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Cite This: Chem. Mater. 2019, 31, 4759−4768
Nanodroplet-Embedded Semiconducting Polymer Layers for Electrochemically Stable and High-Conductance Organic Electrolyte-Gated Transistors Yaena Na and Felix Sunjoo Kim* School of Chemical Engineering and Materials Science, Chung-Ang University, Seoul 06974, Republic of Korea
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ABSTRACT: Organic electrolyte-gated transistors (OEGTs) utilizing high-capacitance ion gel dielectrics for low-voltage electronics often suffer from unintentional electrochemical doping processes (i.e., ion migration and charge transfer) and device degradation due to ion penetration into the semiconducting layer. Here, we report a simple and novel approach to fabricate a nanodroplet-embedded semiconducting polymer layer for reducing the undesired electrochemical doping processes of polymer-based OEGTs using poly(3hexylthiophene) (P3HT) as a semiconducting layer and an insulating polymer/ionic liquid gel as a gate insulator. The semiconductor P3HT is cast from a stock solution blended with a small amount of ionic liquid (IL) as a nanodroplet component. Electrochemical devices employing the IL nanodroplet-embedded P3HT (P3HT/IL) film as an active layer exhibit ideal capacitive polarization and reliable transistor operation. Electrochemical impedance spectroscopy reveals that the P3HT/ IL blends share a structural similarity with the gate dielectric ion gels. The IL nanodroplets dispersed in the active layer secure the local charge balance in the active layer and therefore reduce the slow electrochemical transport processes under applied biases. In OEGT characteristics, the current level of the nanodroplet-embedded devices becomes higher and more stable against operational bias stressing. Additionally, our devices show little hysteresis compared to that shown by the device of pristine P3HT without IL nanodroplets.
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INTRODUCTION The technological evolution of organic thin-film transistors in recent years has been toward manufacturing a reliable and high-performance device with low power consumption. In particular, extensive research is being conducted on increasing the effective capacitance (Ci) of gate-insulating components to lower the operational voltages. High-capacitance dielectric layers have been developed using materials with high dielectric constants,1−3 ultrathin dielectric layers,4−7 and electrolytebased insulators (e.g., ionic liquids (ILs), ion gels, and polyelectrolytes).8−13 Among many methods, ionic electrolytes are highly sought for their prime benefit of enormous Ci on the order of several microfarads per square centimeter, far exceeding that of other conventional dielectrics. Organic transistors using those electrolytes as a gate insulator are referred to as organic electrolyte-gated transistors (OEGTs). In OEGTs, the nanometer-thick electric double layer has a high capacitance at the semiconductor/dielectric interfaces when the gate voltage is applied. It sequentially derives a high charge-carrier density within the bulk semiconductor even at a low gate voltage. Thus, the high density of charge carriers allows OEGTs to operate with a high current level at low voltages, and the charge-carrier mobility of transistors is also highly enhanced by the trap-filling effect.14,15 © 2019 American Chemical Society
Initial studies on OEGTs using polymers as active materials employed inorganic salts such as LiClO4 in poly(ethylene oxide), which exhibited gate-insulating properties at low operational voltages, although the source−drain current was low because of their slow polarization.16−18 Recently, poly(IL)s and solidified polymer composite electrolytes consisting of ILs dispersed in a polymer matrix, known as ion gels, have been studied as novel electrolyte gating systems with high-Ci gating.19 These ion-conducting electrolytes can achieve fast signal switching based on their high ionic conductivities. Especially, solid-state ion gels are in principle practical for high-performance organic transistors. The combination of 1ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]) and poly(vinylidene fluoride-hexafluoropropylene) (P(VDF-HFP)) is a notable example of a solid-state ion gel electrolyte.20 In the past several years, many research groups have varied the ion gel components in OEGTs to enhance the mechanical integrity9,10,21,22 and simplify the filmprocessing strategies.23,24 Received: March 11, 2019 Revised: May 14, 2019 Published: June 5, 2019 4759
DOI: 10.1021/acs.chemmater.9b00995 Chem. Mater. 2019, 31, 4759−4768
Article
Chemistry of Materials
Figure 1. (a) Chemical structures of P3HT, [EMIM][TFSI], and P(VDF-HFP). (b) Schematic of the fabrication procedure for the IL nanodroplet-embedded P3HT (P3HT/IL) films.
processes such as ion penetration and migration when a gate bias is applied. Therefore, the resultant electrochemical device showed ideal polarization and reliable operation under gate biasing. The structure and charge-transport properties of P3HT/IL blend thin films were investigated first, and the idealized polarization due to reduced electrochemical reaction at the ion gel/semiconductor interface was evaluated using electrochemical impedance spectroscopy (EIS). Finally, the stabilized current level under cyclic operation was demonstrated.
Despite the many promising properties of ionic electrolytes, there are common drawbacks in the electrolyte gating process. When a small gate voltage is applied, the field-induced electrostatic charges are normally injected into the semiconductor, forming a two-dimensional channel at the interface. When the gate voltage becomes significantly larger, however, mobile ions initially distributed at the semiconductor/electrolyte interface penetrate the bulk of the semiconductor. As a result, this electrochemical reaction in OEGTs often leads to electrochemical doping of the bulk semiconducting layer and severe degradation of devices.11,14,15,25−28 This volumetric doping of OEGTs results in undesired hysteresis and an inconstant current level due to ion trapping and chemically or physically affected semiconductor structures.29 To resolve the electrochemical doping problems in OEGTs, several attempts have been made in preventing ion diffusion into the semiconductor. Proton-conducting polyanions, which are immobile even after the gate voltage is applied, have been used as electrolytes to reduce the electrochemical doping of the semiconducting layer.11,30,31 In another study, a polar elastomer with a low concentration of ions was used, and its high bias stress stability has been demonstrated.32 Additionally, the restriction of the ion migration into the semiconducting layer by a blocking layer at the semiconductor/electrolyte interface16 or in the few monolayers of the conjugated polyelectrolytes has been studied.33 In spite of various efforts to utilize new materials for preventing the aforementioned issues of ion penetration, however, no method has been proposed for tackling the undesirable doping while maintaining the high performance of the widely studied OEGT systems. Herein, we propose a simple and new method for simultaneously reducing the undesired electrochemical doping processes (i.e., ion migration and charge transfer) and improving the channel conductance of OEGTs employing poly(3-hexylthiophene) (P3HT) as a semiconductor and P(VDF-HFP)/[EMIM][TFSI] ion gel as a gate dielectric. We incorporated a small amount of IL into the p-type polymer (P3HT) as a processing additive and used this nanoscale droplet-embedded film as the active layer in electrolyte-based devices. By making the semiconducting layer and ion gel layer in similar structures with both positive and negative ions in the nanodroplets, we were able to obtain the charge balance in the polymer semiconducting layer and reduce the transport
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RESULTS AND DISCUSSION Figure 1 shows the chemical structures of the polymers and the ionic liquid (IL) and a schematic of the processing procedure of IL nanodroplet-embedded P3HT (P3HT/IL) films. P3HT and P(VDF-HFP) were used as the semiconductor and the dielectric polymer, respectively. P3HT was first dissolved in chloroform at a concentration of 10 mg/mL at 50 °C. Before IL injection, dilution of the IL in either acetone or chloroform was necessary to precisely control the added amount and obtain a homogeneous P3HT/IL blend solution and uniform films. We note that acetone is a good solvent for ILs but not for P3HT. On the other hand, chloroform is the main solvent for P3HT in this work, but it poorly dissolves ILs, unlike other polar IL solvents. The amount of IL in acetone or chloroform was adjusted to a 1:10 volume ratio, and now we call them as “Ac-IL” and “CF-IL”, respectively. Ac-IL or CF-IL solutions were then gradually added into P3HT solutions with various volume ratios (0, 1, 2.5, 5 vol %, and more) and then stirred at 50 °C for additional 1 h. After the solution preparation, the P3HT/IL blend film was deposited onto a glass substrate via spin-coating and used as the active component of electrochemical capacitors and transistors based on the P(VDFHFP)/[EMIM][TFSI] ion gel dielectric. Compared to a neat P3HT film with an average thickness of 63.4 (±0.1) nm, P3HT/IL blend films with 1, 2.5, and 5 vol % Ac-ILs got thicker, having thicknesses of 161 (±4.4), 220 (±1.9), and 225 (±4.6) nm, respectively. In comparison, P3HT/IL blend films mixed with CF-ILs were thinner, having thicknesses of 115 (±9.6), 128 (±9.8), and 131 (±8.2) nm, respectively. The effects of IL injection on the optical absorption properties of P3HT were studied using ultraviolet−visible (UV−vis) spectroscopy.34,35 The normalized absorption 4760
DOI: 10.1021/acs.chemmater.9b00995 Chem. Mater. 2019, 31, 4759−4768
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Chemistry of Materials
indicating that the IL has no influence as a chemical dopant other than its P3HT aggregating effect.39,40 To identify the IL phase at the surface of P3HT/IL blend films, we performed atomic force microscopy (AFM) and scanning electron microscopy (SEM) imaging (Figure 3 and
spectra of dilute P3HT and P3HT/IL blend solutions (0.005 mg/mL) with various Ac-IL and CF-IL compositions are shown in Figure 2a,b. The spectrum of pristine P3HT in a
Figure 2. Normalized UV−vis absorption spectra of P3HT and P3HT/IL blends. (a, b) Absorption spectra of the dilute solution (0.005 mg/mL) in chloroform with (a) Ac-IL and (b) CF-IL. (c, d) Absorption spectra of thin P3HT/IL films processed with (c) Ac-IL and (d) CF-IL.
dilute solution exhibits only one peak at 450 nm, which is associated with the intrachain π−π* transition of P3HT, being free from any intermolecular interactions. When Ac-ILs or CFILs were incorporated into the P3HT solutions, no significant difference was found, except for low-intensity signals at lower energies, which appeared to be vibronic bands at 558 and 608 nm, indicating P3HT molecular crystallites. In contrast, thinfilm absorption spectra of P3HT and P3HT/IL blends show different profiles depending on the amount of injected solutions (Figure 2c,d). The P3HT films have a dominant peak at 530 nm, corresponding to an intrachain π−π* transition, with two shoulders at 580 and 610 nm, indicating intermolecular interactions.36 The P3HT/IL blend films also exhibited a dominant peak and two shoulders. However, the intensity of the shoulders increased, which is attributed to the enhanced molecular ordering and aggregation of P3HT molecules. When Ac-IL was injected, the intensity of the shoulder peaks increased and gradually red-shifted with increasing Ac-IL. In contrast, CF-IL-incorporated P3HT/IL blends presented no dependence on the amount of CF-IL. The intensity and position of the peaks are all similar from 1 to 5 vol % CF-IL. This indicates that Ac-IL induced more effective aggregation on P3HT than CF-IL and had a comparable aggregation effect to other solvent additives.36−38 To further examine the effects of IL on the solid-state molecular packing and electronic structure of P3HT, we monitored the XRD patterns and Raman spectra of P3HT/IL blend thin films (Figure S1). In the out-of-plane XRD profiles, there is no significant difference in the peak position and intensity of the P3HT and P3HT/IL films. Moreover, all the Raman spectra of P3HT/IL blends overlap well with that of pristine P3HT,
Figure 3. Topographic AFM images of nanodroplet-embedded P3HT films after IL removal. IL was selectively removed out by dipping in ethanol. (a−f) P3HT/IL films with Ac-IL ((a) 1 vol %, (b) 2.5 vol %, and (c) 5 vol %) and with CF-IL ((d) 1 vol %, (e) 2.5 vol %, and (f) 5 vol %). (g, h) TOF-SIMS depth profiles of P3HT/IL films with 5 vol % (g) Ac-IL and (h) CF-IL and a corresponding illustration of the distribution of ILs inside the blend films.
Figures S2−S4). Because IL nanodroplets on the surface of the films hindered the morphological observation in AFM, we selectively dissolved the IL phase by dipping the films in ethanol, which is a good solvent for the IL but poor for P3HT (Figure S2). Figure 3a−f shows topographic AFM images of P3HT/IL blend films after the selective removal of IL nanodroplets. All films had isolated pores with submicrometer or a few micrometer scales originating from the removed IL phase. We also observed IL-removed porous regions with a similar size at the bottom side of the films (Figure S3), suggesting that IL nanodroplets could be well dispersed through the entire depth of the blend layers. The size of the pores in P3HT/Ac-IL films largely increased with more Ac-ILs in the blend. We attribute that this is the evidence of molecular aggregation of P3HT induced by Ac-IL and of a large degree of phase separation. In the case of CF-IL, 4761
DOI: 10.1021/acs.chemmater.9b00995 Chem. Mater. 2019, 31, 4759−4768
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Chemistry of Materials
Figure 4. Electrolyte-gated transistor characteristics of P3HT and P3HT/IL blends. (a) Output characteristics and (b) transfer curves (VDS = −1 V) of pristine P3HT, P3HT/Ac-IL 5 vol %, and P3HT/CF-IL 5 vol % OEGTs at a sweep rate of 50 mV/s. The channel width/length ratio is 10.
increased after 25 nm of sputtering, indicating that there was less IL in the upper surface and more in below. When the IL was fully dissolved in acetone (Ac-IL) and incorporated into P3HT, it formed the films of P3HT/IL blends with evenly distributed IL in the vertical direction, similar to an ionic electrolyte. On the other hand, less soluble IL in CF (CF-IL) subsided in the P3HT film and was enriched at the bottom of the film. The schematics in Figure 3g,h illustrate the vertical distributions of IL in P3HT/Ac-IL and P3HT/CF-IL films based on the collective information obtained via AFM, SEM, EDS, and TOF-SIMS analyses. The differences in the IL distribution along the vertical direction depending on the IL processing conditions would affect the electrical properties of P3HT/IL blends. We performed current−voltage analysis, multicycling tests of OEGTs, and electrochemical impedance spectroscopic (EIS) analysis of capacitors to examine the effects of IL incorporation into the semiconducting polymer layer on the electrochemical device performance. We first fabricated ion gel-gated transistors with a top-gate bottom-contact structure using P(VDF-HFP) as a host matrix with [EMIM][TFSI] as the ionic components. Thin films of the active semiconducting channel were prepared by spin-coating a pristine P3HT or P3HT/IL blend solution onto a glass substrate after patterning the source and drain electrodes. The channel width (W) and channel length (L) of transistors were 1 mm and 100 μm, respectively. The freestanding films of the P(VDF-HFP) ion gel and PEDOT:PSS gate electrode were then sequentially transferred onto the transistor channel. Figure 4a,b shows the OEGT output characteristics (IDS vs VDS) and transfer characteristics (IDS vs VG) for the pristine P3HT films and representative P3HT/Ac-IL 5 vol % and P3HT/CF-IL 5 vol % blend films. The output curves at six
the size of IL regions was smaller than that of Ac-IL with the same ratio. As shown by the SEM images after removing IL nanodroplets by ethanol dipping (Figure S2), the average pore diameters of P3HT/Ac-IL and P3HT/CF-IL films, both with 5 vol % addition, were ∼5 μm and ∼300 nm, respectively. The elemental mapping on the surface of P3HT/IL blend films without IL removal was also conducted using the energydispersive spectroscopy (EDS) implemented in the FE-SEM (Figure S4). The samples were stored under high vacuum ( θ > −90°) and resistive (0° > θ > −45°) phase behaviors. We can employ an equivalent circuit comprising two resistor−capacitor (RC) circuits connected in series: one for a bulk ion gel capacitor (CIG) and a resistor (RIG) connected in parallel and the other for an EDL capacitor (CEDL) and a resistor (REDL), also connected in parallel. In electrostatic charging dynamics within the electrolytes, a relatively small CIG has a faster response appearing at a high frequency, and the larger CEDL causes a slower response (i.e., a low frequency). Other than this two4764
DOI: 10.1021/acs.chemmater.9b00995 Chem. Mater. 2019, 31, 4759−4768
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Chemistry of Materials
P3HT. At this point and above, the MIM and MIS capacitors share a structural similarity, and the ion transport across the semiconductor/ion gel interface was invisible. Considering the similarity of both configurations and the properties of the MIS capacitors and OEGTs, it was expected that P3HT/IL blend-based OEGTs would be relatively free from undesirable electrochemical doping. As observed in the charging behavior of the MIS capacitor under VG biasing, because the P3HT/IL blend layer already has ions and the equal numbers of counterions in the nanodroplets similar to the ion gel structure, the electrostatic charges are balanced throughout the operation, and the slow process of ion transport is not required. To demonstrate the reliability and robustness of our OEGTs against electrochemical doping processes, the multicycling stability of the devices was evaluated by sweeping the gate voltage in the forward and reverse directions.47,48 The hysteresis curves for 150 scans of OEGTs of neat P3HT, P3HT/Ac-IL 5 vol %, and P3HT/CFIL 5 vol % are displayed in Figure 6. Pristine P3HT exhibited a
P3HT/IL blends can be attributed to the structural similarity of the ion gel layer and the IL-embedded semiconducting layer. Both layers of the polymer/IL mixtures have charge-balanced ion pairs distributed through the entire film. Although only a single P3HT/IL blend layer in an MSM capacitor cannot act as an EDL charging medium due to the low AC conductance, an MIM capacitor of P(VDF-HFP) mixed with the same ratio of Ac-IL 5 vol % showed a significant increase in the EDL capacitance to ∼1 μF/cm2 (Figure S8). This observation suggests that the structural similarity between the P3HT/IL blend layer and the P(VDF-HFP)/IL blended ion gel layer can be achieved by incorporating a little amount of IL into the active semiconducting polymer layer. Furthermore, the VG dependence of the phase angles at a low frequency decreased significantly after the incorporation of Ac-IL or CF-IL. Especially, for the P3HT/Ac-IL MIS capacitor, the phase behavior started to deviate from the pure P3HT MIS behavior with the addition of just 1 vol % AcIL and became totally VG-independent when 5 vol % Ac-IL was added. The VG-independent behaviors of the MIS capacitors of P3HT/Ac-IL layer with 5 vol % or more of Ac-IL, similar to the behaviors of the ion gel MIM capacitor, suggest a minimal involvement of electrochemical transport processes within the bilayer of ion gel/semiconductor. On the other hand, P3HT/ CF-IL required a larger volume of the IL solution for the transition from an MIS to MIM-like behavior, and complete VG independence at a low frequency was not observed, even at 15 vol % CF-IL addition. P3HT/CF-IL blend layers exhibit only a pseudo-MIM response and are not free from electrochemical reactions within the interfaces. These differences indicate that the vertically homogeneous IL distribution in the P3HT/IL blend film is highly advantageous for reducing penetration of the interfacial anions to the active layer. We also confirmed the transition from MIS to MIM characteristics by incorporating the IL into the semiconducting layer using the frequency-dependent capacitance (Ci vs f) and Nyquist plots (Z″ vs Z′). Figure 5e,f shows the ion gel MIM capacitance and P3HT/IL MIS capacitance. In contrast to the MIM capacitor, which exhibited ideal polarization and high capacitance values, the EDL transition regime for the pure P3HT MIS capacitor was bleached, and the Ctotal value was reduced by a factor of more than two, because reduced EDL capacitance with the addition of CEC lowered the Ctotal.15,45 As the Ac-IL or CF-IL ratio increased, the EDL charging characteristics approached those of the ideal MIM capacitor, and the MIS capacitance value increased to as high as the MIM capacitance. The average ion gel MIM capacitance at 20 Hz was 8.39 (±0.38) μF/cm2, and the MIS capacitances of P3HT with 0, 1, 2.5, and 5 vol % Ac-IL were 2.58 (±0.25), 7.28 (±0.85), 7.25 (±0.44), and 8.01 (±0.92) μF/cm2, respectively. Even if the thickness of the neat P3HT film increased to those of the IL embedded-P3HT blend films, there was no significant difference in the MIS capacitance. The MIS capacitance values at 20 Hz were 1.44 (±0.22) μF/cm2 for 95 nm P3HT and 2.53 (±0.50) μF/cm2 for 220 nm P3HT (Figure S9). Likewise, in the Nyquist plots (Figure S10), the slope of the MIS capacitor with P3HT/IL blends became similar to that of the MIM structure with a consistent slope of typical electrolytes.43,46 On the basis of these EIS results, we conclude that the MIS behavior resembled the MIM behavior as the IL ratio in the semiconducting layer increased. There was a transition point where the ion gel layer and the semiconducting layer behaved as a single layer like when 5 vol % Ac-IL was blended with
Figure 6. Operational stability of OEGTs with (a) pristine P3HT, (b) P3HT/Ac-IL 5 vol %, and (c) P3HT/CF-IL 5 vol % at a sweep rate of 100 mV/s. (d) Relative hole mobility (μsat) of the OEGTs in the saturation region, with respect to the number of cycles.
drastic increase by a factor of 10 in the off-current, caused by the anion penetration into the bulk semiconductor and electrochemical doping. Unlike the off-current, the on-current decreased by a factor of 1.3. This decrease in the on-current is attributed to trapping of the field-induced holes by the polarizable electrolytes.14,46,49 As a result, after 150 cycles of operation, the hole mobility decreased to 70% of its initial value, as shown in Figure 6d. In contrast, in the case of the P3HT/Ac-IL 5 vol % blends, at which the EIS characteristics of the MIS capacitor were identical to those of the MIM capacitor, both on-current and off-current remained almost constant with very small hysteresis. The on-current of P3HT/Ac-IL 5 vol % blend4765
DOI: 10.1021/acs.chemmater.9b00995 Chem. Mater. 2019, 31, 4759−4768
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Chemistry of Materials
deposited on the substrates via thermal evaporation through a shadow mask. A pristine P3HT solution was prepared in chloroform at a concentration of 10 mg/mL at 50 °C. Solutions of P3HT/IL blends were prepared by injecting 10 vol % [EMIM][TFSI] in either acetone or chloroform and stirring the blends at 50 °C for 1 h. The ratios of the IL solution to the P3HT solution were 1, 2.5, and 5 vol %. If necessary, the blends with a higher ratio of IL (7.5−15 vol %) were also tested. The semiconducting channel was formed by spin-coating a solution of the pristine P3HT or P3HT/IL blends (50 °C) at 2000 rpm. The P3HT or P3HT/IL films were then annealed at 120 °C for 30 min. A solution of the P(VDF-HFP)/[EMIM][TFSI] ion gel was prepared by sequentially dissolving the P(VDF-HFP) and [EMIM][TFSI] in acetone at a weight ratio of 1:4:7. The solution was stirred at 70 °C for 1 h, as reported elsewhere.20,23 The ion gel solution was spun onto a separate glass substrate and dried at room temperature to form a freestanding film. The ion gel film was cut into pieces with a razor blade and then carefully transferred onto the transistor channel using tweezers. For the gate electrode, a freestanding film of PEDOT:PSS, which was formed on a Si wafer by drop casting, was cut and transferred onto the ion gel dielectric. EG (10 wt %) was added to the PEDOT:PSS solution prior to deposition for improving the electrical conductivity of the gate electrode. The electrical characteristics of the polymer gel-gated transistors were all measured in a N2-filled glovebox by using a semiconductor parameter analyzer (Keithley 2634B) at voltage sweep rates of 50 and 100 mV/s. Characterization of P3HT/IL Blend Films. The surface morphology of nanodroplet-embedded P3HT/IL blend films was characterized using noncontact-mode atomic force microscopy (AFM; NX-10, Park Systems) in air and field emission scanning electron microscopy (FE-SEM; Sigma, Carl Zeiss) under a vacuum of
