Facile Optimization of Thermoelectric Properties in PEDOT:PSS Thin

Letter Cite This: ACS Appl. Energy Mater. 2018, 1, 336−342

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Facile Optimization of Thermoelectric Properties in PEDOT:PSS Thin Films through Acido-Base and Redox Dedoping Using Readily Available Salts Nitin Saxena,†,‡ Josef Keilhofer,† Anjani K. Maurya,†,§ Giuseppino Fortunato,∥ Jan Overbeck,⊥,# and Peter Müller-Buschbaum*,†,‡ †

Lehrstuhl für Funktionelle Materialien, Physik-Department, Technische Universität München, James-Franck-Strasse 1, 85748 Garching, Germany ‡ Nanosystems Initiative Munich (NIM), Schellingstrasse 4, 80799 München, Germany § Center for X-ray Analytics, Department Materials Meet Life, Empa, Swiss Federal Laboratories for Materials Science and Technology, Lerchenfeldstrasse 5, 9014 St. Gallen, Switzerland ∥ Laboratory for Biomimetic Membranes and Textiles, Empa, Swiss Federal Laboratories for Materials Science and Technology, Lerchenfeldstrasse 5, 9014 St. Gallen, Switzerland ⊥ Swiss Nanoscience Institute, University of Basel, Klingelbergstrasse 82, 4056 Basel, Switzerland # Laboratory for Transport at Nanoscale Interfaces, Department Materials Meet Life, Empa, Swiss Federal Laboratories for Materials Science and Technology, Ü berlandstrasse 129, 8600 Dübendorf, Switzerland S Supporting Information *

ABSTRACT: Poly(ethylene dioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) has emerged as a promising candidate for renewable, clean, and reliable energy generation from waste heat due to its thermoelectric properties. This largely stems from its tunable and potentially high electrical conductivity. However, the resulting small Seebeck coefficients diminish the thermoelectric efficiency. We employ dedoping methods making use of acido-base and redox dedoping in order to optimize its properties. In order to tune the charge carrier concentration in PEDOT:PSS thin films, aqueous solutions of readily available inorganic salts, namely, sodium hydrogen carbonate (NaHCO3), sodium sulfite (Na2SO3), and sodium borohydride (NaBH4), are introduced in different concentrations into PEDOT:PSS solutions before thin film fabrication. This yields optimized thermoelectric properties in terms of power factors up to 100 μW/K2 m. Changes in the electronic structure are characterized using UV−vis spectroscopy and XPS, while changes in the conformation are investigated using Raman spectroscopy. The thermoelectric quantities are compared for the redox dedopants regarding the absolute number of reducing equivalents. KEYWORDS: PEDOT:PSS, thermoelectricity, dedoping, acido-base, redox, oxidation level

O

Thermoelectric efficiency is generally gauged using the dimensionless, temperature-dependent figure of merit ZT, which is given in eq 1. Here, σ represents the electrical conductivity, S the so-called Seebeck coefficient, κ the thermal conductivity, and T the absolute temperature.14

rganic thermoelectric materials have gained increased attention in recent years, due to their promise for facile and continuous generation of electrical power from one of the most abundant, yet least used, sources of energy, which is heat being released from multiple low temperature sources.1,2 Among the potential applications are the use in building integration, along with small and portable power supplies and in textiles. Over time, out of the vast number of possible candidates of organic compounds, within the class of conducting polymers, poly(ethylene3,4-dioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) has emerged. This originates from the easy processability, semitransparency,3 and especially the potentially high and tunable electrical conductivity of thin films through various treatments.4−12 These factors render PEDOT:PSS a promising candidate for thermoelectric generators based on organic materials operating at moderate temperatures.13 © 2018 American Chemical Society

Thermoelectric, dimensionless, temperature-dependent figure of merit. ZT =

σS 2 T κ

(1)

Received: December 22, 2017 Accepted: February 7, 2018 Published: February 7, 2018 336

DOI: 10.1021/acsaem.7b00334 ACS Appl. Energy Mater. 2018, 1, 336−342

Letter

ACS Applied Energy Materials The numerator of this equation σS2 is called the power factor and relates to the electrical power one can obtain from a thermoelectric film. This quantity is usually determined in literature studies, since the measurement of the thermal conductivity of thin films proves to be difficult and requires rigorous sample treatment.13,15,16 Moreover, for applications targeting waste heat recovery the power factor is a reasonable quantity since the extractable power is of the highest importance irrespective of an overall device efficiency. Primary doping describes the introduction of charge carriers in conducting polymers. In PEDOT:PSS, the primary doping of PEDOT chains is mediated through the coordination of PSS and subsequent proton transfer onto the former. Similarly, in PEDOT:Tos thin films, the tosylate anions dope PEDOT chains. One of the commercially available formulations of PEDOT:PSS PH1000, which reaches the highest electrical conductivities through appropriate treatments, consists of short PEDOT chains which are heavily doped by the present PSS.17,18 This increases the number of charge carriers, such that bipolarons are the primary charge carriers. However, the heavy doping also leads to small Seebeck coefficients due to the surplus of charge carriers and therefore only small voltages can be obtained.17 The square dependence of the figure of merit ZT on the Seebeck coefficient explains the need for methods to improve the Seebeck coefficient. Several studies already showed that it is possible to tune the number of charge carriers which are present in PEDOT, or in other words the oxidation level. This in turn allows for variation of the Seebeck coefficient. Bubnova et al. achieved this by treating PEDOT:Tos films with tetrakis(dimethylamino)ethylene (TDAE) vapors.19 Massonnet et al. used redox-active compounds which introduce negative charge carriers into the PEDOT:PSS film and thereby altered the oxidation level.20 As a complementary method, Fan et al. made use of the acido-basic properties of doped PEDOT:PSS and were able to tune the thermoelectric properties by employing a sequential treatment with sulfuric acid and sodium hydroxide.10 In this study, we systematically vary the thermoelectric properties, namely, the Seebeck coefficient, electrical conductivity, and thus the power factor of thin PEDOT:PSS films dedoped with different inorganic salts. The latter include sodium sulfite and sodium borohydride as redox-active salts and sodium hydrogen carbonate as salt with acido-basic properties. Treatment with all three readily available salts yields changes in the electronic structure of PEDOT:PSS thin films, as evidenced through the measurement of thermoelectric quantities and UV−vis spectroscopy. Figure 1 schematically shows the procedure for preparation of the dedoped PEDOT:PSS thin films. The PEDOT:PSS solution is subjected to an ultrasonic bath for 15 min, filtered, and then mixed with the fluorosurfactant Zonyl, as described in earlier work.13,21,22 A 1 M aqueous stock solution of each inorganic salt is prepared and mixed into the PEDOT:PSS solution in different amounts, in order to achieve the desired concentrations. The mixture is then put on a shaker for 30 min, in order to ensure a homogeneous intermixture of the components. The final solution is applied on clean soda-lime glass substrates via spin-coating (1500 rpm, 60 s). The thin films are annealed afterwards at 140 °C for 10 min. A second PEDOT:PSS layer is applied, for which the process is repeated, to gain larger film thicknesses. Post-treatment with ethylene glycol (EG) is conducted, in order to render the thin films electrically conducting, as has been already discussed in literature.23 For this, 1 mL of EG is drop-cast on the thin films, left to take effect for 1 min, spun-off with 1500 rpm

Figure 1. Individual steps of the fabrication procedure for two-layered PEDOT:PSS thin films dedoped with inorganic salts using repeated spin-coating.

for 60 s, and again subjected to an annealing step at 140 °C for 10 min.13 Figure 2 shows the influence of the facile dedoping procedure with sodium hydrogen carbonate, sodium sulfite, and sodium borohydride on the thermoelectric properties, namely, Seebeck coefficient, electrical conductivity, and power factor. In Figure 2a, the results of dedoping of PEDOT:PSS films with sodium hydrogen carbonate are shown. The Seebeck coefficient exhibits a steady, monotonic increase from 24.7 μV/K for the reference sample without dedopant, to a maximum value of 54.8 μV/K for a concentration of 70 mM dedopant, which then levels off. This comes at the cost of electrical conductivity which shows a value of 875 S/cm for the reference sample, slightly increases to approximately 1000 S/cm for low concentrations of dedopant, only to drop almost exponentially to approximately 115 S/cm for the highest concentration of 82 mM. Both trends can be explained by a change in the acido-basic doping of PEDOT chains through the presence and subsequent decomposition of NaHCO3 in aqueous solution, as shown in Scheme 1. The increasing concentration of hydroxyl anions leads to the removal of protons from the PEDOT chains, whereby the latter act as dopants and induce charge carriers. The decreased number of protons leads to a decrease in the charge carrier concentration and therefore the oxidation level, for which the Seebeck coefficient is a direct measure.24 An increase in the Seebeck coefficient is commonly ascribed to a decrease in charge carrier concentration, since there is an inverse dependence of the former on the latter. A decrease in positive charge carriers should therefore also directly affect the electrical conductivity which shows a direct dependency. In sum, an optimum in the power factor is found for a concentration of 24 mM, yielding a value of approximately 100 μW/K2 m, which is an improvement of almost 100% compared to 53 μW/K2 m found for the reference. Notably, especially for high concentrations of NaHCO3, strong inhomogeneities in the film can be observed, as shown in the optical micrographs in Figure S1 of the Supporting Information. This most probably stems from a competitive binding of both PSS chains and hydrogen carbonate anions to the PEDOT chains. While the PSS chains allow for dispersion of PEDOT in water, once the coordination breaks this is no longer the case. Therefore, the induced coagulation leads to inhomogeneous films, 337


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24.4 μV/K for the reference sample without dedopant, to a maximum value of 69.0 μV/K for a concentration of 91 mM, leveling off for higher concentrations. As opposed to dedoping with NaHCO3, treatment with Na2SO3 directly leads to a drop of the electrical conductivity, for values of around 1050 S/cm for the reference to 36 S/cm for the highest concentration of 157 mM. Again, an almost exponential decay of the electrical conductivity as a function of Na2SO3 concentration is found. In total, this leads to an optimum power factor of 90 μW/K2 m at a rather low concentration of Na2SO3 of 12 mM. Notably, in the case of dedoping with Na2SO3, the maximum Seebeck coefficient achievable is significantly larger than for dedoping with NaHCO3. Figure S2 shows optical micrographs of PEDOT:PSS films dedoped with Na2SO3, in which coagulation and therefore inhomogeneities in the film become apparent for high concentrations. Last, sodium borohydride is investigated as the second reducing agent, the results for which can be found in Figure 2c. The decomposition in aqueous solution follows the reaction shown in Scheme 3. Again, optical micrographs of dedoped Scheme 3. Decomposition of Sodium Borohydride in Aqueous Solution

PEDOT:PSS films are shown in the Supporting Information (Figure S3). As compared to Na2SO3, NaBH4 only releases one reducing equivalent (in this case the negatively charged hydride) per mole. As expected, the thermoelectric parameters again vary strongly as a function of the concentration of NaBH4. Regarding the Seebeck coefficient, a behavior similar to that of the previously discussed reducing agent is found. Values rise continuously from 23.0 μV/K for the reference sample, to 58.0 μV/K for the highest concentration of 160 mM NaBH4, which is however lower than the maximum Seebeck coefficient found for dedoping with Na2SO3. Interestingly, the electrical conductivity values are constant within the error up to a NaBH4 concentration of 24 mM. After this plateau it drops continuously from 940 S/cm for the reference sample to 49 S/cm for a concentration of 160 mM. At 24 mM NaBH4, also the maximum of the power factor is found at 96 μW/K2 m, since a high electrical conductivity is present alongside an increased Seebeck coefficient of 31.9 μV/K. However, again, the increase in the latter cannot compensate for the continuous loss in electrical conductivity. An explanation for the evolution of the thermoelectric parameters can be found, when trying to understand the involved processes on an electronic level. For this purpose, UV−vis spectroscopy is employed. The different electronic states, namely, neutral, polaron, and bipolaron states of the electrically conducting PEDOT, can be distinguished through characteristic absorption features in the range of visible light to the infrared. While absorption of neutral species of PEDOT lies around 600 nm, absorption of polarons occurs around 900 nm, and for bipolarons absorption is in the infrared region in a broad band. UV−vis spectroscopy helps to understand the changes in the ratio of the different species within the material as a function of concentration of the different dedopants.20,25,26 Figure 3 shows the absorption coefficients for the dedoped PEDOT:PSS films presented in Figure 2, allowing for the comparison to PEDOT:PSS films post-treated with EG but without dedopant. Normalized absorption coefficients are chosen in order to make the evolution of the spectra as

Figure 2. Power factor (blue ▶), Seebeck coefficient S (green ◆), and electrical conductivity σ (brown ⬢) for PEDOT:PSS thin films dedoped with (a) sodium hydrogen carbonate NaHCO3, (b) sodium sulfite Na2SO3, and (c) sodium borohydride NaBH4, each in varying concentrations. The dashed lines for S and σ and the solid line for the power factor are guides to the eye.

Scheme 1. Decomposition of Sodium Hydrogen Carbonate NaHCO3 in Aqueous Solution

which also become difficult to measure regarding their thermoelectric and optical properties. For this reason, the maximum NaHCO3 concentration investigated was 82 mM. Figure 2b shows the evolution of thermoelectric parameters as a function of sodium sulfite (Na2SO3) concentration, acting as a redox dedopant. Upon decomposition, 1 equiv of Na2SO3 releases 2 equiv of electrons, as shown in Scheme 2. Similar to dedoping with NaHCO3, strong changes in the thermoelectric properties upon dedoping with Na2SO3 are observed. This includes again an increase in the Seebeck coefficient from Scheme 2. Decomposition of Sodium Sulfite Na2SO3 in Aqueous Solution

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Figure 3. Normalized absorption coefficients of PEDOT:PSS films dedoped with (a) NaHCO3, (b) Na2SO3, and (c) NaBH4, with increasing concentrations indicated by transition from black to light green curves as follows: no dedopant (black), 5 mM, 12 mM, 24 mM, 36 mM, 70 mM, 82 mM, 91 mM, and 160 mM (light green). The spectra are normalized to their respective values at 400 nm and stacked, in order to highlight the evolution of features with dedopant concentration. Polaron states appear at wavelengths of around 950 nm, while neutral states can be found at wavelengths around 600 nm.

Figure 4. Top row: High resolution XPS spectra of the S 2p signal. Bottom row: Raman spectra for films dedoped with NaHCO3 (a, d), Na2SO3 (b, e), and NaBH4 (c, f). The black curves in all images represent the reference for PEDOT:PSS films without dedopant, while increasingly lighter shades of green indicate increasing dedopant concentration. The Raman spectra were normalized to the intensity of the band located at 990 cm−1 in order to make the changes in the spectra as a function of dedopant concentration more apparent. All curves are stacked along the y-axis for clarity. The detailed concentrations can be found in the Supporting Information.

the best of our knowledge not yet been assigned to distinct states of PEDOT.10,20,27,28 Summarizing, one can say that a decrease in charge carrier concentration by acido-base dedoping with NaHCO3 is detectable through a shift from bipolaron to polaron and finally neutral states. A similar picture is found for the PEDOT:PSS films treated with the redox-active dedopants Na2SO3 and NaBH4 shown in Figure 3b,c, respectively. Also, for both redox-active dedopants, the tail of the bipolaron absorption found around 1000 nm gradually turns into a broad peak, hinting at an increased polaron concentration. The two peaks at 600 and 680 nm, which appear for concentrations larger than 24 mM and 36 mM for Na2SO3 and NaBH4, respectively, are again attributed to neutral PEDOT, as discussed above. While the ratios of neutral to polaron bands vary slightly, the general trend of the polaron shoulder turning to a peak and the appearance of the neutral bands remains the same. Interestingly, the onset of the appearance of neutral states fits together with the evolution of the power factors, as in every case

a function of dedopant concentration more apparent. The low absorption coefficients for the reference curves in Figure 3a−c (black) already give insight into the electronic structure of PEDOT:PSS without use of any dedopant. Most of the absorption takes place in the infrared region of the spectrum and therefore represents a high concentration of bipolarons, or in other words a heavy electronic doping. Figure 3a shows the different UV−vis spectra for PEDOT:PSS films dedoped with NaHCO3. The acido-base chemistry of NaHCO3 alters the nature of the charge carriers, as can be seen for example from the shoulder which increasingly turns into a peak for intermediate concentrations up to 24 mM. Even though the absorption of bipolarons is not within the wavelength range of these measurements, it is safe to assume that their contribution decreases with increasing NaHCO3 concentration. For concentrations above 36 mM, absorption peaks representing the presence of neutral states appear. While features in the green spectral region have often been observed for reduced PEDOT:PSS, a double feature, as is found here, has to 339


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dedopant. The corresponding values can be found in Table S3. For the most part, this can be ascribed to the vibrational contribution of the quinoid form of PEDOT, which manifests itself as a shoulder located at approximately 1405 cm−1.12 In addition, the band which appears at 1499 cm−1 for PEDOT:PSS without dedoping becomes more pronounced and shifts to higher wavenumbers for increasing concentrations of dedopant. An increase in this spectral region is indicative of rising contributions of the benzoid form of PEDOT. This finding is again in line with the loss of conductivity as a function of dedopant concentration, as the benzoid form of PEDOT does not favor an extended-coil or linear conformation of PEDOT chains the same way the quinoid form does. A loss in the latter most likely leads to more compact conformations of PEDOT chains, which qualitatively manifest as a loss in conductivity due to impeded charge transport. In order to get a full picture, the pH values of aqueous solutions with concentrations of 1 M are listed in Table S4. It is apparent that Na2SO3 solutions are neutral in nature, while NaHCO3 and NaBH4 give basic solutions. Therefore, we investigated the changes in the pH value upon dedoping with NaHCO3 and NaBH4. Values for exemplary concentrations of these two salts in PEDOT:PSS solution are also listed in Table S4. One can see that for optimal concentrations of 24 mM the pH value only decreases slightly, while for high concentrations the PEDOT:PSS solutions turn basic in both cases. Looking at the results, one might argue that in both cases the deprotonation of PEDOT chains is responsible for the observed changes in thermoelectric properties. However, one needs to note that while in the case of dedoping with NaBH4 a shift of the PEDOT contribution to the S 2p band in the XPS measurements toward lower binding energies is observed (Figure 4c), this is not true for dedoping with NaHCO3, where the band does not shift. Thus, even though the resulting pH values are the same for the same concentrations of NaBH4 and NaHCO3, the results indicate that reduction is still part of the mechanism for dedoping with NaBH4. Also, taking dedoping with Na2SO3 (which does not possess basic properties) into account, reduction is a viable explanation for the observed effect on thermoelectric parameters. Still, the effect of all dedopants in terms of chain conformation, as seen from the Raman spectra in Figure 4d−f, indicates that a loss of the quinoid form is present alongside an increase in the benzoid contribution. Both effects are macroscopically observable as a drop in electrical conductivity, as is the increase in Seebeck coefficient for all cases. In summary, the presented results support the notion of a reduced number of charge carriers, respectively, a transition from bipolarons as majority charge carriers to polarons and, subsequently, the formation of neutral states, which can be determined with the help of UV−vis spectroscopy and XPS. This transition is accompanied by conformational changes, as observed from Raman spectra, in which the contribution of the quinoid vibration decreases, while the contribution of the benzoid form strongly increases. The latter is observed irrespective of redox and/or basic properties of the dedopants, as seen from the pH measurements. A question that arises upon closer investigation of the presented results is whether the type of employed reducing agent has an influence on the achievable thermoelectric performance or not. For this purpose, in Figure S4a−c, the three main thermoelectric quantities, namely, Seebeck coefficient, electrical conductivity, and power factor, are, respectively, shown for both reducing agents Na2SO3 and NaBH4, as a function of the actual concentration of reducing equivalents. For both cases, this is

the former marks the point where the optimum dedopant concentration is already exceeded. Therefore, it can be concluded that it is favorable to have a higher concentration of polarons, while the presence of neutral states is unfavorable. In order to investigate the impact of dedoping on the ratio between PSS and PEDOT, a compositional analysis of the dedoped PEDOT:PSS films is performed using X-ray photoelectron spectroscopy (XPS) and compared to the reference original PEDOT:PSS film. The S 2p signals from the XPS spectra probed for the different applied dedopants can be seen in Figure 4a−c. As a general trend, treated films show higher amounts for sodium and sulfur as compared to pristine PEDOT:PSS films (Table S1). In addition, sulfur amounts show the opposite trend of decreasing with increasing salt concentration, except for PEDOT:PSS films dedoped with Na2SO3. In more detail, the S 2p photoelectron transition consists of the spin-split doublet (S 2p 1/2 and S 2p 3/2) for which the energy splitting and relative intensity ratio are 1.2 eV and 1:2, respectively. Furthermore, the S 2p signals for PSS appear at higher binding energies as compared to that for PEDOT.29 Pristine PEDOT:PSS films exhibit a strong PSS signal. Taking the information depth for XPS of 8−10 nm into account, this strong signal originates from a PSS capping layer. In comparison, all of the spectra obtained from dedoped PEDOT:PSS films show a strong decrease in the PSS/PEDOT ratio. Thus, the PSS capping layer is removed. A depletion of PSS for the system PEDOT:PSS was also found by Liu et al. after treatment with selected solvents.30 Corresponding values and a graphical representation of the PSS/PEDOT ratio evolution are given in Table S2 and Figure S5, respectively. The increase in PSS/PEDOT ratio for high concentrations of sodium sulfite can most likely be explained by the presence of reaction products in the form of remaining sulfate ions. In case of 91 mM Na2SO3 in the deconvolution of the S 2p signal, an additional contribution needs to be assumed, as seen in Figure S4g. One advantage of XPS is the ability to detect minute changes in the oxidation levels of investigated atomic species being elucidated by shifts in the binding energies. Thus, the XPS spectra can be used to qualitatively investigate the influence of the different dedopants on the average oxidation level of the thiophene sulfur in PEDOT. From the PEDOT contribution to the S 2p signal probed in the case of dedoping with NaHCO3 (Figure 4a), no significant shift along the binding energy axis can be observed. Therefore, the oxidation level upon dedoping with NaHCO3 is preserved. In contrast, when dedoping with Na2SO3 and NaBH4, the oxidation levels change (Figure 4b,c). Especially, for high dedopant concentrations (light green lines), a maximum shift of 0.3 eV toward lower binding energies of the thiophene sulfur of PEDOT is found, indicating a reduction of the average oxidation level. This finding fits together with the picture of charge carrier reduction through injection of electrons/hydride in the case of Na2SO3 and NaBH4, as opposed to the acido-base dedoping with NaHCO3. Raman spectroscopy gives information about vibrational states in PEDOT and PSS and allows for monitoring of changes in the conformation. Figure 4d−f shows exemplary Raman spectra for samples dedoped with NaHCO3, Na2SO3, and NaBH4, respectively, and also reference spectra of PEDOT:PSS without dedoping. In general, the measured spectra and band positions match the findings of Stavytska-Barba et al. quite well.27 Apart from that, there are several noticeable changes as a function of dedopant concentration, irrespective of the type of dedopant. The most intense band centered at 1420 cm−1 becomes narrower for increasing dedopant concentrations, compared to PEDOT:PSS without 340



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calculated on the basis of the respective chemical equations shown above in Schemes 2 and 3. Independent of the actual concentration of reducing agent, one can immediately see that there is a very good agreement of the magnitudes of the thermoelectric parameters as a function of the reducing equivalent concentration for both cases. Especially, for concentrations of 100 mM, the agreement between the different reducing agents is very good. This, in turn, suggests that irrespective of the nature of the actual agent (the redox potential, number of reducing equivalents per mole), the achievable magnitudes of the different thermoelectric properties depend for the most part on the absolute number of reducing equivalents which interact with PEDOT:PSS. In conclusion, we demonstrate a facile method for variation of thermoelectric parameters in PEDOT:PSS thin films using aqueous solutions of readily available inorganic salts making use of both acido-base and redox dedoping. UV−vis spectroscopy confirms the emergence of absorption features related to polaron and neutral states, with increasing dedopant concentration, while XPS reveals changes in the oxidation level for dedoping with the redox dedopants Na2SO3 and NaBH4. Raman spectroscopy proves conformational changes to the benzoid form for all treatments. Lastly, we show that the degree of dedoping depends less on the type of reducing agent, but instead on the absolute number of reducing equivalents present in solution. At optimal concentrations of dedopant, power factors of 100 μW/K2 m for dedoping with NaHCO3, 95 μW/K2 m for NaBH4, and 90 μW/K2 m for Na2SO3 are reached.

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REFERENCES

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.7b00334. Detailed experimental procedure and characterization methods and optical micrographs of the PEDOT:PSS thin films dedoped with NaHCO3, Na2SO3, and NaBH4 in different concentrations; measurements of pH values for the different dedopants in varying concentrations; elemental compositions of the samples; deconvolutions of S 2p signals obtained from XPS; Raman band positions and fwhm values and details about the measurement of the Raman spectra; and thermoelectric parameters as a function of reducing equivalent concentration (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Peter Müller-Buschbaum: 0000-0002-9566-6088 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by funding the Excellence Cluster Nanosystems Initiative Munich (NIM), the Center for NanoScience (CeNS), and the International Research Training Group 2022 Alberta/Technical University of Munich International Graduate School for Environmentally Responsible Functional Hybrid Materials (ATUMS). We thank L. Ascherl, B. Melzer, and E. Uhl for fruitful discussions. 341


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Facile Optimization of Thermoelectric Properties in PEDOT:PSS Thin

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