Electronic state of charge carriers, in particular, in highly doped regions, in thin-film transistors of a semicrystalline conducting polymer poly(2,5-bis(3-alkylthiophene-2-yl)thieno[3,2-b]thiophene), has been studied by using field-induced electron spin resonance (ESR) spectroscopy. By adopting an ionic-liquid gate insulator, a gate-controlled reversible electrochemical hole-doping of the polymer backbone is achieved, as confirmed from the change of the optical absorption spectra. The edge-on molecular orientation in the pristine film is maintained even after the electrochemical doping, which is clarified from the angular dependence of the g value. As the doping level increases, spin 1/2 polarons transform into spinless bipolarons, which is demonstrated from the spin-charge relation showing a spin concentration peak around 1%, contrasting to the monotonic increase in the charge concentration. At high doping levels, a drastic change in the linewidth anisotropy due to the generation of conduction electrons is observed, indicating the onset of metallic state, which is also supported by the temperature dependence of the spin susceptibility and the ESR linewidth. Our results suggest that semicrystalline conducting polymers become metallic with retaining their molecular orientational order, when appropriate doping methods are chosen.

Conducting polymers (CPs) have been attracting much interest as promising materials for solution-processed organic electronic devices such as thin-film transistors (TFTs), solar cells, or thermoelectric generators.1–4 Poly(3-hexylthiophene) (P3HT) and poly(2,5-bis(3-alkylthiophene-2-yl)thieno[3,2-b]thiophene) (PBTTT) represent a class of semicrystalline CPs exhibiting high mobilities of up to 1 cm2 V−1 s−1 that originate from two-dimensionally ordered lamellar structures.5,6 The electronic state of charge carriers in such high-mobility CPs is one of the key issues to be clarified, which governs both the physical properties of the material and the performance of the device. In general, nonlinear excitations, such as solitons and polarons, are typical spin/charge carriers in lightly doped CPs.7,8 At higher doping levels, doubly charged spinless bipolarons or even a metallic state have been demonstrated both experimentally and theoretically.7–11 

In highly doped semicrystalline CPs, electrical conductivities exceeding 1000 S cm−1 have been reported at room temperature,12–14 implying the formation of metallic states. Recent reports on Hall effect or magnetoconductance measurements also support the metallic state.15,16 However, except for a few cases,13,17 the conductivity tends to decrease at low temperatures presumably due to grain boundaries or structural/energetic disorders introduced by the dopant, following the variable-range hopping mechanism.15,16 In this case, it is indispensable to adopt microscopic methods to clarify the intrinsic electronic state in the crystalline regions.

Electron spin resonance (ESR) spectroscopy is a powerful spin probe in organic materials and devices, which provides microscopic information of charge carriers as well as the local molecular orientation around the carriers.18–28 We have previously demonstrated from the ESR measurements that the edge-on oriented crystalline grains of PBTTT transform into the metallic state when highly doped with fluoroalkylsilane (FTS) molecules based on the following observations:28 (i) temperature-independent Pauli spin susceptibility, expected for metals, and (ii) ESR line broadening due to the scattering of conduction electrons by phonons (Elliott mechanism). In addition, generation of spinless charge carriers, such as polaron pairs or bipolarons, was deduced from the small spin concentrations, although the absence of the spin-charge relation prevented the observation of the onset of the bipolaron formation. Toward the seamless understanding of the electronic state from polarons, bipolarons, to the metallic state in the doped PBTTT films, a wide-range control of the doping level is necessary.

For this approach, we adopt ionic-liquid-gated transistors in the present study. The electrolyte gating of organic TFTs has been widely adopted to achieve high carrier concentration up to 1015 cm−2 at the device interface, which enables to realize a low-voltage operation of the TFT, as well as to control the physical properties of the active layer.12,13,15–17,29–33 In the case of polymer semiconductors, ions can penetrate into the bulk film, causing an electrochemical doping.30 The charge carriers in such TFTs can be sensitively detected by the ESR method under the gate-bias application, similar as in the case of field-induced ESR (FI-ESR) spectroscopy on the solid-gated TFTs.18–27 Marumoto et al. have performed FI-ESR measurements on the ionic-liquid-gated P3HT transistors.34,35 They observed a characteristic angular dependence of the ESR linewidth in the highly doped regime, originating from the two-dimensional (2D) spin interactions of densely produced polarons within the edge-on oriented domains. They also found a saturation of the spin concentration as the gate voltage increases. No information, however, was obtained about the spin-charge relation, which is fundamental to identify the bipolaron formation, due to the absence of doping level estimation. Furthermore, no signature of the metallic state was observed in their previous study.

In this letter, we present the accurate spin-charge relation based on the direct comparison of the spin concentration with the charge concentration determined by the displacement current measurements. We observed a good agreement between spin and charge concentrations in the lightly doped PBTTT thin films, whereas only the spin concentration exhibited a peak around 1% per formula unit and it started to decrease at higher doping levels, indicating the formation of spinless bipolarons. The bipolaron formation was also supported by the optical absorption measurements. As the doping increased further, a certain contribution of the temperature-independent Pauli spin susceptibility was observed together with the anisotropic ESR line broadening due to the Elliott mechanism, characteristic of metals. These results clearly demonstrate the gate-controlled change of the electronic state from polarons via bipolarons to the onset of the metallic state in ionic-liquid-gated PBTTT transistors.

PBTTT with hexadecyl side chains (Fig. 1(a), Mw = 82 800) was purchased from Merck Co., Ltd. The device structure is shown in Fig. 1(a). A 3 mm × 30 mm quartz plate with 30-nm-thick Au/Cr source-drain electrodes was used as a substrate. Typical channel length and width were 0.1 mm and 15 mm, respectively. The substrate surface was treated by hexamethyldisilazane (HMDS). The PBTTT film was fabricated on the substrate by spin coating from dichlorobenzene solution (8 mg mL−1) at 2000 rpm and then annealed at 180 °C for 20 min. The thickness of the PBTTT film (d), determined from the optical absorbance,17 ranged from 20 nm to 35 nm. We separately prepared a viscous ionic-liquid film fabricated on a 3 mm × 25 mm polyethylene-naphthalate (PEN) substrate. An ethyl acetate solution of an ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMIM-TFSI) mixed with 10 wt. % poly(methyl methacrylate) (PMMA; Mw = 996 000) was dropped on the PEN substrate with an Au/Cr gate electrode pre-evaporated. The ionic-liquid film was then annealed at 120 °C. Finally, the PEN substrate was placed on the PBTTT film. For the optical absorption measurements, we used two indium-tin-oxide (ITO)-coated glass substrates instead of the Au/Cr patterned quartz or PEN substrates. By adopting the same procedures as above, a two-terminal capacitor structure of glass/ITO/PBTTT/EMIM-TFSI/ITO/glass was formed. The transistor characteristics were measured with a Keithely 2612 A source measure unit. ESR measurements were performed by using a Bruker E-500 spectrometer. The spin susceptibility was determined from the twice integration of the first-derivative FI-ESR signals calibrated by using CuSO4⋅5H2O as a standard. More details of the FI-ESR measurements were presented elsewhere.18,20,25

FIG. 1.

(a) Schematic illustration of the ionic-liquid-gated TFT in the present study together with the chemical structure of PBTTT (R = C16H33). Typical example of (b) transfer and (c) output characteristics obtained at room temperature. The left and right axes in (b) shows drain and gate currents, respectively.

FIG. 1.

(a) Schematic illustration of the ionic-liquid-gated TFT in the present study together with the chemical structure of PBTTT (R = C16H33). Typical example of (b) transfer and (c) output characteristics obtained at room temperature. The left and right axes in (b) shows drain and gate currents, respectively.

Close modal

Figures 1(b) and 1(c) show typical examples of transfer and output characteristics obtained at room temperature. The left and right axes in (b) show drain and gate currents, Id and Ig, respectively. The device exhibits a low gate-voltage (Vg) operation below 1 V together with a finite Id even at Vg = 0 V. This behavior is consistent with the FI-ESR measurements showing a clear signal at Vg = 0 V as described later. Here, we observe a finite hysteresis in the transfer characteristics due to the slow ionic motion, which can be caused by the ion penetration into the bulk film.12 

Figure 2 shows optical absorption spectra under the application of the gate voltage. Prior to the gate voltage application (0 V), we observe a typical π–π* absorption peak around 2.2 eV together with a weak vibronic shoulder. Under the application of negative Vg, the intensity of the π–π* peak decreases and a new absorption peak appears around 1.6 eV, which is ascribed to the polaron absorption of PBTTT.36 At high Vg region, the π–π* peak loses its intensity and the doped film becomes almost transparent in the visible region. These behaviors are consistent with the previous report,29 directly indicating that the doping takes place within the whole film thickness (∼35 nm), i.e., the doping process is not electrostatic but electrochemical. It is noteworthy that the absorption spectrum recovers almost completely to the initial shape when we apply positive Vg ( + 1 V) as shown in Fig. 2, indicating that the electrochemical doping takes place reversibly.

FIG. 2.

Optical absorption spectra obtained under the application of various gate voltages (Vg). The positive gate bias of +1.0 V was applied after the application of negative biases.

FIG. 2.

Optical absorption spectra obtained under the application of various gate voltages (Vg). The positive gate bias of +1.0 V was applied after the application of negative biases.

Close modal

We note here that the polaron absorption peak shifts to lower energy side at Vg = −1.5 V, as shown in Fig. 2. In accordance with this shift, the intersection point of the absorption spectra deviates from the isosbestic point observed in the spectra of lower Vg cases. This result may indicate the emergence of another absorption bands, presumably due to the formation of bipolarons, which are known to induce a single absorption band in the infrared region.37,38 Similar spectral change including the shift of isosbestic point has indeed been reported in chemically doped P3HT, which is ascribed to the formation of bipolarons.39 

In order to clarify the electronic state of doped charge carriers further, we measure the Vg dependence of the FI-ESR spectrum as shown in Fig. 3(a), where the signals are recorded at room temperature with the magnetic field (H) perpendicular to the substrate. We observe clear FI-ESR signals with g = 2.00315, which corresponds well with the case of π-electron carriers (positive polarons) on the edge-on oriented PBTTT molecules.24 A finite ESR signal observed even at Vg = 0 V is consistent with the transfer characteristics in Fig. 1(b).

FIG. 3.

(a) Gate voltage dependence of the first-derivative FI-ESR spectrum obtained with the magnetic field (H) perpendicular to the substrate at room temperature. (b) Spin-charge relation of a PBTTT TFT as a function of the gate voltage obtained at room temperature. The right axis shows the doping level per formula unit converted from the spin/charge concentrations using the cell dimensions of the PBTTT crystallite.40 

FIG. 3.

(a) Gate voltage dependence of the first-derivative FI-ESR spectrum obtained with the magnetic field (H) perpendicular to the substrate at room temperature. (b) Spin-charge relation of a PBTTT TFT as a function of the gate voltage obtained at room temperature. The right axis shows the doping level per formula unit converted from the spin/charge concentrations using the cell dimensions of the PBTTT crystallite.40 

Close modal

Figure 3(b) shows the Vg dependence of the spin concentration (red), determined from the spin susceptibility assuming the Curie law, compared with the charge concentration (blue) determined by integrating the time evolution of the gate current (displacement current) after the application of a constant Vg as n = (Ig − Igleak)dt/(eSd), where Igleak is the gate leak current in the steady state, e is the electron charge, and S is the film area.17 Here, we assume a uniform carrier distribution within the bulk film with a constant spin/charge concentration by considering the electrochemical nature of the doping process. The right axis shows the doping level per formula unit, calculated from the cell dimensions of PBTTT crystallites reported previously.40 At the low Vg region, spin and charge concentrations agree well with each other, indicating that the charge carriers are positive polarons, which have both spin and charge. On the other hand, spin concentration exhibits a peak around 1%, showing a decrease as the gate voltage increases further. Contrasting to this behavior, charge concentration exhibits a monotonic increase as Vg increases. As a result, a large difference between spin and charge concentrations is observed at the high Vg region, giving a strong evidence of the formation of spinless bipolarons.

Since the electrochemical doping causes an ion penetration into the bulk film, doping-induced orientational disorder can be expected. To explore this possibility, we measured angular dependence of the g value and peak-to-peak linewidth (ΔHpp) of the FI-ESR signal at various doping levels, as shown in Figs. 4(a)–4(c). We observe a monotonic angular dependence of the g value in Fig. 4(a), with the principal values of g = 2.0031 and g// = 2.0019. The g-anisotropy of g > g// is typical for the edge-on oriented PBTTT as schematically illustrated in Fig. 4(e).24 Interestingly, almost similar g-anisotropy is observed at every doping level, indicating that the edge-on orientation is not disturbed by the ion penetration even in the highly-doped state. This result is contrasting to the case of FTS-doped PBTTT, where small amount of the edge-on oriented PBTTT converts into the flat-on orientation due to the doping.28 Then, the ions are expected to be located either between the side chains of PBTTT or at the grain boundaries in the present case. The retention of edge-on orientation is one of the causes of high electrical conductivity reported in ionic-liquid-gated PBTTT TFTs.15–17,33

FIG. 4.

Angular dependence of the g value and peak-to-peak linewidth at various doping levels: (a) 0.25%, (b) 4.1%, and (c) 15%. (d) An example of the gate voltage dependence of ΔHpp obtained with the magnetic field perpendicular to the substrate. (e) Schematic illustration of the edge-on oriented PBTTT molecules on the substrate together with the principal axes of the g tensor. The solid curve in (a) is a guide to the eye. The solid curve in (b) is a fitting curve considering the 2D spin interactions.

FIG. 4.

Angular dependence of the g value and peak-to-peak linewidth at various doping levels: (a) 0.25%, (b) 4.1%, and (c) 15%. (d) An example of the gate voltage dependence of ΔHpp obtained with the magnetic field perpendicular to the substrate. (e) Schematic illustration of the edge-on oriented PBTTT molecules on the substrate together with the principal axes of the g tensor. The solid curve in (a) is a guide to the eye. The solid curve in (b) is a fitting curve considering the 2D spin interactions.

Close modal

Contrasting to the g value, angular dependence of ΔHpp exhibits a marked doping level dependence, as shown in Figs. 4(a)–4(c). At the lowest doping level, the anisotropy is reasonably ascribed to that of the proton hyperfine interaction in the edge-on oriented thiophene units, exhibiting the minimum and maximum when H is applied parallel to the molecular short axis (x-axis) and the chain axis (y-axis), respectively, where x and y axes are defined in Fig. 4(e). This behavior is consistent with the FI-ESR signal of the P3HT TFTs using solid gate insulators.18,20,22 On the other hand, when the doping level reaches several percent, the linewidth exhibits a completely different angular dependence, as shown in Fig. 4(b). The angular dependence in this case is well described by the formula ΔHpp(θ) = A(3cos2θ − 1)2 + B, where A and B are constants and θ is the angle between the magnetic field and the substrate normal, as shown in Fig. 4(e). Following the above expression, ΔHpp exhibits a minimum at an intermediate angle of θ  ∼ 55°, so-called magic angle. This type of angular dependence is derived from the theory of the exchange-narrowed ESR linewidth with 2D dipolar and exchange interactions41,42 and indeed be observed in the case of ionic-liquid-gated P3HT transistors.34,35 The 2D spin interaction is consistent with the high density of polarons as well as the retention of edge-on oriented crystallites forming 2D lamellar structures in the electrochemically doped PBTTT thin films as discussed above.

By increasing the doping level further, ΔHpp exhibits a clear broadening as shown in Fig. 4(d). In such a highly doped region, we observe a further change in the linewidth anisotropy, as shown in Fig. 4(c). The linewidth seems to exhibit a similar angular dependence with that of the g value. This behavior can be explained based on the Elliott mechanism, where the spin-lattice relaxation of conduction electrons, scattered by phonons, governs the linewidth.43,44 In this case, the linewidth depends on the g-shift (Δg) from the free electron value (2.0023) and is described as ΔHpp ∼ (Δg)2/τ, where τ denotes the spin-flip scattering time of conduction electrons. The observed anisotropy change of ΔHpp can thus be a signature of the onset of the metallic state in highly doped devices. Metallic signature is further supported by the temperature dependence of the spin susceptibility and the ESR linewidth by the following discussion.

Figures 5(a) and 5(b) show the temperature dependence of the spin susceptibility χ and ΔHpp in lightly (blue) and highly (red) doped states, respectively, obtained for the identical device. The spin susceptibility is given in the form of χT vs. T plot, where temperature-independent Pauli spin susceptibility gives a constant slope, whereas the Curie spin susceptibility (∝1/T) of isolated polarons gives a constant value proportional to the spin concentration.28 In the lightly doped state, χT is almost constant down to 120 K. ΔHpp in this case exhibits a clear narrowing above 100 K, consistent with the motional narrowing of isolated polarons observed for the PBTTT TFTs with a solid gate insulator.24 This result indicates that the charge carriers are polarons in the lightly doped device. Interestingly, the spin concentration exhibits a clear decrease below 120 K. This may indicate the stabilization of bipolarons at low temperatures, although the entropy effect favors polarons at high temperatures at a given doping concentration.45 

FIG. 5.

(a) Temperature dependence of the spin susceptibility (χT vs. T plot) obtained for highly (red) and lightly (blue) doped states of the same device. Corresponding temperature dependence of ΔHpp, obtained with the magnetic field perpendicular to the substrate, is shown in (b). The solid line in (a) represents the linear fit to the experimental data. The dashed line shows the constant fitting down to 120 K. ΔHpp in the FTS-doped PBTTT, taken from Ref. 28, is also shown in (b).

FIG. 5.

(a) Temperature dependence of the spin susceptibility (χT vs. T plot) obtained for highly (red) and lightly (blue) doped states of the same device. Corresponding temperature dependence of ΔHpp, obtained with the magnetic field perpendicular to the substrate, is shown in (b). The solid line in (a) represents the linear fit to the experimental data. The dashed line shows the constant fitting down to 120 K. ΔHpp in the FTS-doped PBTTT, taken from Ref. 28, is also shown in (b).

Close modal

In the highly doped state, on the other hand, χT exhibits a linear slope characteristic of the Pauli spin susceptibility. The corresponding linewidth exhibits almost linear broadening for high temperatures expected for the Elliott mechanism.11 Similar line broadening is also observed in the case of FTS-doped PBTTT, as shown in Fig. 5(b).28 These results indicate the formation of metallic state in the ionic-liquid-gated PBTTT transistors. However, we note here that the Pauli spin susceptibility obtained for the present device is 7 × 10−9 emu cm−3, which is about 50%–70% of the FTS-doped PBTTT thin films. In the case of the FTS-doped PBTTT thin film, ∼20% of the total film converted into metallic state due to inhomogeneous (surface-rich) doping.28 Thus, the metallic volume fraction in the present TFT is below 15%, although the electrochemical doping takes place homogeneously, as shown in Fig. 2. Such low fraction may be reasonably understood if the system is at the onset of metallic state, since the doping level is slightly (a few times) lower than that of our previous study where the metallic temperature dependence was observed in dc conductivity.17 It should also be emphasized that our present study provides direct evidence for the existence of bipolaronic state as the precursor of metallic state, consistent with the theories,7,8 and the switching between these electronic states can easily be performed reversibly by simply changing the gate-bias of ionic-liquid TFTs at room temperature.

In order to drive the system beyond the insulator-to-metal transition, further increase in the doping level should be necessary. However, the strong ESR line broadening, shown in Fig. 4(d), affects the S/N ratio of the signal as the doping level increases, preventing the ESR observation of “metallic” TFTs at room temperature. In this context, detailed FI-ESR measurements at sufficiently low temperatures, where the line broadening is suppressed, should be an interesting subject. This may clarify more detail about the electronic properties around the insulator-to-metal transition, such as how the metallic domains evolve and govern the charge transport as the doping level increases, which are left open for further studies.

In summary, we have studied the electronic state of charge carriers in highly doped PBTTT thin films in ionic-liquid-gated TFTs by using the FI-ESR technique. Even in the electrochemical nature of the doping process, the ion penetration into the PBTTT film does not disturb the edge-on orientation in the crystallites as confirmed from the g anisotropy. Based on the analyses of the spin-charge relation, spin susceptibility, and ESR linewidth, a clear change in the electronic state of PBTTT is demonstrated in the ionic-liquid-gated TFTs; from polarons via bipolarons to the onset of the metallic state as the doping level increases. These results suggest that, when appropriate doping methods are chosen, semicrystalline CPs form metallic state with retaining their crystallinity.

This work was partially supported by Grants-in-Aid for Scientific Research (Nos. 25287073, 26400315, and 23225005) from the Japan Society for the Promotion of Science (JSPS).

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