Sequential Doping of Carbon Nanotube Wrapped by Conjugated Polymer for Highly Conductive Platform and Thermoelectric Application

Doping of conjugated polymers (CPs) is a promising strategy to obtain solution‐processable and highly conductive films; however, the improvement in electrical conductivity is limited owing to the relatively poor carrier mobility of CPs. Herein, a CP with excellent molecular doping ability, i.e., poly[2‐([2,2'‐bithiophen]‐5‐yl)‐3,8‐difluoro‐5,10‐bis(5‐octylpentadecyl)‐5,10‐dihydroindolo[3,2‐b]indole] (PIDF‐BT) is wrapped onto the surface of single‐walled carbon nanotubes (SWCNTs). The resulting PIDF‐BT@SWCNT simultaneously achieves excellent solution dispersibility and a high electrical conductivity of over 5000 S cm−1 through AuCl3 doping. The doping mechanism is systematically studied using spectroscopic analysis, and the four‐probe field‐effect transistor based on the doped PIDF‐BT@SWCNT confirms a carrier mobility up to 138 cm2 V−1 s−1. The carrier‐transfer barrier energy is related to the Schottky barrier between the SWCNT and PIDF‐BT, which can be controlled by doping. Finally, when the doped PIDF‐BT@SWCNT is applied to a thermoelectric device, a power factor exceeding 210 μW m−1 K−2 is achieved because of its high electrical conductivity, even if the increased carrier density reduces the Seebeck coefficient.


Introduction
[19] However, the electrical conductivity of CP remains unsatisfactory, lower than some highly conductive ceramics such as MXene, let alone metals.Theoretically, carrier density and mobility equally contribute to the conductivity.For highly conductive ceramics, their carrier density and mobility can reach 10 21 -10 22 cm À3 and 250-300 cm 2 V À1 s À1 , respectively. [20,21][24] Therefore, the poor electrical conductivity of the doped CPs is attributed to their low mobility rather than their charge-carrier density.
Carbon nanotubes (CNTs) have excellent electrical properties with an intrinsic carrier mobility of up to 10 5 cm 2 V À1 s À1 , [25] but their strong cohesive interaction limits the solution processability.To obtain a uniformly dispersed CNT solution, dispersants have been used to wrap the CNTs, but they may increase the interfacial resistance between the CNTs and deteriorate their electrical properties. [26,27]Wrapping CNT with semiconducting CPs can effectively avoid a decrease in electrical properties.[30][31] Although studies related to CP-wrapped CNTs have primarily centered on enhancing their semiconductor properties, the electrical properties could be tailored by adjusting the applied CPs.In particular, CP-wrapped CNTs prepared using CP with high doping ability are expected to be excellent conductive platforms through molecular doping due to the high mobility of CNTs and abundant charge carriers generated by CP doping.In this contribution, we demonstrated that when single-walled CNTs (SWCNTs) were wrapped by a CP with good doping ability, the electrical conductivity of resulting CP-wrapped SWCNT can be dramatically improved up to 5000 S cm À1 via doping with conventional molecular dopants, such as 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquino-dimethane (F4TCNQ) and AuCl 3 .The doped CP-wrapped SWCNT exhibited excellent dispersibility, which was applicable to prepare uniform thin films.The doping mechanism was explored using X-ray photoelectron spectroscopy (XPS), confirming that doping of the CP chains could significantly increase the electrical conductivity.In addition, a four-probe FET fabricated with the CP-wrapped SWCNT revealed that the charge-carrier mobility was sensitively affected by the doping level.Finally, it was confirmed that the CP-wrapped SWCNT exhibited excellent thermoelectric performance, which could be further improved by molecular doping.

Results and Discussion
CPs containing indoloindole (IDID) moieties exhibit excellent doping abilities through efficient intermolecular charge transfer to the molecular dopants. [22,32]As shown in Figure 1a Because the electrical properties of SWCNTs strongly depend on their structural heterogeneity, a commercialized SWCNT (TUBALL, diameter ≤ 2 nm), which contains semiconducting and metallic SWCNTs in a 2:1 ratio, was adopted to ensure the versatility of this study.[35] Briefly, tip sonication of SWCNTs and PIDF-BT in o-dichlorobenzene (DCB) spontaneously formed PIDF-BT@SWCNT soluble in DCB.Then, the massive SWCNT aggregates were removed by centrifugation.The DCB solution was filtered with a nylon membrane to remove excess PIDF-BT, and the membrane was washed several times with warm DCB to eliminate the unwrapped PIDF-BT.The membrane filter was mechanically separated to collect free-standing PIDF-BT@SWCNT films, which were redispersed in DCB for characterization and film preparation (Figure 1a).
When the absorption of the redispersed PIDF-BT@SWCNT was analyzed, a clear absorption peak was observed between 400 and 600 nm, which was nearly identical to that of the PIDF-BT film except for an additional redshift of about 10 nm (Figure S1, Supporting Information).In addition, transmission electron microscopy revealed that the SWCNT surface was surrounded by PIDF-BT (Figure 1b), and the elements in PDIF-BT were uniformly distributed along the SWCNT (Figure S2, Supporting Information), confirming that PIDF-BT was successfully wrapped on the SWCNT surface.In addition, when the contents of PIDF-BT@SWCNT were quantified by thermogravimetric analysis, the weight ratio of PIDF-BT to SWCNT in PIDF-BT@SWCNT was %69:31 (Figure S3, Supporting Information).Then, thin films having thickness of about 100 nm was prepared by spin casting the PIDF-BT@SWCNT redispersed in DCB:chloroform (1:2, 3.0 mg ml À1 ).The morphology as well as the electrical and optical properties of the films were characterized before and after sequential doping with F4TCNQ and AuCl 3 .The absorption peak corresponding to PIDF-BT decreased with F4TCNQ doping but completely disappeared after AuCl 3 doping (Figure 1c).In the current densityelectric field ( J-E) characteristic curve (Figure 1d), the electrical conductivity increased in the order of pristine < F4TCNQ-doped < AuCl 3 -doped PIDF-BT@SWCNTs.In addition, the atomic force microscopy (AFM) and scanning electron microscopy images confirmed that the uniform network in PIDF-BT@SWCNT remained unchanged after doping (Figure S4, Supporting Information).The previous results imply that F4TCNQ and AuCl 3 have been successfully doped into PIDF-BT@SWCNT without changing morphologies, and AuCl 3 provides more effective doping to PIDF-BT@SWCNT.
A four-probe resistance meter was used to quantify the electrical conductivity (σ 4p ).The σ 4p of the pristine PIDF-BT@SWCNT was 550 S cm À1 , and it dramatically increased to 3200 S cm À1 after F4TCNQ doping.In particular, AuCl 3 showing a better doping ability than F4TCNQ provided an unprecedented σ 4p exceeding 5200 S cm À1 (avg.4720 S cm À1 ) in organic-CNT hybrid materials, [36][37][38][39] which was even comparable to highly conductive inorganic MXene. [40,41]The high σ 4p of doped PIDF-BT@SWCNT could be attributed to the formation of a uniform network structure favorable for efficient carrier transport as well as the generation of additional charge carriers by doping (Figure S4, Supporting Information).For comparison, a PIDF-BT/SWCNT composite film was prepared with the same composition (69:31 weight ratio) as PIDF-BT@SWCNT.The σ 4p of the PIDF-BT/SWCNT composite was found to be 367 S cm À1 similar to that of pristine PIDF-BT@SWCNT before molecular doping.However, although the σ 4p values increased to 1635 and 2024 S cm À1 after doping F4TCNQ and AuCl 3 into the PIDF-BT/ SWCNT composite, the values were significantly lower than those of doped PIDF-BT@SWCNT (Figure 1e).Compared to the morphologies of PIDF-BT@SWCNT, the PIDF-BT/SWCNT composite contained abundant small particles in the AFM images (Figure S5, Supporting Information).These particles could be undispersed SWCNTs and unwrapped PIDF-BT, which hindered the formation of network structures between SWCNTs, impaired charge transport, and ultimately reduced the σ 4p after molecular doping.
The stability of PIDF-BT@SWCNT was characterized using the change in the electrical conductivity under the N 2 and air conditions with storage time.As shown in Figure 2, the conductivity of pristine PIDF-BT@SWCNT gradually decreased with time, and only 43% and 33% of its initial conductivity were retained after 1260 h in the N 2 and air, respectively.In contrast, the doped PIDF-BT@SWCNT showed relatively stable electrical conductivity.For example, F4TCNQ-doped PIDF-BT@SWCNT retained %81% and 76% of its initial conductivity under the N 2 and air conditions, respectively.For PIDF-BT@SWCNT doped with AuCl 3 , its electrical conductivity exhibited a more stable tendency, retaining 88% and 98% of the initial value under the air and N 2 conditions, respectively.Since the samples stored under nitrogen conditions were partially exposed to the atmosphere during conductivity measurements, the decrease in electrical conductivity over time could be attributed that the resulted charge carriers were scavenged by oxygen and humidity. [17]However, the high stability of the doped PIDF-BT@SWCNT would be beneficial for its practical applications as a stable conductor.In addition, although it is difficult to exactly specify the reason for the improved stability of PIDF-BT@SWCNT after doping, it may be partly related to the barrier energy that the charge carriers generated by PIDF-BT doping should overcome to move into the core SWCNT (vide infra).The lower the barrier energy, the easier the charge carriers formed in the PIDF-BT by the doping can be transferred to the core SWCNT, and the outer PIDF-BT is expected to prevent the access of oxygen and moisture that could impair the electrical stability.
To identify the doping mechanism of PIDF-BT@SWCNT, the binding energies of the C, N, and S atoms that could participate in the doping were analyzed using XPS.As shown in Figure 3a,b, comparing the binding energies of PIDF-BT and pristine PIDF-BT@SWCNT, the peaks of the N and S atoms shifted to higher binding energies, whereas that of the C atom remained unchanged.This indicated that when PIDF-BT@SWCNT was formed, the N and S atoms in PIDF-BT donated electrons to the SWCNT, resulting in partially doped PIDF-BT.Considering the surface defects formed on SWCNTs during synthesis, [42] the electron-rich N and S atoms are likely to passivate the defects through electron donation.The electron spin resonance (ESR) spectra of the SWCNT and PIDF-BT@SWCNT can partly support this interpretation.As shown in Figure S6, Supporting Information, the SWCNT did not exhibit a noticeable ESR signal nor the pristine PIDF-BT film, [22] whereas a distinct ESR signal was observed for PIDF-BT@SWCNT, indicating that radicals were generated while PIDF-BT wrapped the SWCNT.Based on the XPS results, the radical present in PIDF-BT@SWCNT could be regarded as the result of electron donation from PIDF-BT to SWCNT.
When PIDF-BT@SWCNT was doped by dopants, it was observed that N and S atoms of PIDF-BT were further interacted with the dopants.For example, in PIDF-BT@SWCNT doped with F4TCNQ (Figure 3c), some N and S atoms were intercalated with F4TCNQ anions, exhibiting higher binding energies than pristine PIDF-BT@SWCNT.In the case of AuCl 3 doping (Figure 3d), the binding energy change was similar to that of F4TCNQ; however, the intercalation was more pronounced because AuCl 3 could be doped more effectively than F4TCNQ.When comparing the relative portions of N and S atoms intercalated with the dopants, F4TCNQ tended to intercalate to N and S atoms at a similar level (Figure 3e), whereas AuCl 3 exhibited an approximately twofold higher selectivity for N atoms than S atoms (Figure 3f ).In addition, the ESR signals of the doped PIDF-BT@SWCNT were similar to that of the pristine PIDF-BT@SWCNT except for a characteristic signal around a g-factor of 2.003 in the spectrum of F4TCNQ-doped PIDF-BT@SWCNT, which was attributed to the F4TCNQ anion.The ESR spectra suggested that doped PIDF-BT@SWCNT generated the same radicals as the pristine one.Furthermore, as discussed in the XPS analysis, the apparent increase in the ESR intensity for the doped PIDF-BT@SWCNT was due to the generation of additional charge carriers, which increased the electrical conductivity.
The chemical bonds and forces acting between SWCNT, PIDF-BT, and dopant can be partly estimated from the results of XPS analysis.In PIDF-BT@SWCNT, Columbic interactions formed by charge transfer between PIDF-BT and CNT are expected in addition to the π-π interactions known as the driving force for forming polymer-wrapped CNT. [30,31]In the case of doped PIDF-BT@SWCNT, in addition to the interaction between PIDF-BT and SWCNT, PIDF-BT radical cations and dopant anions, which formed through doping, may further interact by Columbic force.A direct interaction between SWCNT and dopant can also be expected, but even if it exists, the level could be negligible because there is no significant change in the binding energy distribution of C atoms after doping as seen in the XPS analysis (Figure 3).
To investigate the characteristics of the doped PIDF-BT@SWCNT in detail, the Raman spectra of PIDF-BT and PIDF-BT@SWCNT were compared before and after doping.In addition, SWCNT wrapped with sodium dodecyl benzene sulfonate (SDBS), i.e., SDBS@SWCNT was prepared.Because SDBS did not interact with the applied dopants, SDBS@SWCNT was used to provide information on the Raman bands of doped SWCNTs in the absence of PIDF-BT.As shown in Figure 4a, the absorptions of PIDF-BT at 1442 and 1471 cm À1 corresponded to the C═C symmetric stretching of thiophene and C═C breathing/stretching of the fused aromatic IDID unit, respectively. [43,44]These peaks were broadened and redshifted with tailing after doping, which could be attributed to the transition from the benzenoid to the quinoid structure of the conjugated backbone in PIDF-BT upon doping. [45,46]he pristine PIDF-BT@SWCNT showed similar Raman bands at 1440 and 1468 cm À1 , which were also redshifted compared to those of PIDF-BT, consistent with the XPS and ESR results, confirming that PIDF-BT in pristine PIDF-BT@SWCNT existed in a partially doped state.In addition, the characteristic Raman bands of PIDF-BT were further redshifted during PIDF-BT@SWCNT doping, indicating that the PIDF-BT was additionally doped by dopants.
Next, the Raman band change, when the SWCNT was directly doped, was interpretated with SDBS@SWCNT.In pristine SDBS@SWCNT, the G band of the SWCNT was split into G À (1573 cm À1 ) and G þ (1591 cm À1 ), and both shifted to higher wavenumbers after doping (Figure S7a, Supporting Information).Similar to the G band, the G' band was also blueshifted after doping (Figure S7b, Supporting Information).The increase in the electrical conductivity of SDBS@SWCNT by doping was confirmed in the J-E characterization curve (Figure S7c, Supporting Information), implying that because the SDBS has not been doped by the applied dopants, the SWCNT of SDBS@SWCNT is directly doped, resulting in a Raman band shift.It is well-known that when SWCNT are doped with p-type dopants, atomic vibrations are screened by electrons, resulting in an increase in vibration energy and a blueshift of the G þ and G' bands. [47,48]In addition, comparison of the J-E characterization curves of PIDF-BT@SWCNT and SDBS@SWCNT reveals that PIDF-BT directly contributes to the increase in electrical conductivity of PIDF-BT@SWCNT by doping.The electrical conductivities of PIDF-BT@SWCNT and SDBS@SWCNT are similar before doping.However, it can be clearly noted that the electrical conductivity of PIDF-BT@SWCNT is significantly higher than that of SDBS@SWCNT after doping.Since SDBS barely generates charge carriers by doping, the significant increase in electrical conductivity after doping of PIDF-BT@SWCNT indicates that the charges generated by doping of PIDF-BT directly contribute to the increase in electrical conductivity of PIDF-BT@SWCNT.
In PIDF-BT@SWCNT, the blueshift tendency of the G þ band was similar to that of SDBS@SWCNT (Figure 4b), but the G 0 band was clearly redshifted (Figure 4c).As shown in Figure 4d, by comparing the relative shifts of the G þ and G' Raman bands of SDBS@SWCNT and PIDF-BT@SWCNT before and after doping, a contrasting change in the G 0 band was clearly observed.It has been reported that the G þ and G 0 Raman bands can also shift under a mechanical stress.For example, the Raman band can be blueshifted when the SWCNTs are compressed, and the G' Raman band can be redshifted when the surface is strained. [49]Therefore, the Raman band shift of PIDF-BT@SWCNT, which is distinct from SDBS@SWCNT, could be interpreted by considering doping and the resulting stress transfer effect together.As described in the absorption spectra (Figure S1, Supporting Information), the absorption corresponding to PIDF-BT of PIDF-BT@SWCNT was additionally redshifted by 10 nm compared to that of PIDF-BT film, implying that the chains formed a more flattened conformation than the film state.To be coiled states where entropy increases, the flattened PIDF-BT chains are likely to compress the core SWCNT.As a result, the G þ (1592 cm À1 ) and G' (2696 cm À1 ) bands of pristine PIDF-BT@SWCNT appeared to be blueshifted from the Raman bands of common SWCNT (1590 and 2667 cm À1 ) without doping. [50]During the molecular doping of CPs, the dopant molecules infiltrated and expanded the polymer chains, and the degree of expansion became more significant for dopants with high doping efficiency, such as AuCl 3 , due to increased dopant loading density (Figure S8a, Supporting Information). [13,32]In addition, when the average distance between the PIDF-BT@SWCNTs was estimated using X-ray diffraction, the distance was found to increase by about 2 nm from 13.4 to 15.3 nm after doping (Figure S8b, Supporting Information).Although it is difficult to directly compare the degree of PIDF-BT expansion due to its random orientation in the PIDF-BT@SWCNT, the enlarged distance between PIDF-BT@SWCNTs can be ascribed to the doping-induced volume expansion of the PIDF-BT.The expansion of PIDF-BT by doping is expected to transfer compressive shear stress to the SWCNT, which partly explains the opposite shift trends of the G þ and G 0 bands of PIDF-BT@SWCNT after doping.
To systematically characterize the electrical properties of PIDF-BT@SWCNT before and after doping, the mobility (μ 4p ) and charge-carrier density, which determine the electrical conductivity, were quantified by four-probe FET devices.As shown in Figure 5a, the source-to-drain current (I SD ) of PIDF-BT@SWCNT decreased linearly with the gate voltage (V G ), implying that holes were the majority carriers for the electrical conductivity regardless of doping.When the mobility was extracted from the I SD -V G transfer curve, the pristine PIDF-BT@SWCNT had a mobility of 97.2 cm 2 V À1 s À1 , which increased to 114.2 cm 2 V À1 s À1 after F4TCNT doping and 138.00 cm 2 V À1 s À1 after AuCl 3 doping.In addition, the charge-carrier density was quantified using the obtained mobility and electrical conductivity.The pristine PIDF-BT@SWCNT showed a charge-carrier density of 0.35 Â 10 20 cm À3 , which increased to 1.76 Â 10 20 cm À3 after F4TCNQ doping and 2.12 Â 10 20 cm À3 after AuCl 3 doping (Figure 5b).As discussed in the spectroscopic analysis, it can be confirmed that PIDF-BT@SWCNT generated additional charge carriers by doping, and AuCl 3 exhibited a better doping efficiency than F4TCNQ.
The significantly higher mobility of PIDF-BT@SWCNT than that of common CPs implies that the SWCNT core of PIDF-BT@SWCNT serves as the main transport channel for charge carriers.[53] To analyze the increase in μ 4p through doping in PIDF-BT@SWCNT, the barrier energy (E a ) for charge-carrier transfer was determined by measuring μ 4p at different temperatures.As shown in Figure 5c, the dependence of μ 4p on temperature followed the conventional Arrhenius model, [53,54] and the E a clearly decreased from 17.88 meV for pristine PIDF-BT@SWCNT, to 10.37 meV after F4TCNQ doping, and 7.85 meV after AuCl 3 doping.The E a values of all samples were lower than 26 meV (the phonon scattering energy at 25 °C [55] ), indicating the formation of conductive channels for efficient charge-carrier transfer at 25 °C.In addition, the decreased E a after doping can explain the high μ 4p of the doped PIDF-BT@SWCNT as well as the absence of an off state in the I SD -V G transfer curve.To investigate the decrease in E a , the energy level of each sample was determined by ultraviolet photoelectron spectroscopy (UPS).The work function estimated from the UPS spectra (Figure S9, Supporting Information) increased in the order of pure SWCNT< pristine < F4TCNQdoped < AuCl 3 -doped PIDF-BT@SWCNT (Figure 5d).The SWCNT used in this study exhibited conductor-like characteristics because they contained about 33% metallic SWCNTs.Therefore, assuming that the SWCNT is a conductor, a Schottky barrier (E sb ) is expected when the SWCNT contacts the semiconducting PIDF-BT in PIDF-BT@SWCNT, and the resulting E sb can restrict carrier transfer (Figure 5e).The Fermi levels of PIDF-BT@SWCNT move toward the valence band after doping, increasing the work function and reducing the E sb level.This tendency became more pronounced as the doping level of PIDF-BT@SWCNT increased.In particular, the work function of PIDF-BT@SWCNT doped with AuCl 3 (5.23 eV) was higher than that of neat PIDF-BT (5.17 eV), indicating a negligible E sb .Therefore, the μ 4p increase in the doped PIDF-BT@SWCNT can be correlated with the E sb values governed by the doping level.Furthermore, the high σ 4p of PIDF-BT@SWCNT after doping can be attributed to the combined result of additional charge generation in the PIDF-BT and fast charge transfer via the SWCNT.
To exploit the excellent electrical properties of PIDF-BT@SWCNT, its thermoelectric performance was characterized.When the Seebeck coefficients of the PIDF-BT@SWCNT were evaluated before and after doping, the thermovoltage was linearly proportional to the temperature gradient (Figure 6a).The Seebeck coefficient extracted from the thermo-voltage trend was the highest for pristine PIDF-BT@SWCNT (56.6 μV K À1 ) and decreased to 25.84 and 15.16 μV K À1 after F4TCNQ and AuCl 3 doping, respectively (Figure 6b).Because the Seebeck coefficient is inversely proportional to the carrier density, [56] the decrease in the Seebeck coefficient after doping can be related to the increased carrier density.That is, the relatively higher carrier density in the AuCl 3 -doped PIDF-BT@SWCNT resulted in higher electrical conductivity, while the increased electronic contribution to the thermal conductivity reduced the entropy difference for the thermal diffusion of electrons, resulting in a lower Seebeck coefficient. [57]However, the excellent electrical conductivity of the doped PIDF-BT@SWCNT compensated for its low Seebeck coefficient and provided an outstanding power factor.In particular, the Seebeck coefficient of PIDF-BT@SWCNT decreased to less than half after F4TCNQ doping, but the power factor was 214.5 μW m À1 K À2 due to the high electrical conductivity.Although the AuCl 3 -doped PIDF-BT@SWCNT exhibits a lower power factor (107.6 μW m À1 K À2 ) than the pristine PIDF-BT@SWCNT (176.2 μW m À1 K À2 ) owing to its low Seebeck coefficient, this value is still higher than the power factors reported for highly conductive CPs formed by molecular doping. [9,17,32]he results indicate that the PIDF-BT@SWCNT has excellent applicability in thermoelectric devices, and their performance can be further improved by adjusting the doping level.

Conclusion
In summary, to produce a solution-processable and highly conductive material system, PIDF-BT@SWCNT was prepared by wrapping PIDF-BT on SWCNT.The obtained PIDF-BT@SWCNT exhibited excellent dispersibility and high electrical conductivity of over 5000 S cm À1 after AuCl 3 doping.The doping mechanism of PIDF-BT@SWCNT was systematically characterized using UV-vis absorption spectroscopy, XPS, Raman spectroscopy, and ESR spectroscopy.In addition, the four-probe FET device based on PIDF-BT@SWCNT confirmed an increase in carrier mobility after doping, which was attributed to the reduced barrier energy for charge-carrier transfer.UPS analysis confirmed that the carrier transfer barrier energy was related to the Schottky barrier at the interface between the SWCNT and PIDF-BT, which could be controlled by doping.Finally, when PIDF-BT@SWCNT was applied to a thermoelectric device, although the Seebeck coefficient decreased due to the increased carrier density after F4TCNQ doping, a power factor exceeding 210 μW m À1 K À2 was achieved because of its high electrical conductivity.The obtained results confirm that doping is an effective strategy to increase the electrical conductivity while maintaining excellent dispersibility for solution-processable PIDF-BT@SWCNT and its analogous system.

Figure 5 .
Figure 5. a) Four-probe field-effect transistor (FET) source-to-drain currentÀgate voltage (I SD ÀV G ) transfer curve of PIDF-BT@SWCNT before and after doping, and b) mobility extracted from the I SD -V G and carrier density calculated with conductivity.c) Temperature-dependent relative mobility and corresponding transfer barrier energy, d) work function estimated using ultraviolet photoelectron spectroscopy, and e) possible schematic energy diagram of PIDF-BT@SWCNT before and after doping.