Stretchable Polymer Dielectrics for Low-Voltage-Driven Field-Effect

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Stretchable Polymer Dielectrics for LowVoltage-Driven Field-Effect Transistors Chien Lu, Wen-Ya Lee, Chien-Chung Shih, Min-Yu Wen, and Wen-Chang Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b06765 • Publication Date (Web): 30 Jun 2017 Downloaded from http://pubs.acs.org on July 3, 2017

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ACS Applied Materials & Interfaces

Stretchable Polymer Dielectrics for Low-VoltageDriven Field-Effect Transistors Chien Lu,1 Wen-Ya Lee,2* Chien-Chung Shih,1 Min-Yu Wen2 and Wen-Chang Chen1 1

Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan, R.O.C. 2

Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, Taipei 106, Taiwan, ROC E-mail: [email protected]

KEYWORDS: Stretchable Electronics, dielectric polymer, conjugated polymer, polymer blend, elastomer

ACS Paragon Plus Environment

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Abstract

Stretchable and mechanical robust field-effect transistor is essential for soft wearable electronics. To realize stretchable transistors, elastic dielectrics with small current hysteresis, high elasticity and high dielectric constants are the critical factor for low-voltage-driven devices. Here, we demonstrate the polar elastomer consisting of poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP): poly(4-vinylphenol) (PVP). Owing to the high dielectric constant of PVDF-HFP, the device can be operated under less than 5V and show a linear-regime hole mobility as high as 0.199 cm2V-1s-1 without significant current hysteresis. Specifically, the PVDF-HFP:PVP blends induce the vertical phase separation and significantly reduce current leakage and reduce the crystallization of PVDF segments, which can contribute current hysteresis in the OFET characteristics. All-stretchable OFETs based on this PVDF-HFP:PVP dielectrics were fabricated. The device can still keep the hole mobility of approximately 0.1 cm2/Vs under a low operation voltage of 3V even as stretched with 80% strain. Finally, we successfully fabricate a low-voltagedriven stretchable transistor. The low voltage operating under strains is the desirable characteristics for soft and comfortable wearable electronics.


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Introduction The expectable deformability and mechanical durability of organic materials are beneficial for the development of wearable electronics and flexible display instruments.1-4 To build a field-effect transistor (FET) with both stretchability and reliable electrical performance requires mechanicallydurable materials, such as polymers or carbon nanotubes, which enable the device to maintain their functionality under stretching.5-8 Stretchable organic FETs (OFETs) are typically composed of multiple layers which may cause the device failure due to the formation of cracks at interfaces.9-10 Compared to the widely-used stretchable electrodes, the reliable performance is mainly limited by brittleness of semiconducting polymers and dielectric polymers. Thus, the development of stretchable semiconducting and dielectric materials are critical in the stretchable electronic applications. Nowadays, there were several reported approaches to impart stretchability to semiconducting polymers,11 consisting of modification of chemical structures,12-15 introduction of flexible and insulating linkers on polymer chains,16 and semiconducting polymer/rubber composites5, 17. However, only few polymer dielectrics (e.g. polyurethane (PU), PVDF-HFP and PDMS) were able to accommodate strain and remain electrical stability.8, 18-20 The hygroscopic or polarizable properties of elastic PU and PVDF-HFP dielectrics generally contributes to undesirable current hysteresis and large leakage current during voltage sweeping.21-22 To reduce current leakage, these polymer dielectrics with larger thicknesses (500~1 μm) are used. Nevertheless, the thick dielectric layer cause low capacitance and increased operation voltages (>60V),23 which is not possible to be exploited for practical applications. Moreover, even though the current leakage can be reduced in a thick dielectric layer, the devices still easily suffer significant current hysteresis, resulting in instability of OFETs. Another possible way to solve low-capacitance issue is to use ion-gel dielectrics. The ion-gel dielectrics, consisting of polymer electrolytes and ion liquid, have


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been introduced into the stretchable OFET due to their high capacitance (near 10 µF cm-2), and flexibility, but the main drawbacks of this dielectric are their low response time and ion diffusion.24-26 Recently, a rubbery e-PVDF-HFP dielectric with large double-layer capacitance ( 300 nF cm-2) was reported by Bao et al in which a variety of organic semiconductors were implemented to make high-performance OFETs at ambient condition.27 However, it is still challenging to solve issues of large electrical hysteresis induced by polarizable PVDF-HFP dielectrics. In this study, we develop a new dielectric material consisting of stretchable poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP): poly(4-vinylphenol) (PVP) blend film as a dielectric layer for stretchable OFET applications, illustrated in Figure 1. In this system, stretchability is provided by the elastomer polymer, PVDF-HFP, with thermally-crosslinked wrinkle morphology. On the other hand, PVP is used to optimize the dielectric property, supress the current hysteresis, and prevent huge current leakage. The OFET with this polymer blend dielectric is expected to be operate in less than 5 V and show high on/off ratio, which are sufficiently employed to fabricate stretchable OFETs which showed reliable performance within specified strains.

Experimental Section Material PVDF-HFP (Fluoropolymer G-801) was purchased from DAIKIN. Poly(4vinylphenol), other chemicals and solvents were purchased from Aldrich. The PEDOT:PSS solution (1–1.3 wt%, CLEVIOS PH1000 from Heraues), polyurethane (PU) (39–41 wt%, Alberdingk U3251 from Alberdingk Boley), Zonyl FS-300 (Sigma). Poly(styrene-b-(ethylene-cobutylene)-b-styrene) (SEBS) (Kraton-D1102A) was purchased from Kraton. PSe-DPP and PII-2T


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were synthesized according to the method reported in literature.28-29 AgNW solution in isopropanol (AW060-L) was purchased from Zhejiang Kechuang Advanced Materials Technology Co., Ltd. Device fabrication PVDF-HFP solution were prepared by dissolving fluoropolymer in propylene glycol monomethyl ether acetate (PGMEA) at a concentration of 50 mg/ml. The mass ratio of PVDF-HFP: BPO: TAIC = 100 : 2 : 6. And poly(4-vinylphenol) (PVP) solution were also prepared by dissolving PVP polymer (50mg/ml) with HDA and TEA as crosslinker and initiator, respectively.23 Then the two polymer solution were blended with each other in volume ratio of 6:1, 3:1, 1:1, 1:3, 1:6. The blend solutions were spin-coated on highly doped n-type Si (100) wafers at a spin rate of 3000 rpm for 60 sec. And these dielectric films were further thermal annealed at 100oC for 2 hours. Then the semiconducting polymers (PSe-DPP and PII-2T, 10mg/ml in chlorobenzene) were fabricated by spin-coating from chlorobenzene at a spin rate of 1000 rpm for 60 sec. The 60-nm-thick gold electrodes were defined by shadow masks, and the channel length (L) and width (W) were 100 and 2000 μm, respectively. The fabrication of stretchable OFET was as follows: PU (Alberdingk U3251) was diluted with Milli-Q water to 40 mg/ml. PEDOT:PSS (10mg/ml) was premixed with 10 wt% DMSO and 1 wt% Zonyl. During blend, the diluted PU was stirred at high speed while PEDOT:PSS were added drop wise. The highly stretchable PEDOT:PSS/PU electrode was prepared as the gate electrode, and silver nanowires (AgNW) were prepared as source/drain electrodes using spray coating. The PEDOT:PSS/PU were filtered through a syringe filter (0.22 μm pore size) before spray coating as the gate electrode. Silicon wafers treated with a self-assembled layer of octadecyltrimethoxysilane (OTS) were prepared by spincoating a solution of OTS in trichloroethylene followed by a vapor treatment in ammonium hydroxide.30 The PEDOT:PSS/ PU solution (prepared in water) was first deposited by spray coating onto hydrophobic octadecyltrimethoxysilane (OTS) treated wafer and


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then the poly(styrene-b-(ethylene-co-butylene)-b-styrene) (SEBS) solution (100mg/ml, in hexanes) was poured onto the OTS-treated wafer. After the solvent is dried in ambient condition, the substrate with the embedded gate electrode can be peeled off from the hydrophobic OTS-treated wafer. The sheet resistance of the PEDOT:PSS/PU electrodes on hydrophobic wafer and embedded in SEBS substrate were 424 ± 14 Ω/sq and 615 ± 31 Ω/sq, respectively. The blend dielectric layer with the PVDF-HFP/PVP ratio of 6:1 was deposited by spin coating and also crosslinked through thermal curing at 100oC for 2 hours. The semiconducting polymer was first spin-coated on hydrophobic wafer and then transferred onto the device. PSeDPP in chlorobenzene (10 mg/mL) was also spin-coated (1000 rpm, 60 sec) on the OTS-coated wafers and then manually transferred onto the dielectric by applying gentle pressure for 10 s. The source/drain electrodes (a channel length (L) of 100 μm and a channel width (W) of 2000 μm) of the OFET devices were defined using spray coated AgNWs with shadow masks heated at 70 oC. The sheet resistance of AgNW electrodes were 32 ± 7 Ω/sq.

Characterization The thickness of polymer film was measured with Microfigure Measuring Instrument (Surfcorder ET3000, Kosaka Laboratory Ltd.) The morphology change of semiconductor layer were performed through field emission scanning electron microscope (FESEM, JEOL JSM- 6330F). SEM samples were sputtered with platinum prior to the images characterization and analysis was operated at accelerating of 10 kV. The depth profiles of the XPS analysis (Thermo Scientific, Theta Probe) were obtained using Argon etching. The electrical performance of the device was recorded in a N2-filled glovebox using a Keithley 4200 semiconductor parametric analyzer. And the parameters such as linear mobility μ(cm2V-1s-1),


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on/off ratios (Ion/off), and threshold voltages (Vth were obtained by using the following equation in the linear regime: 𝑊 𝐼𝑑𝑠 = ( ) 𝜇𝐶(𝑉𝑔 − 𝑉𝑡ℎ − 𝑉𝑑𝑠 /2)𝑉𝑑𝑠 𝐿 where Ids is drain current, Vg is gate voltage, Vth is threshold voltage, Vds is drain voltage, μ is linear mobility, W is the channel width, L is the channel length, and C is the capacitance per unit area of polymer dielectric layer, respectively. The capacitance were collected using an LCR (inductance, capacitance, resistance) meter (Agilent E498E precision LCR meter).

Results and discussion Physical Properties of Polymer Films Before entering the details of OFETs results, we first investigated their surface morphology and the phase separation of the polymer blend film, as shown in Figure 2. In Figure 2(a), the pure PVDF-HFP film thermally crosslinked with peroxide initiator (benzoyl peroxide, BPO) and crosslinking agent (triaryl isocyanurate, TAIC) showed the wrinkled morphology in thin-film surface. The fluorinated elastomer, PVDF-HFP, contains curable site moiety (CSM, CH2=CFBr). Once benzoyl peroxide (BPO) generates free radicals when heating. The radicals will attack the brominated end group of the CSM to give free radical intermediates. The coagent, TAIC, is provides more reaction site for the crosslinking reaction in the peroxide-curing system. The mixture of PVDF-HFP/BPO/TAIC permits efficient thermally crosslinking in the PVDF-HFP elastomer.31 On the other hand, the surface of crosslinked PVP film showed relatively smooth film and no significant phase separation was induced. The PVDF-HFP: PVP blended dielectric films with blending ratios of 6:1, 3:1, 1:1, 1:3 and 1:6 showed different morphologies. The wrinkle morphologies were still observed in the images of films with blending ratios of 6:1, 3:1, 1:1, as


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shown in Fig 2(b), (c) and (d). However, the distribution of wrinkles was not as much as in the pure PVDF-HFP film. When a large amount of PVP (larger than 50%) was added to the PVDFHFP solution, relatively smooth film morphology was produced (Fig 2(e) and (f)). To elucidate the morphological changes in the blends, there are several possible reasons. First, this wrinkled morphology may be induced by thermally crosslinking and thermal expansion.32 During thermal crosslinking, the thermal-crosslinked degree of the top surface is actually higher than that on the bottom. To release thermal compressive stress, the elastic PVDF-HFP film tend to form the wrinkled morphology. Furthermore, the PVDF segment may partially induce crystalline polymorph during thermal treatment at100oC, while the amorphous HFP segment provide elasticity. Furthermore, it has been well-known that, during thermal treatment, the PVDF segment tend to form β phase crystals, which is the polymorph with ferroelectric properties. The formation of the crystalline PVDF segment may also enhance the PVDF-HFP aggregation and induce the wrinkled morphology. Blending with PVP may reduce the formation of β phase crystals. Du et al. have reported that the PVDF-HFP/PMMA blends showed the significant decrease of the fraction of β phase crystals in the polymer film.33 Second, the surface morphology may be also affected by the surface energy of the blended materials. The more hydrophilic PVP polymer with higher surface energy may preferentially migrate to the hydrophilic Si substrate interface during spin-coating, whereas the PVDF-HFP polymers tend to stay on the interface between air and the polymer surface. To evaluate the vertical distribution in the polymer blends, we evaluated the polymer films using contact angle measurement and X-ray photoelectron spectroscopy (XPS). The contact angles (CA) and surface energy of dielectric films were summarized in Figure S2.34 The contact angles of the polymer blends kept in the ranges of 90~110o, which were close to the contact angles of PVDF-HFP (95o),


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even in the low PVDF-HFP content (25%). The fluorine (in PVDF-HFP) and oxygen (in PVP) atomic contents in these polymer blends films (ratios of 1:3 and 1:6) were also analyzed by using X-ray photoelectron spectroscopy (XPS) (Table S1). From the XPS data measured with the different sputtering time, oxygen was not detected at the top surfaces but could be traced in deeper part of the films, which could be attributed to the vertical phase separation between the two polymers. This indicates that the PVP indeed prefers to distribute on the bottom surface, while the PVDF-HFP tends to stay on the top surface. With the increased PVP amount, the oxygen atomic ratio is still not measurable when no sputtering etching. This indicates that the PVDF-HFP form a thin layer on the top of PVP even in a high PVP amount. Note that we also measured the amount of oxygen in the films with ratios of 6:1, 3:1, and 1:1; however, the value of the oxygen amount is not available. According to the above results, this vertical phase separation may be also one of the factors to confine the wrinkle formation of PVDF-HFP.

Electrical Properties of Polymer blended dielectrics The dielectric properties of polymer films were measured in two-terminal metal-insulator-metal (MIM) devices with the film thickness of 230~250 nm for PVDF-HFP/PVP blends and 203 nm for crosslinked PVP. A heavily doped Si substrate was exploited as a bottom electrode, while a bilayer Al (5 nm)/Au (40 nm) as a top electrode (0.0075 cm2) was thermally evaporated onto the surface of polymer blended dielectric films. Intriguingly, the leakage behaviors of polymer blends were similar to that of PVP (Fig. S3). The crosslinked PVP (cPVP) device provided the low leakage current of 10−8∼10−9 A/cm2 in the voltage range from -5V to 5 V, while the pure PVDFHFP device exhibited large leakage current of about 10−5 A cm-2 when only applying a voltage of 2.5 V. When blending with PVP into the dielectric materials, the leakage current density of the


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polymer blends dramatically decreased to 10−7∼10−8 A cm-2, which is two orders lower than that of the pure PVDF-HFP film. This indicates that the crosslinked PVP played a critical role for shielding large current leakage.35-38 The corresponding capacitance of dielectric films showed slight frequency dependence (20 Hz ~3000 Hz) that scaled with blending ratios, as shown in Figure 3(a). The capacitance values decreased with frequency, which is attributed to the limited polarization response time by dipole alignment in high-k dielectrics. The relative dielectric constant (ε) and capacitance of these blended films are shown in Figure 3(b). The capacitance at 100 Hz (dielectric constant,ε) of pure PVDF-HFP and PVP are 34.6 nF cm-2 (ε = 9.7) and 23.5 nF cm-2 (ε = 5.1) at ambient condition, respectively, which are close to the reported values. The amount of highly polarizable PVDF-HFP have a great contribution to the capacitance of the blend dielectric films. The capacitance of the PVDF-HFP/PVP blends measured at 100 Hz were 28.2, 23.1, 20.6, 17.5, and 17.1 nF cm-2, corresponding to the PVDF-HFP/PVP ratios of 6:1, 3:1, 1:1. 1:3, 1:6, respectively.39 The dielectric constant of blended dielectric also decreases as the amount of PVP increases, due to the decreasing amount of high-k PVDF-HFP. Note that the pure PVP film showed a higher dielectric constant and capacitance compared to PVDF-HFP/PVP = 1:1, 1:3 and 1:6. This may be attributed to the hygroscopicity of PVP, which tends to absorb moisture from the air and results in the higher capacitance. Furthermore, PVDF-HFP is a hydrophobic material and prefers to migrate to the film surface due to its lower surface energy, thus leading to forming a water-resistant layer. Therefore, the blended film with small amount of PVP showed relatively lower capacitance than the pristine PVP film. The capacitance values of the dielectric films (ratios of 6:1, 3:1, 1:1) showed higher capacitance and smaller decreases with the frequency. Therefore, we further investigated how the polymer blend dielectrics influences polymer-based OFET performance.


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To fully utilize the merits of blend dielectric layer, we used two high performance p-type polymer semiconductors, PSe-DPP and PII-2T, respectively. Their chemical structures are shown in Figure 1. Figure 3(c) and (d) show the transfer characteristics of the spin-coated PSe-DPP and PII-2T OFET devices with various gate dielectric layers at drain voltages (Vd) of -2 V. Figure 3(e) is the output curves with well-defined linear and saturation regimes from the PSeDPP OFETs. The linear-regime mobility (μ), were determined from linear regime in the gate voltages (Vg ) ranging from -1 to -2 V, as listed in Table 1. The OFETs with a higher ratio of PVDF-HFP showed higher mobilities with on/off current ratios of 103 to 104. The OFETs fabricated with the blend ratio of 6:1 showed exceptional device characteristics with a linear mobility of 0.13 and 0.01 cm2 V-1 s-1 for PSe-DPP and PII-2T, respectively. The mobilities is comparable to the values reported from the devices using bare SiO2 dielectrics. Note that the mobilities of PSe-DPP and PII-2T OFET devices increased as the fraction of PVDF-HFP increased (Figure 3(f)). This may be attributed to PVDF-HFP with high electron-affinity fluorine atoms is able to accumulate a large amount of positive charge carriers at the semiconductor/dielectric interface to refill the traps in the channel. This hypothesis can be supported by the extremely small threshold voltages (Vt) (-0.2 ~ -0.8V). Additionally, no clockwise current hysteresis was observed in all the transfer curves in the OFETs. It is known that OFETs with ferroelectric dielectrics, such as P(VDF-TrFE), tend to show large clockwise hysteresis.37,

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The clockwise current hysteresis originates from bistable dipole

polarizations of crystalline ferroelectric β-phase along the semiconductor–insulator interface, which is generally undesirable for OFET applications. In our devices, the OFETs with PVDFHFP:PVP blend dielectrics showed negligible current hysteresis.41 This indicates that the presence of PVP polymer not only decreases current leakage but also lower electrical hysteresis of transfer


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curves.42 Moreover, the relatively higher capacitance and extremely smaller threshold voltages result in the OFETs only required a small operation voltage below 5V. The low hysteresis and operating voltage of the OFET devices displayed the possibility for practical applications. Moreover, the high linear mobility and subthreshold slope can be achieved under relatively lowvoltage operation (at Vd = -2 V) by the proper selection of the concentrations of PVDF-HFP and PVP. We also evaluate the performance of the devices in the ambient condition (Figure S5). The transfer curves of the devices with the PVDF-HFP/PVP ratio of 6:1 showed small hysteresis even in the high relative humidity (60%). This air stability may be attributed to the water resistance and hydrophobicity of PVDF-HFP. Additionally, we also evaluated the device performance at elevated temperature (Figure S5). It is known that the mobility of polymer semiconductors tend to increase with increased temperatures.43-44 We evaluated the devices at elevated temperature in the ambient conditions. The mobility slightly increased to 0.156 cm2/Vs (50 degree Celsius) from 0.133 cm2/Vs (room temperature). This indicates that the devices were still able to show stable performance and small hysteresis at 50 degree Celsius in the ambient condition without any encapsulation. Furthermore, the elasticity of PVDF-HFP makes the OFETs possible for stretchable electronics. To further investigate the mechanical durability and dielectric properties under mechanical strain, the blend dielectric with the PVDF-HFP/PVP ratio of 6:1 was exploited to fabricate stretchable OFET devices. This is due to the higher fraction of the PVDF-HFP elastomer and the highest mobility among these blends.

Stretchability and Electrical Performance To evaluate both the electrical and stretchable characteristics, PSe-DPP was used for the semiconductor layer due to its higher charge mobility than PII-2T. Figure 4(a) illustrates the


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studied device architecture with an embedded gate electrode in the SEBS substrate. Figure 4(b)(c) showed their transfer curves and device parameters (mobility, Ion/Ioff, and Vth) measured under the different strains. The capacitance of the PVDF-HFP:PVP blended dielectric with ratio of 6:1 on embedded substrate was also measured under stretching and with repeating stretching cycles, as shown in Figure 4(d). Before stretching, the capacitance value of the PVDF-HFP/PVP blend made on the soft SEBS substrate is 23 nF cm-2, close to the value measured on the hard silicon substrate. After stretching, the capacitance could maintained around 19~20 nF cm-2 even with 60%~80% strain. The capacitance decrease slightly to 13~ 17 nF cm-2 after repeating stretching 1000 cycles with 40% strain. The slightly lower capacitance might be attributed to the disordered crystalline domain due to the applied strain. Figure 4(c) summarizes the OFET characteristics of the devices applied with different strains (0% to 80%). The strain is in one direction, and the compression occurs in the other direction. Their transfer and output characteristics of the devices are shown in Figure 4(b) and S8. The device exhibited a high on-current of up to 10-7 A at Vg = -3 V with linear mobility up to 0.199 cm2 V-1 s-1 and on/off ratio of103. The performance of the devices prepared on elastomer substrates is comparable to that of PSeDPP devices made on a hard silicon wafer. The stretchable devices showed small hysteresis about 0.5 ~ 0.6 V. There is no significant increased hysteresis with the higher strains, indicating stable OFET performance under stretching. Note that the hysteresis is larger in the device made on the stretchable substrate than that prepared on the hard Si substrate. This might be due to the different fabrication processes. The stretchable OFETs were made using lamination transfer processes. This may cause the small amount of the air trapping at the semiconductor/dielectric interface, inducing hysteresis. Additionally, the threshold voltages of laminated devices is more positive than the spin-coated ones. This might be also attributed to the


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air trapping at the semiconductor/dielectric interface inducing interfacial doping effect. A slight decrease in the source-drain current (Ids) was observed when the substrate was stretched from 20% to 60% strain, with the channel direction oriented parallel to the stretching direction. The linear mobilities, calibrated with capacitance and W/L under strained, gradually decreased to 0.174, 0.158, 0.141, and 0.097 cm2 V-1 s-1 as the strain was increased to 20%, 40%, 60%, and 80%, respectively. Note that the mobility and on/off current ratio dramatically drop after stretching over 100% strain. This fast degradation in the high strain level is attributed to the mechanical cracking of the SEBS substrate and semiconductors. The degradation of the SEBS substrates is discussed below. As compared to the crosslinking PVP-HFP/PVP dielectric layer, the SEBS substrate without any chemically crosslinking are more vulnerable, resulting in the performance degradation during stretching. Therefore, to understand the mechanical limitation of our devices, we further examined The PVP, PVDF-HFP/PVP blend and SEBS substrates using the cross-sectional SEM technique in different incidence angles. The crosslinked PVP layer deposited on embedded SEBS substrate exhibited the partial aggregations and cracks due to the wrinkle morphology, as shown in Figure S6. The PVP film under strained was broken into smaller islands with denser cracks. This is attributed to the relative higher rigidity of the PVP films and the poor adhesion on the SEBS substrate. On the other hand, the spin-coated PVDF-HFP:PVP blend dielectric (with ratio of 6:1) on the SEBS substrate still showed the wrinkle morphology, as shown in Figure 4 (e), (f) and Figure 5. After applied with strains from 20% to 40%, the wrinkles in SEBS started to be elongated and oriented along the stretching direction. At the 60% strain, the SEBS substrate started to show structural breaks, which should be attributed to the yield limit of thermoplastic SEBS polymer. The cracks became larger when stretching to 80% (Figure 5(e)). The large cracks should


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contribute to the degradation of device performance. For example, the Ids of the device under 80% strains decayed by 50%. The increase in leakage current of stretchable OFET devices at the 80% stretching (Figure 6), which is consistent with the observed breaks and cracks in the dielectric layers. Therefore, our devices under a high level strain showing lower performance is main attributed to the substrate cracking, which causes fast degradation compared to other reported stretchable OFETs.7, 45 This electrical performance applied with repeating stretching cycles could be related to the changes in the yielding properties of the substrate, as shown in Figure 7. Despite the formation of cracks parallel to the strain direction (Figure S8), the continuity of semiconductor and dielectric films still allow the devices to be operated well. The reliability test for OFET device were operated with moderate mechanical strains parallel to charge transport direction, under stretching (40% strain) for 1000 cycles. In Figure 7(c) and (d), Ids and linear mobilities with repeating cycles up to 1000 times were collected and normalized to show the mechanical durability of the stretchable OFET device. The device characteristics were measured both in the released and strained state after completing repeating cycles. Although the Ids and linear mobilities of the devices decrease at a similar rate for both the released and stretched state, the collected values were still partially recovered after released from strain. Both Ids and linear mobility decreased by 60% after 100 cycles and 95% after 1000 cycles. This degradation is mainly attributed to the brittleness of the conjugated polymer films and the yielding limit of thermoplastic SEBS substrate. In our work, we did not observed any delamination of the soft materials even after stretching 80%. However, the mobility drops over 1000 cycles under the 40% strain. This degradation should be attributed to the brittleness of the donor-acceptor copolymers. It was shown that the mobility of isoindigo-based and DPP-based donor-acceptor copolymers dropped one to two orders of magnitude after


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stretching cycles even with only a strain of 20%.12 This is mainly attributed to the formation of cracks on the polymer films. The degradation is more significant when using highly crystalline polymer semiconductors. In our system, the polymers, PSeDPP and PII2T, are highly crystalline.28, 46

Furthermore, the limited mechanical compliance of the thermoplastic SEBS substrate is another

factor for device performance degradation. The significant performance decay emphasizes the importance of appropriate material selection. However, the polymer blend dielectric film still displays the elastic property and high capacitance other than the other elastomer polymer dielectrics, which is beneficial for implementing the low-voltage-operated stretchable OFET devices.

Conclusion In summary, we have investigated the OFET characteristics with polymer blend dielectrics and the application in stretchable device. The PVDF-HFP:PVP blended system provides superior dielectric properties with high capacitance due to the vertical phase separation between polymers. The vertical phase separation caused by different surface energy between these two dielectric materials. Furthermore, the vertical phase separation can significant reduce current leakage and maintain elasticity of the blended dielectrics. The superior elasticity of polymer blend can suppress the formation of cracks in dielectric films. However, the electrical performance under stretching or with repeating is limited by the mechanical cracking in the SEBS substrate. Even though the mechanical limitation of the substrate, the OFET characteristic is able to still operate over 1000 cycles under 40% strain. The low-voltage-operated (< 5V) and negligible hysteresis characteristics could be achieved by the polymer blended dielectric. This thin polymer blend dielectric film not only allows the fabrication of highly integrated devices but also improve the device characteristics


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in a specified strain range, which enables the possibility for future development of flexible and stretchable electronics.

ASSOCIATED CONTENT Supporting Information. XPS analysis, transistor characterizations, SEM images, contact angles are included. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *[email protected] Author Contributions W.-Y. L., C. L. and W.-C. W. conceived the concept, processing and structure details. C.L., C.C. S. and M.-Y. W. assisted with device fabrication and characterization. C. L and W.-Y. prepared the figures and co-wrote the paper. All authors discussed the results and commented on the manuscript. ACKNOWLEDGMENT W.-Y.L. acknowledges funding supports from the Ministry of Science and Technology of the Republic of China.

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Table 1. Electrical parameters of OFET devices using PVDF-HFP:PVP blended dielectrics with the various ratios from 6:1 to 1:6.

polymer

PSeDPP

PII-2T

linear mobilityavg

linear mobilitymax

(cm2V-1s-1)

(cm2V-1s-1)

6:1

(1.36±0.37)×10-1

3:1

Blend ratio

on/off

Vt (V)

1.91×10-1

7.62×104

-0.38

(8.41±0.64)×10-2

9.15×10-2

9.92×103

-0.42

1:1

(5.99±0.81)×10-2

6.96×10-2

1.38×103

-0.14

1:3

(3.47±1.26)×10-2

4.48×10-2

2.65×103

-0.80

1:6

(1.23±0.93)×10-2

2.20×10-2

8.81×103

-0.62

PVP

(3.35±0.77)×10-2

5.20×10-2

5.53×105

-1.34

6:1

(9.97±0.65)×10-3

1.07×10-2

2.85×103

-0.26

3:1

(6.90±0.84)×10-3

7.87×10-3

2.00×103

-0.33

1:1

(4.06±0.76)×10-3

5.01×10-3

1.00×103

-0.48

1:3

(3.85±0.58)×10-3

4.77×10-3

1.29×103

-0.31

1:6

(2.98±1.12)×10-3

4.13×10-3

6.21×1032

-0.50


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Figure 1. Chemical structures of the polymer blend dielectrics and the semiconducting polymers for OFET fabrication. (CSM: CH2=CFBr)


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Figure 2. The SEM analysis of (a) PVDF-HFP, PVDF-HFP:PVP blend dielectric with ratios of (b) 6:1, (c) 3:1, (d) 1:1, (e) 1:3, (f) 1:6, and (g) crosslinked PVP spin-coated on bare silicon substrates at an incident angle of 40o.


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Figure 3. (a) Capacitance vs frequency for dielectric films with different blending ratios. (b) Capacitance and dielectric constant at 10Hz with different blending ratios. Transfer characteristics of OFETs with semiconducting polymers, (c) PSe-DPP and (d) PII-2T, on PVDF-HFP:PVP blended dielectric. (e) Output characteristic of OFET with semiconducting polymers PSe-DPP on on PVDF-HFP:PVP blended dielectric with blending ratio of 6:1. (f) The calculated linear mobility of OFETs with semiconducting polymers, PSe-DPP and PII-2T, with different blending ratios.


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Figure 4. (a) The architecture of stretchable OFET device. (b) The transfer characteristics of the stretchable OFET stretched parallel to charge transport direction. (c) The OFET parameters of the device made from PVDF-HFP:PVP with the blending ratio of 6:1 under different strains (d) The capacitance of PVDF-HFP:PVP blended dielectric with blending ratio of 6:1 on the embedded substrate with different applied strains. The SEM (e) surface image and (f) cross-sectional image of the dielectric film (6:1) on the embedded substrate.


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Figure 5. The SEM surface and cross-sectional images of polymer blended dielectric (ratio of 6:1) on embedded substrate (a) before strained, applied with (b) 20%, (c) 40%, (d) 60%, (e) 80% strain.


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Figure 6. The gate current of stretchable OFET device with strains.


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Figure 7. Transfer curves for (a) released device and (b) strained device that was cycled to 40% strain. Normalized on current and mobility for (c) released device and (d) strained device that was cycled to 40% strain.


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Stretchable Polymer Dielectrics for Low-VoltageDriven Field-Effect Transistors Chien Lu,1 Wen-Ya Lee,2* Chien-Chung Shih,1 Min-Yu Wen2 and Wen-Chang Chen1

Graphic Abstract for Table of content


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Stretchable Polymer Dielectrics for Low-Voltage-Driven Field-Effect

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