Thermoelectric properties of a family of benzodifuranone-based conjugated copolymers in oriented thin films doped sequentially with NDMBI-H

Shubhradip Guchait, Diego Hinojosa, Nathan James Pataki, Said Oummouch, Laurent Herrmann, Mario Caironi, Michael Sommer, Martin Brinkmann

► To cite this version:

Shubhradip Guchait, Diego Hinojosa, Nathan James Pataki, Said Oummouch, Laurent Herrmann, et al.. Thermoelectric properties of a family of benzodifuranone-based conjugated copolymers in oriented thin films doped sequentially with NDMBI-H. Advanced Electronic Materials, In press, 10.1002/aelm.202500047. [hal-04994942]

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Distributed under a Creative Commons Attribution 4.0 International License
Thermoelectric properties of a family of benzodifuranone-based
conjugated copolymers in oriented thin films doped sequentially with
NDMBI-H

Shubhradip Guchait¹, Diego R. Hinojosa², Nathan James Pataki³,⁴, Said Oummouch¹, Laurent

Herrmann¹, Mario Caironi³⁺, Michael Sommer²⁺⁺, Martin Brinkmann¹*

(1) Institute Charles Sadron, UPR022 CNRS - University of Strasbourg, 23 rue du loess,
67034 Strasbourg, France

(2) Technische Universität Chemnitz, Institut für Chemie, Straße der Nationen 62, 09111
Chemnitz, Germany

(3) Center for Nano Science and Technology, Via Rubattino 81, 20134
Milano, Italy

(4) Department of Physics -Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133
Milano, Italy

<footnote>* martin.brinkmann@ics-cnrs.unistra.fr</footnote>

<footnote>+ Mario.Caironi@iit.i</footnote>

<footnote>++ michael.sommer@chemie.tu-chemnitz.de</footnote>
<figure><img src="image_2.png" /><figcaption>Figure for Title of content</figcaption></figure>

Combining high polymer orientation of n-type copolymers by temperature rubbing and sequential doping with N-DMBI results in a strong improvement of electrical conductivity and thermoelectric power factors reaching up to 9.8±1.6 S/cm and 8±3 µW/m·K², respectively.
Abstract

This study demonstrates the possibility to enhance thermoelectric properties of n-type benzodifuranone-based copolymers using a combination of polymer orientation (using high temperature rubbing) and sequential doping with the dopant N-DMBI-H. We focus on the impact of the side chain length and the chemical nature of the comonomer (thiophene *versus* furan) on the efficacy of this methodology that preserves the facile solution-processability of this polymer family and enables effective sequential doping without thermal activation step.

The combination of high temperature rubbing and thermal annealing helps reach high orientation of the copolymers with the thiophene comonomer regardless of the length of the side chains whereas the furan-based polymer is marginally aligned. The high orientation of thiophene-based copolymers results in a strong improvement of electrical conductivity and power factors reaching up to 9.8±1.6 S/cm and 8±3 µW/m·K², respectively.
# I. Introduction.

Thermoelectric generators (TEGs) have the capability to harvest energy from the environment by transforming thermal energy into electrical energy.¹⁻⁴ Given the large amount of waste heat generated e.g. by the combustion of oil and gas, TEGs are appealing devices. In the last decade, much effort has been dedicated to develop polymeric and hybrid materials for TEG applications at moderate temperatures (below 100°C) that require lightweight and flexible systems.³,⁴ TEGs are composed of thermoelectric materials made of doped semiconductors. While inorganic thermoelectric (TE) materials such as $Bi_2Te_3$ are some of the best performing systems with high values of the figure of merit $zT=(S^2\sigma/\kappa)T$ above 2 (S is the Seebeck coefficient, $\sigma$ the electrical conductivity, $\kappa$ the thermal conductivity and T the temperature), polymer-based doped semi-conductors start to become interesting alternative materials to design TEGs with zTs in the 0.2-0.4 range.¹⁻³ Polymeric materials are appealing because they are easy to process using solution-based methods such as blade-coating, spin-coating and ink jet printing, and in addition can be deposited onto flexible polymeric substrates.³ A single TEG is composed of both n-type and p-type “legs”, meaning effective p and n-type thermoelectric materials are needed. Much effort has been dedicated to the fabrication of efficient p-type doped polymer semi-conductors leading to PF on the order of several mW/m$^2$ for the best performing materials.⁴
Despite recent successes, the performance of doped n-type counterparts lags behind p-
type polymers for several reasons.⁵⁻¹¹ Some of the best performing n-type polymers are based
on rigid conjugated backbones such as benzimidazobenzophenanthroline (BBL) that
necessitate processing from harmful solvents such as methylsulfonic acid.⁸⁻¹⁰ Alternatively,
BBL dispersions can be processed from ethanol, delivering conductivities of up to 8 S/cm.⁸
Besides BBL, the group of Pei developed n-type polymers that are easier to process from
organic solvents and are based on benzodifuranone (BDF). Fluorine-substituted BDF
copolymers have been reported to show electrical conductivities as high as 14 S·cm⁻¹ and PF
of up to 28 µW/m·K² when doped with N-DMBI-H.¹¹, ¹², ⁵ In most studies, mixed doping is
used whereby thin films of dopant and polymer are co-processed from a common solvent.
However, the approach of mixed doping usually leads to phase separated morphologies in
thin films with rather large aggregates of dopant molecules dispersed in the polymer
matrix.¹³⁻¹⁵ This is further facilitated upon aggregation of polymer chains already in solution,
and the resulting morphologies are not optimal for charge transport. Sequential doping of
films circumvents phase separation and can improve charge transport.

Beside sequential doping, an effective strategy to enhance thermoelectric
performances of doped polymer semiconductors consists to align the polymer chains and
control crystallization prior to doping of the films.¹⁶,¹⁷,¹⁸ Following this strategy, Brinkmann
and coworkers managed to improve the charge conductivity in the chain direction in rub-
aligned thin films for numerous doped p-type polymers.¹⁹ Incidentally, there was no decrease
in the Seebeck coefficient despite improved charge conductivity resulting in substantial
improvement of the thermoelectric power factor in the chain direction as compared to that of
non-oriented films.¹⁸ To achieve high levels of orientation of polymers, the method of High
Temperature Rubbing (HTR) happened to be very effective.¹⁹⁻²² This method makes use of
shearing forces at the interface between a microfiber-coated rotating cylinder and a polymer
film (see Figure 1). The level of orientation is determined by the temperature of the film
during rubbing and by macromolecular parameters (molecular weight distribution, polymer
nature and length of side chains).<sup>19,21,22</sup> Yet, the strategy of polymer alignment plus sequential
doping has never been demonstrated to enhance thermoelectric properties for f n-type
polymers.

In this contribution, we demonstrate the successful use of rub-alignment and
sequential doping to enhance the thermoelectric performances of n-type polymers using N-
DMBI-H as a dopant. We focus on BDF-based copolymers where the BDF unit is flanked by
two isatin units. The isatin units provide an anchoring point for side chain introduction via N-
alkylation. Given their enhanced solubilities, their easy synthesis and modularity, single
oxygen side chains were chosen.<sup>23</sup> These polymers were previously reported to exhibit
promising conductivities in non-oriented thin films of up to 1 S/cm using mixed doping with
N-DMBI-H.<sup>23</sup> As seen in Figure 1, the BDF-thiophene polymers used in this study have
different side chains consisting of a linear alkyl spacer (C<sub>4</sub>, C<sub>8</sub> or C<sub>12</sub>) and a branched residue
connected via a single ether. As for the sequence of PBTTTs with single ether units in the
side chain recently designed by the Leclerc group, the role of the ether is to introduce some
polar function without perturbing the ordering of the polymer, which is in contrast to widely
used ethylene oxide-based side chains.<sup>24</sup> As a result, single ether side chains favor molecular
interactions with charged counterions of the doping molecules and this interaction can be
tuned by the position of the oxygen atom in the side chain. In the present case, the branched
side chain architecture is required as the BDF backbone structure exhibits a high level of
coplanarity and strong aggregation behaviour.<sup>25</sup> In this study, we exploited the modularity of
the side chain design and varied the linear alkyl spacer length from C<sub>4</sub> to C<sub>8</sub> and C<sub>12</sub>, resulting
in the copolymers BDF-T-4O, BDF-T-8O and BDF-T-12O when using thiophene as
comonomer, respectively. Besides the impact of side chains, we also investigate the impact of
changing the comonomer from thiophene to furan resulting in the copolymer BDF-F-8O (see Figure 1) on the TE properties of rubbed and sequentially doped polymer films. Furan-based conjugated copolymers have been reported to show enhanced backbone planarization and conjugation over thiophene-analogues. 26,27

<figure><img src="image_3.png" /><figcaption>**Figure 1.** a) Molecular structure of the BDF-T-nO (n= 4, 8 and 12) and the BDF-F-8O n-type polymers and the dopant N-DMBI-H. b) Method of preparation of oriented and doped polymer films using the combination of HTR and sequential doping.</figcaption></figure>

In the first part of this contribution, we identify the conditions necessary to observe
high in-plane orientation of the BDF-T-nO (n=4, 8 and 12) and the BDF-F-8O polymers
using a combination of high temperature rubbing and thermal annealing. The second section describes the consequence of sequential doping of the rub-aligned films with N-DMBI-H on spectroscopic, structural and thermoelectric properties of thin films. In particular, we demonstrate that charge conductivities close to 10 S/cm and PF=8 µW/mK² can be reached by the combination of rub-alignment and sequential doping with N-DMBI-H.

## II. Results and discussion.

### a) Orientation and structure of pristine and doped BDF-T-nO films

High temperature rubbing (HTR) has been shown to be a very effective method to align conjugated polymers and improve TE performance across a number of different p-type polymers and dopants. The key parameter to reach the best alignment is the rubbing temperature $T_R$. To quantify the degree of alignment, we used polarized UV-vis-NIR spectroscopy and determined the dichroic ratio $DR=A_{‖}/A_{⊥}$, where $A_{‖}$ and $A_{⊥}$ are the absorbances of the 0-0 absorption band (at 816 nm for BDF-T-8O) for light polarized parallel and perpendicular to the rubbing direction, respectively. Figure 2 and Figure S1 depict the characteristic UV-vis spectra of the best aligned films of BDF-T-nO (n=4, 8 and 12) as well as the evolution of the dichroic ratio with $T_R$, respectively. First, it is noted that the polymer with the $C_4$ spacer aligns best for the highest $T_R$ at 240°C with a DR close to 4. The two other polymers with longer alkyl spacers are observed to align best at a lower $T_R$ around 180°C to obtain a similar DR. It has been observed previously that poly(alkylthiophene)s with longer side chains can align readily at lower $T_R$ than those with shorter side chains, reflecting the fact that the polymers with long side chains are softer at lower $T_R$ when the side chains are molten and can thus better align upon shearing.²² Overall, DR values in the range of 3.5-4 point to limited alignment for this class of polymers, possibly because of the rather rigid
backbone of the BDF copolymers as compared to semi-flexible regioregular poly(3-hexylthiophene) (P3HT).<sup>19</sup> Beside the side chains, we find the chemical nature of the comonomer to be even more important for the success of alignment in rubbed films. Replacing thiophene by furan results in a strong reduction of orientation (see Figure 2b). BDF-F-8O shows a DR reaching a maximum of ~ 2 at most (in the entire range of investigated T<sub>R</sub> up to 240°C). Since this polymer has a similar molecular weight than the other thiophene-based copolymers, the poor alignment of BDF-F-8O must be related to the presence of furan. In conjugated polymers, furan is reported to improve both backbone planarity and conjugation.<sup>26,27</sup> In the present case, this should turn into larger persistence length for the furan copolymer as compared to the thiophene analogues. In our previous studies, we showed that high-T rubbing aligns better semi-flexible polymers such as poly(3-alkylthiophene)s (P3ATs) than rigid-rod polymers such as poly(8,8'-dioctylthiophene) (PFO).<sup>28</sup> Accordingly, we attribute the poor orientation of the furan copolymer to the rigidity of the chains that hamper shear-orientation upon rubbing as opposed to the more flexible thiophene copolymers.

The orientation of the polymer chain in rub-aligned films can be further improved by using post-rubbing thermal annealing of the films close to their melting temperature as demonstrated for some low bandgap polymers.<sup>19,29,30</sup> Such thermal annealing can have serval effects. It can i) help to adjust the chain conformation and promote chain reorganization inside π-stacks without relaxing in-plane chain orientation and/or ii) promote crystal growth from the well-aligned seed crystals out of the surrounding amorphous zones of the polymer.<sup>31</sup> Improving both crystal size and chain packing within crystals is believed to enhance overall orientation in the rub-aligned films. Therefore, we attempted to improve orientation of the polymers by testing different annealing temperatures T<sub>A</sub>. As seen in Figure 2c, annealing at 350°C for 10 min helped substantially improve the orientation of all BDF-T-nO films. DR
increases from 3.5 in as-rubbed films to 4.8 after thermal annealing, whereby similar DR are
obtained for the three polymers without a clear dependence on the length of the alkyl spacer.
It is thus assumed that the improved orientation is mainly related to some re-ordering of the
polymer backbones within π-stacks upon annealing at 350°C. In strong contrast, the level of
orientation in BDF-F-8O films was almost unchanged after thermal annealing indicating that
no molecular re-organization seems possible for this polymer.

Regarding the UV-vis spectra of all BDF-T-nO films, a vibronic structure with a
dominant 0-0 component attributed to a J-type aggregate band can be seen (Figure S1).<sup>32,33</sup>
The 0-0 component is slightly red-shifted at 833 nm for BDF-T-8O with respect to the other
two polymers and the 0-0 feature is more red-shifted in aligned films as compared to
aggregates formed in solution where the 0-0 peak is observed around 812 nm.<sup>23</sup> Interestingly,
the 0-0 component is strongly polarized parallel to the chain direction, as observed previously
for rub-aligned low bandgap polymers.<sup>30</sup>
<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /></figure>

**Figure 2.** a) Polarized UV-vis spectra of a rub-aligned film of BDF-T-8O ($T_R$=180°C and $T_A$=350°C). b) Evolution of the DR with the rubbing temperature $T_R$ for BDF-T-nO films. c) Maximum DR in the as-rubbed state and after thermal annealing at 350°C (1 min).
The structure of the rubbed films was probed by electron diffraction in a transmission Electron Microscope (TEM). Overall, all rubbed and annealed films show limited order, in agreement with previous investigations (see Figure S2).<sup>23</sup> For instance, rub-aligned films of BDF-T-4O show two equatorial reflections corresponding to the interlamellar periodicity d<sub>100</sub> and the π-stacking d<sub>020</sub> at 28.2 Å and 3.45 Å, respectively. The coexistence of the two reflections indicates that films consist of both edge-on and face-on oriented domains. No reflection is seen in the chain direction, indicating poor stacking order in the chain direction. A halo seen at 4.4 Å is attributed to the disordered alkyl side chain layers. Using a longer linker in BDF-T-8O results in an increase of the interlamellar periodicity to 32.3 Å whereas the π-stacking is slightly reduced to 3.40 Å. For BDF-T-12O the films are fully edge-on and only an equatorial 0 2 0 is observed. The π-stacking periodicity is close to that of BDF-T-4O at 3.5 Å.

Upon sequential doping with N-DMBI-H, the films show a sizable change in the lamellar periodicity d<sub>100</sub> (see Table S1). For BDF-T-4O, d<sub>100</sub> increases from 28.2 Å to 29.1 Å whereas for BDF-T-8O, the increase is from 32.3 Å to 34.9 Å. This is consistent with previous results on non-oriented BDF-T-nO films.<sup>23</sup> These are indications that sequential doping with N-DMBI-H implies some intercalation of dopant molecules into the side chain layers of the ordered polymer domains, similarly to reports for FBDPPV.<sup>11</sup> As opposed to the lamellar periodicity, there is marginal change in the π-stacking periodicity upon doping with N-DMBI-H.

b) Doping of BDF-T-nO analyzed by UV-vis-NIR absorption spectroscopy

Doping of the oriented thin films was followed by UV-vis-NIR absorption under polarized light. By selecting the light polarization (POL) with respect to the rubbing direction (R), this
method can probe the doping preferentially in oriented/ordered domains for POL//R and disordered domains for POL⊥R. First, we used incremental concentration doping to identify the impact of dopant concentration on the films. Figure 3 shows the evolution of the UV-vis-NIR spectra for oriented films of BDF-T-12O that are doped by incremental concentration doping (3 min), i.e. dipping a rub-aligned film in solutions in acetonitrile of increasing concentration at room temperature. It is worth noting that the samples were not subsequently annealed at 100-130°C as found in the literature, as such treatment did not improve the doping of the films.<sup>34-37</sup>

Polarized UV-vis-NIR absorption spectroscopy of the films after doping showed that at a dopant concentration of 0.1 g/L, doping is manifested by a slight bleaching of the neutral 0-0 and 0-1 absorption bands. At a higher concentration of 10 g/l (maximum concentration to ensure effective dissolution in acetonitrile), most of the neutral absorption band of for BDF-T-12O is bleached and the characteristic bands related to polarons of BDF-T-12O are found at the positions λ₁=1009 nm, (1.23 eV) λ₂=1340nm (0.926 eV) and λ₃=1689 nm (0.735 eV). Interestingly, these polaronic bands are polarized in the chain direction, alike the neutral absorption bands. The polarized UV-vis-NIR spectra show also that the polaronic bands are very similar for all four polymers and there is no obvious variation in band position upon changing the side chain length or the comonomer (see Figure S3). For POL⊥R, it is observed that the absorption of neutral disordered polymer (amorphous phase) bleaches as well, suggesting that N-DMBI-H is capable of doping the polymers in their amorphous/disordered domains. This is not the case for all polymer/dopant systems. For p-type polymers, phase-selective doping has been demonstrated and was found to determine for instance the long-term stability of the doped polymers.<sup>38</sup>

It is insightful to compare the UV-vis-NIR spectra of the doped species of other BDF-based polymers with those investigated in this study. Incidentally, the observed spectral
features are similar to those reported by Bardagot et al. for doped P(FBDOPV-F) i.e. an
alternating copolymer of BDF and 9,9-dioctylfluorene.³⁶ In stark contrast, conductivities of
up to 14 S/cm were reported by Pei et al. in non-oriented films of the polymer FBDPPV when
doped with N-DMBI-H.¹¹ Incidentally, the polaronic bands for this last polymer were more
red-shifted and significantly different from those found for our series of polymers.¹¹ This
comparison highlights the strong impact of the comonomer on the spectroscopic features of
polarons in doped BDF-based copolymers. It also indicates that the polarons in doped BDF-
T-nO are localized in essence, explaining at least in part the limited electrical conductivities
(vide infra).³⁷,³⁹

<figure><img src="image_7.png" /><figcaption>**Figure 3.** Impact of increasing doping concentration of N-DMBI-H in acetonitrile on the UV-vis-NIR spectra of rub-aligned BDF-T-12O films for polarization parallel (a) and perpendicular (b) to the chain direction. Kinetics of doping of rub-aligned BDF-T-12O films</figcaption></figure>
in a solution of N-DMBI-H (10 g/L, acetonitrile) followed by polarized UV-vis-NIR spectroscopy for light polarization parallel (c) and perpendicular (d) to the chain direction.

Having identified the required concentration of N-DMBI-H to achieve optimal doping of the BDF-T-nO films, we measured the impact of doping time on the UV-vis-NIR spectra of the films. Figure 3c-d shows the evolution of the spectra with increasing doping time using a concentration of 10 g/L for the case of BDF-T-12O (all polymers show similar behavior as seen in Figure S4). The highest level of doping is observed for the film doped three times during one minute at 10 g/L. Notably, sequential doping with N-DMBI-H was realized in acetonitrile without thermal annealing. This is in contrast to most studies making use of thermal annealing at temperatures in the 100°C-130°C range to trigger doping via N-DMBI-H activation via heterolytic C-H bond cleavage.<sup>34,35</sup>

To quantify the potential impact of the side chain linker on the efficacy of doping, we evaluated the ratio of the absorbance of the doped state and the neutral absorption band of the polymer. It can be seen in Figure S3, that the three polymer films show different absorbances of the neutral polymer band after doping with N-DMBI-H in the same conditions. To this aim, we performed a multi-peak fitting using gaussian components for the neutral undoped polymer (N₁ and N₂ components) and the doped state (polaron) with three components at λ₁, λ₂ and λ₃ (see right part of Figure S3). The fitting analysis indicates that the neutral polymer absorption is composed of two contributions: a narrow component (J-type aggregate band) centered at λ<sub>N1</sub>=1.47 eV (844 nm) and a broader component centered at λ<sub>N2</sub>=1.56 eV (795 nm) that might originate from more disordered and non-oriented chain segments. Interestingly, the three components λ<sub>n</sub> (n=1-3) associated to the doped species seem not to belong to a unique vibronic progression as the energetic spacing between them is not constant (0.29 eV and 0.19 eV). This result suggests that there could be a coexistence of two doped
species that may correspond to doped states of the ordered domains (J-type aggregate signature in neutral state) and the less ordered chains of the original undoped polymer. The relative level of doping is estimated from the ratio $r=\frac{Abs(\lambda3)}{Abs(N1)+Abs(N2)}$. As seen in Table S2, the doped films of the BDF-T-nO polymers exhibit relative doping ratios r of 0.74 for BDF-T-4O, 0.77 for BDF-T-8O and 0.68 for BDF-T-12O. The r values are similar for all polymers but apparently BDF-T-8O shows a slightly higher doping level. Overall, this study shows that the alkyl linker in the BDF-T-nO series has only limited influence on the achieved doping level.

### c) Impact of doping on thermoelectric properties and their anisotropy.

Lastly, the electrical conductivity and thermopower in rub-aligned BDF-T-nO films were examined. Figure 4 presents the observed electrical conductivity, Seebeck coefficient and power factor for pristine and rub-aligned films of the four polymers doped with N-DMBI-H. Figure S5 and S6 report the thermoelectric properties versus doping time for the four BDF-based polymers in pristine (unoriented) and rub-aligned thin films respectively. First, we followed the change in conductivity as a function of the doping time for [N-DMBI-H]=10 g/L. We observe an optimum doping time of 2 min whereas conductivities tend to decrease for higher doping times despite reaching higher doping levels at 3 min (inferred from UV-vis-NIR spectroscopy). For non-oriented films, the obtained values of conductivity are in range 0.1-1.3 S/cm in accordance with previous studies.²³ Interestingly, for all BDF-T-nO polymers, the charge conductivity is enhanced in oriented films as compared to non-oriented films by a factor of more than 5 (Figure S5 and S6). For both oriented and non-oriented films of BDF-T-nO, n=8 showed the highest conductivity whereas the lowest conductivity was found for n=12. The highest conductivity is reached for BDF-T-8O, with 9.8±1.6 S/cm in the chain direction (see Table 1) and a typical ratio of conductivity measured parallel $\sigma_{\parallel}$ and perpendicular $\sigma_{\perp}$ to the chains $\sigma_{\parallel}/\sigma_{\perp}$ of 11. The limited influence of the alkyl
linker length on TE performances in oriented films is consistent with UV-vis-NIR absorption showing similar overall doping levels for the three polymers. By contrast, the poor orientation of BDF-F-8O (DR=2) leads to similar charge conductivities in pristine and rub-aligned films with low anisotropy. Accordingly, rather than the length of the alkyl spacer, it is the chemical nature of the comonomer that impacts strongly the thermoelectric performances in rub-aligned BDF-based copolymers.

<figure><img src="image_8.png" /><figcaption>**Figure 4.** Comparison of the conductivity (a), Seebeck coefficient (b) and the thermoelectric power factor (c) probed for rub-aligned films in the chain direction (red) and for non-oriented films (grey) of BDF-T-nO and BDF-F-8O. The films were sequentially doped with</figcaption></figure>
N-DMBI-H (2 doping steps of 1 min at 10 g/L in acetonitrile). d) Correlations between Seebeck coefficient and charge conductivity for rub-aligned films of BDF-T-nO doped with N-DMBI-H. Additional data points in blue correspond to n-type polymers from literature: p(gNDI-gT2)⁴⁰ (blue stars) FBDPPV ¹¹ (blue diamond) and PNDTI-BBT-DP and PNDTI-BBT-DT ⁴¹ (blue inverted triangles). The continuous and dotted lines correspond to the fitting using the scaling law $S \propto \sigma^{-1/4}$ as observed for doped PBTTT.⁴²

As proposed earlier by Kiefer et al. for n-type polymers based on naphthalene bisimide, we verified the scaling relations between the Seebeck coefficient S and the electrical conductivity σ (Figure 4.d) in the directions parallel and perpendicular to the chains.⁴⁰ For the sake of comparison, we also superposed data points from other representative n-type polymers doped with N-DMBI-H such as p(gNDI-gT2), PNDTI-BBT-DP and FBDPPV.⁴⁰,¹¹,⁴¹ In particular, the obtained data points for all rub-aligned BDF-F-nO films tend to distribute around two distinct curves for parallel and perpendicular directions that obey the $S \propto \sigma^{-1/4}$ relation.⁴²

Moreover, the Seebeck coefficient S shows anisotropy after rubbing. As observed previously for p-type polymers, $S_‖$ is always larger than $S_⊥$.²⁰,⁴³ The recent heterogeneous material model developed by Limelette et al. links the apparent anisotropy of the Seebeck coefficient to the coexistence of aligned ordered and disordered domains that have different electrical and thermal conductivities.⁴³ The apparent anisotropy of the Seebeck coefficient results from the inhomogeneous distribution of thermal gradients inside the oriented polymer films. When dealing with an aligned film made of ordered and disordered domains, the thermal conductances of ordered and disordered domains are measured in parallel when the temperature gradient $\nabla T$ is oriented parallel to the rubbing direction whereas they are measured in series when $\nabla T$ is oriented perpendicular to the rubbing. In the low conductivity
regime (when the electronic contribution of the thermal conductivity can be neglected), is it possible to find a relation between $S_\parallel$ and $S_\perp$ following equation (1) : $S_\perp \approx \frac{\chi \kappa_{dis, lat}}{(1-\chi)\kappa_{ord, lat}} S_\parallel$ with $\kappa_{ord, lat}$ the thermal conductivity of the oriented and ordered domains, and $\kappa_{dis, lat}$ the thermal conductivity of the disordered (amorphous) domains. $\chi$ is the volume fraction of ordered domains.

For the doped BDF-T-nO, the ratio $S_\parallel/S_\perp$ is in the range 1.4-2.7 whereas the poorly oriented BDF-F-8O exhibits negligible anisotropy. In the case of BDF-T-8O films, we find $\frac{\chi \kappa_{dis, lat}}{(1-\chi)\kappa_{ord, lat}}=2.7$, a value lower than that observed for p-type polymers such as PBTTT-C$_{12}$ or P3HT. Given the poor order evidenced by TEM in the rub-aligned BDF-T-8O films, we attribute the reduced value of $S_\parallel/S_\perp$ mainly to the low proportion of ordered domains $\chi$ in the films.

<table><caption>**Table 1.** Anisotropic thermoelectric properties in rub-aligned films of BDF-T-nO and BDF-F-8O doped sequentially with n-DMBI-H. Conductivity, Seebeck coefficient and power factors are measured in the direction parallel (//) and perpendicular (⊥) to the rubbing direction.</caption><thead><tr><th>Polymers</th><th>DR</th><th>σ<sub>∥</sub><br>(S/cm)</th><th>σ<sub>⊥</sub><br>(S/cm)</th><th>σ<sub>∥</sub>/<br>σ<sub>⊥</sub></th><th>S<sub>∥</sub><br>(μV/K)</th><th>S<sub>⊥</sub><br>(μV/K)</th><th>S<sub>∥</sub>/<br>S<sub>⊥</sub></th><th>PF<sub>∥</sub><br>(μW/mK<sup>2</sup>)</th><th>PF<sub>⊥</sub><br>(μW/mK<sup>2</sup>)</th><th>PF<sub>∥</sub>/<br>PF<sub>⊥</sub></th></tr></thead><tbody><tr><td>BDF-T-<br>4O</td><td>4.7</td><td>4.5±1.3</td><td>0.6±0.1</td><td>7.5</td><td>-111±10</td><td>-61</td><td>1.8</td><td>5.5±1.7</td><td>0.4±0.1</td><td>13.7</td></tr><tr><td>BDF-T-<sup>8</sup>O</td><td>4.8</td><td>9.8±1.6</td><td>0.9±0.2</td><td>11</td><td>-91±5</td><td>-53</td><td>1.7</td><td>8.0±1.6</td><td>0.4±0.1</td><td>20</td></tr><tr><td>BDF-T-<br><sup>12</sup>O</td><td>4.6</td><td>5.6±0.3</td><td>1.3±0.1</td><td>4.3</td><td>-72±4</td><td>-51</td><td>1.4</td><td>2.2±0.2</td><td>0.2±0.1</td><td>11</td></tr><tr><td>BDF-F-<sup>4</sup>O</td><td>2.0</td><td>1.8±0.5</td><td>1.0±0.5</td><td>1.8</td><td>-105±18</td><td>-99</td><td>1.1</td><td>2.0±0.5</td><td>0.9±0.1</td><td>2.2</td></tr></tbody></table>

Both anisotropies of conductivity and Seebeck coefficient result in a notable anisotropy of the PF which is highest for the best aligned films of BDF-T-8O with PF<sub>∥</sub>/PF<sub>⊥</sub> =20. The best power factor of 8.0±1.6 μW/m·K<sup>2</sup> is obtained for rub-aligned films of BDF-T-
8O, while both BDF-F-8O and BDF-T-12O show lower performance around 2.0±0.5 µW/m·K². This is close to the best value reported by Shi et al. for non-oriented FBDOPVV doped with N-DMBI-H.¹¹ The better TE performance of FBDOPVV films results mainly from the fact that the isatin comonomer is fluorinated which is known to enhance TE properties as opposed to the non-fluorinated BDF-T-nO used in this work.¹¹ Overall, our results demonstrate that the strategy of alignment combined with sequential doping is capable of enhancing thermoelectric performances of n-type polymers, in a similar manner to p-type polymers.¹⁸

### III. Conclusion

A series of four BDF-based n-type polymers BDF-T-nO and BDF-F-8O differing by the length of their alkyl spacer (n= 4, 8 and 12) and the nature of the comonomer (thiophene T and furan F) has been aligned using combined high temperature rubbing and sequential doping with N-DMBI-H. Using this method, films with anisotropy and improved thermoelectric properties are obtained. The furan polymer BDF-F-8O is very difficult to align, in contrast to all thiophene polymers BDF-T-nO that show a high degree of orientation after a combination of rubbing and thermal annealing at high temperature (350°C). The rigidity of the polymer backbone is possibly one factor limiting the alignment of the furan copolymer by high-T rubbing. For the BDF-T-nO series, we observe that the length of the alkyl spacer in the side chain has a limited influence on the thermoelectric properties of the rub-aligned films. Yet, the degree of alignment achieved for these n-type polymers is substantially lower than that obtained for classical p-type polymers such as P3HT or PBTTT, and this is limiting the improvement in TE properties. A second limiting factor is the localized nature of the polarons in these doped copolymers as indicated by UV-vis-NIR spectroscopy.
# IV. Experimental section

## a) Materials and oriented film preparation.

BDF-T-4O and BDF-T-12O were synthesized following a previously published protocol.²³ Synthesis of BDF-F-8O. 2,5-Bistrimethylfuran was prepared following a previous report (https://doi.org/10.1080/00397918308061969). The benzodifuranone monomer 2Br-8O-BDF was prepared according to Hinojosa et al. (ref 23). In a screw cap septum vial 2Br-8O-BDF (100 mg, 0.065mmol, 1eq), 2,5-bistrimethylstannylfuran (25.63mg, 0.065mmol, 1eq), Pd₂dba₃·CHCl₃ (0.7mg, 0.00065 mmol, 0.01 eq) and P(o-tol)₃ (1mg, 0.00325 mmol, 0.05 eq) were added. The vial was subjected to three vacuum/argon refill cycles and was left 15 more minutes sparging with argon after the last cycle. Dry and degassed toluene was then added via syringe until the concentration of the monomers reached 0.01M. The vial was placed in an aluminum heating block and stirred at 100°C for 48h. After this time the reaction mixture was allowed to cool to room temperature and precipitated into 300 mL of methanol with vigorous stirring. The precipitate was collected using filter paper and purified in a Soxhlet extractor with acetone, ethyl acetate and petrol ether. Finally, the material was collected with chloroform and the polymer solution precipitated into methanol to obtain a black solid. Yield 56 mg (60%).

The number average molecular weights ($M_n$) and dispersities ($D$) of the polymers are obtained from high temperature size exclusion chromatography and summarized in **Table 1**. N-DMBI-H (4-(1,3-dimethyl-2,3-dihydro-1H-benzoimidazol-2-yl)phenyl) dimethylamine) was purchased from Sigma Aldrich. Anhydrous solvents (99%) used for doping (acetonitrile) and thin film preparation (chloroform) were also purchased from Sigma Aldrich.
<table><caption>**Table 2.** Summary of the macromolecular parameters of BDF-based polymers. Data is from reference 23 except BDF-F-8O, which was measured under identical conditions.</caption><thead><tr><td>Polymers</td><td>M<sub>n</sub> (kg/mol)</td><td>D</td></tr></thead><tbody><tr><td>BDF-T-4O</td><td>50</td><td>2.0</td></tr><tr><td>BDF-T-8O</td><td>55.3</td><td>1.8</td></tr><tr><td>BDF-T-12O</td><td>54.9</td><td>2.5</td></tr><tr><td>BDF-F-8O</td><td>19.9</td><td>1.2</td></tr></tbody></table>

b) Film preparation, high temperature rubbing and doping

BDF-T/F-nO polymer films were prepared via doctor blade method from a 15g/L solution in CHCl₃ at 65°C on glass substrates cleaned by ultrasonication in acetone, ethanol, hellmanex and deionized water (3 times). To favor alignment by rubbing, we prepared films from low boiling point solvents (chloroform) rather than ortho-dichlorobenzene. Rub-alignment was conducted using a custom-built setup, consisting of a rotating cylinder wrapped with a microfiber cloth and the sample was mounted on a translating hot stage. Thermal annealing at 350 °C was performed on rub-aligned films using a Linkam THMS600 hot stage under N₂ atmosphere. The heating ramp was 30°C/min. The heating time was 10 min and the cooling ramp was 25°C/min. The film thickness was determined from UV-Vis absorbance. The thickness-absorbance calibration curve was obtained from Atomic Force Microscopy measurements on non-oriented films (Figure S7). The AFM images were captured using a Bruker Multimode system operated by a Nanoscope V module. Imaging was performed with a Si₃N₄ tip, featuring a spring constant of 0.4 N/m and a typical resonance frequency of 300 kHz, with a tip radius ranging from 3 to 5 nm.

Solutions of N-DMBI-H in acetonitrile were used to dope pristine and rub-aligned BDF-T-nO and BDF-F-8O films at room temperature following a sequential doping protocol. Doping was performed in a glovebox as well as all subsequent electrical measurements
(Jacomex glove box, p(O₂)< 2 ppm, p(H₂O)< 1 ppm). The optimization of the dopant concentration was conducted on BDF-T-12O films. It involves a complete immersion of the samples for 3 minutes in acetonitrile solution as a function of dopant concentration (0.1-10 g/L). The use of higher dopant concentrations and the incremental concentration doping protocol was constrained by surface contamination issues. To avoid dedoping, rinsing of the doped polymer films with pure solvent was avoided. Instead of varying the concentration, doping was performed by sequentially immersing the films in a solution at a fixed concentration 10 g/L N-DMBI-H, and the doping times was incrementally augmented (1 min per step) at room temperature. The optimized doping conditions were subsequently applied to all BDF-T-nO polymers.

### c) Thin film characterization

**Transmission Electron Microscopy:** Oriented and annealed BDF-T-nO films were coated with a thin amorphous carbon film using an Edwards Auto306 evaporator. Subsequently, the films were floated on distilled water and transferred onto TEM copper grids. The doping by dipping in N-DMBI-H/acetonitrile solution was carried out on the copper grids and under inert atmosphere inside the glove box. After excess dopant solution was removed by blotting, the samples were then briefly exposed to air (20-30 s) only before introducing the sample holder into the vacuum of the TEM. A TECNAI200 FEI microscope (200 kV) equipped with an Eagle 2k CCD camera was employed for electron diffraction (ED) analysis before and after doping. Beam exposure was minimized (low dose system) to avoid de-doping of the oriented films under the electron beam. To calibrate reticular distances, we used the 0 0 2 reflection of oriented poly(3-hexylthiophene) film as a reference.

**Polarized UV-Vis-NIR Absorption:** UV-vis-NIR absorption (350-2500 nm) spectra of doped rub-aligned films were measured using a Cary 5000 spectrometer with a spectral
resolution of 1 nm and polarized incident light, as a function of dopant concentration and doping time. The spectra were collected with the incident light polarization (POL) either parallel (0°) or perpendicular (90°) to the rubbing direction, corresponding to POL//R and POL⊥R, respectively.

**Charge Conductivity and Seebeck Coefficient.** Cleaned glass substrates were dried under nitrogen and treated with plasma before electrode deposition. Gold electrical contacts, 40 nm in thickness, were fabricated on these substrates, with electrodes spaced 1 mm apart and measuring 5 mm in length. A four-point probe geometry was established through the thermal evaporation of Au at an average rate of 4–6 Å/s, using a shadow mask. Before the gold deposition, a 2.5 nm thick layer of chromium was deposited to improve adhesion to the glass substrates, at an evaporation rate of 0.5–1 Å/s. Each sample for transport measurements typically included four individual four-point probe devices, with two devices aligned parallel and two perpendicular to the rubbing direction on a single 20x20 mm² substrate. The BDF-T (F) -nO-polymer films were deposited directly by doctor blade onto the device substrates, carefully aligned by high temperature rubbing along with the electrode patterns on the glass substrates. The rubbing process was meticulously executed to ensure proper alignment of the electrode orientation with the film rubbing direction. The floating method of pristine films deposited on NaPSS was avoided as a trace amounts of water are detrimental to the charge transport properties doped polymer films. The devices with rub-aligned films were annealed at 350 °C prior to doping. Doping was conducted under similar conditions as previously described.

All measurements of electrical conductivity and the Seebeck coefficient were conducted in a Jacomex glovebox under an N₂ atmosphere, with p(O₂) ≤ 2 ppm and p(H₂O) ≤ 1 ppm. Electrical conductivity was assessed using a Keithley 2436b and a Semiprobe station (Lab Assistant 150). The resistivity (ρ) was calculated from the sheet resistance (R) using the
equation ρ = 1.81.R.t, where 't' represents the film thickness. The geometrical correction factor was determined following the methodology described in reference 18. Film thickness was measured using a UV-vis absorbance and thickness calibration plot (Figure S6).

The Seebeck coefficient was measured using a differential temperature method whereby a temperature gradient was established across the sample along or perpendicular to the rubbing direction. The temperature gradient was ramped between 0 and 12K around room temperature and the Seebeck coefficient was extracted by plotting the thermovoltage *versus* temperature gradient. A constantan wire was used to calibrate the Seebeck coefficient. The detailed procedure is given in reference 18. Data analysis and fitting were performed using Kaleidagraph.

**Supporting Information**

Supporting information is available from the Wiley Online Library or from the author.

**Acknowledgements**

M. Schmutz is acknowledged for technical support with TEM. N. Zimmermann is acknowledged for technical support in pre-patterned device preparation. C. Contal is acknowledged for technical support with Atomic Force Microscopy. Fruitful discussions with Olivier Bardagot on n-type doping are gratefully acknowledged. This work was financially supported by ANR grant Thermopolys and by the European Commission through Marie Skłodowska-Curie project HORATES (GA-955837).

**Conflict of interest**

The authors declare no conflict of interest.
## Author Contribution

D. R. H. and S. G. synthesized and characterized the polymers. M. S. supervised the synthesis of the polymers. S. G., N. J. P. and S. O. prepared doped thin films and characterized their anisotropic optical and thermoelectric properties. L. H. contributed to the design of the rubbing machine. M. B. conceived and supervised the rubbing and doping process of thin films. M.B. carried out the TEM and ED measurements. All authors contributed to the manuscript preparation.

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*Supporting information*

**Thermoelectric properties of a family of benzodifuranone-based
conjugated copolymers in oriented thin films doped sequentially with
NDMBI-H**

Shubhradip Guchait¹, Diego R. Hinojosa², Nathan James Pataki³,⁴, Said Oummouch¹, Laurent Herrmann¹, Mario Caironi³, Michael Sommer²⁺, Martin Brinkmann¹*
(2) Technische Universität Chemnitz, Institut für Chemie, Straße der Nationen 62, 09111
Chemnitz, Germany

(3) Center for Nano Science and Technology, Via Rubattino 81, 20134
Milano, Italy

(4) Department of Physics -Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133
Milano, Italy

* martin.brinkmann@ics-cnrs.unistra.fr

+ Mario.Caironi@iit.i

++ michael.sommer@chemie.tu-chemnitz.de

<figure><img src="image_9.png" /></figure>
**Figure S1:** Polarized UV-vis-spectra of rub-aligned BDF-T-nO films and BDF-F-8O for light polarization parallel (//) and perpendicular (⊥) to the rubbing direction *R*.

<figure><img src="image_10.png" /></figure>

**Figure S2:** TEM electron diffraction patterns of BDF-T-nO films after rubbing (at the optimal temperature, see Figure 2.b) and subjected to thermal annealing at 300°C (1 min) and after doping with N-DMBI-H (10 g/L in acetonitrile). The rubbing direction is vertical (*R*).

**Table S1.** Structural parameters in rub-aligned BDF-T-nO films before and after doping with N-DMBI-H.
<table><thead><tr><th>d<sub>hkl</sub> / Polymer</th><th>BDF-T-4O</th><th>BDF-T-8O</th><th>BDF-T-12O</th></tr></thead><tbody><tr><td>d<sub>100</sub> (Å)</td><td>28.2</td><td>32.3</td><td>-</td></tr><tr><td>d<sub>020</sub> (Å)</td><td>3.45</td><td>3.40</td><td>3.5</td></tr><tr><td>d<sub>100</sub> (Å) doped</td><td>29.1</td><td>34.9</td><td>-</td></tr><tr><td>d<sub>020</sub> (Å) doped</td><td>3.5</td><td>3.45</td><td>3.5</td></tr></tbody></table>

<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /></figure>
**Figure S3.** a-c) Comparison of the UV-vis-NIR spectra for rub-aligned BDF-T-nO films doped with N-DMBI-H (10 mg/mL, 3 dipping for 1 min) as recorded for light polarization parallel (//) and perpendicular (⊥) to the chain direction. d-f) Results of the fitting of the UV-vis-NIR spectra (polarization parallel to polymer chains) for rub-aligned BDF-T-nO films doped with N-DMBI-H (10 mg/mL, 3 dipping for 1 min). The components $N_1$ and $N_2$ correspond to the neutral polymers whereas the $\lambda_n$ ($n=1-3$) correspond to the doped state.

<figure><img src="image_14.png" /></figure>

**Figure S4.** Kinetics of doping of rub-aligned films of BDF-T-4O, BDF-T-8O and BDF-F-8O doped in a solution of N-DMBI-H (10 g/L, n-butylacetate) followed by polarized UV-vis-NIR spectroscopy of for light polarization parallel (//) and perpendicular (⊥) to the chain direction (rubbing direction R).
<table><caption>**Table S2.** Result of the fitting of the UV-vis-NIR spectra (polarization parallel to polymer chains) for rub-aligned BDF-T-nO films doped with N-DMBI-H (10 mg/ml, 3 dipping for 1 min). The components N₁ and N₂ correspond to the neutral polymers whereas the λₙ (n=1-3) components correspond to the polaron generation by doping the polymers with N-DMBI-H.</caption><thead><tr><th>Components</th><th></th><th>BDF-T-4O</th><th>BDF-T-8O</th><th>BDF-T-12O</th></tr></thead><tbody><tr><td rowspan="3">λ1</td><td>E₁ (eV)</td><td>0.728</td><td>0.733</td><td>0.722</td></tr><tr><td>Peak area A₁</td><td>0.0078</td><td>0.0046</td><td>0.0019</td></tr><tr><td>HWHM₁ (eV)</td><td>0.070</td><td>0.046</td><td>0.056</td></tr><tr><td rowspan="3">λ2</td><td>E₂ (eV)</td><td>0.915</td><td>0.913</td><td>0.923</td></tr><tr><td>Peak area A₂</td><td>0.0065</td><td>0.009</td><td>0.0088</td></tr><tr><td>HWHM₂ (eV)</td><td>0.080</td><td>0.085</td><td>0.056</td></tr><tr><td rowspan="3">λ3</td><td>E₃ (eV)</td><td>1.205</td><td>1.206</td><td>1.197</td></tr><tr><td>Peak area A₃</td><td>0.050</td><td>0.086</td><td>0.012</td></tr><tr><td>HWHM₃ (eV)</td><td>0.144</td><td>0.167</td><td>0.122</td></tr><tr><td rowspan="3">N₁</td><td>E₄ (eV)</td><td>1.500</td><td>1.48</td><td>1.509</td></tr><tr><td>Peak area A₄</td><td>0.0195</td><td>0.034</td><td>0.014</td></tr><tr><td>HWHM₄ (eV)</td><td>0.10</td><td>0.105</td><td>0.11</td></tr><tr><td rowspan="3">N₂</td><td>E₅ (eV)</td><td>1.63</td><td>1.624</td><td>1.71</td></tr><tr><td>Peak area A₅</td><td>0.048</td><td>0.078</td><td>0.0033</td></tr><tr><td>HWHM₅ (eV)</td><td>0.196</td><td>0.19</td><td>0.092</td></tr></tbody></table>
<figure><img src="image_15.png" /><figcaption>b)</figcaption></figure>

<figure><img src="image_16.png" /><figcaption>c)</figcaption></figure>

<figure><img src="image_17.png" /></figure>
**Figure S5.** Impact of sequential doping (1 min, solution of N-DMBI-H in nBA, 10g/L, 1min per step) for non-aligned films of BDF-T-nO (n=4,8 and 12) on the charge conductivity (a), the Seebeck coefficient S (b) and the power factor (c).

<figure><img src="image_18.png" /></figure>

<figure><img src="image_19.png" /></figure>

<figure><img src="image_20.png" /></figure>
**Figure S6.** Impact of sequential doping (1 min, solution of N-DMBI-H in nBA, 10g/L, 1min per step) for rub-aligned films of BDF-T-nO (n=4,8 and 12) on conductivity (a), the Seebeck coefficient (S) and the power factor (PF) measured in the directions parallel (//) and perpendicular (⊥) to the rubbing direction.

<figure><img src="image_21.png" /></figure>

**Figure S7.** Calibration curve between absorbance at 833 nm and the thickness measured by AFM for pristine non-oriented BDF-T-nO films (n=4,8 and 12). The continuous line is the result of the fit.