PEDOT Films for Thermoelectric Applications - ACS

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Manufacturing Te/PEDOT films for thermoelectric applications Mario Culebras, Ana María Igual-Muñoz, Carlos Rodríguez-Fernández, María Isabel Gómez-Gómez, Clara Gomez, and Andres Cantarero ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 30 May 2017 Downloaded from http://pubs.acs.org on June 4, 2017

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ACS Applied Materials & Interfaces

Manufacturing Te/PEDOT films for thermoelectric applications Mario Culebras,† Ana María Igual-Muñoz,‡ Carlos Rodríguez-Fernández,† María Isabel Gómez-Gómez,† Clara Gómez,‡ and Andrés Cantarero∗,† †Molecular Science Institute, University of Valencia, PO Box 22085, 46071 Valencia, Spain ‡Materials Science Institute, University of Valencia, PO Box 22085, 46071 Valencia, Spain E-mail: [email protected] Phone: +34 963544713. Fax: +34 963543633 Abstract In this work, flexible Te films have been synthesized by electrochemical deposition using PEDOT [Poly(3,4-ethylenedioxythiophene)] nano-films as working electrodes. The Te electrodeposition time was varied in order to find the best thermoelectric properties of the Te/PEDOT double layers. In order to show the high quality of the Te films grown on PEDOT, the samples were analyzed by Raman spectroscopy, showing the three Raman active modes of Te: E1 , A1 , and E2 . The X-ray diffraction spectra also confirmed the presence of crystalline Te on top of the PEDOT films. The morphology of the Te/PEDOT films was studied using scanning electron microscopy, showing a homogeneous distribution of Te along the film. Also an atomic force microscope was used to analyze the quality of the Te surface. Finally, the electrical conductivity and the Seebeck coefficient of the Te/PEDOT films were measured as a function of the Te deposition time. The films showed an excellent thermoelectric behavior giving a maximum power factor of about 320±16 µW m−1 K−2 after 2.5 hours of Te electrochemical

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deposition, a larger value that the reported for thin films of Te. Qualitative arguments to explain this behavior are given in the discussion.

Keywords Thermoelectricity, Seebeck effect, PEDOT, tellurium, thin films, electrodeposition

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Introduction

The increasing demand of electricity, product of our standard way of living, together with the global warming due to the Greenhouse effect are generating irreversible changes in the World environmental conditions. The Greenhouse effect is originated mainly during the energy cycle production when the raw products are of fossil origin, releasing CO2 to the atmosphere. Although nowadays the energy production is mainly based on oil and carbon, and in a minor amount, in nuclear power, it is clearly necessary to favor a more sustainable planet by supporting the development of clean energy resources. One path to improve the sustainability of the planet is to recover part of the heat dissipated into the environment as electrical energy. This is exactly which a thermoelectric generator (TEG) is able to do. 1–4 In order to build a competitive TEG, we need materials able to efficiently turn heat into electricity. The efficiency of a thermoelectric material can be measured in terms of the dimensionless figure of merit, ZT : ZT =

S 2σ T κ

,

(1)

being S the Seebeck coefficient, σ the electrical conductivity, κ the thermal conductivity and T the absolute temperature. Depending on the specific application, values of ZT at least higher than one are required. In order to be competitive with solar cells or other renewable energy resources ZT must be actually higher than 2 (ZT → ∞ corresponds to the efficiency

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of a Carnot cycle). High values of S and σ and low values of κ are required to increase ZT . 4 The most efficient thermoelectric materials developed until now are inorganic semiconductors nanostructures and alloys such as: derivatives of bismuth, tellurium and selenium. 5–8 However, serious drawbacks such as high cost, brittleness, toxicity and scarcity of the raw materials, among others, limit their applicability. Organic materials, such as conductive polymers, are a very promising alternative 1,2,9 to semiconductor compounds. Unfortunately, these organic compounds, although cheap, easy to produce and modify, flexible, lightweight, abundant, and with good mechanical properties, are not able to convert heat into electricity with the efficiently provided by inorganic materials. Different strategies had been employed in the past few years to improve and optimize ZT (or the power factor, P F ≡ S 2 σ) in intrinsically conducting polymers (ICPs), but most of the strategies have been, unfortunately, hardly unexplored. One of the methods most often used to optimize the doping level of ICPs is either chemical and electrochemical doping. 10–14 Using these methods, the thermoelectric performance of poly(3,4-ethylenedioxythiophene) (PEDOT) has reached values of ZT in the range 0.20 to 0.40. The treatment with sulphuric acid vapor reduces the Coulomb interaction between the PEDOT and poly(styrensulfonate) (PSS) chains giving rise to a structural rearrangement between the PEDOT and the PSS chains. This fact results in an increase in the power factor of PEDOT:PSS until 17.0 µW m−1 K−2 . 15 In addition, through sequential post-treatments with sulfuric acid and NaOH, the power factor of PEDOT:PPS was increased until 334 µW m−1 K−2 . 16 However, these values are still lower than those corresponding to inorganic materials. The use of nanocomposites and hybrid organic-inorganic materials have been also studied as a promising alternative to improve the thermoelectric properties of ICPs. Very high values of the P F have been obtained for polyaniline (PANI)/double wall carbon nanotube (DWCNT)/Graphene (1825 µW m−1 K−2 ) 17 and PANI/DWCNTs/PEDOT:PSS/Graphene (2710 µW m−1 K−2 ) 18 synthesized by means of Layer by Layer (LBL) assembly. The incorporation of semiconductor nanostructures such as Te nanorods, 19–21 Te-Cu hybrids nanorods 22 or even Bi2 Te3 nanopar-

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ticles 23–25 produces a large increase in the Seebeck coefficient of the polymer, delivering a high P F , although most of the times the thermal conductivity has not been measured and ZT could not be provided. The incorporation of inorganic nanostructures to an ICP matrix is not an easy path, due to the intrinsic incompatibility between the two components. In particular, Te nanostructures are not very stable in solution, for this reason surfactant or stabilizers are needed. Thus, non homogeneous films with a bad morphology are usually obtained and the thermoelectric performance of the final material has been hardly improved. In this work we have followed a different strategy. The goal of this work is to obtain high quality Te/PEDOT thin films through electrochemical deposition, a strategy which is explored for the first time (as far as we know), since commonly a metal instead of an ICP is used as counter-electrode. 26 The electro-reduction of tellurium over a PEDOT film offers an alternative route to fabricate high-quality inorganic/organic materials displaying good thermoelectric performance with a low cost and an easily handle technique. The fact that both components are semiconductors create a synergistic mixture of thermoelectric properties with a high thermoelectric efficiency. Tellurium films have been grown in by several methods, although never with the purpose of analyzing its capability as a thermoelectric material. By means of hot wall epitaxy, 27 Te films have been obtained on glass and mica, they are polycrystalline with the shape of needles, and at high temperature they have a dentritic growth. It has also been grown by thermal evaporation 28 on glass, and the electrical properties have been measured as a function of the annealing temperature. Recently, it has been grown on Bi2 Te3 , 29 to inspect the predicted topological phase in stress Te. Finally, it has also been grown by electrodeposition, 26 trying to obtain a homogeneous film in the presence of Cd+2 . The homogeneity found in our films is comparable to that of Ref. 29 It is worth to mention the recent discovery of the underwater mountain giving rise to the Canary Islands of Spain. It contains an estimated amount of 2,670 Tons of tellurium. This was an important discovery because Te was one of the expensive elements in the development

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of solar panels. 30

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Results and discussion

Figure 1 shows the TEM images of the cross section of Te/PEDOT films as function of the Te electrodeposition time. The thickness of the Te layer increases as a function of the electrodeposition time due to the accumulation of Te in the PEDOT electrode. Cyclic voltamperometry (CV) was used to optimize and study the deposition of Te over the PEDOT film as a working electrode. The starting solution was formed by TeO2 dissolved 31 in a sulphuric acid solution to form telluric acid (HTeO+ as described in the experimental 2 ),

part. Figure 1(a) shows the corresponding cyclic voltamperogram. The main reduction peak appears at −0.4 V. This cathodic peak can be associated with the reduction of HTeO+ 2 to Te, deposited over the working electrode, according with the following main reaction: 31,32

+ − HTeO+ 2 + 3H + 4e −→ Te(s) + 2H2 O

(2)

Other reduction processes can take place during the Te deposition, for instance the hydrogen production: 2H+ + 2e− −→ H2

(3)

or even other reactions involving Te, occurring at the same time than the Te deposition. For instance: 31,32 + − HTeO+ 2 + 5H + 6e −→ H2 Te + 2H2 O

(4)

Te(s) + 2H+ + 2e− −→ H2 Te

(5)

Figure 1(b) shows the evolution of the current on the working electrode (PEDOT film) during the Te deposition, taking place at −0.4 V with respect to the Ag/AgCl reference electrode. A decrease of the intensity is observed below 60 min of Te electrodeposition, due to the increase of the Te film on the PEDOT substrate. After 60 min, the working 5



electrode saturates and the electrical resistance decreases (probably due to recrystallization), the intensity slightly increases while keeping the voltage of −0.4 V between the electrodes. The occurrence of parallel reactions can be the reason of the efficient growing of Te. 31 Figure 1(c) plots the Te/PEDOT film thickness as a function of Te electrodeposition time. The error bars have been calculated from an average of the height at different regions of the sample. The measurements were performed in different samples. The progressive increment in the thickness indicates the regular deposition of Te on the PEDOT electrode. The thickness data are in agreement with the TEM cross section images observed in Fig. 1(d). (a)

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Figure 1: (a) Cyclic voltamperometry of HTeO+ 2 using a PEDOT film as a working electrode. (b) Galvanometric curve during the Te deposition in an ice bath. (c) Thickness of the Te/PEDOT films as a function of the Te electrodeposition time. (d) TEM images of the cross section of the Te/PEDOT interface for several deposition times.

Figure 2 shows the morphology of the films as a function of the Te electrodeposition time. All the films were homogeneous with different roughness degrees, as detected by SEM. At low deposition times (30 min), it is possible to observe small Te crystals on the PEDOT surface (Fig. 2(a)), the crystals were distributed homogeneously over the PEDOT film. However, 6


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the degree of interconnection between the Te crystals was really low. Figure 2(b) shows the surface of the PEDOT films after 60 min of polymerization, showing how the film surface is practically covered by Te nanoparticles. After 90 min of Te synthesis (Fig. 2(c)), the amount of Te increases, creating a more compact morphology, which improves the contact among Te nanocrystals. Figures 2(d), (e) and (f) show the film surface after 120, 150 and 180 min of Te synthesis, respectively. The SEM images show a Te film homogeneously distributed with a globular morphology typical of electrodeposited materials. 29

Figure 2: SEM images of Te/PEDOT films after: (a) 30, (b) 60, (c) 90, (d) 120, (e) 150 and (f) 180 min of Te electrodeposition. Figure 3 shows the surface topography of a PEDOT film and a Te/PEDOT film synthesized during 2.5 hours of Te deposition, measured using an AFM (NT-MDT Spectrum 7



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Instruments). In the case of the PEDOT surface (see Figure 3(b)), the topology shows a globular morphology, typical of electropolymerized PEDOT, whith a roughness variation around ±30 nm. Figure 3(b) shows the topography of a film composed by Te nanocrystals with certain roughness degree. It is possible to observe domains where the Te crystals are very close between them producing variations in the z-direction smaller than ±20 nm. However, narrow valleys appear, corresponding to Te nanoparticle boundaries in some regions of the sample. These results indicate that the Te deposited does not produce a flat film. As the amount of Te deposited increases, the Te nanocrystals are connecting with each other creating a film with small roughness. The morphology obtained is the typical one for Te synthesized by electrodeposition. 29,32 The AFM results are in agreement with the surface morphology observed in the SEM images (Fig. 2(e)).

Figure 3: Roughness profile corresponding with: (a) PEDOT film and (b) TE/PEDOT film synthesized during 2.5 hours of Te deposition. The insets are the 3D views of AFM image. Figure 4 shows the Raman spectra of the Te/PEDOT films. In all the samples we observe three peaks, located at 88, 117 and 137 cm−1 (±0.2 cm−1 , see Sec. 4), corresponding to the E1 , A1 , and E2 optical modes of Te, respectively. 33 In addition, the vibrational modes of PEDOT are observed until 150 min of Te electrodeposition time. After 150 min, the Te film is thick enough to mask the PEDOT Raman signal. The Raman signal coming from PEDOT can be observed in the high energy region (1300-1700 cm−1 ). The peak located at 1365 cm−1 is related to the Cβ -Cβ stretching; the symmetric stretching mode Cα =Cβ (-O) 8


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appears at 1436 cm−1 , and the asymmetric stretching of C=C split, as it is well known, into two Raman peaks at 1512 and 1565 cm−1 , respectively. The vibrational mode at 1615 cm−1 is attributed to the PET substrate.

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90 min 120 min

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80 120 160 200 240 280

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Raman shift (cm )

Figure 4: Room-temperature Raman-scattering spectra of Te/PEDOT films obtained with the 647.1 nm laser excitation of an Ar-Kr laser.

Figure 5 shows the XRD analysis of Te/PEDOT films from 2θ = 20◦ to 60◦ at different Te electrodeposition times. All the spectra present a peak centered at 2θ=25.8◦ . This peak corresponds to the crystalline domains of the PET substrate. This diffraction peak is attributed to the reflection corresponding to the (100) plane. 34,35 The signal of the PET substrate overcome the PEDOT peaks. Several Te diffraction peaks are also observed in Fig. 5. These peaks correspond to the diffraction of the (101), (102) and (003) planes, centered at 2θ=27.5◦ , 38.2◦ and 45.8◦ , respectively. The intensity of the Te peaks increases with electrochemical deposition time, since the Te content increases as well. The XRD results show a preferred orientation in the [003]-direction during crystal growth. The crystal size,

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Figure 5: XRD analysis of Te/PEDOT films as a function of Te deposition time. D, in this orientation was determined using the Scherrer equation:

D=

Kλ β cos θ

,

(6)

where λ is the X-ray wavelength, β the full-width at half-maximum in radians and θ the Bragg angle. The samples synthesized at 180, 150 and 120 min had a crystal size of 26±3 nm, 27±3 nm and 31±3 nm, respectively. These values are in agreement with the literature. 32,34,35 The electrical conductivity σ has been determined as a function of the Te deposition time. In Fig. 6, the conductivity of the Te/PEDOT double-layer is shown as a function of the deposition time (solid dots, the line is a guide to the eye). It decreases with the Te content from 750±70 S/cm to 59±5 S/cm at 180 min. One of the reasons of this decrease is the fact that the Te deposited using electrochemical methods is polycrystalline, creating

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a film with the appearance of a porous material. The PEDOT conductivity is higher than that of Te by itself due to its semi-metallic character. 36 This behavior comes from the high doping level found in the PEDOT films grown by electrochemical polymerization at oxidative potentials. 10,13 But the huge drop in the electrical conductivity is not only produced by the presence of Te nanocrystals, but also by the decrease of the doping level occurred in the PEDOT during the electrodeposition of Te, since the applied potential during the Te synthesis was −0.4 V. This negative potential produces an electrochemical de-doping of PEDOT, generated by the reduction process taking place in the working electrode. 13

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Figure 6: Electrical conductivity (σ), Seebeck coefficient (S) and Power Factor (P F ) of Te/PEDOT films as a function of Te deposition time. The Seebeck coefficient has been measured at room temperature as a function of the Te deposition time (see Figure 6, right scale -blue online-). The Seebeck coefficient increases a factor of ∼ 25, from 9±1 µV/K to 230±20 µV/K, compensating the decrease of the electrical conductivity. At lower deposition times (30 min), the Seebeck coefficient is closer to the value of the reduced PEDOT, 35-40 µV/K. 10,13,37 This indicates that the Te crystals deposited on the PEDOT film are not well interconnected inhibiting the increase of the Seebeck coefficient, as expected due to the presence of a semiconductor layer. For this 11



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reason, at low electrodeposition times (below 60 min) the Seebeck coefficient is related to the reduced state of PEDOT. 10,13,37 As the deposition time increases, the properties of the Te film dominate over that of PEDOT and a large increase of S is observed. At longer depositions times, the Seebeck coefficient increases until values closer to 230 µV/K. These values are in the same range than that achieved for Te films synthesized by electrochemical polymerization by B. Abad et al. 32 (around 250 ± 40 µV/K), although the values obtained in Ref. 32 were lower, probably due to the presence of PEDOT in the final film. The large increase observed in the Seebeck coefficient at longer deposition times is originated from the good interconnection of the Te nanocrystals. The Te crystallizes producing continuous and homogeneous films as it has been shown by SEM (Fig. 2) and AFM (Fig. 3), providing very good values for the Seebeck coefficient. The power factor of the Te/PEDOT films has been calculated from the values of the electrical conductivity and the Seebeck coefficient (Fig. 6, right scale -red online-). P F increases from 6.5±1.5 µW m−1 K−2 to 308 µW m−1 K−2 with the Te electrodeposition time. This increment in the P F is mainly produced by the increase of the Seebeck coefficient (it goes to the square in the expression of P F ). The optimum Te deposition time was 150 min, giving a P F around 320±16 µW m−1 K−2 . To our knowledge, this is the first time that Te films are synthesized on PEDOT and we cannot compared directly with other works in the literature. However, the Te-PEDOT mixture has been used in the past to increase the P F of PEDOT, achieving a significant improvement, 19,21,22 although below the values reported here. Our results are very promising, being higher than Te films electrochemically deposited (285 µW m−1 K−2 ) 32 or Te films fabricated by a vacuum condensation method (100 µW m−1 K−2 ). 38 However the power factor of Te/PEDOT films continue still being low compared with the highest power factor reported for electrodeposited Bi2 Te3 films (1473 µW m−1 K−2 ), due to their high electrical conductivity (691 S/cm). 39 There are only a few works about the thermoelectric properties of pure PEDOT films with a power factor higher than 320 µW/m K2 . For instance: PEDOT:Tos/PEO-PPO-PEO

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obtained by electrochemical reduction (1270 µW/m K2 ), ? PEDOT:PSS + EG treatment (469 µW/m K2 ), 15 or PEDOT:PSS treated with H2 SO4 and NaOH (334 µW/m K2 ). 16 However, to achieve these values the PEDOT films have been subjected to further chemical treatments not suitable from the point of view of industrial production. In our case, just using raw PEDOT as a working electrode during the electrodeposition of Te, it is possible to obtain very high values of P F without further steps in the synthesis process that can be very interesting from the point of view of industrial manufacture process. Summarizing, in this work we have synthesized Te/PEDOT films with a high P F . However, further research is needed to increase the electrical conductivity keeping a high value of the Seebeck coefficient in order to overcome the reference thermoelectric material, Bi2 Te3 . This work indicates the good synergy between PEDOT and Te.

3

Conclusions

In the present work, flexible Te/PEDOT films have been synthesized by a very simple method, electrochemical deposition. Te has been deposited over PEDOT as working electrodes, and characterized by XRD, Raman, SEM and AFM. The most important results are that, although the electrical conductivity decreases as a function of the Te electrodeposition time, the Seebeck coefficient increases in a similar factor, producing an increase in the P F . The maximum power factor was achieved at 150 min of Te electrodeposition time, a P F = 320 ± 16 µW m−1 K−2 , which is an excellent value in comparison with similar structures found in the literature.

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4 4.1

Experimental Materials

The reactants used in this study are: 3,4-ethylenedioxythiophene (EDOT) 97% purity (Munich, Germany), Lithium Perchlorate (LiClO4 ) 95% purity (Alfa Aesar, Karlsruhe, Germany), tellurium (IV) oxide 99% purity (Across Organics, Geel, Belgium), acetonitrile (reagent grade) and sulphuric acid (95%) (VWR Chemicals, Llinars del Vallés, Spain).

4.2

Synthesis of Te/PEDOT films

The preparation of the Te/PEDOT films was performed in a three electrode cell (Ivium Technologies) by means of electrochemical polymerization. First of all, the films of PEDOT:ClO4 were synthesized on a polyethylene terephthalate (PET) substrate (2 × 4 cm2 ) with a gold coating (20 nm) obtained by metal evaporation in a Univex 300 Evaporation system. The gold layer served as working electrode. PEDOT:ClO4 was polymerized starting from a 0.01 M solution of EDOT and LiClO4 0.1 M, in 100 ml of acetonitrile at a current of 3 mA versus a Ag/AgCl reference electrode for 30 s. PEDOT coated gold PET was washed with acetoni-

Figure 7: TEM images of the cross section Te/PEDOT films Scheme of the Te deposition over the PEDOT working electrode. trile to remove the monomer and oligomeric species from the surface. Afterwards, the gold layer was removed with an acid solution (HNO3 :HCl ratio 1:3). The PEDOT derivative thin films were washed several times with water and finally with ethanol (with this treatment the 14


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gold layer was removed keeping the polymer integrity and its properties 13 ). Te deposited films were obtained by electrochemical deposition on the PEDOT surface using this film as the working electrode (see Figure 7). Tellurium deposition was carried out at −0.4 V from a 0.01 M TeO2 and 0.1 M H2 SO4 solution in 100 ml of ultra pure water in an ice bath. All the films obtained were washed several times with water and finally with ethanol. In all the experiments, a steel electrode was used as the counter electrode and an IVIUM n-stat equipment was employed as a potentiostat.

4.3

Characterization

The van der Pauw method 40 has been used to determine the electrical conductivity of the samples. The electrical conductivity has been obtained from two four-point (A, B, C and D) resistance measurements. For the first resistance measurement, a current IAC is driven through the two contacts A and B and the potential difference VBD is measured between the other two contacts, B and D, giving R1 =VBD /IAC . The second resistance, R2 =VAB /ICD , is obtained by driving a current from C to D and measuring the voltage between A and B. A Keithley 2400 power source was used both as driving source and voltmeter. The conductivity of the sample, σ, can be obtained by solving the van der Pauw equation:

e−πdR1 σ + e−πdR2 σ = 1 ,

(7)

where d is the sample thickness. A homemade setup has been used to measure the Seebeck coefficient. For the Seebeck measurement rectangular samples were prepared with two contacts at the narrow ends of the sample. Temperature differences are generated between the two end points of the sample and the associated potential difference has been measured. A Lakeshore 340 temperature controller has been employed to control the temperature and a Keithley 2750 Multimeter/Switching System to record the potential difference. Two 100 Ω platinum resistors were

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used as temperature sensors. The Seebeck coefficient, S, is determined as the ratio between the electrical potential, ∆V , and the temperature difference, ∆T :

S=

∆V ∆T

.

(8)

The values of σ have the apparatus sensitivity (negligible), while in the case of the Seebeck coefficient we wait until the temperature fluctuates below 0.1 K. With this temperature uncertainty and the fluctuation of the voltage we have calculated the error in the Seebeck coefficient using error propagation. Finally, the power factor error has been also calculated taking into account the uncertainties of σ and S. The thickness of the films were measured by using a Veeco Dektak stylus profilometer. Since the final samples were very smooth, the uncertainty in the thickness is not enough to give an appreciable error bar in Fig. 6. The final error bars shown in Fig. 6 are an average between the measurements taken in three different samples. The X-ray diffraction analysis was carried out using a Bruker AXS D5005 diffractometer. The samples were scanned at 4◦ min−1 using Cu Kα radiation (λ=0.15418 nm) at a filament voltage of 40 kV and a current of 20 mA. The diffraction scans were collected within the range 20◦ ≤ 2θ ≤ 60◦ with a 2θ step of 0.01◦ . The Raman scattering measurements were carried out at room temperature in backscattering configuration using a Jobin Yvon T64000 spectrometer equipped with a liquid-nitrogen cooled open electrode charge-coupled device (OECCD). The excitation line of 647.1 nm was provided by an Ar/Kr laser (Innova 70/Coherent) focused onto the sample using a 100× microscope objective with a numerical aperture N A = 0.90 (Olympus). This setup focuses the light on an area around 1µm on top of the sample. To avoid the sample heating during the experiment, special care was taken, limiting the power down to a few µW. All measurements were calibrated with a Si sample by its characteristic phonon peak at 519.5 cm−1 . 41 After measuring the Si phonon we kept the spectrometer in the same position in order to avoid

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inaccuracies in the calibration, thus our experimental values have a maximum uncertainty of 0.2 cm−1 . Scanning electron microscopy (SEM) images were carried out in a Hitachi S-4800 microscope at an accelerating voltage of 10 kV and a working distance of 14 mm. Small pieces of sample were placed in the sample holder (2 inches). The samples were metallized with an Au-Pd coating before observation. The samples for transmission electron microscopy (TEM) were prepared by embedding a small piece of coated PET in DurcupanT M ACM resin (Sigma Aldrich, Munich, Germany), curing overnight and then cutting cross sections using an Ultra 45◦ diamond knife (Diatome, Hatfield, PA). Samples were imaged on copper grids using a JEOL JEM-1010, coupled with a digital camera MegaView III at 100 kV. The thicknesses of the films were measured after the Te deposition in a DEKTAK 150 profilometer. The surface topography was evaluated using atomic force microscopy (AFM) from an NT-MDT AFM-SPM in semicontact mode using a cantilever NSG10/TiN tip (NTMDT). The AFM height images of each sample were captured in ambient air keeping the humidity bellow the 40% and the temperature at 21◦ C. The Nova PX imaging software was used to analyse the resulting images.

Acknowledgement The authors acknowledge financial support from the Dirección General de Investigación Científica y Técnica through grant CSD2010-0044 of the Programme Consolider Ingenio and grant MAT2016-63955-R. MC would like to acknowledge the Ministry of Education for financial support through the FPU training Programme.

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PEDOT Films for Thermoelectric Applications - ACS

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