Facile and controllable chemical doping of conducting polymers with an ionic liquid dopant

A facile method for chemical doping of conducting polymers is demonstrated with an ionic liquid containing FeCl4 − anions as the oxidizing agents. A drop of the ionic liquid on the film of a typical semicrystalline polymer immediately changed the room temperature conductivity to 500 S cm−1. The highly conductive state originated from both the high doping level and the high crystallinity of the doped film, as confirmed by optical absorption and X-ray diffraction measurements, respectively. Furthermore, the doping level was continuously controlled by the gate voltage of the ionic-liquid-gated transistor structure through an electrochemical dedoping process.

gated transistor structure through an electrochemical dedoping process.© 2024 The Author(s).Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd Supplementary material for this article is available online C harge carrier doping is an essential process in developing various electronic functions with conducting polymers that are applicable to flexible or wearable organic devices.Light-emitting electrochemical cells 1,2) and thermoelectric generators [3][4][5] are typical examples of such applications, and doping control is crucial for improving device performance. Fr optimization in the performance of thermoelectric devices, for example, sensitive control of the doping level is required since the electrical conductivity and Seebeck coefficient generally exhibit opposite trends upon carrier doping, which causes a peak in the power factor at a certain carrier doping level.6) In this context, an efficient and controllable doping method is required to produce polymer-based thermoelectric devices.
Recent progress in chemical doping methods with various dopants [7][8][9][10][11][12][13] has enabled the production of highly conductive polymer films, some of which exhibited extremely high room temperature conductivities exceeding 1000 S cm −1 . I general, however, most chemical doping methods require different samples to change the doping level, which causes a sample-to-sample dependence for the output of doped films/devices.13) Instead, the electrolyte gating method has recently been used to control the doping levels of polymer films, in which an electrolyte such as an ionic liquid is used as the gate insulator of a thin film transistor (TFT) structure.[14][15][16][17][18] Upon applying a gate voltage (V g ), electrochemical doping (dedoping) involving the infiltration (defiltration) of dopant ions into (from) the polymer film occurs.The doping level is sensitively controlled by using a single device up to very high doping levels.17,19) Indeed, we have demonstrated that the typical semicrystalline polymer poly [2,5-bis(3-alkylthiophen-2-yl)thieno(3,2-b)thiophene] (PBTTT) exhibits a macroscopic insulator-to-metal transition with a clear peak of the thermoelectric power factor upon doping with the ionic liquid [N,N-diethyl-N-methyl-N-(2methoxymethyl)ammonium][bis(trifluoromethanesulfonyl) imide] ([DEME][TFSI]) as the gate insulator.19,20,21) The TFSI -anion is frequently used for electrolyte gating, [16][17][18] as well as in recently developed anion exchange doping methods, 10,11,22) to realize a stably doped state.
In contrast to conventional chemical dopants, the ionic liquids adopted for use as gate insulators in TFT devices, such as [DEME][TFSI], exhibit negligible chemical reactivity with polymers.Thus, ionic-liquid-gated transistors usually act as "normally off"-type devices.In the present study, on the other hand, we demonstrate an ionic-liquid-gated "normally on"-type TFT with [1-ethyl-3-methylimidazolium (EMIM)][FeCl 4 ].This ionic liquid acted as an oxidizing agent with the conducting polymer PBTTT when the room temperature conductivity exceeded 500 S cm −1 for the aligned film.Furthermore, the doping level was easily controlled by the application of a positive gate voltage and by adopting the TFT structure through an electrochemical dedoping process.The present ionic liquid also enabled hole doping of poly(2,5-bis(2octyldodecyl)3,6-di(thiophen-2-yl)diketopyrrolo [3,4-c]pyrrole-1,4-dione-alt-thieno [3,2-b]thiophen (DPPT-TT), which has a deeper highest occupied molecular orbital (HOMO) level than that of PBTTT, allowing its use as a versatile oxidizing agent for p-type conducting polymers.
Samples of poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno (3,2-b)thiophene] [PBTTT-C14; Mw = 50,000; Fig. 1(a)] were purchased from Sigma-Aldrich Co., Ltd.DPPT-TT (Mw = 292,200) was purchased from Ossila Co., Ltd.The ionic liquid [EMIM][FeCl 4 ] (purity > 98%) was purchased from Tokyo Chemical Industry Co., Ltd.For the optical absorption measurements, an o-dichlorobenzene solution (20 mg ml −1 ) of PBTTT-C14 was spin coated on a 10 mm × 10 mm cleaned glass substrate (Eagle-XG) at 2000 rpm, followed by annealing at 150 °C for 30 min.For the grazing-incidence X-ray diffraction (GIXD) measurements, an o-dichlorobenzene solution (7.5 mg ml −1 ) of PBTTT-C14 was spin coated on the glass substrate at 1500 rpm, followed by annealing at 250 °C for 30 min.For the conductivity measurements, a uniaxially aligned thin film was fabricated via the flow coating method. 23)In this case, the solution of PBTTT-C14 (15 mg ml −1 ) with the mixed solvents of o-dichlorobenzene and chloroform (6:1) was dropped on the substrate and sheared by a glass blade at a constant speed of 2 mm s −1 , followed by annealing at 250 °C for 30 min.Then we obtained a uniaxially aligned PBTTT thin film with the typical film thickness of 30 nm.The typical optical dichroic ratio of the uniaxially aligned film was as high as 11 (Supplementary Fig. S1).The 30 nm-thick Au electrodes were then evaporated on top of the polymer film to form the side-gate transistor structure [Fig.1(a)], which enabled the gated four-terminal conductivity measurements.The channel length and width were 400 μm and 2 mm, respectively.For chemical doping, [EMIM][FeCl 4 ] was directly dripped on the polymer film, followed by spinning the sample at 6000 rpm to remove the residual ionic liquid on the film surface for absorption measurements.For the GIXD measurements, the residual ionic liquid was removed by syringe aspiration.
Optical absorption measurements were performed with a Varian Cary 5000 spectrometer.GIXD measurements were performed with a Rigaku FR-E Microfocus High Intensity X-ray generator system with a Cu Kα X-ray source (λ = 1.5418Å) at the High Intensity X-ray Diffraction Laboratory at Nagoya University.The room temperature conductivity exceeded 500 S cm −1 along the polymer backbone direction.We also measured the Seebeck coefficient of the doped PBTTT-C14 by using a couple of Peltier elements to induce a temperature gradient, as previously reported. 20,24,25)We observed a positive Seebeck coefficient of 9.4 μV K −1 after dripping the ionic liquid (Supplementary Fig. S2), indicating that the hole carriers were responsible for the observed conduction.These results clearly indicated that the ionic liquid [EMIM][FeCl 4 ] strongly oxidized PBTTT-C14.
To clarify the origin of the efficient charge-transport process, we measured the GIXD patterns of the PBTTT-C14 thin films before and after ionic liquid doping.Figure 2 shows the GIXD patterns obtained for the pristine (a) and doped (b) PBTTT-C14 films together with the out-of-plane (c) and  031002-2 © 2024 The Author(s).Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd in-plane (d) peak profiles.The spectrum of the pristine film exhibited a series of out-of-plane (h00) peaks up to the fourth order and an in-plane (010) peak corresponding to π-π stacking, indicating the formation of a highly ordered crystalline lamellar structure.For the doped state, we observed corresponding peaks with clear shifts toward smaller q z values for the (h00) peak and toward larger q x,y values for the (010) peak.The interlamellar distance changed from 21.4 Å to 26.7 Å with a 5.3 Å lattice elongation, and the π-π stacking distance changed from 3.67 Å to 3.48 Å with a 0.19 Å contraction due to doping.These changes in the lattice parameters were qualitatively consistent with those observed for chemically or electrochemically doped PBTTT thin films, 8,10,11,20) in which the dopant ions were included in the free space of the side chain regions between the lamellae.The dopant anions, i.e., FeCl 4 − as discussed below, were expected to be similarly located in the present doping process.In addition, we observed no broadening of the diffraction peaks after doping, indicating that doping-induced degradation of the crystallites was almost negligible, which was consistent with the observed high electrical conductivity.
Figure 3(a) shows the optical absorption spectra of PBTTT-C14 in the pristine and doped states.The absorption spectra of the pristine films were consistent with those reported previously, with the main π-π* absorption bands observed at approximately 555 nm. 26)After the dripping of [EMIM][FeCl 4 ] on the polymer film, the absorption spectra exhibited drastic changes, including complete bleaching of the π-π* absorption band.][32] In the UV region, we observed split peaks at approximately 250 nm, 320 nm and 370 nm for the doped state, which were reasonably ascribed to the deeply penetrated FeCl 4 -anions inside the polymer films. 11)Although the detailed doping process has not yet been elucidated, this result strongly indicates that the FeCl 4 − ion acted as a dopant in the present system.The penetration of FeCl 4 − ions into the bulk film occurred immediately and thoroughly after the addition of ionic liquid to the film, resulting in complete bleaching of the main absorption peak, followed by a rapid increase in the conductivity due to the generation of dense hole carriers, as shown in Fig. 1(b).
Strikingly, unlike the conventional chemical doping method, the doped state could be controlled if we used [EMIM][FeCl 4 ] as the liquid gate insulator of the transistor.Figure 4 shows the gate voltage dependence of the 4-terminal electrical conductivity (σ) along the chain direction of the aligned PBTTT-C14 film.Without the application of V G , the device exhibited a high conductivity exceeding 500 S cm −1 , as already mentioned.The current-voltage characteristics exhibit a good linearity as shown in Supplementary Fig. S3.Upon applying a positive V G , the conductivity gradually decreased, which indicated the occurrence of electrochemical dedoping due to the removal of FeCl 4 − ions from the bulk film.The chemical doping and electrochemical dedoping processes occurred reversibly with forward and backward sweeps of the gate voltage, as shown in Fig. 4. Thus, the present TFT device with the [EMIM][FeCl 4 ] gate insulator acted as a "normally on" device.][35][36] However, the dedoping mechanism in these cases is to deactivate the PSS dopant by the  031002-3 © 2024 The Author(s).Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd electrolyte ions, which is only applicable to the system where the charge carriers are introduced in advance as in the case of PEDOT:PSS.In contrast, the present method is expected to be applicable to various intrinsic semiconducting polymers since [EMIM][FeCl 4 ] itself acts as the dopant.Then, we checked below the applicability of [EMIM][FeCl 4 ] for doping other polymer materials.Figure 3(b) shows the optical absorption spectrum of DPPT-TT, which has a HOMO energy of −5.33 eV, 37) greater than that of PBTTT (−5.1 eV), 8) before and after doping with [EMIM][FeCl 4 ].The observed result is quite consistent with that for PBTTT; e.g., the main absorption peak at approximately 838 nm for the pristine film 37) was bleached, and the new high-energy polaron band at approximately 1250 nm and the low-energy band of the polaron/bipolaron/free carrier appeared 38) together with split peaks at approximately 250 nm, 320 nm and 370 nm due to the dopant FeCl 4 -ions inside the film.As in the case of PBTTT-C14, the main π-π* absorption band was completely bleached by doping.We also confirmed that the doped DPPT-TT exhibited a clear increase of I D by more than six orders of magnitude with the maximum 4-terminal conductivity of ∼60 S cm −1 , although it exhibited a gradual decrease for longer doping times as shown in Supplementary Fig. S4.These results indicate that the ionic liquid [EMIM][FeCl 4 ] is a strong and versatile dopant for a wide range of conducting polymers.
In summary, we demonstrated chemical doping/electrochemical dedoping of the semicrystalline polymer PBTTT-C14 by using the ionic liquid [EMIM][FeCl 4 ] as the oxidizing agent.The high crystallinity, as well as the high carrier density, resulted in a high room temperature conductivity of >500 S cm −1 along the chain direction of the aligned film in the chemically doped state.These doped states were well controlled by the gate voltage if the ionic liquid was used as the gate insulator of the electrolyte-gated TFT structure through electrochemical dedoping; the device served as a "normally on"-type transistor.The present ionic liquid also enabled the oxidation of DPPT-TT, which has a deeper HOMO level than PBTTT-C14, demonstrating the versatility of the present doping method.

Figure 1 (
b) shows the time dependence of the drain current (I D ) for the aligned PBTTT-C14 film.When the ionic liquid [EMIM][FeCl 4 ] was dripped on the polymer film as the liquid gate insulator of the TFT device, an immediate increase in I D was observed without applying the gate voltage (V G ), as clearly shown in the inset figure.

Fig. 1 .
Fig. 1.(a) Schematic illustration of the side-gate-type transistor structure with PBTTT-C14 as the semiconductor layer and the ionic liquid [EMIM][TFSI] as the gate insulator, together with the chemical structure of PBTTT-C14.(b) Time dependence of the drain current (I D ) of the PBTTT-C14 aligned film during the dripping process of the [EMIM][FeCl 4 ] drop.The applied drain voltage (V D ) was −10 mV.The inset shows the magnified plot around the instant of ionic liquid dripping.

Fig. 2 .
Fig.2.GIXD patterns for the (a) pristine and (b) doped states of the PBTTT thin film with the out-of-plane (c) and in-plane (d) peak profiles relative to the q z and q x,y directions, respectively.

Fig. 3 .
Fig. 3. Optical absorption spectra of (a) PBTTT-C14 and (b) DPPT-TT spin coated thin films in the pristine and doped states.The inset in (b) shows the chemical structure of DPPT-TT.

Fig. 4 .
Fig. 4. Gate voltage dependence of the electrical conductivity (σ) along the chain direction of the aligned PBTTT-C14 thin film chemically doped with the [EMIM][FeCl 4 ] layer, which also acted as a gate insulator for the transistor structure.The sweep rate of the gate voltage is 5 × 10 −4 V sec −1 .The arrows represent the direction of the V G sweep.