Control of Concentration of Nonhydrogen-Bonded Hydroxyl Groups in

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Organic Electronic Devices

Control of Concentration of Non-Hydrogen-Bonded Hydroxyl Groups in Polymer Dielectrics for Organic Field-Effect Transistors with Operational Stability Hyunjin Park, Jimin Kwon, Boseok Kang, Woojo Kim, Yun-Hi Kim, Kilwon Cho, and Sungjune Jung ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b06653 • Publication Date (Web): 25 Jun 2018 Downloaded from http://pubs.acs.org on June 25, 2018

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ACS Applied Materials & Interfaces

Control of Concentration of Non-Hydrogen-Bonded Hydroxyl Groups in Polymer Dielectrics for Organic Field-Effect Transistors with Operational Stability Hyunjin Park,†,‡ Jimin Kwon,†, ¶ Boseok Kang,§ Woojo Kim,¶ Yun-Hi Kim,⊥ Kilwon Cho,§ and Sungjune Jung*, ¶

‡Department

of Electrical Engineering, Pohang University of Science and Technology

(POSTECH), 77 Cheongam-Ro, Nam-Gu, Pohang, Gyeongbuk, 37673, Republic of Korea

¶Department

of Creative IT Engineering, Pohang University of Science and Technology


§Department

of Chemical Engineering, Pohang University of Science and Technology


⊥Department

of Chemistry and Research Institute of Natural Science, Gyeongsang National

University, 501 Jinju Daero, Jinju, Gyeongnam, 52828, Republic of Korea

ACS Paragon Plus Environment

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KEYWORDS: organic field-effect transistors, polymer dielectrics, hysteresis, stability, hydroxyl groups

ABSTRACT

Poly(4-vinylphenol) (PVP) is a promising gate dielectric material for organic field-effect transistors (OFETs) and circuits fabricated on plastic substrates. Thermal crosslinking of PVP with a crosslinker such as poly(melamine-co-formaldehyde) (PMF) methylated at high temperature above 170 °C is widely considered an effective method to remove residual hydroxyl groups that induces polarization effects in the dielectric bulk. However, the threshold voltage shift in transfer characteristics is still observed for an OFET with a PVP–PMF dielectric when it is operated at a slow gate voltage sweep rate. The present study examines the cause of the undesired hysteresis phenomenon and suggests a route to enable a reliable operation. We systematically investigate the effect of the PVP:PMF weight ratio and their annealing temperature on the transfer characteristics of OFETs. We discover that the size of the hysteresis is closely related to the concentration of non-hydrogen-bonded hydroxyl groups in the dielectric bulk, and this is controlled by the weight ratio. At a ratio of 0.5:1, a complete elimination of hysteresis was observed irrespective of the annealing temperature. We finally demonstrate highly reliable operation of small-molecule-based OFETs fabricated on a plastic substrate at a low temperature.


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1. INTRODUCTION Organic field-effect transistors (OFETs) received immense attention over the past two decades for flexible and printable electronic applications such as E-papers, embedded displays, smart labels, and wearable electronics.1–3 Given that OFETs enter the realization of innovative applications, their operational stability is under rigorous scrutiny because it is critical for circuit design and overall device lifetime. A gate dielectric significantly affects the reliable operation of an OFET. Insulating polymers are considered promising materials for gate dielectrics because of their flexibility, solution-processability, and good compatibility with organic semiconductors (OSCs).4–6 Requirements for polymer dielectrics include high environmental and operational stability, low gate leakage current, low processing temperature, and sufficient solvent resistance. To date, several solution-processable polymer dielectrics have been studied, including poly(vinyl alcohol),7 Cytop,8 poly(methyl methacrylate),9 polystyrene (PS),10 benzocyclobutene,11 and poly(4-vinylphenol) (PVP).12 Among various polymer dielectrics, PVP-based dielectrics are promising because of their high dielectric constant (ε ~ 4), excellent chemical resistance to common organic solvents, low gate leakage current, and smooth morphology.12–14 A simple route to high-performance PVP dielectrics was introduced by crosslinking with a crosslinker.12 With respect to most PVP formulations, a PVP and a crosslinker, namely poly(melamine-co-formaldehyde) (PMF), are mixed in propylene glycol monomethyl ether acetate (PGMEA). The typical basic polymer to crosslinker ratio is 5:1.15 Various groups used this formulation to examine the electrical performances of evaporated OFETs,16,17 solutionprocessed OFETs,18–21 and oxide-based FETs.22–24 Different ratios, 3:1,25 2.75:1,26–28 2:1,29–31 and 0.2:1,32 were also examined to optimize device characteristics.


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A major limitation of PVP-based dielectrics is their operational instability of OFETs. It is typically presented as the threshold voltage shift (∆VTH) between subsequent measurements of the transfer characteristics with forward (off-to-on) and reverse (on-to-off) gate voltage sweeps (GVSs). Previous studies claim that residual hydroxyl groups inside a polar polymer dielectric absorb water molecules, and this induces slowly orientating dipoles.33–36 The slowly orientating dipoles causes undesired operational instability of electronic applications such as displays and sensors in their long-term use. Thus, significant efforts focused on reducing the concentration of hydroxyl groups and achieving reliable operation of OFET by increasing crosslinking temperature, varying the PVP to cross-linker ratio, or determining a different crosslinker. Hwang et al.14 fabricated pentacene-based FETs with a PVP–PMF dielectric annealed at 155, 175, and 200 °C to obtain the optimal annealing condition for achieving reliable OFETs. They concluded that 175 °C is the optimal annealing temperature (TA) for PVP–PMF dielectrics because OFETs with optimally annealed dielectrics indicated superior dielectric strength and lower sensitivity to gate bias stress compared with those annealed at other temperatures. Lim et al.34 reported the effects of hydroxyl groups on the electrical reliabilities of pentacene-based OFETs with varying PVP:PMF weight ratios ranging between 1:0 and 1:1.25. They confirmed that the hysteresis of the OFET is significantly related to hydroxyl groups existing within the polymer dielectrics and is reduced by preventing the possible diffusion of hydroxyl-contained species into the polymer dielectrics when the PMF ratio increases to 1.25. The Bao group reported stable operation of OFETs by applying new crosslinkers, namely 4,4’-(hexafluoroisopropylidene)diphthalic anhydride (HDA) and pentaerythritol tetra(3-mercaptopropionate) (4T).37,38 They revealed that OFETs with a PVP dielectric crosslinked with HDA or 4T has the advantages of low hysteresis with low-temperature processing and without significant loss in device performance.


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Figure 1. Linear transfer characteristic of the OFET with a PVP–PMF dielectric at a weight ratio of 5:1 and an annealing temperature of 175 °C. Specifically, VTH,fast and ITH are the threshold voltage and the drain current at the VTH,fast with the fast GVS mode, respectively. The threshold voltage shift (∆VTH,slow) is the difference between the threshold voltages of the forward and reverse GVSs with the slow GVS mode.

Despite reports on hysteresis-free operation, the shift in threshold voltage is still observed in OFETs with crosslinked PVP (cPVP) based on operating conditions. Figure 1 shows cyclic transfer characteristics of bottom-gate, top-contact (BGTC) OFETs based on 6,13bis(triisopropylsilylethynyl)-pentacene (TIPS-pentacene) with a PVP–PMF dielectric. The cPVP was formulated with a typical ratio of 5:1 and annealed at 175 °C; it is considered to be electrically stable without hysteresis. Although the ∆VTH at a fast GVS rate of 0.1 MV·cm-1·s-1 was as low as 1.5 V, we still observed a significantly increased ∆VTH of 8 V when the GVS rate


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decreased to 0.004 MV·cm-1·s-1. The magnitude of the electrical hysteresis phenomenon significantly depends on the start and end gate voltages, GVS rate, and step width of applied voltages during the cyclic transfer characteristic measurements.16,23,30,39–41 Ucurum et al.42 conducted experiments and simulations to verify that changes in the hold time and/or the delay time in the measurement affect the magnitude or direction of electrical hysteresis. They proposed that parameters such as hold/delay time, GVS rate, and start and end gate voltages should be investigated for any comparison on the hysteresis phenomenon with different OFET samples. However, most previous studies lack some information in their evaluation of hysteresis. We speculate that the reliable electrical performance of OFETs with cPVP in previous studies were obtained by measuring a relatively fast GVS rate, and the issue of the reliability is not completely resolved. The duration of measurement in the studies was relatively short to observe electrical instabilities in OFETs such as slow polarization and charge carrier trapping. Thus, it is necessary to apply bias stress on OFETs for a long period to ensure operational stability. In this study, we obtained operational stability in OFETs without electrical hysteresis both at fast and slow GVS rates. We systematically investigated the effect of the conditions of PVP– PMF dielectrics on the operational stability. The effect of the polymer dielectric on fundamental electrical characteristics, including threshold voltage shifts, was observed from the linear transfer characteristics of the OFETs with fast and slow GVS rates. The OFET was also measured under vacuum to investigate the effect of water molecule absorption on the threshold voltage shift, particularly slow polarization effects. Furthermore, the dielectric constant was extracted to indicate that the polarization in the dielectric bulk affects the threshold voltage shift. Fourier transform infrared (FTIR) spectroscopy technique was used to chemically investigate that hydroxyl groups, particularly the non-hydrogen-bonded hydroxyl group, induce hysteresis


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behaviors of OFETs. Thus, we fabricated OFETs based on polymer OSC and small-molecule OSC to demonstrate operational stability enhancement without a hysteresis phenomenon and ensure compatibility with a flexible substrate.

2. RESULTS AND DISCUSSION 2.1. Influence of PVP-PMF dielectric conditions on electrical characteristics of OFETs. We fabricated BGTC OFETs based on TIPS-pentacene to systematically investigate the effect of the PVP:PMF weight ratio and annealing temperature of the dielectric on their electrical characteristics (Figures 2a and b). The annealing temperature was changed from 100 °C to 200 °C with a step size of 25 °C, while different PVP:PMF weight ratios (5:1, 3:1, 2:1, 1:1, 0.5:1, 0.3:1, and 0.2:1) were used to formulate polymer dielectrics. The threshold voltage shift was evaluated by measuring transfer characteristics of an OFET in its linear regime wherein potential across the channel region is uniformly distributed. The measurements were performed at fast and slow GVS rates of 0.1 and 0.004 MV·cm-1·s-1. A detailed description of the threshold voltage shift extraction is given in the Experimental Section.

Figure 2. (a) Schematic diagram of the BGTC OFET based on TIPS-pentacene:PS blend with the PVP–PMF dielectric. (b) Chemical structures of PVP (left) and PMF (right). (c) Contour map


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of normalized threshold voltage shifts (∆VTH/VGSmax×100%) at various weight ratios (y-axis, in log-scale) and annealing temperatures (x-axis) of PVP–PMF dielectrics. Threshold voltage shifts were obtained from OFETs with 35 conditions of PVP–PMF dielectrics at a slow GVS rate of 0.004 MV·cm-1·s-1. Delaunay triangulation is used to compute and draw the contour lines by using a software package (OriginPro 2016, OriginLabs). The plotted references represent only PVP–PMF dielectric conditions and are not the magnitude of threshold voltage shift.

We produced a contour plot from the results on normalized threshold voltage shift measured at a slow GVS rate for a total of 35 fabrication conditions (Figure 2c). The conditions used in previous studies are plotted in the Figure 2c. The PVP:PMF weight ratio can more significantly affect the electrical hysteresis than the annealing temperature. With respect to high PVP contents in the dielectric, an increase in the temperature leads to a more pronounced shift of threshold voltage. A significant threshold voltage shift is still observed in the low temperature condition. In contrast, devices with a PVP concentration that is similar or lower than PMF exhibit significant stability with only 1–2% of threshold voltage shift irrespective of their annealing temperature. Figure 3 shows the typical transfer characteristics of the devices for the representative conditions measured at fast and slow GVS rates. Figures 3a and b compare the electrical performance of devices with PVP:PMF weight ratios of 5:1 and 0.5:1 based on the annealing temperature from 200 °C to 100 °C. A strong ∆VTH was observed at the 5:1 ratio at a slow GVS rate. An increase in TA resulted in an increased threshold voltage shift. Conversely, negligible changes in the transfer characteristics were observed when the PVP contents reduced by 1/10. The linear transfer characteristics with PVP–PMF annealing temperatures of 175 °C and 100 °C with weight ratios ranging from 5:1 through 0.2:1 are compared to observe the effect of


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annealing temperature on electrical performance (Figures 3c and d). Thus, OFETs with highly crosslinked PVP–PMF dielectric (TA = 175 °C) exhibited a significant ∆VTH for all the tested PVP:PMF weight ratios, while this was alleviated for the partially crosslinked PVP–PMF (TA = 100 °C). Notably, optimized weight ratios of 2:1 to 0.3:1 with TA of 100 °C resulted in hysteresis-free characteristics. With respect to a quantitative comparison of the transfer characteristics, the electric field normalized by the thickness of the polymer dielectric layer and drain current scaled by the applied drain voltage were used. Transfer characteristics typically exhibit positive threshold voltage shifts at slow GVS rate. . The transfer characteristics of OFETs with all PVP-PMF dielectric conditions are shown in Figure S1. In addition, the gate leakage current of the OFET is closely related to the conditions of the PVP-PMF dielectric and the hysteresis-free OFET exhibits a low gate leakage current level (Figure S2). Electrical parameters of TIPS-pentacene-based OFETs with all PVP-PMF dielectric conditions are summarized in Table S1.


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Figure 3. Linear transfer characteristics (VDS = 0.1VGS) of OFETs with PVP–PMF dielectrics at fast (left) and slow (right) GVS rates. The weight ratios of PVP–PMF dielectric are (a) 5:1 and (b) 0.5:1 with various annealing temperatures (200 to 100 °C). The annealing temperature (TA) is (c) 175 °C for the highly crosslinked PVP–PMF dielectric and (d) 100 °C for the partially crosslinked PVP–PMF dielectric at various weight ratios (5:1 to 0.2:1).

2.2. Hysteresis Origin Investigation: Effect of Dielectric Bulk vs. Effect of Semiconductor-Dielectric Interface. The experimental results described above suggest that the electrical hysteresis is highly dependent on the PVP:PMF weight ratio, and we determined an optimized ratio for hysteresis-free operation. To investigate the cause of the difference in electrical characteristics of OFETs, we examined whether the hysteresis due to higher reverse sweep drain current originated from slow polarization in the dielectric bulk or from trapped charges at the interface between a dielectric and semiconductor layer. First, we evaluated surface energy and roughness of PVP-PMF films with different conditions, which can affect charge transport, by contact angle measurements and atomic force microscope images.43,44 The PVPPMF films show small difference in contact angles of DI water, which are between 58 ° and 65 ° (Figure S3). Surface morphology of PVP-PMF films was estimated to be lower than a root-mean square (Rq) roughness of 4 Å (Figure S4). Surface energy and roughness of PVP-PMF films were independent of weight ratio and annealing temperatures and show similar values. The results show a good agreement with previous work that examined various PVP:PMF weight ratios.17 To furtherly investigate the interface characteristics, we examined the manner in which different processing conditions of a dielectric affect crystallinity of the TIPS-pentacene thin film. Micron-sized crystalline grains of TIPS-pentacene were observed in polarized microscopic


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images regardless of PVP-PMF dielectric conditions (Figure S5). Moreover, two-dimensional grazing incidence X-ray diffraction (2D-GIXD) studies were performed to obtain the crystalline characteristics of TIPS-pentacene thin films deposited on different PVP–PMF dielectric conditions. 2D-GIXD patterns and the corresponding 1D patterns confirm that the TIPSpentacene thin films grown on four different PVP–PMF dielectrics exhibit similar crystallinity (Figures 4a and b). Moreover, an identical vertical molecular orientation with respect to the substrates was observed (Figure 4c). The results indicate that the surfaces formed by the different dielectric conditions did not affect the growth of the TIPS-pentacene thin films. We attribute this to a small amount of PS blending with TIPS-pentacene solution.45,46 Electrical characteristics of the OFETs depended on the processing conditions of the dielectric as shown in Figures 2c and 3, even though the TIPS-pentacene thin films deposited on the different PVP–PMF dielectric conditions exhibit similar crystallinity at the surface.


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Figure 4. (a) 2D-GIXD patterns of TIPS-pentacene thin films deposited on four different PVP– PMF dielectrics and (b) the corresponding GIXD diffractogram profiles along the out-of-plane direction. (c) Schematic drawing of the TIPS-pentacene crystal with a vertical molecular orientation. Next, we fabricated metal–insulator–metal (MIM) capacitors to investigate the effect of the dielectric bulk layer on the electrical characteristics of the OFETs. Figure 5 shows the dielectric constant (ε) of MIM capacitors at a frequency of 100 Hz and the threshold electric field shift (∆ETH) of the OFETs. The change in the dielectric constant indicates a good agreement with the change in the ∆ETH (Figures 5a and b). The changes in the dielectric constants and threshold electric field shifts follow the same trend based on the PVP:PMF weight ratios. The extracted dielectric constant exhibits dependence on the frequency and conditions of PVP–PMF films including weight ratios and annealing temperatures as shown in Figure S6. Molecular dipoles and mobile ions that exist within the dielectric bulk layer are considered the reason for the change in the dielectric constant of the MIM capacitor, which is related to the variation in the threshold voltage shift. Water molecules can be easily absorbed into the surface of hydroxylcontaining polymer dielectrics and then diffuse through the bulk.16,39,47 To investigate the effect of water molecule absorptions into the PVP-based dielectrics, the linear transfer characteristics of OFETs were measured under vacuum conditions. In the water-free environment, the slow polarization phenomenon did not appear in the transfer characteristics of OFETs with a PVPPMF weight ratio of 5:1 (Figure S7). This shows that water molecules are a major cause that induces the slow polarization effects.


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Figure 5. Variation of the dielectric constant (ε) and the threshold electric field shift (∆ETH) in MIM capacitors with PVP–PMF films with respect to the (a) weight ratios and (b) annealing temperatures (TA).

2.3. Role of Hydroxyl Groups on Slow Polarization Effects. Hydroxyl group species with various hydrogen bonding are considered to be related to the threshold voltage shift in a PVP– PMF dielectric. We finally identified the hydroxyl group that is responsible for the decrease in the shift. Specifically, FTIR spectroscopy was used to measure the IR absorbance of various hydroxyl groups in PVP–PMF films. The intensity of the band was hydrogen-bonded hydroxyl group (≈3340 cm-1), associated hydroxyl group (≈3410 cm-1) and non-hydrogen-bonded (free) hydroxyl group (≈ 3530 cm-1) (Figure 6).48,49 Figures 7a and b compare the IR absorbance of highly crosslinked and partially crosslinked PVP–PMF films with weight ratios of 5:1–0.2:1. The


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IR absorbance of highly crosslinked PVP-PMF films decreased in the overall range through a condensation reaction between PVP and PMF (Figure 7a). Conversely, the absorbance peaks of hydrogen-bonded hydroxyl groups with various weight ratios were almost identical in the partially crosslinked (TA = 100 °C) polymeric films as shown in Figure 7b. However, the intensity of IR absorbance peaks for free hydroxyl groups significantly decreased with increasing PMF contents. To investigate the effect of the weight ratio, the IR absorbance of the PVP-PMF films with weight ratios between 5:1 and 0.5:1 was compared (Figures 7c and d). Although the PVP-PMF film with a weight ratio of 0.5:1 indicates negligible hysteresis in the transfer characteristics, hydrogen-bonded hydroxyl groups were still observed in the polymer film. In contrast, free hydroxyl groups were invisible in the polymer film at all annealing temperatures. As can be seen in Figure 3d, OFETs with 2:1 and 1:1 weight ratios of PVP-PMF dielectrics annealed at 100 °C exhibit small threshold voltage shifts at a slow GVS even though free hydroxyl groups still exist within the dielectrics (Figure 7b). The slow polarization effect increases the reverse sweep drain current, resulting in a positive threshold voltage shift in the transfer curve. In contrast, the lower reverse sweep drain current over forward current and negative threshold voltage shift are caused by the interface charge trapping. It is well known that the subthreshold swing (SS) increases with high charge trap density.41,50 As can be seen in Figure S8, higher PVP concentration in PVP:PMF mixtures leads to increasing in SS of OFETs, which shows that higher free hydroxyl group concentration results in increased interface charge trapping. OFETs with 2:1 and 1:1 weight ratios of PVP-PMF dielectrics were affected by the interface charge trapping as well as the slow polarization effect. Thus, small threshold voltage shifts of the corresponding OFETs are due to the compensation between the slow polarization and interface charge trapping.


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Figure 6. Various hydroxyl groups with different hydrogen bonds.

Figure 7. FTIR spectra of (a) highly crosslinked (TA = 175 °C) and (b) partially crosslinked (TA = 105 °C) PVP–PMF films at various weight ratios. The FTIR spectra of PVP:PMF weight ratios of (c) 5:1 and (d) 0.5:1 at various annealing temperatures.


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It is widely accepted that thermal annealing of PVP at a high temperature approximately above 170 °C is a simple and effective method to fabricate hysteresis-free OFETs by removing hydroxyl groups, which is a source of slow polarization within the dielectric bulk. However, our results indicated that the size of hysteresis heavily depends on the GVS rate, and high threshold voltage shifts were produced in OFETs with PVP-PMF dielectric annealed at 175 °C when it was operated at a fast GVS rate. We reveal that the higher reverse sweep drain current hysteresis due to slow polarization in the dielectric is diminished by reducing the amount of free hydroxyl groups. Our study also suggests that other hydroxyl groups in the dielectric, such as associated and hydrogen-bonded hydroxyl groups, are not closely relevant to slow polarization that causes hysteresis.

2.4. Demonstration of hysteresis-free flexible OFETs. In an extended demonstration, the optimal PVP–PMF dielectric with a 0.5:1 of weight ratio was used to successfully fabricate an OFET on a 3-µm-thick parylene C substrate in which the continuous service temperature is approximately 115 °C (Figure 8). Flexible OFETs exhibited high mechanical flexibility without a significant loss in device electrical performances. The electrical characteristics of flexible OFETs were measured as fabricated, bent, and crumpled. With respect to the bending test, the OFET was wound on a rod with a radius of 10 mm and 5 mm (Figure 8a). The device was also manually crumpled for the crumpling test (Figure 8b). As shown in Figure 8c, the flexible OFET did not exhibit a significant loss of device characteristics. Furthermore, an OFET based on polymer OSC, poly[2,5-bis(7-decylnonadecyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione-(E)-1,2bis(5-(thiophen-2-yl)selenophen-2-yl)ethene] (P-29-DPP-SVS), was fabricated as shown in


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Figure S9. In a manner similar to small-molecule OSC, the optimized polymer dielectric was compatible with the polymer OSC without threshold voltage shifts.

Figure 8. Flexible OFETs on a 3-µm-thick parylene C substrate. Photograph of the flexible OFET for the (a) bending test and (b) crumpling test. (c) Linear transfer characteristics of the flexible OFET were measured as fabricated, bent, and crumpled.

3. CONCLUSION In conclusion, we demonstrated that electrical hysteresis is eliminated by reducing the concentration of non-hydrogen-bonded hydroxyl groups in the PVP–PMF dielectric bulk even at slow GVS rate in this study. With respect to the measurements at the slow GVS rate, the magnitude of the threshold voltage shift did not decrease when the annealing temperature increased above the polymer’s glass transition temperature although it was eliminated when the PMF content increased in the polymer dielectric. Systematic investigation on the effect of the PVP:PMF weight ratio and their annealing temperature indicated that at a ratio of 5:1, an increase in the annealing temperature resulted in increased hysteresis at the slow GVS rate. Interestingly, the OFETs with a ratio of 0.5:1 did not exhibit any measurable threshold voltage


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shift at slow GVS rate, and we obtained the same results irrespective of the annealing temperature. This enabled us to fabricate reliable operation of TIPS-pentacene-based OFETs on an inexpensive parylene C substrate at a low temperature of 100 °C. The results indicated a close correlation between the hysteresis size and the magnitude of the peak for a non-hydrogen-bonded hydroxyl group in the dielectric bulk layer. Device reliability was affected to a less significant extent by other associated and hydrogen-bonded hydroxyl groups. We consider that the reduction in the concentration of free hydroxyl groups in the PVP-PMF dielectric is an effective method to fabricate highly reliable OFETs on a flexible substrate.

4. EXPERIMENTAL SECTION Materials. A small-molecule OSC solution was prepared from blends of 1 wt% TIPSpentacene (>99.9%, Ossila) and 0.2 wt% PS (MW ~280,000, Sigma-Aldrich) dissolved in 1,2dichlorobenzene. A polymer OSC solution was formed by dissolving 0.25 wt% P-29-DPP–SVS in chloroform. A gate dielectric solution was prepared by a 12 wt% mixture of PVP (MW ~11,000, Sigma-Aldrich) and PMF (Mn ~432, 84 wt% in 1-butanol, Sigma-Aldrich) dissolved in PGMEA. All materials were used without further purification. Device Fabrication. OFETs in the BGTC configuration were fabricated on a glass substrate (Eagle XG, Corning) or a 3-µm-thick parylene C substrate. With respect to mechanical support, parylene C was mounted on a glass carrier during the fabrication of a flexible OFET. A 50-nmthick aluminum gate electrode was thermally evaporated through a shadow mask on the substrates. A PVP–PMF solution was deposited by spin coating at 2000 rpm for 60 s and thermally annealed for 60 min. TIPS-pentacene and P-29-DPP–SVS for the semiconducting layer were deposited by spin coating at 2000 rpm for 60 s and thermally annealed at 70 and 100


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°C for 30 min, respectively. Finally, a 40-nm-thick gold was thermally evaporated through a shadow mask for the source and drain electrodes with channel width and length of 1000 and 50 µm, respectively. Device Characterization. The transfer characteristics of the OFETs were measured in a linear regime to have a nearly uniform potential distribution across the channel region and the same gate electric field direction with respect to the gate electrode. With respect to a quantitative comparison of the linear transfer characteristics, the gate-source voltage was applied with a gate electric field of −0.5 MV·cm-1 normalized to the thickness of dielectric. In addition, the drainsource voltage corresponded to 10% of the gate voltage for the same electric field magnitude between the gate electrode and drain electrode. The linear transfer characteristics of OFETs were measured at a fast GVS rate of 0.1 MV·cm-1·s-1 and a slow GVS rate of 0.004 MV·cm-1·s-1 to distinguish the slow polarization effect. The fast GVS rate was not sufficient to induce slow polarization effects in the dielectric layer. At the fast GVS mode, the threshold voltage (VTH,fast) of the OFET was defined by averaging the threshold voltages extracted from the forward and reverse GVSs by the second-derivate method. The threshold voltage at the slow GVS mode was extracted by the constant-current method. The constant current (ITH) was determined as the drain current at the VTH,fast. The threshold voltage shift (∆VTH,slow) was defined as the difference between the threshold voltages of the reverse and forward sweeps at the slow GVS mode. The electrical characteristics of OFETs were measured by using a semiconductor parameter analyzer (4200-SCS, Keithley) under air and vacuum conditions inside a dark box. Material Characterization. The thickness of the PVP–PMF films was measured by a stylus profiler (DektakXT, Bruker). In addition, 2D-GIXD measurements were performed at the 3C and 9A beamlines of the Pohang Accelerator Laboratory. The capacitance of MIM capacitors was


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measured by a LCR meter (ZM2376, NF Corporation) in the frequency range of 10 Hz to 1 MHz. FTIR spectroscopy was recorded on an FT–UV–VIS–IR spectrometer (VERTEX 80, Bruker) with a total of 128 scans with a resolution of 1 cm-1 in medium infrared (600–4000 cm1

).

ASSOCIATED CONTENT Supporting Information Linear transfer characteristics of TIPS-pentacene-based OFETs; gate leakage currents of TIPSpentacene-based OFETs; extracted parameters of OFETs; contact angles of PVP-PMF dielectrics; AFM images of PVP-PMF dielectrics; POM images of TIPS-pentacene thin films; extracted dielectric constants of PVP-PMF dielectrics; and linear transfer characteristics of P-29DPP-SVS-based OFETs.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Author Contributions †These authors contributed equally to this work.

Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT This work was supported by a grant (Code No. 2015M3A6A5072945) from the Center for Advanced Soft Electronics under the Global Frontier Research Program of the Ministry of Science and ICT of South Korea, and by the National Research Foundation of Korea (NRF) (Grant No. NRF-2017R1E1A2A02020984 and NRF-2015R1A2A1A10055620).

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Control of Concentration of Nonhydrogen-Bonded Hydroxyl Groups in

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