# A Label-Like Monolithic Organic Thermoelectric Generator Enabled by Local Inkjet Doping of Aligned Polymer Films

Nathan James Pataki, Shubhradip Guchait, Badr Jismy, Nicolas Leclerc, Adrica Kyndiah, Martin Brinkmann, Mario Caironi

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Nathan James Pataki, Shubhradip Guchait, Badr Jismy, Nicolas Leclerc, Adrica Kyndiah, et al. A Label-Like Monolithic Organic Thermoelectric Generator Enabled by Local Inkjet Doping of Aligned Polymer Films. Advanced Energy Materials, 2025, 10.1002/aenm.202404656. hal-04892779

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# A Label-Like Monolithic Organic Thermoelectric Generator Enabled by Local Inkjet Doping of Aligned Polymer Films

Nathan James Pataki, Shubhradip Guchait, Badr Jismy, Nicolas Leclerc, Adrica Kyndiah, Martin Brinkmann,* and Mario Caironi*

The proliferation of distributed microelectronics and sensors necessitates adaptable, scalable, and cost-effective power supplies. Organic thermoelectric generators (TEGs) that promise to harness heat sustainably and cost-effectively are seen as pivotal elements in shaping future sensor infrastructures. Recent strides in morphological control through the alignment of conjugated polymer backbones have enhanced the thermoelectric performance of doped organic semiconductors to record values, matching expectations for real applications. However, the hurdles in crafting and deploying organic TEGs effectively exploiting aligned polymer films remain unexplored. This work presents a design and fabrication method to incorporate aligned films into a thin label-like TEG. Thin films of regioregular poly(3-hexylthiophene) (P3HT) and poly(2,5-bis((7-butoxyheptyl)thiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT-⁸O) are aligned via high-temperature rubbing technique inducing a high degree of anisotropy in their charge transport properties. The crystal structure and anisotropy of the films are exploited to realize monolithic TEGs by patterning conductive thermoelements via local inkjet doping of films transferred on ultrathin parylene substrates. The TEGs based on aligned P3HT and PBTTT-⁸O exhibit exceptional TEG power factors of 0.33 and 1.04 nW cm⁻² K⁻², respectively. Lastly, as a proof-of-concept use case for the TEGs, a thermoelectrically-powered volume-indicating label is presented as a potential application in the healthcare and food industries.

## 1. Introduction

As the Internet of Things (IoT) expands and diversifies in application scope, there is a growing need for versatile, sustainable, and cost-effective power sources for its network of distributed sensors and microelectronics.[1] Organic thermoelectric

generators (TEGs), which harness low-grade heat waste to generate electricity through the Seebeck effect, are emerging as a viable solution for IoT micro-power requirements.[2,3] With the ongoing miniaturization and improved efficiency of microelectronics and sensors, reducing their power needs to the µW level,[4,5] small-scale organic TEGs, or organic micro-TEGs, are becoming increasingly relevant. Organic TEGs, in particular, offer advantages in sustainable energy harvesting due to their solution-processable nature at room temperature, which facilitates scalable manufacturing and printing techniques.[3,6,7] Moreover, organic semiconductors and conductors are generally made from abundant and non-toxic elements, and their thermoelectric properties are highly tunable through functionalization or modification of the conjugated backbone.[8–10] Recent advancements in organic semiconductors[11–14] and doping strategies[15–17] have led to improved thermoelectric performance, resulting in zT values up to 0.25 for p-type conductors[18] and 0.34 for n-type conductors.[19]

Such advancements indicate the possibility of producing organic TEGs with power outputs relevant for several real-case scenarios.[20–22] Yet, examples in this direction are still scarce. A lack of application-focused design in organic TEGs is a large contributing factor to stagnant progress for devices. In contrast to solar energy conversion, thermal energy conversion lacks a

N. J. Pataki, A. Kyndiah, M. Caironi
Center for Nano Science and Technology
Istituto Italiano di Tecnologia
Via Raffaele Rubattino, 81, Milan 20134, Italy
E-mail: mario.caironi@iit.it

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/aenm.202404656

© 2025 The Author(s). Advanced Energy Materials published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

DOI: 10.1002/aenm.202404656

N. J. Pataki
Department of Physics
Politecnico di Milano
Piazza Leonardo da Vinci 32, Milano 20133, Italy
S. Guchait, M. Brinkmann
CNRS
Université de Strasbourg
ICS UPR 22, Strasbourg F-67000, France
E-mail: martin.brinkmann@ics-cnrs.unistra.fr
B. Jismy, N. Leclerc
CNRS
Université de Strasbourg
ICPEES UMR 7515, Strasbourg F-67000, France
universal and standardized source. The thermal gradients arising from different potential applications vary drastically in spatial distribution, average, and boundary temperatures. Such diversity becomes apparent when comparing potential sources of waste heat like the human body, a hot water pipe, soil, or water. The absence of a reference device implies that a TEG architecture should inherently possess a specific use case, functionality, or intended temperature gradient beyond that of a laboratory hotplate. In addition, the scalability potential of both the design itself and its manufacturing processes is an important aspect in order to leverage the full potential of organics for thermoelectric energy harvesting. Solution-processible organic thermoelectric materials are perfectly positioned to leverage highly scalable planar deposition techniques like inkjet printing, screen-printing, spray-coating, and roll-to-roll printing,[3] thus making them ideal for scalable manufacturing.

Many of the high-value-proposition applications envisioned for organic TEGs are in wearable electronics and remote sensing, which require efficient power sources with small form factors and economical price points.[23] Therefore, spatial constraints and bills of materials cannot be ignored when discussing application-focused organic TEGs. Taking these considerations into account, organic TEGs should aim at minimizing size while maximizing power density and open circuit voltage per unit area, rather than absolute power and voltage alone. The open circuit voltage per unit area and maximum power density of a TEG can be expressed as:[24]

$$V_{oc,density} = \frac{n}{A_{TEG}} S_{pn} \Delta T \quad (1)$$

$$P_{density,max.} = \frac{n^2 S_{pn}^2}{4 R_{TEG} A_{TEG}} \Delta T^2 \quad (2)$$

where n is the number of thermocouples, $S_{pn} = (S_p - S_n)$ and $S_p$ and $S_n$ are the Seebeck coefficients of the materials used for the p- and n-type thermoelements, respectively, $R_{TEG}$ is the internal resistance of the TEG, $A_{TEG}$ is the total cross-sectional area of the TEG, and $\Delta T$ is the temperature gradient across the TEG. To compare the performance of different TEGs, it is often more informative to compare the TEG power factor, $\phi_p$, which is defined as the power output per unit area per degree of temperature difference (Equation 3).

$$\phi_p = \frac{P_{out}}{A_{TEG} \Delta T^2} \quad (3)$$

In summary, advancing organic TEGs toward meeting real-case needs, through application-focused design, necessitates a comprehensive strategy that harmonizes compatibility with the proposed thermal gradient and application scenario, scalability, and efficiency.

As a matter of fact, some of the most successful efforts aimed at developing high-performing organic thermoelectric materials have yet to be incorporated into an exploitable TEG. Studies have consistently reported that macromolecular engineering techniques and secondary crystallization control of polymer thin films have been extremely effective at enhancing thermoelectric properties in conjugated polymers.[25-27] Tech-

niques such as solvent/vapor post-treatment,[28] strain-induced structural transformation,[29] shearing,[30,31] drawing/spinning fibers,[32] and polymer epitaxy[33,34] have been shown to induce high degrees of anisotropy in the morphology of polymer films, which can then be exploited to improve the thermoelectric performance of such materials by orders of magnitude. High-temperature rubbing (HTR) is one of the most effective macromolecular engineering techniques at aligning conjugated backbones of conducting and semiconducting polymers, thus improving the charge transport along the direction parallel to backbone alignment.[25,35] HTR applies a shear mechanical force to the surface of a thin film by way of a rotating cylinder covered with a microfiber cloth that is translated over the polymer thin film on a substrate maintained at a certain temperature, also known as the rubbing temperature, $T_R$.[36] Applying rubbing temperatures, $T_R$, close to the melting temperature of a polymer, such as in the case of regioregular poly(3-hexylthiophene) (P3HT), can lead to significantly increased crystallinity along with alignment.[37]

Key advantages of HTR with respect to other alignment methods are scalability and versatility across a wide number of materials.[25,35,36] HTR can be applied under ambient conditions to large surface areas to produce aligned films at an industrial scale. HTR is also compatible with a variety of substrates and versatile at a large temperature range to meet the needs of the specific polymer.[36] For these reasons, HTR is an ideal macromolecular engineering technique to incorporate into the design and fabrication process of application-focused TEGs. However, in the literature, there currently exist no instances of polymer-based TEGs specifically designed to utilize the superior performance benefits of highly aligned polymeric films.

This work demonstrates the integration of HTR-aligned polymer thin films in a monolithic and label-like TEG architecture, enabled by the application of local inkjet doping. Thin films of P3HT and poly(2,5-bis((7-butoxyheptyl)thiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT-8O) were aligned via the well-studied HTR technique to induce high-degrees of anisotropy in the charge transport properties of the films. The TEGs based on HTR-aligned thin films of P3HT and PBTTT-8O exhibited exceptional normalized power densities of 0.32 nW cm⁻² K⁻² and 1.01 nW cm⁻² K⁻², respectively. To demonstrate a potential real-case application of the proposed TEGs, a proof-of-concept of a thermoelectrically-powered volume indicating label is also reported. This work not only explores the untapped potential of aligned thermoelectric films in TEGs but also shows how the application-focused design of organic TEGs can leverage their potential for innovative applications in printed and flexible electronics.

## 2. Results

### 2.1. Design and Fabrication Approach

The primary goal of the design was to exploit HTR-aligned polymer films and optimize power and voltage density within a form factor suited for practical applications. To this aim, a flexible label-like architecture was devised and is depicted in Figure 1. A pristine, or undoped, aligned polymer film is deposited on a micron-thin substrate with pre-patterned electrodes, such that the direction of rubbing (white arrow labeled with "R", Figure 1) is
<figure><img src="image_3.png" /><figcaption>**Figure 1.** Depiction of the conformable aligned TEG on a hypothetical surface with an existing temperature gradient. The white arrow shows the rubbing direction during the alignment process, such that it is parallel to the direction of the heat flux. The inset to the right shows the various specifications of this architecture including the width of the *p*-type leg, *W*<sub>p</sub>, the width of the *n*-type leg (or metallic interconnection) *W*<sub>n</sub>, the width of the spacing between thermoelements, *W*<sub>i</sub>, and the length of the thermocouples, *L*.</figcaption></figure>

parallel to the direction of the expected heat flux. The TEG architecture is created by locally doping the pristine film in the regions that correspond to the thermoelements. Such a TEG could then be adhered to a complex or non-planar object and capture a heat gradient along its surface. The architecture not only fully capitalizes on the anisotropy of charge transport in the direction of the thermal gradient, but also ensures efficient heat transfer to an ultrathin label-like design. In the proposed architecture, the use of an undoped continuous film and a local doping method serve to maintain the structural integrity of the aligned film. Furthermore, the use of a monolithic TEG allows for a more versatile design, whereby all the specifications of the architecture shown in the inset of Figure 1 may be easily modified to fit any specific use case by simply changing the inkjet printed pattern. The proposed design has potential use cases in situations where 1) a complex or irregular surface could connect two different environments, like a stake connecting soil and air or a buoy exposed to both air and water or 2) a surface that is experiencing a dynamic temperature gradient due to changing volumes, such as an intravenous fluids bag or a glass of beer.

The fabrication process to achieve the architecture shown in Figure 1 is schematically illustrated in Figure 2. The first step (Figure 2a) was to prepare the aligned polymer film. The polymer solution was blade-coated onto a glass substrate that has been pretreated with a water-soluble, sodium polystyrene sulfonate (NaPSS) sacrificial layer. The polymer film was then aligned using the previously reported HTR technique in which the rubbing temperature $T_R$ for each polymer was optimized. The second step (Figure 2b) was to prepare the TEG substrate. Parylene C (Figure 3) is an ideal substrate for a conformable label-like design presented in Figure 1, due to its mechanical flexibility and the ability to precisely control the thickness to within ≈100 nm through vacuum deposition. The ultrathin parylene substrate

was deposited onto a separate glass carrier, and the gold electrodes were patterned by inkjet printing on top of the parylene substrate. In the final stages of fabrication, illustrated in Figure 2c, the undoped polymer film was floated off of the glass carrier by dissolving the NaPSS layer. This floating film was then carefully recovered onto the pre-patterned parylene TEG substrate, ensuring alignment of the rubbing direction with the intended heat flow direction. At this stage, the entire polymer film is undoped and highly electrically resistive. Localized doping of regions corresponding to the thermoelements was achieved using a dopant ink via an inkjet printer. Following this, the fabricated TEG could be delaminated from the glass carrier and transferred onto the desired surface. This fabrication method was preferred over one in which the polymer solution was deposited directly on top of the pre-patterned parylene substrate due to the likelihood that the HTR process could damage the parylene substrate and electrode during the rubbing process.

## 2.2. Validation of Local Doping Process

P3HT and PBTTT.<sup>8</sup>O (Figure 3) were chosen as two candidate conjugated polymers to be incorporated into the design presented in Figure 1. P3HT is a model semicrystalline conjugated polymer, and its electronic, optical, and morphological properties have been studied widely for applications in organic photovoltaics,<sup>[38,39]</sup> transistors,<sup>[40]</sup> and thermoelectrics.<sup>[30,41]</sup> $T_R$ of P3HT was previously optimized to reach high crystallinity (>55%).<sup>[42]</sup> Moreover, it was shown that alignment can improve the thermoelectric figure of merit, $zT$, by more than 1 order of magnitude in *p*-doped P3HT films.<sup>[43]</sup> On the other hand, PBTTT.<sup>8</sup>O was chosen because one of the most highly ordered, chain-extended crystalline microstructures observed in
<figure><img src="image_4.png" /><figcaption>**Figure 2.** a) Preparation of the aligned polymer film on a glass substrate with a sacrificial layer to easily float the film at a later stage. b) Preparation of the TEG thin film substrate and patterning of the architecture by inkjet printing. c) Combination of the aligned polymer film with the TEG substrate through a floating transfer process, and definition of the monolithic TEG architecture through local doping of the aligned film.</figcaption></figure>

organic thin films was achieved with poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT).<sup>[44]</sup> PBTTT has also proved to be amenable to HTR, and most recently it has been reported that substitution of the alkyl side chain in PBTTT with a slightly polar single ether side chain can improve the conductivity of rubbed PBTTT.<sup>8</sup>O films by more than one order of magnitude as compared to poly[2,5-bis(3-dodecylthiophen-2-yl)thieno[3,2-b]thiophene] (PBTTT-C<sub>12</sub>) after *p*-type doping.<sup>[45]</sup>

To pattern the TEG architecture, a dopant ink was optimized such that it would be compatible with an inkjet printer while still being effective at locally doping the desired regions of the aligned film. Tris(4-bromophenyl)ammoniumyl hexachloroantimonate, known as Magic Blue (MB) (Figure 3), was chosen as the oxidant, due to its high electron affinity and acceptable stabil-

ity in air,<sup>[46]</sup> a critical characteristic since the ink would need to be stable for several hours during the printing process.

Besides stabilization of jetting,<sup>[47]</sup> the orthogonality of the dopant ink with respect to the aligned polymer film had to be guaranteed. Acetonitrile is commonly used as an orthogonal solvent for sequential doping procedures since it solubilizes most small molecule dopants without solubilizing most organic semiconductors.<sup>[48,49]</sup> Acetonitrile has a boiling point (BP) of 82 °C and a room temperature viscosity of 0.342 mPa s,<sup>[50]</sup> both of which are slightly low for ideal jetting. Therefore, *n*-butyl acetate, which has a BP of 126 °C and a viscosity of 0.685 mPa s at 25 °C, was added in low-volume fractions to reduce volatility and increase the viscosity of the dopant ink. The final formulation of the acetonitrile-based ink contained 20 vol% of *n*-butyl acetate and 2.5 mg mL<sup>-1</sup> MB, allowing clean jetting and
<figure><img src="image_5.png" /><figcaption>**Figure 3.** Chemical structures of the materials used to fabricate the aligned TEGs, including the aligned polymers regioregular poly(3-hexylthiophene) (RR-P3HT) in red and poly(2,5-bis(((7-butoxyheptyl)- thiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT.<sup>8</sup>O) in orange. The *p*-type dopant, tris(4-bromophenyl)ammoniumyl hexachloroantimonate, known as Magic Blue (MB), is in blue. The flexible substrates are made of the dielectric polymer Prylene-C (in yellow) via vacuum deposition.</figcaption></figure>

minimal satellite droplet formation (Figure 4a). Printing tests verified that the dopant ink could oxidize the aligned polymer film in air, as indicated by the immediate color change from a deep red color to a grey-blue, whereas the aligned polymer structure was not disrupted, as evidenced by polarized optical microscopy (POM) (Figure 4b). Polarized ultraviolet-visible-near infrared (UV-vis-NIR) photospectroscopy was conducted on the pristine and doped films (Figure 4c,d) and confirmed that the MB dopant ink was able to bleach the neutral polymer absorption peak and induce the polaronic absorption bands in the NIR regime.[37,51] POM was also used to verify the edge resolution, that is, the extension of the boundary zone that separates the fully doped and fully pristine areas. The edge resolution defines the shortest critical features adoptable in direct writing the thermoelements. Figure 4b shows that the edge resolution was found to be ≈60 µm. In this design, the leg spacing was set at 200 µm to avoid short circuits between adjacent legs.

The electrical conductivity of the aligned P3HT and PBTTT.<sup>8</sup>O thin films was measured parallel and perpendicular to the original rubbing, upon doping with an increasing number of layers of the MB dopant ink (Figure 4e). In both cases, the number of layers of printed dopant ink has an impact on the resulting conductivity of the films, yet the marginal gains for each additional printed dopant layer diminish after ≈8 layers. Importantly, the local doping method of pre-aligned films adopted here retains anisotropy, with the electrical conductivity of the films measured perpendicular to the rubbing direction 10 times lower than the electrical conductivity measured in the parallel orientation for PBTTT.<sup>8</sup>O films, and 3 times lower for P3HT ones. Lastly, this data shows that the maximum conductivity measured for these P3HT and PBTTT.<sup>8</sup>O samples is σ = 80 S cm<sup>-1</sup> and σ = 200 S cm<sup>-1</sup>, respectively. The thermovoltage of the aligned poly-

mer films was also characterized, yielding Seebeck coefficients measured parallel to the rubbing direction of S = 21.5 µV K<sup>-1</sup> and S = 18.2 µV K<sup>-1</sup> for P3HT and PBTTT.<sup>8</sup>O, respectively.

Doping P3HT with MB was previously shown to have minimal impact on the crystal structure of P3HT films, which are largely preserved after doping. In this work, the structural evolution of the oriented films based on P3HT and PBTTT.<sup>8</sup>O, before and after doping with the dopant and solvent mixture of acetonitrile (80%) and *n*-butyl acetate (20%), was analyzed using electron diffraction (ED). The structure of the aligned P3HT film before and after doping is shown in **Figure 5a,b**, respectively. The ED patterns indicate that the P3HT film orientation remains intact after doping.

In contrast, the aligned films of PBTTT.<sup>8</sup>O exhibit significant structural changes upon doping, as shown in Figure 5c,d. The pristine PBTTT.<sup>8</sup>O films are typically polymorphic with the coexistence of a liquid crystalline phase with d<sub>100</sub> = 19.7 Å and a crystalline phase with d<sub>100</sub> = 14.5 Å.[45,52] In Figure 5c, the rubbed films have a dominant crystalline phase. After doping the aligned PBTTT.<sup>8</sup>O films, most mixed index reflections are suppressed and the ED pattern (Figure 5d) shows only three reflections corresponding to a slightly swollen liquid crystalline phase with d<sub>100</sub> = 21.4 Å. Unlike P3HT, the structure of the PBTTT.<sup>8</sup>O films is altered, suggesting partial inclusion of SbCl<sub>6</sub><sup>-</sup> counterions into the lattice of PBTTT.<sup>8</sup>O. This behavior is in strong contrast to PBTTT-C<sub>12</sub> films doped with MB, which show no structural changes upon doping with MB.[53]

This difference in doping on the structure is possibly related to the single ether groups of PBTTT.<sup>8</sup>O, which likely promote interactions and facilitate the insertion of SbCl<sub>6</sub><sup>-</sup> counterions into the side chain layers. Furthermore, it was verified that simple exposure of the crystalline PBTTT.<sup>8</sup>O films to the solvent mixture
<figure><img src="image_6.png" /><figcaption>**Figure 4.** a) Screenshot of the drop-watcher window on the Fujifilm Dimatix DMP-2850 showing the clean jetting of the MB dopant ink. b) Polarized optical micrograph of the aligned P3HT film after local doping with the MB ink. c) UV-vis-NIR spectra with polarized incident light in the directions parallel and perpendicular to the rubbing direction of the aligned P3HT films before and after doping with MB. d) UV-vis-NIR spectra with polarized incident light in the directions parallel and perpendicular to the rubbing direction of the aligned PBTTT-⁸O films before and after doping. e) Electrical conductivity was measured parallel to and perpendicular to the rubbing direction of aligned P3HT and PBTTT-⁸O films with respect to the number of inkjet printed layers of an MB dopant ink.</figcaption></figure>

does not induce any structural changes, supporting the conclusion that the observed changes result from the doping process rather than swelling caused by the solvent mixture.

## 2.3. Characterization of Monolithic TEGs Based on Aligned Films

To validate the design and fabrication process, a series of monolithic TEGs based on locally doped aligned films were fabricated according to the fabrication scheme shown in Figure 2. The devices were first fabricated with aligned P3HT films and architectures with *n* = 3, 4, and 5 thermocouples. The specifications of the architectures (Table S1, Supporting Information) were slightly adjusted for each device so that the total width did not exceed the width of the aligned film. The power outputs of the TEGs were characterized at increasing applied temperature gradients in a Δ*T* range from 5 K to 50.9 K and load resistances, *R*<sub>load</sub>, between 1 and 10⁸ Ω (Figures S1 and S2, Supporting Information). The maximum power density was found to be 0.8 µW cm<sup>-2</sup> at Δ*T* = 50.9 K for the *n* = 4 device (Figure 6d). The *V*<sub>oc</sub> and maximum *P*<sub>output</sub> at Δ*T* = 50.9 K for the same P3HT device were measured to be 5.5 mV and 0.35 nW, respectively, and an ideal linear

dependence of *V* with respect to *I* and a parabolic relationship between *P*<sub>output</sub> and *I* is observed (Figure 6a).

To quantify the benefit of using aligned P3HT films, a control device was fabricated using unaligned P3HT films. The fabrication process was identical to the aligned one, with the exception that the polymer film was not subjected to the HTR process. The unaligned P3HT *n* = 4 device was found to have a maximum power density of 0.09 µW cm<sup>-2</sup> (Figure 6e), a *V*<sub>oc</sub> of 5.0 mV (Figure 6b), and a maximum *P*<sub>output</sub> of 0.04 nW (Figure 6b) at Δ*T* = 49.8 K. The absolute power output and power density of the aligned P3HT TEG are about one order of magnitude greater than the power output and density of the unaligned TEG, demonstrating the marked benefit of employing HTR to improve performance. On the other hand, the open circuit voltages of the two devices differ by just 9%.

Having validated the use of inkjet-doped, aligned films of P3HT in TEGs, a higher-performing polymer, PBTTT-⁸O, was adopted. Monolithic PBTTT-⁸O TEGs were fabricated with aligned films in a series of different architectures that consisted of *n* = 3, 4, 6, and 10 thermocouples (Table S1, Supporting Information), and the power output was characterized under the same conditions as the P3HT TEGs (Figures S3–S6, Supporting
<figure><img src="image_7.png" /><figcaption>**Figure 5.** a) ED pattern of pristine aligned P3HT. b) ED pattern of aligned P3HT films upon doping with a solution of MB (2.5 mg mL⁻¹) in a mixture of acetonitrile (80% by volume) and *n*-butyl acetate (20% by volume). (a) ED pattern of pristine aligned PBTTT.⁸O. (b) ED pattern of aligned PBTTT.⁸O films upon doping with the same MB (2.5 mg mL⁻¹) solution. Main reflections are indexed using the known structures of P3HT and PBTTT.⁸O.</figcaption></figure>

Information). The maximum power density for the PBTTT.⁸O series of TEGs was found to be 2.7 µW cm⁻² at ΔT = 51.0 K for the *n* = 4 device (Figure 6f). The Voc and maximum Poutput at ΔT = 51.0 K for the same device were measured to be 3.5 mV and 1.2 nW, respectively (Figure 6c), and an ideal linear dependence of V with respect to I and a parabolic relationship between Poutput and I are observed. The maximum absolute power output for all characterized devices demonstrates a quadratic relationship with respect to the applied temperature gradient (Figure 6g,h) in agreement with the expected behavior (Equation 2).

Figure 6i compares the TEG power factor (Equation 3) for all fabricated modules to the measured electrical conductivity of the film used to fabricate that module. It is worth underlining that, whereas PBTTT.⁸O films result in the best-performing TEGs, they are affected by a large variance in TEG power factor and electrical conductivity. The more performing PBTTT.⁸O films produced both the maximum and minimum TEG power factors among the aligned devices, showcasing the unpredictability of the material under these specific doping conditions. On the contrary, TEGs based on P3HT films are more predictable.

The ED analysis of the films (Figure 5) showed that the crystalline structure of P3HT remained unchanged while that of

PBTTT.⁸O changed significantly after doping. The pronounced structural response to the partial inclusion of SbCl₆⁻ counterions into the lattice of PBTTT.⁸O is one potential factor that could contribute to the variability in the performance of the PBTTT.⁸O devices. The change in the structural organization of PBTTT.⁸O after doping is highly relevant in the context of a device where charge transport in the film spans centimeters (*L* = 1.0 – 1.5 cm; *n* = 4–10) and the long-range homogeneity of the rubbed film can vary slightly. In future work, the adoption of a polymer: dopant system, like PBTTT-C₁₂:MB, which shows no structural changes upon doping with MB, might be beneficial.[53]

Furthermore, the large variance seen in the performance of the PBTTT.⁸O devices can be attributed to the large, two-orders-of-magnitude degree of anisotropy in the electrical conductivity observed in the aligned PBTTT.⁸O films (Figure 4). This is significant since the aligned films are floated onto the pre-patterned substrate (Figure 2), where one can expect a small amount of mismatch between the rubbing direction and the channel direction. Therefore, the higher degree of anisotropy observed in the PBTTT.⁸O films could translate into a larger variance in performance caused by the small mismatch. Future automated fabrication procedures would largely correct such errors.
<figure><img src="image_8.png" /><figcaption>**Figure 6.** a) V (left axis) and P<sub>out</sub> (right axis) with respect to *I* for the aligned P3HT (n = 4) TEG. b) V (left axis) and P<sub>out</sub> (right axis) with respect to *I* for the unaligned control P3HT (n = 4) TEG. c) V (left axis) and P<sub>out</sub> (right axis) with respect to *I* for the aligned PBTTT-<sup>8</sup>O (n = 4) TEG. d) P<sub>density</sub> with respect to R<sub>load</sub> for the aligned P3HT (n = 4) TEG. e) P<sub>density</sub> with respect to R<sub>load</sub> for the unaligned control P3HT (n = 4) TEG. f) P<sub>density</sub> with respect to R<sub>load</sub> for the aligned PBTTT-<sup>8</sup>O (n = 4) TEG. g) Comparison of P<sub>out</sub> with respect to ΔT for the aligned and unaligned P3HT TEGs. h) Comparison of P<sub>out</sub> with respect to ΔT for the aligned PBTTT-<sup>8</sup>O TEGs. i) TEG power factor, φ<sub>p</sub>, of all TEGs fabricated in this work with respect to σ of the locally doped polymer films.</figcaption></figure>

To put the overall results into context, **Figure 7** shows the φₚ of the aligned P3HT (n = 4) and the PBTTT-⁸O (n = 4) TEGs alongside other polymer-based TEGs found in literature over the past decade (Table S2, Supporting Information). Monolithic TEGs based on aligned P3HT films already achieve a φₚ value of 0.33 nW cm⁻² K⁻², just below the best ever reported φₚ for organic TEGs, 0.48 nW cm⁻² K⁻², [54,55] whereas the best PBTTT-⁸O based devices with a φₚ of 1.04 nW cm⁻² K⁻², more than double the previous record performance.

## 2.4. A Thermoelectrically Powered Volume-Indicating Label

Here a potential use case is presented that leverages the specific characteristics of the flexible label-like design adopted for the monolithic TEG. The micron-scale dimensions in the out-of-plane direction of the ultrathin design ensure that the aligned TEG is conformable to complex or irregular surfaces, similar to a label or sticker. The planar architecture of the aligned TEG is suitable for temperature gradients that exist parallel, not radial,
<figure><img src="image_9.png" /><figcaption>**Figure 7.** TEG power factor, $\phi_p$, or normalized power density of the aligned P3HT ($n = 4$) and the PBTTT-$^8$O ($n = 4$) TEGs presented in this work (blue and red stars) compared to other organic polymer-based TEGs from literature.[11,15,54–62]</figcaption></figure>

to the complex surface. Combining these two conditions, a thermoelectrically powered volume-indicating label was devised. The design of the label follows a staircase-like architecture, where the thermocouples show increasing length (Figure 8a). More specifically, each thermocouple is elongated by a uniform amount such that the total thermocouple length is written as:

$$L_i = L_0 + (i - 1) x \quad (4)$$

where $L_i$ is the length of thermocouple $i$ in the staircase-TEG, $L_0$ is the length of the shortest thermocouple, and $x$ is the fixed increment by which each successive thermocouple in the staircase-TEG (s-TEG) with $n$ thermocouples is elongated (Figure 8b).

When the s-TEG adheres to the complex surface of a vessel that contains a liquid with variable volume, the changing volume will gradually induce a non-zero temperature gradient across each of the thermocouples. For example, if the fixed end of the device is placed toward the bottom of an intravenous blood bag that is supplying blood to a patient, then more and more of the thermocouples of the s-TEG will be exposed to a non-zero temperature gradient as the blood empties from the bag. The open-circuit voltage and power output of an s-TEG would grow incrementally until $V_{oc,max}$, and $P_{out,max}$, are observed shortly before the bag empties. Flipping the s-TEG such that the fixed end is close to the top of a vessel would be useful for applications in which overflow of a filling vessel should be avoided.

To demonstrate the feasibility of this potential use case, an s-TEG was fabricated using the same methods described above. Considering the necessity for an application-focused TEG to be reproducible, the large variance observed at this stage in the PBTTT-$^8$O modules is not ideal. Therefore, an aligned P3HT film

was used for this proof-of-concept device, since the aligned P3HT TEGs from the previous section exhibited, besides a smaller variance in device-to-device performances, a larger open circuit voltage. The architecture of the aligned TEG was modified to have $n = 3$ thermocouples of different lengths. Furthermore, the width of each thermocouple was adjusted such that each thermocouple would have an equivalent resistance to avoid uneven power dissipation (architecture specifications found in Table S2, Supporting Information). The s-TEG was then delaminated from the glass carrier and adhered to the curved surface of a laboratory vial (Figure 8c), demonstrating that the label-like design is both mechanically robust and flexible as a free-standing film.

To validate this proof-of-concept volume indicator, the transient open circuit voltage of the staircase-like device was measured while the vial was filled and drained with water pre-heated to 50 °C at constant intervals. Pre-heated water was used in this proof-of-principle test to induce a greater temperature gradient and obtain an easily readable signal out of a small device, but a s-TEG with a larger number of thermocouples $n$ would have a greater sensitivity, enabling it to monitor liquids approaching room temperature. The transient voltage curve of the s-TEG (Figure 8d) shows four distinct regions indicated with Roman numerals I–IV in the plot. As depicted by the device insets, Region I corresponds to a growing voltage, as the pre-heated water approaches the fixed end of all three thermocouples. At the crossover from Region I to Region II, voltage is at a maximum since all three legs are exposed to a temperature gradient, although the magnitude of this temperature gradient is different for each thermocouple.

In Region II the voltage decreases as the temperature gradient is gradually reduced across all thermocouples in time, starting with the shortest and ending with the longest. The crossover from Region II to Region III shows that, when the vial is full and there is no significant temperature gradient across any of the thermocouples, the voltage is close to zero. Region III represents a draining of the vial, corresponding to an increasing voltage as more thermocouples are exposed to the temperature gradient caused by the interface between the air and the heated liquid, although the residual heat in the glass vial reduces the magnitude of the voltage when compared to Region II. Similarly, Region IV mimics Region I in that the volume of liquid is below the fixed end of the s-TEG, but again residual heat in the glass reduces the magnitude of the measured voltage. This proof-of-concept of a thermoelectrically-powered volume indicator shows that engineering of the concept could be exploited in the future to reliably estimate the volume, temperature, or other physical properties such as heat capacity or thermal conductivity in such an application. Ultimately, this proof-of-concept staircase TEG is an effective demonstration that the TEG fabrication process based on locally doped, aligned films presented here can be translated into interesting use cases that capitalize on the conformable design and planar architecture.

### 3. Conclusion

This work describes an application-focused design and fabrication approach to leverage the highly improved thermoelectric properties of aligned polymeric films into monolithic organic TEGs. Thin films of regioregular P3HT and PBTTT-$^8$O were
<figure><img src="image_10.png" /><figcaption>**Figure 8.** a) Depiction of the staircase-like architecture of a flexible TEG that can be used to indicate the changing volume of a liquid in a vessel. b) Close-up of the staircase-like architecture that illustrates the difference in the length of each thermocouple. c) Image of the P3HT s-TEG adhered to a glass vial. d) Transient voltage plot of a filling and draining test demonstrating the operating principle of the device as a volume indicator.</figcaption></figure>

aligned via high-temperature rubbing to induce high degrees of anisotropy in the charge transport properties of the films. An optimized multi-stage fabrication technique was developed to integrate aligned, continuous, pristine films into a flexible label-like TEG architecture, where thermoelements were directly written by the application of the local inkjet doping process. The TEGs based on aligned P3HT and PBTTT-⁸O films exhibited exceptional TEG power factors of 0.32 nW cm⁻² K⁻² and 1.01 nW cm⁻² K⁻², respectively. Finally, a thermoelectrically-powered volume indicating label is presented as a potential use case for these aligned TEGs. A proof-of-concept device is fabricated according to a staircase TEG architecture, after which the TEG is transferred onto the non-planar surface of a cylindric container to validate the working principle upon filling and emptying with heated water and demonstrate the flexibility and robustness of the device. The work not only explores the untapped potential of aligned thermoelectric films in TEGs but also sets a precedent for application-focused design in thermoelectric applications, specifically for self-powered sensors, potentially paving the way for enhanced efficiency and effectiveness.

## 4. Experimental Section

**Alignment of Polymer Films:** Regio-regular P3HT was purchased from Ossila (M~w~ = 60.2 kDa, M~n~ = 28.7 kDa, and RR = 97.6%). PBTTT-⁸O was synthesized according to the previously published protocol.[45] The batch used in this study exhibits a molar mass of 37.5 kg.mol⁻¹ and a dispersity

D of 1.26. Polymer solutions were prepared with the purchased P3HT and synthesized PBTTT-⁸O using ortho-dichlorobenzene as a solvent. Polymer thin films were deposited by the doctor blade method from 20 mg mL⁻¹ hot polymer solution on hot substrates maintained at 155 °C. The substrates consist of clean glass slides coated with a sacrificial spin-coated NaPSS film (10 mg mL⁻¹ aqueous solution at 3000 RPM). Aligned polymer films were prepared using a homemade setup for high-temperature rubbing (HTR). It consists of a rotating cylinder covered with a microfiber cloth and a heatable translation stage. Both P3HT and PBTTT-⁸O thin films were oriented with a cylinder speed of 600 RPM and the applied pressure was in the range 2–3 bar. The optimized rubbing temperatures are 200 °C for P3HT and 170 °C for PBTTT-⁸O. 12–15 oriented films for each polymer were prepared and used for this study.

**Dopant Ink Preparation:** MB was purchased from Sigma-Aldrich and mixed with anhydrous (99%) acetonitrile and n-butyl acetate. The volumetric mixture consisted of 80% acetonitrile and 20% n-butyl acetate, and the concentration of the dopant with respect to the solvent mixture was 2.5 mg mL⁻¹. The dopant ink was found to have limited stability in the air (the initial dark blue of the ink fading to completely clear over the course of 2–3 days when stored in the air). Thus the ink was always freshly prepared before local doping of the TEGs.

**Characterization of Locally Doped Aligned Films:** A Varian Cary 5000 spectrometer with polarized incident light (spectral resolution of 1 nm) was used to probe the level of film orientation and the effect of film doping with MB (350–2500 nm). The angle of light polarization was measured with respect to the rubbing direction (0° corresponding to the light polarization POL||R and 90° corresponding to the light polarization POL⊥R).

Electron diffraction by transmission electron microscopy (TEM) was performed on a TECNAI 200 FEI microscope equipped with an Eagle 2k CCD camera. Beam exposure was minimized using the low dose
system in particular to prevent de-doping of the thin films. Doping of the grids was performed in a Jacomex glovebox under N₂ atmosphere (<1 ppm O₂ and <1 ppm H₂O) by dipping the TEM grids supporting the aligned polymer films into solutions of the dopant in a mixture of acetonitrile (80%) and n-butyl-acetate (20%) for 1 min. After extraction from the solutions, the films were blotted with filter paper to remove the excess dopant.

The electrical characteristics in the saturation current regime were determined using a two-point contact method with the aid of a probe station (Wentworth Laboratories) linked to an Agilent B1500A semiconductordevice parameter analyzer. These measurements were taken at ambient temperature within a nitrogen-filled glovebox. To ensure the absence of hysteresis, both forward and reverse scans of the current-voltage (I-V) profiles were conducted. The calculated electrical conductivity was derived from the resistance, which was in turn deduced from the I-V curves, taking into account the shape and size of the samples. For the assessment of the in-plane Seebeck coefficient, a specially designed setup for thin film analysis was utilized, as detailed in the study by Beretta et al.[63] These evaluations were carried out at ambient temperature in a vacuum environment (10⁻⁴ mbar) to minimize the effects of convection and prevent the oxidation of the samples. Only thin film samples exhibiting an electrical resistance of less than 10 MΩ were eligible for measurement due to the constraints of the equipment used.

**Fabrication of Aligned TEGs:** Glass microscope slides were cleaned in an ultrasonic bath of Milli-Q water, acetone, and isopropyl alcohol (10 min of sonication in each solvent). Then, the glass slides were exposed to O₂ plasma at 100 W for 10 min. A 2% aqueous solution of soap was spin-coated onto the glass slides to assist with the delamination of the parylene substrate from the glass carrier. An insulating layer of parylene with a thickness between 1.5 and 2.0 µm was deposited on the glass slides by chemical vapor deposition (CVD). Then the electrodes were printed with an Au nanoparticle ink, supplied by Sigma Aldrich, printed at a plate temperature of 60 °C using a Fujifilm Dimatix Materials Printer DMP-2831 and the electrodes were annealed at 120 °C for 30 min. The NaPSS-treated glass slides carrying the aligned polymer films were then slowly submerged in a bath of deionized water, such that the NaPSS dissolved and the monolithic film floated on the surface. The glass carriers with the parylene substrate and Au electrodes were carefully submerged underneath the floating film and extracted so that the aligned films adhered to the substrate, making sure that the direction of rubbing matched with the direction of the thermocouples. The polymer film and substrate were allowed to dry at room temperature overnight. To dope the aligned films the same Fujifilm Dimatix Materials Printer DMP-2831 was used to print 8 layers of the MB ink on top of the film. Completed TEGs were stored overnight in a nitrogen-filled glovebox, and characterized the following day.

**Characterization of Aligned TEGs:** Type T thermocouples were attached directly on top of the TEG at either end of the printed thermoelements to measure the temperature gradient. One end of the TEG was then fixed to a Linkam THMS 600 stage while the other end was free-floating. The electrodes were connected to an Agilent B2912A Precision Source/Measurement Unit controlled by custom control software such that a current sweep was applied and the voltage was recorded. The temperature of the Linkam stage was increased until the temperature gradient measured by the type T thermocouples was stable for at least 60 s, after which the current sweep was initiated. The target temperature gradients for each device were ΔT = 5, 10, 20, 30, 40, and 50 K, but the true temperature gradient (±0.1 K) from the thermocouples was recorded for each measurement. All characterizations were conducted at room temperature in the air. The stability of the semiconductor:dopant systems in air were found to be limited to 2–3 days; therefore, all fabrication and characterization were completed within a 24-h window. When not in use, devices were stored in a nitrogen-filled glovebox to preserve their integrity.

**Fabrication and Testing of Staircase TEG:** Fabrication of the staircase TEG followed the same procedure as described above, with the exception that the printed Au electrodes and local doping followed the architecture depicted in Figure 6a. The s-TEG was delaminated from the glass carrier

and adhered to a glass laboratory vial for the filling/draining test. Type T thermocouples were attached to the vial such that they measured the temperature gradient across the longest printed thermoelement. Thus, the temperature gradient measured and reported in Figure 6d is representative of just one thermoelement. Water was preheated and maintained at 50 °C, while the vial and the TEG were allowed to reach thermal equilibrium with the atmosphere. The open-circuit voltage over time of the s-TEG was measured using the same Agilent B2912A Precision Source/Measurement Unit and software, beginning with a 120-s baseline followed by the addition of 1 mL of heated water every 30 s. After 18 mL of water was added such that the volume of water inside the vial fully covered all thermoelements of the s-TEG, a 60-s baseline was recorded before the water was withdrawn from the vial at a rate of 1 mL every 30 s.

## Supporting Information

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

## Acknowledgements

This project has received funding from the research and innovation program under the Marie Skłodowska-Curie grant agreement No 955837 – HORATES. The authors thank Prof. Tobias Cramer for sharing the software for the device characterization and recording. Technical support from the Transmission Electron Microscopy platform (M. Schmutz) is gratefully acknowledged. This work is part of the IIT Flagship project on Technologies for Sustainability.

Open access publishing facilitated by Istituto Italiano di Tecnologia, as part of the Wiley - CRUI-CARE agreement.

## Conflict of Interest

The authors declare no conflict of interest.

## Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

## Keywords

energy harvesting, flexible electronics, organic semiconductors, organic thermoelectrics, thermoelectric generators

Received: October 8, 2024
Revised: December 19, 2024
Published online:

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