Contact/Noncontact‐Mode Thermoelectric Characteristics of Polytriarylamine/Lewis Acid Complex Films in Horizontal Device Geometry

Herein, the thermoelectric characteristics of polytriarylamine‐Lewis acid complex films were investigated by employing a horizontal device structure. Poly[N,N′‐bis(4‐butylphenyl)‐N,N′‐bis(phenyl)benzidine] (PolyTPD) is doped with tris(pentafluorophenyl)borane (BCF) via Lewis acid–base reactions by varying the BCF molar ratio (0–300 mol%). The resulting PolyTPD:BCF films are spun on glass substrates and silver electrodes are deposited leading to the horizontal type of organic thermoelectric devices (OTEDs). The OTEDs with the PolyTPD:BCF films are examined by varying temperature differences up to 45 K between two silver electrodes directly contacting hot/cold sources. Both device current and voltage are proportionally increased with the temperature difference, leading to higher powers at larger temperature differences, irrespective of BCF molar ratio. However, the highest current is achieved at 50 mol% owing to the highest electrical conductivity, even though the device voltage is slightly lower at 50 than 20 mol%. The origin of high electrical conductivity is assigned to the formation of radical cations in PolyTPD chains by BCF doping, which is influenced by the reaction time. The device current can be also generated by the illumination of IR radiation (noncontact mode) that is away from the OTEDs with the PolyTPD:BCF films.

DOI: 10.1002/aesr.202300009 Herein, the thermoelectric characteristics of polytriarylamine-Lewis acid complex films were investigated by employing a horizontal device structure. Poly[N,N 0 -bis(4-butylphenyl)-N,N 0 -bis(phenyl)benzidine] (PolyTPD) is doped with tris(pentafluorophenyl)borane (BCF) via Lewis acid-base reactions by varying the BCF molar ratio (0-300 mol%). The resulting PolyTPD:BCF films are spun on glass substrates and silver electrodes are deposited leading to the horizontal type of organic thermoelectric devices (OTEDs). The OTEDs with the PolyTPD:BCF films are examined by varying temperature differences up to 45 K between two silver electrodes directly contacting hot/cold sources. Both device current and voltage are proportionally increased with the temperature difference, leading to higher powers at larger temperature differences, irrespective of BCF molar ratio. However, the highest current is achieved at 50 mol% owing to the highest electrical conductivity, even though the device voltage is slightly lower at 50 than 20 mol%. The origin of high electrical conductivity is assigned to the formation of radical cations in PolyTPD chains by BCF doping, which is influenced by the reaction time. The device current can be also generated by the illumination of IR radiation (noncontact mode) that is away from the OTEDs with the PolyTPD:BCF films.
soluble in organic solvents, such as poly(3-hexylthiophene) (P3HT), poly [2,5-bis(3-tetradecylthiophen-2-yl)thieno [3,2-b]thiophene] (PBTTT), poly(diketopyrrolopyrrole) (PDPP), etc., have recently been revisited by employing various chemical-doping techniques. [35][36][37][38] It has been reported that the thermoelectric performances of OTEDs have been improved by introducing a small molecular acceptor, hexafluorotetracyanonaphthoquinodimethane (F 6 TCNNQ), to the PBTTT polymer via chain orientation process of high-temperature rubbing. [39,40] However, less attention has been paid to the characteristics of OTEDs with triarylamine-based polymers of which electrical conductivity can be enhanced by Lewis acid-doping methods. [41][42][43] In this work, we tried to investigate the thermoelectric characteristics of poly[N,N 0 -bis(4-butylphenyl)-N,N 0 -bis(phenyl)benzidine] (PolyTPD) which is doped with tris(pentafluorophenyl) borane (BCF) at various doping ratios. The OTEDs with a horizontal geometry were fabricated to examine thermoelectric performances by forming two parallel silver (Ag) electrodes on the BCF-doped PolyTPD (PolyTPD:BCF) films. Results showed that the PolyTPD:BCF films could generate electricity at all the BCF-doping ratios when the temperature gap was varied up to 45 K. In particular, the OTEDs with the PolyTPD:BCF films could absorb IR radiation from a remote IR emitter in a noncontact mode leading to electricity generation.

Results and Discussion
The doping reaction between PolyTPD and BCF was carried out by varying the BCF molar ratio (R BCF ), which is based on the molar ratio of BCF to PolyTPD repeating unit, up to 300 mol% (see the reaction scheme in Figure 1a). As observed from Figure 1b, the solution color was changed by the presence of BCF after 72 h. The higher the BCF molar ratio, the darker the color of PolyTPD:BCF solutions. This result supports the occurrence of reactions between PolyTPD and BCF in the broad range of BCF molar ratios (20-300 mol%). As seen from the optical absorption spectra of films in Figure 1c, the BCF-doping reaction led to the formation of new broad absorption peaks in the wavelength (λ) range between %1000 and 3000 nm. The intensity of this broad peak was low at the low BCF molar ratio (R BCF = 20 mol%) but was remarkably enhanced at  R BCF = 50 mol%. No further increase in the peak intensity was observed at the higher BCF molar ratios (R BCF = 100-300 mol%). However, a slight blueshift in the optical spectra was measured and pronounced at the excess molar ratios of BCF to PolyTPD (200 and 300 mol%). This blueshift in optical absorption spectra can be ascribed to the reduced interchain interaction by the dilution role of excess BCF molecules in the films even though the exact reason should be confirmed by further in-depth spectroscopic studies. Note that some aggregations were observed in the spin-coated films with the higher BCF contents (200 and 300 mol%) in the case of short reaction time (≤24 h) (see optical microscopic images in Figure S1, Supporting Information). After spin-coating the PolyTPD:BCF films on glass substrates, a horizontal-type OTED was fabricated by depositing Ag electrodes on the films (see Figure 1 d). For the measurement of thermoelectric performances, as shown in Figure 1e, one Ag electrode was contacted by a hot source and another by a cold source. As shown in Figure 2a, in the case of the pristine PolyTPD films, the horizontal current (I H ) of devices was almost negligibly changed with the temperature difference (ΔT) (see Figure S2, Supporting Information, and Table 1). In contrast, a gradual increase of device current was measured for the OTEDs with the PolyTPD:BCF films. The most pronounced change in device current was obtained at R BCF = 50 mol%, while the highest BCF content (R BCF = 300 mol%) led to a largely reduced device current compared to other BCF contents. This result implies that the whole part of PolyTPD chains might not be fully doped with BCF molecules even at the higher BCF molar ratios (R BCF = 100-300 mol%). Consequently, the undoped BCF molecules that remained in the films could interrupt the charge transport between the BCF-doped PolyTPD chains, leading to the gradually reduced device current with the increasing BCF molar ratio after 50 mol%. Note that the pristine BCF film did not exhibit a meaningful thermoelectric performance (see Figure S3, Supporting Information). Interestingly, as shown in Figure 2b, the largest change in device voltage (V H ) was measured for the OTEDs with the pristine PolyTPD films compared to the PolyTPD:BCF films. Considering that the device voltage is basically formed by the difference in density of thermally activated charge carriers, this result may inform that the pristine PolyTPD films are able to thermally generate more charges than the PolyTPD:BCF films under the thermal gradient condition. However, the poor electrical conductivity (σ = 0.6 μS cm À1 ) of the pristine PolyTPD films, compared to the PolyTPD:BCF films (σ = %260-1470 μS cm À1 ) (see Table 1), might restrict charge transport leading to the confinement of thermally generated charges in the Ag electrode region. As replotted in Figure S4, Supporting Information, the device voltage almost linearly increased with the temperature difference for the OTEDs with the PolyTPD:BCF films. This reflects that the generation of charge carriers in the PolyTPD:BCF films is linearly influenced by temperature change according to the Seebeck effect (S = ΔV/ΔT ). Here, it is worth noting that the highest slope in voltage increase was measured for the OTEDs with the PolyTPD:BCF films at R BCF = 20 mol% (the detailed trend will be discussed again later). As a result, the power (P H ) of OTEDs also gradually and nonlinearly increased with the temperature change owing to the influence of both device current and voltage (Figure 2c). The highest power of %0.25 pW (at ΔT = 40 K) was produced at R BCF = 50 mol% ( Figure 2c and Table 1).
The detailed trend of device parameters at ΔT = 40 K is plotted as a function of BCF molar ratio. As shown in Figure 3 (top panel), the device current quickly increased at R BCF = 20 mol% and a further increase was made at Note that the data points were averaged from more than ten devices. R BCF = 50 mol%. Then, the device current was slowly reduced by adding more BCF. In contrast, as expected from the aforementioned (Figure 2b), the device voltage quickly dropped initially at R BCF = 20 mol% and slowly but gradually decreased with the BCF content. The resulting device power, interestingly, followed a similar behavior with the device current and exhibited a maximum at R BCF = 50 mol%. This result implies that the device current is a governing factor in the present OTEDs. As a consequence, the highest power factor (PF) of %1.6 nW m À1 K À2 was achieved at R BCF = 50 mol%, even though it needs to be further improved via various posttreatment processes such as fibrilization of conducting polymers and compositization with CNTs. [44,45] The highest Seebeck coefficient (S) was obtained for the pristine PolyTPD films, which can be attributed to the highest device voltage as discussed earlier. As the BCF content increased, the Seebeck coefficient was gradually reduced in the case of PolyTPD:BCF films owing to the trend of device voltage (see Figure S4, Supporting Information). However, the electrical conductivity (σ) trend was almost contradictory to the Seebeck coefficient. This result supports that BCF doping could enhance the electrical conductivity of PolyTPD:BCF films but might limit the number of effective sites in PolyTPD chains for thermal activation leading to charge-carrier generation. Here, it is noted that the exact figure of merit (ZT) factor could not be obtained because the thermal conductivity of PolyTPD:BCF films was not properly measured owing to their thin states, etc.
Considering that the thermal conductivity of %0.1 W m À1 K À1 has been reported for several organic materials including chemically doped polyaniline, the ZT factor can be assumed 4.7 Â 10 À6 for the PolyTPD:BCF films (50 mol%). [46][47][48] To understand the influence of BCF doping on the thermoelectric performances, the PolyTPD:BCF films were investigated with X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and electron spin resonance (ESR) spectroscopy. As shown in Figure 4a (top panel), the C1s spectra confirmed the presence of BCF molecules in the PolyTPD:BCF films when it comes to the C1s peak for C-F bonds at a binding energy of % 287.5 eV. This is clearly supported by the F1s peak at 687.5 eV (see the middle panel in Figure 4a). A close look into each spectrum finds a shift in both C1s and F1s peaks, depending on the BCF content, which corresponded to the C-F bonds (see Figure S5, Supporting Information). This result supports that the atom environment of C-F bonds in BCF molecules could be affected by the degree of doping states even though it is due to the unreacted BCF molecules at R BCF = 300 mol% (an excess amount of BCF to PolyTPD). In particular, as observed in Figure 4a (bottom panel), the N1s peak (C-N bonds) revealed the evolution of a new shoulder peak (402 eV) that can be corresponded to a radical cation state of nitrogen atoms. This result delivers that single electrons could be formed in the nitrogen atoms of PolyTPD chains by BCF doping, which is confirmed by the ESR spectra in Figure 4b (no meaningful ESR signal was measured from the pristine BCF film (see Figure S6, Supporting Information)). Note that the ESR intensity was relatively stronger at R BCF = 300 mol% than R BCF = 50 mol%, indicative of more radical cations formed in the case of higher BCF contents. In terms of surface morphology, no peculiar nanostructure was measured in the presence of some holes and slightly reduced surface roughness in the PolyTPD:BCF films. Summarizing all the information provided earlier, it is considered that the thermally generated charges in the hot side of Ag electrodes transport through the PolyTPD:BCF films to the cold side of Ag electrodes by a driving force of charge-carrier density (see Figure 4d).
Next, the time effect of doping reactions was briefly studied by varying the reaction time from 0.1 to 72 h at R BCF = 50 mol%. As shown in Figure 5a (top panel), the increasing device current with the temperature change was measured for all OTEDs with the PolyTPD:BCF films irrespective of reaction time. However, the slope of the device current increase was slightly steeper as the reaction time became longer. In contrast, the device voltage showed almost identical shapes to each other, except for a small gap in the range of temperature difference below ΔT = 25 K at 72 h. This result indicates that the longer reaction time could result in more doping of BCF molecules to PolyTPD chains (see the color change of solutions in Figure S7, Supporting Information), leading to slightly increased electrical conductivity, but the thermal activation sites might not be seriously changed at R BCF = 50 mol% as can be observed from the almost similar Seebeck coefficient with the reaction time (see Figure 5b top). As displayed in Figure 5b (bottom panel), the device power and PF slightly increased with the reaction time, which can  be attributed to the role of device current (conductivity increase). The higher ESR intensity at 72 h than 1 h supports the increased doping reaction of BCF to PolyTPD chains (see Figure 5c). Here, it is noted from the AFM images in Figure 5d that the surface of PolyTPD:BCF films became relatively smoother at 72 h than at 1 h. The different surface morphology can be ascribed to the  www.advancedsciencenews.com www.advenergysustres.com varied amount of doped phases with time in the PolyTPD:BCF films. [49] Finally, the OTEDs with the PolyTPD:BCF films were tested under the illumination of IR radiation (λ = 1 μm % thermal range) which is away and not directly attached to the devices (noncontact mode). As illustrated in Figure 6a, the half of device front was blocked by a mask and the rest half part was exposed to the IR emitter. Upon the IR illumination, the devices with the pristine PolyTPD films did not show a recognizable signal but the OTEDs with the PolyTPD:BCF films delivered clear signals irrespective of BCF content (see Figure 6b). This result basically confirms that the illumination of IR radiation could warm up the exposed part of PolyTPD:BCF films in the devices. Interestingly, the signal intensity (drain current) was dependent on the BCF content under the same IR intensity. The highest signal intensity was measured at R BCF = 50 mol%, which is in good agreement with the trend of thermoelectric performance (see Figure 2 and 3). A close look into the signals finds that the drain current gradually increased with time upon continuous illumination of IR radiation (see Figure S8, Supporting Information, for comparison). As shown in Figure 6c, upon on/off modulation of IR radiation, the intensity of drain current signals was influenced by the distance between the IR emitter and the OTEDs with the PolyTPD:BCF films (R BCF = 50 mol%). This result can be explained by the varied temperature in the PolyTPD:BCF films of devices according to the IR emitter-to-device distance. Therefore, it is briefly summarized that the present OTEDs with the PolyTPD:BCF films can generate electricity even in a noncontact mode of IR source.

Conclusion
The thermoelectric characteristics of PolyTPD:BCF films were investigated by varying the doping ratio and reaction time of BCF to PolyTPD. The PolyTPD:BCF films showed a broad absorption in the IR range of 1000-3300 nm, leading to a varied film color depending on the BCF content. The horizontal-type OTEDs with the PolyTPD:BCF films delivered a gradual increase in current and voltage upon temperature rise despite a negligible change in device current in the case of pristine PolyTPD films. The highest power and PF (PF = 1.587 nW m À1 K À1 ) could be achieved at R BCF = 50 mol%, which can be attributed to the highest electrical conductivity at this BCF content. However, the Seebeck coefficient was measured %104 μV K À1 at R BCF = 50 mol%, which is lower than 664.68 μV K À1 at R BCF = 0 mol%, owing to the low device voltage. The XPS analysis uncovered that radical cations were formed in the nitrogen atoms (tertiary amines) of PolyTPD by the doping reaction of BCF molecules, which was confirmed by the ESR measurement. The surface roughness of PolyTPD:BCF films was slightly reduced as the BCF content increased up to 300 mol%. As the reaction time increased, the device current was slightly increased but almost no change was measured for the device voltage in the case of PolyTPD:BCF films at a fixed BCF content (R BCF = 50 mol%). The OTEDs with the PolyTPD:BCF films could generate charges under the illumination of IR radiation in a noncontact mode. Finally, it is expected that the present study on the OTEDs with the PolyTPD:BCF films can open a new gate for thermoelectric applications of nonconjugated polymer materials. Figure 6. a) Illustration for the illumination test of IR radiation to the OTEDs with the PolyTPD:BCF films: "L" denotes the distance from the IR emitter to the devices, while the mask plays a blocking role for the IR radiation. b) Change of device current for the OTEDs with the PolyTPD:BCF films according to the BCF molar ratio (L = 7 cm). c) Change of device current upon repetitive illumination of IR radiation to the OTEDs (R BCF = 50 mol%): note that L and IR on/off duration times were varied to examine the current change upon modulation test.

Experimental Section
Solutions and Device Fabrication: PolyTPD (weight-average molecular weight = 24 kDa) and BCF were purchased from Luminescence Technology Corp. (Lumtec, Taiwan) and Sigma-Aldrich (USA), respectively. The solutions of PolyTPD and BCF were prepared using chlorobenzene as a solvent by varying the BCF molar ratios (0-300 mol%) to PolyTPD (20 mg mL À1 ). These solutions with various BCF molar ratios were stirred for doping reactions at 25°C for 72 h. To study the influence of reaction time, the main solutions of PolyTPD and BCF (50 mol%) were initially prepared at 25°C, followed by taking out some parts for spin-coating processes. Prior to the spin-coating steps, glass substrates (size = 12 Â 12 mm 2 , thickness = 1.1 mm) were subjected to wet-cleaning processes using acetone and isopropyl alcohol in an ultrasonic bath. The wet-cleaned glass substrates were treated under a UVozone environment (28 mW cm À2 ) for 20 min. On the UV-ozone-treated glass substrates, the PolyTPD:BCF solutions were spun at 1200 rpm for 60 s and dried at 70°C for 2 h (thickness = %87 nm). The PolyTPD:BCF film-coated substrates were moved into a thermal evaporator system equipped inside a nitrogen-filled glove box. Next, the Ag electrodes (60 nm thick) were deposited on the PolyTPD:BCF films when the base pressure of the chamber reached %1 Â 10 À6 torr (see Figure 1d). Note that the PolyTPD:BCF films were coated on quartz substrates for the measurement of optical absorption.
Measurements and Characterizations: The thermoelectric performances were measured using a home-built thermoelectric evaluation system with thermal sensors, an electrometer (6517, Keithley Instruments Inc.), and heat (hot/cold) sources (Cu blocks on Peltier devices, 10 Â 10 mm 2 , TEC1-0706, PTHAUS). The heat sources were controlled by a power supply (SPD1168X, Siglent), while the real-time temperature was measured by a temperature control module (NI-9211, National Instruments, LLC) that is connected to thermocouple sensors. A UV-Visible-IR spectrometer (Lambda 750, PerkinElmer) was used to measure the absorption spectra of solutions and films. An optical microscope (SV-55, Sometech) was used to examine the PolyTPD:BCF films on a microscale. An X-ray photoelectron spectrometer (HPXPS, ESCALAB 250, Thermo Scientific, Inc.) was used to measure the core level atom environments of PolyTPD:BCF films. A scanning probe microscope (NX20, Park Systems) and ESR spectrometer (EMXplus-9.5/2.7, Bruker) were employed for the measurement of film surfaces (AFM images) and single electrons, respectively. An air-cooled IR emitter (ASB-IR-12 K, 11 W, Korea Spectral Products) was used as an IR source to examine the noncontact mode performance of OTEDs.

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