Quantifying anisotropic mobility enhancement in uniaxially aligned polythiophene films via ion-exchange doping

Abstract

We quantitatively investigated the anisotropic enhancement of hole mobility induced by structural modifications upon ion-exchange doping in highly uniaxially aligned poly(3,3′′′-didodecyl-quaterthiophene) (PQT-12) films. Large-area, uniform alignment was achieved via the floating film transfer method (FTM), giving a high dichroic ratio of 15. Anion-exchange p-doping with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) and lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) yielded a carrier density of 8.06 × 10²⁰ cm⁻³ while preserving uniaxial order and improving crystallinity. Compared to undoped films, the hole mobility increased by 850-fold along the backbone direction and 1700-fold along the π–π stacking (perpendicular) direction, demonstrating that doping optimizes structural order for charge transport in both directions, with a more pronounced relative enhancement perpendicular to the chains. Using the stronger oxidant magic blue with LiTFSI further increased the doping level, achieving a higher conductivity of 251 S cm⁻¹. These results reveal how ion-exchange doping and structural anisotropy cooperatively govern charge transport in aligned conjugated polymers.

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1. Introduction

Semiconducting π-conjugated polymers inherently exhibit anisotropic charge transport along the polymer backbone and π–π stacking directions in thin films.^1–3 However, most solution-processed films are macroscopically isotropic due to random chain orientation, which limits the ability to evaluate and exploit directional transport properties. In contrast, uniaxially aligned films enable direct comparison of charge transport along different crystallographic directions, providing fundamental insights into structure–property relationships and offering potential device advantages,^4,5 such as polarization-sensitive organic photodetectors⁶ and direction-selective conduction in organic field effect transistors.^7–10 Reported methods for uniaxial alignment include high-temperature rubbing,¹¹ strain alignment,¹² friction transfer,¹³ confinement in nanoscale patterning,^14,15 magnetic alignment¹⁶ and compression at the air–liquid interface.^17,18 Among these, film formation at the air–liquid interface, which utilizes flow as the driving force for polymer alignment,^19,20 offers a simple yet powerful strategy for producing π-conjugated polymer films with pronounced uniaxiality. With an appropriate choice of liquid subphase and controlled solution flow, uniformly aligned films can be prepared over large areas, enabling precise evaluation of anisotropic properties and their evolution upon post-treatment.

Chemical doping is a common strategy to enhance the electrical conductivity of organic semiconductors by increasing carrier density,^21–23 but it also alters molecular conformation and packing in films.^24–29 Such structural changes—particularly in backbone planarity and π–π stacking order—can either enhance or hinder carrier mobility. In general, polymers with sparse alkyl side chains can provide sufficient free volume for dopant accommodation,³⁰ potentially helping to enhance mobility due to improved polymer chain packing. Recently, ion-exchange doping has emerged as an innovative approach that overcomes the limitations of conventional methods in terms of both achievable doping level and stability.³¹ In this process, the dopant first induces charge transfer from the polymer, which is immediately followed by exchange with a more chemically stable counter ion. The ion-exchange step provides additional thermodynamic driving force, leading to a higher doping level than that obtained with the dopant alone, while simultaneously ensuring long-term stability against dedoping. Mechanistic studies on ion-exchange doping have been conducted to identify the key factors that govern the achievable doping level and to establish optimized doping conditions.^32,33

Although exceptionally high mobility along the backbone direction in aligned polymers has been reported,^34,35 anisotropic enhancement of mobility upon doping has not been addressed and the quantitative relationship between structural modifications and mobility anisotropy remains insufficiently understood. Anisotropic increases and decreases in thermal conductivity resulting from doping-induced structural modifications have been reported, with the observed trends depending on the film's orientation and crystallinity prior to doping.³⁶ Therefore, to elucidate the effect of doping on anisotropic carrier mobility, semiconducting polymer films with well-controlled anisotropy and crystallinity must be adequately doped, and their carrier densities must also be quantitatively evaluated.

In this study, we investigate how ion-exchange doping affects charge transport anisotropy in highly uniaxially aligned poly(3,3′′′-didodecyl-quaterthiophene) (PQT-12) films. Large-area, uniform alignment was achieved via the floating film transfer method (FTM).¹⁹ In addition to the comprehensive structural and optical characterizations, the direction-resolved electrical measurements, combined with carrier density evaluations, elucidated how doping-induced structural modifications influence hole mobility along both the backbone and π–π stacking directions.

2. Results and discussion

2.1. Preparation and optical characterization of uniaxially aligned films

Uniaxially aligned PQT-12 films were fabricated using the previously reported floating film transfer method (FTM)¹⁹ with slight modifications, and then transferred onto glass substrates with a polydimethylsiloxane (PDMS) stamp (Fig. 1a and b). As schematically illustrated in Fig. 1a, PQT-12 films formed at the air–liquid interface exhibited the polymer backbone oriented perpendicular to the film extrusion direction. This modified transfer process using a PDMS stamp enables deposition onto both hydrophobic and hydrophilic substrates (Fig. 1b).

Fig. 1 Schematic illustration of (a) uniaxially aligned film fabrication based on the floating film transfer method (FTM) and (b) film transfer using a PDMS stamp. The mixture of glycerol and water (4 : 1, v/v) maintained at approximately 29 °C was used as the subphase. (c) Pictures of the transferred film taken with polarized light parallel and perpendicular to the backbone direction. (d) Polarized optical microscopy images under crossed-Nicols conditions. Yellow arrows indicate the direction of the backbone.

The films displayed pronounced optical anisotropy. When viewed through a linear polarizer, they showed clear angle-dependent intensity changes (“color fading”) characteristic of linear dichroism (Fig. 1c). Polarized optical microscopy under crossed-Nicols conditions further revealed strong birefringence (Fig. 1d). The films maintained highly uniform morphology over large areas—an advantage over alignment methods such as rubbing or mechanical stretching,^11,12,37 which often damage the films.

Optical anisotropy was examined in more detail by polarization angle-dependent ultraviolet–visible–near-infrared (UV-Vis-NIR) absorption spectroscopy. The UV-Vis-NIR spectra (Fig. 2b) showed strong polarization dependence, where θ denotes the angle between the incident electric-field polarization and the polymer backbone alignment (Fig. 2a). The normalized absorbance at 537 nm followed a cos² θ dependence, confirming that the transition dipole moments are predominantly aligned along the θ = 0° direction, corresponding to the polymer backbone (Fig. 2c). The dichroic ratio (DR) between θ = 0° and 90° was as high as 15.

Fig. 2 (a) A schematic description of the angle θ, which is defined as the angle between the backbone direction and the incident light. (b) Polarization angle-dependent absorption spectra and (c) a plot of normalized absorbance at 537 nm, normalized at θ = 0°. The solid line in (c) represents cos² θ. (d) A schematic illustration of the coordinate system used in the MMSE analysis with a biaxial optical model. Optical constant spectra of an undoped uniaxially aligned film for (e) refractive indices and (f) extinction coefficients.

To quantitatively resolve anisotropy in the optical constants, we applied Mueller matrix spectroscopic ellipsometry (MMSE)^38,39—a generalized form of variable angle spectroscopic ellipsometry—to model the films as optically biaxial media with independent backbone, π–π stacking, and lamellar directions (Fig. 2d). While spectroscopic ellipsometry has been widely applied to spin-coated isotropic polymer films, this is, to our knowledge, the first report of optical constant anisotropy determined for highly aligned conjugated polymer films prepared by FTM. Experimental Mueller matrix elements and best-fit spectra are shown in Fig. S4. The resulting refractive index (n) and extinction coefficient (k) spectra (Fig. 2e and f) showed that k is maximal along the backbone and much smaller along the π–π stacking and lamellar directions, in agreement with the UV-Vis-NIR data. Similarly, n exhibited the largest dispersion along the backbone, as expected from Kramers–Kronig consistency. Measurements were also performed on a control sample of isotropic spin-coated PQT-12 films (Note S1, Fig. S2) for comparison with the aligned samples. The out-of-plane n and k values of the isotropic films closely matched those along the z-axis of the aligned films, whereas the in-plane values corresponded to the orientational average of the backbone and π–π stacking directions in the aligned films. This agreement validates the MMSE-derived optical constants and confirms the pronounced optical anisotropy of the FTM films.

2.2. The effects of ion-exchange doping on optoelectronic properties

Anion-exchange p-doping of uniaxially aligned PQT-12 films was carried out following a reported procedure using a co-dissolved solution of oxidants 2,3,5,6-tetrafluoro-7,7,8,8-tetracyano-quinodimethane (F4TCNQ) and lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) in n-butyl acetate.³¹ In this process, F4TCNQ first oxidizes the polymer, generating unstable F4TCNQ⁻ anions, which are subsequently replaced by the chemically stable TFSI⁻ anions. To achieve a higher carrier density, we also employed a mixed solution of tris(4-bromophenyl)ammoniumyl hexachloroantimonate (TBPA⁺˙:SbCl₆⁻; magic blue)⁴⁰ as a stronger oxidant and LiTFSI in acetonitrile. The resulting films are hereafter referred to as the “F4TCNQ-doped film” and “TBPA-doped film”, respectively (Fig. 3a).

Fig. 3 (a) Schematic illustration of ion-exchange doping: F4TCNQ and LiTFSI are used for the F4TCNQ-doped film, while magic blue and LiTFSI are used for the TBPA-doped film. (b) Pictures of the F4TCNQ-doped film taken with the polarization parallel and perpendicular to the backbone. (c) UV-Vis-NIR spectra of undoped and doped films with polarization parallel to the backbone. (d) Photoemission yield spectra of undoped and doped films. Optical constant spectra of the F4TCNQ-doped uniaxially aligned film for (e) refractive indices and (f) extinction coefficients.

The UV-Vis-NIR absorption spectra (Fig. 3c) clearly demonstrate p-doping with both dopants: a pronounced polaron/bipolaron absorption band emerges in the NIR region, accompanied by bleaching of the neutral absorption peak near 530 nm. Importantly, the high uniaxiality of the films was preserved after doping, as evidenced by the polarization-angle-dependent optical appearance (Fig. 3b) and the strong polarization dependence observed in both neutral and polaronic absorption bands (Fig. S8). Compared with the F4TCNQ-doped film, the TBPA-doped film exhibited more pronounced bleaching of the neutral band and stronger polaron/bipolaron absorption in the NIR region (Fig. 3c).

Photoemission yield spectroscopy (PYS) revealed an increase in the ionization potential (IP) by 0.33 eV upon doping, from 5.12 eV in the undoped film to 5.45 eV in the F4TCNQ-doped film (Fig. 3d). Notably, although the HOMO edge of the F4TCNQ-doped film, deduced from IP, is deeper than the LUMO level of F4TCNQ (−5.2 eV),⁴¹ efficient p-doping is nevertheless achieved through the ion-exchange mechanism. This mechanism stabilizes the oxidized polymer with TFSI⁻ and removes the redox-active dopant, enabling high doping efficiency. In addition, the F4TCNQ-doped film exhibits excellent air stability at room temperature (Fig. S9), consistent with prior reports.⁴² The TBPA-doped films showed a further increase in IP of 5.54 eV (Fig. 3d), indicating a higher doping level than that achieved with F4TCNQ.

Optical constants extracted by MMSE for the F4TCNQ-doped film (Fig. 3e and f) and TBPA-doped film (Fig. S6) show that dispersion remains largest along the backbone direction, confirming that the uniaxial order is retained after doping. A weak polaronic absorption was also detected along the π–π stacking direction. The absorption is absent in the lamellar direction, suggesting that polaron formation in the amorphous region⁴² is not the primary contributing factor. Instead, it is more plausibly attributed to slight misalignment introduced during doping and/or to polaron delocalization along this direction.⁴³ Experimental Mueller matrix elements and best-fit spectra are shown in Fig. S5 and S7.

2.3. Conductivity, carrier density and hole mobility of doped films

For electrical characterization, Au electrodes were patterned along both the backbone (σ_‖) and π–π stacking (σ_⊥) directions, and current–voltage (I–V) characteristics were measured. The F4TCNQ-doped films exhibited currents more than eight orders of magnitude higher than those of the undoped film (Fig. S10), and the linear I–V curves allowed extraction of the conductivity (Fig. 4a). As summarized in Table 1, the average conductivity of the F4TCNQ-doped films along the backbone direction was 165 S cm⁻¹—about three times higher than that along the π–π stacking direction (62 S cm⁻¹)—consistent with the general preference for transport along the backbone. Notably, the TBPA-doped films showed a similar conductivity anisotropy of about 3, with a high σ_‖ of 251 S cm⁻¹, exceeding previously reported values for doped PQT-12.^44,45 Because the electrical conductivity follows σ = q × n × μ (where q is the elementary charge, n is the carrier density, and μ is the carrier mobility), quantifying both n and μ is essential to identify the origin of the conductivity enhancement.

Fig. 4 (a) I–V characteristics of F4TCNQ-doped and TBPA-doped films along the backbone and π–π stacking directions. (b) XPS survey spectra of F4TCNQ-doped and TBPA-doped PQT-12 films.

Table 1 Summary of the anisotropic conductivities, carrier densities, and mobilities of PQT-12 films

	σ ‖ (S cm^−1)	σ ⊥ (S cm^−1)	n (cm^−3)	μ ‖ (cm^2 V^−1 s^−1)	μ ⊥ (cm^2 V^−1 s^−1)
F4TCNQ-doped	165 (6)	62 (2)	8.06 × 10^20	1.28	0.48
TBPA-doped	251 (25)	78 (10)	1.19 × 10^21	1.32	0.41
Undoped (OFET)	—	—	∼5 × 10^18	1.5 (0.1) × 10^−3	2.8 (0.5) × 10^−4

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The number of charges generated within the polymer chains can be quantified by analyzing the corresponding counterions. However, whether these charges behave as free (mobile) carriers or remain partially localized is still under debate, and direct quantification of the mobile carrier density remains challenging. Previous studies have proposed that Coulombic trapping is negligible in ion-exchange doping.⁴⁶ Accordingly, in this study, the upper limit of the carrier density was estimated from the ion concentration determined by X-ray photoelectron spectroscopy (XPS), under the assumption that all generated charges act as mobile carriers. The survey spectrum of the F4TCNQ-doped films (Fig. 4b) shows only elements originating from PQT-12 and TFSI⁻, with negligible signals from Li and F4TCNQ,⁴⁷ confirming that excess ionic components were removed during rinsing. In contrast, TBPA-doped films showed the presence of the residual SbCl₆⁻ anion in the XPS spectra, while no Br signal was detected, confirming the absence of the unreacted dopant.

For quantification of the anion-to-monomer ratios, signals of C 1s, F 1s, S 2p, and O 1s in core-level spectra were analyzed (Fig. S12) following a previously reported procedure. Briefly, the integrated peak intensities were corrected using relative sensitive factors, and the ratios were chosen to minimize the root mean square error (RMSE)⁴⁸ between the measured atomic concentrations and those calculated for a composition of one TFSI⁻ per x monomer units of PQT-12. Details of the calculation are provided in Note S2 of the SI.

For the F4TCNQ-doped film, the minimum RMSE was obtained at x = 1.12 (Fig. S13). The monomer unit volume of PQT-12 was estimated to be 1.10 nm³ using the following structural parameters: a repeating length of 15.5 Å,⁴⁹ a lamellar spacing of 20.0 Å, and a π–π distance of 3.56 Å (from the X-ray diffraction analysis described in the next section). Based on the dopant ratio and unit volume, the carrier density was estimated to be 8.06 × 10²⁰ cm⁻³.

A similar analysis was performed for the TBPA-doped film. In this case, the F 1s peak, the C 1s peak at ∼292 eV and the S 2p peak at ∼169 eV from TFSI⁻ were smaller than those in the F4TCNQ-doped film, while a noticeable amount of Sb was detected (Fig. S12). Based on the procedure in Note S2, the anion ratio was determined to be TFSI⁻ : SbCl₆⁻ = 1 : 4, yielding a carrier density of 1.19 × 10²¹ cm⁻³. This higher carrier density compared with the F4TCNQ-doped film is consistent with the more pronounced polaron/bipolaron absorption observed in the optical spectra.

The hole mobilities (μ) of the F4TCNQ-doped film, calculated from the measured σ and n, were μ_‖ = 1.28 cm² V⁻¹ s⁻¹ and μ_⊥ = 0.48 cm² V⁻¹ s⁻¹ (Table 1). For the TBPA-doped film, the corresponding values were μ_‖ = 1.32 cm² V⁻¹ s⁻¹ and μ_⊥ = 0.41 cm² V⁻¹ s⁻¹, respectively. Note that because the upper limit of the hole density was used, the mobilities reported here correspond to the lower limit. The close similarity between the two sets of values indicates that the higher conductivity of the TBPA-doped films arises primarily from their increased carrier density rather than the change in mobility.

For comparison, the anisotropic mobilities of the undoped aligned film were determined from organic field effect transistors (OFETs) operated in the saturation regime, with an estimated carrier density of ∼5 × 10¹⁸ cm⁻³ (Table 1) assuming an effective accumulation thickness of 2.5 nm. Relative to the undoped film, the doped films exhibited mobility enhancements of approximately 850-fold along the backbone and 1700-fold along the π–π stacking direction (Note S3, Fig. S15). Because the relative increase was larger in the π–π stacking direction, the mobility anisotropy decreased from 5.4 in the undoped films to approximately 3 in the doped films.

2.4. Doping-induced structural modifications

The film orientation and lattice parameters were investigated by two-dimensional grazing-incidence wide-angle X-ray scattering (2D GIWAXS) while rotating the sample azimuthally (φ-axis) to control the X-ray incidence direction relative to the polymer backbone (Fig. 5a and Fig. S17).

Fig. 5 (a) Schematic illustration of the symbols used to indicate the direction of incident X-ray irradiation. (b)–(e) 2D GIWAXS patterns of undoped and F4TCNQ-doped films under different X-ray irradiation directions. (f) Line-cut profiles along the out-of-plane and in-plane directions, with the vertical axis shown on a logarithmic scale.

The undoped aligned PQT-12 film exhibited a predominantly edge-on orientation. When the beam was aligned parallel to the backbone, the (010) π–π stacking diffraction appears mainly in-plane (Fig. 5b), whereas it disappeared when the beam was perpendicular to the backbone (Fig. 5c), confirming strong uniaxial alignment. In addition, π–π stacking diffraction was also observed in a diagonal direction in Fig. 5b, indicating that some domains possess π–π stacking tilted toward the out-of-plane direction. No clear lamellar (h00) diffraction was observed along the out-of-plane direction, except for a small, diagonal feature, possibly from a tilted lamellar peak. These characteristics suggest limited lamellar coherence consistent with backbone torsional disorder. Line-cut profiles (Fig. 5f and Table S3) gave an in-plane π–π stacking distance of 4.07 Å, which was similar to the spin-coating films (Fig. S20 and Tables S8 and S9).

In contrast, the F4TCNQ-doped film exhibits pronounced structural ordering. The 2D GIWAXS pattern showed a strong series of lamellar reflections along the out-of-plane direction, together with enhanced in-plane (010) intensity when the beam is parallel to the backbone (Fig. 5d), confirming retention of uniaxial order after doping. Line-cut profiles (Fig. 5f and Table S3) reveal that the (010) peak shifted to higher q, corresponding to a reduction in π–π spacing from 4.07 Å to 3.56 Å. The lamellar series appearing in the doped film exhibited d-spacings of 20.0 Å, 10.0 Å, 6.63 Å, and 5.14 Å for the (100), (200), (300), and (400) orders, respectively. The TBPA-doped film exhibited a similar reduction in π–π stacking distance to 3.44 Å and showed a distinct lamellar peak with a d-spacing of 21.1 Å (Fig. S16 and Table S3). Higher-order lamellar diffractions were less pronounced, likely due to the coexistence of differently shaped TFSI⁻ and SbCl₆⁻ anions. Spin-coated PQT-12 films also exhibited comparable structural modifications and d-spacings, although the changes were less pronounced (Fig. S20, Tables S8 and S9), demonstrating the generality of the doping-induced structural changes irrespective of whether the film is isotropic or uniaxially oriented.

The substantial shortening of the π–π distance, together with lamellar expansion beyond the typical value for undoped PQT-12 (17.2 Å),⁵⁰ indicates that the TFSI⁻ counter anions reside within the side-chain lamellae without disrupting backbone packing, thereby improving overall crystallinity. This scenario is consistent with established mechanisms in which p-doping promotes quinoidal character, reduces backbone torsional disorder, and tightens π–π stacking in the polymer.^29,37,51 Indeed, the peak width of the π–π stacking diffraction decreased upon doping, corresponding to the increase in the crystal coherence length from 3.9 nm in the undoped film to 4.5 nm in the F4TCNQ-doped film. The observed structural changes rationalize the mobility enhancements: the 850-fold increase along the backbone direction suggests backbone planarization, while the even larger 1700-fold increase along the π–π stacking directions reflects improved interchain packing and more favorable charge transport across the π-conjugated network.

Considering that dopants have preferential locations in polymer films and that the evolution of the thin-film structure upon doping is affected by the initial crystallinity and molecular orientation,^35,36,42 it is important to clarify how PQT-12 films with different initial structures result in different doping efficiencies and mobility enhancements. Two isotropic spin-coated films with different crystallinities were prepared using either chloroform or chlorobenzene as the solvent, followed by doping. As detailed in Note S4, a similar effect of doping-induced structural modification on mobility enhancement was observed for the spin-coated films. However, the carrier densities and conductivities of the spin-coated films were lower than those of the uniaxially aligned film, indicating that the presence of amorphous regions in the spin-coated films limits the doping efficiency.³⁵ Nevertheless, an exceptionally high conductivity of 265 S cm⁻¹—comparable to that of the uniaxially aligned film—was achieved for the TBPA-doped spin-coated film prepared from a chlorobenzene solution. The superior conductivity achieved by spin-coating from chlorobenzene suggests that the initial crystallinity is also a key factor determining the doping efficiency and charge-transport properties of doped films. These findings imply that optimizing the film-processing conditions to achieve high crystallinity and maximal molecular alignment could pave the way toward applications such as highly conductive plastics.

3. Conclusion

Highly uniaxially aligned PQT-12 films with large-area uniformity were fabricated by the floating film transfer method (FTM) and transferred onto substrates by PDMS stamps. Optical and structural analyses confirmed that the uniaxial alignment was retained after ion-exchange p-doping with TFSI⁻, enabling clear evaluation of direction-dependent charge transport. XPS-based carrier density measurements, combined with electrical characterization, revealed mobility enhancements of 2–3 orders of magnitude along both the backbone and π–π stacking directions, attributable to doping-induced backbone planarization and reduced π–π stacking distance. These findings demonstrate that structural anisotropy and doping act cooperatively to optimize charge transport in aligned conjugated polymers. The combination of uniaxial alignment and efficient ion-exchange doping thus offers a promising strategy for achieving highly conductive polymer films.

4. Experimental section

4.1. Preparation of uniaxially aligned films

PQT-12 was purchased from Merck (906921, Lot: MKCV4226). The molecular weight of PQT-12 (Fig. S1) was determined by gel permeation chromatography (GPC, HLC-8420GPC, Tosoh) using CHCl₃ as the eluent at 40 °C and calibrated with a polystyrene standard. The uniaxially aligned film was prepared using the floating film transfer method (FTM).¹⁹ Specifically, a CHCl₃ solution of PQT-12 (10 mg mL⁻¹, 20 μL) was dropped on the interface between a PTFE plate and a liquid subphase composed of a glycerol : water mixture (4 : 1, v/v). For reproducibility, the subphase temperature was maintained at approximately 29 °C by placing the FTM container in a water bath with a temperature controller (TR-1α, AS ONE). The temperature and mixing ratio of glycerol and water were optimized to avoid the use of toxic ethylene glycol, which is typically employed in FTM. After confirming uniformity and a high dichroic ratio by eye using a polarizer (SPF-30C-32, Sigmakoki Co., Ltd), the film was transferred to a PDMS stamp (Sylgard 184, Dow Corning; precursor : crosslinker = 12 : 1, g/g) and subsequently transferred onto the substrate by pressing the PDMS stamp at 40 °C. The thickness of the FTM films was measured using a stylus profiler (Dektak 6M, Bruker), with the film-to-film thickness variation ranging from 10 to 16 nm.

4.2. Molecular doping

Anion-exchange doping was performed to prepare F4TCNQ-doped films according to a previous report.⁵² F4TCNQ (5 mM) and LiTFSI (100 mM) were co-dissolved in n-butyl acetate. Substrates with uniaxially aligned PQT-12 films were immersed in the doping solution at 60 °C for 10 min, then cooled to room temperature and briefly immersed in pure n-butyl acetate for 5 s to remove excess F4TCNQ and LiTFSI. TBPA-doped films were prepared by immersing the uniaxially aligned PQT-12 film in the mixed solution of magic blue (1.5 mM) and LiTFSI (30 mM) in dehydrated acetonitrile at 45 °C for 10 min, then cooled to room temperature and briefly immersed in pure dehydrated acetonitrile for 5 s. Because of the lower stability of magic blue combined with TFSI⁻,⁵³ the dopant solution was quickly prepared and used. F4TCNQ, LiTFSI and magic blue were weighed under ambient conditions, and the doping was conducted in a N₂-purged glove box. Photoemission yield spectroscopy (PYS) was performed in air using an AC-2 instrument (RIKEN KEIKI) with a monochromated D₂ lamp.

4.3. Optical evaluations

Polarized UV-Vis-NIR spectra were obtained using a spectrophotometer (V-670, JASCO) equipped with a rotary sample holder (RSH-744) and a polarizer (GPH-506) in transmission mode. Polarized optical microscopy images were acquired using a polarizing microscope (ECLIPSE LV100N POL, Nikon Instruments Inc.) under crossed-Nicols conditions.

Ellipsometry measurements were performed with a spectroscopic ellipsometer (RC2, J.A. Woollam). Four layers of uniaxially aligned films were transferred to quartz substrates using a PDMS stamp, and variable angle MMSE measurements were carried out. 4 × 4 Mueller matrices of the reflected spectra were measured at angles from 45° to 75° (in 10° increments), and the transmitted spectra at angles from 0° to 50° (in 5° increments), as well as the transmittance at normal incidence. The backbone of the polymer was aligned to the x-axis as defined by the instrument. To correct for slight misalignment between the backbone and the x-axis, the Euler angle φ (corresponding to rotation in the in-plane direction, i.e., rotation along the z-axis) was used as a fitting parameter.

Analyses were based on a biaxial optical model, in which Gaussian functions along the x-, y-, and z-axes were used to model each oscillation, assuming that the width and energy of the Gaussians were the same in all directions (with only the oscillation intensity differing among axes). Six and eight Gaussian functions were used for the undoped and doped films, respectively. Based on this fitting, spectra of the refractive index and extinction coefficient along the x-, y-, and z-axes were derived.

4.4. Electrical evaluations

The I–V characteristics of the doped films were evaluated using a Keithley 6430 source meter under vacuum (below 1 × 10⁻² Pa). A single layer of uniaxially aligned film prepared on a liquid subphase was transferred to a glass substrate using a PDMS stamp. Molecular doping was performed as described in Section 4.2, followed by deposition of Au (40 nm) by thermal evaporation through a metal shadow mask under a pressure of approximately 10⁻⁴ Pa. The films remained uniformly transparent, indicating that de-doping does not occur during deposition (Fig. S11). The channel width (W) and length (L) were 1000 μm and 200 μm, respectively, with both the parallel and perpendicular directions to the polymer backbone present on a single substrate.

4.5. Elemental analysis

XPS measurements were performed for a single layer of uniaxially aligned films prepared on ITO-coated glass substrates with a photoelectron spectroscopy system (PHI 5000 Versa Probe II, ULVAC-PHI Inc.) using monochromated Al Kα (1486.6 eV) radiation. The take-off angle was 45° to the sample substrate. Shirley backgrounds were subtracted from the XPS spectra to numerically calculate the areal intensity for each atom.

4.6. Structural evaluations

Two layers of uniaxially aligned films prepared on a liquid subphase were transferred to Si(100) substrates using a PDMS stamp and 2D GIWAXS measurements were performed at the BL13XU of SPring-8. Samples were mounted on a manual rotation stage (RS-313, Chuo Precision Industrial Co., Ltd) to control the in-plane rotation angle (φ-axis rotation). The X-ray energy was 12.40 keV (wavelength = 0.1 nm), and the angle of incidence was fixed at 0.12° using a Huber diffractometer. The 2D GIWAXS patterns were recorded with a two-dimensional image detector (Pilatus 300K, Dectris). The crystal coherence length was calculated as 0.9 × 2π/FWHM based on the Scherrer equation, where FWHM was derived by fitting with a Lorentz function.

Conflicts of interest

The authors declare no competing financial interests.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information: molecular weight distribution, VASE of spin-coated films, experimental raw data and fitting of MMSE, polarization dependent UV-Vis-NIR spectra of doped films, stability test of doped films, I–V characteristics of undoped films, details of XPS measurements, OFET characteristics of undoped films, X-ray irradiation-direction-dependent 2D GIWAXS, 2D GIWAXS patterns of TBPA-doped films, summary of d-spacing, and experiments on doped spin-coated films. See DOI: https://doi.org/10.1039/d5tc03170b.

Acknowledgements

This work received financial support from JSPS KAKENHI (JP 20H00393). We thank Mr. Jyunya Takashima (J. A. Woollam Japan) for fruitful discussions on the measurements and analysis based on Mueller matrix spectroscopic ellipsometry. 2D GIWAXS measurements were performed with the approval of the Japan Synchrotron Radiation Research Institute (JASRI; Proposal 2025A1652) at the BL13XU of SPring-8 with support from Dr. Tomoyuki Koganezawa (JASRI). We thank Dr. Hyeonwoo Jung (RIKEN) for his assistance with 2D GIWAXS measurements and Dr. Kyohei Nakano (RIKEN) for insightful comments and fruitful discussions.

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