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Facile Route to Control the Ambipolar Transport in Semiconducting Polymers Dongyoon Khim, Ye Rim Cheon, Yong Xu, Won-Tae Park, Soon-Ki Kwon, Yong-Young Noh, and Yun-Hi Kim Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b00298 • Publication Date (Web): 18 Mar 2016 Downloaded from http://pubs.acs.org on March 19, 2016

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Chemistry of Materials

Facile Route to Control the Ambipolar Transport in Semiconducting Polymers Dongyoon Khim,† Ye Rim Cheon,§ Yong Xu,† Won-Tae Park,† Soon-Ki Kwon,# YongYoung Noh,†* and Yun-Hi Kim§* §

Department of Chemistry, Gyeongsang National University and Research Institute of for Green Energy Convergence Technology (RIGET), Jinju 660-701, Republic of Korea.

†

Department of Energy and Materials Engineering, Dongguk University, 26 Pil-dong, 3-ga, Jung-gu, Seoul 100-715, Republic of Korea.

#

School of Materials Science and Engineering & Research Institute for Green Energy Convergence Technology (REGET), Gyeongsang National University, Jinju 600-701, Republic of Korea

Keywords: Ambiplar charge transport, conjugated polymers, organic field-effect transistors, copolymer, donor/acceptor building block

ABSTRACT Control of electron and hole transport in conjugated molecules is a challenging but essential task for deeply understanding the intrinsic charge transport behaviors as well as technological benefits for optimising the performance of various opto-electronic devices. Here we suggest a facile route to controlling ambipolar charge transport in conjugated polymers by precise regulation of the copolymerisation ratio between a relatively large electron donor and acceptor building block as a repeating unit. By varying the ratio between poly[2,5-bis(2octyldodecyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione-(E)-[2,2-bithiophen]-5-yl)-3-(thiophen2-yl)acrylonitrile] (DPP-CNTVT) as an electron transport unit and DPP-selenophenevinylene-selenophene (DPP-SVS) as a hole transport unit, mobility (µFET) and onset voltage (Von) in organic field-effect transistors are effectively modulated from p-channel [µFET,h = 6.23 ± 0.4 cm2 V-1 s-1] to n-channel [µFET,e = 6.88 ± 1.01 cm2 V-1 s-1] dominant transport. The same two DPP-based building blocks can lead not only to precise controllability of the 1 ACS Paragon Plus Environment


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transport mode but also significantly increased mobility without distortion of polymer backbone co-planarity. We also investigate bias stability of hole and electron in ambipolar transistors. Our methodology provides a new strategy for developing high-performance ambipolar polymer semiconductors for application in printed flexible integrated circuits and light-emitting transistors.

INTRODUCTION Recently, the rapid progress in the organic electronics industry has led to the resurgence of old dyes and pigments, such as diketopyrrolopyrrole (DPP) and isoindigo (IID), via inserting them into conjugated polymers as organic semiconducting materials.1-5 This dramatic revitalisation is mainly driven by their excellent opto-electric and electric properties.1-3 In particular, fastidious design of molecular structures by randomly combining electron donor and acceptor (D-A) building blocks and side chains can generate new polymeric structures with controlled charge transport properties and solubility for application in printed organic field-effect transistors (OFETs). There have been many attempts to control the charge transport of conjugated molecules by adjusting the D-A system,1-8 but this has a fundamental problem predicting charge transport properties precisely in semiconductors to achieve well-balanced hole and electron mobility with a high value. This is because it does not yet quantitatively predict the hole and electron transport ability of commonly used D-A units. In addition, the D-A polymer should maintain a high co-planarity and a proper energy level for efficient charge transport and injection.9 Therefore, most ambipolar transporting polymers would be synthesised by means of trial and error experiments at the present technology level. Here we introduce a general and simple chemical methodology to control ambipolar charge transport properties by precise control of the copolymerisation ratio between the 2 ACS Paragon Plus Environment

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electron

transport

semiconductor

poly[2,5-bis(2-octyldodecyl)pyrrolo[3,4-c]pyrrole-

1,4(2H,5H)-dione-(E)-[2,2-bithiophen]-5-yl)-3-(thiophen-2-yl)acrylonitrile]

(DPP-CNTVT)

and the hole transport semiconductor DPP-selenophene-vinylene-selenophene (DPP-SVS), which for convenience’s sake we refer to in this paper as CNTVT and SVS, respectively. Importantly, these random copolymers are not made by combination of a small functional unit as a unit cell, such as DPP, thiophenes, isoindigo and naphthalenedicarboximide, but use a relatively large unit of DPP-CNTVT and DPP-SVS as the n- and p-type building blocks, respectively. This approach with the DPP-based large building blocks can precisely tune electron and hole transport, and enable significantly high charge carrier mobility to be achieved at the same time by keeping a high co-planarity and better packing motif. By varying the ratio of copolymer CNTVT (n-type):SVS (p-type) from 1:9 to 3:7, 5:5, 7:3, and 9:1, a type of majority carrier was gradually converted from hole to electron, and the maximum hole (µFET,h) and electron mobility (µFET,e) showed as high as 6.23 ± 0.4 cm2 V-1 s-1 at CNTVT:SVS (1:9) to 6.88 ± 1.01 cm2 V-1 s-1 at CNTVT:SVS (9:1) in the top-gate bottomcontact (TG/BC) OFETs, respectively. In addition, the well-balanced µFET,h and µFET,e of 3.15 ± 0.2 cm2 V-1 s-1 and 3.03 ± 0.15 cm2 V-1 s-1 were achieved at CNTVT:SVS (5:5).

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RESULTS AND DISCUSSION

Figure 1. Chemical structure of copolymer CNTVT:SVS. Five series of CNTVT(m):SVS(n) = (1:9, 3:7, 5:5, 7:3, and 9:1) by precise regulation of the copolymerisation ratio between oligomer-sized electron acceptor (DPP-CNTVT) and donor (DPP-SVS) as a repeating block.

Optical, Thermal, and Electrochemical Properties The synthetic scheme for the random copolymers is displayed in Scheme S1 of Supporting Information. The composition of the random copolymer was controlled by adjusting mole ratio of CNTVT to SVS (Figure 1). The chemical structure of the polymers was confirmed by H-NMR (Figure S1-5), and IR spectra (Figure S6), and elemental analyses (see Materials and Methods section in SI). The copolymers showed excellent solubility enough to process thin film device as active layer in common organic solvents, such as chloroform and chlorobenzene, and even in non-chlorinated and environmentally friendly solvents, such as tetrahydrofuran (THF) and tetralin. The excellent solubility could be explained by increasing monomer irregularity in the random copolymer as well as long alkyl substituent even though the polymers maintain highly coplanar backbone structures.10


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Table 1. Summary of optical and electronic chemical properties for CNTVT:SVS copolymers. Band gap [eV]

λmax [nm] Solution [nm]

Film (r. t.) [nm]

Film (200 °C) [nm]

Optical

CV

CNTVT:SVS (1:9)

809

484, 774, 817

489, 772, 818

1.26

1.92

CNTVT:SVS (3:7)

466, 777, 824

476, 768, 829

476, 767, 827

1.28

1.86

CNTVT:SVS (5:5)

467, 779, 821

475, 769, 829

474, 770, 829

1.29

1.84

CNTVT:SVS (7:3)

465, 769, 816

472, 771, 825

466, 760, 825

1.31

1.82

CNTVT:SVS (9:1)

471,761, 818

477, 759, 828

474, 759, 830

1.31

1.81

Semiconductor

LUMOUPS (LUMOCV) [eV]

HOMOUPS (HOMOCV) [eV]

Mn [KDa] (PDI)

-4.19 (-3.48) -4.32 (-3.50) -4.30 (-3.52) -4.48 (-3.55) -4.51 (-3.56)

-5.45 (-5.40) -5.60 (-5.36) -5.59 (-5.36) -5.79 (-5.37) -5.82 (-5.37)

203 (1.74) 240 (1.58) 226 (1.62) 160 (1.79) 149 (1.64)

The optical and electronic chemical properties for the CNTVT:SVS polymers studied are listed here. The bandgap for each material was determined from the linear absorption spectra (Figure 2c) and cyclic voltammetry (CV) (Figure S10). The highest occupied molecular orbital (HOMO) was determined by means of CV and ultraviolet photoelectron spectroscopy (UPS) measurement (Figure S11).

The optical properties of the polymers were measured in chloroform solution. All polymers showed similar π–π* transitions and intramolecular charge transfer bands at around 460 nm and 820 nm, regardless of the different mole ratio of the building blocks. The optical band gap was gradually increased from 1.26 eV for CNTVT:SVS (1:9) to 1.31 eV for CNTVT:SVS (9:1) with increasing mole ratio of CNTVT. The UV-vis absorption bands of the films were slightly broadened and showed distinct dual bands compared with those of solutions (Figure 2a-c and Figure S10). However, no evidence of the absorption spectra change could be observed after the film annealed at 200 oC (Figure S10). Cyclic voltammetry (Figure S11) and ultraviolet photoelectron spectroscopy (Figure S12) were carried out to investigate the lowest unoccupied molecule orbital (LUMO) and the highest occupied molecule orbital (HOMO) energetic level of all copolymers, which are summarized in Figure 2a and Table 1. We mainly used ultraviolet photoelectron spectroscopy data measured as thin film form owing to its better accuracy to predict charge injection properties in OFETs. The LUMO level gradually increased with increasing mole ratio of CNTVT from CNTVT:SVS



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(1:9) [-4.19 eV] to (9:1) [-4.51 eV] owing to the strong electron withdrawing property of the nitrile (C≡N) group in TVT, expecting a favourable electron injection from the Au electrode.

Film Morphology and Crystallinity To understand the electronic structure and transitions, and molecular geometry of the copolymers, density function theory calculations were conducted at the B3LYP/6-31G level.11 We constructed four repeating units and ten modelling molecules by varying the position and mole ratio of CNTVT and SVS units (Figure S13 and Table S2 in SI). The HOMO and LUMO orbitals were mainly located on the DPP-SVS and DPP-CNTVT units, respectively. Both HOMO and LUMO energy levels gradually decreased with increasing mole ratio of CNTVT, indicating the controllability of hole and electron balance in the polymer. The surface morphologies of all copolymer films annealed at 310 °C for 20 min were observed by atomic force microscope. The atomic force microscope images with height mode and the corresponding root means square roughness of those films are shown in Figure 2d and Figure S14. No evidence of morphological change could be observed in atomic force microscope images, but all thin films showed a very smooth surface with a root means square roughness of 1–2 nm, indicating potentially favourable morphology for charge transport in TG transistors, where charge transport occurs at the top surface of the semiconductor layer. We carried out 2D grazing incidence X-ray diffraction (2D-GIXD) studies to investigate the molecular ordering and crystalline characteristics in the microstructures of all polymers. The 2D-GIXD patterns and the corresponding 1D profiles are shown in Figure 2e–g, and extracted crystallographic parameters are summarized in Table S3. The (h00) and (010) diffraction peaks were predominantly observed along the out-of-plane (qz) and in-plane (qxy) directions, which corresponded with (100) lamella d-spacing of 27.3 Å and (010) π–π 6 ACS Paragon Plus Environment

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stacking distance of 3.63–3.76 Å, respectively. This indicated that all polymers showed longrange ordering with strong edge-on and weak face-on orientation relative to the substrate. Notably, these π–π stacking values are very close to the d-spacing of the single DPP-SVS polymer, indicating that the microstructure of DPP-based copolymers would be predominantly influenced by the backbone structure and alkyl chain length rather than the mole ratio of CNTVT and SVS.10

Figure 2. Optophysical properties, morphologies, and crystallinities of CNTVT:SVS thin films. (a) Energy level diagram of the CNTVT:SVS polymers. UV-vis absorption spectra of polymer (b) in chloroform solution and (c) thin films. (d) Height mode atomic force microscope (AFM) images of



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CNTVT:SVS (1:9, 5:5, and 9:1) thin films. (e) 2D-GIXD images of CNTVT:SVS thin films and (f) corresponding 1D-GIXD profiles in out-of-plane and in-plane directions (please see supplementary information for more information).

Charge Transport Characteristics of FETs To explore the charge transport properties of the series of CNTVT:SVS copolymers, we fabricated TG/BC FETs with poly(methyl-methacrylate) (PMMA) gate dielectric (t ≈ 500 nm). The detailed fabrication procedures are described in the experimental section. Figure 3a–b shows the transfer characteristics of OFET devices based on the series of CNTVT:SVS copolymers at Vd = -80 V and Vd = +90 V for p- and n-channel operations, respectively, and all electrical parameters are summarized in Table 2 (see Figure S16-17 in SI). As expected, hysteresis-free ambipolar transport is observed for all devices. The ambipolarity of each polymer is precisely modulated by mole ratio between DPP-CNTVT (n-type) and DPP-SVS (p-type). By varying the ratio of copolymer from 1:9 to 3:7, 5:5, 7:3, and 9:1, the predominant charge carrier is gradually converted from hole to electron. The µFET,h is 6.23 ± 0.4 cm2 V-1 s-1 at a CNTVT:SVS ratio of 1:9 and gradually decreases with increasing DPPCNTVT as the electron accepting unit, and drops to 0.60 ± 0.28 cm2 V-1 s-1 at a ratio of 9:1. The µFET,e is 0.077 ± 0.02 cm2 V-1 s-1 at a CNTVT:SVS ration of 1:9, and increases with reducing DPP-SVS as the electron donating unit, and eventually exhibits as high as 6.88 ± 1.01 cm2 V-1 s-1 at a CNTVT:SVS ration of 9:1. Note that the dominant electron and hole mobilities after copolymerisation to CNTVT:SVS (9:1) and (1:9) are comparable with the uniform DPP-CNTVT and DPP-SVS reported, respectively.12-13 The well balanced µFET,h [3.15 ± 0.2 cm2 V-1 s-1] and µFET,e [3.03 ± 0.15 cm2 V-1 s-1] is obtained at a CNTVT:SVS ratio of 5:5. The factor defined as µFET,h/µFET,e gradually decreases from 80.9 at a CNTVT:SVS ration of 1:9 to 0.087 at a CNTVT:SVS ratio of 9:1). Importantly, as the mole ratio of CNTVT increases, the onset voltage (Von) is elevated from -17.0 V (1:9) to -49.2 V (9:1) and 8 ACS Paragon Plus Environment

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is reduced from 71.0 V (1:9) to 35.6 V (9:1) for p- and n-channel operation regimes, respectively. In ambipolar transistor, evolution of Von is referred as relative change of hole and electron charge density.14-15 This indicates that the accumulated mobile hole and electron densities in the transistor channel are changed accordingly with the mole ratio. Such a change in mobile carrier concentration can be calculated as ∆n = ∆Von × Ci/q, where ∆Von is the shift of Von, Ci is the gate dielectric capacitance per unit area, and q is the elementary charge. The calculated ∆n is summarised in Table 2.

Figure 3. Electrical characteristics of CNTVT:SVS FETs. (a), (b) Transfer characteristics of TG/BC CNTVT:SVS (1:9, 3:7, 5:5, 7:3, and 9:1) FETs with PMMA gate dielectrics. (c) The evolution of hole and electron mobilities of CNTVT:SVS OFETs as a function of the ratio between the CNTVT and SVS moieties. (d) The polarity balance defined as the ratio of hole/electron mobility. (e) The evolution of



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Von of p-channel and n-channel CNTVT:SVS OFETs as a function of the ratio between the CNTVT and SVS moieties. (f) Output characteristics of p-channel of CNTVT:SVS (1:9) and n-channel of CNTVT:SVS (9:1) FETs. (g) Transfer and (h) output characteristics of low-operating voltage CNTVT:SVS (1:9) FET with high-k P(VDF-TrFE-CTFE) dielectric.

Table 2. Fundamental electrical parameters of TG/BC OFETs based on CNTVT:SVS copolymers with -2 PMMA [Ci = 6.2 nF cm ] gate dielectrics. The hole and electron mobilities were evaluated in the saturation region at Vd = -80 V and 90 V, respectively, using gradual approximation equations [W/L = 1.0 mm/30 µm]. Semiconductor CNTVT:SVS (1:9) CNTVT:SVS (3:7) CNTVT:SVS (5:5) CNTVT:SVS (7:3) CNTVT:SVS (9:1)

Ambipolarity [µFET,h / µFET,e] 80.9 2.94 1.03 0.14 0.087

µFET,h [cm2/Vs]

VTh,h [V]

6.23 (± 0.4) 3.77 (± 0.23) 3.15 (± 0.2) 0.84 (± 0.087) 0.60 (± 0.28)

-45.8 (± 1.4) -47.2 (± 1.8) -49.7 (± 1.4) -47.3 (± 0.98) -45.5 (± 1.80)

Von,h [V]

∆nh (1:9 to x) [1013 cm2 ]

-17.5

N/A

-28.7

-4.25

-32.7

-5.75

-42.9

-9.82

-49.2

-12.25

µFET,e [cm2/Vs]

VTh,e [V]

0.077 (± 0.02) 1.28 (± 0.21) 3.03 (± 0.15) 5.7 (± 0.8) 6.88 (± 1.01)

51.9 (± 1.47) 55.5 (± 0.42) 61.9 (± 1.47) 64.3 (± 2.53) 57.0 (± 5.3)

Von,e [V]

∆ne (1:9 to x) [1013 cm2 ]

71.0

N/A

48.7

+8.65

48.3

+8.79

42.4

+11.07

35.6

+12.54

Calculated mobilities are average values from over 10 devices.

The accumulated mobile hole density is decreased by a value of ∆nhole = - 12.25 × 1013 cm-2 and the electron density is increased as ∆nelectron = + 12.54 × 1013 cm-2 for CNTVT:SVS ratios varying from 1:9 to 9:1. The CNTVT:SVS OFETs exhibited solvent and thermal annealing temperature-dependent device characteristics (see Figure S18-24 and Table S4-9), which is presumably attributed to the improved crystallinity of DPP-based polymer films by solvent or thermal effect.5 We have further demonstrated OFETs for low-voltage operation, which demands a strong capacitive coupling between the semiconductor and gate electrode. To this end, P(VDF-TrFE-CTFE) was used as high-k polymer dielectric.16 Figure 3g and h (also Figure S25 and Table S10 in SI) display a representative set of transfer and output characteristics, measured from a TG/BC CNTVT:SVS (1:9) transistor (L = 30 µm, W = 1.0 mm). The device shows promising characteristics, enabling high µFET,h of ~3.0 ± 0.2 cm2 V-1 s-1 and low threshold voltage of ~1.2 V to be achieved for a linear regime at low Vd = -3 V.


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Figure 4. Low-temperature measurement of CNTVT:SVS FETs. (a) Electron mobility extracted in linear regime (at Vd = 5 V) versus temperature for three different mole ratios. (b) Activation energy extracted from Arrhenius plots for electron and hole mobilities. (c) Schematic of electron transport profile at CNTVT:SVS ratios of 5:5 and 9:1.

To better comprehend the mobility variation caused by modulating the copolymerisation ratio, we performed low-temperature measurements. The mobility exhibits thermal activation behaviours from which the activation energy (EA) is estimated (Figure 4a-b). A puzzle appears as to why the EA for n-channel transport is even raised as the CNTVT:SVS ratio increases from 5:5 to 9:1, likewise for p-channel, since one may simply expect a better transport profile for higher mobility and thus with lower EA. We then implemented mobility calculations17 considering three factors: (1) density of states (DOS), (2) localized and delocalized states hybridization, and (3) DOS broadening. More details are included in Figure S26-30 and Table S11 of SI. The calculations reveal that different ratios of D-A units in the copolymer considerably change the DOS’s value, composition and distribution. At a CNTVT:SVS ratio of 9:1 for instance, the DOS in the LUMO contains a higher density of accessible states with lower energetic disorders for better electron transport (Figure 4c). As the charge transport in disordered organic semiconductors takes place mainly by hopping from site to site,18-19 the holes and electrons have to find their respective ways to transport in the channel, namely hopping efficiency, which is affected by the ratio of n- and p-type units in the copolymer. Another reason for mobility variation is the charge injection. We evaluated the contact resistance (RC) was evaluated by the method reported by Wang et al.,20 where the 11 ACS Paragon Plus Environment


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output characteristics and the transfer characteristics in saturation regime were applied to the extractions of the threshold voltage (VT), the saturation current at a specific gate voltage (ID,sat), and the transition voltage (Vtr). The contact resistance is calculated as:

ܴ஼ ≈ 2ܴௌ = 2

ඥܸீ − ்ܸ (ඥ2ܸ௧௥ − ඥܸீ − ்ܸ ) ‫ܫ‬஽,ௌ௔௧

As RC is gate-voltage dependent, we took only a constant gate voltage VG = 50 V or -50 V to compare the contact resistance’s evolution at various mole ratios for the sake of simplicity. The evaluated parameters are all listed in Table 3. We found it differs largely (see Table 3) owing to the equally influenced access transport profile as the channel transport, and altered injection barrier arising from the gradual shift of LUMO and HOMO levels (see Figure 2a), which regulate bulk and interface charge injection, respectively. It means that the diverse copolymerisation ratios not only modulate the channel transport but the charge injection as well, resulting in changed mobilities, as observed.9,21-22

Table 3. Parameters related to the contact resistance extraction. CNTVT:SVS VT (V) Vtr (V) IDsat (µA) RC (kΩ.cm)

1:9 -36.2 -30 63.8 46.9

p-Channel (at VG = −50 V) 3:5 5:5 7:3 -36.2 -33 -37.3 -29 -38 -27 44.3 34.5 18.0 65.4 109.7 149.4

9:1 -38.9 -22 6.4 342.5

1:9 49.9 19 0.9 400.7

n-Channel (at VG = 50 V) 3:5 5:5 7:3 49 46.4 49.2 20 22 21 4.1 8.8 13.4 259.0 202.5 74.5


9:1 46.8 24 38.4 47.9

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Figure 5 ǀ Bias and Bending Stability of CNTVT:SVS FETs. (a) Bias stress test of CNTVT:SVS (1:9 and 9:1) FETs with PMMA dielectric. The normalised current decay for CNTVT:SVS (1:9 and 9:1) devices as a function of time under the constant biases of Vg = -60 V and Vd = -5 V for p-channel and Vg = 60 V and Vd = 5 V for n-channel, respectively. Photographs of (b) flexible CNTVT:SVS FETs based on PEN substrate and bending test. (c) Transfer characteristics of CNTVT:SVS FET based on PEN substrate during repeated bending cycles at a bending radius of 6.0 mm.

Stability of Ambipolar FETs To verify device stability, constant-current bias stress were implemented on CNTVT:SVS (1:9) and CNTVT:SVS (9:1) OFETs, for p-channel and n-channel regime, respectively, See Figure 5a and Figure S31 in SI. The stability was monitored by measuring the output drain current as a function of time. Both devices showed high bias stability, and we found an interesting relationship between the dominant charge transport and the corresponding bias stability. CNTVT:SVS (1:9) devices, in which hole transport dominates, show higher stability at negative gate voltages (for hole accumulation) compared with positive gate voltages (for electron accumulation), while CNTVT:SVS (9:1) devices where electron transport prevails show superior stability at positive gate voltage compared to negative gate voltage. We also found a similar tendency in bias stability using other ambipolar semiconductors such as poly[[2,5-bis(2-octyldodecyl)-2,3,5,6-tetrahydro-3,6dioxopyrrolo[3,4-c]pyrrole-1,4-diyl]-alt-[[2,2'-(2,5-thiophene)bis-thieno(3,2-b)thiophene]5,5'-diyl]] (DPPT-TT) as p-dominant semiconductor and poly([N,N'-bis(2-octyldodecyl)naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)) (P(NDI2OD-T2)) 13 ACS Paragon Plus Environment


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as n-dominant semiconductor. (see Figure S32 in SI). It turned out that the minority carriers are generally more susceptible to trap states because their concentration at thermal equilibrium is relatively much lower than that of majority carriers, for the reason that NaNd=Ni2, where Na and Nd are the concentrations of acceptor and donor, respectively. Ni is the intrinsic carrier concentration and is constant at a fixed temperature, so that more majority carriers (e.g., Na) will accordingly reduce minority ones, which is Nd. In p-channel dominant semiconductors such as CNTVT:SVS (1:9) and DPPT-TT, it is obvious that electrostatically induced hole concentration by negative gate biasing for p-channel is much higher than electrostatically induced electron by positive gate bias for n-channel. Assuming identical densities for hole- and electron-traps in a p-dominant semiconductor, the more pronounced hole accumulation could fill the hole-traps more effectively, and likewise for n-dominant semiconductor. This trend is more significant in the semiconductors with severely unbalanced ambipolarity such as P(NDI2OD-T2) against those semiconductors having well-balanced ambipolar transport properties, such as CNTVT:SVS. So, the charge carrier concentration is a crucial factor for determining the stability of OFETs operating in different transport regimes. In addition, electron as the minority carrier in CNTVT:SVS (1:9) corresponds to a rapid decrease of drain current under continued bias stress, also signifying a higher density of electron traps distributing over a wider energy range within the energy gap, probably resulted from humid oxygen23,24 The bending stability of the flexible transistors on polyethylene naphthalene (PEN) substrate was evaluated by mechanical stress test. The bending test was carried out using a home-built bending machine (Figure 5b). The OFETs on polyethylene naphthalene substrate [thickness (D) = 0.125 mm] were repetitively bent with a bending radius (Rc) of 6.0 mm; a strain (ε) of 1.04% can be calculated using the equation ε = D/2Rc. After 3000 bending cycles, the transfer characteristics of OFETs remained uncompromised with respect to those of the as-fabricated devices, but the on- and off-current level slightly 14 ACS Paragon Plus Environment

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increased after over 5000 cycles (Figure 5c and Figure S33). This presumably resulted from exposure to air during the long bending test.

Figure 6. Ambipolar Inverter Characteristics. (a) Voltage transfer curves and (b) corresponding voltage gain of ambipolar CNTVT:SVS (1:9. 3:7, 5:5, 7:3, and 9:1) inverters. (c) Inverting voltages of ambipolar inverters at Vdd = 60V. (d) Drain current level of CNTVT:SVS OFETs at Vg = -60 V, Vd = -60 V for p-channel and Vg = 60 V, Vd = 60 V for n-channel, respectively.

Ambipolar Inverter Characteristics We applied the polymer transistor as the active layer in a logic NOT gate, which consists of two identical ambipolar transistors. Figure 6a and b show the voltage transfer characteristics and voltage gain for CNTVT:SVS inverters. More detailed information about the inverter characteristics is included in Figure S34. It is noted that the inverting voltages (Vinv) were precisely controlled depending on the mole ratio of the two building blocks, while the values of voltage gain were as high as 20–30. For ideal inverter device, Vinv is a half of the supply voltage (Vdd). However, most of the ambipolar inverters showed a shift of Vinv 15 ACS Paragon Plus Environment


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from the ideal owing to unbalanced mobility. By increasing the SVS ratio from CNTVT:SVS 1:9 to 9:1, the Vinv shifts to a lower input voltage (Vin), as shown in Figure 6c. Interestingly, even though balanced hole and electron mobility (µhole/µelectron ≈ 1.04) was achieved with the CNTVT:SVS (5:5) device, the CNTVT:SVS (7:3) inverter showed a more ideal Vinv locating at 1/2 Vdd. This is owing to more balanced hole and electron saturation current levels with the CNTVT:SVS (7:3) transistor than others (Figure 6d).9,25

CONCLUSIONS In summary, we report a facile route to control ambipolar charge transport in conjugated polymers by precise regulation of the copolymerisation ratio between oligomersized electron acceptor and donor as a repeating block. By varying the ratio between two large building blocks, µFET and Von are effectively modulated from p-channel to n-channel dominant transport. In addition, the same DPP-based backbone of the two building blocks can lead to facilitating high co-planarity and better packing motif, enabling high mobility. Our methodology provides a new strategy for synthesising optimised materials by assembling two different materials while maintaining the innate properties of each polymer, such as band structures, enabling future solution-processed, flexible, lightweight electronics with low manufacturing costs based on conjugated polymer materials.

EXPERIMENTAL SECTION Device Fabrication. Corning Eagle glass and PEN as substrates were cleaned in an ultrasonic bath with deionised water, acetone and isopropanol for 10 min each. Au/Ni (15/3 nm) source and drain (S/D) were patterned onto substrates by conventional lift-off photolithography methods. CNTVT:SVS copolymers were dissolved in anhydrous trichlorobenzene, chlorobenzene, p-xylene and tetralin to make a semiconducting ink of 5 16 ACS Paragon Plus Environment

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mg/ml, which was heated at 80 °C overnight. The S/D patterned substrates were modified by oxygen plasma treatment for improving adhesion and then semiconductor inks were spincoated. The thin films were thermally annealed at 110, 150, 200, 250 and 310 °C for 20 min in a nitrogen-filled glovebox. For the PEN devices, thin films were annealed under 200 °C to avoid thermal degradation of the PEN substrate. For gate dielectric layers, PMMA (SigmaAldrich, Mw = 120 kD) and P(VDF-TrFE-CTFE) (Solvay) were dissolved in n-butyl acetate and 2-butanone, respectively. After spin-coating of the dielectric layers on top of the semiconducting films, they were baked at 80 °C for more than 30 min. The devices were finally completed by deposition of Al using a metal shadow mask as the gate electrode. Inverter devices were fabricated using the same procedures. Materials and Methods. See Supporting Information. Thin Film Characterisation: The surface morphologies and microstructures of the polymer thin films were investigated using tapping-mode atomic force microscopy (Nanoscope, Veeco Instrument, Inc.) at the Korea Basic Science Institute and 2D GIXD measurements from the Pohang Accelerator Laboratory (PAL). UV-Vis absorption spectra were measured using a UV/VIS/NIR spectrophotometer (Lambda 750, Perkin-Elmer). The surface profilers were measured using a surface profiler (Ambios, XP-1). All photoemission measurements were carried out in a PHI-5000 ultrahigh vacuum surface analysis system equipped with a He-discharge lamp (21.22 eV) and a monochromatic Al kα X-ray. All spectra were measured at a pressure of 1 × 10−6 Pa. The semiconducting layers were spin-coated Au deposited on glass and then annealed at 310oC as same condition of OFET process. Device Measurement and Characterisation. The electrical characteristics of the transistors and inverters were measured using a Keithley 4200-SCS instrument in a nitrogen-filled glove box. The µFET and VTh values were calculated at the saturation region (Vd = - 80 V for pchannel and Vd = 90 V for n-channel) using the gradual channel approximation equation. The 17 ACS Paragon Plus Environment


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capacitance-voltage characteristics were measured using an Agilent 4284A precision LCR meter and a Keithley 4200-SCS. ASSICIATED CONTENT Supporting Information Available: Detailed experimental procedures and characterizations for copolymers as well as additional figures. This materials is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author [email protected] (yk), [email protected] (yn) Author Contributions D. Khim and R. Cheon contributed equally to this work. Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENT This work was supported by the Center for Advanced Soft-Electronics funded by the Ministry of Science, ICT and Future Planning as a Global Frontier Project (2013M3A6A5073183 and 2013M3A6A5073172).

REFERENCES (1) Nielsen, C. B.; Turbiez, M.; McCulloch, I. Recent Advances in the Development of Semiconducting DPP-Containing Polymers for Transistor Applications. Adv. Mater. 2013, 25, 1859-1880.


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