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In Situ Complementary Doping, Thermoelectric Improvements and Strain-Induced Structure within Alternating PEDOT:PSS/PANI Layers Virgil Andrei, Kevin Bethke, Fani Madzharova, Aafke Cecile Bronneberg, Janina Kneipp, and Klaus Rademann ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10106 • Publication Date (Web): 05 Sep 2017 Downloaded from http://pubs.acs.org on September 5, 2017

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ACS Applied Materials & Interfaces

In Situ Complementary Doping, Thermoelectric Improvements and Strain-Induced Structure within Alternating PEDOT:PSS/PANI Layers

Virgil Andrei,

†, ¶

Kevin Bethke,

†

Kneipp,

Fani Madzharova,

†

†

Aafke C. Bronneberg,

and Klaus Rademann

‡

Janina

∗, †

†Department of Chemistry, Humboldt-Universität zu Berlin, Brook-Taylor-Straÿe 2, 12489

Berlin, Germany. ‡Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Institute for Solar Fuels,

Hahn-Meitner-Platz 1, 14109 Berlin, Germany ¶Current address: Department of Chemistry, University of Cambridge, Lenseld Road,

Cambridge CB2 1EW, UK. E-mail: [email protected] Phone: +49 30/2093-5565. Fax: +49 30/2093-5559

1

ACS Paragon Plus Environment


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Abstract Although the deposition of alternating layers from poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) and polyaniline (PANI) salts has recently provided a breakthrough in the eld of conductive polymers, the cause for the conductivity improvement has remained unclear. In this work, we report a cooperative doping eect between alternating PANI base and PEDOT:PSS layers, resulting in electrical conductivities of 50-100 S cm−1 and power factors of up to 3.0±0.5 µW m−1 K−2 , which surpass some of the recent values obtained for protonated PANI/PEDOT:PSS multi-layers by a factor of 20. In this case, the simultaneous improvement in the electrical conductivity of both types of layers is caused by the in situ protonation of PANI, which corresponds to the removal of the excess acidic PSS chains from the PEDOT:PSS grains. The interplay between the functional groups' reactivity and the supramolecular chain reorganization leads to an array of preparation-dependent phenomena, including a step-wise increase in the lm thickness, an alternation in the electrical conductivity and the formation of a diverse surface landscape. The latter eect can be traced to a build-up of strain within the layers, which results in either the formation of folds or the shrinkage of the lm. These results open new paths for designing nanostructured thin-lm thermoelectrics.

Keywords thermoelectric polymers; PEDOT:PSS/PANI composites; alternating layers; surface topology; strain

1 Introduction Transparent conductive polymers are a fundamental part of the next generation of renewable energy sources,

1

such as exible solar cells,

25

and wearable thermoelectrics.

gated chains are intrinsically semi-conductive, and electrochemical

14

1113

610

While conju-

several strategies, including chemical

doping, solvent-driven chain reorganization,

2


1518

6,12

and the choice of

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counteranion,

13,19,20

have increased the electrical conductivity by more than several orders of

magnitude, reaching 0.007-4600 S cm (PEDOT:PSS),

13,15,16,18,21

10

−7

−1

-300 S cm

for poly(3-alkylthiophene) derivates. posites

7,29

for poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)

−1

2325

for polyaniline (PANI),

Although carbon-

7,2628

19,22

and 10

−4

-1000 S cm

−1

and inorganic-based com-

have enabled further improvements, few reports have focused on combining dier-

ent polymers. Concerning this aspect, 2016 oered outstanding breakthroughs, with several reports describing nearly simultaneously the exciting properties of PEDOT:PSS/PANI salt multilayered composites.

3032

For example, repeatedly deposited quadlayers of PANI:HCl/graphene-PEDOT:PSS/PANI: HCl/PEDOT:PSS-double-walled carbon nanotubes (DWNTs) reached a high Seebeck coefcient (S) of 120 µV K

−1

and a remarkable electrical conductivity ( σ ) of 1885 S cm

sulting in power factors, PF

= σS 2 ,

in excess of 2710 µW m

−1

K

−1

, re-

−2 31 . These values cur-

rently hold the record for the thermoelectric performance of organic materials,

31

indicating

a massive potential for further development from alternating PEDOT:PSS/PANI layers. Assuming a thermal conductivity ( κ) between 0.424.6 W m merit ZT

= (σS 2 /κ)T

34,3638

K

−1

, the resulting gure of

of 0.03-2 would make the reported multilayered lms highly compet-

itive with more conventional compounds, and oxides.

−1

31

including chalcogenides,

6,3335

skutterudites,

34

In this case, the orientating eect of the carbon nanotubes on the PANI

chains seems essential to improve the performance. While PANI:HCl/DWNT-PEDOT:PSS bilayers presented a similarly impressive power factor of approximately 1000 the PANI:HCl/graphene-PEDOT:PSS bilayers could only reach 0.14 0.45 S cm

−1

after 80 deposition cycles.

31

µW m−1 K−2 ,

µW m−1 K−2

with

A similar enhancement of up to 1585 S cm

−1

σ

=

has

also been reported for ve PEDOT:PSS/PANI:CSA bilayers, where the PANI salt was synthesized using camphorsulfonic acid (CSA).

32

In this case, a physical explanation for the

improvement has been proposed based on chain stretching and band alignment; this would favor an enhanced

σ

while maintaining an optimal S.

The discrepancy between the last two cases is notable because the only dierences are

3



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the use of graphene and the acid employed for the PANI salt, namely, hydrochloric acid or CSA. This raises the question as to whether improvement can come from a dierent source. While it has been demonstrated that a signicant conductivity enhancement can be obtained even for single-component lms by spin-coating several layers on top of each other, underlying chemistry can also play an important role.

41

18,39,40

the

On the one hand, the conductivity

of PEDOT:PSS lms is aected by a high amount of PSS counteranions, which are employed in excess as a surfactant to obtain aqueous dispersions. On the other hand, the conductivity of PANI is commonly increased by using organic acids with large anions, often resulting in insoluble salts.

19

In other words, while the electrical conductivity of the soluble basic form of PANI is enhanced in the presence of sulfonic acids,

19

poly(styrenesulfonate) chains are found in ex-

cess in the PEDOT:PSS micelles. This piece of knowledge opens the door for the rational design of highly conductive lms using in situ complementary doping between PANI and PEDOT:PSS. Although the cooperative doping was suggested in 2011, ductivity of the respective bilayers only amounted to 0.003-0.7 S cm

42

the electrical con-

−1 42 . Although further

results (summarized in Table S2 of the Supporting Information) have revealed improvements of up to 232 S cm

−1

, the thermoelectric investigations did not previously expand beyond a

single PANI/PEDOT:PSS bilayer.

4246

Other than thermoelectrics, only a single example of

multiple PEDOT:PSS/PANI:HCl layers on cellulose nanobers has been reported for supercapacitor applications.

47

Since all earlier works have either focused on PEDOT:PSS/PANI

bilayers or on protonated PANI/PEDOT:PSS multilayers, no proof of the potentially favorable interactions between several PANI base and PEDOT:PSS layers has been previously reported.

In this work, we demonstrate that the cooperative doping can signicantly in-

crease the electrical conductivity of multiple PEDOT:PSS/PANI alternating layers, even without pre-protonating the PANI species. The exciting surface landscape of the obtained lms is also characterized in detail, to provide the most accurate explanation to date for the reported improvements. Accordingly, our work contributes towards explaining the large

4


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dierences in the previous data and the observed morphological changes from a molecular point of view, which may enable further progress in the rational design of supramolecular polymer interactions for energy applications.

2 Experimental Section 2.1

Sample preparation

The briey described experimental procedures are based on our previous work from Ref. 18. First, 200 mg of PEDOT:PSS from Sigma-Aldrich/Agfa-Gevaert and 19.8 mL distilled water were stirred for 48 h at room temperature to obtain a homogeneous 1 wt.% suspension. PANI (33 mg) from Sigma-Aldrich and 3.0 mL dimethyl sulfoxide (DMSO) were also stirred for 48 h at room temperature, resulting in a saturated solution with partially undissolved PANI. The latter solution was decanted, and only the liquid part was employed. The detailed specications of these chemicals are presented in Table S1 of the Supporting Information. The multilayered lms were spin-coated using the polar 1 wt.% PEDOT:PSS aqueous suspension, noted as PP, and alternately the non-polar saturated solution of the PANI base in DMSO, abbreviated as PB. Both solutions were spin-coated onto

∼25×25 mm2

glass slides,

which were cleaned for 20 min in a Piranha solution. Since the polarity of the two polymers diers before they interact with each other, no spontaneous spreading of the solutions occurred along the already deposited layers. Instead, a manual pre-spreading of the solutions using a glass rod was required before starting the spin-coating, as seen from the Supporting Video. A Chemat Technology KW-4A Spin-Coater was employed and operated in two steps: at 500 RPM for 15 s and then at 2000 RPM for 60 s. To gain a detailed image of the resulting chain interactions, the alternating layers were deposited by either a so-called "dry" or "wet" spin-coating procedure. The eect of either PANI or PEDOT:PSS as the rst bottom layer was investigated. For the "dry" procedure, all intermediate lms were annealed in an oven at 403 K for 5 min after depositing each layer.

5



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In contrast, the "wet" spin-coating occurred directly on the freshly coated moist lms, which were held together by the interactions between the complementary polymers. For the latter method, the alternating layers were quickly deposited one after another, and the lms were only dried after the last step. The dry lms were coated with two stripes of conductive silver paste, to ensure a good electrical contact.

2.2

Characterization

All thermoelectric properties were determined at room temperature. To calculate the Seebeck coecient, a homemade heating plate varied the temperature stepwise along the sample within approximately age.

±10 K, while a Fluke 289 multimeter recorded the corresponding volt-

A VersaSTAT3 current source was employed to measure the quasi-4-point electrical

resistance of the samples. The physical properties of the lms were determined by atomic force microscopy (AFM), UV-vis, Raman and X-ray photoelectron spectroscopy (XPS) measurements. Except where explicitly noted, the data refer to homogeneous macroscopically smooth regions with no visible PANI:PSS precipitates, which account for the vast majority of the total lm area (see Figs. S2-S5). The thickness of the samples and their surface topology were investigated using a Nanosurf Mobile S atomic force microscope. The thickness was averaged from the surface proles of three scratches, which were made using a common needle. An example is shown in Fig. S2 for a PANI/PEDOT:PSS "dry" bilayer. The position of the scratches is indicated by the black markings in Figs. S2-S5. The UV-vis spectra were measured using a Jasco V-670 UV-vis spectrometer, while the Raman spectra were recorded employing a Raman microscope. The excitation wavelength was 633 nm; the average power at the sample was 8 mW, corresponding to an intensity of

9 −2 1.5×10 W m ; and the accumulation time was 10 s. The excitation light was focused on the surface of the thin lm samples by a 10 × microscope objective (NA=0.3). The diameter of

6


r

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the focal spot in xy-directions was calculated to be 2.6 focal volume in the z-direction was approximately 12

µm, while the height of the cylindrical

µm.

The recorded Raman spectra were

frequency calibrated using a 50:50 vol. acetonitrile to toluene mixture.

2 The XPS analysis of smaller ( ∼12×12 mm ) samples was carried out using a SPECS PHOIBOS 100 hemispherical XPS analyzer and a monochromatic X-ray source (SPECS FOCUS 500 monochromator, Al K α radiation, 1486.74 eV). Survey and ne spectra were collected at a normal angle from the surface. The pass energy was set to 30 eV and 10 eV for survey and ne spectra, respectively, with step sizes of 0.5 eV and 0.05 eV. The photoemission lines in the ne spectra were tted with Voigt proles using a Shirley background subtraction. The area ratio of the 2p 1/2 and 2p3/2 was xed to 2. All spectra were calibrated using the C 1s peak at 284.8 eV as a reference.

3 Results and Discussion Fig. 1a-d reveals the thermoelectric results for the lms where PANI is the rst layer in direct contact with the glass substrate. The blue points denote the "dry" lms, while the red circles correspond to the "wet" ones. Even though the Seebeck coecient of non-doped PANI is aected by its very low electrical conductivity of 5.83 ×10 bilizes at approximately 12

µV K−1

is comparable to the reported 9-13 and to the 2-17 µV K

−1

−6

±1.59×10−6 S cm−1 , the value sta-

for the subsequent layers, as shown in Fig. 1a. The value

µV K−1

of PEDOT:PSS products from Agfa-Gevaert

found for various PANI salts.

6,46,4851

13,18

Taking into account the sam-

ple inhomogeneity (Figs. S2-S5), the variation of the recorded data points is also maintained within common experimental error,

18

ranging from 9 to 15

are not unusual for conjugated polymers,

6,18

µV K−1 .

Although these values

thermoelectric applications often require a fur-

ther improvement in the Seebeck coecient. This improvement may be obtained by adding inorganic and organic components such as silicon nanoparticles, and tellurium nanorods,

7,29

52

carbon nanotubes

or by varying the amount of charge doping on the chains.

7


7,30,31

6,12,14


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The similarity of the Seebeck coecient with the one found for pristine PEDOT:PSS is not unexpected since the average height also increases from h

≈ 18.9 nm

for a PANI single layer

to approximately 117 nm for a PANI/PEDOT:PSS (PB/PP) bilayer, i.e., PEDOT:PSS is the major component. This also translates to a layer, which increases to 1.48 ±0.21 S cm

−1

σ

of 0.69±0.15 S cm

−1

for the "dry" PB/PP bi-

for the corresponding "wet" lm. These values are

already higher than the conductivity of pristine PEDOT:PSS, 0.46 ±0.11 S cm

−1 18 , indicating

a favorable interaction between the two types of polymers. A further improvement of the electrical conductivity occurs after the third step, namely, the spin-coating of PANI; whereby,

σ

reaches 63.1±15.9 S cm

the "wet" layers.

−1

for the "dry" lms, and a maximum of 103.4 ±15.5 S cm

−1

for

Generally, good wetting results in a favorable performance, which can

be explained through the washing of PEDOT:PSS by the DMSO solvent from the PANI solution. Concerning the polymer grain size, the AFM images in Fig. S2 and Fig. S4 reveal an inplane diameter ( ∅xy ) between 50-150 nm and a surface root mean square (RMS) of 3.26 nm for the PANI grains. In the case of the PEDOT:PSS micelles, the two parameters amount to

∼ 40 nm

and 1.49 nm.

The thickness of the PEDOT:PSS and PANI layers and their

grain size suggest a near monolayer coverage of the soft PANI colloids, while the smaller PEDOT:PSS grains are spread more evenly to form a smoother layer. The higher solubility of the PEDOT:PSS grains in their respective solvent also explains the larger thickness of the PEDOT:PSS layer. This dierence in height between the two types of polymer layers produces a stepwise evolution of the total lm thickness, which then results in alternating values of

σ.

The alternation is also reected in the values of the power factor, which reveal

a maximum of approximately 1

µW m−1 K−2

after three layers. Both eects are indicated by

the green dotted lines in Fig. 1b-d and Fig. 1f-h. Similar results can also be observed in Fig. 1e-h, where PEDOT:PSS is rst spin-coated. In this case, no data could be recorded for the "wet" samples beyond the PP/PB bilayer, since the subsequent PP deposition leads to a shrinkage of the entire lm, which is illus-

8


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Figure 1: Thermoelectric properties of alternating PANI base (PB) and PEDOT:PSS (PP) layers at room temperature: (a, e) Seebeck coecient, (b, f ) lm thickness, (c, g) electrical conductivity, and (d, h) power factor. The rst deposited layer consists of PANI in frames ad and of PEDOT:PSS in frames e-h. The green dotted lines indicate the stepwise alternating trends in the values. Because of lm shrinkage, no data are available beyond the PP/PB "wet" bilayer in frames e-h.

trated in Fig. 2k-o and described in detail below. Again, the stepwise increase in the lm thickness produces alternating values of the electrical conductivity, between approximately 100 S cm

−1

and 50 S cm

−1

. The thin PANI layers are particularly benecial for the

σ

value,

since they decrease the resistance by the same amount as PEDOT:PSS, while maintaining a relatively similar lm thickness. Accordingly, the samples with an even number of layers present the highest power factors, as observed in the cases of the "dry" and "wet" PP/PB bilayers and the "dry" PP/PB/PP/PB quadlayer. In particular, the power factor amounts to 3.0±0.5 µW m

−1

K

−2

for the "wet" PP/PB bilayer, which is at least 20 times higher than

the reported values of up to 0.14 bilayers.

31

µW m−1 K−2 for PANI:HCl/graphene-PEDOT:PSS multiple

Moreover, this value greatly surpasses the power factors of pristine PEDOT:PSS

lms, which range between 8-30 nW m compares favorably even with the

−1

K

−2

for correspondingly multilayered samples,

∼1 µW m−1 K−2

18

and

of solvent reorganized PEDOT:PSS.

9


18


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Although other reports achieve electrical conductivities approaching 1000 S cm DOT:PSS,

6,16

Page 10 of 33

−1

for PE-

they employ the highly conductive PH1000 product from Heraeus Clevios,

whereas we used the Orgacon Dry PEDOT:PSS from Agfa-Gevaert. Since the thermoelectric performance has been commonly observed to vary depending on the vendor and product,

6,16,17

further improvements may be achieved just by choosing more conductive products.

Nevertheless, our current results compare favorably with previous reports of Agfa-Gevaert products,

13,18

which highlights complementary doping as a promising strategy for enhancing

the thermoelectric properties of multilayered composites. Beyond the

σ

improvement, the spin-coating of the PANI base on top of PEDOT:PSS

produces an entire new range of surprising surface eects, which are exemplied in Fig. 2 and in Figs. S1-S6 of the Supporting Information. Since PANI salts are insoluble in most common solvents, the contact of the basic PB solution with an acidic PEDOT:PSS lm results in a spontaneous precipitation of a protonated PANI ring, which follows the initial shape of the droplet. These dark traces are noticeable from the optical images of the PP/PB "dry" bilayer in Fig. 2a and b. The composition of the macroscopic precipitate is conrmed by Raman spectroscopy, which is in this case a very convenient method because of its focused excitation. A comparison between the spectra of the precipitate and of the clear region is shown in Fig. S10. The height of this visible precipitate is also estimated between 0.5-2

µm

using AFM measurements, as illustrated in Fig. S6. A weaker precipitation corresponding to a higher macroscopic homogeneity is observed for the "wet" lms in Figs. S3 and S5, making them more suitable for practical applications. With this in mind, a further improvement in the physical properties of the alternating lms may still be achieved by a better control of the pH, as discussed in Section 2.2 of the Supporting Information. A better pH distribution within the wet surface might explain this behavior since the dissolved protons are not spatially bound to their respective PSS chains. This assumption can be rationalized based on general physicochemical principles. Within a "dry" PEDOT:PSS layer, the protons are bound to the anionic sulfonate groups, as expected for any common

10


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ionic solid. If the PEDOT:PSS layer is "wet", then an equilibrium is formed between their bound and dissolved states, which would ensure a homogeneously spread pH distribution within the lm solution.

As a PANI droplet is initially dropped onto the PEDOT:PSS

lm, an acid-base reaction occurs at their interface, even before the spreading of the droplet, leading to the formation of the macroscopic precipitate. In the case of the "dry" PEDOT:PSS lms, the PANI base near the droplet edge can only react with the protons in its immediate vicinity, thereby creating a pH imbalance between the initially covered and non-covered areas. In contrast, proton diusion from the initially non-covered PEDOT:PSS area towards the contact surface is likely within the "wet" lms, resulting in a better pH distribution.

Figure 2: Topological eects within alternating PEDOT:PSS/PANI layers.

(a-e) Surface

landscapes of a "dry" PP/PB bilayer, from the macroscopic precipitate to the nanometer scale folding.

(f-j) The nanofolding of the alternating "dry" lms, where PEDOT:PSS is

the rst layer: (f ) smooth PEDOT:PSS layer, (g) PP/PB bilayer, (h) PP/PB/PP lm, (i) PP/PB/PP/PB quadlayer, (j) PP/PB/PP/PB/PP lm. Signicant folding is observed in frames (g) and (i), where PANI is the top layer. The larger folds can also pierce through the subsequent PEDOT:PSS layers, as seen from frames (h) and (j). (k-o) The shrinkage of the "wet" PP/PB bilayer as a function of time while spreading a third PEDOT:PSS layer.

A similar micro- and nanoscopic folding also occurs in the apparently at regions, as revealed by atomic force microscopy (AFM) in Fig. 2c-e.

The central role of the in situ

formed PANI:PSS salts in forming the microscopic landscape is also illustrated in Fig. 2f-

11



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Page 12 of 33

j, which reveals the surface of the "dry"-prepared samples with an initial PP layer, at a 5 ×5 µm

2

resolution. As the basic form of PANI is deposited onto PEDOT:PSS, its granular

structure is lost, giving way to smooth folds.

This rst indicates a reorganization of the

polymer grains towards extended and more linear PANI chains. The folds can broaden with increasing numbers of layers, as seen from Fig. 2g and 2i. Because of their height (roughly 10200 nm), the PANI folds can also pierce through the relatively smooth PEDOT:PSS layers, as seen in Fig. 2h and 2j.

Otherwise, the thicker PEDOT:PSS layers maintain a largely

granular topography as seen in Fig. 2f,h, and j, corresponding to the surface that has been reported for pure PEDOT:PSS lms.

53,54

Another interesting eect occurs when spreading the PEDOT:PSS solution over a PP/PB "wet" bilayer, which results in a shrinkage of the existing polymer lm along the glass surface.

Its timely evolution is presented in Fig. 2k-o, where an image is recorded every

2 s, and in the top frame of the Supporting Video.

In this case, the micro-scale folding

of PANI could induce a certain elastic tension within the supported layers, which would overcome the weak interaction forces between the aqueous PEDOT:PSS lm and the glass substrate.

This whole-lm shrinkage does not occur when PANI is the rst "wet" layer,

which indicates a stronger interaction between the solvent DMSO and glass. Instead, only a shrinkage of the three upper layers is observed when spreading a fourth PEDOT:PSS solution over a PB/PP/PB trilayer, which causes small round ruptures within the organized layered structure.

These ruptures are visible in Fig. S1 and in the bottom frame of the

Supporting Video. This folding and shrinkage of the PEDOT:PSS/PANI base multilayers is particularly unique since previous works have only reported an either granular, smooth

6,15,39,54

6,15,53

or

PEDOT:PSS surface. While heating the PEDOT:PSS lms normally results

in a vertical shrinkage of the individual polymer grains through solvent loss and thereby in smooth lms,

55,56

no reorganization leading to folds of such magnitude has been previously

reported. The resulting shrinkage of the entire conductive wet lm in the horizontal direction also represents an unprecedented observation to the best of our knowledge. At a molecular

12


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level, the folding and its corresponding strain might be caused by the removal of the PSS chains from the PEDOT:PSS grains by the basic form of PANI, which would result in a reorganization of the interface between the PANI and PEDOT:PSS layers. To verify these assumptions, the samples are analyzed by UV-vis, Raman and X-ray photoelectron spectroscopy (XPS), with examples of the spectra given in Fig. 3a, b, c, and d.

Further discussions are also found in the Supporting Information, covering the data

from Figs. S7-S13. While the alternation in the characteristic bands is visible in both the UV-vis and Raman spectra, a small UV-vis band appears at approximately 450 nm upon adding multiple layers, conrming the in situ formation of the PANI:PSS salt. band assignment is found in Tables S3 and S4.

39,4246,5759

46

A detailed

A more signicant change is seen

in Figs. S7 and S8 for the "wet" ve-layered lms, which might indicate a direct charge transfer from the positively charged PEDOT to the initially neutral PANI chains. As earlier mentioned, the Raman microspectroscopy also attests to the presence of the PANI:PSS salt within the macroscopic precipitate. These two methods oer complementary information to XPS and to the previously presented AFM data, which can provide a better understanding of the interfacial interactions between the PEDOT:PSS and PANI layers. The strain-inducing chain reorganization is strongly supported by XPS measurements, which quantitatively reveal the chemical composition within approximately 10 nm of the top surface from the ratio of the XPS signals.

The ratios in Table 1 are based on the

nitrogen 1s signals from Figs. 3c and S11 and on the sulfur 2p features from Figs. 3d and S11-S13. and N

0

The nitrogen 1s signals at approximately 402 eV and 400 eV correspond to N species, respectively.

47,49,60

+

The sulfur 2p doublets at approximately 169 eV and

165 eV can be assigned to the sulfonate groups in PSS and to the thiophene rings from PEDOT, respectively.

39

In particular, the ratio of the sulfur 2p signals of PSS and PEDOT

is signicantly increased just below the thin PANI layers, which indicates an accumulation of PSS at the PP/PB interface. The accumulation of PSS at the outer surface of a PEDOT:PSS layer indicates that a second PEDOT rich phase is formed within the PEDOT:PSS layer,

13



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Page 14 of 33

contributing to an increased electrical conductivity. This type of chain rearrangement stands out since the conductivity improvement is typically obtained by a solvent post-treatment of the PEDOT:PSS lm, which leads to a washing of the PSS chains and therefore to a PEDOT-rich outer surface.

15,53

An interface reorganization is also supported by the nitrogen

1s spectra, which reveal that only a fth of the top PANI layer is protonated. These eects are less pronounced for lms that are deposited using the "wet" spin-coating procedure indicating a less localized interface reorganization, which is consistent with the higher macroscopic homogeneity.

Figure 3: Various spectra of the alternating "dry" layers, where PANI is rst spin-coated. The spectra are colored blue for the basic form of PANI and green for the subsequent layers, resembling the tones of the samples. (a) UV-vis spectra. (b) Raman spectra ( λex

= 633 nm).

(c) XPS nitrogen 1s ne spectra. (d) XPS sulfur 2p ne spectra.

Table 1: Relative composition of the polymer species near the top surface, as obtained from their S 2p and N 1s XPS signals.

Sample

Ratio PSS/PEDOT dry wet PB/PP 3.62 3.08 PB/PP/PB 6.14 4.64 PB/PP/PB/PP 3.28 2.42 PB/PP/PB/PP/PB 8.53 2.92

Ratio [N+ ]/([N+ ]+[N0 ]) dry wet 1.00 1.00 0.20 0.26 1.00 0.84 0.22 0.24

Based on these ndings, a model of the polymer distribution within the alternating layers is proposed in Fig. 4a, while the underlying chain reactivity is presented in Fig. 4b. This

14


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


model reveals the plausible grain reorganization and chain interactions leading to the PSSrich PP/PB interface, considering the existing models for single-component PEDOT:PSS and PANI lms.

3,15,18,40,61,62

Overall, the sequential deposition of the PEDOT:PSS and PANI lay-

ers can be followed from the alternation of their bands in both UV-vis and Raman spectra. As demonstrated by the UV-vis measurements, the acid-base reaction between the acidic PSS chains and the PANI base leads to the in situ formation of a green PANI:PSS salt. This salt is found to be the main component of the macroscopic precipitate, as revealed by Raman microspectroscopy. According to AFM measurements, the salt formation leads to a reorganization of the PEDOT:PSS and PANI grains near their interface, resulting in smooth PANI folds. An accumulation of the PSS chains at the interface, which is illustrated in Fig. 4a as dark blue margins, is also observed from the XPS data. This reorganization of the PEDOT:PSS grains can lead to more PEDOT within the PEDOT:PSS layers, as indicated by the red-violet tones in Fig. 4a. Moving further away from the PEDOT:PSS/PANI interface, AFM suggests that the granular structure for the most inner PEDOT:PSS micelles is maintained, while XPS indicates that some of the outer PANI chains from the top layer preserve their basic form. Although further optimization is still required to obtain highly uniform surfaces, these preliminary results indicate the potential advantages of the PEDOT:PSS/PANI lms for large-scale sustainable applications. In contrast to the complex carbon-based composites,

31

our alternating layers present a relatively high degree of transparency, which is comparable to the one of PEDOT:PSS

18,63

or even to the more conventional doped oxides.

63,64

This

transparency can be observed not only from the optical images of the lms in Figs. 3a and S2-S5 but also from the UV-vis spectra in Figs. 3a and S7. The latter indicate an absorbance of 0.1-0.4 for most alternating lms, corresponding to a transmittance between 40-80%. By depositing thinner layers and avoiding the formation of macroscopic PANI precipitates, the transparency of the insoluble pH-neutral lms could be further increased, making them a valuable alternative to PEDOT:PSS for thin-lm photovoltaics.

15


65,66

Moreover, precise control


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 4:

Page 16 of 33

Molecular and supramolecular interactions within alternating PANI base and

PEDOT:PSS layers. (a) Model for the chain reorganization, which causes the conductivity improvements and the topology eects.

(b) The in situ formation of the PANI:PSS salt,

which drives the chain reorganization.

of the lm shrinkage would enable the formation of self-sustained membranes with excellent stability and exibility, which could be employed for thermoelectric clothing or in exible solar cells with very low bending radii.

To obtain the higher macroscopic homogeneity

required for such large-scale applications, an automated spray deposition process might be even more suitable. The strain-induced eects of the alternating lms may also provide advantages for other applications.

In particular, the separation of the PP/PB/PP wet trilayers from the glass

substrate may result in the development of self-sustained thermoelectric membranes, which would stand out due to their ultra-low weight and thickness. Because of the phonon scattering at the chain, grain and fold boundaries, such lms could benet from a very low thermal conductivity, which would be similar to the multiscale strategy of Kanatzidis et al. for solidstate composites.

67,68

This thermal conductivity could be lower than the 0.17 W m

typically reported for PEDOT:PSS,

14,39,58

and signicantly below the 0.424.6 W m

−1

K

−1

−1

K

−1

suggested for the PANI:HCl/graphene-PEDOT:PSS/PANI:HCl/PEDOT:PSS-DWNT quadlayers.

31

The alternating lms would also represent a straight-forward solution-processed

16


Page 17 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


alternative to more complex layer-by-layer deposition methods, such as atomic layer deposition, which aim to achieve the 2D quantum conned structures proposed by Hicks and Dresselhaus.

69,70

4 Conclusions To conclude, this work reveals that chemical interactions play a fundamental role in enhancing the thermoelectric properties of alternating PEDOT:PSS/PANI layers.

Notably, the

improvement in the electrical conductivity can be explained by a complementary doping between both conjugated polymers: the conductive PANI:PSS salt is formed in situ, while the excess acidic PSS chains are removed from the PEDOT:PSS grains. This chain reorganization at the layers' interface induces a certain elastic strain, which results in richly patterned surfaces at the supramolecular scale. This rational design of the polymer interactions enables not only a high performance using less components, i.e., no additional counter anions or carbon allotropes, but also opens new pathways towards complex 2D nanostructures. By controlling the folding and shrinkage of these lms, further thermoelectric improvements could be achieved since self-sustained ultra-thin thermoelectric membranes could be produced.

Such lms would not only provide the 2D quantum connement proposed by

Dresselhaus but also a very low thermal conductivity through the phonon scattering at the folds' boundaries, resembling the multiscale approach of Kanatzidis and coworkers.

Acknowledgement We are grateful to DFG for the support through the SPP 1415 Program. V.A. is most grateful to the Gesellschaft Deutscher Chemiker for the August-Wilhelm-von-Hofmann-Scholarship and to the German Federal Government and the Bayer Foundations for the Deutschlandstipendium scholarship.

K.B. is most grateful for the scholarship from the International

Max-Planck Research School at the Fritz Haber Institute. Funding by FCI (Chemiefonds

17



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Fellowship) to F.M. is gratefully acknowledged.

Supporting Information Available The following les are available free of charge.

•

Art-Alternating-2017-Supplementary.pdf: further experimental results, literature data and discussions.

•

Art-Alternating-2017-Supporting Video.mp4: video of the polymer lm shrinkage.

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