Low-Voltage Polymer Field-Effect Transistors Gated via a Proton Conductor** By Lars Herlogsson, Xavier Crispin, Nathaniel D. Robinson, Mats Sandberg, Olle-Jonny Hagel, Göran Gustafsson, and Magnus Berggren* Organic field-effect transistors (OFETs)[1,2] and other “plastic” electronic devices[3] are currently being investigated for potential use in printed flexible integrated electronics and displays. Ideally, these systems are fast, operate at low voltage, and are robust enough to be manufactured using standard printing techniques.[1,4,5] Current printing technology allows for a separation between the source and drain electrodes in OFETs of less than 1 lm.[6] In traditional OFETs, the organic semiconductor film is separated from the gate electrode by a thin insulating dielectric film. The gate/insulator/semiconductor sandwich can be seen as a capacitor, where the charge density in the semiconductor, and thus also the conductance of the semiconductor channel, is tuned with the applied voltage. Tremendous effort has been devoted to reaching a high capacitance (per area) Ci between the gate and the channel, in order to allow transistors to operate at low voltage.[7,8] Since the dielectric constant k of organic materials is usually quite low, very thin gate-insulator layers are required in order to obtain a high capacitance. Molecular assembly and self-organization techniques have been utilized to manufacture gatedielectric layers only a few nanometers thick, resulting in a large capacitance (Ci of up to 1 lF cm–2).[8–10] Using an electrolyte, for example, a salt in a polymer matrix, to gate a silicon-based transistor[11] or a polymer-based electrochemical transistor[12,13] was first demonstrated more than two decades ago. Recently, polymer-based transistors have been gated with hygroscopic insulators (e.g., poly(vinyl
– [*] Prof. M. Berggren, L. Herlogsson, Dr. X. Crispin, Dr. N. D. Robinson ITN, Linköpings Universitet Campus Norrköping, SE-601 74 Norrköping (Sweden) E-mail:
[email protected] Dr. M. Sandberg, O.-J. Hagel Thin Film Electronics AB Westmansgatan 27, SE-582 16 Linköping (Sweden) Dr. G. Gustafsson Acreo AB Bredgatan 34, SE-602 21 Norrköping (Sweden) [**] The authors gratefully acknowledge The Swedish Foundation for Strategic Research (COE@COIN), VINNOVA, the Royal Swedish Academy of Sciences, and the Swedish Research Council, for financial support of this project. In addition, the authors wish to thank Rhodia for providing the P(VPA-AA) material, and Robert Forchheimer, Tommi Remonen, and Olle Inganäs for stimulating discussions. Supporting Information is available online from Wiley InterScience or from the author.
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DOI: 10.1002/adma.200600871
phenol)),[14,15] with a polysaccharide derivative containing LiBF4,[16] or with LiClO4 in poly(ethylene oxide)[17–21] and poly(3,4-ethylenedioxythiophene)–poly(styrene sulfonic acid).[22,23] In these devices, the redistribution of mobile ions within the conjugated-polymer layer and the solid electrolyte together with the charge injection from the source/drain electrodes leads to electrochemical doping/undoping of the polymer channel. Bulk electrochemistry controls the on/off state of the transistor. We classify such transistors as electrochemical transistors. They operate at low voltage (< 2 V) but switch slowly (requiring up to several seconds).[13–17] Besides their necessity for electrochemical reactions, an interesting feature of electrolytes is their ability to form electric double-layer capacitors (EDLCs). When a difference in electric potential is applied between two ion-blocking electrodes sandwiching a common electrolyte, the anions/cations in the electrolyte move towards the positively/negatively charged electrode to form an electric double layer, composed of a compact (Helmholtz) layer and a diffuse layer. Such EDLCs can have extraordinarily high capacitance (up to Ci = 500 lF cm–2)[24] and respond quickly, as a result of a charge separation of only a few angstroms within the Helmholtz layers formed in a few tens of microseconds.[25] Here, we demonstrate that by using a solid-state polyanionic proton-conducting electrolyte, a new generation of OFETs, called EDLC-OFETs, can be gated by using an EDLC. The EDLC-OFET resides at the interface between electrochemical transistors and field-effect transistors. The use of a polyanionic electrolyte, in which the anions are virtually immobile, prevents the penetration of anions into the conjugated polymer when the gate is negatively biased in relation to the channel, and thus prevents electrochemical doping of the bulk semiconducting polymer layer, as is the case in electrochemical transistors. The rapid formation of an electric double layer with a large capacitance at the conjugated polymer/electrolyte interface results in a fast and robust field-effect transistor with a high current throughput, capable of operating at low gate voltages. The electrolyte we have chosen to work with is a random copolymer of vinyl phosphonic acid and acrylic acid, P(VPAAA) (Fig. 1a). The phosphonic acid groups are strongly acidic (the first acidic constant, pKa1, is about 2.5 ± 0.5 in water), and therefore provide plenty of potentially mobile protons (ca. 8 mmol g–1, counting one dissociated proton per phosphonic acid group). EDLCs were fabricated by sandwiching
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Figure 1. a) The chemical structure of P(VPA-AA) protonated (left) and deprotonated (right), as it occurred near the poly(3-hexylthiophene) (P3HT) channel when a negative gate potential was applied. b) The chemical structure of P3HT, both neutral (left) and doped (right). c) The effective capacitance (䊉) and phase angle (&) versus frequency for a capacitor comprising a 54 nm thick P(VPA-AA) polyelectrolyte and Ti electrodes (see inset). TE: top electrode; BE: bottom electrode.
P(VPA-AA) between two Ti metal electrodes (see the inset in Fig. 1c) and characterized by using impedance spectroscopy. The effective capacitance and the phase angle as a function of frequency are presented for a 54 nm thin P(VPA-AA) film in Figure 1c. The phase-angle curve indicates a more resistive behavior at higher frequencies (> 170 kHz), whereas it suggests a more capacitive character at lower frequencies (< 170 kHz). The resistive behavior is attributed to protons that migrate away from the polymer chains.[26] The effective capacitance increased with decreasing frequency and was of the order of 10 lF cm–2 at 100 Hz. The main contributions to the capacitance at low frequencies were interpreted to be the response of the double layers formed at the P(VPA-AA)/Ti interfaces, and the polarization within the dielectric media (polyanion and condensed counterion relaxation).[26–28] The EDLC-OFETs fabricated (Fig. 2a) were top-gate devices with Au source/drain bottom electrodes (channel length L= 9 lm and width W = 200 lm), a regioregular poly(3-hexylthiophene) (P3HT, Fig. 1b) layer (18 nm thick), and the proton-conducting “insulator” P(VPA-AA) (54 nm thick) capped with a Ti gate electrode. The output characteristics of the EDLC-OFETs (Fig. 2b) showed clear current modulation for drive voltages of less than 1 V. All applied voltages were 1 V or less to avoid electrochemical reactions in the electrolyte
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layer. The drain current ID saturates at a drain voltage VD lower than the applied gate voltage VG (e.g. the saturated VD, VDsat = –0.5 V for VG = –1 V). The large saturation current (IDsat = –1.3 lA for W/L= 22) obtained for a low operation voltage (VG = –1 V) reflects the large Ci of P(VPA-AA). The relationship between these quantities is interpreted through conventional semiconductor theory, which reproduces the general trends in OFET characteristics[29,30] sat ID
W l Ci VG 2L
VT
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1
where Ci is the capacitance per unit area of the gate insulator, l is the charge-carrier mobility, and VT is the threshold voltage. Note that a high capacitance allows for greater channel lengths, making such devices more suitable for manufacture with printing techniques.[1] The transfer characteristics (Fig. 2c) revealed that (–ID)1/2 is linearly proportional to VG at saturation (VD = –1 V). The extracted field-effect mobility is ca. 0.012 cm2 V–1 s–1 (calculated for VT = –0.29 V and Ci = 20 lF cm–2, at VG = –1 V and VD = –1 V), which is of the same order of magnitude as other P3HT-based OFETs.[31,32] The on/off current ratio (Ion/Ioff) of 140 is rather low. Several reasons for this have been identified, giving the potential for improvement: 1) the contact resistance diminishes the on current; 2) an ionic displacement current between source and drain; and 3) a high residual doping level of the P3HT layer. The leakage current (gate current) is typically two orders of magnitude lower than ID. P(VPA-AA), like other electrolytes, focuses the electric field caused by a potential applied between two electrodes (in our case the gate and the channel) within electric double layers very close to each electrode. For small negative gate voltages, protons are attracted to the gate electrode, and form a Helmholtz layer. At the channel, deprotonated P(VPA-AA) anions close to positively doped P3HT chains form an electric double layer, which cannot pack itself into a Helmholtz layer (sketched in Fig. 2e). The doping level in the channel reflects the density of excess anions at the P3HT/P(VPA-AA) interface resulting from the depletion of protons. Thus, the conduction through the channel can be controlled by altering the gate potential. In addition to the current–voltage characteristics, the time response of a transistor is of tremendous importance in electronic applications. The transient response to a square-shaped pulse in gate potential, from 0 V to –1 V and vice versa, while the drain electrode was held at a constant potential of –1 V, is shown in Figure 2d. The rise/fall occurs in 3.5 ms (< 0.3 ms) and includes 90 % of the response. Note that the rise time for 60 % of the response is only about 0.1 ms. These transient times are about a hundred times faster than those previously reported for electrolyte-based transistors.[13,16] The relatively slow rise in current is attributed to the time required to form an electric double layer (proton-depletion layer) at the P3HT/ P(VPA-AA) interface over the complete length of the channel. The fast fall is associated with the ease of disrupting the
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channel by removing small areas of the double layer when the gate potential is set to 0 V. An obvious question is whether electrochemistry, that is, the doping of the bulk of the P3HT film, causes current modulation in the P3HT channel, or whether the channel is opened with an analogous process at the interface with the polyanionic P(VPA-AA). The following arguments suggest that the latter is the case. First, the polyanionic chains cannot penetrate the P3HT film/channel when the gate is negatively biased because they are effectively immobile. Second, the acidic character of P(VPA-AA) decreases the concentration of hydroxide (OH–) anions (naturally found because of the dissociation of water) in humid air, which could conceivably penetrate into the P3HT layer. Third, charge consumption in the P3HT electrochemical cells typically occurs over a timescale of seconds or hundreds of milliseconds (depending on the length scales and the concentration of ions involved),[33] whereas the devices presented here exhibited transient times
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below 1 ms. For similar channel dimensions, P3HT-based electrochemical transistors respond slowly, requiring up to several seconds, to a gate-voltage step of –0.6 V.[13,15] The resulting slow response of the transistor, because of ion migration in the semiconductor, typically implies a variation of the transfer characteristic for different gate-voltage sweep rates.[17] Such dependence was not observed in the EDLC-OFET (see Fig. 2c). Fourth, the trends and nature of the current–voltage characteristics of this device agree well with “traditional” OFET characteristics, with the exception that the applied voltage necessary for device operation is markedly lower. In order to show the detrimental effect of small anions on the transistor response and study whether the principal mechanism involved for the EDLC-OFET is bulk electrochemistry, a transistor with 1 wt % LiClO4 blended into the P(VPA-AA) film was studied (Fig. 3). The large amount of P(VPA-AA) in the film still led to a fast rise and fall in the drain current. After the capacitive current peak following the rise, the addi-
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tional salt led to a progressive increase in current over several seconds, attributed to the enlargement of the conducting channel resulting from the penetration of perchlorate anions into the P3HT layer.[33,34] When the transistor was turned off after having been on for 5 s, the off current was three times larger than in the original off state (before the applied gate pulses), which was probably because of the stability of the P3HT+–ClO4– in the channel. To completely undope the channel and restore the low conductivity of the neutral semiconducting polymer, a positive potential of +1 V needed to be applied to the gate. This behavior is typical for electrochemical transistors, but was not present in the EDLC-OFET, for which the current level of the off state was independent of the history of the gate bias. According to the electrolyte model presented here, a polymer transistor gated with a polyanionic electrolyte should essentially have the same operating voltages independent of the thickness of the electrolyte layer, and of the position of the gate electrode. In the second type of EDLC-OFET, illustrated in Figure 4a, the gate electrode of a neighboring transistor was utilized to modulate the conductance of the channel (gate lines separated by 1.1 mm). The drain current in the output characteristics (Fig. 4b) was lower than that observed from the vertically gated EDLC-OFET (Fig. 2b), possibly because the induced charges in the P3HT were located over the source electrode rather than in the channel. Adjusting the position of the gate electrode relative to the channel could improve this. In the third example (Fig. 4c) a much thicker P(VPA-AA) layer was cast from a drop on the P3HT film. The output characteristics displayed clear current modulation (Fig. 4d), but the drain current was much lower than in the other two transistor configurations. This could be explained with a weaker capacitive coupling between the gate and channel, because of poorer contact between the metal wire and the electrolyte and/or the decreased area of the gate electrode. These two experiments demonstrated that the device behavior was robust
-0.8 V
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Figure 3. The switching characteristics of an electrochemical OFET. The chronoamperometric response for a P3HT transistor (L= 7 lm, W = 200 lm) with a 54 nm thick P(VPA-AA) gate insulator “contaminated” with 1 wt % of LiClO4, in order to show the typical behavior for electrochemical transistors. The plot shows the response of ID at constant VD when VG pulses of –1 V and +1 V are applied. The insets show the applied VG and an enlargement of the drain current.
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Figure 4. Alternative gating of EDLC-OFETs. a) A schematic cross section of a laterally gated transistor manufactured as the EDLC-OFET shown in Fig. 2a. b) Typical output curves obtained for the device shown in (a) (L= 9 lm and W = 200 lm). c) A schematic cross section of a transistor with a hemispherical P(VPA-AA) gate insulator formed on top of the P3HT layer by letting a drop of polyelectrolyte solution dry. A metal wire was pushed against the droplet to act as the gate electrode. d) Output curves obtained for the device sketched in (c) (L= 43 lm and W ≈ 500 lm).
with respect to the alignment and patterning of the electrolyte and gate electrode, a typical challenge when taking printed organic electronics to production. In summary, we have shown that a polyanionic proton conductor can be used as a high-capacitance layer in an OFET, significantly improving its performance. The novelty of this
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Experimental The EDLCs were manufactured by spin-coating P(VPA-AA) (Rhodia) from a mixture of 1-propanol and deionized water onto a Si wafer with a global layer of thermally evaporated Ti (bottom electrode). The 54 nm thick pinhole-free polymer film was subsequently annealed under vacuum at 110 °C for 1 min, after which circular Ti top electrodes were deposited, by using thermal evaporation in a vacuum through a shadow mask. The overlap between the bottom and top Ti electrodes defined the area of the capacitors, which was ca. 7.6 × 10–4 cm2. The EDLC-OFETs were prepared first by thermally evaporating, under vacuum, a thin global Au layer (on a Cr adhesion layer) on a Si wafer with thermally grown oxide. Source/drain electrodes were patterned from this film by using photolithography and wet-etching. P3HT (Sigma-Aldrich), with a regioregularity greater than 98.5 % and a molecular weight of about 90 000, was then spin-coated from chloroform onto the substrate and subsequently dried at 60 °C under nitrogen, giving a thickness of ca. 18 nm. To construct the EDLC-OFETs sketched in Figures 2a and 4a, a 54 nm thick layer of P(VPA-AA) was spin-coated on top of the organic semiconductor. A Ti gate electrode was thermally evaporated in vacuum through a shadow mask to complete the transistor. To fabricate the transistor shown in Figure 4c, a hemispherical P(VPA-AA) gate insulator was formed on top of the P3HT layer by letting a drop of P(VPA-AA) solution dry for 10 min at 100 °C. A metal wire was pushed against the droplet to act as the gate electrode. The capacitance and phase-angle characteristics for the capacitors were recorded for an ac voltage of 0.1 V with an Alpha high-resolution dielectric analyzer (Novocontrol GmbH). The frequency of the applied-voltage signal was swept from high to low frequencies. The effective capacitance was calculated from the equation: C = 1/(2pfZ″), where f is the frequency and Z″is the reactance. The characteristics of all the transistors reported here were measured with a Hewlett Packard 4155B parameter analyzer. A voltage sweep rate of 0.03 V s–1 was used in the current–voltage (I–V) measurements, unless otherwise stated. An Agilent 33120A arbitrary-waveform generator was also used, together with the parameter analyzer, when performing the switch-speed measurements. Note that the large gate capacitor-charging current peaks (because of a substantial overlap between source/drain and gate electrodes) were directed opposite to the change in the drain current. Consequently, the actual switch times could be even shorter than those reported. All measurements were made in ambient air (relative humidity ca. 40 %) at room temperature. Measuring at another humidity would
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be likely to change the water content in the electrolyte film and consequently the electrical characteristics of the devices. Most devices functioned and behaved in a reproducible manner. Received: April 21, 2006 Revised: September 25, 2006
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electrolyte-gated field-effect transistor resides in the mechanism that opens the channel through the quick formation of highly capacitive electrical double layers at the P3HT/polyelectrolyte and polyelectrolyte/gate interfaces as the transistor is gated. Electrochemical doping of the P3HT bulk was prevented by using only immobile anions in the electrolyte. This resulted in a fast-responding (< 0.3 ms), low-voltage (< 1 V) transistor. Employing a polyelectrolyte as gate insulator also eases design and manufacturing requirements. Controlling the alignment of the gate electrode and the thickness of the gate insulator becomes less critical because a nanometer-scale “capacitor” spontaneously forms at the semiconductor/insulator interface as the gate is biased. This enables one to produce low-voltage and relatively fast transistors in robust device structures, which is of great importance for flexible electronics.
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