydroxyl groups.
Although the weight loss of the first event is the same for the non- and
cross-linked PVA, it decreased significantly in the second event, since
the hydroxyl groups are linked to the cross-linking agent ADC changing
the chain scission mechanism. Therefore, there was an increase in the
third weight loss and residue content. The second peak of the chain scission overlaps the temperature where the pure ADC undergoes decomposition, around 180 °C [30]. The residual mass fraction of the crosslinked PVA samples was about 8–11 times larger than that of the noncross-linked ones. Differences on the curves thermal profiles, on the
weight loss and larger residual mass indicate the occurrence of a reaction between ADC and PVA that delayed overall PVA thermal degradation and induced char formation. No effect of the molecular weight on
the thermal degradation of PVA was observed.
Fig. 2 presents the DSC endothermic curves and Table 3 summarizes
the thermal transitions of the PVA films. The annealing of the PVA films
110 °C for 7 min (1st run) eliminated any moisture of the cross-linked
samples and no endothermic peak due to water evaporation was seen
in the 2nd endothermic run. Non-cross-linked PVA films presented a
small amount of moisture after the annealing and a small endothermic
peak was also present in 2nd run.
The non-cross-linked PVA films presented glass transition (Tg) and
melting (Tm) temperatures, as expected. The PVA samples with higher
hydrolysis degree (PVA2 and PVA4) presented higher crystallinity
(high enthalpy) and crystallites that are more homogeneous (welldefined Tm peak) compared to the samples with lower hydrolysis degree (PVA1 and PVA3). PVA with higher hydrolysis degree has higher
hydroxyl content and higher density of intermolecular interactions between these groups that favors the polymer crystallization. On the
other hand, a higher interaction between the hydroxyl groups decreases
the chain mobility; therefore PVA2 and PVA4 present higher Tg (75 °C)
than PVA1 and PVA3 (68 °C).
The cross-linked PVA films did not present thermal transition related
to Tm, since the reticulation process with ADC avoids any chain crystallization. The wide endothermic peak over 150 °C is due to the endothermic sublimation of ADC–PVA compounds, formed during the PVA
-5
-10
Table 3
DSC thermal properties of the PVA films.
PVA1-ADC
PVA2-ADC
PVA3-ADC
PVA4-ADC
PVA1
PVA2
PVA3
PVA4
50
100
150
Temperature [°C]
Fig. 2. DSC curves of the PVA films.
200
250
Sample
Tg
(°C)
TMAX(water loss)
(°C)
ΔHwaterlLoss
(J/g)
Tm
(°C)
ΔHmelting
(J/g)
PVA1
PVA2
PVA3
PVA4
PVA1–ADC
PVA2–ADC
PVA3–ADC
PVA4–ADC
68
75
69
75
–
–
–
–
127
127
127
124
182
195
181
196
3
4
4
3
99
238
135
260
194
221
189
227
–
–
–
–
43
90
32
65
–
–
–
–
114
E.A. Van Etten et al. / Thin Solid Films 568 (2014) 111–116
Current [A]
0
-10n
PVA1
PVA2
PVA3
PVA4
PVA1-ADC
PVA2-ADC
PVA3-ADC
PVA4-ADC
-20n
-30n
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
Electric Field [MV/cm]
Fig. 3. I–V Characteristics of the PVA films.
reticulation process, and detected by TGA analysis. The cross-linked PVA
films were amorphous and presented more rigid structure than the
non-cross-linked ones. No Tg could be detected until 150 °C by this
technique.
Fig. 3 presents the DC insulation response of the PVA films. As the
substrate is p-type Si, the measurements were done applying a negative
voltage to the top contact to maintain the MIS structure in accumulation
and to assure that the voltage drop fell only on the PVA layer. Considering that the insulation effect on any layer is strongly dependent on its
thickness, the responses to the electrical field were normalized, dividing
the applied voltage by the PVA thickness.
Cross-linking reduces leakage current by more than an order of magnitude, due to the reduction in chain mobility and due to the reduction
of PVA hydrophilicity. This result is in agreement with what was previously presented in literature [26]. The electric insulation response of all
cross-linked PVA films was very slight. On the non-cross-linked samples, Mw plays an important role. PVA3 and PVA4 with higher Mws
allow lower leakage currents. High Mw is known for reducing chain mobility of high polymer chains, since the density of chain entanglements
and the density of chain extremities are, respectively, higher and
lower than low Mw polymers. In a polymer, the extremities have higher
mobility than the chain segments and both entanglements and extremities stay on the amorphous region of a semi-crystalline polymer [31]. In
a semi-crystalline polymer, the charge transport involves mostly the
amorphous region, leaving a less important role for crystalline region.
Figs. 4 and 5 show the variation of the relative dielectric constant
and tan δ, respectively, of the different PVA films versus temperature
and frequency. An increase of dielectric constant is related to polarization which is dependent on chain freedom of movement which,
in turn, is dependent on time and temperature.
For non-cross-linked PVA films the dielectric response decreases
with the increase of frequency and increases with the increase of temperature. The coordinated bond movements necessary for polarization
are time dependent. For each temperature, there is a resonance frequency where the dipole movements are in phase with the electric
field. At this resonance frequency there is a jump in capacitance; loss
is minimized and tan δ reaches a maximum. The higher the temperature
is, the freer the chain movements are, resulting in a higher resonance
frequency. For lower temperatures, low resonance frequencies were
not reached due to equipment limitations.
Polarization is also dependent on the concentration of permanent dipoles in the polymer chain. At lower temperature and lower frequencies, PVA2 and PVA4, due to their higher density of hydroxyl groups,
have a higher dielectric constant. Hydroxyl groups function as strong
permanent dipoles that can align to the field, increasing polarization.
PVA2, having a lower Tg and less rigid amorphous region, has a higher
dielectric constant than PVA4.
The largest increase of dielectric constant occurs around the polymer
Tg, between 50 °C–110 °C, and at lower frequencies when the polymer
has sufficient time to align its dipoles to the electric field.
For cross-linked polymers, temperature has a less pronounced effect
on the chain mobility. The rigidity of the structure and the lower density
of dipoles (cross-linking compromises hydroxyl groups) generate lower
dielectric constant. Nevertheless, PVA2–ADC and PVA4–ADC present
higher amounts of dipoles when compared with PVA3–ADC and
PVA1–ADC and therefore show higher dielectric constants.
Another effect found in cross-linked PVA films is that the increase of
temperature reduces the dielectric constant due to water loss. Water
molecules, when present in the PVA structure, function as plasticizers.
Rel. dielectric constant
40
30
20
10
0
Rel. dielectric constant
24°C
50°C
80°C
110°C
150°C
200°C
PVA1
PVA2
PVA3
PVA1-ADC
PVA2-ADC
PVA3-ADC
PVA4
PVA4-ADC
12
10
8
6
4
1k
10k
100k
Frequence [Hz]
1M 1k
10k
100k
Frequence [Hz]
1M 1k
10k
100k
1M 1k
Frequence [Hz]
Fig. 4. Relative dielectric constant vs temperature and frequency of the PVA films.
10k
100k
Frequence [Hz]
1M
E.A. Van Etten et al. / Thin Solid Films 568 (2014) 111–116
1.5
24°C
50°C
80°C
110°C
150°C
200°C
PVA1
tan δ
1.0
115
PVA3
PVA2
PVA4
0.5
0.0
PVA2-ADC
PVA1-ADC
PVA4-ADC
PVA3-ADC
tan δ
0.3
0.2
0.1
0.0
1k
10k
100k
1M 1k
Frequence [Hz]
10k
100k
1M 1k
Frequence [Hz]
10k
100k
1M 1k
10k
Frequence [Hz]
100k
1M
Frequence [Hz]
Fig. 5. Tan δ vs temperature and frequency of the PVA films.
Hysteresis has been a commonly reported problem arising from the
use of polymers as gate dielectric in OFETs. The hysteresis and memory
behaviors in OFETs are normally proposed to be due to the charge storage, slow polarization of the gate dielectric and electron trapping in the
semiconductor [10]. Although hydroxyl density plays a hole on the film
hysteresis [33], a more relevant part is played by the ionic contaminates
that are generally present in the dielectric film. Impurities such as Sodium Acetate or ADC residual ions move with the applied electrical field
leading to charge separation within the electrically neutral dielectric
giving rise to strong electrode effects [23]. It was found that remnant
cations, especially for solutions containing larger amounts of crosslinking agent, resulted in a lowering of the electric field strength of
the c-PVA film. This suggests that Cr+5 and NH4+ radicals play an important role with respect to the gate leakage current of a transistor
with a cross-linked gate insulator [26,34].
They embed the polymer chains increasing the free volume and reducing Tg. A plasticizing effect of small amounts of water on Tg can lead to
slight differences in the Tg values reported elsewhere [32]. Elimination
of these water molecules, therefore, decreases chain mobility and consequently reduces dielectric constant. This effect is stronger in crosslinked PVA because chain mobility is less sensitive to the rise of
temperature.
The dielectric constants obtained in this work are in agreement with
what have been reported in different evaluations of PVA as a gate dielectric, between 5 and 10 [23,25–28].
Fig. 6 shows the hysteresis C–V curves of the studied films and
Table 4 presents the resulting sheet concentrations of mobile charges
calculated from these graphs. These concentrations are obtained by
multiplying the voltage differences between both curves by the maximum capacitance in accumulation. Cross-linked films present a higher
hysteresis than non-cross-linked films. This indicates the presence of
contaminates in the film structure that are most likely introduced by
the reticulation process. The cross-linking process introduces ionic species that, once hydrated, behave as mobile charges considerably increasing the hysteresis of the film. On non-cross-linked PVA films, the excess
moisture absorbed functions as fixed charges, allowing high current
passage and reducing hysteresis.
4. Conclusions
PVA is a promising candidate to be used as a gate dielectric in OFET
structures due to its suitable properties such as low leakage current,
high dielectric constant, simple application and good film properties.
The engineering of its structure can provide a more efficient dielectric
Capacitance [F]
16p
12p
8p
4p
Capacitance [F]
0 PVA1
PVA2
PVA3
PVA4
16p
12p
8p
4p
0 PVA1-ADC
-4 -3 -2
PVA2-ADC
-1
0
Voltage [V]
1
2
-4
-3
-2
PVA3-ADC
-1
0
Voltage [V]
1
2
-4
-3
-2
PVA4-ADC
-1
0
Voltage [V]
1
2
-4
-3
-2
-1
0
Voltage [V]
Fig. 6. C–V curves measured from accumulation to inversion and from inversion to accumulation for all PVA films.
1
2
116
E.A. Van Etten et al. / Thin Solid Films 568 (2014) 111–116
Table 4
Sheet concentration of the mobile charge in the PVA films.
Sample
Non-cross-linked PVA
(cm−2)
PVA1
PVA2
PVA3
PVA4
0.46
0.50
0.32
1.31
×
×
×
×
1011
1011
1011
1011
Cross-linked PVA
(cm−2)
5.90
1.43
5.96
4.73
×
×
×
×
1011
1011
1011
1011
material considering the device structure and its components. Control
of cross-linking, hydrolysis degree, molecular weight and moisture content can be used to optimize the characteristics of the film, depending of
the desired properties. Insulation and dielectric response are in antithesis; the increase of one results in the decrease of the other since both are
affected, either positively or negatively, by the chain mobility. This generates the need to compromise between these two properties.
The major impact on the polymer properties is given by reticulation.
The largest drawback encountered for the studied PVA films was the
high sensitivity to environmental conditions, especially moisture.
The methodology used for this study proved to be efficient in the
sense of concentrating on the analysis of the gate dielectric properties
and associating them with the characteristics encountered through
polymer structural analysis. Classical tools for polymer thermal analysis,
such as TGA and DSC, provided a good characterization of the structures
and allowed for complete structure and properties analysis.
The choice of PVA for an organic insulator must be on cross-linked
polymer with high Mw and incomplete hydrolyzation. The large hysteresis presented could be reduced by additional optimization of the crosslinker concentration and purification of the polymer, which must be investigated further.
Acknowledgment
This work was partially supported by Conselho Nacional de
Desenvolvimento Científico e Tecnológico (CNPq), INCT-NAMITEC,
Proc. 573.738/2008-4, Coordenação de Aperfeiçoamento de Pessoal de
Nível Superior (CAPES) and Fundação de Amparo à Pesquisa do Estado
do Rio Grande do Sul (FAPERGS).
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