Spontaneous Doping at the PolymerâPolymer Interface for High

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Organic Electronic Devices

Spontaneous doping at the polymer-polymer interface for high-performance organic transistors Eun-Sol Shin, Won-Tae Park, Young-Wan Kwon, Yong Xu, and Yong-Young Noh ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b21090 • Publication Date (Web): 08 Mar 2019 Downloaded from http://pubs.acs.org on March 16, 2019

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ACS Applied Materials & Interfaces

Spontaneous

Doping

at

the

Polymer-Polymer

Interface for High-Performance Organic Transistors Eun-Sol Shin§, Won-Tae Park§, Young-Wan Kwon±, Yong Xu†,* and Yong-Young Noh§,*

§Department

of Chemical Engineering, Pohang University of Science and Technology, 77

Cheongam-Ro, Nam-Gu, Pohang 37673, Republic of Korea †Electronic

and Optical Engineering, Nanjing University of Posts and Telecommunications,

Nanjing 210023, Jiangsu, China ±KU-KIST

Graduate School of Converging Science and Technology, Korea University, Seoul

02841, Republic of Korea

Keywords: Organic field effect transistors, fluorinated low-k gate dielectric, doping effect, amorphous polymer, charge carrier transport

ACS Paragon Plus Environment

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ABSTRACT: Low-k amorphous fluorinated polymers such as poly(perfluoroalkenylvinyl ether), CYTOPTM have widely been used as gate dielectrics for organic field-effect transistors (OFETs) because of their strong hydrophobicity to prevent the penetration of moisture and other contaminants and their perfect solvent orthogonality with organic semiconductors. Here, we report a new functionality of the fluorinated low-k polymer dielectrics, which is spontaneous p-doping at the dielectric-semiconductor interface in OFETs. This functionality makes the ambipolar charge transport a unipolar p-type. In the OFETs based on indacenodithiophene-co-benzothiadiazole (IDT-BT) and diketopyrrolopyrrole-thieno[3,2-b]thiophene (DPPT-TT), the charge transport is obviously ambipolar when paired with common polymer dielectrics such as poly(methyl methacrylate); however, it is perfectly modulated to the unipolar p-type by applying the fluorinated dielectrics of CYTOP and polytetrafluoroethylene (Teflon). We propose that this modulation of charge transport results from the rearrangement C-F bonds at the interface between the fluorinecontaining dielectrics and the conjugated polymer semiconductors by a proper thermal annealing. These well-aligned dipole moments lead to an abrupt downshift of the Fermi level of the semiconductor toward the highest occupied molecular orbitals near the dielectric-semiconductor interface, which provides a p-doping effect on the channel transport and results in unipolar p-type characteristics in the composed OFETs. This study reveals a new functionality of the fluorinated dielectrics for future organic electronics.


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INTRODUCTION Solution processable conjugated polymers have extensively been exploited as promising materials for upcoming plastic electronics such as flexible displays and logic circuits based on organic field-effect transistors (OFETs).1-4 There have been numerous endeavors to improve the OFET performance, particularly the field-effect mobility, by designing a molecular structure to obtain the high ordering and connectivity of semiconducting polymers.5-7 The OFETs based on state-of-the-art conjugated polymers recently exhibited an impressive mobility over 10 cm2V−1s−1.8-9 The unexpected high performance has been achieved by applying electron donoracceptor (D-A) copolymers as the active layer. In the carefully designed D-A conjugated polymers with excellent coplanarity and interchain connectivity, the charge carriers can mainly transport through the extended π-orbitals over the polymer backbone with a few interchain hoppings.10-11 Thus, the mainstream conjugated polymer-based OFET studies focus on the polymers that consist of D-A building blocks such as diketopyrrolopyrrole (DPP), isoindigo, naphthalenediimide, benzothiadiazole, and indacenodithiophene.8, 12-17 The D-A copolymers typically show ambipolar charge transport due to their relatively small band gap, which originates from the neighboring presence of the electron-rich and electrondeficient moieties, i.e., the electron push-pull geometry.18-19 The ambipolar OFETs commonly display a notably high off-state current, which causes a high power consumption. Specific


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applications such as light-emitting transistors may require this balanced ambipolar transport, but most circuit applications such as complementary circuits and display drivers require unipolar charge transport to minimize the off-state current. In addition, the simultaneous presence of electrons and holes in a transistor channel distorts the typical device characteristics and troubles the precise extraction of device parameters such as the field-effect mobility.20-21 Recently, there have been a few approaches to modulate the charge transport from ambipolar to unipolar, i.e., incorporation of electron-withdrawing groups in the donor part and doping a D-A conjugated polymer to deplete the minority charge carriers.22-24 However, those methods require the synthesis of new materials or addition of heterogeneous dopant molecules to the semiconducting layer, which increases the manufacturing cost and may degrade the semiconductor film morphology. In this paper, we report a facile method to modulate the charge transport from ambipolar to unipolar in D-A polymer-based OFETs by applying fluorinated low-k dielectrics such as poly(perfluoroalkenylvinyl ether) (CYTOP) and polytetrafluoroethylene (Teflon). (Figure 1) These fluorinated dielectrics induce many dipole moments at the semiconductor-dielectric interface to give a strong p-type doping effect because C-F bonds are formed between the dielectrics

and

the

semiconductor.

indacenodithiophene-co-benzothiadiazole

The

observed

(IDT-BT)

ambipolar

and

charge

transport

in

diketopyrrolopyrrole-thieno[3,2-

b]thiophene (DPPT-TT) OFETs with the common polymer dielectrics such as poly(methyl methacrylate) (PMMA) is changed to unipolar hole transport by applying the fluorinated dielectrics. This interesting modulation of charge transport is a result of the well aligned C-F bonds at the dielectric-semiconductor interface leading to electrical doping, as confirmed by ultraviolet photoelectron spectroscopy (UPS) and electron spin resonance (ESR) spectroscopy. This electrical doping is mechanistically different from the widely reported chemical doping that involves a


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chemical reaction or charge transfer between the host and the dopant. The Fermi energy level of IDT-BT and DPPT-TT is shifted by approximately 1 eV toward their highest occupied molecular orbitals (HOMO) by depositing CYTOP atop. However, no ESR signal is observed, which indicates that there is no conventional red-ox type doping between CYTOP or Teflon and the organic semiconductor (OSC). This result proves that the C-F bonds align at the OSC-dielectric interface and produce a strong interface dipole, which causes an abrupt upshift of the OSC bands, e.g., equivalent p-doping at the channel interface; hence, the holes accumulate, and electrons are depleted. However, no chemical reaction is involved. The low-k fluorinated dielectrics have the advantages of both low random dipole effect in the bulk and high charge concentration at the interface to make high-performance OFETs.


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RESULTS Characteristics of devices according to dielectric.

Figure 1. Chemical structure of the semiconductor (IDT-BT and DPPT-TT) and dielectric (PMMA, CYTOP, and Teflon). OFET structure with the top-gate and bottom-contact configuration, where the shaded region illustrates the interface dipole (∆) between the dielectric and the OSC, because of the C-F bonds formed between the fluorinated dielectrics and the conjugated polymer semiconductors. These interface dipoles are well aligned to provide an abrupt upshift of the vacuum level from the semiconductor to the fluorinated dielectric, so the Fermi level in the OSC layers near the interface is shifted toward the HOMO, and the holes accumulate in the OFET channel. This phenomenon is similar to p-doping. Here, for simplicity, the Fermi level is determined by the gate, and the flat-band condition is supposed. In the UPS measurement without a gate, the Fermi level is determined by the ITO substrate.


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The dielectric properties of the fluorine-containing polymers (Teflon and CYTOP) and fluorine-free polymers (PMMA) are investigated based on the capacitance–frequency (C–f) characteristics and leakage current density using a metal-insulator-metal (MIM) sandwich structure. Typical plots of C-f and the leakage current density versus the electric field (J vs. E) in an applied field of 2 MV cm−1 are shown in Figure S1 c and f, respectively. The measured capacitance of PMMA (t ~ 500 nm), CYTOP (650 nm) and Teflon film (500 nm) is 6.8, 2.5, 2.8, and 2.8 nF·cm−2 at 100 kHz. The measured leakage current densities at 2 MV cm−1 are 1.3 × 10−9, 4.3× 10 −10, 2.9 × 10−10, and 1.4× 10−10 A·cm−2 for PMMA, Teflon, and CYTOP, respectively.

Figure 2. Transfer characteristics of p-type for a) IDT-BT OFETs and b) DPPT-TT OFETs with PMMA, Teflon and CYTOP gate dielectrics. c) P-channel turn-Von and d) hole mobility of IDTBT and DPPT-TT OFETs with PMMA, Teflon and CYTOP gate dielectrics


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These low leakages ensure the reliable operation of the composed OFETs. The top-gate and bottom-contact IDT-BT and DPPT-TT OFETs were fabricated with three dielectrics. The characteristics of IDT-BT and DPPT-TT OFETs strongly depend on the applied gate dielectrics. (Figure 2) The OFETs with PMMA dielectric shows typical ambipolar characteristic. However, this ambipolar charge transport is completely modulated to unipolar p-type by replacing PMMA with CYTOP and Teflon, as shown in Figure 2a and b. Figure S2 show n-type transfer curves for IDT-BT and DPPT-TT with PMMA, CYTOP and Teflon. The basic parameters of all transistors are summarized in Table 1 and Table S1 in the supporting information. In particular, Figure 2c and d show that the turn-on voltage (VON, as defined as the gate voltage at which Id reaches its minimum for an ambipolar FET or Id starts to increase for unipolar FETs) and field-effect mobility change with various dielectrics. VON for the p-channel is dramatically shifted in the positive direction from -17 ~ -15 V with PMMA to 5.5 ~ 3 V with Teflon. The large decease in VON or the operation shift from the enhancement mode to the depletion mode indicates that the hole accumulation is enhanced while electrons are depleted from the channel using the fluorinate dielectrics. Notably, the hole mobility shows notably different tendencies between IDT-BT and DPPT-TT OFETs. In IDT-BT OFETs, the hole mobility strongly depends on the dielectric, and the highest hole mobility is obtained with CYTOP: up to 1.71 ± 0.14 cm2V−1s−1 compared with that of Teflon and PMMA (0.1 ~ 0.5 cm2V−1s−1 ). In contrast, the hole mobility in DPPT-TT OFETs is less dependent on the dielectrics, and the highest value is obtained with PMMA. In order to minimize the effect on the difference between the devices, more than 10 transistors are measured. The results for IDT-BT are consistent with the previous report that the carrier mobility decreases when the gate dielectrics with higher dielectric constant (k) are used because the increased dipole


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moments impose more disorder to the transporting charge carriers at the dielectric-OSC interface.25 For example, the mobility of amorphous triarylamine (PTAA) OFETs dramatically increases by 20 times when k decreases from 3.6 to 2.0.25 In this case, the hole mobility of amorphous IDT-BT OFETs increases by 10 times when k decreases from 3.5 to 2.1. However, the mobility of the polycrystalline DPPT-TT OFETs shows a notably weak dependence on k as shown in Table 1.26 In addition, the operating stability of IDT-BT and DPPT-TT FETs based on PMMA, Teflon and CYTOP dielectric were performed using 100 cycling test. Most of the device is very stable under 100 cycling test but hole mobility and threshold voltage are slightly changed in IDT-BT FETs based on CYTOP. Table 1. Parameters of IDT-BT and DPPT-TT OFETs with PMMA, CYTOP, Teflon dielectrics.

IDTBT

DPPTTT

Dielectric

Dielectric constant

μh, Avg

VON

SS

EA

T0

( cm2V−1s−1)

(V)

(Vdec−1)

(meV)

(K)

PMMA

3.5

0.17±0.02

−15

16.63

107.6

327.3

Teflon

2.1

0.57±0.20

4

6.31

72.0

315.8

CYTOP

2.1

1.71±0.02

5.5

5.26

63.4

322.9

PMMA

3.5

0.59±0.12

−17

27.60

83.4

268.9

Teflon

2.1

0.18±0.03

−2

12.00

64.7

722.8

CYTOP

2.1

0.22±0.05

3

10.00

47.9

792.4


9


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Characteristics of films between the semiconductor and dielectric.


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Figure 3. Ultraviolet photoelectron spectroscopy (UPS) and electron spin resonance spectra (ESR) analysis. UPS spectra of the CYTOP or Teflon (t ~ 650 nm) coated on semiconductor films before the Ar sputtering, interface between the dielectric and the semiconductor after Ar sputtering, ITO substrate and bare semiconductor films for a) CYTOP on IDT-BT, b) Teflon on IDT-BT, c) CYTOP on DPPT-TT and d) Teflon on DPPT-TT. Electron spin resonance spectra of e) CYTOP/IDT-BT, CYTOP/DPPT-TT, Teflon/IDT-BT and Teflon/CYTOP and f) DPPT-TT doped by FeCl3.

To identify the reason for the diverse characteristics with different dielectrics, particularly fluorinated CYTOP and Teflon, ultraviolet photoelectron spectroscopy (UPS) and X-ray


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photoelectron spectroscopy (XPS) were performed to investigate the Fermi level (EF) and element compositions at the OSC-dielectric interface. (Figure 3 a-d and Figure S5) All samples were prepared on an ITO substrate, and the conjugated polymer semiconductors were coated on ITO and subsequently covered by CYTOP or Teflon. The coating conditions were identical to those for the OFET fabrication. UPS spectra were not observed from the top surface of the CYTOP and Teflon layers because of their insulating property. The dielectric layers (t ~ 650 nm) were etched by Ar sputtering (5-kV ion gun) for 30 min to expose the dielectric-OSC interface. The appearance of the interface was confirmed by the XPS results. (Figure S5) The PMMA sample could not be measured because the PMMA layer with a soft property collapsed when Ar sputtering. At the interfaces with CYTOP and Teflon, the cut-off binding energy decreases by approximately 0.65 ~ 1.53 eV compared to the bare conjugated polymer films. As listed in Table 2, the cut-off binding energy is 16.94, 17.03, 16.96, 16.38, 15.82, 15.43, and 15.72 eV, and the distance of the EF level from the OSC HOMO is -0.20, 1.04, 0.95, 0.10, 0.07, 0.36, and -0.03 eV for ITO substrate, bare IDT-BT, bare DPPT-TT, IDT-BT/CYTOP interface, IDT-BT/Teflon, DPPT-TT/CYTOP, and DPPT-TT/Teflon, respectively. The ionization potential of IDT-BT decreases from 4.40 eV to 5.04 eV (CYTOP) and 5.60 eV (Teflon). In the case of DPPT-TT, the ionization potential of DPPT-TT also decreases from 4.47 eV to 6.0 eV and 5.76 eV when interfaced with CYTOP and Teflon, respectively. These increased ionization energies and the shifted Fermi level at the interface indicate that there is an abrupt downshift of the OSC bands from the dielectric to the OSC, as illustrated in Figure 1 and Figure S6. Equivalently, an abrupt band upshift occurs when the OSC is interfaced with the fluorinated dielectrics. As a result, the OSC Fermi level at the interface is abruptly shifted toward the HOMO, which is a direct evidence of p-doping because the hole concentration increases, as consistent with the observed device characteristics. Thus, p-doping


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occurs at the IDT-BT and DPPT-TT surface when fluorinated dielectrics are deposited atop. However, this phenomenon is not the conventional organic doping through chemical and red-ox reactions via charge transfer between the host and the dopant because of the insulating properties of CYTOP and Teflon with a notably large bandgap.

Table 2. Parameters calculated from the UPS data for the ITO substrate, IDT-BT, DPPT-TT, interface of IDT-BT/CYTOP, IDT-BT/Teflon, DPPT-TT/CYTOP and DPPT-TT/Teflon layers.

Cut off energy

Ionization potentials

(eV)

(eV)

Distance of the Fermi level from the OSC HOMO

HOMO (eV)

(eV) 4.48 ITO/glass substrate

16.94

(Work function)

−0.20

-

IDT-BT

17.03

4.40

1.04

−5.43

DPPT-TT

16.96

4.47

0.95

−5.41

IDT-BT interfacing with CYTOP

16.38

5.04

0.10

−5.14

IDT-BT interfacing with Teflon

15.82

5.60

0.07

−5.67

DPPT-TT interfacing with CYTOP

15.43

6.00

0.36

−6.35

DPPT-TT interfacing with Teflon

15.72

5.76

−0.03

−5.73

To explore the underlying doping mechanism, we performed electron-spin resonance (ESR) spectroscopy measurements at ~9.4 GHz microwave frequency, modulation frequency of


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100 KHz with 10 G modulation width and at 1 mW incident power. ESR spectroscopy has been widely used to detect magnetic resonance absorption of microwaves according to the Zeeman splitting of electron spins.27 The ESR samples were prepared on the insulating polyethylene terephthalate (PET) substrate, and the coating condition and structure of the layer were identical to those for the OFET fabrication. The conjugated polymer was coated on the PET substrate; then, a 650-nm-thick dielectric layer was coated on the conjugated polymer layer. As shown in Figure 3e, no ESR signals supported that the charge carriers in OSCs with the fluorinated dielectrics are not originated from an any chemical red-ox reactions. As a reference, we measured the DPPT-TT film doped with FeCl3, which is a well-known and efficient p-dopant for OSCs including DPPTTT.28 The purchased FeCl3 was dissolved in 2-ethoxyethanol (5 mg·ml−1) and blended with the DPPT-TT solution (5 mg·ml−1). The blended solutions of DPPT-TT with 2 wt% FeCl3 were prepared by mixing two different solutions and stirring at 80 °C overnight. In contrast to the OSCs interfaced with CYTOP and Teflon, the DPPT-TT film doped with FeCl3 (2 wt%) exhibited a notably strong ESR signal with a g-value (~ 2.002). (Figure 3f) The ESR spectroscopy for the DPPT-TT film doped with FeCl3 (2 wt%) was also performed at room temperature using a Jeol JES-FA200 ESR spectrometer (X-band: ~9.4 GHz) with 1 mW incident power. The ESR and UPS results show that the interface p-type doping does not stem from the conventional doping via charge transfer with red-ox chemical reactions. We can reasonably speculate that the interface pdoping is a result of the interface dipoles that arise from the strong C-F bonds between the fluorinated dielectrics and the conjugated polymer semiconductors.29 The aligned dipole moment is formed by rearrangement of fluorine atoms on the semiconductor caused by an appropriate thermal annealing process.30 These microscopic dipole moments are well aligned at the interface and produce a strong downshift of the OSC bands (or vacuum level shift). An identical large shift


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of the Fermi level (~1 eV) was observed on the copper(II) phthalocyanine surface on a highly aligned trifluoromethyl functionalized thiol self-assembled monolayer on Au.31 Thus, the shift of the UPS spectra proves the good alignment of C-F dipole moments in the amorphous fluorinated polymers. The Fermi level of the OSC near the interface suddenly moved toward the HOMO, which enhanced the hole-accumulation even without the gate voltage. This p-doping at the channel interface can modulate the ambipolar charge transport in OFETs into unipolar p-type because the electron concentration is significantly reduced. From another viewpoint, this interface dipole does not involve any charge transport between dielectric and OSC, so no ESR signal was detected. We can define this type of effect as electrical doping instead of chemical doping. In addition, the UV−vis−NIR absorption spectrum was measured to verify the electrical doping effect at the interfaces. As expected, the peaks in the near infra-red (NIR) region, which often indicates chemical doping, was not observed as shown in Figure S7.

Low temperature measurement.


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Figure 4. Field-effect mobility in the linear regime versus temperature of a) IDT-BT and b) DPPTTT OFETs with various dielectrics. Characteristic temperature (T0) and saturation mobility (μh, sat) for c) IDT-BT and d) DPPT-TT OFETs.

To obtain more insights into the effects on charge transport, we conducted low-temperature measurements on IDT-BT and DPPT-TT OFETs with various dielectrics. The activation energy (EA) was first evaluated using the mobility’s temperature dependency: µeff = µ0 exp(−Ea/kT)

(2)

where µ0 is the characteristics mobility at infinite temperature. EA is often used to characterize the average energy between the populated states and the mobile states,32-35 so it depends on the profile


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of the charge transport and the Fermi level. The lowest EA always pairs with CYTOP for both IDTBT and DPPT-TT OFETs. The fluorinated CYTOP induces strong p-doping to the neighboring conjugated polymer, whose Fermi level is significantly shifted toward the HOMO; i.e., the energetic barrier for hopping is reduced. The EA values of IDT-BT are 107.6, 72.0 and 63.4 and DPPT-TT are 83.4, 64.7 and 47.9 for PMMA, Teflon and CYTOP. As a result, CYTOP devices show the lowest EA. (Figure 4 a and b) Here, we must mention that the dipoles incorporated in the dielectric are unlike the interface dipoles discussed in this work. The dielectric dipoles in PMMA are often randomly orientated and cause disorder to the transport charge carriers at the dielectricsemiconductor interface, whereas the interface dipoles in CYTOP are well aligned, and the energetic landscape for charge transport is not altered. Moreover, we applied transport models to attain the transport parameters, such as the broadening of the density of states (DOS) or Urbach energy, which correlates with the characteristic temperature (T0) extracted from the gate-voltage dependency of the OFET drain current at variable temperatures36: ID ∝(VG −VTH )γ

(3)

where γ is the gate-voltage dependence. Since the charge transport in high-performance polymer OFETs with top-gate configuration generally shows a 2D distribution, γ and T0 should satisfy the following relationship37 γ = T0 / T + 1

(4)

Surprisingly, the IDT-BT and DPPT-TT OFETs show very different behaviors when the dielectric changes from PMMA to CYTOP. With regard to IDT-BT, the observed T0 values are comparable for all dielectrics, but T0 of DPPT-TT OFETs constantly increases from high-k PMMA to low-k


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CYTOP. (Figure 4 c and d) The fitting examples for T0 and γ are in Figure S8. The difference should arise from the distinct characteristics of IDT-BT and DPPT-TT. It has been reported that the charge transport in IDT-BT approaches the disorder-free state for its specific side-chain conformation.34 The Urbach energy is as low as 24 meV; hence, almost all states are thermally accessible at room temperature. This phenomenon may explain the nearly invariable T0 with different dielectrics. However, the energetically ordered and morphologically amorphous IDT-BT is susceptible to the external disorder caused by dielectric dipoles, so only the low-k CYTOP can retain its intrinsic disorder-free characteristics to deliver the highest mobility. In contrast, the Urbach energy for DPPT-TT is up to 33 meV, which is significantly higher than the thermal energy at room temperature. In other words, the charge transport in DPPT-TT is not sensitive to the negative effect of dielectric dipoles. Instead, the accessible states, which are represented by T0, are significantly improved by the higher charge concentration, because of the higher specific capacitance from the higher-k dielectrics. The doping and high k have similar effects on improving the charge concentration and carrier mobility, but high k appears to play a more important role in the transport enhancement for the energetically disordered and morphologically polycrystalline DPPT-TT. Thus, the lowest T0 and highest mobility are observed to pair with the highest-k dielectric of PMMA.

CONCLUSIONS


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In conclusion, we have demonstrated a simple method to modulate the charge transport in polymer OFETs from ambipolar to unipolar p-type by applying fluorinated dielectrics such as CYTOP and Teflon. The UPS, ESR, NIR absorption measurements show that this charge transport modulation is attributed to an electrical p-doping at the dielectric-semiconductor interface. During the deposition of the fluorinated dielectric on the conjugated semiconductor, C-F bonds are spontaneously formed in-between and produce many dipole moments at the interface. Since the C-F bonds are well directed from the conjugated polymer semiconductor to the fluorinated polymer dielectric, the microscopic dipoles are well aligned to induce a strong interface dipole, which causes an abrupt upshift of the OSC bands near the channel interface. The Fermi level is abruptly moved toward the HOMO, holes accumulate in the channel, and electrons are depleted from the channel. Perfect unipolar p-type characteristics with improved charge transport are consequently observed in highly ordered IDT-BT OFETs. This p-doping does not involve any real charge transfer or chemical reaction between the dielectric and the semiconductor, so it is mechanistically different from the typical molecular doping for organic semiconductors. This spontaneous doping significantly simplifies the OFET fabrication for unipolar devices to decrease the off current and power consumption. Our results also show that the low-k fluorinated dielectrics can take advantages of both low random dipole effect and high charge concentration. Therefore, this work paves a new path for future organic electronics to achieve high-performance devices and circuits.

EXPERIMENTAL METHODS


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Device fabrication Glass substrates with pre-patterned Au/Ni electrodes (3 nm/13 nm in thickness) were cleaned by deionized water, acetone, and isopropanol for 10 min each in ultrasonic bath. Then, the substrates were treated by UV ozone for 30 min. IDT-BT and DPPT-TT were dissolved in dichlorobenzene with overnight annealing at 80 ℃. The IDT-BT solution was spin-coated onto the treated substrates at 2000 rpm for 60 s and subsequently annealed at 100℃ for 30 min in an N2 glove box. The DPPT-TT solution was similarly spin-coated, but the annealing was at 250℃ for 30 min. Poly(methyl methacrylate) (PMMA, Sigma-Aldrich, Mw ¼ 120kD) in n-butyl acetate, polytetrafluoroethylene (Teflon, Sigma-Aldrich) in FC-40 with 80 mg.ml−1 concentration, and poly(perfluoroalkenylvinyl ether) (CYTOP) polymer were prepared as dielectric layers. PMMA and Teflon were spin-coated on the conjugated polymer layer at 2000 rpm for 60 s and subsequently annealed at 80 ℃ for 2 hours in the N2 glove box. Pure CYTOP was spin-coated in 2 steps (first at 500 rpm for 5 s and then at 4000 rpm for 60 s) and annealed at 80℃ for 2 hours in N2 glove box. Gate electrodes of Al (50 nm thick) were deposited through a shadow mask using thermal evaporation. Measurement of device characteristics The electrical characteristics of the OFETs and leakage current density were measured using a semiconductor characterization system (Keithley 4200-SCS) in an N2-filled glove box. The mobility and threshold voltage were derived from the equations for classical silicon MOSFETs in the saturation regime. The capacitance-frequency characteristics for various dielectric layers were measured using an Agilent 4284A precision LCR meter, which was controlled by Keithley 4200SCS. The interfaces between CYTOP, Teflon and conjugated polymer semiconductors were analyzed by XPS and UPS in an ultrahigh vacuum surface analysis system (ULVAC, PHI-5000),


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which was equipped with a monochromatic Al kα X-ray (1486.6 eV) at a base pressure of 1 × 10-6 Pa. The doping effect of the device was characterized by the electron spin resonance (ESR) spectroscopy using a JES-FA200 X-band spectrometer (JEOL, Japan) and UV−vis−NIR absorption spectroscopy (JASCO V-770 UV-Vis spectrophotometer, USA).

ASSOCIATED CONTENT Supporting Information. Current and normalized current decay versus time, Capacitancefrequency characteristics and leakage current density versus electric, N-type transfer curve, Transfer curve after 100 cycling measurements, X-ray photoelectron spectroscopy (XPS) analysis, UV−vis−NIR absorption spectra, Parameters of IDT-BT and DPPT-TT transistors in n-type. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (Prof. Y.-Y. Noh). * E-mail: [email protected] (Prof. Yong Xu).

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT


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This work was supported by the Center for Advanced Soft-Electronics (2013M3A6A5073183) funded by the Ministry of Science & ICT through the NRF grant funded by the Korea government.


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REFERENCES 1.

Gelinck, G.; Heremans, P.; Nomoto, K.; Anthopoulos, T. D., Organic Transistors in

Optical Displays and Microelectronic Applications. Advanced materials 2010, 22 (34), 3778-3798. 2.

Yi, H. T.; Chen, Z.; Facchetti, A.; Podzorov, V., Solution‐Processed Crystalline

n‐Type Organic Transistors Stable against Electrical Stress and Photooxidation.

Advanced Functional Materials 2016, 26 (14), 2365-2370. 3.

Guo, X.; Ortiz, R. P.; Zheng, Y.; Kim, M.-G.; Zhang, S.; Hu, Y.; Lu, G.; Facchetti,

A.; Marks, T. J., Thieno [3, 4-c] pyrrole-4, 6-dione-based Polymer Semiconductors: Toward High-Performance, Air-Stable Organic Thin-Film Transistors. Journal of the

American Chemical Society 2011, 133 (34), 13685-13697. 4.

Lüssem, B.; Tietze, M. L.; Kleemann, H.; Hoßbach, C.; Bartha, J. W.; Zakhidov,

A.; Leo, K., Doped Organic Transistors Operating in the Inversion and Depletion Regime.

Nature communications 2013, 4, 2775. 5.

Caironi, M.; Bird, M.; Fazzi, D.; Chen, Z.; Di Pietro, R.; Newman, C.; Facchetti, A.;

Sirringhaus, H., Very Low Degree of Energetic Disorder as the Origin of High Mobility in an n‐channel Polymer Semiconductor. Advanced Functional Materials 2011, 21 (17), 3371-3381. 6.

Holliday, S.; Donaghey, J. E.; McCulloch, I., Advances in Charge Carrier Mobilities

of Semiconducting Polymers used in Organic Transistors. Chemistry of Materials 2013,

26 (1), 647-663. 7.

Lee, W. H.; Cho, J. H.; Cho, K., Control of Mesoscale and Nanoscale Ordering of

Organic Semiconductors at the Gate Dielectric/Semiconductor Interface for Organic Transistors. Journal of Materials Chemistry 2010, 20 (13), 2549-2561. 8.

Kim, G.; Kang, S.-J.; Dutta, G. K.; Han, Y.-K.; Shin, T. J.; Noh, Y.-Y.; Yang, C., A

Thienoisoindigo-Naphthalene Polymer with Ultrahigh Mobility of 14.4 cm2/V· s that Substantially Exceeds Benchmark Values for Amorphous Silicon Semiconductors.

Journal of the American Chemical Society 2014, 136 (26), 9477-9483. 9.

Yamashita, Y.; Hinkel, F.; Marszalek, T.; Zajaczkowski, W.; Pisula, W.;

Baumgarten, M.; Matsui, H.; Müllen, K.; Takeya, J., Mobility Exceeding 10 cm2/(V· s) in


23


Page 24 of 28

Donor–Acceptor Polymer Transistors with Band-like Charge Transport. Chemistry of

Materials 2016, 28 (2), 420-424. 10.

Kim, H. G.; Kang, B.; Ko, H.; Lee, J.; Shin, J.; Cho, K., Synthetic Tailoring of Solid-

State Order in Diketopyrrolopyrrole-based Copolymers via Intramolecular Noncovalent Interactions. Chemistry of Materials 2015, 27 (3), 829-838. 11.

Qiu, M.; Zhu, D.; Yan, L.; Wang, N.; Han, L.; Bao, X.; Du, Z.; Niu, Y.; Yang, R.,

Strategy to Manipulate Molecular Orientation and Charge Mobility in D–A type Conjugated Polymer through Rational Fluorination for Improvements of Photovoltaic Performances.

The Journal of Physical Chemistry C 2016, 120 (40), 22757-22765. 12.

Nielsen, C. B.; Turbiez, M.; McCulloch, I., Recent Advances in the Development

of Semiconducting DPP‐Containing Polymers for Transistor Applications. Advanced

Materials 2013, 25 (13), 1859-1880. 13.

Song, H.; Deng, Y.; Gao, Y.; Jiang, Y.; Tian, H.; Yan, D.; Geng, Y.; Wang, F.,

Donor–Acceptor Conjugated Polymers Based on Indacenodithiophene Derivative Bridged

Diketopyrrolopyrroles:

Synthesis

and

Semiconducting

Properties.

Macromolecules 2017, 50 (6), 2344-2353. 14.

Stalder, R.; Mei, J.; Graham, K. R.; Estrada, L. A.; Reynolds, J. R., Isoindigo, a

Versatile Electron-Deficient Unit for High-Performance Organic Electronics. Chemistry of

Materials 2013, 26 (1), 664-678. 15.

Kim, F. S.; Guo, X.; Watson, M. D.; Jenekhe, S. A., High‐mobility Ambipolar

Transistors and High‐gain Inverters from a Donor–Acceptor Copolymer Semiconductor.

Advanced Materials 2010, 22 (4), 478-482. 16.

Guo, X.; Kim, F. S.; Seger, M. J.; Jenekhe, S. A.; Watson, M. D., Naphthalene

Diimide-based Polymer Semiconductors: Synthesis, Structure–Property Correlations, and N-channel and Ambipolar Field-Effect Transistors. Chemistry of Materials 2012, 24 (8), 1434-1442. 17.

Chen, Z.; Lee, M. J.; Shahid Ashraf, R.; Gu, Y.; Albert‐Seifried, S.; Meedom

Nielsen, M.; Schroeder, B.; Anthopoulos, T. D.; Heeney, M.; McCulloch, I., High‐Performance Ambipolar Diketopyrrolopyrrole‐Thieno [3, 2‐b] Thiophene Copolymer Field‐Effect Transistors with balanced Hole and Electron Mobilities. Advanced materials 2012, 24 (5), 647-652.


24

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


18.

Fei, Z.; Gao, X.; Smith, J.; Pattanasattayavong, P.; Buchaca Domingo, E.;

Stingelin, N.; Watkins, S. E.; Anthopoulos, T. D.; Kline, R. J.; Heeney, M., Near Infrared Absorbing Soluble Poly (cyclopenta [2, 1-b: 3, 4-b′] dithiophen-4-one) vinylene Polymers Exhibiting High Hole and Electron Mobilities in Ambient Air. Chemistry of Materials 2012,

25 (1), 59-68. 19.

Moral, M.; Garzon, A.; Garcia, G.; Granadino-Roldan, J. M.; Fernandez-Gomez,

M., DFT Study of the Ambipolar Character of Polymers on the Basis of s-Tetrazine and Aryl Rings. The Journal of Physical Chemistry C 2015, 119 (9), 4588-4599. 20.

Baeg, K.-J.; Kim, J.; Khim, D.; Caironi, M.; Kim, D.-Y.; You, I.-K.; Quinn, J. R.;

Facchetti, A.; Noh, Y.-Y., Charge Injection Engineering of Ambipolar Field-Effect Transistors for High-Performance Organic Complementary Circuits. ACS applied

materials & interfaces 2011, 3 (8), 3205-3214. 21.

Caironi, M.; Gili, E.; Sakanoue, T.; Cheng, X.; Sirringhaus, H., High yield, Single

Droplet Electrode Arrays for Nanoscale Printed Electronics. ACS nano 2010, 4 (3), 14511456. 22.

Kim, J. H.; Yun, S. W.; An, B. K.; Han, Y. D.; Yoon, S. J.; Joo, J.; Park, S. Y.,

Remarkable Mobility Increase and Threshold Voltage Reduction in Organic Field‐Effect Transistors by Overlaying Discontinuous Nano‐Patches of Charge‐Transfer Doping Layer on Top of Semiconducting Film. Advanced Materials 2013, 25 (5), 719-724. 23.

Lee, B. H.; Bazan, G. C.; Heeger, A. J., Doping‐Induced Carrier Density

Modulation in Polymer Field‐Effect Transistors. Advanced materials 2016, 28 (1), 57-62. 24.

Zaumseil, J.; Sirringhaus, H., Electron and Ambipolar Transport in Organic Field-

Effect Transistors. Chemical reviews 2007, 107 (4), 1296-1323. 25.

Yan, H.; Chen, Z.; Zheng, Y.; Newman, C.; Quinn, J. R.; Dötz, F.; Kastler, M.;

Facchetti, A., A High-Mobility Electron-Transporting Polymer for Printed Transistors.

Nature 2009, 457 (7230), 679. 26.

Zhao, N.; Noh, Y. Y.; Chang, J. F.; Heeney, M.; McCulloch, I.; Sirringhaus, H.,

Polaron Localization at Interfaces in High‐Mobility Microcrystalline Conjugated Polymers.

Advanced Materials 2009, 21 (37), 3759-3763.


25


27.

Page 26 of 28

Weil, J. A.; Bolton, J. R., Electron Paramagnetic Resonance: Elementary Theory

and Practical Applications. John Wiley & Sons: 2007. 28.

Long, D. X.; Choi, E.-Y.; Noh, Y.-Y., Manganese Oxide Nanoparticle as a New p-

Type Dopant for High-Performance Polymer Field-Effect Transistors. ACS applied

materials & interfaces 2017, 9 (29), 24763-24770. 29.

Baeg, K.-J.; Khim, D.; Kim, J.; Han, H.; Jung, S.-W.; Kim, T.-W.; Kang, M.;

Facchetti, A.; Hong, S.-K.; Kim, D.-Y., Controlled Charge Transport by Polymer Blend Dielectrics in Top-gate Organic Field-Effect Transistors for Low-Voltage-Operating Complementary Circuits. ACS applied materials & interfaces 2012, 4 (11), 6176-6184. 30.

Lee, W. H.; Suk, J. W.; Lee, J.; Hao, Y.; Park, J.; Yang, J. W.; Ha, H.-W.; Murali,

S.; Chou, H.; Akinwande, D., Simultaneous Transfer and Doping of CVD-grown Graphene by Fluoropolymer for Transparent Conductive Films on Plastic. Acs Nano 2012, 6 (2), 1284-1290. 31.

Chen, W.; Gao, X. Y.; Qi, D. C.; Chen, S.; Chen, Z. K.; Wee, A. T. S.,

Surface‐Transfer

Doping

of

Organic

Semiconductors

Using

Functionalized

Self‐Assembled Monolayers. Advanced Functional Materials 2007, 17 (8), 1339-1344. 32.

Letizia, J. A.; Rivnay, J.; Facchetti, A.; Ratner, M. A.; Marks, T. J., Variable

Temperature Mobility Analysis of n‐Channel, p‐Channel, and Ambipolar Organic Field‐Effect Transistors. Advanced Functional Materials 2010, 20 (1), 50-58. 33.

Noriega, R.; Rivnay, J.; Vandewal, K.; Koch, F. P.; Stingelin, N.; Smith, P.; Toney,

M. F.; Salleo, A., A General Relationship between Disorder, Aggregation and Charge Transport in Conjugated Polymers. Nature materials 2013, 12 (11), 1038. 34.

Venkateshvaran, D.; Nikolka, M.; Sadhanala, A.; Lemaur, V.; Zelazny, M.; Kepa,

M.; Hurhangee, M.; Kronemeijer, A. J.; Pecunia, V.; Nasrallah, I., Approaching DisorderFree Transport in High-Mobility Conjugated Polymers. Nature 2014, 515 (7527), 384. 35.

Xu, Y.; Sun, H.; Li, W.; Lin, Y. F.; Balestra, F.; Ghibaudo, G.; Noh, Y. Y., Exploring

the Charge Transport in Conjugated Polymers. Advanced Materials 2017, 29 (41). 36.

Brondijk, J.; Roelofs, W.; Mathijssen, S.; Shehu, A.; Cramer, T.; Biscarini, F.; Blom,

P.; De Leeuw, D., Two-Dimensional Charge Transport in Disordered Organic Semiconductors. Physical review letters 2012, 109 (5), 056601.


26

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


37.

Kronemeijer, A. J.; Pecunia, V.; Venkateshvaran, D.; Nikolka, M.; Sadhanala, A.;

Moriarty, J.; Szumilo, M.; Sirringhaus, H., Two‐Dimensional Carrier Distribution in Top‐Gate Polymer Field‐Effect Transistors: Correlation between Width of Density of Localized States and Urbach Energy. Advanced Materials 2014, 26 (5), 728-733.

SYNOPSIS TOC


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