Visible Luminescence of Dedoped DBU-Treated ... - ACS Publications

Article pubs.acs.org/JPCC

Visible Luminescence of Dedoped DBU-Treated PEDOT:PSS Films Isidro Cruz-Cruz,†,‡ Marisol Reyes-Reyes,† Israel A. Rosales-Gallegos,† Andrei Yu Gorbatchev,† José M. Flores-Camacho,† and Román López-Sandoval*,‡ Instituto de Investigación en Comunicación Ó ptica, Universidad Autónoma de San Luis Potosí, Á lvaro Ó bregon 64, San Luis Potosí 78000, Mexico ‡ Advanced Materials Department, IPICYT, Camino a la Presa San José 2055, Col. Lomas 4a sección, San Luis Potosí 78216, Mexico †

S Supporting Information *

ABSTRACT: Dedoping of PEDOT:PSS by a simple and physical process has been performed by the addition of 1,8diazabicyclo[5.4.0]undec-7-ene (DBU), a strong organic base, in the PEDOT:PSS aqueous dispersion. The DBU-treated PEDOT:PSS samples showed a strong absorption band at 600 nm (π−π* transition) and a small band at 900 nm (polaronic band), which indicates that some PEDOT chains were being reduced. The dedoping efficiency depended of the amount of DBU added to the aqueous PEDOT:PSS dispersion. The DBU-treated PEDOT:PSS aqueous dispersions and films showed a strong yellow emission, visible with the naked eye, when they were excited with a 488 nm laser. In addition, the same emission features were observed using another reducing agent like sodium hydroxide (NaOH), which indicates that the emission originates from neutral PEDOT chains rather than artifacts. Finally, Raman and FTIR spectroscopies were used to figure out the mechanism responsible for the PEDOT reduction. The reduction of PEDOT chains is due to the formation of hydroxide anions, which come from the interaction of DBU with water present in the PEDOT:PSS aqueous solution and interact with the oxidized thiophene molecules, resulting in a neutralization of the PEDOT backbone.

■

INTRODUCTION Organic compounds have attracted a great interest over the past years due to the possibility of obtaining flexible, stable, and lowcost electronic devices.1 One of these compounds that have attracted great interest is the conductive material poly(3,4ethylenedioxythiophene) doped with the counteranion poly(styrenesulfonate) (PEDOT:PSS).2 As is well-known, PEDOT is considered a promising material in applications where the electronic transport is crucial since, in its oxidized state, it has excellent conductivity and high transparency in the visible spectrum, among other properties.3,4 In the case of the polymeric complex PEDOT:PSS, which is available commercially as an aqueous dispersion, PSS improves its processability for the fabrication of thin films, but reduces the conductivity down to around 0.1 S/cm. In this regard, many efforts have been performed in order to improve its conductivity and the importance of the molecular rearrangement in the thin films was discussed widely,5−22 because better molecular arrangement promotes an increase in the crystallinity degree and improves the charge carriers delocalization. In particular, oxidizing agents have been used to increase the PEDOT:PSS © XXXX American Chemical Society

conductivity by the partial replacement of some segments of PSS−1 by ionic groups such as SO4−2 or HSO4−1 and the corresponding elimination of the insulating PSS from the thin films.20,21 Thus, the majority of the applications are related with PEDOT in its oxidized state (p-doped). However, in this work, we performed the partial reduction of this polymer to its neutral state (PEDOT0), which shows enough stability. As reported in the literature, it is possible to modulate the electrical, thermoelectric, and optical properties of PEDOT:PSS23−26 by using reducing agents like polyethylenimine,24 hydrazine,25 and sodium hydroxide (NaOH).26 In the former two cases, it was possible to obtain a low oxidized form of PEDOT,24,25 whereas, in the latter, no change in the doped state of the polymer was observed, but only a change in the relative population between polarons and bipolarons.26 In the present work, the use of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) as a reducing agent in PEDOT:PSS is reported. DBU is Received: April 27, 2015 Revised: July 23, 2015

A

DOI: 10.1021/acs.jpcc.5b04016 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

using a different equipment (IR-VASE Mark II Ellipsometer) for the 1:0.01 and 1:0.2 PEDOT:PSS-DBU films (see the Supporting Information). Photoluminescence (PL) spectra were measured at room temperature using the 488 nm line of the argon-ion laser for excitation. The spectrum was dispersed with a 0.5 m monochromator. In the experimental setup, a Ge photodetector operating at room temperature, conventional lock-in amplifiers operating at 100 Hz, and a long pass filter with a wavelength cut off at 530 nm were used to measure the emission intensity of the DBU-treated PEDOT:PSS samples. To obtain the PL efficiency of the PEDOT:PSS-DBU films, an integrating sphere (INS125 International Light Technologies) was used.

a strong non-nucleophilic base used as reagent and catalyst in organic synthesis,27 and particularly, it is utilized in “green chemistry”.28−30 PEDOT:PSS treated with DBU presents a strong absorption band in the visible spectrum (∼600 nm), which is associated with the success in the PEDOT:PSS reduction.31−35 The change in the electronic structure of PEDOT:PSS and its remarkable absorption band in the visible region of the electromagnetic spectrum (attributed to a π−π* transition) have motivated us to carry out photoluminescence studies. PEDOT0chains showed emission in the visible spectrum, which opens the possibility of its use in potential optical applications in optoelectronics devices. For comparison, the PEDOT:PSS dedoping was also carried out using another base, sodium hydroxide (NaOH), with the aim of exploring whether the absorption band, the emission band, and overall features of the modified PEDOT:PSS were intrinsic to the polymer and not due to the used base. Note that NaOH has different characteristics to DBU, such as its high hygroscopicity, its nucleophilic character, and its high toxicity. On the basis of the results, we have shown that the emission originates from the dedoped or low oxidized PEDOT:PSS. Additionally, the reduced PEDOT:PSS was studied using other spectroscopic techniques such as Raman and Fourier transform infrared (FTIR) spectroscopies to figure out the mechanism responsible for the PEDOT reduction.

■

RESULTS AND DISCUSSION Initially, the conductivity of the samples was determined by using the four-point probe technique. The pristine PEDOT:PSS films exhibited conductivities around 0.12 S/cm and a film thickness of 100 nm, whereas the film conductivities for 1:0.005 samples were of the order of 0.05 S/cm with a thickness of 170 nm. On the other hand, the 1:0.01 (v/v) PEDOT:PSS-DBU film presents a thickness of 250 nm, and the film conductivity could not be obtained, because its value was very low, below the measuring range of the multimeter. It is in agreement with a dedoping process of the PEDOT chains. For the v/v ratios of 1:0.03 and 1:0.2, due to discontinuities in the films and nonuniformity in their thicknesses (see Figure S1 in the Supporting Information), it was not possible to obtain their conductivity; because of the conductivity calculation by the four-point technique, it is necessary to have samples with a uniform thickness. Figure 1 shows the measured UV−vis absorption spectra of PEDOT:PSS-DBU films for the different v/v ratios of the two

■

EXPERIMENTAL METHOD The solutions were prepared by dissolving 1,8-diazabicyclo[5.4.0]undec-7-ene (Sigma-Aldrich 98% purity, in liquid state) in an aqueous dispersion of PEDOT:PSS (Clevios P). The volume ratios (PEDOT:PSS-DBU) were set to values of 1:0.2, 1:0.03, 1:0.01, and 1:0.005 (v/v). The solutions were stirred (1100 rpm) for 1 h at 70 °C. Thin films were prepared using these solutions and deposited on Dow Corning glass substrates (2.5 cm in length). Previously, the substrates were successively ultrasonically cleaned in acetone, methanol, and isopropyl alcohol for 20 min each time; immediately, after evaporation of the solvents, the substrates were kept in a UV−ozone ambient for 45 min. The PEDOT:PSS-DBU solutions were subsequently deposited on the substrates by spin-coating (1870 rpm for 5 s, followed by 40 s at 2500 rpm) at 22 °C and a relative humidity around 45−50%. Finally, the samples were dried at 100 °C for 15 min. In the case of the films dedoped with NaOH (ACS Fermont, in the form of pellets), the NaOH pellets were first dissolved in deionized water through an ultrasonic bath for 20 min. Afterward, this blend was mixed with the polymer using the same procedure as the one employed for DBU. Regarding film thickness, we performed six measurements on each sample using an Alpha Step 500 surface profiler along two parallel scratches going from the center to an edge of the film. The PEDOT:PSS films with a determined DBU volume added to the PEDOT:PSS dispersion show little differences in their thickness. The reported thicknesses correspond to an average of these measurements made on the film. Absorption measurements were performed with a UV−vis− NIR Varian Cary 5E spectrophotometer. Raman measurements were carried out by using a Jobin-Yvon T64000 spectrometer in backscattering configuration with a 514 nm Ar-ion laser. FTIR spectra were collected with a PerkinElmer 1600 FTIR spectrophotometer. In this case, KBr pellets were used as a substrate for the FTIR measurements. Additionally, to corroborate the FTIR measurements, these were performed

Figure 1. Absorbance spectra for PEDOT:PSS and PEDOT:PSS-DBU films using different volume ratios of 1:0.2, 1:0.03, 1:0.01, 1:0.005.

components. From the figure, we observe that, for pristine PEDOT:PSS, the absorption spectrum showed the same features previously reported in the literature.20,21 In contrast, all the samples with different PEDOT:PSS-DBU v/v ratios show absorption bands around 600 and 900 nm. The first band at 600 nm is attributed to a π−π* transition, characteristic of the neutral state in PEDOT,31,32 i.e., PEDOT0, whereas the band at 900 nm is related with the polaronic band,20−22,33−35 characteristic of the oxidized state, i.e., PEDOT+. In addition, B


Article

The Journal of Physical Chemistry C from the figure, we can observe that the intensity of the π−π* band is bigger than the polaronic band, indicating that a large number of PEDOT:PSS chains in the solution are dedoped.31−35 Moreover, the dedoping efficiency depends on the amount of DBU added to the PEDOT:PSS solution; the band at 600 nm becomes more intense, whereas the band at 900 nm begins to decrease33−35 as a function of DBU added to the PEDOT:PSS dispersion. However, the presence of the polaronic band at 900 nm is a clear indication that the PEDOT chains in the aqueous dispersion are not completely reduced. Additionally, another experiment was performed in a glovebox under a N2 atmosphere: PEDOT:PSS films were dipped into pure DBU during 10 min, and their corresponding absorbance spectra were obtained (see Figure S2 in the Supporting Information) and compared to those shown in Figure 1. As can be observed, the absorption spectrum of the DBU dipped PEDOT:PSS film shows also a strong absorption band around 600 nm and the polaronic band located around 900 nm has almost completely disappeared. In this way, this suggests that DBU or a DBU complex can transfer electrons to PEDOT:PSS. Under these conditions, PEDOT:PSS films changed from transparent to a strong deep blue color (Figure S3 in the Supporting Information), which matches very well with reduced and doped PEDOT films using an electrochemical cell.36 The spin-coating technique was employed to obtain thin films from the PEDOT:PSS-DBU mixtures at the v/v ratios previously used in the UV−vis−NIR studies. We have observed that the deposited films using mixtures with PEDOT:PSS-DBU v/v ratios of 1:0.2 and 1:0.03 were not uniform in thickness and discontinuous; it was possible to observe some aggregates (Figure S1 in the Supporting Information). In contrast, the use of a very low content of DBU in the mixture (1:0.01 and 1:0.005 v/v) facilitated the formation of a uniformity in thickness of PEDOT:PSS layer on the glass substrate. Then, for the following studies, we have focused on films of PEDOT:PSS treated with DBU at a 1:0.01 v/v ratio because these films exhibited a good morphology and good dedoping level (the band at 900 nm is small compared with the 600 nm band). It has been reported that the neutral PEDOT:PSS is not stable34,35 in room conditions because these can be reoxidized due to the oxygen in the air. For this reason, we studied if the same effect is observed when the dedoped PEDOT:PSS samples were exposed to the environment. Figure 2 shows the evolution in time of the absorption spectrum of DBUtreated PEDOT:PSS film. This figure presents the absorption spectrum taken immediately after deposition (named “week 0”) and its evolution over time under environmental conditions. It is observed that, after the first week of environment exposition, the film showed remarkable changes; both bands decrease. However, the 600 nm band decreased faster than the 900 nm band, and in this way, the 900 nm band becomes more pronounced. This feature is important evidence supporting the polymer reoxidation. In addition, another type of optical characterization used on DBU-treated PEDOT:PSS film was PL measurements. Photoluminiscence allowed us to contrast between the optical response of the pristine PEDOT:PSS films and those of the DBU-treated films. Figure 3 shows the room-temperature PL spectra of pristine and DBU-treated PEDOT:PSS samples. These PL spectra were obtained using a 15 mW laser power. From the figure, we observe that neither the pristine PEDOT:PSS film nor the pristine PEDOT:PSS solution

Figure 2. Absorbance spectra for PEDOT:PSS-DBU thin films with a 1:0.01 volume ratio exposed to the environment during some weeks.

Figure 3. Photoluminescence spectra at room conditions under a 488 nm laser excitation for PEDOT:PSS, PEDOT:PSS-DBU (1:0.01), and PEDOT:PSS-NaOH in solutions and films.

shows an emission band. In contrast, the DBU-treated PEDOT:PSS both in film and in solution show an emission band, whose emission peak depends on whether the DBUtreated PEDOT:PSS sample is in solid (910 nm) or liquid phase (640 nm). The differences in the emission spectrum between both phases are related to reoxidation of the PEDOT layers during PL measurements in the films, particularly the dedoped PEDOT chains, because they are simultaneously in direct contact with the laser excitation and with the oxygen in the environment. In the case of samples in the liquid phase, the dedoped PEDOT chains are not in direct contact with the laser excitation nor oxygen in the environment. Considering that PEDOT is hydrophobic while PSS is hydrophilic, we deduced that, in the PEDOT:PSS-DBU sample in the liquid phase, as the excess of PSS chains tends to be located on the surfaces of vial walls, they protect the dedoped PEDOT chains from the aforementioned photochemistry process, avoiding, in this way, its possible reoxidation and degradation. Moreover, the laser power used for performing these PL measurements is large, and this could be damaging the dedoped PEDOT chains, which might be an important parameter responsible for the difference between both emission peaks. Furthermore, the maximum of C


Article

The Journal of Physical Chemistry C the emission peak of the PEDOT:PSS-DBU sample in the liquid state is at 630 nm, and it is so intense that it can be observed with the naked eye (Figure 4a,b). In addition, to

Figure 5. Photoluminescence spectra of (1:0.01) PEDOT:PSS-DBU films under a 488 nm laser excitation at different laser power (2, 5, and 10 mW).

presence of oxygen during the PL measurements. In addition, from Figure 5, we can observe that DBU-treated encapsulated films present the emission peak at 630 nm when a low laser power is used in the PL measurements; however, when the laser power was increased to 10 mW, the emission peak is again at 910 nm. This modification in the absorption peak at high laser powers is related to the film reoxidation, because the film encapsulation is not perfect, which leads to the degradation of the film due to the higher laser power used for the PL measurement. The measured PL efficiency of these PEDOT:PSS-DBU films was around 2% (see the Supporting Information). Raman spectroscopy is a powerful nondestructive technique used for the characterization of the oxidation state of PEDOT:PSS.33−35 It is known that the intensities of the Raman bands are increased due to a resonance effect, which implies that the excitation wavelength provided by the laser is important for detecting the degree of oxidation of PEDOT chains.35 When the laser wavelength is close to the π−π* transition energy, the neutral form of PEDOT shows intense and well-defined resonant Raman bands, due to the resonant effect, whereas the Raman bands of a doped PEDOT structure are less intense and not well-defined.35 Figure 6 shows Raman spectra of pristine and DBU-treated films (at volume ratio of 1:0.01) using a wavelength of 514 nm. The Raman spectrum of pristine PEDOT:PSS shows the characteristic peaks of doped PEDOT:PSS films.34,35 These peaks are less intense and less well-defined as compared to the peaks coming from the DBUtreated films. On the other hand, the main peak of the pristine film located at 1453 cm−1 was red-shifted in the DBU-treated film to 1434 cm−1 and is narrower. This band is associated with CαCβ symmetric stretching35 and is considered as a combination of two separate bands corresponding to the neutral (at around 1440 cm−1) and oxidized (at around 1456 cm−1) structures in PEDOT chains.37,38 The location and the shifting of this peak as a consequence of the decrease in the intensity band of the oxidized structure characterize neutral PEDOT.35 This clearly shows the dedoping of the DBU-treated films, i.e., the change from the doped states of some PEDOT chains to neutral states. In addition, we have performed FTIR measurements of PEDOT:PSS pristine films as well as those that were treated with DBU to observe the changes that occur

Figure 4. Images of DBU-treated PEDOT:PSS (a, b) solution and (c) film under a 488 nm laser excitation. Note that the luminescence originating from the DBU-treated PEDOT:PSS samples can be observed with the naked eye.

corroborate that the emission originates from of the dedoped PEDOT:PSS and to rule out the presence of an artifact, we have used an aqueous dispersion of sodium hydroxide, another base, for the PEDOT:PSS dedoping. The PL emission of the dedoped PEDOT:PSS using sodium hydroxide shows essentially the same features as those observed using DBU (Figure 3). In order to avoid the possible reoxidation of the dedoped PEDOT:PSS films during the PL measurements, we modified two experimental conditions: first, the laser intensity was reduced, and second, the films were encapsulated. Figure 5 shows the PL emission of the DBU-doped PEDOT:PSS encapsulated samples. From the figure, we can observe that the encapsulated sample shows an emission peak at 630 nm, which is also observed with the naked eye (Figure 4c). From the absorption spectrum and from the PL measurements of the encapsulated samples, we can establish that the PL peak at 900 nm of the DBU-treated PEDOT:PSS films shown in Figure 3 is related to a reoxidation of the PEDOT chains due to the D


Article


Figure 6. Raman spectra of pristine PEDOT:PSS and DBU-treated PEDOT:PSS films at low DBU loadings.

Figure 8. FTIR spectra of PEDOT:PSS and DBU-treated PEDOT:PSS films (1:0.01) in the 2750−4000 cm−1 region.

in the films due to treatment. The studies of FTIR spectroscopy were also used to elucidate the possible mechanism of PEDOT:PSS reduction. The FTIR spectrum obtained from DBU-treated PEDOT:PSS film (at volume ratio of 1:0.01) was divided, for a better study, in two regions (Figures 7 and 8; see

our case, we do not observe this band in the PEDOT:PSS pristine film, which is in its oxidized state, so this IR vibration may come from the CN bond of the DBU molecule, which has been reported at 1610 cm−1.30,40 Another possibility is that this band is due to the C−N+ stretching vibration or the N+−H bending vibration of the DBU resonant structure, which is formed due to the interaction of water with DBU.30 Figure 8 shows the region of the FTIR spectrum corresponding to higher wavenumbers, where new bands of DBU-treated films are present. The bands centered at 3047, 3125, and 3257 cm−1 are associated with the N−H stretching vibration,30,41 while the band located at 3465 cm−1 corresponds to the hydroxyl functional group (OH).30,41 Moreover, some other bands around 2800 cm−1 can be assigned to vibrations of free DBU molecules.40 Using the FTIR information, a possible mechanism for the dedoping process on DBU-treated films is described in Figure 9. This mechanism can be understood in the following way: Because of the water interaction with DBU, the DBU structure is changed to a resonant structure (Figure 9 (1)). The resulting structure is composed of the DBU molecule, which has been protonated and a N−H bond is formed, and a hydroxide anion.30 Subsequently, the hydroxide anion coming from the resonant structure reduces the thiophene molecule, resulting in a neutralization of the PEDOT backbone (Figure 9 (2)). Finally, when the PSS anion is not acting as a PEDOT counterion anymore, this interacts with the protonated DBU molecule (Figure 9 (2)). The consequence of the dedoping process can be easily observed as nonuniformity in thickness and discontinuity in the PEDOT:PSS-DBU films with a high content of DBU. This fact can be attributed to a PSS segregation caused by the dedoping process.

Figure 7. FTIR spectra of PEDOT:PSS and DBU-treated PEDOT:PSS films (1:0.01) in the 900−1800 cm−1 region.

also Figure S4 in the Supporting Information the corresponding spectrum related with the volume ratio 1:0.2). As shown in Figure 7, the FTIR spectra of the pristine film20,21,25 and the DBU-treated film presented similar position bands; only four new peaks are observed in this particular region of the spectrum. Three bands at around 1068, 1330, and 1488 cm−1 are characteristic of the neutral PEDOT.35 These IR bands show that the DBU separates some PSS chains from the PEDOT:PSS complex and, as consequence, some PEDOT chains are reduced to the neutral state. In addition, the processability of PEDOT:PSS dispersion diminishes because the PSS counterions also provide its solubility. This solubility loss results in PEDOT films with low morphological quality when a high amount of DBU is used in the PEDOT:PSS aqueous dispersion (1:0.2 and 1:0.03 PEDOT:PSS-DBU films).The other IR band at 1646 cm−1 has been assigned to the quinoid structure of the oxidized PEDOT.39 However, in

■

CONCLUSIONS In this work, it has been shown that PEDOT:PSS is dedoped by the addition of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), a strong organic base, into the PEDOT:PSS aqueous dispersion. In this way, the dedoping process was carried out using simple physical processes; no electrolytic techniques were used, as is commonly performed in the literature. In addition, the process is very simple. These PEDOT:PSS-DBU samples showed a strong absorption band at 600 nm (π−π*) and a small band at E


Article


Figure 9. Proposed reduction mechanism of PEDOT:PSS solutions and films.

■

ACKNOWLEDGMENTS The authors acknowledge C. G. Elı ́as-Alfaro, F. Ramı ́rezJacobo, D. Partida-Gutierrez, B. A. Rivera-Escoto, E. AlvaradoGómez, G. Labrada-Delgado, and H. G. Silva-Pereyra for technical assistance as well as to A. López-Valdivieso (Lab. Quı ́mica de Superficies, Instituto de Metalurgia, UASLP) and LINAN at IPICyT, for providing access to its facilities. This work was supported at UASLP and IPICYT through grants no. SEP-PROMEP/103.5/15/3228 (M.R.-R., J.M.F.-C., R.L.-S.) and by CONACYT no. 2009-123861 (UASLP-Instituto de Metalurgia) and S-3957 (IPICYT), and research scholarship (I.A.R.-G.).

900 nm (polaronic band), which is a clear indication of the PEDOT reduction. The dedoping efficiency depended on the amount of DBU added to the aqueous PEDOT:PSS dispersion. Moreover, the dedoped PEDOT samples, aqueous dispersion and films, show strong emission in the visible range with its maximum emission at 630 nm when they were excited with a 488 nm laser. To corroborate that the observed emission is related to neutral PEDOT chains and that it is not an artifact, the dedoping process of PEDOT:PSS was performed using NaOH, an inorganic strong base. These dedoped films also show the same emission spectra. Using Raman and FTIR optical spectroscopy measurements, a mechanism responsible for the PEDOT dedoping was proposed. This reduction mechanism can be understood in the following way: The interaction between DBU and water, present in the PEDOT:PSS dispersion, changes the structure of the DBU molecule to a complex formed by a protonated DBU molecule and hydroxide anion. Then, the hydroxide anion of this complex reduces the thiophene molecule, resulting in a neutralization of the PEDOT backbone.

■

■

(1) Wallace, G. G.; Spinks, G. M.; Kane-Maguire, L. A. P.; Teasdale, P. R. Conductive Electroactive Polymers: Intelligent Polymer Systems; CRC Press, Taylor & Francis Group: Boca Raton, FL, 2009. (2) Elschner, A.; Kirchmeyer, S.; Lövenich, W.; Merker, U.; Reuter, K. PEDOT: Principles and Applications of an Intrinsically Conductive Polymer; Taylor and Francis: Boca Raton, FL, 2011. (3) Levermore, P. A.; Chen, L.; Wang, X.; Das, R.; Bradley, D. D. C. Fabrication of Highly Conductive Poly(3,4-Ethylenedioxythiophene) Films by Vapor Phase Polymerization and Their Application in Efficient Organic Light-Emitting Diodes. Adv. Mater. 2007, 19, 2379− 2385. (4) Fabretto, M. V.; Evans, D. R.; Mueller, M.; Zuber, K.; HojatiTalemi, P.; Short, R. D.; Wallace, G. G.; Murphy, P. J. Polymeric Material with Metal-Like Conductivity for Next Generation Organic Electronic Devices. Chem. Mater. 2012, 24, 3998−4003. (5) Ouyang, J.; Chu, C.-W.; Chen, F.-C.; Xu, Q.; Yang, Y. Polymer Optoelectronic Devices with High-Conductivity Poly(3,4-Ethylenedioxythiophene) Anodes. J. Macromol. Sci., Part A: Pure Appl.Chem. 2004, 41, 1497−1511. (6) Huang, J. S.; Miller, P. F.; Wilson, J. S.; de Mello, A. J.; de Mello, J. C.; Bradley, D. D. C. Investigation of the Effects of Doping and PostDeposition Treatments on the Conductivity, Morphology, and Work Function of Poly (3,4-Ethylenedioxythiophene)/poly (styrene Sulfonate) Films. Adv. Funct. Mater. 2005, 15, 290−296. (7) Kim, Y. H.; Sachse, C.; Machala, M. L.; May, C.; MullerMeskamp, L.; Leo, K. Highly Conductive PEDOT:PSS Electrode with Optimized Solvent and Thermal Post-Treatment for ITO-Free Organic Solar Cells. Adv. Funct. Mater. 2011, 21, 1076−1081.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b04016. Photographic images of thin films at different PEDOT:PSS-DBU v/v ratios, DBU dipped PEDOT:PSS films, and their corresponding FTIR and absorbance spectra (PDF)

■

REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Tel: +52 444 834 2000. Fax: +52 444 834 2010. E-mail: [email protected]. Notes

The authors declare no competing financial interest. F


Article

The Journal of Physical Chemistry C (8) Xia, Y.; Ouyang, J. Salt-Induced Charge Screening and Significant Conductivity Enhancement of Conducting Poly(3,4ethylenedioxythiophene):Poly(styrenesulfonate). Macromolecules 2009, 42, 4141−4147. (9) Kim, J. Y.; Jung, J. H.; Lee, D. E.; Joo, J. Enhancement of Electrical Conductivity of Poly(3,4-Ethylenedioxythiophene)/poly(4Styrenesulfonate) by a Change of Solvents. Synth. Met. 2002, 126, 311−316. (10) Ashizawa, S.; Horikawa, R.; Okuzaki, H. Effects of Solvent on Carrier Transport in Poly(3,4-Ethylenedioxythiophene)/poly(4-Styrenesulfonate). Synth. Met. 2005, 153, 5−8. (11) Jonsson, S. K. M.; Birgerson, J.; Crispin, X.; Greczynski, G.; Osikowicz, W.; Denier van der Gon, A. W.; Salaneck, W. R.; Fahlman, M. The Effects of Solvents on the Morphology and Sheet Resistance in Poly (3,4-Ethylenedioxythiophene)-Polystyrenesulfonic Acid (PEDOT-PSS) Films. Synth. Met. 2003, 139, 1−10. (12) Crispin, X.; Jakobsson, F. L. E.; Crispin, A.; Grim, P. C. M.; Andersson, P.; Volodin, A.; van Haesendonck, C.; Van der Auweraer, M.; Salaneck, W. R.; Berggren, M. The Origin of the High Conductivity of Poly (3, 4-Ethylenedioxythiophene)-Poly (styrenesulfonate) (PEDOT-PSS) Plastic Electrodes. Chem. Mater. 2006, 18, 4354−4360. (13) Dimitriev, O. P.; Grinko, D. A.; Noskov, Y. V.; Ogurtsov, N. A.; Pud, A. A. PEDOT:PSS filmsEffect of Organic Solvent Additives and Annealing on the Film Conductivity. Synth. Met. 2009, 159, 2237−2239. (14) Badre, C.; Marquant, L.; Alsayed, A. M.; Hough, L. A. Highly Conductive Poly(3,4-ethylenedioxythiophene):Poly (styrenesulfonate) Films Using 1-Ethyl-3-Methylimidazolium Tetracyanoborate Ionic Liquid. Adv. Funct. Mater. 2012, 22, 2723−2727. (15) Zhi-Hui, F.; Yan-Bing, H.; Quan-Min, S.; Li-Fang, Q.; Yan, L.; Lei, Z.; Xiao-Jun, L.; Feng, T.; Yong-Sheng, W.; Rui-Dong, X. Polymer Solar Cells Based on a PEDOT:PSS Layer Spin-Coated Under the Action of an Electric Field. Chin. Phys. B 2010, 19, 038601. (16) Cruz-Cruz, I.; Reyes-Reyes, M.; Aguilar-Frutis, M. A.; Rodriguez, A. G.; López-Sandoval, R. Study of the Effect of DMSO Concentration on the Thickness of the PSS Insulating Barrier in PEDOT:PSS Thin Films. Synth. Met. 2010, 160, 1501−1506. (17) Xia, Y. J.; Sun, K.; Ouyang, J. Y. Solution-Processed Metallic Conducting Polymer Films as Transparent Electrode of Optoelectronic Devices. Adv. Mater. 2012, 24, 2436−2440. (18) Kim, N.; Kee, S.; Lee, S. H.; Lee, B. H.; Kahng, Y. H.; Jo, Y.-R.; Kim, B.-J.; Lee, K. Highly Conductive PEDOT:PSS Nanofibrils Induced by Solution-Processed Crystallization. Adv. Mater. 2014, 26, 2268−2272. (19) Mukherjee, S.; Singh, R.; Gopinathan, S.; Murugan, S.; Gawali, S.; Saha, B.; Biswas, J.; Lodha, S.; Kumar, A. Solution-Processed Poly(3,4-Ethylenedioxythiophene) Thin Films as Transparent Conductors: Effect of p-Toluenesulfonic Acid in Dimethyl Sulfoxide. ACS Appl. Mater. Interfaces 2014, 6, 17792−17803. (20) Reyes-Reyes, M.; Cruz-Cruz, I.; López-Sandoval, R. Enhancement of the Electrical Conductivity in PEDOT:PSS Films by the Addition of Dimethyl Sulfate. J. Phys. Chem. C 2010, 114, 20220− 20224. (21) Cruz-Cruz, I.; Reyes-Reyes, M.; López-Sandoval, R. Formation of Polystyrene Sulfonic Acid Surface Structures on Poly(3,4-Ethylenedioxythiophene): Poly(styrenesulfonate) Thin Films and the Enhancement of Its Conductivity by Using Sulfuric Acid. Thin Solid Films 2013, 531, 385−390. (22) Palumbiny, C. M.; Heller, C.; Schaffer, C. J.; Körstgens, V.; Santoro, G.; Roth, S. V.; Mü l ler-Buschbaum, P. Molecular Reorientation and Structural Changes in Cosolvent-Treated Highly Conductive PEDOT:PSS Electrodes for Flexible Indium Tin OxideFree Organic Electronics. J. Phys. Chem. C 2014, 118, 13589−13606. (23) Kim, H.; Nam, S.; Lee, H.; Woo, S.; Ha, C. S.; Ree, M.; Kim, Y. Influence of Controlled Acidity of Hole-Collecting Buffer Layers on the Performance and Lifetime of Polymer:Fullerene Solar Cells. J. Phys. Chem. C 2011, 115, 13502−13510.

(24) Xuan, Y.; Sandberg, M.; Berggren, M.; Crispin, X. An AllPolymer-Air PEDOT Battery. Org. Electron. 2012, 13, 632−637. (25) Lee, S. H.; Park, H.; Son, W.; Choi, H. H.; Kim, J. H. Novel Solution-Processable, Dedoped Semiconductors for Application in Thermoelectric Devices. J. Mater. Chem. A 2014, 2, 13380−13387. (26) de Kok, M. M.; Buechel, M.; Vulto, S. I. E.; van de Weijer, P.; Meulenkamp, E. A.; de Winter, S. H. P. M.; Mank, A. J. G.; Vorstenbosch, H. J. M.; Weijtens, C. H. L.; van Elsbergen, V. Modification of PEDOT:PSS as Hole Injection Layer in Polymer LEDs. Phys. Status Solidi 2004, 201, 1342−1359. (27) Ishikawa, T. Superbases for Organic Synthesis: Guanidines, Amidines, Phosphazenes and Related Organocatalysts; Ishikawa, T., Ed.; John Wiley & Sons: Chichester, U.K., 2009. (28) Shieh, W. C.; Dell, S.; Repic, O. 1,8-Diazabicyclo Undec-7-Ene (DBU) and Microwave-Accelerated Green Chemistry in Methylation of Phenols, Indoles, and Benzimidazoles with Dimethyl Carbonate. Org. Lett. 2001, 3, 4279−4281. (29) Shieh, W. C.; Dell, S.; Repic, O. Nucleophilic Catalysis with 1, 8Diazabicyclo Undec-7-Ene (DBU) for the Esterification of Carboxylic Acids with Dimethyl Carbonate. J. Org. Chem. 2002, 67, 2188−2191. (30) Cota, I.; Chimentao, R.; Sueiras, J.; Medina, F. The DBU-H2O Complex as a New Catalyst for Aldol Condensation Reactions. Catal. Commun. 2008, 9, 2090−2094. (31) Dietrich, M.; Heinze, J.; Heywang, G.; Jonas, F. Electrochemical and Spectroscopic Characterization of Polyalkylenedioxythiophenes. J. Electroanal. Chem. 1994, 369, 87−92. (32) Kvarnström, C.; Neugebauer, H.; Blomquist, S.; Ahonen, H. J.; Kankare, J.; Ivaska, A. In Situ Spectroelectrochemical Characterization of Poly(3,4-Ethylenedioxythiophene). Electrochim. Acta 1999, 44, 2739−2750. (33) Łapkowski, M.; Proń, A. Electrochemical Oxidation of Poly(3,4Ethylenedioxythiophene) − “in Situ” Conductivity and Spectroscopic Investigations. Synth. Met. 2000, 110, 79−83. (34) Ouyang, J.; Xu, Q.; Chu, C.-W.; Yang, Y.; Li, G.; Shinar, J. On the Mechanism of Conductivity Enhancement in Poly(3,4Ethylenedioxythiophene):poly(styrene Sulfonate) Film through Solvent Treatment. Polymer 2004, 45, 8443−8450. (35) Garreau, S.; Louarn, G.; Buisson, J. P.; Froyer, G.; Lefrant, S. In Situ Spectroelectrochemical Raman Studies of Poly(3,4-Ethylenedioxythiophene) (PEDT). Macromolecules 1999, 32, 6807−6812. (36) Kirchmeyer, S.; Reuter, K. Scientific Importance, Properties and Growing Applications of poly(3,4-Ethylenedioxythiophene). J. Mater. Chem. 2005, 15, 2077−2088. (37) Chiu, W. W.; Travas-Sejdic, J.; Cooney, R. P.; Bowmaker, G. A. Synth. Met. 2005, 155, 80−88. (38) Chiu, W. W.; Travas-Sejdic, J.; Cooney, R. P.; Bowmaker, G. A. J. Raman Spectrosc. 2006, 37, 1354−1361. (39) Xiong, S.; Zhang, L.; Lu, X. Conductivities Enhancement of Poly(3,4-Ethylenedioxythiophene)/poly(styrene Sulfonate) Transparent Electrodes with Diol Additives. Polym. Bull. 2013, 70, 237−247. (40) Spectral Database for Organic Compounds. http://sdbs.db.aist. go.jp/sdbs/cgi-bin/cre_index.cgi. (41) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds, 6th ed.; Wiley-Interscience: Hoboken, N.J., 2009.

G


Visible Luminescence of Dedoped DBU-Treated ... - ACS Publications

Recommend Documents