(Poly(3,4-ethylenedioxythiophene):Polystyrene ... - ACS Publications

Dec 21, 2015 - School of Materials Science and Engineering, Yeungnam University, Gyeongsan, 712-749, South Korea. •S Supporting Information...
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(Poly(3,4-ethylenedioxythiophene):Polystyrene Sulfonate):Polytetrafluoroethylene for Use in High-Performance and Stable Bottom-Contact Organic Field-Effect Transistors Jiye Kim,† Hyekyoung Kim,*,§ Se Hyun Kim,*,‡ and Chan Eon Park*,† †

Polymer Research Institute, Department of Chemical Engineering, Pohang University of Science and Technology, Pohang, 790-784, South Korea ‡ School of Chemical Engineering, Yeungnam University, Gyeongsan, 712-749, South Korea § School of Materials Science and Engineering, Yeungnam University, Gyeongsan, 712-749, South Korea S Supporting Information *

ABSTRACT: We successfully fabricated highly stable pentacene-based bottom-contact organic field-effect transistors (OFETs) with good charge injection properties at the electrode/organic semiconductor interface, obtained by optimizing the composition of solution-processed (poly(3,4ethylenedioxythiophene):polystyrene sulfonate):polytetrafluoroethylene ((PEDOT:PSS):PTFE)-treated Au source/drain (S/D) electrodes. The (PEDOT:PSS):PTFE layer was deposited on the Au layer by spin-coating a mixture solution. The work function of the electrode increased from 4.84 to 5.21 eV as the PTFE concentration increased, accompanied by an interface dipole at the electrode surface. The optimized (PEDOT:PSS):PTFE (0.95:0.05)-treated electrodes significantly reduced the charge injection barrier at the electrode/semiconductor interface to achieve efficient charge transfer in the OFETs. Bottom-contact OFETs prepared with the optimized (PEDOT:PSS):PTFEtreated S/D electrodes had a field-effect mobility of 0.16 cm2/(V·s), which exceeded that of PEDOT:PSS-treated S/D electrodes (0.073 cm2/(V·s)). The operational stability of the optimized device was remarkable under gate-bias stress (VG = −40 V over 3 h).

1. INTRODUCTION Organic electronic devices such as field-effect transistors, lightemitting diodes, photovoltaics, and sensors have potential utility in commercial electronics goods (e.g., displays, lighting, and radio frequency identification (RFID) tags), as well as in military and healthcare applications.1−3 The significant progress in this field may be attributed to the development of material synthesis approaches, understanding of device physics, and novel processing methods.4−6 Organic field-effect transistors (OFETs) are expected to be useful in emerging next-generation flexible and wearable electronics.2,3,7−9 However, several bottlenecks still remain to the commercialization of OFETs. Although the charge carrier mobilities of organic semiconductors can reach values in the range of 10−20 cm2/(V s) (in the case of solution-processed polymer semiconductors),10−12 the device stabilities in the air or under electrical stress, as well as the contact resistance between the source/ drain electrodes and the channel, require further improvements. Research groups recently addressed these issues and proposed several effective solutions.13−17 Among these solutions, the introduction of a fluorine-containing moiety into the components of OFETs (especially the gate dielectrics and the source/drain electrodes) improves device stability during OFET operation and increases charge injection at the source/channel contact.14,18,19 Fluorine has a small atomic © 2015 American Chemical Society

radius that is comparable to that of the hydrogen atom, as well as the lowest polarizability among all elements. Because of this low polarizability, molecular assemblies that contain fluorine have good hydrophobicity and chemical inertness.20 Therefore, fluorine-modified dielectrics can reduce the number of interactions with trap-causing species such as H2O, thereby preventing charge-trap formation and increasing the stability of device operation under ambient air or electrical stress. The high electronegativity and permanent dipole of fluorine introduce a built-in potential into the molecular assembly; this potential can modulate the work function (Φ) of the electrode surface.18,19 For example, fluorinated self-assembled monolayers at the semiconductor/electrode interface can improve the hole injection from the Fermi level of the electrode to the highest occupied molecular orbital (HOMO) level of a p-type semiconductor.18,19,21−24 In addition, the permanent dipole imposes an energetic barrier to charge transport at the semiconductor/dielectric interface. A fluorinated polymer at the semiconductor/dielectric interface can increase the HOMO energy of the dielectric, thereby increasing the energetic barrier Received: August 19, 2015 Revised: December 14, 2015 Published: December 21, 2015 956

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calculated in the saturation regime (drain voltage (VD) = −40 V) using the equation: ID = μFETCiW(2L)−1(VG − Vth)2, where ID is drain current, Ci is capacitance, and VG is gate voltage. The surface morphologies of the (PEDOT:PSS):PTFE films and pentacene layers were measured using an atomic force microscope (AFM, Dimension 3100, VEECO). 2D-GIXD measurements were performed at the 3C and 9A beamlines of the Pohang Accelerator Laboratory (PAL), Korea. Ultraviolet photoemission spectroscopy (UPS) was used to characterize the electronic structure features of an electrode/ semiconductor interface in normal emission mode at the 4D beamline at PAL, Korea.

to charge transfer from semiconductor to dielectric (charge trapping).14 The present study demonstrates that use of a solutionprocessed electrode consisting of poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) and polytetrafluoroethylene (PTFE) can simultaneously improve the charge injection behavior and device stability of an OFET. We investigated the crystalline morphologies and electronic structures of a pentacene film grown on the electrodes treated with pure PEDOT:PSS or PTFE-containing PEDOT:PSS ((PEDOT:PSS):PTFE) by using 2D grazing-incidence X-ray diffraction (2D-GIXD) and in situ ultraviolet photoemission spectroscopy (UPS), which monitored the hole injection barrier by depositing pentacene onto the electrodes. We also determined how fluorine moieties in the electrode affect not only the hole injection but also the stability of the bottomcontact OFET devices. Consequently, OFETs employing the optimized (PEDOT:PSS):PTFE-treated S/D electrodes showed a higher field-effect mobility (μFET) and a greater device stability under gate-bias stress conditions compared to the device prepared with pure PEDOT:PSS-treated Au.

3. RESULTS AND DISCUSSION Figure 1 shows the chemical structures of the materials used in this work. Various amounts of PTFE were dissolved in a water-

2. EXPERIMENTAL SECTION 2.1. Materials and Sample Preparation. Highly doped ntype (100) Si wafers with 300 nm thick thermally grown SiO2 layers were used as gate substrates. CYTOP (CTL-809A, Asahi Glass) was used as a surface modification layer without further purification. Surface-modified oxide dielectrics were fabricated by first cleaning the substrates with acetone, then exposing the surfaces to UV-ozone plasma. CYTOP was dissolved (10 wt %) in its solvent (CT-solv. 180, Asahi Glass). The solution was spin-coated onto the substrates in ambient air, and the resulting films were annealed at 130 °C for 1 h to remove residual solvent. Subsequently, a 3 nm thick Ti film (adhesion layer) was formed on the CYTOP-treated SiO2 substrate by thermal evaporation through a shadow mask. Au source/drain electrodes with channel length L = 100 μm and width W = 1500 μm were deposited by thermal evaporation through a shadow mask. PTFE solution dispersed in water (with solid content of 60 wt %) was purchased from Aldrich and used as received. To prepare (PEDOT:PSS):PTFE solution, PTFE solutions were loaded in ambient conditions at weight ratios of 1.00:0.00, 0.95:0.05, and 0.90:0.10. The solutions were stirred for >12 h in ambient conditions, then sonicated for 30 min to increase their homogeneity. The (PEDOT:PSS):PTFE solutions were spin-coated onto the Au-patterned substrate. Residual water was removed from the (PEDOT:PSS):PTFE-treated electrodes by heating the substrates for 2 h at 120 °C. The (PEDOT:PSS):PTFE solution wetted the hydrophilic Au surface and dewetted the hydrophobic CYTOP surface. For this reason, the (PEDOT:PSS):PTFE could be specifically spin-coated onto the Au electrodes. Finally, 50 nm thick pentacene (Aldrich) films as active layers were deposited onto the (PEDOT:PSS):PTFEtreated substrates by organic molecular beam deposition (deposition rate = 0.2 Å/s; vacuum pressure = 10−7 Torr; substrate temperature = 25 °C). 2.2. Characterization. Electrical characteristics of the OFETs were measured using a Keithley 4200 SCS at room temperature in a N2-purged glovebox (H2O < 0.1 ppm and O2 < 0.1 ppm) and dark condition to exclude the effect of H2O, O2, and visible light on the electrical performance of OFETs. Field-effect mobility (μFET) and threshold voltage (Vth) were

Figure 1. Schematic diagram of a bottom-contact OFET and chemical structures of the polymers used as electrode and dielectric modifiers.

based PEDOT:PSS solution, and the (PEDOT:PSS):PTFE solutions were stirred for several hours. The (PEDOT:PSS):PTFE films with a thickness of ca. 25 nm were prepared by spin-coating the (PEDOT:PSS):PTFE solutions onto Au substrates and then annealing the samples at 120 °C for 2 h. Figure 2 shows optical microscope (OM) and AFM images of the (PEDOT:PSS):PTFE films prepared with various PTFE content ratios. The surface morphologies of the (PEDOT:PSS):PTFE films significantly depended on the

Figure 2. (a−c) OM images and (d−f) AFM images of (PEDOT:PSS):PTFE surfaces: (a, d) 1.00:0.00, (b, e) 0.95:0.05, and (c, f) 0.90:0.10. Insets in (d−f): height profiles along the yellow lines. 957

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Figure 3. (a) UPS energy distribution curves for (PEDOT:PSS):PTFE film with various PTFE contents. Left side: secondary electron emission. Right side: magnified view of the region near the Fermi edge. (b) Water contact angle on Au, CYTOP, and PEDOT:PSS films and their surface energies. (Insets: optical images of water drops on these films.)

Figure 4. (a−c) AFM topographies of pentacene films on (PEDOT:PSS):PTFE layers with various PTFE contents (a) 1.00:0.00, (b) 0.95:0.05, (c) 0.90:0.10, and (d) on channel (CYTOP), respectively. (e−h) The 2D-GIXD patterns of 50 nm thick pentacene on (PEDOT:PSS):PTFE films with various PTFE contents (e) 1.00:0.00, (f) 0.95:0.05, (g) 0.90:0.10, and (h) on channel (CYTOP), respectively.

PTFE content. As shown in Figure 2d, the PEDOT:PSS films exhibited very smooth surfaces with a root-mean-square roughness (Rq) of 1.57 nm. As the PTFE content in the (PEDOT:PSS):PTFE solution increased, the film roughness increased to 4.15 nm (see the cross-sectional height profiles shown in Figure 2d−f). The PTFE was difficult to disperse in the water-based PEDOT:PSS solution due to its hydrophobic properties, and thus the PTFE aggregated within the (PEDOT:PSS):PTFE films. As a result, the (PEDOT:PSS):PTFE (0.90:0.10) surface was very rough, with an Rq of 4.15 nm. The OM images shown in Figure 2a−c also reveal that the (PEDOT:PSS):PTFE (0.90:0.10) films consisted of large aggregates, whereas the PEDOT:PSS film prepared without PTFE formed a uniform surface. The Φ’s of the (PEDOT:PSS):PTFE-treated Au surfaces prepared with various compositions were determined based on the UPS energy distribution curves (Figure 3a). The Φ of an electrode is defined as the energy difference between the Fermi level and vacuum level and is one of the most important factors that affect the charge carrier injection between an electrode and a semiconductor in OFETs.25 For a p-type organic semiconductor, such as pentacene, which provides a highest occupied molecular orbital (HOMO) of 5.0 eV,17,26,27 a high value of Φ (≥5.0 eV) should be required to achieve Ohmic contact between an electrode and a semiconductor. Φ was calculated by subtracting the width of the spectrum from the

source energy (90 eV). The strong electron-withdrawing properties of the PTFE increased the Φ of the (PEDOT:PSS):PTFE layers; Φ was 4.84 eV in the 1.00:0.00 film, 5.16 eV in the 0.95:0.05 film, and 5.21 in the 0.90:0.10 film. To fabricate OFETs with the (PEDOT:PSS):PTFE-treated S/D electrodes, we first patterned Au electrodes on the CYTOP dielectric surface. The CYTOP surface was much more hydrophobic than the surfaces of the Au or the PEDOT:PSS (or (PEDOT:PSS):PTFE) layers (Figure 3b). Therefore, the (PEDOT:PSS):PTFE solutions dewetted from the CYTOP, but spread out to form a complete film on the Au surface; i.e., the (PEDOT:PSS):PTFE films only modified the Au electrode and not the channel region covered with CYTOP (Figure S1 in the Supporting Information (SI)). This controlled wettability method offered an easy and effective approach to improving hole injection at the electrode/ semiconductor interface by decreasing the hole injection barrier and matching the semiconductor crystalline morphology of the electrode to that of the channel. The use of CYTOP as a dielectric layer clarified how the physicochemical properties of the electrode/channel interface affect the device stability and charge injection at the interface. Previous reports have described excellent device stability against a gate-bias stress in OFETs prepared with CYTOP dielectrics, possibly due to the high energetic barrier to trap creation.13,28−30 Therefore, the device instability behavior in this study was attributed to the 958

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The Journal of Physical Chemistry C electrode/channel interface rather than to the channel/ dielectric interface. The crystalline morphologies of the pentacene layer between the channel and the S/D electrodes significantly affected the contact resistance in the bottom-contact OFETs; therefore, we investigated the crystalline morphologies of the pentacene layer on the (PEDOT:PSS):PTFE electrodes and the dielectric surface. Figure 4a−d shows AFM images of pentacene films grown on the (PEDOT:PSS):PTFE-treated Au electrodes and CYTOP channel surface. The pentacene on the (PEDOT:PSS):PTFE surface formed crystals with a small grain size of 100−200 nm (Figure 4a−c). This morphology did not depend significantly on the PTFE ratio (Figure 4b,c). However, the pentacene layer on the channel region formed large crystals > 500 nm (Figure 4d). The crystallinities of the pentacene crystals on the (PEDOT:PSS):PTFE surfaces were further investigated by measuring the 2D-GIXD patterns of the pentacene films. 2D-GIXD patterns (Figure 4e,h) of pentacene films grown on the (PEDOT:PSS):PTFE (1.00:0.00)-treated Au surface and channel region exhibited intense X-ray reflections of (00l) crystal planes along the qz axis (out-ofplane), with a d-spacing of 15.3 Å, as well as {1,±1}, {0,2}, and {1,±2} reflections along the qxy axis (in-plane) that correspond to a herringbone in-plane molecular packing structure. These reflections indicate that the pentacene films have a “thin-film phase” crystalline structure. By contrast, the 2D-GIXD patterns (Figure 4f,g) of the pentacene films grown on the (PEDOT:PSS):PTFE (0.95:0.05 and 0.90:0.10)-treated Au surfaces revealed the coexistence of a “thin-film phase” and “bulk phase” crystals. The corresponding crystal planes were indicated by the (00l)* reflections along the qz axis and the 8°tilted peaks with respect to the qz axis. In the “bulk phase”, the crystals show poor π-overlap, and the crystal planes are less ordered with respect to the substrate than are “thin-film phase” crystals. We expected that the presence of fluorocarbons in the (PEDOT:PSS):PTFE layer would degrade the ordered crystalline morphologies of the pentacene films. The fluorine groups of PTFE yielded a lower surface energy than PEDOT:PSS. Local differences between the surface energies of PEDOT:PSS and PTFE induced variations in the pentacene crystal growth mode and resulted in structural misalignment among the pentacene crystals formed on the (PEDOT:PSS):PTFE surface. The surface roughness of a dielectric layer has a significant influence on the growth behavior of the pentacene layer (such as nucleation, molecular ordering, and crystal size).31,32 Therefore, the rough surfaces of the (PEDOT:PSS):PTFE layers can lead to disordered crystals compared with pentacene layers on the PEDOT:PSS and channel region. The drain current−gate voltage (ID−VG) transfer characteristics (Figure 5a) of the bottom-contact OFETs prepared with the (PEDOT:PSS):PTFE-treated electrodes were used to calculate their electrical parameters (Table 1). The OFET using the (PEDOT:PSS):PTFE (0.95:0.05) yielded a saturation ID that was almost twice the values measured in the other devices. The electrical performances of the devices prepared with the (PEDOT:PSS):PTFE (0.95:0.05) were better than those of the other devices, despite having poor crystalline morphologies of the pentacene layer compared to the PEDOT:PSS device. The value of μFET for the OFETs prepared with the (PEDOT:PSS):PTFE (0.95:0.05) was 0.16 cm2/(V·s), whereas those of the devices prepared with the

Figure 5. (a) Transfer characteristics of bottom-contact OFETs containing (PEDOT:PSS):PTFE electrodes with various PTFE contents: 1.00:0.00 (red), 0.95:0.05 (green), and 0.90:0.10 (blue). Output characteristics of OFETs with (PEDOT:PSS):PTFE with (b) 1.00:0.00, (c) 0.95:0.05, and (d) 0.90:0.10, respectively.

Table 1. Electrical properties of bottom-contact OFETs containing (PEDOT:PSS):PTFE electrodes with various PTFE content (PEDOT:PSS):PTFE

1.00 : 0

0.95 : 0.05

0.90 : 0.10

Mobility (cm2/Vs) Vth (V) SS (V/decade) Ion/Ioff

0.073 −4.77 −1.76 1.03 × 106

0.160 −6.23 −0.51 2.55 × 106

0.063 −3.16 −0.75 1.10 × 106

(PEDOT:PSS):PTFE (1.00:0.00) and (0.90:0.10) were 0.073 and 0.063 cm2/(V s). The output curves (Figure 5b−d) revealed that all OFETs exhibited linear I−V dependence behaviors in the linear regime, at VD ≪ VG, indicating that all OFETs displayed Ohmic contact characteristics. The introduction of the (PEDOT:PSS):PTFE layers on bare Au electrodes allows the bottom-contact pentacene OFETs to achieve dramatically improved electrical performance (Figure S2). The good electrical performance of the (PEDOT:PSS):PTFE (0.95:0.05) device appeared to derive from the low hole injection barrier between the source/drain electrode and the pentacene layer. In the bottom-contact configuration, hole injection depends on both the electronic structure features (including energy level alignment and the band bending at the electrode/semiconductor interface) and the crystalline morphology at the electrode/semiconductor interface. Therefore, the hole injection barrier at the pentacene/(PEDOT:PSS):PTFE interface should be investigated in detail. The hole injection barrier between the pentacene and the (PEDOT:PSS):PTFE-treated Au layer was characterized using UPS for various thicknesses of pentacene layers from 0 to 15 nm (Figure S3). The UPS spectra (Figure 6a−c) obtained at the pentacene/(PEDOT:PSS):PTFE interfaces established the 959

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DOT:PSS):PTFE (0.90:0.10) surface showed a very high injection barrier despite the high PTFE content. The PTFE component at the (PEDOT:PSS):PTFE (0.90:0.10) surface appeared to be more aggregated than the (PEDOT:PSS):PTFE (0.95:0.05) component; this difference may be the cause of the reduction of the surface dipole effect on the (PEDOT:PSS):PTFE (0.90:0.10) surface. The contact resistance of the (PEDOT:PSS):PTFE-treated electrodes was obtained using the transmission line method (details in the SI). The contact resistance (RCW) as a function of channel length for various VG (Figure S4) was much lower in OFETs with the (PEDOT:PSS):PTFE (0.95:0.05) layer than in the OFETs with the (PEDOT:PSS):PTFE (1.00:0.00) and (0.90:0.10) layers. This result is consistent with the above discussions of the surface morphologies of the (PEDOT:PSS):PTFE layers (Figure 2) and the hole injection barrier (Figure 7). Figure 8a−c shows the gate-bias stress stability behavior of the OFETs prepared with the (PEDOT:PSS):PTFE-treated Au S/D electrodes. A sustained gate bias of −40 V (VD = 0 V) was applied to the OFETs over 3 h. Device measurement was carried out in a nitrogen-purged glovebox (H2O < 0.1 ppm and O2 < 0.1 ppm) and dark condition to exclude the effect of H2O, O2, and visible light on the electrical performance of OFETs. The transfer curves of the OFETs prepared with the (PEDOT:PSS):PTFE-treated Au S/D electrodes shifted toward negative values, but the μFET values did not change. The relative threshold voltage shifts (ΔVth) in the devices are summarized in Figure 8d. Compared with the (PEDOT:PSS):PTFE (1.00:0.00) and (0.90:0.10) devices, the ΔVth under a sustained bias stress of VG = −40 V was smaller in the (PEDOT:PSS):PTFE (0.95:0.05) device than in the other devices (Figure 8d). The values of ΔVth measured for each of the three devices after applying the gate-bias stress were −3.6 eV (1.00:0.00), − 2.7 eV (0.95:0.05), and −4.7 eV (0.90:0.10). The OFETs with CYTOP dielectrics showed excellent device stability against the gate-bias stress. Hence, the differences in the device stabilities were attributed to the electrode/pentacene interface. Interestingly, the trend in ΔVth of devices is similar to the variation of the hole injection barrier formed at the electrode/pentacene interfaces: decreasing the hole injection barrier increased the gate-bias stability. The hole injection barrier impedes the flow of hole carriers at the electrode/ semiconductor interface when the gate electrode is negatively biased. As a result, immobile positive charges accumulate near the interface; i.e., a hole injection barrier may act as a trap state at the electrode/semiconductor interface. The presence of immobile positive charges can delay hole injection from a source electrode to semiconductor during application of a

Figure 6. UPS energy distribution curves for pentacene films deposited on (PEDOT:PSS):PTFE layers with various PTFE contents: (a) 1.00:0.00, (b) 0.95:0.05, and (c) 0.90:0.10. Right side: magnified view of the region of the HOMO peak on comparative valence band structures of pentacene layers deposited on (PEDOT:PSS):PTFE surfaces.

band diagrams (Figure 7a−c) of each interface. The hole injection barriers at the interfaces between the pentacene layers and the electrodes were 0.32 eV for (PEDOT:PSS):PTFE (1.00:0.00), 0.26 eV for (PEDOT:PSS):PTFE (0.95:0.05), and 0.46 eV for (PEDOT:PSS):PTFE (0.90:0.10). The (PEDOT:PSS):PTFE (0.95:0.05) sample provided the lowest hole injection barrier; this result is consistent with the variations in electrical properties. The (PEDOT:PSS):PTFE surface has electron-withdrawing properties because the C−F interface dipole induces formation of charge carriers (holes) at the interface between pentacene and (PEDOT:PSS):PTFE. As a result, the vacuum level shift from electrode to pentacene increased, thereby reducing the hole injection barrier at the pentacene and (PEDOT:PSS):PTFE interface. However, the (PE-

Figure 7. Band diagrams of the interfaces between pentacene and (PEDOT:PSS):PTFE layers with various PTFE contents: (a) 1.00:0.00, (b) 0.95:0.05, and (c) 0.90:0.10. 960

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interfaces, and the contact resistance of OFET with (PEDOT:PSS):PTFE layers (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel.: +82538102536 (H.K.). *E-mail: [email protected]. Tel.: +82538102788 (S.H.K.). *E-mail: [email protected]. Tel.: +82542792269 (C.E.P.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a grant from the Korea Science and Engineering Foundation (KOSEF), funded by the Korean Government (MEST) (NRF-2014R1A2A1A05004993, NRF2014R1A1A1005896, and NRF-2012H1B8A2025602).

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Figure 8. (a−c) Transfer characteristics of OFETs containing (PEDOT:PSS):PTFE electrodes with various PTFE contents before, during, and after an applied gate-bias stress of VG = −40 V (VD = 0) for 3 h: (a) 1.00:0.00, (b) 0.95:0.05, and (c) 0.90:0.10. (d) Threshold voltage shift (ΔVth) as a function of stress time. Device measurement was carried out in a nitrogen-purged glovebox (H2O < 0.1 ppm and O2 < 0.1 ppm) and dark condition.

sustained negative gate bias, thereby leading to the negative shift of Vth.

4. CONCLUSIONS We fabricated high-performance and more operationally stable OFETs with the optimized (PEDOT:PSS):PTFE-treated S/D electrodes. The (PEDOT:PSS):PTFE films with various PTFE contents were prepared, and the surface properties of these films were characterized. As the PTFE content increased, the Φ of the (PEDOT:PSS):PTFE-treated Au electrodes increased from 4.84 to 5.21 eV. The pentacene layer prepared on the (PEDOT:PSS):PTFE film formed disordered crystalline structures as a result of the PTFE aggregates, presented in the (PEDOT:PSS):PTFE layer. At the optimized (PEDOT:PSS):PTFE ratio (0.95:0.05), the hole injection barrier at the electrode/pentacene interface decreased by 0.26 eV, so a bottom-contact OFET had higher performance with a μFET value of 0.16 cm2/(V·s) (vs 0.073 cm2/(V·s) for devices prepared with PEDOT:PSS-treated electrodes). The use of the optimized (PEDOT:PSS):PTFE-treated Au electrodes in OFETs improved their charge injection characteristics and, therefore, significantly improved their operational stability.

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REFERENCES

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b08065. The S/D electrodes and channel area before and after (PEDOT:PSS):PTFE treatment, the electrical performance of OFET with bare Au S/D electrodes, the hole injection barriers of (PEDOT:PSS):PTFE/pentacene 961

DOI: 10.1021/acs.jpcc.5b08065 J. Phys. Chem. C 2016, 120, 956−962

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DOI: 10.1021/acs.jpcc.5b08065 J. Phys. Chem. C 2016, 120, 956−962