This is an open access article published under a Creative Commons Non-Commercial No Derivative Works (CC-BY-NC-ND) Attribution License, which permits copying and redistribution of the article, and creation of adaptations, all for non-commercial purposes. Article Cite This: Macromolecules 2020, 53, 609−620 pubs.acs.org/Macromolecules Closing the Stability−Performance Gap in Organic Thermoelectrics by Adjusting the Partial to Integer Charge Transfer Ratio Osnat Zapata-Arteaga,† Bernhard Dörling,† Aleksandr Perevedentsev,† Jaime Martín,‡,§ J. Sebastian Reparaz,† and Mariano Campoy-Quiles*,† † Institute of Materials Science of Barcelona (ICMAB-CSIC), Campus of the UAB, 08193 Bellaterra, Spain ‡ POLYMAT and Polymer Science and Technology Department, Faculty of Chemistry, University of the Basque Country UPV/EHU, Manuel de Lardizabal 3, 20018 Donostia-San Sebastián, Spain § Ikerbasque, Basque Foundation for Science, E-48011 Bilbao, Spain * S Supporting Information ABSTRACT: Two doping mechanisms are known for the well-studied materials poly(3-hexylthiophene) (P3HT) and poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT), namely, integer charge transfer (ICT) and charge transfer complex (CTC) formation. Yet, there is poor understanding of the effect of doping mechanism on thermal stability and the thermoelectric properties. In this work, we present a method to finely adjust the ICT to CTC ratio. Using it, we characterize electrical and thermal conductivities as well as the Seebeck coefficient and the long-term stability under thermal stress of P3HT and PBTTT of different ICT/CTC ratios. We establish that doping through the CTC results in more stable, yet lower conductivity samples compared to ICT doped films. Importantly, moderate CTC fractions of ∼33% are found to improve the long-term stability without a significant sacrifice in electrical conductivity. Through visible and IR spectroscopies, polarized optical microscopy, and grazing-incidence wide-angle X-ray scattering, we find that the CTC dopant molecule access sites within the polymer network are less prone to dedoping upon thermal exposure. 1. INTRODUCTION For p-type doping, charge transfer (δ) of effective charges is Fine control of the doping level in organic semiconductors is said to take place when the electron affinity of the dopant is higher than the ionization potential of the host (EAD > IPH).6,7 necessary to optimize their electrical properties for applications Then a two-step process occurs, that is, (i) the transfer of one such as thermoelectrics, transistors, and light-emitting devices. electron (δ = 1e) from the host to the dopant, generating a The realization of organic thermoelectric devices relies, indeed, ground state integer charge transfer (ICT), followed by (ii) on doping pristine organic semiconductors materials to tune thermal dissociation into a localized charge on the dopant and their electrical conductivity (σ), Seebeck coefficient (α), and a mobile polaron on the host.8 The alternative mechanism thermal conductivity (κ). Previous reports have explored the involves partial hybridization of the frontier orbitals between trade-off between these parameters to improve the dimension- the dopant and host, creating new bonding and antibonding less figure of merit ZT = α2σΤ/κ,1,2 where T stands for the orbitals leading to a noninteger δ, often termed charge transfer absolute temperature. complex (CTC).9,10 Overall, the difference between both Organic semiconductors were first doped by using alkali mechanisms suggests that CTCs need to interact with the metals and halides, although their tendency to diffuse or react unreacted host and then dissociate into free carriers; thus, an with the ambient or other layers has limited further extra step is required compared to the ICT species.10,11 development.3 Molecular dopants have emerged as more Evidence of these processes can be obtained by techniques promising candidates as they appear to be sufficiently stable for such as optical absorption spectroscopy. For p-type doping, the (at least short) lab tests. Particularly, dopant/host systems ICT mechanism is characterized by the appearance of new such as 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane subgap transitions from the localized radical anion (A− → (F4TCNQ) doped poly(2,5-bis(3-alkylthiophen-2-yl)-thieno- A−*) and two transitions from the mobile polaron (P+ → P+*) [3,2-b]thiophene) (PBTTT) have become materials test beds which, at high doping level, become a single transition from a for studying the mechanisms involved in doping. Currently, two main doping mechanisms have been proposed, namely, Received: October 28, 2019 integer charge transfer (ICT)4 and charge transfer complex Revised: December 17, 2019 (CTC) formation.5 Published: January 8, 2020 © 2020 American Chemical Society 609 DOI: 10.1021/acs.macromol.9b02263 Macromolecules 2020, 53, 609−620 Macromolecules Article newly formed bipolaron (BP+ → BP+*).12 In the case of the “codeposition method” (e.g., complexation in solution32 and CTC mechanism, the band gap between the newly formed thus a limit on the amount of dopant that can be added while orbitals is understood to follow a Hückel model: maintaining solubility) and keeps morphology changes to a minimum.32−34 This results in a comparatively high electrical CTC IPH + EAD Egap = ± (IPH − EAD)2 + 4β 2 conductivity due to the undisturbed morphology and 2 (1) concomitant good charge carrier mobility (μ) of oriented or which is affected by the IPH, the EAD, and a resonance integral semicrystalline polymers.35,36 Typically the sequential methods β that changes exponentially with intermolecular distance and involve doping a precast polymer film either from the vapor orientation.13−18 For inferring the value of δ for tetracyano- phase or from solutions in orthogonal solvents.26,37,38 Contrary quinodimethane (TCNQ) derivatives, IR spectroscopy is used to codeposition, however, sequential methods do not offer as there is a shift to lower energies of the main −CN stretch precise control of the dopant loading and suffer from dopant vibrational band upon charge transfer.19,20 Moreover, rational diffusion limitations. This has motivated further developments synthetic design can provide highly specific physicochemical of the related processing schemes that utilize photochemical, and electronic characteristics to certain molecular dopants and thermal, and reactive dedoping33,39,40 to control the doping hosts,18,21−24 whereby modifying their functional groups or level and elsewhere optimization of multilayer processing size can effectively change their electron affinity, ionization techniques41 to address the diffusion-limited problem. Specific potential, miscibility, and stability.22,25,26 processing methods that provide controlled ICT/CTC ratios Doping of P3HT and PBTTT with F4TCNQ is known to are yet to be reported. proceed predominantly by an ICT mechanism, yet exceptions Here we demonstrate that controlling the substrate temper- have been reported for specific processing techniques17 or ature and time during vapor doping results in a varying degree suggested for very high dopant loadings.19 For instance, early of the associated CTC and ICT states in the F4TCNQ/ experiments on the F4TCNQ/quaterthiophene system awoke PBTTT system. (Where appropriate, comparisons are drawn interest because it was found to follow a CTC mechanism (δ < with the more widely studied P3HT/F4TCNQ “reference” 1), even though ICT was expected due to the structural system to establish the generality of the findings.) Hence, this similarity to P3HT.13 Subsequently, Jacobs et al. showed that allows us to produce a set of samples with controlled ICT/ increased temperature during codeposition of the F4TCNQ/ CTC ratios and explore their thermoelectric performance and P3HT system can, in fact, yield CTC, which was later its stability under thermal stress. We find that samples attributed to a polymorph of P3HT.17 In line with conven- processed to feature a moderate fraction of the CTC exhibit tional theory, the electrical conductivity of such systems was improved long-term stability without sacrificing electrical lower than in the ICT case, which additionally showed very conductivity compared to samples where ICT dominates. short lifetimes under ambient conditions. Moreover, recent These findings are rationalized by using extensive spectro- experiments by Kiefer et al. demonstrated a particular case of scopic analysis as well as microscopy and structural character- “double doping” (δ = 2) for some PBTTT derivatives doped ization techniques. Importantly, since ICT and CTC doping with F4TCNQ. In their work, it was shown that a polymer mechanisms have been found in many different organic with a sufficiently low IP is capable of donating a second systems (see Table S1), our here presented conclusions electron to the new EA level of the F4TNQ radical anion.6 On regarding stability are likely to be very general. the other hand, Goetz et al. indicated that there is an optimal value between the degree of charge transfer δ and the peak 2. MATERIALS AND METHODS electrical conductivity of small molecular semiconductors.27 2.1. Materials and Sample Preparation. Poly(3-hexylthio- Whether conjugated polymers follow the same behavior phene) (P3HT; regioregular; Mn = 50−100 kDa) from Sigma-Aldrich, remains an open question. poly(2,5-bis(3-tetradecylthiophen-2-yl)-thieno[3,2-b]thiophene) Long-term stability of doped conjugated polymers is highly (PBTTT-C14; hereafter termed PBTTT, Mn = 50 kDa) from 1- desirablea fact that has motivated numerous studies related Material, and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane to the diffusion and migration of the dopant within the (F4TCNQ) were purchased from TCI Chemicals. Chlorobenzene polymer network and/or layers. Typically the dopant/host (>99%) was purchased from Sigma-Aldrich and conductive silver interactions are mostly weak van der Waals and Coulombic paint from RS Components. All chemicals were used as received. forces, which lead to the eventual dedoping of the polymer Polymer films were fabricated from 30 mg/mL (P3HT) or 20 mg/ network.28,29 It has also been suggested that the flexibility of mL (PBTTT) solutions in chlorobenzene. Solutions were stirred at 85 and 110 °C, respectively, for 3 h and used at this temperature for the polymer backbone and the available space within the side deposition. Films were deposited in ambient atmosphere by using a chains dictate not only the doping limit but also the mobility blade coater preheated at 110 °C, with a blade height of 200 μm at a and diffusion of the dopant. 30 To this end, several speed of 30 mm/s. Finally, P3HT and PBTTT films were annealed at straightforward approaches to improve doping stability have 150 and 180 °C, respectively, for 30 min and then allowed to cool been tested, such as the use of bulkier dopants26 and slowly to room temperature. For vapor doping, a closed container modification of the chemical structure of dopants and/or beaker with similar dimensions to the setups reported in the hosts.21,23,31 However, using bulkier dopant often carries literature42,43 was chosen as a doping vessel (Figure S1) and a hot processability penalties, while changing the chemical structure plate at 210 °C as the heat source. Then, large (∼75 × 25 mm2) can lead to undesirable shifts of the respective energy levels. substrates precoated with polymer films were placed on the lid and exposed to F4TCNQ vapor for a doping time (td) of 5 and 50 min in Because the presence of ICT and/or CTC depends on a ambient conditions. These samples are referred to in the upcoming combination of energy levels and morphology, the doping text as partially and maximally doped, respectively (Figure S2). method itself can also play a key role in determining their Dedoping of the maximally doped samples was done on a Kofler relative contributions. Currently, the “sequential processing bench by using a continuous dedoping temperature (Tdd) range of method” is the most widely used one for molecular doping: it 70−120 °C for a dedoping time (tdd) of 20 min (Figure S1). Then avoids processability issues inherent to the alternative they were sliced into several rectangular sections, as shown in Figure 610 DOI: 10.1021/acs.macromol.9b02263 Macromolecules 2020, 53, 609−620 Macromolecules Article S1. Each ∼6 mm wide section is presumed to be quasi-homogeneous 3. RESULTS AND DISCUSSION regarding the dopant content. Small deviations are expected either 3.1. Doping Method To Obtain a Desired CTC/ICT from thickness variations or from the Kofler bench, which exhibits a temperature gradient of 6.7 K/cm. Yet, this only results in a variation Ratio. Owing to the success of doping from the vapor phase in temperature of ±2 K for each 6 mm wide sample. Samples doped and thermally induced dedoping, we employ these methods while controlling the polymer substrate temperature were fabricated together with the use of temperature gradients for the in a second setup, which, as seen in Figure S3, consisted of a modified dedoping of F4TCNQ/P3HT and F4TCNQ/PBTTT sys- reactor beaker with inbuilt heating elements. tems. As a first series of experiments, we prepared samples 2.2. Characterization. Vis−NIR spectra were measured by using following the protocols established in the literature and both a Bruker HYPERION FTIR microscope connected to a thereafter investigated which processing parameters are VERTEX 70 spectrometer and a GES5E ellipsometer from Sopra/ relevant for the formation of CTC and/or ICT states. For Semilab. The dopant content was estimated by integrating the main this, our “maximally doped” samples (see the Materials and transitions of the F4TCNQ anion (A−), the neutral polymer (NP), Methods section for details) of P3HT and PBTTT were Beer’s law, and a known attenuation coefficient.11,43 The fraction (χ) dedoped by thermal annealing in a continuous range of of CTC (χCTC) and ICT (χICT) states was estimated as the ratio temperatures (Tdd = 70−120 °C) by using a Kofler bench, with between the integrated intensity of their respective bands. IR the evolution of the charge transfer states tracked via IR spectroscopy was measured using a PerkinElmer Spectrum One spectroscopy and optical absorption spectroscopy (Figure S4). spectrometer from 4000 to 500 cm−1. The degree of charge transfer was calculated from the shift Δν of the −CN stretch vibrational Figures 1a and 1b show the evolution of the IR spectra after band, which is observed for tetracyanoquinodimethanes (TCNQ) partially dedoping the polymer films. Here, for P3HT and derivatives upon electron transfer ÅÄ ÑÉ−1 2Δν ÅÅÅÅ ν12 ÑÑÑ ÅÅ1 − 2 ÑÑÑ ν0 ÅÅÅÇ ν0 ÑÑÑÖ δ= (2) where ν0 and ν1 correspond to the neutral and anion stretch modes, respectively.19,44 Raman spectra were taken with a WITec alpha 300 RA+ confocal microscope, coupled to an Olympus objective with 100× magnifica- tion (NA = 0.9), and excited with a 488 or 785 nm solid state laser. The Seebeck coefficient and electrical conductivity were measured by using a custom-built setup with a four-probe configuration. All samples were sliced so that the typical sample area was approximately 20 mm × 6 mm. These were contacted with silver paint at the corners, which served as electrodes. Electrical conductivity was measured by using the van der Pauw method. Subsequently, for the Seebeck coefficient measurement, the temperature gradient between two contacts was slowly ramped from 0 to 40 K and measured with two T- type thermocouples, while the voltage was recorded with a Keithley 2400 SourceMeter. The Seebeck coefficient was extracted from a linear fit of the measured slope. The thickness of the deposited samples was determined by using a P16+ profilometer from KLA Tencor. Figure 1. IR spectra for F4TCNQ-doped (a) P3HT and (b) PBTTT, The thermal conductivity of the samples was measured by using a centered on the −CN stretching region. In both cases, the data custom-built frequency-domain thermoreflectance setup which will be correspond to samples dedoped at equivalent temperatures and times fully described in a future publication. In brief, a gold transducer with (Tdd and tdd, respectively, as labeled in (a)). Arrows indicate the 60 nm thickness was thermally evaporated onto the surface of the evolution of each peak with increasing Tdd; the dashed line (N) samples. The wavelength of the pump and probe lasers was set at 405 indicates the position of neutral F4TCNQ. Data are normalized to the and 532 nm, respectively, and the frequency range spanned between 1 SC peak (2187 cm−1), and background from polaron absorption is kHz and 10 MHz. The maximum estimated temperature rise at the removed. surface of the transducer was about 2 K. In simple words, a sinusoidal high-power excitation at 405 nm (about 2 mW) was focused onto the PBTTT, we see three peaks at 2194 cm−1 (B), 2187 cm−1 surface of the transducer, creating a local sinusoidal temperature (SC), and 2170 cm−1 corresponding to the shifted −CN oscillation. The resulting sinusoidal surface temperature wave was stretching modes for δ = 1, which for neutral F4TCNQ appear monitored through the low-power probe laser. Finally, the phase lag at 2227 cm−1 (N) and 2216 cm−1 (not resolved here). between the pump harmonic waves was measured by using a lock-in Moreover, Hase et al. reported a correlation between the amplifier and modeled by numerically solving the spherical heat dopant’s location within the polymer network and the B and equation. Thus, fitting of the phase lag vs the applied frequency SC modes.20 Their work suggested that B peaks correspond to renders the thermal conductivity of the thin films. A general detailed dopant located within the backbone of the polymer while the description of the operational principle of this approach can be found elsewhere.45 SC peak was mostly affected by the placement of dopant Grazing incidence wide-angle X-ray scattering (GIWAXS) experi- within the side chains. In addition, their analysis highlighted ments were conducted at the NCD-SWEET beamline in ALBA the different stability behaviors for the dopant at each synchrotron (Spain). The thin films were typically exposed for 1−5 s position.20 For our experiments, both polymers show the to an X-ray beam with a wavelength of 0.998 Å at an incidence angle presence of an additional peak at 2218 cm−1 (Figures 1a and 0.12°. 2D scattering patterns were collected by employing a WAXS 1b), which, according to eq 2, corresponds to a CTC with a δ LX255-HS detector from Rayonix (pixel size = 40 μm), = 0.2. We also see that for P3HT (Figure 1a) the SC mode 611 DOI: 10.1021/acs.macromol.9b02263 Macromolecules 2020, 53, 609−620 Macromolecules Article Figure 2. (a) Absorption spectra for PBTTT doped with F4TCNQ by using (a) fixed doping time td at various substrate temperatures Tsub and (b) fixed Tsub at various td. In (a), solid gray lines show the data for Tsub = 70−130 °C (bottom to top). The black dashed line corresponds to PBTTT conventionally doped by using an unheated substrate. Deep red curves represent the conditions depicted in the upcoming text as containing a “moderate CTC fraction” (c) IR spectra centered on the −CN stretching region for PBTTT doped with F4TCNQ. The black curve corresponds to the unheated substrate. In (a) the data are normalized to the PBTTT NP transition while the data in (b) are normalized by thickness and offset for clarity due to the nearly absent NP transition at 90 min. decreases in intensity upon heating while the feature at 2218 observe an increase in the A− features up to Tsub = 130 °C cm−1 (CTC) increases with temperature. For PBTTT (Figure before they drop again from 150 to 190 °C. We attribute the 1b), the intensity of the CTC peak is clearly higher and at a latter to simultaneous doping and dedoping (see Figure S6 and similar ratio than the SC peak after annealing at 120 °C the discussion below). (contrary to P3HT). Also, there is a clear change in the ratio In addition, the absorption feature at 640 nm is now well between the B and SC modes as a function of Tdd, which separated from the A− and NP bands, which unambiguously differs from the stability behavior previously seen in the indicates that it belongs to CTC features observed by IR literature.20 Yet, it must be noted that doping protocols are spectroscopy in Figure 1. Reassuringly, the electrical different and, thus also, the final crystalline structure of the thin conductivity follows the same trend, reaching 85 S/cm at layer. Moreover, microscopy and thermoelectric analyses 130 °C and dropping to 28 S/cm at 190 °C (Table S2). Glass (Figures S5 and S6) show clear dedoping signs after thermal transition (Tg) and crystallization of the side chains in PBTTT annealing, further evidencing different stability behaviors for occur at around 100 °C.46 Hence, we rationalize these changes the CTC and ICT states. to come from three different effects. First, an increased Tsub The above experiments show (i) that there are clear avoids excessive dopant condensation on the polymer film spectroscopic signatures of both ICT and CTC states in the surface, which would normally act as a “barrier” for the maximally doped polymers and (ii) that temperature may be incoming dopant (see also micrographs in Figure S6 and the used to change the relative ratio between both doping discussion below). Second, increased alkyl stacking distance mechanisms. due to thermal expansion improves diffusion through the Recent experiments by Lim et al. indicated that doping from polymer structure. Third, after thermal expansion (presumably the vapor phase at elevated substrate temperatures can from the side chains), inter- and intramolecular distances improve doping.43 They identified that after using a td of 15 between dopant and host change, leading to changes in d as min, polymer substrate temperature (Tsub) in their setup seen in other similar blends.15 reached 75−80 °C. They observed a linear increase in the Next, we evaluate the effect of different doping times (td = alkyl- and π-stacking distance during in situ GIWAXS 10, 60, and 90 min) at a fixed substrate temperature (Tsub = measurements for the temperatures reached during doping. 130 °C). Figure 2b shows that the CTC band increases while the A− features from the ICT and NP bands decrease before This suggested an improved dopant diffusion through side bleaching entirely at 90 min. Accordingly, the electrical chains caused by thermal expansion.43 In their experiments, conductivity drops from 85 to 3 S/cm (Table 1). These like in our initial tests, the substrate temperature was only results are in line with the reports for P3HT and quater- indirectly “controlled” by the doping time. The doping vessel thiophene.13,17 As established in the literature, exciton consisted of a 200 mL crystallization beaker with a Petri dish as dissociation of the ground state ICT readily occurs at room the lid and sample holder while heating was done by placing the beaker on a hot plate at 210 °C, meaning that the lid could Table 1. Dopant Fractions χ, Electrical Conductivity, and easily have reached 75−80 °C in the case of maximally doped Seebeck Coefficient as a Function of Doping Conditions samples. This suggests that a temperature-assisted vapor (Data Are Reported for the As-Doped Samples) doping process can improve dopant diffusion and allow for a more controlled selection of the doping mechanism. Following electrical Seebeck this line of thought, we designed a more versatile setup, conductivity coefficient capable of independently controlling the temperatures of both Tsub/td χCTC χICT [S cm−1] [μV K−1] the evaporation source and the substrate (instrumentation nonheated/ (low) 92 60 ± 3 shown in Figure S3). 10 min Hereafter, we analyze samples that were doped with 130 °C/ (moderate) 0.66 85 62 ± 4 10 min 0.34 controlled substrate temperature Tsub. Figure 2 shows the 130 °C/ (high) 0.54 0.46 9 121 ± 14 screening process of different doping conditions. The first 60 min approach (Figure 2a) consisted of increasing Tsub while 130 °C/ (maximum) 3 175 ± 31 keeping the doping time constant (td = 10 min). Here, we 90 min 612 DOI: 10.1021/acs.macromol.9b02263 Macromolecules 2020, 53, 609−620 Macromolecules Article Figure 3. Thermoelectric properties of doped PBTTT films as a function of dedoping time at 100 °C: (a) electrical conductivity, (b) Seebeck coefficient, (c) thermal conductivity, and (d) figure of merit ZT. Samples with high, moderate, and low χCTC correspond to open black squares, solid black squares, and green circles, respectively. Dashed lines are a guide to the eye. temperature,8 whereas CTC can be considered charge neutral though this process is less efficient, it still contributes to the due to a higher binding energy.10 In this sense, the drop in macroscopic conductivity with mobile charges. Most impor- electrical conductivity comes from an interplay of CTC/ICT tantly, from Table 1 we see that keeping χCTC below a certain fractions, where each one has a different free charge carrier threshold level (χ CTC ≤ 0.34) allows to maintain a contribution. comparatively high electrical conductivity. Additional characterization using IR spectroscopy (Figure Table 1 summarizes the estimated ICT and CTC fractions 2c) reveals a pronounced CTC peak. Here, the B mode is now (χ) and the corresponding electrical conductivity and Seebeck visible and in a higher ratio than the SC mode. As these modes coefficient values as a function of doping conditions for the have been associated with dopant position with respect to the previous sample set. For simplicity, “low”, “moderate”, “high”, polymer, one may argue that temperature-assisted vapor and “maximum” will be used in the following text to identify doping results in dopant located at different places with samples with the respective CTC content. respect to “conventional” vapor doping with uncontrolled In this section, we have demonstrated that by controlling the substrate temperature (vide infra). Moreover, as in the work of substrate temperature and doping time, different ratios of CTC Neelamraju et al., fitting of the IR spectra would suggest and ICT can be obtained. The fact that increased Tsub and td different coexisting charge-transfer states in all samples (Figure result in higher CTC content suggests that the two doping S7).19 We thus infer that a low χCTC fraction is present even in mechanisms may evolve differently if provided with enough samples doped by using an unheated substrate (Figure 2a, black dashed line), which then increases both with Tsub and td, energy, which in return can result in different stability trends. reaching χCTC = 0.54 when doped at 130 °C for 60 min (see In the next section, we explore this interplay between the long- Figure S7 and the related text for examples of how these term stability of the thermoelectric properties and the CTC fractions were obtained from spectroscopic data). fraction. In line with the literature, increasing the χCTC within the 3.2. Stability of the Thermoelectric Properties as a total dopant content reduces the electrical conductivity. As Function of CTC Content. While different binding energies stated before, although CTC is considered charge neutral,10 have been presumed for the two doping mechanisms, there is some experiments were still able to measure small, nonzero little to no information about how stable these species are. values of σ for doped samples, even though no sign of ICT Long-term stability is, however, crucial for thermoelectric states was detected.17 This suggests that carrier generation applications. Research on heavier/bulkier dopants and the probably still occurs with further interaction from the present modification of polymer structures to increase molecular species, namely between neutral polymer and CTC.11 Even interactions are clear signs of its importance.22,26 In the 613 DOI: 10.1021/acs.macromol.9b02263 Macromolecules 2020, 53, 609−620 Macromolecules Article Figure 4. Absorption spectra of doped PBTTT films as a function of dedoping time at 100 °C for samples with (a) low and (b) moderate χCTC. (c) Peak intensity from fitted spectra and intensity ratio between peaks associated with CTC and anion bands as detailed in Figure S7. Arrows highlight the evolution of selected bands with increasing tdd. Data are normalized at the NP transition for both samples. following section, we explore the effect of χCTC on the thermal trend: ICT is less stable than CTC (Figure S8), which would stability of PBTTT. suggest that this is indeed a fundamental effect occurring We have chosen to evaluate the stability of thermoelectric within the bulk of the film affecting the two doping properties by measuring the change in electrical conductivity σ, mechanisms for the F4TCNQ/PBTTT system. Seebeck coefficient α, and thermal conductivity κ of doped The Seebeck coefficient α follows an opposite trend to σ PBTTT samples as a function of annealing time at elevated (Figure 3b): starting at 60 μV K−1 and changing to 90 and 53 temperatures (in essence, an “accelerated aging” test). μV K−1 after 30 min for the samples with low and moderate Annealing conditions were chosen on the premise that most χCTC, respectively. Then, after 10 h, α increases in both series dopant within the side chains leaves the polymer structure up to 173 and 110 μV K−1 for the low and moderate χCTC above 100 °C, while small time increments allow screening a cases, respectively. These values agree with the final higher full picture of the dedoping process.20 Specifically, annealing electrical conductivities found in samples that originally had conditions were set in two regimes: (i) Tdd = 100 °C and tdd = higher χCTC. 60 min in 10 min increments and (ii) extending the Interestingly, the thermal conductivity κ of the as-doped measurements to longer intervals of 1, 2, 5, and 10 h. In samples in the same set (Figure 3c) starts at lower values (∼0.3 essence, this would allow visualizing both the intermediate and W m−1 K−1) than the reference undoped PBTTT (∼0.8 W the extreme cases of the doped/dedoped polymer. During this m−1 K−1). This is likely due to a scattering effect for thermal section, we compare samples containing low, moderate, and transport induced by the interdiffusion of the large molecular high fractions of CTC states. Note also that although the dopants and/or changes in the degree of crystallinity upon selected dedoping temperature Td = 100 °C is sufficiently close doping (see next section). An in-depth study of the scattering to the glass transition temperature of PBTTT, any concom- mechanisms will be the topic of a future work, as it goes itant morphological changes of the polymer itself are unlikely beyond the scope of the present piece. The thermal since all samples were precrystallized by thermal annealing conductivity values do not change significantly during the prior to doping (see Materials and Methods section). first 30 min of annealing but then begin to increase, finally Figure 3 presents the thermoelectric properties of the three reaching values of around 1 and 5 W m−1 K−1 for samples with samples with different χCTC. The first noteworthy observation low and moderate χCTC, respectively. The significant drop in is that at the starting point samples containing low and electrical conductivity within the same range (tdd = 2, 5, and 10 moderate χCTC exhibit almost identical values of electrical conductivity, Seebeck coefficient, and thermal conductivity. h) indicates a much lower number of charge carriers for those This suggests that the ICT fraction is dominating the electrical samples compared to the as-doped which, following the transport properties of the different samples, and the Wiedemann−Franz law, implies that the increase in κ is not corresponding microstructures are sufficiently similar to result due to the electronic contribution of the thermal conductivity in equivalent thermal conductivities. but rather the lattice one (Figure S9). The similar κ values for As with previous experiments, electrical conductivity (Figure the undoped PBTTT reference and the highly dedoped 3a) in the as-doped samples (tdd = 0) yields nearly identical samples (tdd = 2, 5, and 10 h) with low χCTC suggest that these values of ∼82 and 90 S/cm for samples with moderate and low samples are structurally similar to the reference. As we will see χCTC, respectively. Then, after 10 h, σ drops to 6 and 0.01 S/ below, the larger κ (5 W m−1 K−1) for strongly dedoped cm for the samples with moderate and low χCTC, respectively. sample with initial moderate χCTC is likely to be due to the Interestingly, the sample containing a high χCTC fraction did formation of F4TCNQ crystals on the surface of the sample not exhibit any degradation of its electrical conductivity for at upon continued thermal stress. least the measured period (60 min at 100 °C, see details in Figure 3d combines these results into ZT values for both Figure S8), maintaining a value of ∼9 S/cm. We draw series which after 30 min (tdd) show peak values of 0.08 and attention to the fact that the σ value at which the sample with a 0.05 for the low and moderate χCTC cases, respectively. Here, moderate CTC saturates coincides with the σ value for the the evolution of highly dedoped samples (tdd = 10 h) shows ZT sample with a high χCTC. This match strongly suggests that values of 1 × 10−5 and 4 × 10−4 for the low and moderate χCTC CTC is a much more stable state than ICT under thermal cases. Although the electrical conductivity decreases in both stress and that the drop in conductivity observed for the cases, the samples with moderate χCTC are more stable, as moderate χCTC sample is due to loss of the ICT (see also next expected, even when taking into account the (probably section). Interestingly, thicker samples also show the same overestimated) high thermal conductivity. A more realistic 614 DOI: 10.1021/acs.macromol.9b02263 Macromolecules 2020, 53, 609−620 Macromolecules Article Figure 5. Transmitted-light microscopy for the (a) low and (b) moderate χCTC series, showing PBTTT films as a function of dedoping time (as indicated). Unpolarized (top row) and cross-polarized (bottom row) micrographs are shown. Constant imaging settings were used. Scale bar = 50 μm. Figure 6. Raman spectra of doped PBTTT samples with different χCTC content as well as reference spectra of undoped/neat PBTTT and neutral F4TCNQ (as indicated). Panels a−c show different spectral regions. Vertical dotted markers indicate selected peak positions for the samples with low/moderate χCTC. Excitation wavelength λex = 785 nm; spectra are peak-normalized for clarity. The data in (a) and (c) are magnified ×5 and ×20, respectively. ZT estimate (correcting for κ) for the moderate χCTC case after the peaks associated with the ICT (A−) are quenched much 10 h would be 0.002 (cf. 1 × 10−5 for the low χCTC case). faster than those associated with the CTC (Figure S8). This 3.3. Correlation of Stability and Doping Mechanism indicates that the rate of depletion of ICT state dopants is by Spectroscopic and Structural Characterization. To substantially faster than that of CTC state dopants. In the further understand the thermal stability of these samples, following, we try to assess whether this is due to the dopant additional characterization was performed. Figure 4 shows the being situated in different locations with respect to the evolution of the optical absorption spectra upon continuous polymer or, alternatively, is a result of a larger binding energy annealing for two samples featuring low and moderate χCTC. for the dopant to the polymer. Here, both series show clear signs of dedoping as the Optical microscopy (Figure 5) corroborates the differences absorption spectra tend toward the shape of the undoped in doping stability between the two sets. Here, the as-doped pristine film. Nonetheless, the A− and CTC bands are still samples with a low χCTC (Figure 5a) show a considerable present at the end of the experiment (black curve, after 10 h) amount of F4TCNQ crystals on the surface, previously in samples with moderate χCTC (as opposed to the low χCTC suggested to have a detrimental effect on doping by acting as case) and exemplified in the peak analysis in Figure 4c and a “barrier” for the incoming dopant during processing. These Figure S7. For thicker samples, it is very clearly observed how crystals are particularly well detected by using cross-polarized 615 DOI: 10.1021/acs.macromol.9b02263 Macromolecules 2020, 53, 609−620 Macromolecules Article Figure 7. Raman spectra of doped PBTTT samples with different χCTC content as well as reference spectra of undoped/neat PBTTT and neutral F4TCNQ (as indicated), showing an expanded view of the 250−500 cm−1 spectral region. Spectra were recorded with excitation at (a) 488 nm and (b) 785 nm. Vertical dotted markers indicate selected peak positions for the samples with low/moderate χCTC. Data are peak-normalized in the 1380−1460 cm−1 region (as in Figure 6). microscopy. Interestingly, they disappear entirely after 1 h. On peak shifts follow the differences in conductivity between the the other hand, the as-doped samples with a moderate χCTC samples, from the film with higher conductivity (low χCTC) (Figure 5b) show no visible dopant crystals on the film surface, toward the reference undoped PBTTT film. presumably due to the fabrication method that employs longer Higher energy vibrations (around 2500−3200 cm−1) are deposition times at temperatures above F4TCNQ sublimation. typically related to modes involving lighter atoms, such as the Then, following postannealing, dopant crystals emerge from C−H stretching vibrations. In Figure 6c, we examine two the polymer network onto the surface (tdd = 1, 2, and 5 h) modes centered at 2890 and 3090 cm−1 which are assigned to before finally disappearing at the final step (tdd = 10 h). These C−H2 stretching and aromatic C−H stretching, respectively.47 crystals may be responsible for the increase in thermal The weaker 3090 cm−1 mode cannot be well resolved in all conductivity after long thermal stress, as measured by samples due to variations in the signal-to-noise ratio in the frequency-domain thermoreflectance (Figure 3c). data. However, the more intense 2890 cm−1 mode belonging Also noteworthy are the changes in color upon dedoping. to the tetradecyl side chains of PBTTT is only observed for Here, for samples with a low χCTC, a rapid recovery of the pink samples with low/moderate CTC content while being absent color of pristine PBTTT can be seen due to pronounced for the sample with maximum χCTC as well as the reference dedoping and re-emergence of the absorption band of neutral PBTTT film. The latter suggests that the side-chain packing for PBTTT (Figure 5a). On the other hand, the series with the sample with maximum χCTC closely matches that of the moderate χCTC (Figure 5b) shows a more gradual change, undoped polymer, implying that in the CTC state the dopant consistent with the higher thermal stability of doping, with the is unlikely to be located in the vicinity of the side chains. On emergence of pink color seen only after 10 h. Hence, both the other hand, the appearance of the 2890 cm−1 mode for absorption and optical microscopy analyses confirm that samples with low/moderate χCTC indicates that the side-chain samples featuring a moderate/high χCTC are thermally more packing of the polymer is altered,48 from which we infer that stable than samples dominated by the ICT doping mechanism. the dopant molecules forming the ICT state are indeed Through our previous experiments, we have shown that the preferentially located around the side chains, in agreement with presence of CTC states within the total doped content the IR data (Figure 2c). Therefore, besides possible differences improves the long-term stability. Moreover, our IR analysis for CTC and ICT in the dopant−polymer binding energy, suggests that the processing conditions that lead to increased dopants are, at least partially, located in different parts of the χCTC are also accompanied by the emergence of vibrational film. modes associated with the dopant allocated within the polymer Finally, low-energy vibrations (centered at a few hundred backbone, suggesting changes in morphology similar to those cm−1) are associated with either vibrations involving heavy reported for codeposition techniques. These results motivate atoms or vibrations extended over a larger group of atoms. us to further analyze the structural changes of the polymer This is, for instance, the region where signatures of different network. polymorphs are often observed. Given that the studied 3.4. Raman Spectroscopy. Figure 6 shows the Raman materials do not include heavy atoms, the spectral changes spectra of three representative samples with low, moderate, in this region are thus interpreted in terms of variations in and maximum χCTC as well as that of the neat (undoped) polymer/dopant microstructure. Figure 6a shows, along with PBTTT and F4TCNQ references. The central region of the the above-discussed softening of the PBTTT-related peak, the spectrum centered at ∼1500 cm−1 (Figure 6b) corresponds to emergence of two strong peaks at ∼298 and 344 cm−1. By vibrations related to the backbone of the polymer, mainly inspection of the reference spectra, it is clear that these peaks carbon single/double bonds symmetric/asymmetric stretch- are related to F4TCNQ, with their enhancement ascribed to ing/bending modes. Accordingly, these vibrations localized in the effect of excitation at 785 nm being resonant with the the conjugated core are strongly affected by the generated absorption of F4TCNQ radical anions (see Figure 2a). It is carriers which navigate through the π-system. As a result, the worth noting that these peaks exhibit a 5 cm−1 shift to higher 616 DOI: 10.1021/acs.macromol.9b02263 Macromolecules 2020, 53, 609−620 Macromolecules Article Figure 8. (a) 2D GIWAXS patterns for selected samples. (b) Large integrated patterns for the same samples as in the left panel as well for that with maximal χCTC. (c) Sketch of suggested dopant position with increased CTC content. energies for the sample with maximum χCTC relative to the 3.5. Grazing-Incidence Wide-Angle X-ray Scattering samples with low/moderate χCTC. This is indicative of (i) (GIWAXS). To further investigate the changes in the crystalline increased intermolecular interactions for the CTC state and part, we have analyzed the samples using GIWAXS. X-ray (ii) preferential placement of the corresponding dopant within scattering has been recently coupled with FTIR experiments to the crystalline polymer fraction of PBTTT given that here the deduce dopant position within host/dopant systems,20 excitation is also preresonant with the absorption of crystalline revealing, in the case of P3HT, that doping generally modifies PBTTT. the spacing between (h00) crystalline planes (associated with A natural extension of the analysis above is to compare the lamellar packing of aromatic backbones and aliphatic side Raman spectra recorded with different excitation wavelengths, chains) and (0k0) crystalline planes (associated with the π−π λex, which, by virtue of resonant enhancement, would stacking). preferentially select different species within the doped samples. Specifically, five changes have been observed and correlated Figure 7 shows Raman spectra for the same samples and to other optical and/or electrical properties. (1) First, low spectral region as in Figure 6a recorded with excitation at 785 dopant content diffuses primarily into the amorphous regions, which does not influence directly the scattering patterns but nm (as above; preferentially exciting F4TCNQ anions and changes significantly the electrical conductivity. (2) Increasing crystalline PBTTT) and 488 nm (preferentially exciting the dopant content broadens scattering features but does not amorphous PBTTT). As before, we focus on the peaks at contribute to any particular shift or new peaks. (3) Further ∼300 and 340 cm−1. Comparison of the data obtained with the increase in the dopant content shifts the (100) peak toward two different excitations shows that, remarkably, these peaks lower q, indicating an expansion of the lamellar packing. (4) are entirely absent for the maximum χCTC sample for λex = 488 The (0k0) peaks attributed to the π−π stacking, e.g. (010), nm (unlike for λex = 785 nm; cf. Figures 7a and 7b), shift to higher q, indicating a contraction of the lattice in that corroborating the proposal that the CTC state dopant is direction. (5) Finally, in high dopant content regimes, the indeed located primarily within crystalline PBTTT regions. (010) reflection splits into new patterns, which has been Conversely, a new peak is seen at 322 cm−1 for samples with associated with mixed crystalline phases with different tilting low/moderate χCTC for λex = 488 nm which is absent in the λex angles of the polymer backbone. It has also been attributed to = 785 nm data. Although its origin is unclear, the contrast with the intercalation of dopant between the polymer backbones the maximum χCTC sample nevertheless allows us to propose and the formation of dopant/polymer cocrystals.19,43,49−51 that the ICT state is formed by dopants located primarily PBTTT proceeds in a similar fashion, yet to our knowledge, within the amorphous/interstitial regions of the polymer there are no literature reports that show patterns as those network. indicating mixed crystalline phases in the (010) orientation. 617 DOI: 10.1021/acs.macromol.9b02263 Macromolecules 2020, 53, 609−620 Macromolecules Article Our GIWAXS patterns, shown in Figure 8, mostly agree with molecules leading to CTC preferentially affect the crystalline previously reported studies.35,51 A detailed analysis of these domains, thus being locked within the crystal structure. GIWAXS patterns including the position (i.e., the Q value) and fwhm of the diffraction peaks as well as the corresponding d- spacing is provided in Table 2. Our results confirm a reduction ■ * ASSOCIATED CONTENT S Supporting Information The Supporting Information is available free of charge at Table 2. Summarized Data from GIWAXS Patterns https://pubs.acs.org/doi/10.1021/acs.macromol.9b02263. (100) (010) Summary of organic systems and reported doping d- d- mechanisms, experimental setup, absorption spectra of Q (peak fwhm spacing Q (Peak spacing doped P3HT and PBTTT, experimental setup with max, nm−1) (nm−1) (nm) max, nm−1) (nm) added heating elements, absorption and IR spectra of pristine PBTTT doped PBTTT, Seebeck coefficient and electrical annealed 3.1 0.42 2.03 17.4 0.36 conductivity measurements, micrographs of doped nonannealed 3.16 0.29 1.99 17.4 0.36 P3HT, electrical conductivity of doped PBTTT as a doped PBTTT function of Tsub, peak fitting of IR and absorption low χCTC 2.84 0.41 2.12 18 0.35 spectra, absorption spectra of dedoped PBTTT, moderate 2.8 0.19 2.24 estimated electrical conductivity and thermal conductiv- χCTC ity and GIWAXS patterns with selected processing max χCTC 2.76 0.33 2.28 17.9 0.35 conditions (PDF) of the π−π stacking distance and an increase of the lamellar ■ AUTHOR INFORMATION Corresponding Author packing distance upon doping the polymer (Figure 8b, dotted *E-mail mcampoy@icmab.es, m.campoy@csic.es. lines). Additionally, we see the appearance of new peaks in the ORCID typical Q-region where the (010) shows up for the samples Osnat Zapata-Arteaga: 0000-0002-0844-2773 with moderate and maximum χCTC (Figure 8b). However, Aleksandr Perevedentsev: 0000-0003-0146-3560 these features are partially masked by the appearance of peaks Jaime Martín: 0000-0002-9669-7273 associated with neutral F4TCNQ crystals both on the surface Mariano Campoy-Quiles: 0000-0002-8911-640X and within the polymer, which increase with increased doping time (please refer to Figure S10 for the all GIWAXS patterns as Notes The authors declare no competing financial interest. ■ a function of doping conditions). Therefore, in agreement with the Raman spectra at low energies, the crystalline structure of the PBTTT is distorted a little by the ICT and more strongly ACKNOWLEDGMENTS when the CTC fraction increases (see Table 2). We thank Dr. Martijn Kemerink for fruitful discussions at the These distortions of the molecular packing are compatible early stages of this project. The authors acknowledge financial with the positioning of the dopant molecules within interstitial support from the Spanish Ministry of Economy, Industry, and regions between aromatic backbones. The fact that the samples Competitiveness through the “Severo Ochoa” Program for with increased CTC content show a decreased intensity in the Centers of Excellence in R&D (SEV-2015-0496), MAT2015- SC peak in IR (compared to B), or the high-energy Raman 70850-P, PGC2018-095411-B-I00, and No. MAT2017-90024- peaks (Figure 6c), might be related to (i) spectroscopy P (TANGENTS)-EI/FEDER, UE projects; from the General- preferentially probing amorphous domains or (ii) CTC and itat de Catalunya through grants 2017SGR488 and AGAUR ICT forming different polymorphs or cocrystals. The 2018 PROD 00191; from CSIC through project 201560I032; appearance of new peaks in the GIWAXS data supports the and from the European Research Council (ERC) under grant second option. agreement no. 648901. O.Z.-A. acknowledges CONACYT- SENER for his Ph.D. scholarship (No. 472571). J.M. 4. CONCLUSIONS furthermore thanks MINECO for the Ramón y Cajal contract We have established a doping method to obtain a desired CTC and the PGC2018-094620-A-I00 grant. We acknowledge to ICT fraction. The method relies on controlling the support of the publication fee by the CSIC Open Access temperature of the substrate during evaporation of the dopant. Publication Support Initiative through its Unit of Information Resources for Research (URICI). ■ This enabled us to assess the role of different CTC fractions in doped PBTTT films in terms of their thermoelectric properties, namely, the electrical conductivity, Seebeck REFERENCES coefficient, and thermal conductivity. Moreover, we followed (1) Wang, H.; Yu, C. Organic Thermoelectrics: Materials the thermal stability of the thermoelectric parameters upon Preparation, Performance Optimization, and Device Integration. continuous exposure to 100 °C for ca. 10 h. We found that Joule 2019, 3 (1), 53−80. samples rich in CTC are more thermally stable than those (2) Bubnova, O.; Khan, Z. U.; Malti, A.; Braun, S.; Fahlman, M.; Berggren, M.; Crispin, X. Optimization of the Thermoelectric Figure based on higher ICT fractions. Interestingly, samples exhibiting of Merit in the Conducting Polymer Poly(3,4-Ethylenedioxythio- intermediate CTC fractions result simultaneously in good phene). Nat. Mater. 2011, 10 (6), 429−433. thermoelectric properties and improved stability. To under- (3) Kar, P. Doping in Conjugated Polymers; John Wiley & Sons, Inc.: stand this, we performed a combined study including Hoboken, NJ, 2013. absorption, polarized optical microscopy, Raman spectroscopy, (4) Lüssem, B.; Riede, M.; Leo, K. Doping of Organic Semi- and GIWAXS. The resulting structural picture is that dopant conductors. Phys. Status Solidi A 2013, 210 (1), 9−43. 618 DOI: 10.1021/acs.macromol.9b02263 Macromolecules 2020, 53, 609−620 Macromolecules Article (5) Salzmann, I.; Heimel, G.; Duhm, S.; Oehzelt, M.; Pingel, P.; and Thermal Stability of a Molecularly p-Doped Polythiophene. Adv. George, B. M.; Schnegg, A.; Lips, K.; Blum, R.-P.; Vollmer, A.; et al. Mater. 2017, 29 (24), 1700930. Intermolecular Hybridization Governs Molecular Electrical Doping. (23) Li, J.; Zhang, G.; Holm, D. M.; Jacobs, I. E.; Yin, B.; Stroeve, P.; Phys. Rev. Lett. 2012, 108 (3), 035502. Mascal, M.; Moulé, A. J. Introducing Solubility Control for Improved (6) Kiefer, D.; Kroon, R.; Hofmann, A. I.; Sun, H.; Liu, X.; Organic P-Type Dopants. Chem. Mater. 2015, 27 (16), 5765−5774. Giovannitti, A.; Stegerer, D.; Cano, A.; Hynynen, J.; Yu, L.; et al. (24) Scholes, D. T.; Yee, P. Y.; McKeown, G. R.; Li, S.; Kang, H.; Double Doping of Conjugated Polymers with Monomer Molecular Lindemuth, J. R.; Xia, X.; King, S. C.; Seferos, D. S.; Tolbert, S. H.; Dopants. Nat. Mater. 2019, 18 (2), 149−155. et al. Designing Conjugated Polymers for Molecular Doping: The (7) Li, J.; Duchemin, I.; Roscioni, O. M.; Friederich, P.; Anderson, Roles of Crystallinity, Swelling, and Conductivity in Sequentially- M.; Da Como, E.; Kociok-Köhn, G.; Wenzel, W.; Zannoni, C.; Doped Selenophene-Based Copolymers. Chem. Mater. 2019, 31 (1), Beljonne, D.; et al. Host Dependence of the Electron Affinity of 73−82. Molecular Dopants. Mater. Horiz. 2019, 6 (1), 107−114. (25) Koech, P. K.; Padmaperuma, A. B.; Wang, L.; Swensen, J. S.; (8) Tietze, M. L.; Benduhn, J.; Pahner, P.; Nell, B.; Schwarze, M.; Polikarpov, E.; Darsell, J. T.; Rainbolt, J. E.; Gaspar, D. J. Kleemann, H.; Krammer, M.; Zojer, K.; Vandewal, K.; Leo, K. Hexafluorotetracyanonaphthoquinodimethane (F6-TNAP): A Con- Elementary Steps in Electrical Doping of Organic Semiconductors. ductivity Dopant for Organic Light-Emitting Devices. Chem. Mater. Nat. Commun. 2018, 9 (1), 1182. 2010, 22 (8), 3926−3932. (9) Jacobs, I. E.; Moulé, A. J. Controlling Molecular Doping in (26) Li, J.; Rochester, C. W.; Jacobs, I. E.; Friedrich, S.; Stroeve, P.; Organic Semiconductors. Adv. Mater. 2017, 29 (42), 1703063. Riede, M.; Moulé, A. J. Measurement of Small Molecular Dopant (10) Salzmann, I.; Heimel, G.; Oehzelt, M.; Winkler, S.; Koch, N. F4TCNQ and C 60 F 36 Diffusion in Organic Bilayer Architectures. Molecular Electrical Doping of Organic Semiconductors: Fundamen- ACS Appl. Mater. Interfaces 2015, 7 (51), 28420−28428. tal Mechanisms and Emerging Dopant Design Rules. Acc. Chem. Res. (27) Goetz, K. P.; Vermeulen, D.; Payne, M. E.; Kloc, C.; McNeil, L. 2016, 49 (3), 370−378. E.; Jurchescu, O. D. Charge-Transfer Complexes: New Perspectives (11) Pingel, P.; Neher, D. Comprehensive Picture of P-Type Doping on an Old Class of Compounds. J. Mater. Chem. C 2014, 2 (17), of P3HT with the Molecular Acceptor F4TCNQ. Phys. Rev. B: 3065−3076. Condens. Matter Mater. Phys. 2013, 87 (11), 115209. (28) Li, J.; Rochester, C. W.; Jacobs, I. E.; Aasen, E. W.; Friedrich, (12) Enokida, I.; Furukawa, Y. Doping-Level Dependent Mobilities S.; Stroeve, P.; Moulé, A. J. The Effect of Thermal Annealing on of Positive Polarons and Bipolarons in Poly(2,5-Bis(3-Hexadecylth- Dopant Site Choice in Conjugated Polymers. Org. Electron. 2016, 33, iophen-2-Yl)Thieno[3,2-b]Thiophene) (PBTTT-C16) Based on an 23−31. Ionic-Liquid-Gated Transistor Configuration. Org. Electron. 2019, 68, (29) Müller, L.; Rhim, S.-Y.; Sivanesan, V.; Wang, D.; Hietzschold, S.; Reiser, P.; Mankel, E.; Beck, S.; Barlow, S.; Marder, S. R.; et al. 28−34. (13) Méndez, H.; Heimel, G.; Winkler, S.; Frisch, J.; Opitz, A.; Electric-Field-Controlled Dopant Distribution in Organic Semi- conductors. Adv. Mater. 2017, 29 (30), 1701466. Sauer, K.; Wegner, B.; Oehzelt, M.; Röthel, C.; Duhm, S.; et al. (30) Andersson, M. R.; Thomas, O.; Mammo, W.; Svensson, M.; Charge-Transfer Crystallites as Molecular Electrical Dopants. Nat. Theander, M.; Inganäs, O. Substituted Polythiophenes Designed for Commun. 2015, 6, 8560. Optoelectronic Devices and Conductors. J. Mater. Chem. 1999, 9 (9), (14) Coropceanu, V.; Cornil, J.; da Silva Filho, D. A.; Olivier, Y.; 1933−1940. Silbey, R.; Brédas, J.-L. Charge Transport in Organic Semiconductors. (31) Liu, J.; Qiu, L.; Portale, G.; Torabi, S.; Stuart, M. C. A.; Qiu, X.; Chem. Rev. 2007, 107 (4), 926−952. Koopmans, M.; Chiechi, R. C.; Hummelen, J. C.; Anton Koster, L. J. (15) Hu, P.; Wang, S.; Chaturvedi, A.; Wei, F.; Zhu, X.; Zhang, X.; Side-Chain Effects on N-Type Organic Thermoelectrics: A Case Li, R.; Li, Y.; Jiang, H.; Long, Y.; et al. Impact of C−H···X (X = F, N) Study of Fullerene Derivatives. Nano Energy 2018, 52, 183−191. and π−π Interactions on Tuning the Degree of Charge Transfer in (32) Jacobs, I. E.; Aasen, E. W.; Oliveira, J. L.; Fonseca, T. N.; F6TNAP-Based Organic Binary Compound Single Crystals. Cryst. Roehling, J. D.; Li, J.; Zhang, G.; Augustine, M. P.; Mascal, M.; Growth Des. 2018, 18 (3), 1776−1785. Moulé, A. J. Comparison of Solution-Mixed and Sequentially (16) Zhu, L.; Kim, E.-G.; Yi, Y.; Brédas, J.-L. Charge Transfer in Processed P3HT:F4TCNQ Films: Effect of Doping-Induced Molecular Complexes with 2,3,5,6-Tetrafluoro-7,7,8,8-Tetracyanoqui- Aggregation on Film Morphology. J. Mater. Chem. C 2016, 4 (16), nodimethane (F4TCNQ): A Density Functional Theory Study. 3454−3466. Chem. Mater. 2011, 23 (23), 5149−5159. (33) Jacobs, I. E.; Aasen, E. W.; Nowak, D.; Li, J.; Morrison, W.; (17) Jacobs, I. E.; Cendra, C.; Harrelson, T. F.; Bedolla Valdez, Z. I.; Roehling, J. D.; Augustine, M. P.; Moulé, A. J. Direct-Write Optical Faller, R.; Salleo, A.; Moulé, A. J. Polymorphism Controls the Degree Patterning of P3HT Films Beyond the Diffraction Limit. Adv. Mater. of Charge Transfer in a Molecularly Doped Semiconducting Polymer. 2017, 29 (2), 1603221. Mater. Horiz. 2018, 5 (4), 655−660. (34) Kang, K.; Watanabe, S.; Broch, K.; Sepe, A.; Brown, A.; (18) Valencia, A. M.; Cocchi, C. Electronic and Optical Properties of Nasrallah, I.; Nikolka, M.; Fei, Z.; Heeney, M.; Matsumoto, D.; et al. Oligothiophene-F4TCNQ Charge-Transfer Complexes: The Role of 2D Coherent Charge Transport in Highly Ordered Conducting the Donor Conjugation Length. J. Phys. Chem. C 2019, 123 (14), Polymers Doped by Solid State Diffusion. Nat. Mater. 2016, 15 (8), 9617−9623. 896−902. (19) Neelamraju, B.; Watts, K. E.; Pemberton, J. E.; Ratcliff, E. L. (35) Patel, S. N.; Glaudell, A. M.; Peterson, K. A.; Thomas, E. M.; Correlation of Coexistent Charge Transfer States in F4TCNQ-Doped O’Hara, K. A.; Lim, E.; Chabinyc, M. L. Morphology Controls the P3HT with Microstructure. J. Phys. Chem. Lett. 2018, 9 (23), 6871− Thermoelectric Power Factor of a Doped Semiconducting Polymer. 6877. Sci. Adv. 2017, 3 (6), e1700434. (20) Hase, H.; O’Neill, K.; Frisch, J.; Opitz, A.; Koch, N.; Salzmann, (36) Hamidi-Sakr, A.; Biniek, L.; Bantignies, J.-L.; Maurin, D.; I. Unraveling the Microstructure of Molecularly Doped Poly(3- Herrmann, L.; Leclerc, N.; Lév ê q ue, P.; Vijayakumar, V.; Hexylthiophene) by Thermally Induced Dedoping. J. Phys. Chem. C Zimmermann, N.; Brinkmann, M. A Versatile Method to Fabricate 2018, 122 (45), 25893−25899. Highly In-Plane Aligned Conducting Polymer Films with Anisotropic (21) Liu, J.; Qiu, L.; Portale, G.; Koopmans, M.; ten Brink, G.; Charge Transport and Thermoelectric Properties: The Key Role of Hummelen, J. C.; Koster, L. J. A. N-Type Organic Thermoelectrics: Alkyl Side Chain Layers on the Doping Mechanism. Adv. Funct. Improved Power Factor by Tailoring Host-Dopant Miscibility. Adv. Mater. 2017, 27 (25), 1700173. Mater. 2017, 29 (36), 1701641. (37) Fujimoto, R.; Yamashita, Y.; Kumagai, S.; Tsurumi, J.; (22) Kroon, R.; Kiefer, D.; Stegerer, D.; Yu, L.; Sommer, M.; Müller, Hinderhofer, A.; Broch, K.; Schreiber, F.; Watanabe, S.; Takeya, J. C. Polar Side Chains Enhance Processability, Electrical Conductivity, Molecular Doping in Organic Semiconductors: Fully Solution- 619 DOI: 10.1021/acs.macromol.9b02263 Macromolecules 2020, 53, 609−620 Macromolecules Article Processed, Vacuum-Free Doping with Metal−Organic Complexes in an Orthogonal Solvent. J. Mater. Chem. C 2017, 5 (46), 12023− 12030. (38) Patel, S. N.; Glaudell, A. M.; Kiefer, D.; Chabinyc, M. L. Increasing the Thermoelectric Power Factor of a Semiconducting Polymer by Doping from the Vapor Phase. ACS Macro Lett. 2016, 5 (3), 268−272. (39) Jacobs, I. E.; Wang, F.; Hafezi, N.; Medina-Plaza, C.; Harrelson, T. F.; Li, J.; Augustine, M. P.; Mascal, M.; Moulé, A. J. Quantitative Dedoping of Conductive Polymers. Chem. Mater. 2017, 29 (2), 832− 841. (40) Fuzell, J.; Jacobs, I. E.; Ackling, S.; Harrelson, T. F.; Huang, D. M.; Larsen, D.; Moulé, A. J. Optical Dedoping Mechanism for P3HT:F4TCNQ Mixtures. J. Phys. Chem. Lett. 2016, 7 (21), 4297− 4303. (41) Zuo, G.; Andersson, O.; Abdalla, H.; Kemerink, M. High Thermoelectric Power Factor from Multilayer Solution-Processed Organic Films. Appl. Phys. Lett. 2018, 112, 083303. (42) Hynynen, J.; Kiefer, D.; Yu, L.; Kroon, R.; Munir, R.; Amassian, A.; Kemerink, M.; Müller, C. Enhanced Electrical Conductivity of Molecularly P-Doped Poly(3-Hexylthiophene) through Understand- ing the Correlation with Solid-State Order. Macromolecules 2017, 50 (20), 8140−8148. (43) Lim, E.; Peterson, K. A.; Su, G. M.; Chabinyc, M. L. Thermoelectric Properties of Poly(3-Hexylthiophene) (P3HT) Doped with 2,3,5,6-Tetrafluoro-7,7,8,8-Tetracyanoquinodimethane (F4TCNQ) by Vapor-Phase Infiltration. Chem. Mater. 2018, 30 (3), 998−1010. (44) Dixon, D. A.; Calabrese, J. C.; Miller, J. S. Crystal and Molecular Structure of the 2:1 Charge-Transfer Salt of Decamethyl- ferrocene and Perfluoro-7,7,8,8-Tetracyano-p-Quinodimethane: [[Fe- (C5Me5)2]+.]2[TCNQF4]2-. The Electronic Structure of [TCNQF4]n (n = 0, 1-, 2-). J. Phys. Chem. 1989, 93 (6), 2284−2291. (45) Schmidt, A. J.; Cheaito, R.; Chiesa, M. A Frequency-Domain Thermoreflectance Method for the Characterization of Thermal Properties. Rev. Sci. Instrum. 2009, 80 (9), 094901. (46) Root, S. E.; Alkhadra, M. A.; Rodriquez, D.; Printz, A. D.; Lipomi, D. J. Measuring the Glass Transition Temperature of Conjugated Polymer Films with Ultraviolet−Visible Spectroscopy. Chem. Mater. 2017, 29 (7), 2646−2654. (47) Corsetti, S.; Rabl, T.; McGloin, D.; Kiefer, J. Intermediate Phases during Solid to Liquid Transitions in Long-Chain n-Alkanes. Phys. Chem. Chem. Phys. 2017, 19 (21), 13941−13950. (48) Miller, N. C.; Sweetnam, S.; Hoke, E. T.; Gysel, R.; Miller, C. E.; Bartelt, J. A.; Xie, X.; Toney, M. F.; McGehee, M. D. Molecular Packing and Solar Cell Performance in Blends of Polymers with a Bisadduct Fullerene. Nano Lett. 2012, 12 (3), 1566−1570. (49) Scholes, D. T.; Hawks, S. A.; Yee, P. Y.; Wu, H.; Lindemuth, J. R.; Tolbert, S. H.; Schwartz, B. J. Overcoming Film Quality Issues for Conjugated Polymers Doped with F 4 TCNQ by Solution Sequential Processing: Hall Effect, Structural, and Optical Measurements. J. Phys. Chem. Lett. 2015, 6 (23), 4786−4793. (50) Duong, D. T.; Wang, C.; Antono, E.; Toney, M. F.; Salleo, A. The Chemical and Structural Origin of Efficient P-Type Doping in P3HT. Org. Electron. 2013, 14 (5), 1330−1336. (51) Cochran, J. E.; Junk, M. J. N.; Glaudell, A. M.; Miller, P. L.; Cowart, J. S.; Toney, M. F.; Hawker, C. J.; Chmelka, B. F.; Chabinyc, M. L. Molecular Interactions and Ordering in Electrically Doped Polymers: Blends of PBTTT and F4TCNQ. Macromolecules 2014, 47 (19), 6836−6846. 620 DOI: 10.1021/acs.macromol.9b02263 Macromolecules 2020, 53, 609−620