PEDOT:Poly(4-styrenesulfonate) (PEDOT:PSS) and PEDOT:Tos films (~100 nm thick) were deposited on glass substrates pre-patterned with metal contacts (~100 nm thick). While exposed to ambient air (40 % Relative Humidity (RH), 20 °C), a temperature gradient Δ𝑇 is applied between the two electrodes and the open circuit voltage Voc between those two electrodes is measured to estimate the Seebeck coefficient 𝑆 = Voc ⁄Δ𝑇 (Fig. S1.1). The linewidth and inter-distance separation of the metal electrodes were chosen such to minimize the error associated with the estimation of the Seebeck coefficient (Fig. S1.2).7 Five different metals were explored, Al, Ni, Ag, Au, and Pt, covering a range of work function values from 3.7 to 5.2 eV, as measured by ultraviolet photoelectron spectroscopy (Fig. S2.1). Note that the presence of humidity does not affect the metal work functions20, as the latter measured by Kelvin probe for Au, Ag, and Al substrates have similar values both in vacuum and in air (40 %RH) (Table S2.1).
We observe that the Seebeck coefficient of PEDOT:Tos [PEDOT:PSS] increases with the work function of the metal contact, from 22.7 µV/K [9.4 µV/K] with Al to 50.5 µV/K [26.3 µV/K] with Pt (Fig. 1a). This is a large improvement achieved just by replacing the metal contact and altering the WF. Notably, there is no correlation between the Seebeck coefficients of the metal contacts (open symbols) and the measured Seebeck coefficients (closed symbols). Previous works have reported surprisingly large variations of Seebeck coefficient for both PEDOT:PSS (~10 µV/K with Ag contacts1,2,9,12 and ~20 µV/K with Au contacts2,6) and PEDOT:Tos (40 µV/K with Au contacts2). Our results seem to suggest that the large variations in Seebeck coefficient reported in the literature might be due to the different energetics of the chosen metal-organic interface. Furthermore, even for the same type of metal electrode, the effective Seebeck coefficient is also expected to vary slightly, since the effective metal WF – being a surface property – is affected by the metal surface crystallinity 23, morphology, and also by impurities introduced during metal deposition.
Two metal contacts, set at different temperatures (ΔT = 1 °C around room temperature) and connected to the PEDOT film, constitute a single leg OTEG. The generated electrical power is then measured for various load resistances (Fig. S2.3) aiming at finding the maximum output power for Rload = Rdev. The measured maximum output power density Pmax of the OTEGs based on PEDOT:Tos [PEDOT:PSS] is also increasing with the metal WF, from 0.017 μW/cm2 [0.003 μW/cm2] with Al to 0.715 μW/cm2 [0.105 μW/cm2] with Pt (Fig. 1b). Note that the measured Pmax (closed symbols) is in rather good agreement with the calculated value based on Eq. 1 (open symbols). As P ~ ΔT 2, the extrapolated power values for ΔT = 30 °C are 643.5 μW/cm2 for the Pt/PEDOT:Tos/Pt and 94.5 μW/cm2 for Pt/PEDOT:PSS/Pt (Fig.1b). The maximum power density values cannot be explained solely based on the measured Seebeck coefficients. We find that the device resistance of the PEDOT legs also depends on the metal WF. For PEDOT:PSS, Rdev varies from 6 kΩ for Al to 0.15 kΩ for Pt (Fig. 1c), while for PEDOT:Tos Rdev decreases from 0.7 kΩ (Al) to 0.01 kΩ (Pt). Since the channel resistance, measured with the 4-probe method, is constant for the various films (Fig. 1c), we attribute the change in Rdev to the variation of the contact resistance Rc with the contact metal WF (Fig. 2c). In order to provide further evidence on that, we extracted Rc values also for the PEDOT:PSS/Ag contact (6.7 ohm cm) and PEDOT:PSS/Au contact (2.4 ohm cm) and indeed observed a similar dependence for the total device characteristics. Hence, the WF of the metal contact in an OTEG impacts the resulting Seebeck coefficient, the device resistance, and thus its electrical power conversion performance. These findings raise two questions: what is the origin of the interfacial effects and what is the true Seebeck coefficient of PEDOT?
Before designing experiments to answer these two fundamental questions, it is important to remind that the humidity affects the thermoelectric properties of PEDOT:PSS. The reason to this is that the thermodiffusion of ions generates an additional contribution to the Seebeck voltage that has time-dependent and diminishing characteristics.10,24 In our measurements, carried out at 40 %RH, the Seebeck coefficient is constant over an extended period of time (Fig. S3.1), thus indicating the absence of an ionic Seebeck effect. Moreover, there is no additional static voltage as indicated by the fact that the intercept of the thermovoltage vs ΔT curve runs through zero (see Fig. S2.2). The voltage contribution from the ionic Seebeck effect in PEDOT:PSS is also known to be negligible at relative humidity levels below 40 %25, and has never been observed in PEDOT:Tos even at elevated humidity levels9. With this in mind, we then decided to focus on PEDOT:Tos exposed to 40 %RH in order to investigate the effect of the metal contact WF. Even though PEDOT:Tos does not display any ionic Seebeck effect, due to its low ionic conductivity, computational microscopy26 supports the presence of water molecules within the film at ambient conditions, which could potentially trigger an interfacial redox reaction.
PEDOT:Tos films were deposited inside a glovebox in an inert and dry atmosphere ([O2]=3.9 ppm, [H2O]=3 ppm). The Seebeck coefficients of PEDOT:Tos/Al, PEDOT:Tos/Au, and PEDOT:Tos/Pt were not dependent on the metal WF, and reach 21±1 μV/K at room temperature, thus matching previously reported values for experiments conducted under the same conditions (Fig. 2a)4. We believe that this value is the true Seebeck coefficient of the PEDOT:Tos films (i.e., without any contribution introduced by the metal contact). When the samples were exposed to ambient atmosphere (40 %RH at room temperature), the Seebeck coefficient increased from 21 μV/K to 50 μV/K for OTEGs with Pt contacts (Fig. 2b). Interestingly, when the samples were re-introduced into the glovebox, the Seebeck coefficient returned back to its original value of 21 μV/K. Next, we identified whether the relative humidity and/or the presence of oxygen were key-parameters to control the Seebeck coefficient. We conducted the thermoelectric measurements in three different atmospheres: dry nitrogen, dry oxygen-nitrogen mixture and humid nitrogen. After transferring the samples to a glovebox filled with dry air (O2+N2, -30 °C dew point), we observed a slight increase in the Seebeck coefficient value reaching 25±3 µV/K (Fig. 2c). One set of samples was then kept in dry nitrogen atmosphere while it was characterized over an extended period of time. These OTEGs displayed a constant Seebeck coefficient over the entire measurement period. Another set of samples was instead exposed to humid nitrogen and underwent an immediate and rapid increase of the Seebeck coefficient, starting at 25 μV/K and finally reaching 43 μV/K after 20 h of exposure. After 20 h, the exposure to humidity was terminated and dry nitrogen was fed into the chamber. We then observed that the Seebeck coefficient slowly reduced towards its original value of 21 μV/K. The power output recorded from this Au/PEDOT:Tos/Au OTEG leg also displays an evolution that strongly depends on the relative humidity level (Fig. S2.3). Consequently, we can then conclude that humidity is a key-parameter to observe the effect of WF on the measured Seebeck coefficient for PEDOT films.
The way S and Rdev depend on the WF of the metal contacts pinpoints that the phenomenon responsible for these measured trends relates to the energetics of the metal-polymer interface. Interfacial dipoles are known to form at the conducting polymer-metal electrode interface, and depend on their WFs15,27-31. The WF of PEDOT:Tos and PEDOT:PSS is typically 4.5± 0.2 eV eV and 5.0 ± 0.1 eV 32,33, respectively. The equalization of the chemical potential of the electrons at the metal-polymer interface leads to a reorganization of the electronic density (integer charge transfer across the interface) that produces an interfacial dipole15. If the Fermi level of the metal is higher [lower] than that of the PEDOT film, PEDOT is oxidized [reduced]. The extension of the interfacial dipole into polymer semiconductor films, measured under vacuum conditions, is typically localized over a few nm.15 However, when a PEDOT film is exposed to ambient air with 40 %RH, ions from the polymer film can potentially diffuse within the polymer bulk as the interchain interactions are dominated by electrostatic and Van der Waals forces. In other words, PEDOT films are mixed ion-electron conductors with an ionic conductivity that increases exponentially with the relative humidity of the environment34. The presence of mobile ions accompanied by the interface charge transfer, driven by electronic chemical potential equilibration between PEDOT and the metal, is expected to form an inhomogeneous oxidation level profile in the PEDOT bulk starting from the interface. This spontaneous reorganization of the electrons and ions due to the Fermi level alignment at the PEDOT-metal interface has not been identified to date. However, it is known that under an external applied bias, generated by two electrodes connecting a PEDOT layer that is also coated with an electrolyte, a potential gradient is created along the PEDOT-electrolyte interface. This then generates an oxidation profile that results in an electrochromic gradient, a phenomenon that can be observed for instance along the channel of an electrochemical transistor35.
Vibrational spectroscopy is known to be sensitive to the oxidation level of PEDOT36-38 and can thus be used to probe the metal-PEDOT interface. To identify the intrinsic vibrational transitions of the polymer, we first measured ATR-FTIR (attenuated total reflectance Fourier transform infrared spectroscopy) of PEDOT:PSS and PEDOT:Tos films on a non-conducting IR-transparent CaF2 substrate (Fig. 3b,c and SI, Section 4). Two important vibrational transitions are the asymmetric C=C stretching mode doublet of the thiophene ring located at 1533 & 1547 cm-1 and its symmetric C=C counterpart at 1415 cm-1.39 Next, infrared reflection absorption spectroscopy (IRAS)40-42 was used to probe changes in those vibration bands induced by a modification in the oxidation level at the metal-PEDOT interface. From the IRAS spectra for PEDOT:PSS and PEDOT:Tos thin films (thickness ~100 nm) deposited on top of Al, Ag, Au and Pt (Fig. S.4:1-4), we focus on the evolution of the frequency of the C=C bonds, see Figs. 3b-d. The asymmetric [symmetric] C=C vibration peak maxima of the thiophene unit are located at 1525 cm-1 [1417 cm-1] on Al, 1530 cm-1 [1415 cm-1] on Ag, 1533 cm-1 [1415 cm-1] on Au, and 1535 cm-1 [1413 cm-1] on Pt. Compared to the pristine PEDOT film, measured on the insulating CaF2 substrate, the symmetric peak shift towards lower wavenumbers indicates that PEDOT chains undergo oxidation on the Pt substrate and reduction on the Al substrate.37 This observation is fully consistent with the expected spontaneous electron transfer occurring between the metal and the PEDOT films. Thus, if the polymer WF is smaller than the metal WF (e.g. platinum), the polymer becomes oxidized at the interface15; while if the polymer WF is larger than the metal WF (e.g. aluminum), the polymer gets reduced at the interface compared to the pristine oxidation state of PEDOT. Since the asymmetric peak position of the Au sample is the same as with the pristine PEDOT:PSS, there is no apparent interfacial redox reaction. However, there is an interfacial redox reaction between PEDOT:Tos and Au, as the asymmetric peak of the pristine PEDOT:Tos is lower than that of PEDOT:Tos/Au, which is likely due to the lower WF of PEDOT:Tos compared to PEDOT:PSS.
In IRAS, the incident infrared light is reflected at grazing incidence by the metal surface so that the absorbed wavelengths correspond to transitions in the vibrational modes of PEDOT at the metal interface, as well as in the bulk of the thin film. Tuning the thickness of the PEDOT film provides us with a probe to study the volumetric extension of this redox phenomenon. When there is no interfacial redox reaction, as identified for the PEDOT:PSS/Au system, the peak positions remain constant while increasing the polymer thickness from 100 to 800 nm, although we do observe a slight trend in the peak ratios of the asymmetric C=C doublet. In contrast, the C=Casym peak of PEDOT:PSS on Al [Ag] varies with the thickness from 1525 cm-1 [1531 cm-1] for a 100 nm thick film to 1531 cm-1 [1533 cm-1] for a 800 nm thick film (Fig 3e-h, Fig. S4.5). This supports the formation of an oxidation level profile within the PEDOT film that extends from the metal-interface into the polymer bulk, reaching up to several hundreds of nanometers of thickness.
Electrochemical doping/dedoping of PEDOT thin films is known to alter its crystalline structure43. We studied changes in morphology, induced by the oxidation level profile triggered by the redox interfacial chemistry, by using grazing incidence wide angle X-ray scattering (GIWAXS). As the metals themselves have a strong scattering contribution, we deposited a very thin layer (10 nm) of Au and Al on Si (with native oxide) followed by the deposition of the PEDOT films on top. 2D GIWAXS scattering patterns (Fig. S5.1) were recorded and a background (Si+metal) subtraction was carried out. This allowed us to decouple the pure PEDOT contribution (Fig. 4 inset), which was also integrated to obtain the 1D scattering patterns. The scattering features observed for both PEDOT:Tos and PEDOT:PSS agree well with those previously reported in literature43,44. Interestingly, a higher degree of order (i.e. a higher integrated (100) peak intensity) is observed for PEDOT in contact with Au as compared to Al, which is consistent with the higher oxidation level of PEDOT:Tos on Au43.
The interfacial redox chemistry found at the interface between the metal contact and the PEDOT film in OTEG legs is promoted by the WF differences of the metal and the polymer, along with the kinetics of ions that are boosted by the presence of humidity. In order to better understand the full impact and opportunity of the WFs in OTEGs, we propose that the interfacial energetics should be taken into the account while characterizing and optimizing OTEGs in the future. We believe that the description of the electric potential profile along the metal-polymer-metal OTEG leg, submitted to a temperature gradient, should include the drop in Seebeck potential within the PEDOT (Sm=dV/dT) film, as well as the two dissimilar interfacial potential drops located at the hot (Vint(TH)) and cold sides (Vint(Tc)). With such a description (Fig. S6.1), the temperature dependence of the interface potential drop Vint(T) = f(T) dictates if the measured Seebeck coefficient is larger or smaller than the true Seebeck coefficient of the polymer, through Smeas= Sm+[Vint(Tc)-Vint(TH)]/dT. Beyond the scope of this article, more efforts should also be devoted to investigate the details of the origin and temperature evolution of those interfacial phenomena in order to reach a quantitative model of the effective Seebeck coefficient and generated power in OTEGs. Note also that the temperature dependence of those interfacial potentials also offer yet another degree of freedom to optimize OTEGs. Recently, dissimilar metallic contacts have suggested a modification of the Seebeck coefficient by surface polarization effects45. In our case, OTEGs based on different metal contacts, as in the case with Pt/PEDOT:PSS/Ag (Smeas= 29.5 μW/K) and Pt/PEDOT:Tos/Ag (Smeas= 52.5 μW/K) devices, exhibit higher effective Seebeck coefficient (see Fig. S7.1) than with same metals (Smeas= 50.5 μV/K).
Finally, we highlight the importance of the interfacial energetics on the OTEG performance, by comparing the remarkably high value of Pmax obtained with Pt contacts with values found in the literature (Fig.5). Since our devices are based on a single element, as a proof of concept for the phenomenon, we expect that even higher power output could be achieved through proper device fabrication (i.e., by optimizing the number and dimensions of the thermoelectric elements). It is evident from Fig.5 that interfacial energetics can severely enhance the output power of OTEGs, we manage to significantly enhance the thermoelectric performance and reach a record-high OTEG output power. Our study thus opens up a new pathway towards the optimization of organic thermoelectric technology.
Moreover, our discovery of a doping gradient within the polymer bulk, extending away from the various metal contacts, is of direct relevance for the field of organic electronics in general, as metal-conducting polymer interface is a fundamental elementin many devices. A better understanding of these polymer-metal interfaces will impact the performance and efficiencies of several devices, from solar cells to supercapacitors and batteries, to organic electrochemical transistors and more.