The optical properties of the pristine PEDOT:PSS and those modi fied with de-ionized water, ethylene glycol and MWCNT were investigated.. These studies have revealed that the optical para[r]
Trang 1Original Article
Vinamrita Singha,*, Tanuj Kumarb
a Department of Applied Sciences and Humanities, Ambedkar Institute of Advanced Communication Technologies & Research, Geeta Colony,
Delhi, 110031, India
b Department of Nanosciences & Materials, Central University of Jammu, Rahya-Suchani, Jammu, 181143, India
a r t i c l e i n f o
Article history:
Received 13 February 2019
Received in revised form
25 July 2019
Accepted 4 August 2019
Available online xxx
Keywords:
PEDOT:PSS
Optical properties
Refractive index
Ethylene glycol
MWCNT
Optical conductivity
a b s t r a c t The present work focuses on studying the optical properties of the pristine PEDOT:PSS and the PEDOT:PSS modified with de-ionized water, ethylene glycol and MWCNT The effect of various additives
on the absorption, the refractive index, and the dielectric constant has been inspected The refractive index dispersion has been analyzed using the single oscillator model developed by Wemple and DiDo-menico The optical constants, such as the dispersion energy, the single oscillator energy, the average oscillator strength, the average interband oscillator strength, the long wavelength refractive index, and the plasma resonance frequency have been determined The energy bandgap was found reduced with the addition of EG and MWCNT representing a red shift and a conformational change in the PEDOT:PSS from a benzoid to a quinoid structure The UV-visible absorption spectrum indicates the creation of free charges The increase in the refractive index with doping suggests the formation of localized energy states within the energy bandgap These localized states act as recombination centers and increase the low energy electronic transitions The dielectric constant was also found increased in the modified samples, exhibiting advantages for the formation of conducting thin films A phase segregated morphology was obtained for the solvent treated PEDOT:PSS, and the MWCNTs were observed to be uniformly distributed throughout the polymer Furthermore, the optical conductivity has been calculated
to give comprehensive information about material properties and their systematic selection for desired applications
© 2019 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)
1 Introduction
Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PED
OT:PSS) has proved to be a remarkable conducting polymer due to
its vast application and purposes in numerous devices PEDOT:PSS
is widely used in, but not limited to, solar cells, thermoelectric
devices, sensors, fuel cells, carbon capturing membranes and
supercapacitors [1e8] Its widespread practical applications are
attributed to its unique properties PEDOT:PSS being water soluble
is commercially available as an aqueous dispersion with different
conductivity grades, and hence can easily form good uniform thin
films by using different preparation techniques[9e13] Along with
a reliable electrical conductivity, PEDOT:PSS has a high work function and is optically transparent (>90%) in the visible range, making it suitable as an intermediate layer or electrodes in opto-electronic devices[3,14] Moreover, its electrical and optical prop-erties can be readily tuned using simple chemical or additive methods and various post-treatments[15e17] This gives the pos-sibilities to modify its conductivity and transparency for particular purposes with an aim to eventually enhance the device performance
The properties of PEDOT:PSS have been improved using addi-tives like dimethyl sulfoxide, ethylene glycol (EG), sorbitol, N, N dimethylformamide, multiwalled carbon nanotubes (MWCNT), de-ionized (DI) water, etc in the fabrication[2,17,18] The changes are brought about via different mechanisms, such as the effect of the dielectric constant of the additive material, the particle size change, the removal of PSS, or the reorientation of PEDOT polymeric chains [18,19] Kim et al.[15] improved the conductivity of PEDOT:PSS from ~1 S∙cm1 to 1418 S∙cm1 using EG and post-treatment
* Corresponding author.
E-mail addresses: vinamritasingh.phy@gmail.com , drvinamrita@aiactr.ac.in
(V Singh).
Peer review under responsibility of Vietnam National University, Hanoi.
Contents lists available atScienceDirect Journal of Science: Advanced Materials and Devices
j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j s a m d
https://doi.org/10.1016/j.jsamd.2019.08.009
2468-2179/© 2019 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license ( http://creativecommons.org/licenses/by/4.0/ ).
Trang 2methods, which also changed the absorption/transmission of the
resultingfilms The authors proposed that the transformation of the
coil-shaped PEDOT to the elongated, well-connected grains, and
the reduction of insulating PSS resulting in compactfilms were the
causes for the observed changes Further, the dilution of PEDOT:PSS
with DI water affected the particle size and thefilm morphologies
due to the depletion of excess inter-particles PSS[18] The addition
of MWCNT in the PEDOT:PSS significantly enhanced the work
function and the conductivity along with changing the optical
transparency of thefilms [2,20,21] This led to an increased ef
fi-ciency of organic solar cells and organic LEDs [21,22] Thus, the
electrical and optical properties of the PEDOT:PSS and its modi
fi-cation have become an essential area for investigations as it will
have a considerable impact on the performance of thefinal device
In view of this, the present work focuses on studying the optical
properties of the pristine PEDOT:PSS and the modified PEDOT:PSS
Three additives, namely DI, EG and MWCNT in different
concen-trations have been used, and their effects on the absorption, the
refractive index, and the dielectric constant have been inspected
Along with the absorption changes, it becomes essential to
un-derstand the correlation of the optical properties with the electrical
properties as these two together will govern the charge conduction
through the devices The refractive index dispersion has been
analyzed using the single oscillator model developed by Wemple
and DiDomenico [23], and the optical constants, such as the
dispersion energy, single oscillator energy, average oscillator
strength, average interband oscillator strength, long wavelength
refractive index, and plasma resonance frequency have been
determined Scanning electron microscopy (SEM) was used to
analyze the morphological changes in the PEDOT:PSS upon
modi-fication Furthermore, the optical conductivity has been calculated
to give comprehensive information about material properties and
their systematic selection for the desired application
2 Experimental details
Thinfilm samples of the pristine PEDOT:PSS and its solution
mixed with DI water, ethylene glycol (EG), and EG with 2%, 4%, 8%,
10% MWCNT were prepared on cleaned glass substrates The thin
films were prepared using the spin coating technique at
500e2000 rpm for 1 min to obtain a thickness of ~50 nm The
samples were then annealed in a vacuum oven at 100C for 20 min
The UVevisible absorption spectra of the prepared films were
measured using the Shimadzu UV 2600 system The sample
no-menclatures used in the present paper are: PP ¼ pristine
PEDOT:PSS; PDI1/PDI2 ¼ PEDOT:PSS þ DI in 4:1 and 2:1 ratio;
PEG1/PEG2 ¼ PEDOT:PSS þ EG in 4:1 and 2:1 ratio; PEM ¼
PEDOT:PSSþ EG þ MWCNT at different mentioned concentrations
The morphology of the thinfilms was studied using SEM images
acquired by the Zeiss, MA15 The PEDOT:PSS solution with 2.2e2.6%
in H2O was obtained from Sigma Aldrich and MWCNTs were
ob-tained from NanoShell
3 Results and discussion
The optical properties of the thinfilms of the pristine PEDOT:PSS
and the modified PEDOT:PSS were investigated using the UV-visible
spectroscopy The absorption spectra of the films are shown in
Fig 1(aec) It can be observed that the absorption is very less in the
visible range, i.e., the transmission is high which makes it suitable for
solar cell application as most of the active layers absorbing in the
400e700 nm range Moreover, the dip in the absorption spectra near
500 nm accords with the wavelength at which most of the polymers
absorb to the maximum The spectral response of the PEDOT:PSS in
the concerned range is dominantly due to PEDOT as PSS does not
show any absorption above 310 nm[24] The characteristic peak at
~380 nm is due to the transition from n top* in the PEDOT backbone [14,24], while the broad absorption plateau beyond 700 nm corre-sponds to polarons and uncoupled bipolaron transitions in the benzoid or quinoid structure of PEDOT
Addition of DI water to the PEDOT:PSS (Fig 1a) decreases the absorption as it simply dilutes the solution On the other hand, the modification of the PEDOT:PSS with EG enhances the absorption, and the difference in absorbance increases along the higher wavelengths
as depicted inFig 1b FromFig 1c, it is found that on further adding MWCNT at different concentrations in PEG2 samples, the absorption intensity rises and is maximum for 10% MWCNT It may be noticed that all the modified samples retain the spectral response of the PEDOT:PSS The increase in conductivity obtained by doping PEDOT:PSS results from the formation of the freely moving solitons, polarons and bipolarons[14] This increase in the self-localized ex-citations is indicated from an enhancement in the absorption in the near infrared (NIR) region observed for EG and MWCNT doped samples Due to the increase in the carrier concentration, the bipo-laron subgap transition[25]takes place, which is further confirmed
by the calculated energy bandgap values tabulated inTable 1 The energy bandgap of the EG and MWCNT doped samples decreases, indicating a red shift and a conformational change in the PEDOT:PSS from the benzoid to the quinoid structure The energy bandgap was calculated using the Tauc relation:
a,h,n¼ Ah,n Eg
n
whereais the absorption coefficient;nis the frequency; h is the Planck's constant; A is a constant; Egis the energy bandgap; and
n¼ 1/2 for the allowed direct transition and 2 for the allowed in-direct transition The extrapolation of the linear segment of the plot
of (a∙h∙n)1/2vs h∙n gives the value of the energy band gap as shown inFig 1d for one sample The Tauc plots for all samples are provided in the supplementary information
The complex refractive index and other optical parameters are important quantities for assessing the usability of a material for various applications The refractive index is also closely related to the electronic polarizability and the localfields inside the materials Therefore, the optical parameters were studied in order to deter-mine the effect of doping PEDOT:PSS on these properties In order
to calculate the refractive index, the following relation was used[26]:
nðlÞ ¼ 4r
ðr 1Þ2 k2
!1=2
rþ 1
r 1
In this expression, k is the extinction coefficient given bya∙l/4p
with a being the absorption coefficient, which is calculated as
a∙d¼ 2.303A Here, d is the film thickness and A is the absorbance The reflectance (r) in the above expression is derived from the
reflection and transmission coefficients (R and T) obtained from UV-Visible spectroscopy using an iterative process between the expressions for reflectance (r) and transmittance (t) given by[26]:
1þ t2þh
1þ t22
4t2Rð2 RÞi1=2
ð1 rÞ2þhð1 rÞ4 4T2r2i1=2 The variation of the refractive index (n) with the wavelength for different samples is presented in Fig 2(aec) As observed, the
Trang 3refractive indexfirst decreases and then increases with the
wave-length The refractive index is ~1.6 for the pristine PEDOT:PSS, while
it increases for modified samples with n ~1.8 for PDI and PEG A
similar change was also observed for MWCNT modified samples
The increase in the refractive index with doping may be attributed
to the formation of localized energy states within the energy
bandgap These localized states act as recombination centers and
increase the low energy electronic transitions[27] The refractive
index for all samples initially decreases with an increase in
wave-length showing the normal dispersion behavior But beyond a
certain wavelength, it increases with the increasing wavelength
The region of the normal dispersion can be probed using the
Wemple and DiDomenico single oscillator model[23] According to
this model, the refractive index is related to the dispersion energy,
Ed, and the single oscillator energy, Eo, parameters through the following relation[28]:
n2 1 ¼ EdEo
E2o ðhnÞ2 The Edand Eoparameters were calculated by linearlyfitting the plots of (n2-1)1as a function of (h∙n)2, shown inFig 2d The values
of Ed were in the range 1.41e3.58 eV and Eo in the range of 3.96e4.17 eV The obtained values are given inTable 1, which also tabulates the values of the average oscillator strength (So), the long wavelength refractive index (n∞) and the average interband
Fig 1 (aec) The UV-visible absorption spectrum of different PEDOT:PSS samples and (d) The Tauc plot for the calculation of the energy band gap.
Table 1
Values of optical parameters for the pristine and the modified PEDOT:PSS samples.
Sample E g (eV) n∞ lo (nm) S o 10 6 (nm2) E o (eV) E d (eV) N/m* 10 48 ε∞ up 10 10 (Hz)
PP 3.64 1.16 313.16 3.61 3.96 1.41 4.74 2.38 7.59
PEG1 3.62 1.26 305.28 6.26 4.07 2.43 3.72 2.78 6.22
PEG2 3.62 1.24 306.78 5.77 4.05 2.24 7.93 3.02 8.71
PDI1 3.64 1.27 305.76 6.58 4.07 2.56 4.56 2.98 6.65
PDI2 3.65 1.25 306.76 6.04 4.05 2.34 4.67 3.1 6.60
PEM2% 3.62 1.21 309.12 4.75 4.02 1.85 7.08 2.56 8.94
PEM4% 3.61 1.23 306.45 5.58 4.06 2.17 8.19 2.76 9.27
PEM8% 3.59 1.17 311.16 3.76 3.99 1.47 8.36 2.48 9.87
PEM10% 3.59 1.36 298.39 9.49 4.17 3.58 7.88 3.35 8.25
Trang 4oscillator wavelength (lo) The graph between (n2-1)1and l2
were linearlyfitted to obtain the values of So, n∞, andlousing the
relations[28]:
n2
∞ 1
n2 1¼ 1
lo=l
2
So¼n2∞ 1
l2
o
Another important aspect of material properties to investigate is
the dielectric constant The real and imaginary parts of the
dielectric constant are evaluated using[28]:
ε1¼ n2 k2¼ ε∞e2,N,l2
p,m*,c2
ε2¼ 2n,k ¼ε∞,u2
p,l3
8p,c3,t
whereup¼ (e2N/εo∙ε∞m*)1/2is the plasma resonance frequency N/
m* gives the ratio of the free carrier concentration to the effective mass andt¼ the optical relaxation time The values of the dielectric constant indicate the presence of localized energy states within the energy bandgap The variation of the real and imaginary parts of the dielectric constant with energy is shown inFig 3 It may be stated that the dielectric constant increases for the modified PEDOT:PSS The presence of the free carriers in an increased number may be responsible for the higher dispersion observed
Fig 2 (aec) Refractive index versus wavelength of different PEDOT:PSS samples and (d) plot of (n 2 -1)1with (h ∙n) 2 (symbols) andl2 (solid line).
Fig 3 (a) Real and (b) imaginary parts of the dielectric constant versus energy for different PEDOT:PSS samples.
Trang 5within the material The dielectric constant of a material relates to
the nature of the Columbian forces among the charges in the
ma-terial A high dielectric constant leads to smaller attractive forces
between the electron-hole pairs, which is desirable for the use of
the PEDOT:PSS as a conductive layer Thus, the modified PEDOT:PSS
has an enhanced conductivity[2] The values of N/m*,ε∞andup
calculated using the above equations are provided inTable 1 These
values vary with the addition of DI water, EG and MWCNT An
in-crease in the number of free carriers as well as in the plasma
frequency is further supported by the increase in N/m* for the modified PEDOT:PSS samples
Fig 4 shows the optical conductivity with the energy of the PEDOT:PSS samples The optical conductivity was calculated using
s ¼a∙n∙c/4p[29] It is observed that the optical conductivity is increased in the modified samples This is attributed to the increased refractive index and the ability of the material to scatter electrons more easily as light is incident on it Moreover, it has been shown that the electron-phonon coupling is weakened after the addition of EG into the PEDOT:PSS[30] The electrical conductivity of PEDOT:PSS also increases with the addition of EG The presence of EG not only reduces the excess PSS but also results in the reorganization of PSS along the PEDOT chain The particle size of the EG doped PEDOT:PSS is found decreased, which leads to a higher order packing of the chains This gives rise to a greater number of polarons and hence of free charge carriers This effect has been associated with the transformation of the PEDOT:PSS from the benzoid to the quinoid structure
The morphological changes in the PEDOT:PSS films due to DI water, EG, and MWCNT were investigated by SEM images Signi fi-cant variations in the morphology are observed, as shown inFig 5 The pristine film is smooth, whereas, the appearance of the segregation and a rougher surface is observed for the DI water and
EG based PEDOT:PSSfilms The addition of solvents resulted in the phase segregation and a granular structure of PEDOT:PSS thinfilms [18,31] Further addition of MWCNT resulted in a rough surface with uniformly distributed MWCNTs which are embedded in the polymer matrix (Fig 5d) These results are consistent with those in
Fig 4 Variation of the optical conductivity with energy for different PEDOT:PSS
samples.
Fig 5 SEM images of (a) pristine PEDOT:PSS, (b) PEDOT:PSS with de-ionized water, (c) PEDOT:PSS with ethylene glycol and (d) PEDOT:PSS with ethylene glycol and MWCNT.
Trang 6our previous work[2], in which the surface roughness calculated
using the atomic force microscopy images, was increased after the
treatment of the PEDOT:PSS with additives The granular structure
correlates with the increase in conductivity as well, since the
removal of PSS and the coalescing of the grains lead to a better
charge transfer through the conductive PEDOT A rougher film
morphology also increases the scattering and hence, the absorption
of light by thefilm, as it has been observed in the present work
Thus, these studies reveal the dependency of the optical and
morphological properties of the PEDOT:PSS on the additives used,
which can be used to fabricate materials of desired properties
4 Conclusion
The optical properties of the pristine PEDOT:PSS and those
modified with de-ionized water, ethylene glycol and MWCNT were
investigated These studies have revealed that the optical
param-eters are dependent on the additives and also varied with the
wavelength The energy bandgap was found reduced with the
addition of EG and MWCNT The creation of free charges was
indicated by the rise in the absorption in the NIR region of the
UV-visible absorption spectrum The refractive index dispersion was
analyzed using the Wemple and DiDomenico single oscillator
model, and the optical constants, such as the dispersion energy,
single oscillator energy, average oscillator strength, average
inter-band oscillator strength, long wavelength refractive index, and
plasma resonance frequency were found to vary with the energy of
the incident light The SEM images have revealed the phase
segregation of PEDOT:PSSfilms upon the addition of DI and EG The
dielectric constants are also found increased for the modified
samples due to the presence of localized states within the band gap,
which is advantageous for the formation of conducting thinfilms
Acknowledgment
The authors are grateful to AIACT&R and Central University of
Jammu for their support and encouragement
Appendix A Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.jsamd.2019.08.009
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