Structure Control of a Ï-Conjugated Oligothiophene-Based Liquid

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Structure Control of a π‑Conjugated Oligothiophene-Based Liquid Crystal for Enhanced Mixed Ion/Electron Transport Characteristics Ban Xuan Dong,† Ziwei Liu,‡ Mayank Misra,§ Joseph Strzalka,∥ Jens Niklas,⊥ Oleg G. Poluektov,⊥ Fernando A. Escobedo,*,§ Christopher K. Ober,*,‡ Paul F. Nealey,*,†,# and Shrayesh N. Patel*,†,⊥ †

Pritzker School of Molecular Engineering, University of Chicago, Chicago, Illinois 60637, United States Department of Materials Science and Engineering and §School of Chemical and Biomolecular Engineering, Cornell University, Ithaca, New York 14853, United States ∥ X-ray Science Division, ⊥Chemical Sciences and Engineering Division, and #Materials Science Division, Argonne National Laboratory, Lemont, Illinois 60439, United States ‡

S Supporting Information *

ABSTRACT: Developing soft materials with both ion and electron transport functionalities is of broad interest for energy-storage and bioelectronics applications. Rational design of these materials requires a fundamental understanding of interactions between ion and electron conducting blocks along with the correlation between the microstructure and the conduction characteristics. Here, we investigate the structure and mixed ionic/electronic conduction in thin films of a liquid crystal (LC) 4T/PEO4, which consists of an electronically conducting quarterthiophene (4T) block terminated at both ends by ionically conducting oligoethylenoxide (PEO4) blocks. Using a combined experimental and simulation approach, 4T/PEO4 is shown to self-assemble into smectic, ordered, or disordered phases upon blending the materials with the ionic dopant bis(trifluoromethane)sulfonimide lithium (LiTFSI) under different LiTFSI concentrations. Interestingly, at intermediate LiTFSI concentration, ordered 4T/PEO4 exhibits an electronic conductivity as high as 3.1 × 10−3 S/cm upon being infiltrated with vapor of the 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) molecular dopant while still maintaining its ionic conducting functionality. This electronic conductivity is superior by an order of magnitude to the previously reported electronic conductivity of vapor co-deposited 4T/F4TCNQ blends. Our findings demonstrate that structure and electronic transport in mixed conduction materials could be modulated by the presence of the ion transporting component and will have important implications for other more complex mixed ionic/electronic conductors. KEYWORDS: π-conjugated liquid crystals, mixed ion/electron conduction, molecular dopant, molecular dynamics simulation, X-ray diffraction, spectroscopic ellipsometry organic electronics which utilizes π-conjugated small molecules and polymers as electronic conductors has experienced tremendous growth. Compared to traditional silicon-based electronics, organic electronics offer advantages of low cost processability, mechanical flexibility, biocompatibility, and nearly limitless capacity to tailor the physical properties via

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he molecular design of materials with the ability to simultaneously transport electronic and ionic charge carriers is of broad interest spanning energy storage and conversion, bioelectronics, sensors, and electrochromic applications. Such examples include electrochemical transistors,1 lithium-ion battery electrodes,2−4 and dye-sensitized solar cells.5 Previously, the simultaneous electronic and ionic conduction properties as well as the methods outlined for measuring dual conduction have been extensively investigated in various inorganic materials.6 More recently, the field of © 2019 American Chemical Society

Received: February 6, 2019 Accepted: June 13, 2019 Published: June 13, 2019 7665

DOI: 10.1021/acsnano.9b01055 ACS Nano 2019, 13, 7665−7675

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Cite This: ACS Nano 2019, 13, 7665−7675

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ACS Nano facile chemical design.7−11 Together with the expansion of the organic electronics field, there have been growing interests in developing organic materials such as polymeric12−19 or liquid crystal (LC) systems20,21 with mixed electronic/ionic conducting properties that couple the electronic conduction of the π-conjugated moieties with other ionic conduction moieties. Given their enormous potential, understanding the structure−processing−transport relationships in organic mixed conductors are of keen scientific and technological interests. Until now, only a few structure−mixed conduction studies have been performed on organic mixed conductors, and most of them only focus on polymeric systems.12−19 Understanding fundamental mixed conduction in conjugated polymers, however, is not straightforward due to the complex semicrystalline structures that make both structure characterization and simulation challenging.19 Compared to polymeric systems, LCs hold advantages of simple assembly and structural reorganization because of their intrinsic fluidity. The nanostructure of LCs such as nematic, smectic, columnar, or bicontinuous cubic can be achieved spontaneously via nanoscale segregation of immiscible parts of the molecules.22−25 Their small size also makes detailed simulations of their structure and properties less computationally intensive than simulation of polymeric systems. Moreover, their synthesis is more precise, which makes them ideal material testbeds for new chemical strategies without the major effort that developing new monomers would entail. While either ionic or electronic transport behaviors in LCs have been widely observed,26−30 so far, there have been only a few reports on mixed ion/electron conduction behavior of LCs. Kato et al. reported electrochromic behaviors of LCs as a result of the materials’ ability to conduct ions and electrons, but no detailed investigation of structure and ionic/electronic conduction behaviors was shown.20,21 Toward studying mixed conduction in LCs, recently, we synthesized a coil−rod−coil π-conjugated LC compound 4T/ PEO4 consisting of a quarterthiophene (4T) rigid mesogen and flexible tetra(ethylene oxide) (PEO4) segments on both ends of the mesogen (Figure 1a).31 The π-conjugated 4T mesogen serves as the electronically conducting unit, and the PEO4 segments provide ion conducting properties when blended with salt. The volume ratio of thiophene and the PEO block is chosen in order to enable an ion transport function of PEO4 while permitting π−π interaction between 4T units to achieve smectic ordering. As seen in the polarized optical image (POM) of neat 4T/PEO4 (Figure 1b), the material exhibits birefringent characteristics typically observed in LC materials. We have shown that 4T/PEO4 self-assembles to form a highly ordered smectic phase, resulting in high ionic conductivity of 5.2 × 10−4 S cm−1 at 70 °C upon blending with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) at a concentration of r = [Li+]/[EO] up to 0.05. Additionally, the observation of π−π interaction between 4T units makes 4T/PEO4 a promising candidate to achieve the desired mixed ionic and electronic conduction. In this work, we combine experimental and computational approaches to investigate the influence of the underlying 4T/ PEO4 self-assembled structure on its mixed conduction characteristics. We show that blending 4T/PEO4 with LiTFSI is a versatile route in generating LCs with different structures, which affects both ionic and electronic conduction. At a LiTFSI concentration r of 0.05 and below, 4T/PEO4 adopts ordered smectic morphology, whereas at the intermediate

Figure 1. (a) Chemical structures of 4T/PEO4 and LiTFSI together with their corresponding representations used for MD simulation described later. The radius of Li+ ion is enhanced, and hydrogen atoms are hidden for clarity. (b) Exemplary polarized optical microscope image of a neat 4T/PEO4 sample.

concentration r = 0.10, the 4T units start to penetrate into the PEO phase and connect the smectic layer together, resulting in a more ordered structure. At a high LiTFSI concentration of r = 0.20, the 2D arrangement starts to break apart and the system becomes disordered. While 4T/PEO4-LiTFSI samples exhibit ionic conducting behavior, their electronic transport is realized by exposing the samples to vapor of the 2,3,5,6tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ), which is a method known to introduce electronic dopants into organic semiconductors without disrupting the morphology of the host material.32 The optimum ion transport is observed at low LiTFSI concentration, where 4T/PEO4 adopts smectic morphology, and the highest electronic conductivity is seen at r = 0.10, where favorable electronic percolation pathways exist. Our results demonstrate the strong connection between processing, morphology, and mixed conduction properties of LCs.

RESULTS AND DISCUSSION Structure Control of 4T/PEO4 by the Addition of LiTFSI. To study the evolution of 4T/PEO4 morphology upon addition of the ionic dopant LiTFSI (chemical structure shown in Figure 1a), we employ grazing incidence wide-angle X-ray scattering (GIWAXS), UV−vis−NIR absorption spectroscopy, and variable-angle spectroscopic ellipsometry (VASE), in combination with molecular dynamics (MD) simulations. The representative GIWAXS detector images of ca. 80 nm 4T/ PEO4-LiTFSI thin film at different r are shown in Figure 2a. The GIWAXS pattern of the r = 0 sample shows narrow diffraction peaks (00h) in the out-of-plane direction up to the fourth order, indicative of an ordered smectic phase in which the layers are oriented parallel to the substrate. This is further supported by our MD results showing that the top view of 4T 7666


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Figure 2. (a) Representative GIWAXS patterns of 4T/PEO4-LiTFSI thin films at different blending concentration of r = [Li+]/[EO]. (b) Snapshots of 4T/PEO4-LiTFSI structures at three representative LiTFSI concentrations of r = 0.05, 0.10, and 0.20 from MD simulation together with the top-view and side-view 4T unit arrangements (color code is the same as that in Figure 1a). (c) Simulated GIWAXS patterns from the MD snapshots.

units self-assemble into a typical herringbone packing comparable to that in the P21/c crystal structure reported for pure 4T.33 In our previous publication, we showed that the herringbone packing of 4T drives the self-assembly of the 4T/ PEO4 smectic structure and results in the two distinctive π−π interaction reflections marked A and B in our experimental and simulated GIWAXS diffraction patterns.31 The elongation of these reflections suggests strong in-plane and weak out-ofplane ordering of the π−π interacting units. This is likely due to the 2D π−π interaction of the 4T units within each smectic plane. For r ≤ 0.05, the diffraction patterns of 4T/PEO4LiTFSI are qualitatively similar to those of the neat samples, indicating that the smectic morphology of 4T/PEO4 is maintained. This is confirmed by the MD simulation snapshot and calculated GIWAXS patterns of r = 0.05, as shown in Figure 2b,c. For r = 0.1, we observe a significant change in the structure of the sample compared to the neat and lower salt concentration samples. The GIWAXS image of the r = 0.1 sample reveals a different diffraction pattern indicative of a more ordered 4T/PEO4-LiTFSI structure. The appearance of extra diffraction peaks in this sample might indicate either a more periodic arrangement between 4T and PEO domains

and/or an increase in crystallinity upon interacting with LiTFSI. Moreover, we do not observe the peak elongation of the off-axis reflections along the vertical direction, as seen in the r = 0 or 0.05 samples, suggesting a better 3D registration between 4T units along the π−π interaction direction across the whole film thickness. In the MD-generated structure at r = 0.1, we observe 4T/PEO4 fragments acquiring enough translational entropy to break apart from the herringbone arrangement of 4T, penetrate the PEO phase, and thus connect the smectic layers. To quantify the smectic order disruption from MD simulation, we performed a cluster analysis using hierarchical clustering for thiophene rings. Two thiophene rings are considered part for the same cluster if they are 0.6 nm or less apart. We merge all the clusters given the same criteria until we are left with smallest number of clusters, Nc, which are at least 0.6 nm apart. We found that Nc = 2 for r = 0.01, 0.02, and 0.05, and Nc = 1 for r = 0.10 and 0.2, indicative of a layer merging at high LiTFSI concentrations. As demonstrated in Figure 2c, the calculated GIWAXS pattern of the r = 0.1 sample shows key similarities to the experimental GIWAXS patterns with the appearance of diffraction reflections near |qr| = 0.6, 0.9, 1.3, 1.5, and 2 Å−1. The appearance of these diffraction reflections in the r = 0.1 sample thus most likely 7667


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Figure 3. (a) UV−vis−NIR absorption spectroscopy of 4T/PEO4 at r = 0 and r = 0.1 (b) (c) In-plane and out-of-plane imaginary permittivity of 4T/PEO4 at r = 0 and r = 0.1 measured by spectroscopic ellipsometry.

Figure 4. (a) Schematic representation for F4TCNQ vapor doping procedure of 4T/PEO4-LiTFSI thin film. (b,c) Evolution of UV−vis− NIR absorption spectra of r = 0 and r = 0.1 samples during the F4TCNQ vapor doping process.

the addition of LiTFSI. VASE measurement was performed to further determine the average orientation of the conjugated backbone 4T with regard to the underlying substrates. For both r = 0 and r = 0.1 samples, the best fits were achieved by employing the uniaxial anisotropic model which assumes different dielectric functions for the in- and out-of-plane direction, but no preferred orientation within the x−y plane (ε″x = ε″y ≠ ε″z).34 The imaginary part of the complex permittivity ε″ of the r = 0 and r = 0.1 samples is shown in Figure 3b,c. The r = 0 sample exhibits pronounced anisotropic behavior which has an out-of-plane imaginary permittivity that is stronger than that of the in-plane component. This is indicative of the average tendency of the conjugated backbone 4T to lie perpendicular to the substrate, which is consistent with the observation from GIWAXS data. The preferred outof-plane orientation of the conjugated backbone of 4T/PEO4 is probably due to the favorable interaction of the PEO side chain with the SiO2 substrate. On the other hand, the in-plane and out-of-plane imaginary permittivities of the r = 0.1 sample are more comparable, suggesting that, unlike the neat sample,

originated from the additional periodicities between 4T and PEO domains introduced by the interlayer bridge rather than the increase in the crystallinity of the samples. Finally, at very high salt concentration of r = 0.2, the smectic peaks in the outof-plane direction significantly broaden and the signature π−π interacting peaks at higher q disappears, indicating that the LC structure of 4T/PEO4 is lost by the excess amount of LiTFSI, which is observed in the corresponding MD simulation snapshots and the calculated GIWAXS patterns. To corroborate the structure insights acquired from GIWAXS and MD simulations, we perform UV−vis−NIR absorption spectroscopy and VASE experiments. We particularly focus on the r = 0.1 sample where the most dramatic 4T/PEO4 structural change occurs. As seen in Figure 3a, for both r = 0 and r = 0.1 samples, we observe the appearance of the vibronic (or “aggregate”) shoulders as indicated by the arrows. These vibronic shoulders suggest the formation of π−π aggregates within the thin film sample. Interestingly, in the r = 0.1 sample, the intensity of these aggregate shoulders greatly enhances, suggesting better π−π interaction of 4T/PEO4 upon 7668


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Figure 5. (a) Optical images of an IDE devices used for conductivity measurement. (b) Nyquist plots of the r = 0.1 sample at different F4TCNQ doping time. The inset of each Nyquist plot represents the simplified equivalent circuit used to fit the impedance data. (c) Electronic conductivity of r = 0 and r = 0.10 samples as a function of F4TCNQ doping time. The dashed line represents the maximum 4T/ F4TCNQ electronic conductivity reported by Méndez and Heimel et al.35 (d) Evolution of ionic/electronic conductivity of r = 0.1 samples during F4TCNQ vapor doping process.

To reveal the extent of molecular doping as a function of F4TCNQ vapor exposure time, we performed UV−vis−NIR absorption measurements as shown for 4T/PEO4 r = 0 and r = 0.1 thin films in Figure 4b,c. Overall, the evolution of the UV− vis−NIR spectra is consistent with formation of hole charge carriers from the molecular doping process. For both thin films, vapor doping leads to a decrease in the primary absorption peaks between 2.5 eV and 3.5 eV and the appearance of subgap absorption peaks in the 0.5−1 eV range, whose intensity increases with doping time. The saturation doping exposure time is around 25 min for both r = 0 and r = 0.1 samples for the experimental conditions of this work. Additionally, for both samples, the intensity of the two subgap peaks is relatively strong compared to the main absorption peaks, suggesting that F4TCNQ is homogeneously distributed through the thickness of the film beyond 25 min of deposition. This conclusion is further supported by GIWAXS depth profile measurements in Figure S7, where nearly homogeneous diffraction profiles are observed at different depths of the sample doped with F4TCNQ vapor for 25 min. With respect to the doping mechanism, the appearance of the characteristic subgap peaks between 0.5 eV and 1.0 eV in UV−vis−NIR spectra is indicative of the ground-state charge transfer complex (CPX) doping mechanism, which has been similarly observed in 4T vapor co-deposited with F4TCNQ.35 CPX is described to occur through frontier orbital hybridization of 4T and F4TCNQ, which leads to the formation of a hole transporting 4T/PEO4-F4TCNQ mixed phase. The formation of 4T/PEO4-F4TCNQ mixed phase is further supported by the small increase in 4T/PEO4 π−π interaction distance from 0.47 nm to 0.52 nm upon introducing F4TCNQ for the r = 0 sample (Figure S6). Interestingly, for the r = 0 sample, we observe absportion peaks from F4TCNQ anions near 1.5 eV for short vapor exposure times. The appearance of a F4TCNQ anion absorption peak is indicative of the integer

quarterthiophene units in this sample are no longer preferably oriented in the out-of-plane direction. It is likely the smectic phases destabilize at r= 0.1, and thus creates a mesophase with stronger π−π interaction and percolation between 4T units as indicated by GIWAXS and MD simulations above. Overall, we have shown that blending 4T/PEO4 with LiTFSI is an effective method to control its structure, which will directly influence ionic and electronic transport properties as described in the next sections. Introducing Electronic Conduction Functionality via F4TCNQ Vapor Doping. 4T is formally a semiconducting material and insulating in its pristine state. To enable and modulate electronic conduction in 4T/PEO4 and 4T/PEO4LiTFSI requires the incorporation of molecular dopants. Specifically, the small molecule electron acceptor F4TCNQ has been shown to be a suitable dopant molecule for 4T.35 Note that different strategies of solution mixing and sequential doping have been employed to introduce molecular dopants into conjugated organic semiconductors.32,36,37 Motivated to understand how controlling the underlying self-assembled structure with LiTFSI influences the charge transport properties, we choose to follow the sequential doping method where one first casts a thin film and then infiltrates F4TCNQ from the vapor phase using different exposure times, as illustrated in Figure 4a. Critically, previous reports have shown that vapor doping of polymeric semiconductors with F4TCNQ leads to homogeneous doping through the thickness of the thin film and with minimal disruption of the underlying self-assembled structure.32,37,38 For our 4T/PEO4 r = 0 and r = 0.1 thin films, GIWAXS experiments reveal that vapor doping does not lead to significant disruption to the underlying self-assembled structure as indicated by similar GIWAXS patterns before and after vapor doping with F4TCNQ (see Supporting Information Figure S4). 7669


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Figure 6. (a) Ionic conductivity and maximum electronic conductivity upon introducing F4TCNQ and (b) predicted percolation factor and electronic mobility from MD simulation as functions of LiTFSI concentration. (c,d) Electron paramagnetic resonance spectra of r = 0 and 0.1 samples as a function of F4TCNQ doping time. The saturated spin concentration was calculated to be ca. 1020 cm−3 for both samples (see Supporting Information Figure S3). (e) Snapshots of structure and electron transfer rate ki of r = 0 and 0.1 samples. In structure snapshots, all atoms except for the 4T units are omitted for clarity.

electronic conductivity of 4T/PEO4 without F4TCNQ was (4.9 ± 1.5) × 10−7 S/cm, as determined by the DC method. The intrinsic electronic conductivity without F4TCNQ of 4T/ PEO4-LiTFSI samples was below 2 × 10−8 S/cm for all values of r. These values of intrinsic electronic conductivity are sufficiently low to enable the measurement of ionic conductivity using the EIS method.13 Figure 5b depicts Nyquist impedance plots of the r = 0.1 sample at different F4TCNQ doping times. As frequently observed for ion conducting materials, the Nyquist plot of the sample without F4TCNQ shows a semicircle at high frequencies and a capacitive tail at low frequencies, indicative of dominating ion conducting behavior. Upon introducing vapor F4TCNQ to the sample for 3 min, the Nyquist impedance data drastically change. The observation of two semicircles in the Nyquist plot together with the absence of the capacitive tail at low frequency indicates the presence of both ionic and electronic transport.13 At 30 min of F4TCNQ

charge transfer (ICT) doping mechanism where electron transfers occur from the highest occupied molecular orbital (HOMO) of the host organic semiconductor to the lowest unoccupied molecular orbital (LUMO) of the acceptor dopant. A similar coexistence of CPX and ICT observed in our 4T/PEO4 r = 0 sample was recently shown in P3HT doped with F4TCNQ.39 Mixed Ion/Electron Conduction in 4T/PEO4 Measured by Electrochemical Impedance Spectroscopy. To determine the influence of F4TCNQ molecular doping on the charge transport properties of 4T/PEO4-LiTFSI thin films, we performed electrochemical impedance spectroscopy (EIS) measurement at room temperature. All of our electronic and ionic conductivity measurements were performed on samples fabricated atop gold interdigitated electrode (IDE) devices (Figure 5a) to increase the effective surface areas and enhance the signal-to-noise ratio of electrical measurements in thin films.40 We note that, at room temperature, the intrinsic 7670


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dictating charge transport properties. Thus, we expand and characterize the electronic and ionic conduction characteristics of all LiTFSI concentrations from r = 0 to r = 0.20 after vapor doping with F4TCNQ. As shown in Supporting Information Figure S2, all samples show similar trends for both ionic and electronic conductivity as a function of F4TCNQ doping time. In Figure 6a, we summarize the ionic conductivity of 4T/ PEO4-LiTFSI samples before F4TCNQ infiltration as well as the maximum electronic conductivity of 4T/PEO4-LiTFSI samples upon introducing F4TCNQ vapor. Similar to our previous results,31 the room temperature ionic conductivity of 4T/PEO4-LiTFSI is relatively high for r = 0.01 [(2.3 ± 0.2) × 10−5 S/cm)] and r = 0.05 [(4.2 ± 0.3) × 10−5 S/cm] samples due to the ordered smectic morphology of 4T/PEO4. The stable smectic morphology and high ionic conductivity up to r = 0.05 in 4T/PEO4 are similar to those in other reports by Ohtake et al., where the authors found that lithium saltcomplexed rod−coil−rod LCs exhibited a stabilized mesophase with high ionic conductivity.42,43 However, at r = 0.10 and r = 0.20, where the smectic layer ordering is disrupted, we found a significant decrease in ionic conductivity for both concentrations. Although the decrease in ionic conductivity at high concentrations could originate from ion-pair formation and the reduction in segmental mobility that arises from transient cross-linking between Li+ and EO units,44 we partially attribute the decrease in ionic conductivity to the disruption of the smectic ordering that hinders the ionic conduction pathways within the samples. On the other hand, the electronic conductivity remains relatively constant for r ≤ 0.05, where 4T/PEO4 adopts a smectic morphology but increases by an order of magnitude at r = 0.1, as was shown earlier. At r = 0.2, the electronic conductivity significantly decreases, which is also consistent with the poorly ordered structure suggested by both GIWAXS and MD simulations. To provide further insights into the origin of the maximum electronic conductivity at r = 0.1, we calculate the electronic mobility μe for the MD structures of 4T/PEO4-LiTFSI (without F4TCNQ) shown in Figure 2d by using constrained density functional theory on pairs of dimers to estimate the electron hopping rate from the following equation:

doping, the Nyquist impedance data become a single semicircle with a significantly reduced radius, indicating that the electronic conductivity has increased considerably compared to ionic conductivity. The ionic and electronic resistance values were calculated for F4TCNQ-doped 4T/ PEO4-LiTFSI samples by fitting the EIS data to the appropriate equivalent circuits shown in the inset of each Nyquist plot in Figure 5b. The rationale for the equivalent circuit selection and conductivity calculation is reported elsewhere13 and is also detailed in the Supporting Information. We note for all samples that the electronic conductivity can be measured using either the EIS or the DC method, both yielding good agreement, suggesting the accuracy of the model fit in our EIS data. Shown in Figure 5c is the room temperature electronic conductivity of the two samples as a function of F4TCNQ doping time. As expected, vapor doping F4TCNQ leads to an increase in lateral thin film electronic conductivity for both samples. The neat r = 0 sample shows more than an order of magnitude increase in electronic conductivity from (1.3 ± 0.89) × 10−5 S/cm to (3.2 ± 0.54) × 10−4 S/cm after 25 min of doping. The r = 0.1 sample initially has a conductivity lower than that of the 4T/PEO4 neat sample but exhibits a larger increase in conductivity from (1.3 ± 0.16) × 10−7 S/cm to (3.1 ± 0.45) × 10−3 S/cm after 25 min of doping. We believe the higher electronic conductivity of the r = 0 sample at lower vapor doping times relates to the presence of ICT states in coexistence with CPX states. It has been proposed that ICT states lead to more mobile charge carriers relative to CPX.41 In turn, the formation of either CPX or ICT states is intimately linked to the nature of mobile charge carriers, indicating the magnitude of electronic conductivity. Interestingly, near the optimal doping time, the maximum electronic conductivity of (3.1 ± 0.45) × 10−3 S/cm in the r = 0.1 sample is nearly an order of magnitude higher than the maximum electronic conductivity of the r = 0 sample. Moreover, electronic conductivity of the r = 0.1 sample is superior to the maximum conductivity of 4T co-deposited with F4TCNQ reported by Méndez and Heimel et al.35 as marked by the dashed line in Figure 5c. Considering that both samples yield only CPX states at the optimal doping time and a comparable doping level, the improvement in electronic conductivity most likely arises from the difference in the underlying self-assembled structure as suggested by the structural changes observed in GIWAXS and MD simulation results discussed earlier. Figure 5d summarizes the evolution of ionic and electronic conductivity of the r = 0.1 sample as a function of F4TCNQ vapor doping time. The charge transport processes can be divided into three stages: the early stage where ionic transport dominates (gray-shaded area), the intermediate stage where ionic and electronic transports are comparable (orange-shaded area), and the late stage where the electronic transport dominates (blue-shade area). Here, we would like to note that, at the late stage, we believe that ionic conduction still remains. However, the EIS response only captures the lower resistance electronic transport contribution as we are using electronically reversible and ion-blocking Au metal electrodes.13 Our results indicate that 4T/PEO4-LiTFSI samples still maintain their ion conducting functionality upon doping with F4TCNQ. Influence of 4T/PEO4-LiTFSI Morphology on Mixed Conduction. It is evident that the underlying self-assembled structure with the addition of LiTFSI plays a vital role in

μed =

k e ∑ diki i ∑i ki 6kBT i

(1)

Here, di is the distance between the molecules in the ith dimer; ki is the rate constant for electron hopping, which is calculated using Marcus theory;45 kB is the Boltzmann constant, and T is the temperature. Subsequently, we calculate electron mobility μe using the following equation:46 μe = θpμed

(2)

where θp is the percolation factor that serves to scale the electron hopping rates between pairs of dimers to bulk electronic conductivity. We estimated θp by using a numerical method to find the fraction of open connections in each MDgenerated structure. Additional details of electron mobility estimation are shown in the Supporting Information. Note that the reorganization energy in Marcus theory has both inner and outer contributions, with the former coming from the nuclear reorganization of the reactants and products and the latter from the reorganization of the environment around the reactants and products.47 However, as we are primarily interested in capturing the qualitative effect of varying LiTFSI 7671


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molecular dopant. Using GIWAXS, spectroscopic ellipsometry measurement in combination with molecular dynamics simulation, we showed that 4T/PEO4-LiTFSI samples underwent a transition from a smectic morphology at LiTFSI concentration of r = 0.05 to a more ordered structure with better percolation between 4T units at r = 0.1 before losing its ordered structure at r = 0.20. The highest ionic conductivity was observed for r = 0.05 and below whereas the ionic conductivity decreased at r = 0.10 and 0.20 partially due to the disruption of ion conduction pathways. The electronic conductivity upon doping with vapor F4TCNQ was shown to increase from r = 0 to r = 0.10 due to the improvement of percolation pathways for electronic transports, whereas the r = 0.20 sample exhibited the lowest electronic conductivity because of the disordered structure. UV−vis−NIR absorption spectroscopy measurement indicated that CPX was the major F4TCNQ doping mechanism for both r = 0 and r = 0.1 samples, suggesting the formation of a cocrystal between F4TCNQ and 4T/PEO4. Importantly, for the r = 0 sample, in addition to the expected CPX process, we also observe the weak ICT process at the early state of F4TCNQ doping, indicating the presence of both electronic doping mechanisms in one single material. The results presented in this work provide several important contributions to the rational design of electronic and ionic transporting LC materials. First, we introduce the design strategy of using LC multiblock oligomers that provide both electroactive and ion-active properties. Second, we describe a detailed connection between processing, self-assembly, and ion/electron transport behavior in LC systems. In addition, we show how the use of LC materials introduces opportunities in elucidating molecular doping mechanisms, which could significantly help in advancing the field of organic semiconductors. Our results thus enable additional, more detailed studies of the interconnection among LC self-assembly, electronic properties of 4T/PEO4 (and related molecules), and molecular doping; work along these lines is currently under way. By utilizing the ease of synthesis, straightforward self-assembly, and less intensive computational calculation, we showed that LCs could be use as material testbeds to understand mixed conduction behavior of soft materials in general.

concentration, we only calculate the inner contribution and assume that the outer contribution, although significant in absolute value, is relatively constant (i.e., less sensitive to salt concentration). As shown in Figure 6b, the predicted percolation factor θp and electron mobility μed follow similar quantitative trends as a function of LiTFSI concentration r. Both θp and μe are relatively constant from r = 0 to r = 0.05, where 4T/PEO4 adopts a smectic morphology. However, both θp and μe increase and reach a maximum at r = 0.1. These simulation results reveal that the change in the self-assembled structure, indicative of 3D interconnectivity between smectic layers, accounts for the enhanced electronic conductivity at r = 0.1. As the structure of 4T/PEO4 becomes disordered at r = 0.2, both θp and μe decrease significantly, which is consistent with the poor electronic conductivity measured experimentally. To further emphasize the role of electronic charge percolation on electronic conductivity, we perform electron paramagnetic resonance (EPR) spectroscopy measurements on r = 0 and r = 0.1 samples as a function of F4TCNQ vapor doping time, the results of which are shown in Figure 6c,d. Before doping, no EPR signals are observed, which is indicative of negligible intrinsic charge carriers in 4T/PEO4. Upon introducing F4TCNQ, the EPR signal increases as a function of doping time and saturates around 25 min for both samples, which is consistent with UV−vis−NIR results. This evolution is indicative of increase in spin concentration arising from the electronic charge carriers formed from molecular doping. Importantly, for the same F4TCNQ doping time, the EPR signal of the r = 0.1 sample is much sharper compared to that of the r = 0 sample. For EPR measurement of organic semiconductors, the narrowing of EPR signals typically originates from delocalization of spin (shallow trapped electron spins). This is due to the averaging of the anisotropic part of magnetic interactions and/or decrease of hyperfine interaction. The increased delocalization of electronic charge could be due either to (i) increasing crystallinity and/or (ii) increasing percolation of charge transporting pathways. Based on our observation from MD simulation in Figure 2, we postulate that the bridging of the smectic layers enhances the percolation pathways for charge carrier, which gives rise to the sharpening of the EPR signal in r = 0.1 samples. We emphasize that the final spin concentration at the optimal doping time is determined to be around 1020 cm−3 for both r = 0 and r = 0.1 samples (see Supporting Information Figure S3 for more details). As conductivity is the product of charge carrier mobility and carrier concentration, the similar carrier concentration suggests that the improvement in electronic conductivity of the r = 0.1 sample originates from a higher “apparent” electronic carrier mobility (assuming all carriers possess equal mobility). The change in carrier mobility is supported by the MD snapshots of structure and electron transfer rate ki for r = 0 and r = 0.1 samples, which indicates better percolation pathways for electronic carrier transport in the r = 0.1 sample (Figure 6e,f). Our combined experimental and computational results thus provide complementary pieces of the connection between processing, morphology, and ion/ electron transport behaviors in LC systems.

EXPERIMENTAL SECTION Materials and Sample Preparation. The synthesis of 4T/PEO4 was accomplished via two-fold Kumada cross-coupling of 5,5′dibromo-2,2′-bithiophene with 3 equiv of the corresponding brominated thiophene-PEO4 precursors in the presence of Ni(dppp)Cl2 as catalyst. More details of the synthesis procedure are provided in our previous publication.31 All substrates used in this study were cleaned by ultrasonication in acetone and 2-propanol for 15 min each followed by ozone plasma treatment for 5 min. Grazing incidence wide-angle X-ray scattering measurements were performed on films deposited on Si substrates with 1.5 nm of native SiO2. UV−vis−NIR absorption measurement was performed on films deposited on top of glass substrates. Spectroscopic ellipsometry measurements were performed on films supported by Si substrates possessing different thermal oxide layer thicknesses. Conductivity measurements were performed on films deposited atop custom-fabricated interdigitated gold electrodes on Si substrates with 1000 nm of thermally grown SiO2. Electroparamagnetic resonance measurement was performed on films supported by quartz substrates. Solutions of 4T/PEO4 and lithium bis(trifluoromethane)sulfonimide (LiTFSI) salt (Sigma-Aldrich, battery grade) were

CONCLUSIONS In conclusion, we have investigated the ionic and electronic conduction characteristics in connection to the self-assembled structure of a π-conjugated oligothiophene-based liquid crystal 4T/PEO4 upon doping with the LiTFSI salt and the F4TCNQ 7672


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intensity along the qz direction. All of the GIWAXS data processing and extraction were executed using the GIXSGUI package for MATLAB.51 UV−Vis−NIR Absorption Spectroscopy. The UV−vis−NIR absorption spectroscopy measurements were performed using a Shimadzu UV-3600 Plus UV−vis−NIR dual beam spectrophotometer at the Soft Matter Characterization Facility (SMCF) at the University of Chicago. Molecular Dynamics Simulation. We simulate all of the systems using LAMMPS with optimized parameters for liquid simulations allatom as the force field. The force field parameters for 4T/PEO4 are the same as the ones used in our previous works,31 and for LiTFSI, we use the parameters used by Park et al.52 The initial structures are prepared using the methodology shown in our previous work, where 200 units of PEO4 along with the varying concentrations of LiTFSI are equilibrated as a bilayer system and then attached to the 4T crystal to create a smectic structure with LiTFSI in the PEO regime. To equilibrate the 4T/PEO4 systems containing LiTFSI, we thermal cycled all of the systems between 300 and 380 K for 10 times at a rate of 10 K/ns followed by 20 ns simulation at 300 K. Details of electronic mobility calculation are shown in Supporting Information Figure S7. To simulate GIWAXS patterns, we calculate the structure factor for the thiophene rings, S(q) = ∑n exp(iq·rn) 2 , where rn is the center of mass of each non-hydrogen atom in the thiophene ring as done in our previous study.31 The two-dimensional S(q) shown in Figure 2c results from averaging the qx and qy components along circles of

prepared separately by dissolving the materials in anhydrous tetrahydrofuran (Sigma-Aldrich) with a concentration 10 mg/mL each and shaken on a vortexer for at least several hours before mixing. 4T/PEO4 and LiTFSI solutions were then mixed at the appropriate ratios to achieve the desired concentrations. The mixed solutions were then shaken overnight before being spun onto the prepared substrates at 1000 rpm for 2 min to make both pristine 4T/PEO4 and 4T/ PEO4-LiTFSI thin films. Film thicknesses of all samples in this study were kept at ca. 80 nm, as confirmed by spectroscopic ellipsometry measurement. All sample preparation was performed in an argon-filled glovebox. Vapor Doping Procedure of F4TCNQ. Vapor doping of F4TCNQ was performed using the previously described procedure.32 In brief, 4T/PEO4-LiTFSI thin films were vapor-doped with F4TCNQ in an argon-filled glovebox. Approximately 5 mg of dopant was placed in a glass jar (diameter ≈ 5 cm; height ≈ 4.5 cm), and then the glass jar was inserted into a stainless-steel container preheated at ∼200 °C for at least 30 min on top of a hot plate. The sample was placed underneath the cap using double-sided tape. The samples were taken out of the doping chamber at different times to achieve samples at different doping times (doping levels). Electrochemical Impedance Spectroscopy. Conductivity measurements were performed on a sample fabricated on top of the gold IDE using a Gamry 600+ potentiostat inside an oxygen- and moisture-free argon-filled glovebox. An optical image of the IDE device is exemplified in Figure 5a. Prior to conductivity measurement, the excess amount of materials on the electrode pads was removed. The electrical connection from the IDE to the potentiostat was made using two custom-built microprobes. The EIS characterization was performed from 1 MHz to 0.1 Hz with an oscillatory peak potential of 50 mV. The collected impedance data were then fit to the appropriate equivalent circuits in order to extract the sample ionic resistance, Rion, and electronic resistance, Re. Details of selecting equivalent circuits are shown in the Results and Discussion. The conductivity σ of the sample was then calculated according to the following equation:48

σion/e =

1 d R ion/e l(N − 1)h

constant qr =

Variable-Angle Spectroscopic Ellipsometry. The optical anisotropy of 4T/PEO4-LiTFSI samples was measured using the αSE variable angle spectroscopic ellipsometer (J.A. Woollam Co.). Measurements were performed on three identical films spun on Si substrates; one possessed ca. 1.5 nm of native SiO2 thickness, and two possessed a thermally grown oxide layer of thicknesses ca. 100 and 1000 nm. The thermally grown SiO2 layers were used for interference enhancement to increase the out-of-plane signal. VASE measurements were performed in the reflection mode at three angles: 65, 70, and 75°. A multisample analysis of three samples was used to increase the goodness of the results because of the strong correlations between the fitting parameters.34 All of the data analyses were performed using the CompleteEASE software, provided by J.A. Woollam Co. The optical constants of Si, native oxide, and thermal oxide were also taken from CompleteEASE software’s library database. Polarized Optical Microscopy. An Olympus BX51 polarizing optical microscope was used for visual observation of optical textures of 4T/PEO4. Electron Paramagnetic Resonance Spectroscopy. Continuous wave X-band (9−10 GHz) EPR experiments were carried out with a Bruker ELEXSYS II E500 EPR spectrometer (Bruker Biospin, Rheinstetten, Germany), equipped with a TE102 rectangular EPR resonator (Bruker ER 4102ST). Measurements were performed at room temperature (T = 295 K). Thin films on the substrate with dimensions of 2 mm × 15 mm were placed in the EPR quartz tubes with 4 mm in diameter. For spin quantification, a single crystal of CuSO4·5H2O with known spin concentration was used as a reference sample. Spin quantification was done by comparing double integrals of the experimental and reference EPR signals.

(3)

Here, d = 8 μm is the separation distance between electrodes; l = 1000 μm is the electrode length; N = 160 is the number of electrodes, and h = 80 nm is the thickness of the film. Grazing Incidence Wide-Angle X-ray Scattering. GIWAXS measurements were performed at beamline 8-ID-E of the Advanced Photon Source, Argonne National Laboratory, with 10.86 keV (λ = 0.11416 nm) synchrotron radiation. Samples were enclosed and measured inside a low-vacuum chamber (10−3 mbar) to minimize concerns about radiation damage as well as to prevent extraneous scattering from ambient air. The measurement time was chosen to be 4 s per frame. For each sample, three data sets were taken from three adjacent spots on the sample and then averaged in order to enhance the signal-to-noise ratio. In our work, the samples were tilted at an angle of incidence of 1° with respect to the incoming beam. This angle was chosen to be above the estimated critical angle of the sample (ca. 0.13°) but also above the critical angle of the Si substrates (ca. 0.17°) in order to probe the whole film thickness and minimize signal from reflected beam at the same time.49,50 The scattering signal was recorded with a Pilatus 1MF pixel array detector (pixel size = 172 μm) positioned 228 mm from the sample. Each data set was stored as a 981 × 1043 32-bit tiff image with a 20-bit dynamic range. The Pilatus detector has rows of inactive pixels at the border between detector modules. In order to fill these gaps, after each measurement, the detector was moved to a new vertical direction and the measurement on each spot was repeated, then the gaps were filled by combining the data from two detector positions. The signals were reshaped and output as intensity maps in qz versus qr (=

qx2 + qy2 .

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.9b01055.

qx2 + qy2 )

Details of equivalent circuit selections, mixed conduction at different LiTFSI concentrations, spin concentration of r = 0 and r = 0.1 samples at different F4TCNQ doping times, GIWAXS of r = 0 and r = 0.1

space. We also performed detector nonuniformity, detection efficiency, the polarization effect, and a solid-angle variation for each image. Vertical line cuts were performed as a function of 7673


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samples before and after F4TCNQ infiltration, line-cuts from GIWAXS patterns, F4TCNQ penetration depth and detailed calculation of electronic mobility by MD simulation (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: *E-mail: *E-mail: *E-mail:

[email protected]. [email protected]. [email protected]. [email protected].

ORCID

Ban Xuan Dong: 0000-0002-2873-5207 Mayank Misra: 0000-0002-2700-1228 Joseph Strzalka: 0000-0003-4619-8932 Jens Niklas: 0000-0002-6462-2680 Oleg G. Poluektov: 0000-0003-3067-9272 Fernando A. Escobedo: 0000-0002-4722-9836 Christopher K. Ober: 0000-0002-3805-3314 Paul F. Nealey: 0000-0003-3889-142X Shrayesh N. Patel: 0000-0003-3657-827X Author Contributions

C.K.O., F.A.E., P.F.N., and S.N.P. conceived the project. B.X.D. completed all sample preparations, device fabrication, performed GIWAXS, UV−vis−NIR, spectroscopic ellipsometry, F4TCNQ vapor doping, EIS, analyzed all experimental data and oversaw experiments. Z.L. and C.K.O. synthesized 4T/PEO4 and performed optical microscope experiments. M.M. and F.A.E. performed MD simulation. S.N.P. designed vapor doping experiments. O.G.P. and J.N. performed EPR experiments. J.S. helped with GIWAXS measurements. All authors participated in manuscript preparation and editing. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by NSF DMREF Award Number 1629369. B.X.D. thanks Dr. Patrice Rannou for useful discussions. This research used resources of the Advanced Photon Source, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. The EPR study was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences, under Contract No. DE-AC02-06CH11357 at Argonne National Laboratory (J.N. and O.G.P.). Parts of this work were carried out at the Soft Matter Characterization Facility of the University of Chicago. REFERENCES (1) Khodagholy, D.; Rivnay, J.; Sessolo, M.; Gurfinkel, M.; Leleux, P.; Jimison, L. H.; Stavrinidou, E.; Herve, T.; Sanaur, S.; Owens, R. M.; Malliaras, G. G. High Transconductance Organic Electrochemical Transistors. Nat. Commun. 2013, 4, 2133. (2) Delacourt, C.; Laffont, L.; Bouchet, R.; Wurm, C.; Leriche, J.-B.; Morcrette, M.; Tarascon, J.-M.; Masquelier, C. Toward Understanding of Electrical Limitations (Electronic, Ionic) in LiMPO4 (M = Fe, Mn) Electrode Materials. J. Electrochem. Soc. 2005, 152, A913− A921. (3) Kwon, Y. H.; Minnici, K.; Huie, M. M.; Takeuchi, K. J.; Takeuchi, E. S.; Marschilok, A. C.; Reichmanis, E. Electron/Ion 7674


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