Effect Transistors

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Shellac Films as a Natural Dielectric Layer for Enhanced Electron Transport in Polymer Field-Effect Transistors Seung Woon Baek, Jong-Woon Ha, Minho Yoon, Do-Hoon Hwang, and Jiyoul Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b03288 • Publication Date (Web): 14 May 2018 Downloaded from http://pubs.acs.org on May 15, 2018

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ACS Applied Materials & Interfaces

Shellac Films as a Natural Dielectric Layer for Enhanced Electron Transport in Polymer FieldEffect Transistors Seung Woon Baek†, ‡, Jong-Woon Ha#, ‡, Minho Yoon§, Do-Hoon Hwang*,#, Jiyoul Lee*,† †

Department of Graphic Arts Information Engineering, Pukyong National University, Busan

48547, Republic of Korea #

Department of Chemistry, Pusan National University, Busan 46241, Republic of Korea

§

Department of Physics, Yonsei University, Seoul 03722, Republic of Korea

*E-mail: [email protected] (D.-H. Hwang) or [email protected] (J. Lee)

KEYWORDS: natural resin, dielectric, field-effect transistor, polymer semiconductor, n-type, ambipolar, charge transport.

ACS Paragon Plus Environment

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Abstract Shellac, a natural polymer resin obtained from the secretions of lac bugs, was evaluated as a dielectric layer in organic field-effect transistors (OFETs) on the basis of donor (D)–acceptor (A)type conjugated semiconducting copolymers. The measured dielectric constant and breakdown field of the shellac layer were ~3.4 and 3.0 MV/cm, respectively, comparable with those of a poly(4-vinylphenol) (PVP) film, a commonly used dielectric material. Bottom-gate/top-contact (BG/TC) OFETs were fabricated with shellac or PVP as the dielectric layer and one of three different D–A-type semiconducting copolymers as the active layer: poly(cyclopentadithiophenealt-benzothiadiazole)

[P(CDT-BTZ)]

bis(dicarboximide)-alt-bithiophene)

with

p-type

[P(NDI2OD-T2)]

poly(dithienyl-diketopyrrolopyrrole-alt-thienothiophene)

characteristics, with

n-type

poly(naphthalenecharacteristics,

[P(DPP2T-TT)]

with

and

ambipolar

characteristics. The electrical characteristics of the fabricated OFETs were then measured. For all active layers, OFETs with a shellac film as the dielectric layer exhibited a better mobility than those with PVP. For example, the mobility of the OFET with a shellac dielectric and n-type P(NDI2OD-T2) active layer was approximately two orders of magnitude greater than that of the corresponding OFET with a PVP insulating layer. When P(DPP2T-TT) served as the active layer, the OFET with shellac as the dielectric exhibited ambipolar characteristics, whereas the corresponding OFET with the PVP dielectric operated only in hole-accumulation mode. The total density of states was analyzed using technology computer-aided design (TCAD) simulations. The results revealed that, compared to the OFETs with PVP as the dielectric, the OFETs with shellac as the dielectric had a lower trap-site density at the polymer semiconductor/dielectric interface and much fewer acceptor-like trap sites acting as electron traps. These results demonstrate that shellac is a suitable dielectric material for D–A-type semiconducting copolymer-based OFETs, and the


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use of shellac as a dielectric layer facilitates electron transport at the interface with D–A-type copolymer channels.


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Introduction Recent developments in the field of organic electronics have led to potential new devices based on the concepts of “printed electronics,” in which an electronic circuitry is produced using costeffective graphic printing processes, and “flexible electronics,” which originates from the inherent mechanical flexibility of organic semiconductors1–5. On the basis of the merits of printed and flexible electronics, the applications of organic electronics are currently expanding in order to include biomedical devices that can be attached to or inserted into the human body6–8. Organic field-effect transistors (OFETs) control the current supplied to connected functional components such as organic light-emitting diodes, organic photodiodes, and various sensors. Thus, OFETs are considered to be fundamental units that can determine the potential applications of organic electronics9, 10. OFETs typically comprise electronic materials with a wide range of conductivities, including conductors, insulators, and semiconductors. Although OFETs’ performance depends on the properties of the organic semiconductor that serves as the active channel layer, the properties of the gate dielectric, which comes into contact directly with the active layer, also has a strong effect on OFETs’ performance. The primary role of this gate dielectric is to insulate the source/drain (S/D) electrodes from the gate electrode and to induce charge carriers in the semiconductor channels. Thus, the key properties of gate dielectric materials are (1) dielectric strength (electric breakdown voltage), which reflects the ability to maintain a strong electric field between the S/D electrodes and the gate electrode; (2) dielectric constant or permittivity, which indicates the efficiency of charge–carrier induction in the organic semiconductor channel; and (3) interfacial compatibility, which reflects how smoothly charge carriers are transferred at the interface between the semiconductor and the gate dielectric layer11–13. In addition, when an OFET is to be applied in


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the human body as a biomechanical device, biocompatibility is also considered as an important feature in order to prevent unwanted side effects. Therefore, several natural resins that are renowned to be environment-friendly and harmless to the human body have recently been introduced as dielectric material candidates for OFET devices.14-19 Shellac, a natural resin secreted by female lac bugs on trees, is a promising material for organic biomedical devices because of its biocompatibility with the human body. Shellac is safer to the human body than other natural resins; therefore, it has been extensively used as a coverage for citrus fruits to prevent moisture loss, as a glazing layer for chocolate and candies, and as an enteric coating in some niche medical applications to perform taste masking of unpleasant drugs that are orally administered. Shellac has also been employed in electrical applications because of its good electrical insulating properties and its ability to seal out moisture17–19. In this study, we investigated the feasibility of using shellac as a gate dielectric material in bottom-gate/top-contact (BG/TC)structured OFETs with donor (D)–acceptor (A)-type conjugated semiconducting copolymers as the active layers. The basic dielectric properties of shellac film were first compared with those of a poly(4-vinylphenol) (PVP) film, a widely used polymer dielectric layer11, 20–22. OFETs were then prepared

using

three

different

D–A-type

poly(cyclopentadithiophene-alt-benzothiadiazole)

semiconducting

[P(CDT-BTZ)]

with

copolymers:

primarily

p-type

characteristics, poly(naphthalene-bis(dicarboximide)-alt-bithiophene) [P(NDI2OD-T2)] with dominant n-type characteristics, and poly(dithienyl-diketopyrrolopyrrole-alt-thienothiophene) [P(DPP2T-TT)] with ambipolar characteristics23–26. The results indicate that shellac is a suitable material for use as a gate dielectric layer in OFETs based on D–A-type semiconducting copolymers. Importantly, for all the tested D–A-type copolymer films, the OFET’s performance was better when shellac was used as the dielectric layer compared to when PVP was employed.


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The shellac dielectric resulted in a superior OFET performance in the electron-accumulation mode compared to the PVP dielectric, indicating that the interface between the shellac and the semiconducting copolymer films is suitable for electron transfer.

Figure 1: Device configuration of the OFETs tested in this study (left) and the chemical structures of the three critical components of the shellac (right). Experimental Methods Sample Preparation: BG/TC-structured OFETs (shown in Figure 1) were fabricated as follows. First, an aluminum (Al) gate electrode was thermally evaporated through a shadow mask onto a soda-lime glass substrate (Corning EAGLE Glass) that has been cleaned by sequential sonication in acetone, isopropanol, and deionized (DI) water for 10 min each. The PVP solution that was used to create the reference dielectric film was prepared by dissolving PVP (105 mg) and poly(melamine-co-formaldehyde) (5.3 mg) in propylene glycol monomethyl ether acetate (1.0 mL). The resulting solution was then spin-coated on the glass substrate with the patterned Al gate electrode at 3,000 rpm for 30 s, followed by annealing in an oven at 200 °C for 1 h. Wax-free shellac comprising 85% of the three main components (aleuritic, jalaric, and shellolic acids, which are depicted in Figure 1) and 15% of the remaining components (e.g., natural pigments) was purchased from Sigma-Aldrich Co. and used without further purification. To prepare a shellac solution, the solute was dissolved in methanol (90 mg/mL) and stirred at room temperature


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overnight. The filtered shellac solution was spin-coated onto the Al gate electrode-patterned glass substrate at 3,000 rpm for 30 s followed by annealing at 100 °C for 30 min. The thicknesses of the PVP and shellac films were measured to be ~430 and ~320 nm, respectively, using field-emission scanning electron microscopy. Subsequently, a 5.0 mg/mL solution of D–A-type semiconducting copolymer dissolved in chloroform was spin-coated on the PVP or shellac films. For annealing, the entire assemblies (glass substrate + PVP or shellac + copolymer) were then placed on a hot plate at approximately 200°C for 1 h in a glove box (oxygen and water contents below 1.0 ppm). In order to form the S/D electrodes, a 50 nm thick gold (Au) layer was thermally evaporated on the semiconducting copolymer films. The typical widths (W) and lengths (L) of the S/D electrodes were 1 mm and 50 m, respectively. Device Characterization: The current–voltage (I–V) characteristics of the OFETs were recorded using a Keithley 236 Source Measure Unit in combination with a Keithley 2635 Source Meter controlled using LabVIEW software. The electrical measurement unit was connected to an MSTech Vacuum Probe Station with a chamber pressure of less than 10−3 Torr at room temperature. In order to evaluate the properties of the insulating dielectric layer, we fabricated metal–insulator– metal (MIM) structural devices, in which the insulating layer is sandwiched between an Al bottom electrode and an Au top electrode. The capacitance of the insulating layers in the MIM devices was measured using an Agilent 4284A Precision LCR Meter, and the leakage current was measured with the aforementioned Keithley 236 Source Measure Unit. Thin-Film Characterization: The surfaces of the dielectric thin films were analyzed by atomic force microscopy (AFM; Bruker Icon-PT-PLUS) in tapping mode. The surface energies of the dielectric films were obtained by measuring the contact angles of DI water and diiodomethane on their surfaces.


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Results

Figure 2: AFM images of the reference PVP film in (a) height mode and (b) amplitude mode and the shellac film in (c) height mode and (d) amplitude mode. Surface Characterization of the Spin-Coated Dielectric Thin Films: The surface of the dielectric layer that interfaces with the semiconducting copolymer film strongly affects the performance of the OFET, especially those with a BG/TC structure. This effect is primarily attributed to the formation of a channel of moving charges at the interface during OFETs’ operation. In this study, we first investigated the surfaces of the PVP and shellac thin films using AFM. Figure 2 shows AFM images (5.0 × 5.0 m) of the shellac and PVP films taken in both height and amplitude modes. The root-mean-square surface roughness of the PVP film (1.8 nm) was negligibly lower than that of the shellac thin film (1.9 nm). Next, the contact angles of DI water (a polar liquid) and diiodomethane (a nonpolar liquid) on the PVP and shellac thin films were measured to estimate the processability of the polymer semiconductor to be coated on the shellac


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film. The measured contact angles of DI water on the PVP and shellac films were 39.9° and 54.4°, respectively, whereas those of diiodomethane were 24.3° and 17.8°, respectively (Figure 3). These contact angles indicate that the shellac surface was more hydrophobic than that of the PVP. The surface energies  L were calculated using the Owens–Wendt model as follows 27: 

 L 1  cos   2  SN   LN  2  SP   LP

,

(1)

where  is the contact angle of the test liquid with the sample;  LN and  LP are the nonpolar and polar components of the test liquid, respectively; and  SN and  SP are the nonpolar and polar components of the surface energy of the sample, respectively. The calculated surface energy of the shellac film (57.63 mJ/m2) was lower than that of the PVP film (65.89 mJ/m2). However, the surface energy of the shellac film is sufficient enough to ensure that to form a semiconducting polymer film is formed on top of the shellac film.

Figure 3: PVP contact angle images for (a) DI water and (b) diiodomethane and shellac contact angle images for (c) DI water and (d) diiodomethane.


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Figure 4: (a) The capacitance–frequency curve of the shellac film that was measured at 20 Hz–1 MHz. The inset shows a schematic illustration of the MIM device. (b) Plot of the current density versus the electric field for the PVP and shellac dielectric films. The arrows indicate the breakdown points.

Dielectric Properties of the PVP and Shellac Films: The electric and dielectric properties of the PVP and shellac films were characterized using MIM (Au/insulator/Al) devices. Figure 4(a) depicts the capacitance–frequency curve of the shellac film measured at 20 Hz–1 MHz; the inset shows a schematic representation of the MIM device. As depicted in Figure 4(a), the areal capacitance of the shellac film was 10.9 nF/cm2 at 20 Hz, which gradually decreased as the frequency increased. The frequency-dependent capacitance values are mainly observed to occur due to the relatively slow response of the hydroxyl groups in the shellac resin. At the relatively high frequency of 1 MHz, the PVP and shellac films exhibited areal capacitance of 8.50 and 9.36 nF/cm2, respectively (see Figure S1 in the Supporting Information). However, the calculated dielectric constant (r) of the shellac film (3.38) was lower than that of the PVP film (4.12) because the PVP film was thicker than the shellac film. Figure 4(b) shows a plot of the current density versus the electric field for the PVP and shellac films. The current density of the MIM device with


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the PVP thin film exceeded 10−7 A/cm2, the threshold at which the current density between the top and the bottom electrodes begins to break down the insulator, at an electric field of 3.4 MV/cm. In contrast, for the MIM device with the shellac film, the current density exceeded the above threshold at an electric field of 3.0 MV/cm, lower than that for the PVP thin film. However, the current density across the PVP film between the top and the bottom electrodes increased rapidly after exceeding the current density threshold. In contrast, after exceeding the current density of 10−7 A/cm2, the MIM device with the shellac film maintained a current density of ~2.0 × 10−7 A/cm2 until the electric field reached 4.5 MV/cm. In consideration of these results, the electric strength of the shellac film was comparable or superior to that of the PVP film.

Figure 5: (a) The chemical structure of P(CDT-BTZ) with p-type characteristics. (b) The transfer curves of P(CDT-BTZ)-based OFETs employing PVP or shellac films as the dielectric layer in the saturation regime (drain voltage, VD = −60 V). ). The solid lines depict the drain current (ID), whereas the dashed lines depict the gate leakage current (IG). Output curves of the P(CDT-BTZ)based OFETs employing the (c) shellac film and the (d) PVP film.


11


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Electrical Properties of OFETs with a PVP or Shellac Film as the Dielectric Layer: The I–V characteristics of the OFETs containing different semiconducting polymers and PVP or shellac dielectrics were investigated. Figure 5 shows the chemical structure of P(CDT-BTZ) along with the electrical properties of the corresponding OFETs employing shellac or PVP films as the dielectric layers. The transfer and output characteristics of OFETs containing P(CDT-BTZ) active layers exhibited the typical characteristics of p-channel transistors. Specifically, the output curves of the OFETs with shellac and PVP films (Figs. 5(c) and 5(d), respectively) exhibited appropriate saturation at high drain biases. The transfer and output curves of the OFETs with shellac films as the dielectric layers showed no hysteresis between the forward and the reverse bias sweeps. In contrast, the curves of the OFETs with PVP films exhibited a slight hysteresis, indicating a relatively unstable operation. It should be noted that the leakage current of P(CDT-BTZ)-based OFETs using both shellac and PVP is rather large. However, considering that the leakage current is approximately 50% of the ON current, it can be concluded that most of the measured current is the drain current that flows from the source to the drain through the semiconductor channel. Additionally, we speculate that the relatively high OFF current of the OFETs is originated from the unpatterned semiconducting polymer films. The field-effect mobility (μFET) of each P(CDTBTZ) OFET was determined in the saturation regime (VD = −60 V) on the basis of the |ID|1/2 versus VG curves28, 29. The calculated μFET of holes in the saturation region was approximately one order of magnitude higher for the OFET with the shellac film (1.1 × 10−3 cm2/V·s) compared to the OFET with the PVP film (1.0 × 10−4 cm2/V·s). These results can be primarily attributed to the lower interface trap states between the shellac film and the P(CDT-BTZ) semiconducting layer as compared with those between the PVP film and the P(CDT-BTZ) layer, which will be discussed in detail in the subsequent section.


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Figure 6: (a) The chemical structure of P(NDI2OD-T2) with dominant n-type characteristics. (b) The transfer curves of P(NDI2OD-T2)-based OFETs with PVP or shellac films as the dielectric layer in the saturation regime (VD = 60 V). The solid lines depict the drain current (ID), whereas the dashed lines depict the gate leakage current (IG). Output curves of the P(NDI2OD-T2)-based OFETs with a (c) shellac film and a (d) PVP film as the dielectric layer. Figure 6 shows the chemical structure of P(NDI2OD-T2) along with the transfer and output curves of the P(NDI2OD-T2)-based OFETs with shellac or PVP dielectric layers. Unlike the P(CDTBTZ)-based OFETs, which displayed a transistor operation in the hole-accumulation mode, P(NDI2OD-T2)-based OFETs showed an n-type behavior, which indicated that the channel was opened by electron accumulation for the application of positive gate bias. The electron mobility of


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the P(NDI2OD-T2) OFET with the shellac film (1.8 × 10−2 cm2/V·s) was approximately three orders of magnitude higher than that of the OFET with the PVP film (2.7 × 10−5 cm2/V·s). This result is consistent with the higher currents observed for the OFETs with shellac films compared to the OFETs with PVP films under the same gate and drain biases. On the other hand, it is observed that the OFF level of the OFETs with shellac films in the transfer characteristics is also much higher than the one of the PVP counterpart. As stated above in the experimental method section, the shellac contains three main acid components as well as some impurities that are not controlled yet, which may lead an unintended doping effect in the polymer semiconductor layer in the OFET with shellac and cause the discrepancy with the one of the OFET with PVP. However, we believe that this can be improved by the further purification of the shellac material. It is also worth noting that the currents in the output curves [Figs. 6(c) and 6(d)] of both P(NDI2OD-T2)based OFETs were independent of the gate bias, and their I–V curves for different gate biases “pinched” together at a low drain bias. These phenomena indicate that a relatively large contact resistance originated from the large Schottky barrier between the Fermi level of the Au electrodes and the lowest unoccupied molecular orbital (LUMO) level of the P(NDI2OD-T2) polymer semiconductors29.


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Figure 7: (a) The chemical structure of the ambipolar P(DPP2T-TT) semiconductor. (b) The transfer and output curves of P(DPP2T-TT)-based OFETs with a PVP film. (c) The transfer and output curves of P(DPP2T-TT)-based OFETs with a shellac film). The solid lines depict the drain current (ID), whereas the dashed lines depict the gate leakage current (IG). Figure 7 shows the I–V characteristics of the OFETs with P(DPP2T-TT), which is known to be ambipolar (i.e., capable of operating in either the p-type or the n-type mode)

25-26

. When a PVP

film was employed as the dielectric layer in the P(DPP2T-TT)-based OFET, only p-type characteristics were observed (i.e., the channel only opened for negative bias), and the calculated μFET of holes in the saturation region was approximately 0.10 cm2/V·s. In contrast, when a shellac film was applied as the dielectric layer, the OFET exhibited ambipolar properties, with hole and


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electron accumulation for negative and positive biases, respectively, and the calculated values of μFET for holes and electrons were 8.4 × 10−2 and 0.24 cm2/V·s, respectively.

Table 1: Summary of the calculated charge–carrier mobilities for polymer-based OFETs. Polymer semiconductor (PSC)

P(CDT-BTZ)

P(NDI2OD-T2)

P(DPP2T-TT)

(p-type)

(n-type)

(ambipolar)

\

Hole

Electron

Hole

Electron

Dielectric

h (cm2/V·s)

e (cm2/V·s)

h (cm2/V·s)

e (cm2/V·s)

PVP

1.0×10−4

2.7×10−5

0.1

-

Shellac

1.1×10−3

1.8×10−2

8.4×10−2

0.24

The μFET values of the OFETs with D–A-type copolymers and polymer dielectrics are summarized in Table 1. The hole mobilities of the P(CDT-BTZ)-based OFETs with the shellac and PVP films differed by one order of magnitude. This difference was much smaller than that observed for the electron mobilities in the P(NDI2OD-T2)-based OFETs (three orders of magnitude). For devices based on ambipolar P(DPP2T-TT), the OFET with PVP as the dielectric exhibited only p-type characteristics, whereas the OFET with shellac as the dielectric showed ambipolar characteristics. These results indicate that the shellac film maintained or improved electron transport in the D–Atype semiconducting copolymer channel layer of the OFET, whereas the PVP film suppressed electron transport. Hole transport in the channel was not negatively affected by either the PVP film or the shellac film.


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Trap Density of States (DOS) in the OFETs: In general, the suppression of charge transport in the channels of transistor devices is closely related to the trap sites at the interface or semiconductor bulk21, 30, 31. Thus, we extracted the total trap DOS for each OFET using technology computeraided design (TCAD) simulations. The total trap DOS is composed of an acceptor-like conduction band [i.e., the highest occupied molecular orbital level; DOS gA(E)] and a donor-like valence band [i.e., the LUMO level; DOS gD(E)] as follows:

g(E) = g A (E) + g D (E), where g A (E) = NA exp (

E−EC k TA

) and g D (E) = ND exp (

EV −E k TD

),

(2)

where NA and ND are the DOSs of acceptor and donor traps, respectively; TA and TD are the characteristic temperatures of acceptor and donor traps, respectively; and k is the Boltzmann constant32, 33. Trap DOS was extracted by fitting the I–V curves calculated using the ATLAS organic simulator module (SILVACO) with basic semiconductor equations (e.g., Poisson’s equation, carrier continuity equation, drift-diffusion transport equation, energy balance transfer equation, displacement current equation, and organic defect model) to the experimentally obtained transfer curves of the OFETs (see Figure S2 in Supplemental Information)34. Figure 8 shows the calculated DOS profiles that were extracted from the transfer curves of the OFETs. The extracted DOS parameters are summarized in Table 2.


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Figure 8: Total DOS functions [g(E)] of the OFETs based on (a) P(CDT-BTZ), (b) P(NDI2ODT2), and (c) P(DPP2T-TT). Trap DOS functions were extracted using SILVACO TCAD simulation.


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Table 2: The extracted total trap DOS parameters for the OFETs based on P(CDT-BTZ), P(NDI2OD-T2), and P(DPP2T-TT).

PSC/Dielectric \ Parameters

P(CDT-BTZ) (p-type)

P(NDI2OD-T2) (n-type)

P(DPP2T-TT) (ambipolar)

PVP

Shellac

PVP

Shellac

PVP

Shellac

NA (cm eV−1)

9.35×1019

7.35×1019

2.71×1019

7.91×1018

4.03×1019

8.03×1018

ND (cm−3eV−1)

2.60×1018

2.20×1018

2.60×1019

3.23×1019

1.46×1019

7.73×1018

kTA (eV)

0.241

0.241

0.250

0.448

0.241

0.241

kTD (eV)

0.448

0.448

0.241

0.241

0.241

0.345

−3

As shown in Fig. 8, the P(CDT-BTZ)-based OFET with the shellac dielectric had slightly fewer donor-like trap sites (i.e., hole traps) than the OFET with the PVP dielectric. In contrast, for the P(NDI2OD-T2)-based OFETs, the DOS of donor-like traps was lower in the device with the PVP dielectric. The P(DPP2T-TT)-based OFETs with the shellac dielectrics were shown to have lower donor-like traps compared to the P(DPP2T-TT) OFETs with the PVP film near the valence band, and the donor-like traps in the P(DPP2T-TT) OFETs with the shellac film have been gradually lowered, compared to those of the P(DPP2T-TT) OFETs with the PVP film, as the energy level gets closer to the conduction band. In contrast, for all three D–A-type semiconducting copolymers, the OFETs with shellac dielectrics had lower acceptor-like trap densities than those with PVP dielectrics. In particular, the acceptor-like trap densities of the OFETs based on P(NDI2OD-T2) and P(DPP2T-TT) were much smaller when shellac was used as the dielectric compared to when the PVP film was employed. These results suggest that the shellac film interfaces contain fewer


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acceptor-like traps which hinder electron transport through the semiconducting polymer channels in OFETs based on D–A-type semiconducting copolymers. Therefore, the shellac thin film is regarded as a suitable dielectric layer for the fabrication of OFETs with efficient electron transport operated in the n-type mode.

Discussion Chua et al. reported that hydroxyl groups on the surfaces of dielectric materials such as SiO2 and PVP suppress the accumulation of electrons in semiconducting polymer films by trapping electrons at the interface between the semiconducting polymer and the dielectric layer, particularly in OFETs with BG structures35. The results for the OFETs with PVP dielectrics in this study are consistent with the findings of Chua et al. In contrast, the OFETs with shellac dielectric layers, which also have surface hydroxyl groups, exhibited electron-accumulating channels in n-type dominant or ambipolar semiconducting copolymers interfacing with the hydroxyl groups, and they have relatively lower acceptor-like trap DOS in their sub-bandgap. In order to understand the low acceptor-like trap DOS of the OFETs with shellac dielectrics, we considered that the chemical structure of shellac is similar to that of aliphatic alcohols to polyvinyl alcohol (PVA) with hydroxyl groups had been reported as dielectric materials for n-type or ambipolar pentacene-based OFETs 36, 37

. Chang et al. reported that the proton-dissociation constants of aliphatic alcohols in PVA are

higher than those of aromatic alcohols in PVP; this means that the hydroxyl groups of aliphatic alcohols are more protonated compared to the hydroxyl groups of the aromatic alcohols. Dissociated protons at the interface can act as electron traps in conjugated polymer films, thereby increasing the number of acceptor-like trap states12, 38. The hydroxyl groups of the aliphatic


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alcohols in shellac remain protonated, thus suppressing electron trapping in the interfacing channels of OFETs.

Conclusion We investigated the feasibility of using shellac (a natural polymer resin) as a gate dielectric material in OFETs. The results show that shellac meets the general requirements for dielectric materials of OFETs in terms of high capacitance and low leakage current. Compared to the commonly used PVP films, the hydroxyl groups of the aliphatic alcohols on the surfaces of the shellac films remained protonated, which helped suppress electron trapping by reducing acceptorlike trap DOS. Therefore, shellac films are suitable for application in OFETs with n-type or ambipolar characteristics in which not only p-type polymer semiconductor material but also electron accumulation is important, and this characteristic is expected to be of great help in the production of high-speed circuits (based on complementary metal-oxide-semiconductor devices) composed of OFETs. In addition, because of its biocompatibility, shellac shows promise as a natural material in OFETs used in biomedical applications.


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ASSOCIATED CONTENT The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. ; Transfer curves of the OFETs fitting with TCAD simulations.

AUTHOR INFORMATION Corresponding Author *

E-mail: [email protected] (D.-H. H.) or [email protected] (J. L.).

Author Contributions ‡These authors contributed equally. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT This research was supported by the Basic Science Research Program through the National Research Foundation (NRF) funded by the Ministry of Science, ICT & Future Planning of Korea (Code No. 2018R1A1A1A05021060 and No. 2011-0030013 through GCRC-SOP). The authors gratefully appreciate to Mr. Man-Gyu Hwang, CEO of SILVACO Korea, for his kind supports to use the TCAD software developed by SILVACO.


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TOC GRAPHICS


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Effect Transistors

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