Highly conductive PEDOT:PSS films prepared through a treatment with geminal diols or amphiphilic fluoro compounds

Article abstract of DOI:10.1016/j.orgel.2012.05.039

Poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) films with high conductivity can have important application as the transparent electrode of optoelectronic devices. In this paper, we report the significant conductivity enhancement of PEDOT:PSS through a treatment with germinal diols which have two hydroxyl groups connected to one carbon atom or amphiphilic fluoro compounds which have hydrophobic fluorocarbon groups and hydrophilic hydroxyl or carboxylic groups. Several compounds, including hexafluoroacetone, cyclohexanehexone, formaldehyde, acetaldehyde, and perfluorobenzophenone, which could convert into geminal diols, were used to treat PEDOT:PSS films. The conductivity enhancements are generally consistent with the equilibrium constants for the conversion of these compounds into geminal diols. PEDOT:PSS films were also treated with several amphiphilic fluoro compounds. The conductivity was significantly enhanced when PEDOT:PSS films were treated with hexafluoroisopropanol, trifluoroacetic acid and heptafluorobutyric acid, while it hardly changed when they were treated with 2,2,2-trifluoroethanol. Conductivities of more than 1000 S cm^-1 were observed on the treated PEDOT:PSS films. The mechanism for the conductivity enhancement of PEDOT:PSS through the treatment with geminal diols or amphiphilic fluoro compounds is attributed to the phase segregation of PSSH from PEDOT:PSS and conformational change of the PEDOT chains as the results of the compounds-induced reduction in the Coulombic attraction between the positively charged PEDOT and negatively charged PSS chains.

Full text of DOI:10.1016/j.orgel.2012.05.039

Organic Electronics 13 (2012) 1785–1792
Contents lists available at SciVerse ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Highly conductive PEDOT:PSS films prepared through a treatment
with geminal diols or amphiphilic fluoro compounds
Yijie Xia a,b, Jianyong Ouyang a,
⇑
a Department of Materials and Engineering, National University of Singapore, Singapore 117574, Singapore
b Institute of Materials Research and Engineering (IMRE), Singapore 117602, Singapore
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 6 April 2012
Received in revised form 10 May 2012
Accepted 12 May 2012
Available online 15 June 2012
Poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) films with high
conductivity can have important application as the transparent electrode of optoelectronic
devices. In this paper, we report the significant conductivity enhancement of PEDOT:PSS
through a treatment with germinal diols which have two hydroxyl groups connected to
one carbon atom or amphiphilic fluoro compounds which have hydrophobic fluorocarbon
groups and hydrophilic hydroxyl or carboxylic groups. Several compounds, including hexa-
fluoroacetone, cyclohexanehexone, formaldehyde, acetaldehyde, and perfluorobenzophe-
none, which could convert into geminal diols, were used to treat PEDOT:PSS films. The
conductivity enhancements are generally consistent with the equilibrium constants for
the conversion of these compounds into geminal diols. PEDOT:PSS films were also treated
with several amphiphilic fluoro compounds. The conductivity was significantly enhanced
when PEDOT:PSS films were treated with hexafluoroisopropanol, trifluroacetic acid and
heptafluorobutyric acid, while it hardly changed when they were treated with 2,2,2-trifluo-
roethanol. Conductivities of more than 1000 S cmꢀ1 were observed on the treated
PEDOT:PSS films. The mechanism for the conductivity enhancement of PEDOT:PSS through
the treatment with geminal diols or amphiphilic fluoro compounds is attributed to the
phase segregation of PSSH from PEDOT:PSS and conformational change of the PEDOT
chains as the results of the compounds-induced reduction in the Coulombic attraction
between the positively charged PEDOT and negatively charged PSS chains.
Keywords:
PEDOT:PSS
Conductivity enhancement
Geminal diol
Amphiphilic compound
Insulator-to-metal transition
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
However, ITO has problems of high price, scarcity of in-
dium on earth and high mechanical brittleness. These
Optoelectronic devices, such as liquid crystal displays
(LCDs), light-emitting diodes (LEDs), solar cells, touch
panel displays, lasers, and detectors, have important appli-
cation in our life. They are indispensable units for many
technologies. The market for optoelectronic devices has
been huge and is still rapidly expanding. At least one elec-
trode of an optoelectronic device is required to be trans-
parent in order to emit or harvest light. Conventionally,
indium tin oxide (ITO) is the most popular material as
the transparent electrode of optoelectronic devices.
problems cause ITO unsuitable for flexible electronic
devices that are regarded as the next-generation electronic
devices and bring a concern of the long-term availability
[1,2]. Hence, new transparent conductive materials are
urgently needed. Many materials have been proposed as
the transparent electrode of optoelectronic devices, includ-
ing conducting polymers [3–15], carbon nanotubes [16–
19], graphenes [20,21], and metal wires [22–24]. Among
them, poly(3,4-ethylenedioxythiophene):poly(styrenesul-
fonate) (PEDOT:PSS, chemical structure shown in Scheme
1) is promising as the next-generation transparent elec-
trode material. PEDOT:PSS is the most successful conduct-
ing polymer in terms of the practical application thanks to
⇑
Corresponding author.
E-mail address: mseoj@nus.edu.sg (J. Ouyang).
1566-1199/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.orgel.2012.05.039

1786
Y. Xia, J. Ouyang / Organic Electronics 13 (2012) 1785–1792
time. HFA hydrolyzes into 1,1,1,3,3,3-hexafluoropropane-
2,2-diol (HFP2OH) with water, which is a geminal diol with
two –OH groups connected to one C atom. Because normal
polyols can significantly enhance the conductivity of
PEDPT:PSS and geminal diols are special polyols, it is
expected that the conductivity of PEDOT:PSS can be en-
hanced by other geminal diols and the conductivity
enhancement should be related to the structure of the
geminal diols. In addition, HFP2OH is an amphiphilic com-
pounds owning to the two highly hydrophilic –OH groups
and the two highly hydrophobic –CF3 groups. An amphi-
philic compound can strongly interact with PEDOT:PSS,
as PEDOT is highly hydrophobic while PSS is highly hydro-
philic. Due to the high hydrophobicity of fluorocarbon
groups, such as –CF3, it is reasonably expected that other
amphiphilic fluoro compounds can significantly enhance
the conductivity of PEDOT:PSS as well. In this paper, we
report the conductivity enhancements of PEDOT:PSS films
through a treatment with various geminal diols and
amphiphilic fluoro compounds. The conductivity of
PEDOT:PSS can be enhanced from 0.3 S cmꢀ1 to more than
1000 S cmꢀ1, and the conductivity enhancement depends
on the structure of these compounds.
Scheme 1. Chemical structure of PEDOT:PSS.
its advantages in processability, mechanical and optical
properties. It can be dispersed in water and some organic
solvents, and high-quality PEDOT:PSS films can be readily
prepared by conventional solution processing techniques,
such as spin coating and printing [25–27]. Moreover,
PEDOT:PSS films have high transparency in the visible
range, high mechanical flexibility, and excellent thermal
stability. However, PEDOT:PSS does suffer a problem of
low conductivity. As-prepared PEDOT:PSS films from aque-
ous PEDOT:PSS solution usually have a conductivity below
1 S cmꢀ1, which is saliently lower than that of ITO [26,28].
It is thus important to significantly enhance the conductiv-
ity of PEDOT:PSS films for their application as the transpar-
ent electrode of optoelectronic devices.
2. Experimental
2.1. Treatment of PEDOT:PSS films
Several methods have been developed to significantly
enhance the conductivity of PEDOT:PSS since the report
by Kim et al. by adding dimethyl sulfoxide (DMSO) or
dimethylformamide (DMF) into PEDOT:PSS aqueous solu-
tion [29]. The addition of other organic compounds, includ-
ing ethylene glycol (EG), glycerol, other polyols
(compounds with two or more OH groups), ionic liquids,
anionic surfactants, and dimethyl sulfate, into PEDOT:PSS
aqueous solution can also significantly improve the con-
ductivity of PEDOT:PSS [13,30–38]. Similar conductivity
enhancement was also observed by treating PEDOT:PSS
films with polar organic compounds, salts, zwitterions, car-
boxylic or inorganic acids, or cosolvents [9,39–45]. Most of
the research works have been carried out on PEDOT:PSS
aqueous solutions commercially supplied by H.C. Starck,
which was taken over by Heraeus, or Agfa, and the conduc-
tivity of the treated PEDOT:PSS films depends on the grade
of the PEDOT:PSS aqueous solutions. The conductivity of
PEDOT:PSS prepared from Clevios P is usually below
200 S cmꢀ1 after a treatment, while the conductivity of
PEDOT:PSS films prepared from Clevios PH1000 can be en-
hanced to 680 50 S cmꢀ1 or higher after a treatment with
EG or DMSO [46]. Recently, Kim et al. prepared PEDOT:PSS
films from Clevios PH1000 solution added with EG and
then treated the PEDOT:PSS films in EG bath again [47].
PEDOT:PSS aqueous solution (Clevios™ PH 1000) was
purchased from H.C. Starck. The concentration of
PEDOT:PSS was 1.3% by weight, and the weight ratio of
PSS to PEDOT was 2.5 in solution. All other chemicals were
obtained from Sigma–Aldrich. All the materials were used
without further purification.
PEDOT:PSS films were prepared by spin coating the
Clevios™ PH 1000 aqueous solution on 1.3 ꢁ 1.3 cm2 glass
substrates, which were pre-cleaned sequentially with
detergent, de-ionized (DI) water, acetone and isopropyl
alcohol. The PEDOT:PSS films were dried at 120 °C on a
hot plate for 15 min. The treatment was performed by
dropping 100 lL geminal diol or fluoro compound or aque-
ous solution of a compound on a PEDOT:PSS film on a hot
plate at 140 °C. The films dried after about 5 min. They
were cooled down to room temperature, and then were
rinsed with deionized water and dried at 140 °C again.
2.2. Characterization of PEDOT:PSS films
The conductivities of dried polymer films were mea-
sured by the van der Pauw four-point probe technique
with a Keithley 2400 source/meter. The electrical contacts
were made by pressing indium on the four corners of each
PEDOT:PSS film on glass substrate. The temperature
dependences of the resistivities of the untreated and trea-
ted PEDOT:PSS films were tested using a Janis Research
VPF-475 dewar with liquid nitrogen as coolant and a
Conductus LTC-11 temperature controller. The UV–Vis-
NIR absorption spectra of the polymer films were taken
with a Varian Cary 5000 UV–Vis-NIR spectrometer, and
the AFM images were obtained using a Veeco NanoScope
The conductivity could be enhanced up to 1418 S cmꢀ1
.
Recently, we reported the treatment of PEDOT:PSS films
prepared from pristine Clevios PH1000 aqueous solution
with a fluoro compound, hexafluoroacetone (HFA, chemi-
cal structure shown in Scheme 2) [48]. The conductivity
of the PEDOT:PSS films is enhanced to 1164 S cmꢀ1 after
treated with HFA for one time, which is even higher than
that of PEDOT:PSS films treated with EG or DMSO for one

Y. Xia, J. Ouyang / Organic Electronics 13 (2012) 1785–1792
1787
O
O
O
O
O
F3C
CF3
O
hexafluoroacetone
cyclohexanehexone
F
O
F
F
F
F
F
F F
CH3COCH3
acetone
HCHO
F
F
perfluorobenzophenone
CH3CHO
formadehyde
acetaldehyde
OH
F3C
CF3
CF3CH2OH
hexafluoroisopropanol
2,2,2-trifluoroethanol
O
F
F
F3C
OH
F
F
CF3COOH
trifluoroacetic acid
heptafluorobutyric acid
Scheme 2. chemical structure of some germinal diols and amphiphilic fluoro compounds.
IV Multi-Mode AFM with the tapping mode. The X-ray
photoelectron spectroscopy (XPS) spectra were collected
with an Axis Ultra DLD X-ray photoelectron spectrometer
that the conductivity of PEDOT:PSS could be significantly
enhanced through a treatment with HFA [48]. HFA hydro-
lyzes into HFP2OH, a geminal diol,
equipped with an Al K X-ray source (1486.6 eV). The
a
thicknesses of the polymer films were determined with
an Alpha 500 step profiler.
CF3COCF3 þ H2O ! CF3—CðOHÞ2—CF3:
Other geminal diols may improve the conductivity of
PEDOT:PSS as well. Geminal diols are usually generated
through the hydrolysis of ketones or aldehydes. Chemical
structures of some molecules which can convert into gem-
inal diols are listed in Scheme 2. The equilibrium constant
for the conversion of a ketone or aldehyde into a geminal
diol depends on the molecular structure. It is about 106
for the conversion of HFA into HPF2OH. Such a high con-
version constant is related to the strong electron-with-
drawing ability of the two –CF3 groups, which causes the
middle C atom of HFA to be partially positively charged
so that the hydrolysis can take place. The equilibrium con-
stant decreases with the decrease in the positive charge on
3. Results and discussion
3.1. Conductivity enhancement of PEDOT:PSS films by geminal
diols
The conductivity of PEDOT:PSS can be significantly en-
hanced by EG and other polyols, while the conductivity is
negligible when alcohols with only one OH group are used
[32]. This suggests that a geminal diol, which has two –OH
groups connected to one C atom and can be considered as a
special polyol, can significantly enhance the conductivity
of PEDOT:PSS. This is evidenced by our recent observation

1788
Y. Xia, J. Ouyang / Organic Electronics 13 (2012) 1785–1792
the middle C atoms. For example, it becomes 103 for the
conversion of formaldehyde into its geminal diol,
H–C(OH)2–H,
Table 2
Conductivities of PEDOT:PSS films treated with amphiphilic fluoro
compounds.
Compounds
Melting
point (°C)
Boiling
point (°C)
Conductivity
a
(S cmꢀ1
)
HCHO þ H2O ! H2CðOHÞ2:
HFA hydrate
ꢀ129
ꢀ3
ꢀ28
58
74
1164
1022
11
It further decreases to 10ꢀ3 for the conversion of ace-
tone into CH3–C(OH)2–CH3,
Hexafluoroisopropanol
2,2,2-Trifluoroethanol
Heptafluorobutyric
acid
ꢀ45
ꢀ18
120
578 (5.6 M)
CF3COCF3 þ H2O ! CH3—CðOHÞ2—CF3:
Trifluoroacetic acid
ꢀ15
72
473
a
The conductivities of PEDOT:PSS films treated with HFA,
formaldehyde and acetone are consistent with the equilib-
rium constants for the conversion of these compounds into
geminal diols (Table 1). The melting points and boiling
points of these compounds are also listed in Table 1. A
treatment of PEDOPT:PSS films with 12.2 M formaldehyde
aqueous solution, which corresponds to the concentration
of 36.7% by weight, enhances the conductivity from 0.3 to
862 S cmꢀ1, while a treatment with acetone hardly im-
prove the conductivity. The conductivity of the PEDOT:PSS
films treated with acetaldehyde is 83 S cmꢀ1, which is
higher than that treated with acetone but lower than that
treated with formaldehyde. This can be understood in
terms of the chemical structures of these compounds. It
is inferred that the equilibrium constant for the conversion
of acetaldehyde into geminal diol should be between those
of formaldehyde and acetone.
Conductivity of PEDOT:PSS after treated with a neat compound or
aqueous solution of a compound with the highest concentration. The
number in parenthesis indicates the concentration of the aqueous solu-
tion used for the treatment.
3.2. Conductivity enhancement of PEDOT:PSS films by
amphiphilic fluoro compounds
The geminal diol, HFP2OH, is also an amphiphilic mole-
cule. Amphiphilic compounds can shield the Coulombic
attraction between PEDOT and PSS and lead to the conduc-
tivity enhancement of PEDOT:PSS, since PEDOT is hydro-
phobic while PSS is hydrophilic. Fluorocarbon groups are
highly hydrophobic. These inspire us to investigate the
treatment of PEDOT:PSS with other amphiphilic fluoro
compounds. The chemical structures of the fluoro com-
pounds are listed in Scheme 2 as well.
We also investigated the treatment of PEDOT:PSS with
cyclohexanehexone. Only its geminal diol structure, dod-
ecahydroxycyclohexane, exists in water [49]. Cyclohexane-
hexone is a solid at room temperature, and it has limit
solubility in water. We found that the conductivity of PED-
OT:PSS films was enhanced to 349 S cmꢀ1 after a treatment
with 0.1 M cycloehexanehexone aqueous solution. This
conductivity is even higher than the conductivity
(177 S cmꢀ1) of PEDOT:PSS films treated with 0.1 M HFA.
In addition, we investigated the treatment of
PEDOT:PSS with perfluorobenzophenone which has a car-
bonyl group connected with two fluorobenzene rings. It
is a solid at room temperature. Its aqueous solution gives
rise to no change in the conductivity of PEDOT:PSS. It
may be difficult for this molecule to convert into geminal
diol because the attachment of two –OH groups to the C
atom of the carbonyl group can destroy the conjugation
of the carbonyl group with the two fluorobenzene rings.
Table 2 lists the conductivities of the PEDOT:PSS films
treated with various fluoro compounds. Their melting
points and boiling points are listed as well. All these fluoro
compounds are liquid at room temperature. Conductivity
as high as 1022 S cmꢀ1 was observed on the PEDOT:PSS
films treated with hexafluoroisopropanol (HFP). This con-
ductivity is quite close to that treated with HFA [48]. The
conductivity enhancement cannot be attributed to the
effect by geminal diol, because HFP does not convert to a
geminal diol in water. But HFP has the amphiphilicity sim-
ilar to HFP2OH owing to the two –CF3 groups and the –OH
group.
100
80
Table 1
Conductivities of PEDOT:PSS films treated with geminal diols.
HFA treated
HFP treated
formaldehyde treated
60
Name
Melting
Boiling
Conductivity
a
point (°C)
point (°C) (S cmꢀ1
)
Cyclohexanehexone
octahydrate
99
345
349 (0.1 M)
40
HFA trihydrate
Formaldehyde
Acetaldehyde
Acetone
ꢀ129
ꢀ92
ꢀ28
ꢀ19
20
56
359
1164
862 (12.2 M)
83 (18.2 M)
1
20
0
ꢀ124
ꢀ95
Perfluorobenzophenone 90–95
0.3 (1 M)
400
450
500
550
600
650
700
a
Conductivity of PEDOT:PSS after treated with a neat compound or
Wavelength (nm)
aqueous solution of a compound with the highest concentration. The
numbers in parentheses indicate the compound concentrations of aque-
ous solutions used for the treatments.
Fig. 1. Transmittance spectra of PEDOT:PSS films treated with HFA, HFP
and 12.2 M formaldehyde solution.

Y. Xia, J. Ouyang / Organic Electronics 13 (2012) 1785–1792
1789
conductivity of PEDOT:PSS films, the treatment does not
affect the transparency of the polymer films in the visible
range. Fig. 1 presents the transmittance spectra of PED-
OT:PSS films treated with HFA, HFP and formaldehyde.
These PEDOT:PSS films are about 50 nm thick. Their trans-
mittances are more than 90% in the visible range from 400
to 700 nm. As we observed that the HFA-treated PED-
OT:PSS films could have transmittance and surface resis-
tance comparable to ITO on plastic [48], the HFP- and
formaldehyde-treated PEDOT:PSS films can have compara-
ble transmittance and surface resistance to ITO on plastic
as well.
The conduction mechanism of PEDOT:PSS films treated
with HFA, HFP and formaldehyde were studies by measur-
ing their resistances from room temperature down to
110 K (Fig. 2a). At temperature below 200 K, the resis-
tances of these PEDOT:PSS films monotonically increase
with the lowering temperature, which is similar to that
of untreated PEDOT:PSS films. But the temperature depen-
dences of the resistances of these treated PEDOT:PSS films
are different from that of untreated PEDOT:PSS films when
the temperature is higher than 200 K. The resistances are
almost constant, especially for the PEDOT:PSS films treated
with HFA and HFP.
(a)
1.0
0.8
0.6
0.4
0.2
0.0
HFA treated
HFP treated
formaldehyde treated
untreated
100
150
200
250
300
Temperature (K)
1
0.8
0.6
0.4
HFA treated
HFP treated
formaldehyde treated
untreated
Fig. 2b shows the analysis of the temperature depen-
dences of the resistances with the one-dimensional vari-
able range hopping (VRH) model [29,34,50],
"
#
0.2
ꢀ
ꢁ
1=2
(b)
0.10
T0
T
RðTÞ ¼ R0 exp
;
0.06
0.07
0.08
T-1/2 (K-1/2
0.09
)
2
where T0 = 16/kBN(EF)L//L\ is the energy barrier between
localized states, N(EF) is the density of the states at the
Fermi level, and L// (L\) is the localization length in the par-
allel (perpendicular) direction. The untreated PEDOT:PSS
film has a linear logR vs Tꢀ1/2 in the whole temperature
range from 110 to 310 K, and the T0 value estimated from
this linear relationship is 1901 K. The logR vs Tꢀ1/2 data
of the HFA- and HFP-treated PEDOT:PSS films have a linear
relationship only at the low temperature range below
200 K, and the T0 value estimated from the linear relation-
ships are 99 and 127 K, respectively. The T0 value of form-
aldehyde-treated PEDOT:PSS film at T < 200 K is 194 K. The
T0 values in the low temperature range are consistent with
the conductivities of these PEDOT:PSS films at room tem-
perature, which are 0.3, 1164, 1022, and 862 S cmꢀ1 for
the untreated, HFA-, HFP-, and formaldehyde-treated
PEDOT:PSS films, respectively. T0 decreases with the
increasing conductivity. It thus suggests that the treatment
with a geminal diol or an amphiphilic fluoro compound
can lower the energy barrier for the interchain and inter-
domain charge hopping.
Fig. 2. Temperature dependences of the normalized resistances of
PEDOT:PSS films. (a) Normalized resistances of untreated and treated
PEDOT:PSS films versus temperature. (b) Analyses of resistance–temper-
ature relationships of untreated and treated PEDOT:PSS films with the
one-dimensional VRH model. The resistances are normalized to those of
the corresponding PEDOT:PSS films at 110 K.
However, the conductivity enhancement became less
significant when 2,2,2-trifluoroethanol was used for the
treatment. The conductivity was only 11 S cmꢀ1 after a
treatment with 2,2,2-trifluoroethanol. It seems that the
presence of two hydrophobic groups or two hydrophilic
groups in the molecule is important for the significant con-
ductivity enhancement.
This assumption was further confirmed by the investiga-
tion on the treatments of PEDOT:PSS films with trifluoroace-
tic acid and heptafluorobutyric acid. The carboxylic group
can be considered as two polar groups, a carbonyl group
and a hydroxyl group. Though trifluoroacetic acid is different
from 2,2,2-trifluoroethanol only by the carbonyl group, the
former can enhance the conductivity to 473 S cmꢀ1, much
more significant than the latter. The conductivity enhance-
ment becomes even more significant for heptafluorobutyric
acid, which has more fluoro atoms on the molecule.
The logR–Tꢀ1/2 curves of the PEDOT:PSS films treated
with HFA, HFP and formaldehyde suggest that the polymer
films experience a phase transition at around 230 K. The T0
value of PEDOT:PSS treated with formaldehyde decreases
to 54 K at the temperature range from 230 to 310 K. The
resistances of the PEDOT:PSS films treated with HFA and
HFP are almost insensitive to temperature at the tempera-
ture range of more than 230 K. Thus, the phase transition
at around 230 K for the PEDOT:PSS films treated with
3.3. Optical and electrical properties of treated PEDOT:PSS
films
Though a treatment with a geminal diol or an amphi-
philic fluoro compound can significantly enhance the

1790
Y. Xia, J. Ouyang / Organic Electronics 13 (2012) 1785–1792
HFA and HFP can be an insulator-to-metal phase transition.
Similar insulator-to-metal phase transition was also ob-
served on polyaniline by Lee et al. [51]. The phase transi-
tion of polyaniline is related to the crystalline structure
of the polymer. However, we did not observe any X-ray dif-
fraction band for untreated PEDOT:PSS films and
PEDOT:PSS films treated with HFA, HFP and formaldehyde.
vaporize during the treatment and/or be rinsed away by
water.
These results suggest that the mechanism for the con-
ductivity enhancement of PEDOT:PSS by these geminal
diols and amphiphilic fluoro compounds can be similar to
that by HFA and polyols. It has been understood that the
polyol treatment of PEDOT:PSS can give rise to the phase
segregation of PSSH chains from PEDOT:PSS and the
conformational change of the PEDOT chains [32,33]. The
amphiphilic fluoro compounds can also give rise to
changes in the composition and structure of PEDOT:PSS.
The amphiphilic fluoro compound-induced phase segrega-
tion of the PSSH chains can be ascribed to the shielding of
the Coulombic attraction between PEDOT and PSS by the
amphiphilic flruoro compounds. Some of these PSSH
chains can be removed from the polymer films by the rinse
with water. The wash-away of some PSSH chains from the
PEDOT:PSS films can reduce the less conductive shell for
the conductive PEDOT chains and lead to the conductivity
enhancement. The phase segregation can not only reduce
the amount of insulator PSSH in the polymer film but also
the conformation of the PEDOT chains [54].
The surface morphology of the PEDOT:PSS films chan-
ged after the treatment with geminal diols and fluoro
compounds as studied by AFM (Fig. 5). The rms rough-
nesses are 1.63 nm, 1.68 nm, 1.44 nm, and 2.27 nm for
the untreated, HFA-, HFP-, and formaldehyde-treated
PEDOT:PSS films. The apparent rms roughness increases
to 4.88 nm for the PEDOT:PSS film treated with 2,2,2-tri-
fluoroethanol. Obviously, that is mainly due to the
appearance of pores in the 2,2,2-trifluoroethanol-treated
PEDOT:PSS film, which may be due to the swelling of
the polymer chains in 2,2,2-trifluoroethanol and the
3.4. Mechanism for the conductivity enhancements
The mechanism for the conductivity enhancements of
PEDOT:PSS though the treatment with geminal diols and
amphiphilic fluoro compounds was studied by various
chemical and physical characterizations. The UV absorp-
tion spectra of PEDOT:PSS films are shown in Fig. 3. The
intensities of the two absorption bands, which originate
from the aromatic rings of PSS, drop after a treatment with
HFA or HFP. This change indicates the decrease of the PSSH
amount in PEDOT:PSS after the treatment. On the other
hand, the intensities of the two absorption bands hardly
change after a treatment with 2,2,2-trifluoroethanol. It
suggests that the treatment with 2,2,2-trifluoroethanol
does not give rise to remarkable change in the composition
of PEDOT:PSS. The changes in the UV absorption spectra of
PEDOT:PSS films after the treatment with different com-
pounds are consistent with the effects of these compounds
on conductivity of PEDOT:PSS.
The reduction of PSSH from the PEDOT:PSS films after a
treatment is confirmed by the XPS spectra of the PED-
OT:PSS films (Fig. 4). The two XPS bands with binding ener-
gies between 166 and 172 eV are the S 2p bands of the
sulfur atoms in PSS, whereas the two XPS bands with bind-
ing energies between 162 and 166 eV are the S 2p bands of
the sulfur atoms in PEDOT [52,53]. The S 2p XPS intensity
ratio of PEDOT to PSS increases after the HFA and HFP
treatment, while it only slightly changes after the treat-
ment with 2,2,2-trifluoroethanol.
vaporization of 2,2,2-trifluoroethanol.
A close-up look
on the AFM image can find the change in the morphol-
ogy of the PEDOT:PSS films after the treatments. Fibrous
structures can be observed on the PEDOT:PSS films trea-
ted with HFA, HFP and formaldehyde, while they are ab-
sent for the untreated and 2,2,2-trifluoroethanol-treated
PEDOT:PSS films. The change in the AFM images can be
attributed to the phase segregation of PSSH chains from
Moreover, no fluorine XPS signal was detected on
PEDOT:PSS films treated with HFA and other fluoro
compounds. Hence, these compounds do not remain in the
PEDOT:PSS films after the treatment. They may completely
1.0
untreated
HFA treated
HFP treated
2,2,2-trifluoroethanol
treated
1.0
untreated
2,2,2-trifluoroethanol treated
HFP treated
HFA treated
0.8
0.8
0.6
0.4
0.2
0.0
0.6
0.4
0.2
0.0
172
170
168
166
164
162
200
250
300
350
Wavelength (nm)
Binding Energy (eV)
Fig. 3. UV absorption spectra of PEDOT:PSS films untreated and treated
Fig. 4. S2p XPS spectra of PEDOT:PSS films untreated and treated with
with HFA, HFP, and 2,2,2-trifluoroethanol.
HFA, HFP, and 2,2,2-trifluoroethanol.

Y. Xia, J. Ouyang / Organic Electronics 13 (2012) 1785–1792
1791
Fig. 5. AFM images of PEDOT:PSS films (a) untreated and treated with (b) HFA, (c) HFP, (d) formaldehyde solution, and (e) 2,2,2-trifluoroethanol. The unit
for the AFM images is m.
l
PEDOT:PSS films after the treatment. It also suggests the
conformational change of the PEDOT chains during the
treatment.
The treatment does not affect the transmittance of
PEDOT:PSS films in the visible range, so that PEDOT:PSS
films with high transparency and low sheet resistance
can be obtained. The conductivity enhancement is attrib-
uted to the phase segregation of PSSH from PEDOT:PSS
and conformational change of the PEDOT chains, induced
by the geminal diols and amphiphilic fluoro compounds.
4. Conclusions
In conclusion, the conductivity of PEDOT:PSS films can
be significantly enhanced through a treatment with a gem-
inal diol or amphiphilic fluoro compound. The conductivity
can be enhanced to more than 1000 S cmꢀ1. The conductiv-
ity enhancement is related to the equilibrium constants for
the conversion of the compounds into geminal diols or the
chemical structure of the amphiphilic fluoro compounds.
Acknowledgement
This research work was financially supported by a
research grant from the SMART, National Research Founda-
tion of Singapore (R-284-000-094-592).

1792
Y. Xia, J. Ouyang / Organic Electronics 13 (2012) 1785–1792
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