Study of modified PEDOT:PSS for tuning the optical properties of its conductive thin films

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 DiDomenico.

Journal of Science: Advanced Materials and Devices (2019) 538e543 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Original Article Study of modified PEDOT:PSS for tuning the optical properties of its conductive thin films Vinamrita Singh a, *, Tanuj Kumar b 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 a b s t r a c t Article history: Received 13 February 2019 Received in revised form 25 July 2019 Accepted August 2019 Available online September 2019 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 DiDomenico 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/) Keywords: PEDOT:PSS Optical properties Refractive index Ethylene glycol MWCNT Optical conductivity 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 * Corresponding author E-mail addresses: vinamritasingh.phy@gmail.com, drvinamrita@aiactr.ac.in (V Singh) Peer review under responsibility of Vietnam National University, Hanoi 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 optoelectronic devices [3,14] Moreover, its electrical and optical properties can be readily tuned using simple chemical or additive methods and various post-treatments [15e17] This gives the possibilities 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 additives like dimethyl sulfoxide, ethylene glycol (EG), sorbitol, N, N dimethylformamide, multiwalled carbon nanotubes (MWCNT), deionized (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∙cmÀ1 to 1418 S∙cmÀ1 using EG and post-treatment 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/) V Singh, T Kumar / Journal of Science: Advanced Materials and Devices (2019) 538e543 methods, which also changed the absorption/transmission of the resulting films 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 compact films were the causes for the observed changes Further, the dilution of PEDOT:PSS with DI water affected the particle size and the film 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 the films [2,20,21] This led to an increased efficiency of organic solar cells and organic LEDs [21,22] Thus, the electrical and optical properties of the PEDOT:PSS and its modification have become an essential area for investigations as it will have a considerable impact on the performance of the final 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 concentrations 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 understand 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 modification Furthermore, the optical conductivity has been calculated to give comprehensive information about material properties and their systematic selection for the desired application Experimental Thin film 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 to obtain a thickness of ~50 nm The samples were then annealed in a vacuum oven at 100  C for 20 The UVevisible absorption spectra of the prepared films were measured using the Shimadzu UV 2600 system The sample nomenclatures used in the present paper are: PP ¼ pristine PEDOT:PSS; PDI1/PDI2 ẳ PEDOT:PSS ỵ DI in 4:1 and 2:1 ratio; PEG1/PEG2 ẳ PEDOT:PSS ỵ EG in 4:1 and 2:1 ratio; PEM ẳ PEDOT:PSS ỵ EG ỵ MWCNT at different mentioned concentrations The morphology of the thin films 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 obtained from NanoShell Results and discussion The optical properties of the thin films 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 539 show any absorption above 310 nm [24] The characteristic peak at ~380 nm is due to the transition from n to p* in the PEDOT backbone [14,24], while the broad absorption plateau beyond 700 nm corresponds 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 in Fig 1b From Fig 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 excitations 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 bipolaron subgap transition [25] takes place, which is further confirmed by the calculated energy bandgap values tabulated in Table 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: À a,h,n ¼ A h,n À Eg Án where a is the absorption coefficient; n is the frequency; h is the Planck's constant; A is a constant; Eg is the energy bandgap; and n ¼ 1/2 for the allowed direct transition and for the allowed indirect transition The extrapolation of the linear segment of the plot of (a∙h∙n)1/2 vs h∙n gives the value of the energy band gap as shown in Fig 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 local fields inside the materials Therefore, the optical parameters were studied in order to determine 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 À 1Þ !1=2 À k2 À rỵ1 r1 In this expression, k is the extinction coefcient given by a∙l/4p with a being the absorption coefficient, which is calculated as a∙d ¼ 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]: r¼ 2R hÀ i1=2 Á2 ỵ t ỵ ỵ t 4t R2 Rị tẳ 2T i1=2 h rị2 ỵ rị4 4T r The variation of the refractive index (n) with the wavelength for different samples is presented in Fig 2(aec) As observed, the 540 V Singh, T Kumar / Journal of Science: Advanced Materials and Devices (2019) 538e543 Fig (aec) The UV-visible absorption spectra of different PEDOT:PSS samples and (d) The Tauc plot for the calculation of the energy band gap Table Values of optical parameters for the pristine and the modified PEDOT:PSS samples Sample Eg (eV) n∞ lo (nm) So  10À6 (nmÀ2) Eo (eV) Ed (eV) N/m*  1048 ε∞ up  1010 (Hz) PP PEG1 PEG2 PDI1 PDI2 PEM2% PEM4% PEM8% PEM10% 3.64 3.62 3.62 3.64 3.65 3.62 3.61 3.59 3.59 1.16 1.26 1.24 1.27 1.25 1.21 1.23 1.17 1.36 313.16 305.28 306.78 305.76 306.76 309.12 306.45 311.16 298.39 3.61 6.26 5.77 6.58 6.04 4.75 5.58 3.76 9.49 3.96 4.07 4.05 4.07 4.05 4.02 4.06 3.99 4.17 1.41 2.43 2.24 2.56 2.34 1.85 2.17 1.47 3.58 4.74 3.72 7.93 4.56 4.67 7.08 8.19 8.36 7.88 2.38 2.78 3.02 2.98 3.1 2.56 2.76 2.48 3.35 7.59 6.22 8.71 6.65 6.60 8.94 9.27 9.87 8.25 refractive index first decreases and then increases with the wavelength 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 wavelength 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 ẳ  Ed Eo E2o hnị2  The Ed and Eo parameters were calculated by linearly fitting the plots of (n2-1)À1 as a function of (h∙n)2, shown in Fig 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 in Table 1, which also tabulates the values of the average oscillator strength (So), the long wavelength refractive index (n∞) and the average interband V Singh, T Kumar / Journal of Science: Advanced Materials and Devices (2019) 538e543 541 Fig (aec) Refractive index versus wavelength of different PEDOT:PSS samples and (d) plot of (n2-1)À1 with (h∙n)2 (symbols) and lÀ2 (solid line) oscillator wavelength (lo) The graph between (n2-1)À1 and lÀ2 were linearly fitted to obtain the values of So, n∞, and lo using the relations [28]: ε1 ¼ n2 À k2 ¼ ε∞ À e2 ,N,l p,m* ,c2 ε2 ¼ 2n,k ¼ 2  n2∞ À ¼ À lo=l n À1 So ¼ n2∞ À l2o 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]: ε∞ ,u2p ,l 8p,c3 ,t where up ¼ (e2N/εo∙ε∞m*)1/2 is the plasma resonance frequency N/ m* gives the ratio of the free carrier concentration to the effective mass and t ¼ 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 in Fig 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 (a) Real and (b) imaginary parts of the dielectric constant versus energy for different PEDOT:PSS samples 542 V Singh, T Kumar / Journal of Science: Advanced Materials and Devices (2019) 538e543 Fig Variation of the optical conductivity with energy for different PEDOT:PSS samples within the material The dielectric constant of a material relates to the nature of the Columbian forces among the charges in the material 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∞ and up calculated using the above equations are provided in Table These values vary with the addition of DI water, EG and MWCNT An increase 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 shows the optical conductivity with the energy of the PEDOT:PSS samples The optical conductivity was calculated using s ¼ a∙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 Significant variations in the morphology are observed, as shown in Fig 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:PSS films The addition of solvents resulted in the phase segregation and a granular structure of PEDOT:PSS thin films [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 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 V Singh, T Kumar / Journal of Science: Advanced Materials and Devices (2019) 538e543 our 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 the film, 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 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 parameters 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 UVvisible 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 interband 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:PSS films 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 thin films Acknowledgments [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] The authors are grateful to AIACT&R and Central University of Jammu for their support and encouragement [21] Appendix A Supplementary data [22] Supplementary data to this article can be found online at https://doi.org/10.1016/j.jsamd.2019.08.009 [23] References [1] S Arora, V Singh, M Arora, R Pal Tandon, Evaluating effect of surface state density at the interfaces in degraded bulk heterojunction organic solar cell, Phys B Condens Matter 407 (2012) 3044e3046, https://doi.org/10.1016/ j.physb.2011.08.086 [2] V Singh, S Arora, M Arora, V Sharma, R.P Tandon, Characterization of doped PEDOT: PSS and its influence on the performance and degradation of organic solar cells, Semicond Sci Technol 29 (2014), https://doi.org/10.1088/02681242/29/4/045020 [3] K 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(aec) The UV-visible absorption spectra of different PEDOT:PSS samples and (d) The Tauc plot for the calculation of the energy band gap Table Values of optical parameters for the pristine and the. .. absorption of light by the film, 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