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Tuning the Work Function of Printed Polymer Electrodes by Introducing a Fluorinated Polymer to Enhance the Operational Stability in Bottom-Contact Organic Field-Effect Transistors Se Hyun Kim, Jiye Kim, Sooji Nam, Hwa Sung Lee, Seung Woo Lee, and Jaeyoung Jang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b16259 • Publication Date (Web): 20 Mar 2017 Downloaded from http://pubs.acs.org on March 21, 2017

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ACS Applied Materials & Interfaces

Tuning the Work Function of Printed Polymer Electrodes by Introducing a Fluorinated Polymer to Enhance the Operational Stability in BottomContact Organic Field-Effect Transistors Se Hyun Kim,a,† Jiye Kim,b,† Sooji Nam,c Hwa Sung Lee,*,d Seung Woo Lee,*,a and Jaeyoung Jang*,e

a

School of Chemical Engineering, Yeungnam University, Gyeongsan, North Gyeongsang 38541,

South Korea b

Department of Chemical Engineering, Pohang University of Science and Technology, Pohang,

37673, South Korea c

Smart I/O Control Device Research Section, Electronics and Telecommunications Research

Institute, Daejeon, 305-700, Republic of Korea d

Department of Chemical & Biological Engineering, Hanbat National University, Daejeon 305-

719, Republic of Korea e

Department of Energy Engineering, Hanyang University, Seoul, 133-791, Republic of Korea

KEYWORDS: Organic field-effect transistors (OFETs); work function tuning; poly(3,4ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS); Nafion; electrohydrodynamic printing; bottom-contact electrodes

ACS Paragon Plus Environment

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ABSTRACT

Poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) is a promising electrode material for organic electronic devices due to its high conductivity, good mechanical flexibility, and feasibility of easy patterning with various printing methods. The work function of PEDOT:PSS needs to be increased for efficient hole injection and the addition of a fluorinecontaining material has been reported to increase the work function of PEDOT:PSS. However, it remains a challenge to print PEDOT:PSS electrodes while simultaneously tuning their work functions. Here, we report work function tunable PEDOT:PSS/Nafion source/drain electrodes formed by electrohydrodynamic printing technique with PEDOT:PSS/Nafion mixture solutions for highly stable bottom-contact organic field-effect transistors (OFETs). The surface properties and work function of the printed electrode can be controlled by varying the Nafion ratio, due to the vertical phase separation of the PEDOT:PSS/Nafion. The PEDOT:PSS/Nafion electrodes exhibit a low hole injection barrier, which leads to efficient charge carrier injection from the electrode to the semiconductor. As a result, pentacene-based OFETs with PEDOT:PSS/Nafion electrodes show increased charge carrier mobilities of 0.39 cm2/V·s compared to those of devices with neat PEDOT:PSS electrodes (0.021 cm2/V·s). Moreover, the gate-bias stress stability of the OFETs is remarkably improved by employing PEDOT:PSS/Nafion electrodes, as demonstrated by a reduction of the threshold voltage shift from -1.84 V to -0.28 V.


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1. INTRODUCTION Organic field-effect transistors (OFETs) have been intensively investigated for applications in flexible displays, radio-frequency identification tags, sensors, and new military and healthcare devices.1-3 The great interest in OFETs is due to their organic components, which can be fabricated in a simple, large-scale, and low-cost manner. Additionally, their particular physical and chemical properties, including their flexibility and biocompatibility, are desirable.4-6 Considerable efforts in the development of OFET technologies have brought about new organic electronic materials (semiconductors, electrodes, and dielectrics) with high OFET performance, surpassing that of amorphous Si.7-8 However, there is still room to commercialize OFETs in current electronic display, memory, and sensor markets. Among the critical issues facing this commercialization, solution-processable and/or printable organic conductive materials must be developed to replace rare and expensive metals such as gold and indium.9-13 Poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) is a promising conductive material for organic electronic devices because it enables cost-effective and flexible devices as well as printing-based production.14-15 A water-dispersed PEDOT:PSS showed high transparency (84–85% transmittance) and excellent thermal stability,16-17 but its intrinsic conductivity is very low (~1 S/cm) relative to metals and indium tin oxide (ITO) that currently see commercial use. Several types of treatments have been introduced to improve the conductivity of PEDOT:PSS. The usage of various additives (polar solvents,18-20 surfactants,21-22 and acids23-24) can allow PEDOT:PSS to assemble conductive PEDOT chains to create conducting pathways.24,25 Considering the applicability of PEDOT:PSS as a source/drain (S/D) electrode in OFETs, its work function is a critical factor in determining the charge carrier transport ability of the overall OFET; this is the case because the charge carrier injection from


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the source electrode to the semiconductor can be modulated by the energetic difference between the Fermi level of the electrode and the valence or conduction bands of the semiconductor. For example, OFETs with p-type semiconductors require electrodes with work functions that are higher than the energy of the valence band of the semiconductor in order to provide efficient hole injection.25 One method for increasing the work function of PEDOT:PSS is through the addition of a fluorine-based material.21, 26 Fluorine has the highest electronegativity among all elements, and moieties containing fluorine generally have high permanent dipoles that introduce a built-in potential into the molecular assembly; this potential can be used to modulate the work function.27 Recently, several groups have reported that fluorine-containing small molecules and polymers can modulate the work function of PEDOT:PSS at the interface between PEDOT:PSS electrodes and p-type semiconductors.28-30 In addition, fluorine has a small atomic radius, comparable to that of the hydrogen atom, and the lowest polarizability among all elements. Based on this, fluorine-containing materials exhibit good hydrophobicity and chemical inertness, which can improve the electrical and chemical stability of OFETs.26 Another advantage of PEDOT:PSS as a S/D electrode of OFETs is the feasibility of facile patterning with direct-printing processes. Photolithography is generally used to make patterns for OFET components, but this method is not appropriate for patterning PEDOT:PSS because several chemicals used in this process (including the developer) can damage PEDOT:PSS. In addition, printed PEDOT:PSS S/D electrodes can provide a favorable surface for crystal growth of the overlying organic semiconductors in bottom-contact OFETs,31 whereas metal electrodes cause significant contact resistance due to the formation of an unfavorable interface dipole layer and/or to the poor crystallinity of the organic semiconductor.13 Moreover, printing processes have several technical advantages compared to photolithography, including their simplicity, low


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resource requirements, and cost-effectiveness.32-34 Nevertheless, more work is required to obtain high pattern fidelity and resolution for the mass production of such systems.35 In this study, we investigate fluorine-containing PEDOT:PSS bottom-contact S/D electrodes fabricated by an electrohydrodynamic (EHD) printing process. The fluorine-containing PEDOT:PSS solution is made by adding Nafion to the PEDOT:PSS master solution at several mixing ratios. The EHD printing used in our experiment offers an excellent means for direct patterning of organic conductors because the electric field enables uniform jetting without disruption, and the patterns can be smaller than the nozzle diameter (on the sub-micrometer level).36-38 We discuss the physical and chemical properties of the EHD-printed PEDOT:PSS/Nafion electrodes as a function of Nafion content, including surface hydrophobicity, work function, and hole injection barrier height at the interface between the PEDOT:PSS/Nafion electrode and the overlying pentacene layer. Furthermore, we investigate how the PEDOT:PSS/Nafion electrode influences OFET performance and electrical stability under sustained gate-bias stressing compared with a neat PEDOT:PSS electrode.

2. EXPERMINTAL SECTION Conductive ink preparation: PEDOT:PSS, dimethyl sulfoxide (DMSO), and Triton X-100 surfactant were purchased from Sigma-Aldrich. A PEDOT:PSS solution was prepared with DMSO at a volume ratio of 5:1. The surfactant was then mixed with PEDOT:PSS at 0.1 wt% to increase the wetting properties of the PEDOT:PSS solution. Nafion (~5% in a mixture of lower aliphatic alcohols and water) was purchased from Sigma-Aldrich. To fabricate the


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PEDOT:PSS/Nafion solutions with various Nafion ratios, 0.5, 1, 3, and 5 wt% Nafion were dispersed in PEDOT:PSS solutions and then stirred for 1 h. EHD printing process for patterning electrodes: For EHD printing, a PEDOT:PSS/Nafion solution was loaded into a glass syringe and ejected from a metallic nozzle at a flow rate of 0.15 µL/min (Enjet, Korea). A power supply installed on the equipment generates an electric field between the nozzle and the aluminum substrate ground. An x- and y-axis stage was used to control the printing speed and working distance, allowing variation of the line widths. The whole process was interfaced with a computer and monitored using a CCD camera. The PEDOT:PSS/Nafion patterns were printed in a stable cone-jet mode at an applied voltage of 1.05–1.1 kV and a working distance of 250–300 µm. Uniform jet streams are ejected out of the conical meniscus, which is elongated by the strong field effect, enabling printing of the desired patterns. Device fabrication: Highly n-doped Si wafers were used as substrates. These were cleaned in a hot piranha solution and then sonicated in distilled water multiple times. A 50-nm-thick Al2O3 dielectric layer was deposited on the substrate by atomic layer deposition (ALD). The Al2O3deposited substrates were cleaned by organic solvent treatment (acetone) and exposed to a UV/ozone cleaner for 20 min. For surface modification, a 5 wt% divinyltetramethyldisiloxane bis-benzoclobutane (BCB) (Dow Chemical) solution in mesitylene (Dow Chemical) was spincoated onto the Al2O3/Si substrate and then thermally cured at 250 °C for 1 h. The PEDOT:PSS/Nafion solutions were printed onto the surface-treated substrate and then annealed at 150 °C for 30 min. The fabrication of OFET devices was completed by depositing a 50-nmthick pentacene film onto the substrate using an organic molecular beam deposition system (deposition rate = 0.2 Å/s; vacuum pressure = 10–7 Torr; substrate temperature = 25 °C).


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Characterization: The contact angles of the samples were measured using a contact angle meter system (OCA20, Dataphysics Instruments GmbH, Germany). The menisci formed under various jetting conditions were monitored using a CCD camera. All electrical measurements were performed using a Keithley 4200 SCS in an N2-rich glove box. Sheet resistances of the PEDOT:PSS/Nafion composite films were characterized by a CMT-SR2000N instrument, and the morphologies of the PEDOT:PSS/Nafion and pentacene films were investigated using atomic force microscopy (AFM), optical microscopy (OM), and X-ray photoelectron spectroscopy (XPS). 2D grazing incidence X-ray diffraction (2D-GIXD) measurements were performed at the 3C and 9A beamlines of the Pohang Accelerator Laboratory (PAL) in Korea. Ultraviolet photoemission spectroscopy (UPS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy were used to characterize the PEDOT:PSS/Nafion films in the normal emission mode at the 4D beamline of the Pohang Accelerator Laboratory, Korea.

3. RESULTS AND DISCUSSION Figure 1a shows the chemical structures of the materials used as the S/D electrodes in this work. Nafion, in a mixture of water and alcohol, was dissolved in a water-based PEDOT:PSS solution at various ratios (0, 1, 3, and 5 wt%). With the PEDOT:PSS/Nafion solutions, highly stable lines were successfully printed via EHD printing in the stable cone-jet mode. The sheet resistance of the PEDOT:PSS/Nafion films increased gradually from 204 Ω/sq (PEDOT:PSS) to 272 Ω/sq (Nafion 5.0 wt%) as the Nafion ratio increased (Figure 1b). The increased sheet resistance of the PEDOT:PSS/Nafion film may be attributed to the insulating properties of Nafion. The water contact angles (θwater) for the PEDOT:PSS and PEDOT:PSS/Nafion films are shown in Figure 1b. The PEDOT:PSS film exhibited a hydrophilic surface (θwater = 14.7º),


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whereas a slight addition of Nafion (1 wt%) made the film fairly hydrophobic (θwater > 100º); the angles were almost saturated and did not change much, even with an increased amount of Nafion up to 5 wt%. The images of θwater for all samples are shown in Figure S1 in the Supporting Information (SI). The results suggest that a substantial amount of hydrophobic Nafion is distributed on the surface, which may be due to the vertical phase separation of the film containing both hydrophilic and hydrophobic components.28,

39

To clearly determine the

chemical composition of the surface of the PEDOT:PSS/Nafion film, XPS and UPS were utilized. As shown by the XPS spectra in Figure 1c, an intense F 1s core level peak (near a binding energy of 689 eV) corresponding to Nafion was clearly detected. This peak is dependent on the Nafion content of the PEDOT:PSS/Nafion; its intensity increases as the Nafion content of the films increases. Here, we also prepared samples containing 0.5 wt% Nafion in order to detect the gradual increase of the peak intensity. As expected, the peak was not detected in the neat PEDOT:PSS film. The surface potentials of the PEDOT:PSS/Nafion films were characterized by the secondary electron emission spectra of UPS, as shown in Figure 1d (left). The kinetic energy for the onset of the secondary electrons, which is defined as the energy difference between the vacuum level of the sample and the detector,40 can be used as an indicator to determine the degree of the surface potential; namely, a higher kinetic energy represents a higher surface potential. In our system, the kinetic energy increased with increasing Nafion content ratios from 5.9 (0 wt%, pristine PEDOT:PSS) to 6.4 (5 wt%). This is likely due to the strong electronwithdrawing character of the surface C-F bonds. The work functions (Φ) of the PEDOT:PSS/Nafion electrodes with various Nafion contents were determined based on the UPS energy distribution curves (Figure 1d) and calculated using the energy difference between the Fermi level and vacuum level. The Φ of the PEDOT:PSS/Nafion electrode strongly depended on


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the Nafion content because Φ was significantly affected by the electrode surface properties as well as the bulk properties. The Φ increased from 4.99 eV to 5.79 eV as the Nafion content increased. This result might be associated with the strong electron-withdrawing properties of the Nafion present at the PEDOT:PSS/Nafion electrode surface and the strong interface and dipole arrangement at the surface, as observed in previous reports.41-42


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Figure 1. (a) Chemical structures of electrode materials. (b) Sheet resistance and water contact angle of the PEDOT:PSS/Nafion films as a function of Nafion ratio. (c) XPS spectra of the F 1s core level for PEDOT:PSS with various Nafion ratios. (d) UPS energy distribution curves for the PEDOT:PSS/Nafion film with various Nafion ratios. Left side: secondary electron emission. Right side: the region near the Fermi edge.


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In order to characterize the chemical composition of the whole PEDOT:PSS/Nafion film, NEXAFS spectroscopy profiles of the PEDOT:PSS and PEDOT:PSS/Nafion films were obtained using a fixed photon incidence angle of 55º from the surface normal (≈ the “magic angle” of 54.7º) (Figure 2). NEXAFS spectroscopy is an element-specific and highly surface-sensitive technique, which detects electrons emerging from the outermost surface region (≈ 6 nm) in the partial-electron-yield (PEY) detecting mode as well as electrons from bulk films in totalelectron-yield (TEY) detecting mode. Therefore, the difference between the TEY and PEY spectra can provide information about compositional changes in the films. As shown in Figure 2a, the TEY and PEY spectra of the PEDOT:PSS film are almost identical, indicating that the chemical composition of the PEDOT:PSS film is nearly homogeneous throughout the whole film. By contrast, Figures 2b–d of the PEDOT:PSS/Nafion showed very different TEY and PEY spectra, with increased intensity of the σ* (C-F) resonance peak in the PEY spectra compared to the TEY spectra.43-44 This difference became more pronounced as the Nafion content increased. These results imply that Nafion is mainly located at the surface of the film, indicating the vertical phase-separated film structure of Nafion and PEDOT:PSS, as proposed in Figure 2e.


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Figure 2. C K-edge NEXAFS spectra acquired from the PEDOT:PSS/Nafion layers in the TEY and PEY modes: (a) PEDOT:PSS, (b) Nafion 1 wt%, (c) Nafion 3 wt%, (d) Nafion 5 wt%, and (e) schematic diagram showing vertical phase separation in the PEDOT:PSS/Nafion layer.

Figure 3 shows OM and AFM images of the PEDOT:PSS/Nafion films fabricated with various Nafion contents. The PEDOT:PSS/Nafion S/D electrodes were clearly patterned by EHD printing with a channel length of 100 µm, regardless of the Nafion content (Figures 3a–d). AFM images of the PEDOT:PSS/Nafion films present similar surface properties (Figures 3f–h), whereas the neat PEDOT:PSS films showed rougher surface morphologies than their Nafioncontaining counterparts (Figure 3e). As shown in Figures 3e–h, the surface roughness (Rq) of the PEDOT:PSS/Nafion films decreased from 8.01 nm to 5.83 nm as the Nafion content increased. We believe that the phase-segregated structure of PEDOT:PSS, with PEDOT:PSS grains consisting of a PEDOT-rich core and PSS-rich shell, induced this surface roughness. However,


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since Nafion covered the surface of the PEDOT:PSS/Nafion films, due to the vertical phase separation, it could reduce the surface roughness.

Figure 3. (a–d) OM images and (e–h) AFM images of the PEDOT:PSS/Nafion electrode: (a,e) PEDOT:PSS, (b,f) Nafion 1 wt%, (c,g) Nafion 3 wt%, and (d,h) Nafion 5 wt%.

The contact resistance of bottom-contact OFETs is significantly affected by the crystalline morphology of the semiconductor layer on the electrodes and in the channel region.13,

31, 45

Pentacene grown on the EHD-printed PEDOT:PSS or PEDOT:PSS/Nafion electrode region is shown in the AFM images (Figure 4a–d). The pentacene on the PEDOT:PSS/Nafion formed granular crystals with grain sizes of 100–300 nm, similar to those on the neat PEDOT:PSS. The crystalline morphology of pentacene on the PEDOT:PSS/Nafion did not change significantly with increasing Nafion content. These results might be explained by the Nafion-rich surface, regardless of the Nafion content, due to the vertically phase-separated PEDOT:PSS/Nafion layer.


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The crystallinities of the pentacene layer on the PEDOT:PSS/Nafion films were investigated by measuring the 2D-GIXD patterns of the pentacene layers (Figures 4e–h). The 2D-GIXD patterns of the pentacene layers on PEDOT:PSS/Nafion and PEDOT:PSS exhibited intense (00l) X-ray reflections along the qz-axis (out-of-plane) with a d-spacing of 15.4 Å, as well as {1,±1}, {0,2}, and {1, ±1} reflections along the qxy-axis (in-plane). These reflections correspond to a herringbone molecular packing structure and indicate a ‘thin-film phase’ crystalline structure in the pentacene layer. In addition, the 2D-GIXD patterns of the pentacene layer on PEDOT:PSS/Nafion and PEDOT:PSS revealed an intense (00l)* X-ray reflection along the qzaxis and the 8º-tilted peaks with respect to the qz-axis. These results indicate the coexistence of a ‘thin-film phase’ and ‘bulk phase’ crystals. The ‘bulk phase’ crystals show poor molecular overlap and a less-ordered crystalline structure, resulting in the degradation of the device performance.


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Figure 4. (a–d) AFM images and (e–h) 2D GIXD patterns of 50-nm-thick pentacene films on PEDOT:PSS/Nafion layers: (a,e) PEDOT:PSS, (b,f) Nafion 1 wt%, (c,g) Nafion 3 wt%, and (d,h) Nafion 5 wt%.

The morphological and crystallographic studies that we performed on the pentacene layers did not focus on the molecular orientation near the interface between the electrodes and the overlying pentacene layers. In order to clarify the in-plane π-conjugated overlap of the pentacene layer near the PEDOT:PSS/Nafion and PEDOT:PSS surfaces, where charge carrier injection mainly occurs, NEXAFS spectroscopy was performed on in-situ-deposited pentacene monolayers on PEDOT:PSS/Nafion and PEDOT:PSS in PEY mode. NEXAFS spectroscopy offers information about the average orientation of the π-conjugated planes in pentacene by collecting the carbon K-edge spectra obtained from various incidence angles of the synchrotron photon beam from the surface plane. Furthermore, the tilt angle of the conjugated plane can be calculated by determining the intensity of the corresponding resonance as a function of the incidence angle.46 Figure 5 shows NEXAFS spectra of the pentacene layers on the PEDOT:PSS/Nafion and PEDOT:PSS layers. The peaks at 284.3 and 285.8 eV originated from π* (C=C) orbitals, while the σ* (C-C) orbitals expressed peaks at 292–297 eV. The tilt angle (α) between the C=C bond in the conjugated planes and the PEDOT:PSS/Nafion surface was determined by the following equation: ௉

ଵ

‫ܫ‬௩ ∝ ቂ ଷ ቄ1 + ଶ (3 cos ଶ ߠ − 1)(3 cosଶ ߙ − 1)ቅ +

(ଵି௉) ଶ

sinଶ ߙቃ

(1)

Here, θ is the polarization angle of the incident synchrotron light with respect to the direction normal to the surface, and P = 1 is used for the degree of polarization.46 The dichroic ratio (R) is


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calculated as the maximum resonance intensity of the carbon 1s → π* transition divided by the minimum resonance intensity. The intensity of the peaks from the π* (C=C) orbitals at 284.3 and 285.8 eV decreased as the incident angle increased, as shown in Figure 5. These results indicate that the pentacene molecules adopted an edge-on orientation on the PEDOT:PSS/Nafion (PEDOT:PSS) layer. As shown in Figure 5e, the α values for the pentacene layer on the PEDOT:PSS and PEDOT:PSS/Nafion layers with various Nafion ratios were 63.5º (R=0.34) for PEDOT:PSS, 63.9º (R=0.35) for 1 % Nafion, 64.7º (R=0.38) for 3 wt% Nafion, and 64.6º (R=0.38) for 5 wt% Nafion. The similar α values revealed that the Nafion surface did not significantly contribute to the crystalline structure of the pentacene layer near the PEDOT:PSS/Nafion surface. Therefore, charge injection from the PEDOT:PSS/Nafion electrode to the pentacene layer may be dominantly affected by the surface potential of the PEDOT:PSS/Nafion electrodes and not by the crystalline structure of the pentacene layer.


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(a) 3.5

30 Å 45 Å 55 Å 70 Å

Intensity (a.u.)

3.0 2.5 2.0 1.5 1.0

(b) 3.5

30 Å 45 Å 55 Å 70 Å

3.0

0.5

2.5 2.0 1.5 1.0 0.5

0.0

0.0 280

290

300

310

320

280

3.5

30 Å 45 Å 55 Å 70 Å

3.0

Intensity (a.u.)

300

310

2.5 2.0 1.5 1.0

(d) 3.5

320

30 Å 45 Å 55 Å 70 Å

3.0

Intensity (a.u.)

(c)

290

Photon Energy (eV)

Photon Energy (eV)

0.5

2.5 2.0 1.5 1.0 0.5

0.0

0.0 280

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Photon Energy (eV)

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Photon Energy (eV)

(e) 1.2 1.1

Intensity (a.u.)

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Intensity (a.u.)

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1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.0

PEDOT:PSS α=63.5° Nafion 1.0 wt% α=63.9° Nafion 3.0 wt% α=64.7° Nafion 5.0 wt% α=64.6°

0.2

0.4

0.6

0.8

1.0

2

cos θ

Figure 5. Angle-dependent C K-edge NEXAFS spectra acquired from the top interface of 1.5nm-thick pentacene on PEDOT:PSS/Nafion films in the PEY mode: (a) PEDOT:PSS, (b) Nafion 1 wt%, (c) Nafion 3 wt%, (d) Nafion 5 wt%, and (e) intensities of π* transitions versus incidence angle. The solid lines represent the fitted curves.


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Figure 6a shows the typical drain current-gate voltage (ID-VG) transfer characteristics of the pentacene-based bottom-contact OFETs with the PEDOT:PSS/Nafion and PEDOT:PSS electrodes. The charge carrier mobility (µ) of the OFETs with PEDOT:PSS/Nafion electrodes increased from 0.021 cm2/V·s for PEDOT:PSS to 0.39 cm2/V·s for Nafion 3 wt% (Table 1). The typical drain current-drain voltage (ID-VD) output characteristics reveal good linear and saturation behavior as well as Ohmic contact characteristics in the PEDOT:PSS/Nafion electrodes (Figure S2 in SI). As the Nafion content increased up to 3 wt%, the electrical performance of OFETs with PEDOT:PSS/Nafion electrodes was gradually enhanced. However, the device with 5 wt% Nafion exhibited slightly lower performance than its 3 wt% counterpart, which may be due to the larger amounts of insulating Nafion at the electrode surface. The electrical stabilities of OFETs with PEDOT:PSS/Nafion electrodes under a gate-bias stress also improved with increasing Nafion content from 0 to 3 wt% (Figures 6b and c). Figure 6c shows the gate-bias stress stability behavior of the OFETs with the PEDOT:PSS/Nafion electrodes. A gate bias of −10 V (at VD = 0 V) was applied to the devices for 3 h in a nitrogen-purged glove box (H2O < 0.1 ppm and O2 < 0.1 ppm) in the dark. After gate-bias stressing for 1 h, the threshold voltage shift (∆Vth) decreased remarkably from -1.84 V (for PEDOT:PSS) to -0.28 V (for Nafion 3 wt%) as the Nafion ratio increased (Figure 6b). These results indicate that the incorporation of Nafion (up to 3 wt%) into PEDOT:PSS S/D electrodes improved both the µ and the gate-bias stability. Since all of the devices tested here adopted the same semiconductor/dielectric interface (pentacene/Al2O3),

the

resulting

device

performances

can

be

attributed

to

the

electrode/semiconductor interface properties.


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Figure 6. (a) Transfer characteristics of OFETs with the PEDOT:PSS/Nafion electrodes. (b) Threshold voltage shift (∆Vth) as a function of stress time (VG = −10 V, VD = 0 V). (c) Transfer characteristics of OFETs with the PEDOT:PSS/Nafion electrodes with various Nafion ratios before, during, and after an applied gate-bias stress of VG = −10 V (VD = 0 V).


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Table 1. Electrical performance of bottom-contact OFETs with PEDOT:PSS/Nafion electrodes.

μ (cm2/Vs)

Ion/Ioff

Vth (V)

SS (V/decade)

PEDOT:PSS

0.021 (±0.015)

1.07×105

-1.97 (±0.22)

-0.11 (±0.02)

Nafion 1 wt%

0.053 (±0.021)

1.89×105

-1.85 (±0.23)

-0.09 (±0.03)

Nafion 3 wt%

0.39 (±0.050)

5.63×105

-1.83 (±0.20)

-0.10 (±0.02)

Nafion 5 wt%

0.23 (±0.13)

2.98×105

-1.00 (±0.25)

-0.09 (±0.02)

In general, the µ of bottom-contact OFETs is significantly affected by the energetic difference between the Fermi level of an electrode and the highest occupied molecular orbital (HOMO) energy level of a semiconductor (referred to as the “hole injection barrier”) because the charge carriers at the Fermi level of the source electrode move to the HOMO level of the semiconductor under applied source-gate and source-drain electric fields.11, 31, 40, 45 Given that all of the devices possess the same energy values for the HOMO level of each semiconductor, the Φ of the electrodes determines the hole injection barrier during device operation. However, the hole injection barrier is influenced by a variety of interfacial properties, including electronic structure features (e.g., band bending and energy level alignment) as well as the crystalline structure of the semiconductor at the electrode/semiconductor interface.11, 13, 31, 40, 45 This implies that the hole injection barrier cannot be completely explained by a simple calculation with the values of Φ and the HOMO level. A more exact hole injection barrier can be experimentally characterized using UPS on in-situ-deposited semiconductor layers (of various thicknesses) on the electrode surface under vacuum (< 10–7 Torr; to prevent contamination from H2O and O2). Figures 7a–d show the UPS spectra for the interfaces between the pentacene and PEDOT:PSS/Nafion electrodes with


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various Nafion contents (0–3 wt%). Here, pentacene layers (0 to 15 nm thick) were deposited onto the electrodes in situ. The resulting band diagrams of the interface are described in Figures 7e–h. The hole injection barrier for the PEDOT:PSS electrode was found to be 0.33 eV (Figure 7e). However, the Φ of PEDOT:PSS/Nafion electrodes gradually increased as the Nafion ratio increased from 1 wt% to 5 wt%, thereby lowering the hole injection barrier (Figures 7f–h). As a result, the energy of the pentacene HOMO level at the interface became even higher than the Fermi level of the PEDOT:PSS/Nafion electrode (Figures 7f-h). Therefore, holes in the electrode can be efficiently injected into the pentacene layer if electrodes with higher Nafion contents are utilized. This might be one reason why the µ of OFETs improved as the Nafion content increased up to 3 wt%. When the Nafion content increased to 5 wt%, we observed a slight reduction in µ, which is presumably due to the presence of excess insulating Nafion on top of the electrodes.

0 nm 1.5 nm 6.0 nm 20.0 nm

(a) Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(b)

(c)

0.49 eV

0.24 eV

0.36 eV

0.18 eV

0.33 eV

83

84

85

86

Kinetic Energy (eV)

(e) Φ=4.99eV 0.33 eV 0.49 eV

84

85

86

87

Kinetic Energy (eV)

(-)0.07 eV

0.24 eV

(-)0.03 eV

87

(d)

(-)0.01 eV

(-)0.14 eV

(-)0.13 eV

(-)0.23 eV

84

85

86

87

Kinetic Energy (eV)

84

85

87

Kinetic Energy (eV)

(f)

(g)

(h)

Φ=5.41 eV

Φ=5.71 eV

Φ=5.79 eV

0.03 eV 0.24 eV

86

0.13 eV 0.06 eV

0.23eV

0.07 eV

PEDOT:PSS Pentacene

Nafion 1.0 wt%

Pentacene

Nafion 3.0 wt%

Pentacene

Nafion 5.0 wt%

Pentacene


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Figure 7. (a–d) Magnified view of the region of the HOMO peak on the comparative valence band structures of the pentacene layer deposited on PEDOT:PSS/Nafion films and (e–h) band diagram of the interface between the pentacene and PEDOT:PSS/Nafion electrodes: (a,e) PEDOT:PSS, (b,f) Nafion 1 wt%, (c,g) Nafion 3 wt%, and (d,h) Nafion 5 wt%.

It is interesting to note that the gate-bias stability of the OFETs tested here is significantly improved by using PEDOT:PSS/Nafion electrodes with higher Nafion contents. In fact, the ∆Vth under gate-bias stressing is generally caused by charge trapping at the semiconductor, dielectric, or the interface between the two.47-48 However, there have been few investigations into the gatebias stability of OFETs caused by charge trapping at the electrode/semiconductor interface. We expect that the mechanism for charge trapping at the interface between the PEDOT:PSS/Nafion electrodes and pentacene layer is also highly associated with the hole injection barrier. As discussed previously, the presence of a hole injection barrier can disturb hole injection from the electrode to the semiconductor; therefore, holes that cannot overcome the hole injection barrier might be trapped at the electrode/semiconductor interface. During the application of sustained gate-bias stressing, trapped holes are accumulated at the interface and act as additional obstacles for hole injection. Consequently, a higher VG is required to inject holes from the electrode to form conducting channels by compensating for immobile trapped holes during the application of a gate-bias stress; this results in a negative Vth shift. Another possible scenario involves the hygroscopic and acidic character of the PSS components of PEDOT:PSS, which causes charge trapping in the electrode. Leo et al. have recently reported that using PEDOT:PSS electrodes as the anodes of organic photovoltaics can degrade the device performance when the cells are exposed to air.18 As mentioned previously, fluorinated materials have good hydrophobic and chemical inertness. Hence, the introduction of a Nafion layer on the PEDOT:PSS surface can


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passivate the underlying PSS components, thereby preventing the charge trapping that can deteriorate the gate-bias stability of the overall OFET.

4. CONCLUSIONS In conclusion, we printed polymer electrodes and tuned their work functions simultaneously by introducing PEDOT:PSS/Nafion S/D electrodes via EHD printing to fabricate highly stable bottom-contact OFETs. PEDOT:PSS/Nafion solutions were made by adding small amounts of Nafion to a water-based PEDOT:PSS solutions at various ratios from 1 wt% to 5 wt%. We found that introducing Nafion induced vertical phase separation in the PEDOT:PSS/Nafion electrodes, allowing the surface properties, including the surface potential, hydrophobicity, and roughness, to be effectively controlled. As a result, the PEDOT:PSS/Nafion electrodes showed higher work functions compared to the neat PEDOT:PSS electrode. They also had reduced charge injection barriers at the interface between the electrodes and the overlying pentacene layers. The injection barrier gradually decreased as the Nafion ratio increased, leading to significantly enhanced device performance at an optimal Nafion ratio of 3 wt%. Furthermore, the use of PEDOT:PSS/Nafion electrodes significantly improved the operational stability of OFETs under sustained electrical stressing, as compared to devices with neat PEDOT:PSS electrodes, which was likely caused by the reduced injection barrier.


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ASSOCIATED CONTENT Supporting Information. Images of water contact angles θwater for the neat PEDOT:PSS (i.e., 0% Nafion) film and PEDOT:PSS/Nafion films with different Nafion ratios and the drain current-drain voltage (IDVD) output characteristics of the pentacene-based bottom-contact OFETs with the neat PEDOT:PSS electrodes and PEDOT:PSS/Nafion electrodes. This material is available free of charge via the Internet at http://pubs.acs.org.

Notes The authors declare no competing financial interests.

Author Information Corresponding Author *E-mail: [email protected], Fax: +82-2-2291-5982, Tel: +82-2-2220-2334 (J. Jang) *E-mail: [email protected] (S. W. Lee) *E-mail: [email protected] (H. S. Lee) Author Contributions †

Se Hyun Kim and Jiye Kim contributed equally to this work as first authors.

ACKNOWLEDGMENT This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (NRF-2014R1A1A1005896). This work was also supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2015R1D1A1A02062369).


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