Flexible, plastic transistor-based chemical sensors

Organic Electronics 10 (2009) 377–383

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Flexible, plastic transistor-based chemical sensors Mark E. Roberts, Stefan C.B. Mannsfeld, Randall M. Stoltenberg, Zhenan Bao * Department of Chemical Engineering, Stanford University, Stauffer III, 381 North-South Mall, Stanford, CA 94305-5025, USA

a r t i c l e

i n f o

Article history: Received 19 September 2008 Received in revised form 2 December 2008 Accepted 2 December 2008 Available online 11 December 2008

PACS: 70

a b s t r a c t Flexible, plastic chemical sensors were fabricated using a thin polymer gate dielectric layer and polymer electrodes patterned via selective wetting directly on the surface of the organic semiconductor film. Low-voltage transistors based on DDFTTF with PEDOT:PSS electrodes had a mobility as high as 0.05 cm2/Vs with an on–off ratio of 1.2  104 on ITO/PET substrates. These devices demonstrated stable operation in water with sensor characteristics similar to those reported on rigid silicon substrates, with sub-ppm detection for cysteine and 2,4,6-trinitrobenzene (TNB). Ó 2008 Elsevier B.V. All rights reserved.

Keywords: Organic transistor Sensor Flexible Patterning

Recent progress in organic materials has given a great deal of promise to plastic electronics, especially for organic light-emitting diodes (OLEDs) [1] and radio-frequency identification cards (RFIDs) [2–4]. The rapid progress of the field has been, in part, due to improvements in fabrication processes and new material design, with noticeable improvements in electronic properties for the semiconducting [5– 7], conducting [8] and insulating [9–12] components. Recently, organic field-effect transistors (OTFTs) have gained considerable attention with the demonstration of potentially scalable patterning processes [13–18] and improved semiconductor and gate insulator performance, laying the groundwork for low-power circuits [11] and devices [19]. Due to the versatility of chemical synthesis, organic materials have a niche application in chemical sensors based on OTFTs [20], in which the semiconductor functions simultaneously as the active transport layer and the sensing component. Functional derivatives of organic semiconductors have already demonstrated utility in chemical and biological sensors based upon the ability to impart specific * Corresponding author. Tel.: +1 650 723 2419. E-mail address: [email protected] (Z. Bao). 1566-1199/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2008.12.001

functionality to direct the interaction with target analytes [21,22]. However, most devices used a rigid, planar substrates and required high operating voltages. The development of low-cost, disposable sensors for environmental monitoring, health diagnostics or detection of chemical warfare agents requires incorporation of simple processing on inexpensive, large area substrates. The ability to detect chemical species in water adds another degree of versatility to OTFT sensors, allowing for in situ measurements with these applications for non-volatile species. For example, detection of amino acid concentrations in situ could have a profound impact on disease detection and prevention [23], while the ability to monitor munitions disposal is important for maintaining safe water supplies [24]. In this report, we combine the fundamental advantages of organic materials for the demonstration of flexible, transparent chemical sensors based on plastic thin-film transistors in aqueous solutions. Our OTFT sensors are fabricated using organic and polymeric materials with simple processing on a conductive indium tin oxide (ITO) film on polyethylene terephthalate (PET) substrates. These devices were operable at low bias in vapor and liquid solutions enabled by the use of a

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Fig. 1. Fabrication scheme for flexible sensors with gold and PEDOT:PSS electrodes. (i) A 35 nm PVP–HDA film is deposited via spin-coating on ITO/PET. (ii) 30 nm DDFTTF is thermally evaporated on the PVP–HDA films. (iii) Gold electrodes are deposited through a shadow mask. (iv) DDFTTF surface is modified with UV-ozone through a shadow mask. (v) PEDOT:PSS solution is selectively patterned on the UV-ozone treated areas. (vi) The solvent is removed in vacuum leaving conductive polymer electrodes.

cross-linked gate dielectric layer [9]. Additionally, we show the chemical sensing performance of these devices is comparable to that of OTFTs fabricated on rigid, planar substrates, with detection of cysteine and trinitrobenzene (TNB) down to 100 parts per billion. The fabrication scheme for the flexible OTFTs is shown in Fig. 1. A previously reported polymer dielectric film, [9] poly(4-vinylphenol) (PVP) cross-linked with 4,40 -(hexafluoroisopropylidene)diphthalic anhydride (HDA), was deposited via spin-coating directly on the ITO surface at 3000 rpm and cured at 100 °C for 2 h, resulting in a film with a thickness of 35 nm and a capacitance of 115 nF/ cm2. Fig. 2B shows the atomic force micrograph (AFM) of the PVP–HDA surface with a surface roughness of 2.6 nm, moderately rougher than that reported on a planar surface. DDFTTF, an organic semiconductor demonstrating longterm stability in water, was thermally deposited on the cross-linked polymer film at a rate of 0.2–0.3 Å/s and a substrate temperature of 105 °C to a thickness of 30 nm. Top-contact electrodes were deposited on the semiconductor surface using two methods. Gold (40 nm) was deposited via thermal evaporation (1.0 Å/s) and PEDOT:PSS was deposited via selective wetting. The surface energy of the organic semiconductor surface can be considerably modified as shown by a decrease in water contact angle (First Ten Angstroms FTA200 equipped with a CCD camera) upon UV-ozone exposure.

Without any treatment, the DDFTTF surface is very hydrophobic (112°), due to the vertically-aligned, densely-packed alkyl chains. After treating the organic semiconductor surface with UV-ozone for 12 min, the water contact angle decreases substantially from 112° to 23° (shown in Supporting information, figure S1), due to the complete removal of the alkyl chains. Using X-ray photoelectron spectroscopy (SSI S-Probe Monochromatized XPS Spectrometer, Al(ka) radiation (1486 eV)), we showed that a dodecyl silane monolayer is completely removed from a silicon oxide surface after 12 min (Supporting information figure S2). The unique contrast in surface energy is the premise for the deposition of poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) [25] via selective wetting. Immediately after UV-ozone exposure, the substrate with a patterned DDFTTF surface (with features varying from 50 lm to 400 lm) is briefly held in a solution vortex [26] of PEDOT:PSS (Baytron P, 1.2–1.4% aqueous solution, Bayer Corp.) diluted with water and ethylene glycol (EG) at a ratio of 47:47:6, 90:0:10, or 50:50:0 (Baytron P:water:EG). As the substrate is withdrawn from the solution, the contrast in surface energy results in the deposition of PEDOT:PSS solution on the regions previously exposed to UV-ozone. The substrates are then placed on a hot plate at 90 °C for 1 h to dry the film. The addition of ethylene glycol is known to lead to a significant increase

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Fig. 2. (A) AFM images (10  10 lm2, phase left, height right) of the top-contact PEDOT:PSS electrode interface with DDFTTF. The line profile (red line in height image) shows good conformity of the PEDOT:PSS electrodes that are actually smoother than the top surface of the DDFTTF film. (B) AFM (10  10 lm2, height) of the PVP–HDA surface on an ITO/PET substrate.

in the conductivity of PEDOT:PSS films [27]. We found that ethylene glycol can either be directly added to the PEDOT:PSS formulation as described above or the sample can be subsequently treated by dipping it into ethylene glycol. In the latter case, the significant surface energy contrast between DDFTTF and the already deposited PEDOT:PSS electrodes leads to an exclusive wetting of the ethylene glycol on the PEDOT:PSS electrodes (Supporting information, figure S4). Both treatments resulted in a reduction of the surface resistivity of the PEDOT:PSS electrodes by several orders of magnitude (compared to untreated PEDOT:PSS electrodes) to values as low as 20 X/sq. Atomic force micrographs (AFM) (Digital Instruments Nanoscope IV operated in tapping mode (350 kHz frequency, Si tip)) images of the PEDOT:PSS electrodes on the DDFTTF surface are shown in Fig. 2A. The PEDOT:PSS makes a good, conformal contact with the surface of the DDFTTF film (see line profile, red line in height image), partially filling the pores on the top surface of the DDFTTF film. The resultant PEDOT:PSS electrode surface is therefore significantly smoother than the top surface of the DDFTTF film. Aside from the top-contact gold electrodes, the resultant devices shown in Fig. 3 are both flexible and transparent, highlighting the advantageous properties of organic

materials. Fig. 3C and D show optical micrographs of PEDOT:PSS electrodes patterned on the semiconductor surface. While the DDFTTF surface was patterned with UV-ozone using a shadow mask with channel lengths (L) of 50 lm and 100 lm, and respective channel widths (W) of 1 mm and 2 mm for a W/L ratio of 20, the final electrodes had a W/L ratio ranging between 4 and 6 for L = 50 and 100 lm. As the solvent (water and ethylene glycol) evaporated, the polymer in solution was drawn to the center leaving slight remnants along the periphery of the UVozone treated regions while maintaining the defined L, shown by the light extensions of the electrodes in the images. The lighter regions of the electrodes, however, were not conductive. The feature size can potentially be reduced by stamping hydrophobic masks [28], for example, using a PDMS elastomeric template for dimensions down to 10 lm [29]. 5,50 -Bis-(7-dodecyl-9H-fluoren-2-yl)-2,20 -bithiophene, DDFTTF, was chosen for the semiconductor layer based on its proven stability in ambient and water [9], thin-film transistor characteristics, and long alkyl side chains that promote molecular packing and water stability. Previously, we showed that thin evaporated films of DDFTTF grow perpendicular to the substrate with highly two-dimensional films promoted by the interactions between the alkyl side

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Fig. 3. Digital photograph of an OTFT with PEDOT:PSS electrodes on black and white text (A) and a flexed OTFT with gold electrodes (B); optical micrographs of OTFTs with PEDOT:PSS electrodes with 50 lm (C) and 100 lm (D) channels. Panel C shows the device on a dark surface while D shows the device on a white surface, illustrating the transparency. Note that the electrodes are invisible when the light is focused on the substrate.

chains [30–32]. Additionally, it is well known that the charge transport occurs within the first 5 nm (in proximity to the dielectric) [33], therefore, the gentle UV-ozone surface modification of the film should not affect the transport properties, as shown in the following section. OTFTs with top-contact gold and PEDOT:PSS were electrically characterized in ambient and water to determine their suitability for sensing in water environments. By incorporating a thin, cross-linked polymer gate dielectric, we were able to operate at gate and source-drain biases below 1 V. The OTFTs fabricated with gold electrodes (W/ L = 20) on DDFTTF showed a mobility of 0.06 cm2/Vs with an on–off ratio of 4  103. The rough ITO surface, and correspondingly rough dielectric surface, lead to the decreased electrical performance compared to using rigid, planar substrates. OTFTs fabricated with PEDOT:PSS electrodes (W/L = 4–6) performed comparable to those with gold electrodes with a mobility of 0.05 cm2/Vs with an on–off ratio of 1.2  104. Although the performance of the two systems was quite similar, the OTFTs with PEDOT electrodes had a lower on-current due to the shorter electrode width from the solution deposition process. A slight increase in leakage current was observed for OTFTs on ITO/PET (3 nA at 1 V) as compared to devices on planar silicon substrates (0.2 nA at 1 V), both of which are much less than the source-drain current. The influence of bending on the OTFT performance for devices fabrication with small molecule organic semiconductors and polymer dielectrics has been reported previously [9,34,35], and we expect similar mechanical properties for these sensors. Electrical characterization in water is performed by placing a droplet of deionized water across the entire electrode (channel) region. It was critical to reduce the sourcedrain bias (VDS) to 0.6 V in order to limit the ionic conduction through water. The transfer characteristics of the OTFT

with gold electrodes are shown in Fig. 4A in ambient (black) and water (grey). The influence of water on the electronic properties is similar to that reported in a previous report, with a slight increase in the mobility (0.12 cm2/Vs), positive threshold shift, and a reduction in the on–off ratio due to the finite ionic current through water. The output characteristics under water are shown in Fig. 4D. As mentioned above, a slight leakage current is observed as a result of using a thin insulating layer on a rough electrode surface. Similar plots for the OTFT with PEDOT:PSS electrodes are shown in Fig. 4C and D. Interestingly, only a small change in transfer characteristics is observed when exposing these devices to water. Other than an increase in the off-current, as previously discussed, and a marginal increase in mobility, the characteristics are nearly identical. This is most likely due to the fact that the semiconductor was previously exposed to water during the PEDOT:PSS deposition process. We demonstrated chemical sensing by adding a small volume of analyte solution to a droplet of water spanning the source and drain electrodes. Based on the sensitivity to various analytes in a previous report, we investigated the influence of cysteine, trinitrobenzene (TNB), and pH on the OTFT performance. In the sensing trials, the source-drain and gate electrodes are continuously biased at 0.6 V (in the saturation regime), and a droplet of analyte solution is added after current stabilized (approx. 30–60 s). The relative drain current change, DIDS = IDS/IDS0 (IDS0 is the drain current baseline before the analyte addition) is plotted versus time for each analyte system. We observed similar DIDS responses on the flexible ITO/PET substrates as we previously observed on silicon for cysteine solutions and pH [9], which was performed on OTFTs with PEDOT:PSS electrodes. Top-contact OTFTs with PEDOT:PSS electrodes were covered in water (5 ll) and biased according to the condi-

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Fig. 4. (A) Semi-log plot of the transfer characteristics (IDS versus VG) for the OTFT on ITO/PET with 35 nm PVP–HDA, 30 nm DDFTTF and gold electrodes in ambient (black) and under water (grey). IDS1/2 versus VG is also plotted on the right axis. (B) Output characteristics (IDSVDS) for the device in (A) under water. (C) Log plot of transfer characteristics with PEDOT:PSS electrodes in ambient (black) and under water (grey) with the square root shown on the right axis. (D) Output characteristics for device in (C) under water. The structure of a top-contact OTFT with the structure of DDFTTF is shown above.

Fig. 5. (A) Optical micrograph of an OTFT under operation in water. (B) DIDS of an OTFT with PEDOT:PSS electrodes when exposed to solutions of TNB with concentrations as low as 100 ppb, (inset) resistive current of PEDOT electrodes in air, water, and 100 ppb TNB solution. (C) DIDS (IDS/IDSbaseline) when exposed to solutions of cysteine with various concentrations, (inset) relative saturation current change with concentration for OTFTs fabricated on silicon and ITO/ PET substrates. (D) DIDS with solution pH on ITO/PET substrates with PEDOT:PSS electrodes, (inset) DIDS as a function of change in [H+] concentration.

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tions described. Fig. 5A shows the droplet of water spanning the channel region. After establishing a baseline, 2 ll of 300 ppb TNB was added to the droplet, and an increase in IDS was observed. The same procedure was done using 3 parts per million (ppm) TNB, and the composite current characteristics are shown in Fig. 5B in the form of DIDS versus time. A higher degree of noise relative to the signal is observed due to the contact between the gold wire probes and the soft, conducting polymer electrodes. Previously, we observed a decrease in current when the OTFTs (silicon substrates and gold electrodes) were exposed to TNB; however, with the polymer electrodes we observed an increase in current. To explain this, we measured the resistive current of the electrodes in water and in the saturated (300 ppb) TNB solution. Indeed, the conductivity of the electrodes increases by an average of 31% in the TNB solution due to a charge transfer between the analyte and conducting polymer [36,37], indicating, in this case, that a change in contacts plays a major role. Therefore, when using electrode materials, such as conducting polymers, that can dope and de-dope under specific solution conditions, it is important to consider the influence of the analyte on the electrode conductivity. In fact, the use of polymer doping/de-doping can be used to fabricate organic electrochemical transistors [38]. We also investigated the influence of pH on the polymer electrode conductivity since the conductivity has been shown to vary with acidity [39] or alkalinity [40]. However, we observed a change in resistive current of less than 3% when exposing the polymer electrode to solutions of pH 3 (based on HCl) and pH 11 (based on NaOH). Therefore, the DIDS response was similar for OTFTs fabricated with gold and PEDOT:PSS electrodes (Fig. 5D). The inset shows a plot of DIDS versus change in hydronium ion, H+, concentration. It is important to note that the parasitic current between the S-D electrodes in negligible for the acidic or basic solutions under the bias conditions applied [9]. Fig. 5C shows the drain current response to solutions of cysteine with concentrations ranging from 0.1 ppm to 10 ppm. For comparison, the inset of Fig. 5C shows the concentration dependent OTFT drain current change (IDS at saturation minus IDS of the baseline, divided by IDS of the baseline) for plastic and rigid substrates. With the demonstration of chemical sensing using organic thin-film transistors fabricated on flexible substrates, we have taken a step toward the realization of low-cost, disposable sensors. In our fabrication process, we maintain the requirements of low input power and device stability in air and water environments. Our fabrication process relies on the simplicity of solution methods with the ability to deposit a polymer insulating layer and patterned polymer electrodes from solution without damaging the organic semiconductor or plastic substrate. However, consideration must be given to the choice of electrode material, which can potentially influence the OTFT response toward certain analytes. Acknowledgement M.E.R. acknowledges the NASA GSRP fellowship. Z.B. acknowledges the NSF-sponsored Center for Polymer Inter-

face Macromolecular Assemblies (CPIMA), NSF DMR Solid State Chemistry and NSF-EXP on sensors.

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