Water-Gated n-Type Organic Field-Effect Transistors for Complementary Integrated Circuits Operating in an Aqueous Environment.

The first demonstration of an n-type water-gated organic field-effect transistor (WGOFET) is here reported, along with simple water-gated complementary integrated circuits, in the form of inverting logic gates. For the n-type WGOFET active layer, high-electron-affinity organic semiconductors, including naphthalene diimide co-polymers and a soluble fullerene derivative, have been compared, with the latter enabling a high electric double layer capacitance in the range of 1 μF cm-2 in full accumulation and a mobility-capacitance product of 7 × 10-3 μF/V s. Short-term stability measurements indicate promising cycling robustness, despite operating the device in an environment typically considered harsh, especially for electron-transporting organic molecules. This work paves the way toward advanced circuitry design for signal conditioning and actuation in an aqueous environment and opens new perspectives in the implementation of active bio-organic interfaces for biosensing and neuromodulation.

Introduction

Organic semiconductors have been widely adopted for flexible and large-area micro- and opto-electronics applications[1−8] and are also emerging as ideal materials for several biomedical uses, from wearable healthcare devices to applications in neuroscience and biotechnology, including: biosensors; actuators; devices for efficient, sensitive, and reliable neural stimulation, and/or recording.[9−15] Electrochemical detectors, as well as field-effect transistors, and photodetectors were successfully integrated into living systems,[9,14,16−21] both in vitro and in vivo, being able to establish functional interconnections between electronic and ionic conduction.

Following a route similar to that for the fast growth and success of more conventional electronics, the development of organic biointerfaces would experience a strong expansion of possibilities and further opportunities with the realization of logic and analog circuits. Several applications can be envisaged, from drug delivery to neuromodulation and from control of intracellular calcium waves to highly sensitive biosensing and spatially and temporally resolved release of biochemicals for controlled cell growth and differentiation.[22,23]

There have been mainly two approaches to biointerfaced circuits, a purely ionic approach and a mixed ionic/electronic one. The former adapts to and integrates the ionic nature of biological signal transduction and aims at the development of devices relying on the control of ion density and motion. Interesting and widely demonstrated examples are: nanofluidic devices; field-effect ion transistors;[24,25] and ionic, bipolar junction transistors, showing the ability to modulate the transport of both cations and anions under physiological conditions.[26,27]

An alternative approach is ionic gating of an electronic channel,[9,14,28−30] which has also been considered a general strategy for the reduction of the operative voltage of an organic field-effect transistor (OFET).[31] Such a device actually inherits and benefits from at least two decades of development toward the improvement of the electronic and transport properties of organic semiconductors and can interface with the biological world through liquid ion gating in an aqueous environment.[25] Therefore, it potentially represents an ideal hybrid system that bridges biology and electronics. The development of water-gated organic field-effect transistors (WGOFETs)[32] is a natural step toward such technology. WGOFETs combine: the use of water (or, more generally, saline aqueous solutions) as the gating medium; a very low operating voltage, typically in the range of 1 V; and recently developed high-mobility printable small molecules or polymers, which may enable cost-effective fabrication of integrated biosensors and bioactuators in the future. The low voltage is made possible by the high capacitance arising at the water/organic semiconductor interface, as a consequence of the electric double layer (EDL) generated by mobile ions in the electrolyte and charge carriers within the organic transistor channel.[33−35] Several organic semiconductors, evaporated small molecule thin films, single crystals, and solution-processed semiconducting polymers have been recently tested as active materials for the realization of WGOFETs in different configurations,[25,32,36−42] showing promising properties in terms of electronic conduction, biocompatibility, easiness of processability, and environmental and electrochemical stabilities. The common feature among most of the semiconductors adopted so far in the literature for WGOFETs is that they are good hole-transporting materials, enabling so-called “p-type” devices. The critical aspects that are now under study and still represent a challenge for such devices are the control of parameters and stability issues.[42] These same criticalities have probably frustrated so far any convincing attempt to demonstrate n-type WGOFETs by employing electron-transporting organic semiconductors, apart from cases in which a pretreatment of the semiconductor surface with acetonitrile allows for latent observation of n-type functionality.[38] Yet, the demonstration of n-type WGOFETs would enable more robust, complementary logic requiring less power to be operated[43] and a greatly expanded opportunity for the design of analog circuitry for signal processing and amplification.

In this work, we investigate the possibility of fabricating water-gated transistors with solution-processable electron-transporting semiconductors and report the demonstration of an n-type polymer FET that is gated only through the use of water, in the absence of any interfacial treatment. We report WGOFETs based on three different electron-transporting semiconductors, namely, poly{[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bisthiophene)} (PNDIT2),[44] poly[(E)-2,7-bis(2-decyltetradecyl)-4-methyl-9-(5-(2-(5-methylselenophen-2-yl)vinyl)selenophen-2 yl)benzo[lmn][3,8] phenanthroline-1,3,6,8(2H,7H)-tetraone] (PNDISVS),[45] and [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM).[46,47] Quite surprisingly, according to the current understanding, all employed n-type materials permit operation of water-gated devices at a low voltage, with width-normalized currents in the range of 3–55 pA/μm at 0.8 V, and, combined with suitable p-type WGOFETs, enable the realization of water-gated complementary inverting logic gates. Electrochemical impedance spectroscopy (EIS) reveals the possibility of formation of a high-capacitance EDL only in the case of PCBM, whereas a much lower capacitance is observed in the case of PNDIT2 and PNDISVS, owing to partial degradation of the transport properties, triggered by contact with water, thus rationalizing the higher currents registered in PCBM devices despite the lower charge mobility. Demonstration of just marginal degradation over 1 h of continuous operation under water exposure indicates a reasonably promising future for n-type WGOFETs and their integration into robust biointerfaced logic circuits and sensors.

Results and Discussion

Fabrication and Characterization of n-Type WGOFETs and Complementary Logic Gates

Figure a depicts the configuration employed for the realization of WGOFETs.[32] We adopt a bottom-contact, top-gate geometry, realized by patterning with photolithography Source and Drain electrodes (channel width W = 20 000 μm, channel length L = 20 μm) on glass and by depositing the electron-transporting semiconductors by spin-coating from solution. A drop of water is then cast on the semiconductor surface, and a tungsten probe works as a gate electrode. When a positive voltage is applied between the gate and the electrodes, ions present in the water droplet start migrating: anions are accumulated toward the gate and cation, toward the semiconductor. This should electrostatically induce accumulation of electrons in the semiconductor, at the semiconductor/water interface, forming a Helmholtz double layer.[33−35] As the two oppositely charged layers at this interface are very close, with the typical thickness of the double layers being in the range of a few angstrom in electrolytes, a large capacitive coupling arises, enabling operation of the transistor at a very low bias (generally, VGS < 1 V). It is worth noting that such a gating mechanism can also be achieved starting from highly resistive, purified water, as in this work. Indeed, due to air-exposure and handling, the resistivity of water drops by orders of magnitude as an effect of rapid contamination by carbon dioxide from the air, which leads to rapid production of HCO3–. In our case, we adopted ultrapure (Milli-Q, Millipore) water, characterized by a nominal resistivity of 10 MΩ cm at 25 °C. This resistivity decreases to hundreds of kΩ cm upon contamination. We can expect that in the case of p- and n-gating HCO3– and H+ ions, respectively, will migrate toward the interface with the organic semiconductor, thus contributing to the formation of the EDL. Other ionic species may be active, and the local polarization induced by the aqueous environment may also facilitate the migration process.[48] However, a detailed analysis is beyond the goal of the present work.

Figure 1

(a) Sketch of the n-channel WGOFET architecture; (b) molecular structures of the n-type semiconductors employed as active phases in WGOFETs, namely, PNDIT2, PNDISVS, and PCBM; (c) transfer characteristics of the n-channel WGOFETs made with PNDIT2, PNDISVS, and PCBM; all curves are taken at VDS = 0.8 V; source-to-gate leakage currents (IGS) are also reported.

The chemical structures of the n-type semiconductors investigated in this work are shown in Figure b. The semiconductors were chosen because of their relatively low-lying lowest unoccupied molecular orbital (LUMO) levels. PNDIT2 and PNDISVS are two naphthalene diimide-based donor–acceptor co-polymers featuring a bithiophene and selenophene–vinylene–selenophene donor moiety, respectively. They have LUMO levels at −3.91 and −3.98 eV, respectively, owing to the high electron affinity of the NDI unit,[49] and both are known to yield electron mobilities higher than 1 cm2/V s in solid-state (SS) field-effect devices.[5,45] PCBM is a soluble fullerene derivative, often used as an electron acceptor for bulk heterojunction organic solar cells,[50−53] yielding good electron mobilities in conventional SS OFETs, in the 0.01–0.21 cm2/V s range,[54−57] and showing a similar LUMO level to that of NDI-based materials (∼−4.0 eV).[58] The stability of the adopted semiconductors in water and/or oxygen, clearly relevant for the development of WGOFETs, has been partially addressed in the literature,[59−63] and different degradation pathways, involving both oxygen and water molecules, have been identified. However, on the basis of the available environmental stability data, it is not yet trivial to predict whether such semiconductors can work in n-type WGOFETs, in which the active layer is in direct contact with liquid water. We verified that it is possible to record n-type field-effect behavior in all cases, as demonstrated by transfer curves for WGOFETs (Figure c). A clear modulation of the source-to-drain current (IDS) can be observed in all WGOFETs using a low positive VGS. All curves display similar and relatively high leakage currents through the water drop (IGS values up to 0.05 μA at VGS = 1 V); however, for VGS > 0.5 V, IDS is higher than IGS in all cases, thus allowing to reveal a genuine field effect in our water-gated devices. The highest IDS values are measured in PCBM WGOFETs (1.1 μA already at VGS = 0.8 V); slightly lower current values are measured in PNDISVS (0.3 μA at VGS = 0.8 V), which however displays a higher threshold voltage, VTh, of 0.49 V, compared to 0.18 V for the PCBM device. The lowest currents are shown by the PNDIT2 device, with 0.06 μA at VGS = 0.8 V and VTh = 0.17 V. Devices based on PCBM and PNDIT2 display similar Ion/Ioff values of ∼103, one order of magnitude superior to those for PNDISVS-based devices, mostly in virtue of the early onset with respect to that for PNDISVS. Importantly, for the following interpretation of the data, at VGS = VDS = 0.8 V (with VDS being the bias applied between the source and drain electrodes during operation), all WGOFETs are well into the accumulation regime, beyond the subthreshold, as evidenced by the good linear fitting of IDS1/2 versus VGS.[64]

The demonstrated possibility of realizing n-type WGOFETs enables the realization of electrical circuits integrating both n- and p-type transistors. As a first proof-of-concept of an integrated complementary organic circuit gated through water, we fabricated complementary inverters (Figure a),[65] using well-established p-type WGOFETs, based alternatively on (poly(2,5-bis(3-hexadecylthiophen-2-yl)thieno[3,2-b]thiophene)) (pBTTT) or (poly(3-hexylthiophene-2,5-diyl)) (P3HT).[32,41] To fabricate the integrated inverters, the two semiconductor layers are deposited from solution and patterned on interdigitated channels, one next to the other, using a photolithographic method developed by Chang et al.[66] In Figure b, a picture of a typical device is shown: the p-type (in figure pBTTT) and n-type (in figure PNDIT2) layers are, respectively, visible in the same substrate as active layers of the two interconnected transistors. A drop of water is employed as the gating medium and is common to both transistors. A tungsten tip (radius 300 μm) serves as an input node of input voltage Vin and is immersed in the water drop at approximately the same distance from both transistors and at ∼1 mm from the active material surface. Using this approach, we have realized various inverters combining different p- and n-type WGOFETs. The output characteristics of the single devices in the inverters and the voltage-transfer characteristics (VTC) of the inverter are plotted in Figure . The combination of PNDIT2 and pBTTT (L = 40 μm and W = 20 000 μm) produces an inversion threshold, Vinv, of 0.28 V (calculated as the intercept between the bisecting line of the quadrant corresponding to Vin values from 0 to 1 and the inverter curve), thus shifted to lower values with respect to the ideal value corresponding to half of the supply voltage (VDD/2) (Figure c–e). The shifted transition between the two logic states is attributed to the mismatch between n- and p-type transistors, originating from the differences in mobility, capacitance, and threshold voltages. It is possible to observe hysteresis between the forward (inversion from logic states “1” to “0”) and backward (from “0” to “1”) scans. On quantifying it by considering the difference Vin,forward – Vin,backward at Vout = VDD, a value of 0.29 V is obtained. The gain of the inverter, a significant parameter for establishing the quality of the device, extracted as the slope in the transition region (Vinv) of the VTC, is about ∼2 (Figure S2). The combination of PNDISVS and pBTTT (Figure f–h) (p-type: L = 40 μm, W = 15 000 μm; n-type: L = 5 μm, W = 20 000 μm) is characterized by a more balanced VTC, with Vinv = 0.38 V at VDD = 0.8 V; a reduced hysteresis of 0.18 V (compatible with the hysteretic behavior shown by the transfer characteristics of both p- and n-type transistors, see Figure S3); and an increased gain of ∼3 at the logic transition (Figures S3 and S4). The best inverter was obtained by combining PCBM and P3HT (L = 5 μm and W = 10 000 μm) (Figure i–k, where data refer in this case to an inverter obtained by external wiring of two WGOFETs): we have obtained Vinv = 0.39 V at VDD = 0.8 V, a limited hysteresis of 0.07 V, and an increased gain of ∼6 (Figure S5 and S6).

Figure 2

(a) Circuit configuration of a complementary inverter; (b) exemplary picture of real inverters realized in this work; n-type output (c, f, i), p-type output (d, g, j), and VTC as a function of input voltages (e, h, k) of a water-gated inverter realized with PNDIT2 (c, d, e), PNDISVS (f, g, h), and PCBM (i, j, k) WGOFETs as the n-channel.

Thanks to the possibility of observing an n-type field effect for all three electron-transporting semiconductors presented here, we showed that it is possible to adopt a complementary design for water-gated electronic circuits, exemplified here by the very simple case of an inverting device, thus overcoming the strong limitations of unipolar design toward water-gated circuits: on the one hand, more robust water-gated logic gates can be exploited, and on the other, as most well-known analog architectures require both n- and p-type FETs, signal-conditioning stages, such as amplifiers operating in an aqueous environment, become feasible.[67]

Investigation of the Capacitive Coupling and Transport Mechanism of n-Type WGOFETs

As already outlined above, the devices show different currents (Figure c), and it is important to clarify the origin of such differences. Interestingly, the WGOFET currents do not simply reflect the mobility of the OFETs fabricated with the polymer films under exactly the same conditions as those adopted for WGOFETs (details of the devices and the corresponding transfer characteristics are reported in the Supporting Information). In fact, we have extracted a mobility in saturation, μsat, of 2.5 cm2/V s for PNDIT2, μsat = 1.9 cm2/V s for PNDISVS, and μsat = 8 × 10–3 cm2/V s for PCBM from OFETs. Therefore, although PCBM shows the smallest field-effect mobility among conventional transistors, it leads to the best n-type WGOFETs reported. This observation may imply that the field-effect mobility in water-gated devices is drastically different from that in conventional FETs and/or that the capacitance of all WGOFETs is not the same and is affected by the specific nature of the semiconductor. The latter has been previously observed, for example, in p-type WGOFETs, in which it was shown that the capacitance induced by the Helmholtz double layer thickness can have sensible contributions from the polymer structure (side chains) and/or microstructural features at the interface with water.[42] Moreover, in the case of n-type materials that are typically subjected to oxygen/water-induced operational instabilities, the effectiveness of the charge-accumulation process could have been compromised by the direct contact with water.

From the WGOFET transfer characteristic curves, we first extract the capacitance–mobility product in the saturation regime, C·μsat, a safely extracted figure of merit, accounting also for the efficiency of the channel accumulation process. We have extracted C·μsat values of 3 × 10–4, 2.5 × 10–3, and 7 × 10–3 μF V–1s–1 for PNDIT2, PNDISVS, and PCBM, respectively. For comparison, the same quantities extracted for conventional OFETs (see Table ) are as follows: 1.45 × 10–2, 1.1 × 10–2, and 4.6 × 10–5 μF V–1s–1 for PNDIT2, PNDISVS, and PCBM, respectively. Evidently, only PCBM-based WGOFETs congruently benefit from the high capacitance of the water-gating process, resulting in values of C·μsat that are 2 orders of magnitude higher than those in conventional OFETs, characterized by a low capacitance of 5.8 nF cm–2. Conversely, NDI polymer-based WGOFETs show C·μsat values that are even lower than those for the corresponding conventional OFETs, in agreement with a scenario of water-induced degradation of accumulation and/or transport properties. To solve this discrepancy, we performed EIS measurements to extract the capacitance values at the water–semiconductor interface for the three cases. Electrochemical characterization was carried out on Au/water/polymer/Au capacitor structures by means of a potentiostat, in a two-electrode configuration (polymer/gold interface area ∼5 mm2; gold counter-electrode area ∼9 cm2). Capacitances values were extracted from impedance measurements according to an equivalent RC frequency-dependent circuit (Figure S8a), in which the Helmholtz double layer capacitance is considered to be well approximated by C(ω) in the low-frequency range. Figure a shows the extracted capacitances as a function of frequency at gate voltages of 0.4 V, for which the accumulation regime is achieved. Bode plots are reported in the SI (Figure S9).

Table 1

Summary of the Main Electrical Parameters Extracted from Au/Pure Water/OSC/Au Structures, WGOFETs, and SS OFETs of This Worka

	C (μF/cm2)	C·μsat WGOFET (μF/V s)	VTh WGOFET (V)	μ SS-OFET (cm2/V s)
PNDIT2	0.08b	0.0003c	0.17	2.5
PNDISVS	0.13b	0.0025c	0.49	1.9
PCBM	1.0	0.0070	0.18	0.008

Note that C represents the capacitance values as extracted from EIS measurements at 0.1 Hz (in agreement with the slow VGS sweep rate applied during transfer curve acquisition); as commented in the text, in the case of semiconductor degradation, this C value does not coincide with the capacitance responsible for the field effect within the channel.

Corresponding to the geometrical capacitance of the semiconducting film (0.068 and 0.123 μF cm–2 for PNDIT2 and PNDISVS, respectively).

Apparent values, extracted by approximating the active area to the one defined by the source and drain pattern (W, L).

Figure 3

(a) EIS characterization of PNDIT2, PNDISVS, and PCBM films within Au/pure water/OSC/Au structures (Au counter-electrode voltage, 0.4 V); (b) tentative sketch of the charge-accumulation and transfer mechanisms in WGOFET architectures in the case of fully preserved (up), i.e., PCBM, and partially degraded (down), i.e., PNDIT2 and PNDISVS, semiconductors.

The capacitance values at low frequencies, before the cutoff (<100 Hz), are remarkably higher for the case of PCBM, reaching almost 1 μF cm–2 at 0.1 Hz. This value is of the same order of magnitude of that recorded in other p-type WGOFETs.[32,40,42,68] It can be observed that the capacitance of PCBM increases for frequency values lower than 1 Hz (Figure a), strongly suggesting transition to a different charge-accumulation mechanism, enabled by slower bias modulation. Ion migration through PCBM films, with the formation of a volumetric capacitance and therefore an extended gated volume, may likely account for such behavior;[69] a detailed understanding of the charge-accumulation mechanism in PCBM/water interfaces would require deep investigations, which are out of the scope of this work. If we use the EIS extracted capacitance at 0.1 Hz from the C·μsat parameter of the PCBM WGOFET, we estimate a μsat of 0.007 cm2/V s, a value that is strongly consistent with the conventional OFET mobility (0.008 cm2/V s). This demonstrates that no evident degradation owing to the direct contact with water precludes the electron accumulation and transport mechanism in water-gated PCBM in the whole channel area.

Conversely, in the case of NDI-based polymers, much lower capacitance values are recorded, approximately 120 and 75 nF cm–2 (at 1 Hz) for PNDISVS and PNDIT2, respectively. These values are not in agreement with the formation of an EDL over the whole channel area and are instead very close to the geometrical capacitance (Cg) of the polymer layer considered as a perfect dielectric: from the thicknesses of the semiconducting layers (25 nm for PNDIT2 and 45 nm for PNDISVS), we estimated capacitances of 123 and 68 nF cm–2 for PNDIT2 and PNDISVS, respectively (assuming εr = 3.5).[70,71] However, if no charge at all could be accumulated in the semiconductor, as the capacitance measurements seem to imply, we would obviously not be able to record any field-effect current in these devices. As a matter of fact, because both the NDI co-polymers show working WGOFETs (Figure c), electrons must, first, be able to accumulate in the semiconductor with a capacitance high enough to operate at |VGS| < 1 V and, further, to be transported through the channel. This can only be possible if, following a partial degradation process precluding the accumulation of charges on most of the device width, some limited areas, negligible with respect to the total area, hence almost not recordable with EIS measurements, are actually active. Such a degradation mechanism might be related to the trapping of accumulated carriers, which occupy deep electronic states and cannot move under the oscillating field of the impedance measurement. Such scenario is schematically depicted in Figure b: within the matrix of a degraded polymer, mostly impeding charge injection and accumulation, few conducting paths exist, forming a much lower effective channel width compared to that geometrically defined by the source and drain pattern. Accordingly, the estimated values of C·μsat are just apparent values and are largely underestimated.

It is quite surprising that despite the strong degradation (not more than 10% of the channel is likely preserved) charge percolation is still possible along the 20 μm long channels of the WGOFETs. We can speculate that the high level of interconnectivity of the polymer, as recently reported for PNDIT2,[72] along with the demonstration that long-range crystalline order extended along the channel is not necessary to provide effective charge percolation,[73] may be at the origin of the preservation of charge percolation paths even in the case of strong degradation.

It is worth mentioning that a 1 order of magnitude of difference in C·μ is observed between PNDIT2 and PNDISVS, possibly owing to the superior stability of PNDISVS.[45]Although on the basis of the estimated energetic levels of the two polymers electrochemical reactions of H2O molecules and/or hydrated oxygen with the neutral and charged conjugated segments should be similarly favored, it is possible that the improved performance of PNDISVS derives from a more effective self-insulation of the backbones from a combination of effects related to side chains (C8C10 for PNDIT2 vs C10C12 for PNDISVS) and micro- and meso-structural features (with evidence of superior crystallinity in PNDISVS films[45]), as the different packings can act as a kinetic barrier against penetration of aqueous reactive species into the film.[74,75]

Stability of PCBM-Based WGOFETs

Having analyzed the origin of the better performance of the PCBM WGOFETs, we performed tests to first assess the stability of operation of the devices while gated through water (p-type polymer stability has already been established elsewhere[39,76]). In Figure a, the continuous cycling voltage test of PCBM WGOFETs is reported; we performed 100 cycles by switching the device on (VGS = 0.8 V) and off (VGS = −0.2 V), with a duration of 9 s/cycle (total test duration 15 min). In the first 15 cycles of the test, no evidence of degradation was observed; after the first 15 cycles, a gradual decrease in the on current was observed, also accompanied by an increase in the off current. After 85 cycles a reduction of 10% in the on current and on/off reduction from 744 to 620 is registered, providing proof of a promising, yet limited so far, robustness of the device to operation in contact with water.

Figure 4

(a) Continuous cycling voltage test of PCBM WGOFETs at VDS = VGS = 0.8 V; the duration of each cycle was 9 s, and the test was carried out for a total time of 15 min; (b) PCBM shelf-life test: transfer characteristics (VDS = 0.8 V) at different times of exposure to air-free water, for a total exposure time of 55 min.

We have also tested shelf-life stability, by maintaining PCBM continuously in contact with water and measuring the transfer curve of the device just after depositing the droplet and after 3, 9, and 55 min (Figure b). After 3 min, we recorded a marked threshold voltage shift toward lower voltages. The curve recorded after 9 min is identical to the one recorded after 3 min. After 55 min, only a modification in the subthreshold region was recorded, whereas the current on full accumulation was almost unmodified. The off current increase registered in the cycling test is thus due to a shift in the threshold voltage, solely induced by the prolonged exposure to water, as observed in the shelf-life stability test, and is not triggered by the charge-accumulation process; on the other hand, the shelf-life stability test does not evidence any on current drop over a time of exposure 4 times longer than that during the cycling test, suggesting that simple exposure of the PCBM surface does not affect the electron mobility within the film and that the 10% reduction in the on current in the cycling test is mostly due to electrical stress factors.

Conclusions

In this work, we have reported the first demonstration of n-type transistors gated through water using low-lying LUMO-level organic semiconductors, that is, two NDI-based co-polymers and the fullerene derivative PCBM. In particular, PCBM-based WGOFETs preserve full functionality, displaying high capacitance values approaching 1 μF/cm2 and mobility values totally comparable to those of solid-state standard devices. As a proof of concept, we realized water-gated inverters, using as the counterpart of n-type WGOFETs the well-established p-type WGOFETs based on PBTTT and P3HT, resulting in well-balanced low voltage inversion and gain values of up to 6. This work demonstrates the possibility of realizing a water-gated, complementary circuit design based on organic semiconductors. Reasonable, short-term stability of water-gated n-type devices was also reported, a key and most critical aspect for the future development of any real application. Although embryonic and still requiring extension to physiological solutions, the reported results open new perspectives in the realization of complex circuits for signal conditioning and actuation at the interface between biological matter and organic electronics.a

Methods

Materials

PNDIT2 was purchased from Polyera Corporation (Activink N2200) and PCBM, from Nano-C. PNDISVS was synthesized by following the previously reported procedure.[45] P3HT was purchased from Sigma–Aldrich and pBTTT, from Ossila Ltd. (Sheffield, UK). The purchased materials were used without any further purification. Water was purified with a Milli-Q water system (Millipore). Purified water shows a resistivity of 10 MΩ cm just after purification, which decreases to hundreds of kΩ cm because of handling, due to the well-known process of contamination by carbon dioxide from air, which leads to the rapid production of HCO3–.

Samples Preparation

For both WGOFETs and SS OFETs, we adopted a bottom-contact architecture. Thoroughly cleaned 1737F glass was used as the substrate. Interdigitated Au contacts were defined by a lift-off photolithographic process, with a 0.7 nm thick Cr adhesion layer. The thickness of the Au contacts was 30 nm. The substrates were cleaned in a sonic bath in isopropyl alcohol for 2–3 min before deposition of the semiconductor.

PNDIT2 was dissolved in toluene (5 g/L), and the obtained solution was deposited by spin-coating at 1000 rpm for 30 s in air. In this way, 45 nm thick PNDIT2 films were obtained and finally annealed at 120 °C for 10 min to remove residual solvent. PNDISVS was dissolved in chlorobenzene (12 g/L), and the obtained solution was deposited by spin-coating at 2000 rpm for 60 s in air. In this way, 25 nm thick PNDISVS films were obtained and finally annealed at 280 °C for 10 min to remove residual solvent and improve the structural order; they were then cooled slowly to ambient temperature. PCBM was dissolved in chlorobenzene (20 g/L), and the obtained solution was deposited by spin-coating at 1500 rpm for 1 min in a nitrogen glovebox. PCBM films (thickness 55 nm) were annealed at 100 °C for 10 min to remove residual solvent and improve the structural order; they were then cooled slowly to ambient temperature. The P3HT solution was prepared by dissolving P3HT in chloroform at a concentration of 3 g/L. pBTTT was dissolved in a solution of anhydrous dichlorobenzene at a concentration of 5 g/L on a hotplate at 100 °C. The solutions were filtered with a 0.2 lm polytetrafluoroethylene (PTFE) syringe filter before spin-coating.

Concerning SS OFETs, in the case of PNDIT2 and PNDISVS, PMMA (Sigma–Aldrich) was spun from n-butylacetate (80 g/L, filtered with a 0.45 μm PTFE filter) at 1300 rpm for 60 s. Dielectric-layer thicknesses of ≈550 nm were obtained. In the case of PCBM, the perfluorinated polymer CYTOP CTL-809M dielectric (Asahi Glass) was spun as received at 6000 rpm for 90 s (film thickness, 550 nm) as the dielectric layer.

After dielectric deposition, the devices were annealed under nitrogen, on a hotplate, at 120 °C for 14 h. Gate Al electrodes (30 nm thick) were thermally evaporated as gate contacts.

Characterization

EIS and cyclic voltammetry were performed on a Au/water/OSC/Au structure. The measurements were performed with a potentiostat (Metrohm Autolab PGstat 302N) working in a two-electrode configuration. The first working electrode has a contact area for the OSC/Au interface that is much smaller (5 mm2) than the area of the second Au counter-electrode (9 cm2), to avoid the contribution of the latter in the measured complex impedance. Data analyses were performed using Nova 1.8 software. The impedance spectra were recorded in the 0.1 Hz to 100 kHz frequency range by applying a VAC = 0.02 V (root-mean-square value) sine-wave voltage signal. A 0.4 V continuous bias (DC signal, referred to as the gold counter-electrode) was superposed to the AC signal. For the details of the equivalent electrical circuit used to fit the EIS data, please refer to the Supporting Information.

The electrical characteristics of the transistors and inverters in this work were measured in a nitrogen glovebox on a Wentworth Laboratories probe station with an Agilent B1500A semiconductor device analyzer.