Electroluminescence in Ion-Gel Gated Conjugated Polymer Field

Oct 7, 2012 - electroluminescence efficiency.20 In the ion-gel gated FETs we distinguish two separate regimes: When the drain source voltage (Vds) is ...
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Electroluminescence in Ion-Gel Gated Conjugated Polymer FieldEffect Transistors Shrivalli N. Bhat, Riccardo Di Pietro, and Henning Sirringhaus* Cavendish Laboratory, Department of Physics, University of Cambridge, J J Thomson Avenue, Cambridge CB3 0HE, United Kingdom S Supporting Information *

ABSTRACT: We report electroluminescence from ion-gel gated, field-effect transistors based on the conjugated polymer, poly(9,9′-dioctylfluorene-co-benzothiadiazole) gated by an 1-ethyl-3methylimidazoliumbis (trifluoromethylsulfonyl) imide/poly (styrene-block-ethylene oxide-blockstyrene) ion gel, and investigate the mechanism for light emission. The devices emit light from near the electron-injecting drain electrode when the drain source voltage exceeds approximately the energy gap of the polymer (Vds > Eg/e). Charge accumulation spectroscopy is used to demonstrate the significant penetration of the negative TFSI− ions into the F8BT assisted by the application of negative gate voltages, where they lead to significant p-type doping of the bulk of the F8BT film. In contrast, no evidence for diffusion of positive ions with positive gate voltages is observed, and this is consistent with the location of the recombination zone in the proximity of electron injecting electrode and the absence of a comparable electron current at positive gate voltages. We conclude that in the light-emission regime the devices operate more akin to a holecurrent dominated light-emitting electrochemical cell than a transistor. KEYWORDS: ion gel, low voltage operation, light emitting transistors, light emitting electrochemical cells

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INTRODUCTION Ionic liquids1−3 and ion gels4 are an interesting class of materials to be used instead of conventional gate dielectrics in organic semiconductor field-effect transistors (FETs). Ion-gel and ionic liquid gated molecular crystal5−7 and conjugated polymer8,9 transistors with high carrier mobilities under low voltage operation have been recently reported. Reports of hole,9,10 electron,11 and ambipolar12 charge transport with different semiconductor materials and ion gels render these transparent, environmentally friendly materials with high specific capacitance highly versatile. The ion-gel gated FET also provides a means for studying the charge transport physics under high charge carrier density,13 provided that the transistor operation can be controlled to be in an electrostatic mode due to the electrical double layer14 at the semiconductor and ion-gel interface. However, the possibility of ion penetration from the ion-gel layer into the active semiconductor layer on application of the gate voltage needs to be taken into account in such studies. For both ion gels and polymer electrolyte gated transistors there have been careful studies15,16 of the penetration of ions into the semiconductor layer depending on the magnitude of the gate voltage8 and the measurement conditions. Whether the transistor is operating in an electrostatic or electrochemical mode determines the interpretation of measurements and the device physics. For example, in the electrochemical mode the photoluminescence of the organic semiconductor is quenched even in the bulk,17 and the injection of the charge carriers at the source-drain contacts is enhanced.18 In pure electrostatic operation, the organic © 2012 American Chemical Society

semiconductor remains in its pristine state, and the charge carrier mobility can be calculated in linear and saturated regimes using the standard field effect transistor equations based on the gradual channel approximation.19 The possibility of the penetration of ions into the organic semiconductor layer on application of the gate voltage complicates the interpretation of charge transport as the bulk ion concentration is generally not known. Due to the low voltage operation and high current densities achievable, ion gel or polyelectrolyte gated FETs are of interest also for the realization of high performance light-emitting field effect transistors. This requires that the device can be operated in an ambipolar mode with simultaneous electron and hole accumulation layer formation and with high electroluminescence efficiencies.12,20 A potential application of such high current density light emitting FETs could be in electrically pumped organic laser.21 In this respect, transparent ion gels, which also offer the flexibility of having the gate electrode not necessarily on top of the channel,9 present another key advantage over traditional dielectric materials. However, the possibility of penetration of ions from the ion gel into the polymer layer on the application of a gate voltage also requires us to consider that such devices might operate in a mode that is more akin to that of light emitting electrochemical cells (LECs)18,22 than that of electrostatically gated unipolar23 or ambipolar20 light emitting FETs. Received: May 24, 2012 Revised: October 4, 2012 Published: October 7, 2012 4060

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from Polymer Source Inc.25 Ionic liquid and triblock copolymers were used as received without any further purification. The ion gel solution (triblock copolymer/ionic liquid/solvent of 0.7/9.3/90 wt %) was spin-coated from anhydrous acetonitrile (which is an orthogonal solvent for F8BT), on the top of 40 nm thick F8BT films, and annealed at 70 °C for about 15 h in a glovebox(nitrogen atmosphere). A conductive polymer ink, PEDOT:PSS, was used for the gate electrode. PEDOT:PSS from Clevios was used as received. In order to deposit the gate electrode, the devices were taken outside the glovebox (in air) for about 2−5 min on a 70 °C preheated hot plate, and PEDOT:PSS was drop-cast for the gate electrode. The devices were then taken back into the glovebox and annealed at 70 °C for about 15 h and stored in nitrogen glovebox at room temperature. The chemical structures of ionic liquid, triblock copolymer, and semiconducting polymer used in this study are shown in Figure 1a. A schematic of a bottom-contact transistor with offset top gate electrode is shown in Figure 1b.

In this paper, we report light emission from an 1-ethyl-3methyl imidazolium bis (trifluoromethyl sulfonyl)imide/poly(styrene-block-ethylene oxide-block-styrene) (EMIM-TFSI/ SOS) ion-gel gated poly(9,9′-dioctyl fluorene-co-benzo thiadiazole) (F8BT) polymer FET. We selected F8BT because in light-emitting FETs with a conventional gate dielectric such as PMMA it provides excellent ambipolar transport with balanced electron and hole mobilities and exhibits a high photo- and electroluminescence efficiency.20 In the ion-gel gated FETs we distinguish two separate regimes: When the drain source voltage (Vds) is well below the energy gap (Eg) of the polymer the device behaves as a stable FET with a well-defined current saturation regime. On the other hand, when Vds becomes approximately greater than Eg/e (where e is the elementary charge) the drain current starts to increase rapidly with Vds and we observe corresponding light emission from the channel. On the basis of the observation of increasing drain current in consecutive sweeps with same drain-source and gate voltages in the output characteristics and significant hysteresis in the transfer characteristics we investigate the role of ions behind the light emission. Using charge accumulation spectroscopy (CAS) we study the gate voltage dependent ion diffusion from the ion-gel layer to the polymer semiconductor layer. We see a clear gate voltage dependence of the diffusion of negative ions (TFSI−) into the polymer layer in the hole accumulation regime (negative gate voltages). The diffusion of negative ions increases with increasing gate voltage. Further, we do not see evidence for the diffusion of positive ions (EMIM+) under electron accumulation conditions, which is consistent with the fact that we do not observe n-type device characteristics for positive gate voltages and the location of the recombination zone always remains in the proximity of the electron injecting electrode. Moreover, we compare the behavior of the three-terminal FETs with two terminal bilayer devices (without gate electrode). The latter also emit light at room temperature, with similar increase in the current and corresponding light emission, when Vds is approximately greater than Eg/e of the semiconductor polymer. In these two-terminal devices the potential difference between the source and drain electrodes helps ion displacements and diffusion which eventually lead to light emission similar to the work by Sandstrom et al.22 at high temperatures. These results independently suggest that ion-gel gated FETs operate more akin to planar bilayer LECs when Vds > Eg/e.

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Figure 1. (a) Molecular structure of (i) ionic liquid, 1-ethyl-3methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIM-TFSI), (ii) triblock copolymer poly(styrene-block-ethylene oxide-blockstyrene) (SOS), where R is coupling unit, −CH2−phenyl−CH2−,25 (iii) semiconducting polymer, poly(9,9′-dioctylfluorene-co-benzothiadiazole) (F8BT). (b) Cross sectional schematic of the top, offset gated, bottom-contact transistor. (c) Microscope image of a transistor. The top, offset gate electrode (PEDOT:PSS) is about a millimeter away from the edge of interdigitated pattern. (d) Frequency dependence of the capacitance at various dc voltages on gate electrode, with source-drain electrodes grounded (oscillation amplitude 100 mV).

EXPERIMENTAL SECTION

Transistors were fabricated on glass substrates cleaned by ultra sonic treatment in H2O, acetone and isopropyl alcohol, and oxygen plasma. Source and drain electrodes defining the channel length of 40 μm and width of 2 cm were patterned by photolithography. The thickness of gold for source and drain electrode was about 30 nm. Spin coated films of poly(9,9′-dioctylfluorene-co-benzothiadiazole), F8BT, (from Cambridge Display Technology, used as received) were spin-coated from xylene solution (4 mg/mL), annealed at 290 °C for about 30 min,24 and then cooled to room temperature to obtain polycrystalline F8BT films and to remove any traces of solvent. The films were dried overnight under nitrogen atmosphere in a glovebox before spin coating the ion gel solution. Ionic liquid 1-ethyl-3methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]) was purchased from Sigma Aldrich. The triblock copolymer, poly(styrene-block-ethylene oxide-block-styrene) (SOS), with polystyrene (PS) block molecular mass, Mw of 10 kg mol−1, and polyethyleneoxide (PEO) block Mw of 40 kgmol−1, was purchased

The devices were tested in a nitrogen glovebox. Electrical characterization was performed using an Agilent 4155B semiconductor parameter analyzer, and capacitance measurements were done using an HP 4192A impedance analyzer (5 Hz to 13 MHz). Light intensity measurements were done using a Hamamatsu S1133-01 silicon photodiode, with sensitivity of 0.3 A/W at λ = 560 nm. Electroluminescence spectra were recorded using an Ocean Optics HR4000 spectrometer. Images and videos were recorded using ULead video studio software interfaced with the charged couple device (CCD) camera and a single objective (20×) directly attached to the camera. All measurements were acquired in a nitrogen atmosphere. Recombination zone profiles were obtained by analyzing reference images with external illumination to detect the position of the electrodes and comparing these to images taken in the dark while the device was emitting light. Origin Pro data analyzing software is used to 4061

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analyze the pixels of images and hence to evaluate the width of the recombination zone. Charge accumulation spectroscopy (CAS) measurements were performed using a Cary 6000i double beam spectrophotometer equipped with a controlled atmosphere sample holder. A Keithley 2612 dual-channel source measure unit was used to bias the transistor while taking the absorption spectra. During the measurements the sample holder was filled with nitrogen and kept at 300 K.

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RESULTS AND DISCUSSION The dependence of capacitance on frequency was studied at different voltages. Studying this variation can provide useful insights that help to determine whether the device is operating in an electrostatic mode or electrochemical mode. Measurements were done while source and drain were grounded and voltage was applied on the offset gate electrode. A schematic diagram and microscope image of the device can be seen in Figure 1b,c. The variation of capacitance with frequency at various dc gate voltages is shown in Figure 1d. The capacitance values are measured on a 1 mm2 area capacitor. Between gate voltages of −1 and −2 V the capacitance increases by nearly 1 order of magnitude at lower frequencies, indicating the formation of an accumulation layer with charge carrier concentration of 1.9 × 1012 /cm2 and 1.8 × 1013 /cm2, respectively. These values are calculated from the measured capacitance values by multiplying with the gate voltage. It is interesting to note that, in the low frequency regime, below 100 Hz, the absolute capacitance value for −2 V is higher than the capacitance values correspond to −3 and −2.5 V. Such a decrease in the value of the capacitance with increasing voltages in the low frequency regime is not expected for a stable electrical double layer capacitance14 but might indicate a pseudocapacitance.26 A pseudocapacitance is the capacitance measured when ions have penetrated into the active polymer layer on the application of voltage, as opposed to the electrical double layer capacitance measured when ions are only present at the polymer−ion-gel interface.27 This provides a first indication that at higher gate voltages ions are able to penetrate into the polymer semiconductor layer. The capacitance at the interface is due to the movement of ions from the bulk of the ion-gel to the interfaces with the semiconductors and the gate electrode, until the bulk of the ion-gel is field-free.14,27 This allows the ion-gel to be made thick and also the gate electrode to be spaced away from the channel9 unlike conventional gate dielectrics relying on capacitance derived from a thickness-dependent dielectric polarization. Having the gate electrode offset introduces certain advantages, primarily when the device is light emitting, as there is no absorption of the light by the gate electrode. The gate-offset configuration is ideal for spectroscopic studies as well, as the incident light beam does not have to pass through the gate electrode thus reducing potential interference artifacts.

Figure 2. (a) Transfer characteristics of a transistor with gate electrode offset by about 1 mm, with channel length L = 40 μm and width W = 2 cm, at Vds = −1 V (sweep rate 5 mV/s). Source (Is), drain (Id), and gate (Ig) currents are plotted. The arrows indicate the direction of the voltage sweep. (b) Output characteristics, with pulsed voltage on Vds and constant voltage on Vds. Sweep rate 20 mV/s. Vg starts at −3 V and goes up to −3.4 V in steps of 0.1 V.

approaches −0.3 V during the backward scan. However, clear linear (Vds ≤ (Vg −Vth)) and saturation regimes (Vds ≥ (Vg − Vth)) without much hysteresis can be seen in the output characteristics, where Vds is swept with fixed Vg. The transfer characteristics were taken with slow (5 mV/s) sweep rates with Vg and Vds voltages continuously applied without any pulsing. During the measurement of the output characteristics, Vds was pulsed with a pulse period and width of 0.5 s and 20 ms, respectively (20 mV/s sweep rate), with constant Vg. Pulsing Vds with fixed Vg resulted in lower hysteresis in the output characteristics. However, pulsing the gate voltage during the transfer scan did not help in reducing the hysteresis in the transfer characteristics.

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ELECTROLUMINESCENCE We observed light emission in the ion-gel gated F8BT transistors when the swept drain source voltage (Vds) exceeds approximately the energy gap of the polymer divided by the elementary charge. Figure 3 shows the Vds sweep to higher voltage, up to −3.6 V with two fixed Vg values. The variation of the drain current in the low Vds regime shows current saturation behavior as shown for the characteristics in Figure 2, where the drain source voltage was kept below −3 V. In the higher Vds

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TRANSISTOR CHARACTERISTICS Figure 2a,b shows the transfer and output characteristics of the transistor. The transfer characteristics exhibit an increase of drain current around −1.2 V of gate voltage (Vg). Around Vg = −3 V the rate of increase of the drain current with gate voltage begins to reduce, when the drain source voltage (Vds) was kept constant at −1 V. On−off ratio is on the order of 105 . Considerable hysteresis can be seen in the transfer characteristics as the gate voltage is swept back and forth. The onset voltage is around −1.2 V during the forward scan, and it

Figure 3. (a) Variation of the drain current (Id) with Vds while Vds was swept up to −3.6 V, in steps of 0.01 V, at two different gate voltages, −2.9 and −3.0 V (continuous measurement, Vds sweep rate 60 mV/s; due to both the continuous measurement condition without voltage pulsing and the larger voltage range the hysteresis is larger than in Figure 2b). (b). Microscope image of the light emitting transistor with channel length 40 μm, superimposed on the reference image; the recombination zone is in close proximity of the electron injecting electrode, and the width of this recombination zone as measured on the optical microscope image is about 4 μm. 4062

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Table 1. Gate Voltage Dependence of Vonset (Drain Current), Vonset (Light Emission), and the Corresponding EQE at Vds = −3.6 V

regime, the drain current starts to increase rapidly, deviating from a typical unipolar, p-type transistor behavior, and this is accompanied with light emission. The variation of Id with Vds follows an approximate power law in the light emitting regime. In order to obtain a quantitative estimate of the onset voltage (Vonset), for the increased drain current at high drain source voltages we fitted the current−voltage characteristics to an empirical formula Id = b + k(Vds − Vonset)c, where k and b are arbitrary constants. c = 4 is the power law exponent that best fits the data. On the basis of the fit for the forward scans values of Vonset = −2.62 V at Vg = −3 and −2.42 V at Vg = −2.9 V are extracted. The energy gap between the HOMO and LUMO levels in F8BT is approximately 2.6 V;20 i.e., the current increase and light emission occur when Vds exceeds approximately the energy gap divided by the elementary charge. Figure 3b shows an optical microscopy image of the light emitting transistor with a channel length of 40 μm. The recombination zone width is about 4 μm and is in the proximity of the electron injecting electrode; i.e., we are unable to move the recombination zone across the channel by biasing the device into an ambipolar transport regime as in F8BT FETs with conventional gate dielectric.20 In Figure 3a we note further that on the initial scan at the first applied gate voltage (Vg = −2.9 V) the current at small drain source voltage increases superlinearly in the forward voltage sweep (toward more negative Vds), while on the reverse sweep as well as on subsequent sweeps this superlinearity is absent. This is interpreted as evidence for a contact resistance being present in the initial forward scan, which is reduced in subsequent sweeps. This might indicate the presence of negative ions in the semiconducting layer, which on application of a sufficiently large drain source bias drift toward the source electrodes and aid the hole injection. For more details on the observation of this sweep dependent contact resistance feature see the Supporting Information. Figure 4a shows the current−voltage characteristics for subsequently acquired sweeps on the same device together with

Vg (V)

Vonset(Id) (V)

Vonset(Iph) (V)

EQE at Vds = −3.6 V (%)

−2.8 −2.9 −3

−2.62 −2.82 −2.65

−2.77 −2.76 −2.55

2.64 × 10−4 3.12 × 10−4 1.37 × 10−4

curve fitting in the light emission regime during the forward scan. From the table, it is evident that the onset voltages for both drain current and electroluminescence are well correlated. Figure 4b shows the corresponding variation of the external quantum efficiency (EQE), ηext, with Vds for three different gate voltages. This is calculated as ηext = 1.538 × (Iph/Id), where Iph is the photocurrent measured by the photodiode, Id is the drain current, and the numerical factor 1.538 accounts for the sensitivity of the photodiode.28 No geometrical corrections are included in this numerical factor as the dimension of the photodiode matches approximately with the dimension of the photodiode. We can see from the Figure 4b and from Table 1 that the external quantum efficiency at Vds = −3.6 V increases slightly from Vg = −2.8 V to Vg = −2.9 V. However, for Vg = −3.0 V, the EQE decreases to values below those observed during the previous scans at Vg = −2.8 and −2.9 V. This decrease, together with that of the current observed in Figure 4a, is attributed to a degradation of the device when operated at such high voltages. Vds values are not swept further in order to remain within the electrochemical window of the ion gel used. The electrochemical window for the pure ionic liquid, 1-ethyl-3methylimidazolium bis(trifluoromethylsulfonyl) imide, is −3 to 2.5 V.29 For the ion gel, however, which is a mixture of the SOS triblock polymer and the ionic liquid, the voltage limits we applied during electrical characterization of devices are consistent with report by Cho et al.9 which is an extensive report on in gel gated transistors. Cho et al.9 had applied gate voltages up to −4 V. To discuss the complex hysteresis behavior in Figures 3 and 4, we note that in the sweeps at higher gate voltage the current in the reverse scan is initially higher than in the forward scan which could be a manifestation of penetration of ions and electrochemical doping of the F8BT film progressing with time while the high gate voltage is applied. However, this appears to be superimposed with a device degradation effect, which leads to the current on the reverse sweep decreasing below that on the forward scan at smaller Vds. This degradation also leads to an overall decrease in current if voltage sweeps are measured repeatedly at the same gate voltage.

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Figure 4. (a) Variation of drain current (Id) with Vds at gate voltages −2.8, −2.9, and −3.0 V. Vds was swept up to −3.6 V in steps of 0.01 V (Vds sweep rate 60 mV/s). (b) Corresponding variation of the external quantum efficiency (EQE) as a function of Vds.

CHARGE ACCUMULATION SPECTROSCOPY In order to understand the gate voltage dependent ion diffusion into the semiconductor film on the application of the voltages, we carried out charge accumulation spectroscopy (CAS) measurements. This technique compares optical transmission scans of the device with Vg = 0 V and scans with an applied gate voltage and is able to detect the charge-induced optical absorption induced by polarons on the conjugated polymer in the accumulation layer as well as a corresponding bleaching signal of the neutral π−π* absorption. This technique is related to charge modulation spectroscopy (CMS) measurements reported previously.30 In CMS a lock-in modulation technique is used to detect only mobile charges which can respond to the

the corresponding efficiency for the detected electroluminescence. For this measurement a photodiode was placed directly on the top of the light emitting channel while recording the current−voltage characteristics, and the photocurrent due to the electroluminescence and the corresponding drain current were recorded simultaneously. The red dotted line in Figure 4a for the forward scans represents the power law fit. Table 1 lists the onset voltages for both drain current and electroluminescence for three different gate voltages based on the 4063

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The induced absorption and bleaching bands increase monotonously with gate voltage until about −1.6 V, but at higher gate voltages the incremental increase in ΔT/T with increasing gate voltage becomes smaller and smaller, and we approach a saturation of the signals around gate voltages of −3 V. This suggests that there is no further increase of the charge concentration at higher gate voltage, and this appears to be correlated with a saturation of the transistor current at higher gate voltage (Figure 2a). Output characteristics obtained after finishing each optical scan are shown in Figure 6a. Variation of

frequency of the gate voltage modulation, which is typically in the range 10−100 Hz.31 The present technique is also sensitive to trapped charges as well as to charges induced by electrochemical doping, because any charges that can respond on the time scale of several minutes between the two optical transmission scans are detected in CAS. These measurements were performed in situ under nitrogen atmosphere with top, offset gate electrode transistors (Figure 1b) while the source and drain electrodes were grounded. Each scan (while obtaining the spectra over the energy range shown from lower to higher energies) takes approximately 5 min to complete. This necessitates the application of the gate voltage during this time interval. The variations of the optical transmission spectra with negative gate voltages are shown in Figure 5a. All spectra are

Figure 6. (a) Output characteristics obtained after each CAS optical scan. (b) Variation of ΔT /T with gate voltages at different energies (0.7, 1.8, and 2.6 eV). The graph also shows the variation of the drain current with Vg. Figure 5. (a) CAS spectra in the hole accumulation regime: Variation of ΔT /T with photon energy at different gate voltages starting from −1 to −3 V. (b) Electron accumulation regime: Variation of ΔT /T with photon energy at different gate voltages starting from 1 to 4 V.

ΔT /T with gate voltages at different energies and the variation of the drain current with Vg are shown in Figure 6b. The current magnitude follows the increase in the polaron induced absorption (PIA) at 1.8 eV and bleaching for gate voltages below −1.9 V. Once the gate voltage exceeds −1.9 V, the current and the 1.8 eV PIA signal saturate and no longer increase anymore, while the intensity of the CMS signals at 0.7 and 2.6 eV continue to increase. This subtly different gate voltage dependence of the charge induced absorption bands at 0.7 and 1.8 eV is noteworthy. In the CAS spectra of Figure 5a for small gate voltage of −1 and −1.5 V the absorption band at 1.8 eV is more intense while at higher gate voltages the band at 0.7 eV becomes more intense. The strength of the absorption band at 1.8 eV saturates relatively early, i.e., around gate voltages of −1.6 to −2 V, while the band at 0.7 V continues to increase. However, the increase in the bleaching beyond −1.6 to −2 V suggests that new charge is injected when the gate voltage is increased beyond −1.6 to −2 V. This might indicate that at higher gate voltages/higher charge concentrations a different charge species, such as a bipolaron, is formed preferentially with a characteristic absorption contributing mainly to the 0.7 eV absorption band. Bipolarons are expected to form only when the charge concentration is sufficiently high and their formation could be facilitated by the presence of a negative counterion that has penetrated into the polymer semiconductor.35 The saturation in the current is an indication that bipolaron mobility is a few orders of magnitude lower than polaron mobility (Figure 6a,b). By applying the Beer−Lambert law, to a semiconducting film with both charged and neutral molecules, the ΔT/T signal can be expressed as

referenced and normalized to the transmission spectrum of the grounded device Vg = 0 V, by plotting (TVg − ToV)/ToV for different gate voltages. We detect three broad charge induced absorption (negative ΔT/T) bands, at 0.7, 1.8, and 3.4 eV, and two associated bleaching features of the neutral polymer absorption (positive ΔT/T) at 2.6 and 3.8 eV. The spectral features are consistent with those observed in CMS measurements of F8BT FETs with conventional gate dielectrics.32 In particular, we attribute the absorption band at 1.8 eV to singly charged hole polarons. These are also formed in the CMS measurements, which involve much smaller charge concentrations. The weak induced absorption feature at 3.4 eV is interpreted as a higher energy transition of the polaron. It is possible that this spectral band overlaps with the bleaching feature making a direct assessment of its intensity challenging. The differential transmission values on the order of 0.1−0.2 for both the charge-induced absorption and bleaching peaks of the ion gel gated polymer transistors are approximately 2 orders of magnitude higher than what would be expected if the device was operated in purely electrostatic mode.33,34 If the devices were operated in pure electrostatic field effect mode, the measured ion gel capacitance of 1 μF/cm2 at −3 V (corresponding to a charge density of 1 × 1013 cm−2) would give a peak value of ΔT/T only of the order of 2 × 10−3 as calculated using the Beer−Lambert law and the known optical cross section for the radical cation in F8BT.32 Instead, we observe values on the order of 0.1−0.2. As discussed in more detail below this observation is not compatible with the formation of a thin charged layer at the interface with the iongel due to electrostatic effects only and suggests the presence of a higher polaron concentration in the polymer semiconductors due to electrochemical doping.

ΔT /T = exp[−(σN(E). ΔnN) + (σP(E)nP)]t0 −1

where ΔnN = n′N − nN. n′N is the density of neutral site remaining in the charged film, and nN = 7.7 × 1020 cm−3 is the density of neutral sites in pristine film. The latter is estimated on the basis of estimating the unit cell volume of F8BT from X4064

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ray measurements.36 σN(2.6 eV) = 2 × 10−16 cm2 is the optical cross section for the neutral absorption, determined by measuring the optical transmission of a F8BT film of known thickness t0 = 40 nm. σp is the optical cross section for the polaron absorption, and nP is the density of polarons in the charged film. It is possible to estimate a cross section for the polaron absorption of σp(1.8 eV) = 2 × 10−16 cm2 and a number of bleached F8BT repeat units per polaron of 1. This has been measured previously for charge transfer doping of F8BT films with MoO3 giving a high localization of the polaron over just one repeating unit.32,36 If we assume the polaron absorption cross-section to be zero in the region of the bleaching signal, it is possible to estimate the amount of F8BT sites that are bleached upon application of the gate bias, from the intensity of the bleaching. For the maximum gate voltage of −3 V, the peak value for the bleaching is ΔT/T = 0.28. This gives a density n′N = 4.6 × 1020 cm−3; i.e., 60% of the F8BT units remain uncharged and the remaining 40% of repeating units become charged and are bleached. A similar calculation can be applied to estimate the polaron concentration. The polaron induced absorption at 1.8 eV gives a concentration of polarons in the film of np = 1.27 × 1020 cm−3. Given the estimate for the polaron delocalization from charge transfer doping, these account for about half of the bleached F8BT repeat units. The remainder of the charges might be in bipolaron states that contribute to the bleaching signal, but not to the induced absorption around 1.8 eV. We conclude from this consideration that such a high charge concentration is not consistent with the device operating in electrostatic mode at the gate voltages above −1.5 V, which are needed to operate the devices in light-emission mode. If the device was operating with a double-layer of charges formed in the first few monolayers at the interface, it would be impossible to explain how 40% of the F8BT film can become bleached upon gate voltage application. This suggest that there is a prominent gate voltage enhanced drift/diffusion of TFSI− ions into the bulk of the F8BT film, which results in p-type doping of F8BT and results in much higher charge concentrations than what would be estimated from the measured capacitance. This needs to be taken into account in order to extract meaningful carrier mobility values in this operating mode, as discussed below. Further spectroscopic evidence for this stems from experiments, where after sweeping the gate to −3 V we grounded the gate and took repeated CAS scans at 0 V to study the evolution of the charge accumulated in the device. As can be seen from Figure 5b after the first recovery (0Vrec1) scan, a significant portion of the charge is still present in the channel and the charge concentration takes about 15 min to decrease down to 10% of the maximum as can be seen in subsequent scans (oVrec2 and 0Vrec3) . This long response time to the removal of a gate bias also suggests clearly that doping of F8BT due to diffusion of the TFSI− ions into the semiconducting film is taking place. Finally, the same set of measurements was done for positive gate voltages and showed no signs of electron accumulation at the polymer semiconductor−ion-gel interface (Figure 5b). No charge-induced absorption characteristic of the radical anion, which would be expected to absorb at similar energy as the radical cation/hole polaron,37 was observed for voltages up to 4 V. This is consistent with our observation that we cannot operate the devices successfully in unipolar electron mode at positive gate voltage; i.e., in contrast to F8BT devices with

conventional gate dielectrics, the EMIM-TFSI/SOS ion-gel FETs do not allow formation of mobile electron polarons at the interface. The much weaker residual spectral features in the ΔT/T scan might be due to interference artifacts, degradation of the device due to unscreened electric fields, or the presence of positive EMIM+ ions inside the F8BT film. We cannot exclude the possibility that some EMIM+ ions penetrate into the F8BT layer with positive gate voltages, but they are unable to induce n-type doping of the F8BT to the same extent that the TFSI− induce p-type doping. The absence of mobile electrons in the film is consistent with our observation that the recombination zone is always located in the proximity of the electron injecting electrode.

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EXTERNAL QUANTUM EFFICIENCIES AND CARRIER MOBILITIES Compared to EQE values of ambipolar, poly(methyl methacrylate) (PMMA) gated F8BT light-emitting FETs, for which we have observed typical EQE of 0.5−1%20 and more recently 8−12%,38 EQE values of the ion-gel gated F8BT transistors are considerably lower (10−4 %) (Figure 4b). This could have a number of reasons. The ion-gel FETs operate in a unipolar hole mode with light-emission from near the negative, electron injecting drain electrode, while in the ambipolar PMMA devices electron and hole currents are perfectly balanced with typical hole mobilities, μh = 6 × 10−4 cm2 V −1 s−1, and electron mobilities, μe = 5 × 10−4 cm2 V −1 s−1, and the light-emission can be made to occur from within the channel. The ion-gel gated devices are operated by a very large hole current owing to the large hole concentration due to electrochemical doping. Many of these holes are unlikely to encounter one of the few electrons that are injected from the drain contact and get extracted through the drain current without contributing to light emission. Other reasons for the low efficiency could be quenching of excitons associated with the proximity of the drain electrode39 or luminescence quenching due to the large hole polaron concentration in the F8BT films. We note that the absolute detected light output/ photocurrent in PMMA gated F8BT ambipolar light-emitting FETs and that in the ion-gel gated F8BT unipolar transistors are in fact of the same order of magnitude. This implies that the number of formed excitons which recombine radiatively in both cases are comparable. This could indicate that the efficiency of the device is indeed limited by the number of electrons that can be injected from gold into F8BT without being assisted by electrochemical n-type doping in the vicinity of the contacts. Hole injection, on the other hand, is likely to be facilitated by the significant p-type doping of the film as discussed above. We would also like to comment on the field-effect mobility for hole transport in the ion-gel gated devices. If we calculate the mobility value using the standard FET equation in the linear regime using the following equation Id =

WC iμh(Vg − VT)Vds L

That is, not taking into the account the presence of any ions in the active polymer film due to electrochemical doping, we obtain a hole mobility, μh ≈ 1 cm2 V −1 s−1. Here Id is the drain current, Vg is the gate voltage, W = 2 cm is the channel width, L = 40 μm is the channel length, Ci is the specific capacitance, and VT is the threshold voltage. 4065

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sweep of this device. In the previous, first sweep, with the same voltage conditions, Id and the electroluminescence are an order of magnitude lower. This indicates that ions require some time to diffuse into the device, particularly when the penetration of negative TFSI− ions is not driven by an applied negative gate voltage as in the FETs. The light emission behavior when Vds > Eg/e is similar and reveals that the three terminal transistors operate as a transistor only at low Vds, but behave more like an LEC in the high Vds regime. While the gate voltage facilitates the ion diffusion/drift into the channel, the ions then rearrange along the channel diffusion in response to the applied sourcedrain bias like in an LEC.

It is incorrect to neglect the electrochemical doping of the bulk of the F8BT film. A better estimate of mobility is obtained if we use the value of the polaron concentration P = 1.5 × 1020 cm−3, as estimated from the CAS data and use the equation

PμheWtVds L where t = 40 nm is the thickness of the polymer film. P = 1.27 × 1020 cm−3 is the polaron concentration as estimated from the CAS data, at Vg = −3 V, and e is the elementary charge. We obtain a hole mobility value, μh = 7.37 × 10−2 cm2 V −1 s−1, which is much closer to the value measured for FETs with standard polymer gate dielectrics.20 The former calculations are related to faster measurements8 of FETs with the minimum probability of ion penetration into active polymer layer, and latter calculations are associated with slow measurements. Nonetheless, we cannot exclude the possibility of the enhanced mobility values even with faster measurement conditions. Id =

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SUMMARY AND CONCLUSIONS We have reported light emission in ion-gel gated polymer semiconductor transistors and have shown that the ion-gel gated transistor exhibits characteristics of both a transistor and an LEC. At the high voltages required to induce light-emission the transistor characteristics gradually disappear during successive measurements due to the increased conductivity of the polymer film due to the diffusion of ions. The location of the recombination zone and the CAS results provide unambiguous evidence for gate-voltage assisted penetration of the TFSI− ions and associated p-type doping in the bulk of the F8BT film. No evidence for n-type doping or the formation of an electron accumulation layer was observed. The mechanism behind the light emission in these ion gel gated polymer transistors is more similar to that of light emitting electrochemical cells than that of ambipolar, light-emitting FETs. Further work is needed to investigate whether through suitable choice of ions and the polymer it is possible to enter a regime in which n-type doping of the bulk near the negative electrode can be induced. This would allow moving the recombination zone into the middle of the channel, and hence increase EQEs and make better use of the large current densities that can be achieved in such devices.

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ION-GEL BILAYER DEVICES Finally, we discuss measurements on a two-terminal device structure without a gate electrode. A planar F8BT, ion-gel bilayer device with only source-drain electrodes and no gate electrode is used. Such a bilayer approach, as opposed to a mixture of polymer and electrolyte or ionic liquid,40 eliminates any phase separation issues or processing issues associated with solvent incompatibility. Bilayer LECs due to their simplicity, in principle, make it possible to conduct more controlled studies for understanding the electric field distribution41,42 in LECs which has been the subject of the debate43 in recent years. Figure 7 shows the variation of the current and the corresponding photocurrent due to electroluminescence with

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ASSOCIATED CONTENT

S Supporting Information *

Additional figure and details. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

Figure 7. Variation of current and photocurrent with voltage in twoterminal F8BT/ion-gel device without a gate electrode. The red dotted line shows the power law fit during the forward scan in the light emission regime.

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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voltage. In the light emitting regime, similar to ion gel gated transistors, the variation of Id and Iph with Vds follows an approximate power law. In order to obtain onset voltages, we fitted the current−voltage characteristics to the equation Id = k(Vds − Vonset)c where k is arbitrary constant, and c = 4 is the power that best fits the data. On the basis of the fits for the forward scan in the light emitting regime, Vonset for the current is Vonset(Id), −2.99 V. Similarly, with the photocurrent−voltage characteristics we obtain Vonset for the light emission, and Vonset(Iph) = −2.95 V. Again, the recombination zone is in the proximity of the electron injecting electrode. More interestingly, both the increase in current and the light emission gradually build up. The Vds sweep shown in Figure 7 is in fact the second voltage

ACKNOWLEDGMENTS S.N.B. thanks Gates Cambridge trust and Cambridge Overseas trust for funding. REFERENCES

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