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Precise Side-chain Engineering of ThienylenevinyleneBenzotriazole based Conjugated Polymers with Coplanar Backbone for Organic Field Effect Transistors and CMOS-like Inverters Min-Hye Lee, Juhwan Kim, Minji Kang, Jihong Kim, Boseok Kang, Hansu Hwang, Kilwon Cho, and Dong-Yu Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b14701 • Publication Date (Web): 22 Dec 2016 Downloaded from http://pubs.acs.org on January 6, 2017

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ACS Applied Materials & Interfaces

Precise Side-chain Engineering of ThienylenevinyleneBenzotriazole based Conjugated Polymers with Coplanar Backbone for Organic Field Effect Transistors and CMOS-like Inverters Min-Hye Lee,† Juhwan Kim,‡ Minji Kang,† Jihong Kim,† Boseok Kang,∥ Hansu Hwang,† Kilwon Cho,∥ and Dong-Yu Kim*,†

†

School of Materials Science and Engineering, Gwangju Institute of Science and Technology, 123,

Cheomdangwagi-ro, Buk-gu, Gwangju, 61005, Republic of Korea ‡

Department of Chemical Engineering and Materials Science, University of California, Irvine, Irvine,

California, 92697, United States ∥

Department of Chemical Engineering, Pohang University of Science and Technology, 77, Cheongam-

ro, Nam-gu, Pohang, Gyeongbuk, 37673, Republic of Korea

KEYWORDS Conjugated Polymer, Organic Field Effect Transistor, Coplanar Backbone, CMOS-like Inverter, Side-chain Engineering, Donor-Acceptor Configuration

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ABSTRACT

In this article, two donor−acceptor (D−A) alternating conjugated polymers based on thienylenevinylene-benzotriazole (TV-BTz), PTV6B with a linear side chain and PTVEhB with a branched side chain, were synthesized and characterized for organic field effect transistors (OFETs) and Complementary metal–oxide–semiconductor (CMOS)-like inverters. According to density functional theory (DFT), polymers based on TV-BTz exhibit a coplanar and rigid structure with no significant twists, which could cause to an increase in charge-carrier mobility in OFETs. Alternating alkyl side chains of the polymers impacted neither the band gap nor the energy level. However, it significantly affected on the morphology and crystallinity when the polymer films were thermally annealed. To investigate the effect of thermal annealing on the morphology and crystallinity, we characterized the polymer films using atomic force microscopy (AFM) and 2D-grazing incidence X-ray diffraction (2DGIWAXD). Fibrillary morphologies with larger domains and increased crystallinity were observed in the polymer films after thermal annealing. These polymers exhibited improved charge-carrier mobilities in annealed films at 200 °C, and demonstrated optimal OFET device performance with p-type transport characteristics with charge-carrier mobilities of 1.51 cm2/Vs (PTV6B) and 2.58 cm2/Vs (PTVEhB). Furthermore, CMOS-like inorganic (ZnO)-organic (PTVEhB) hybrid bilayer inverter showed that the inverting voltage (Vinv) was positioned near of the ideal switching point at half (½) of supplied voltage (VDD) due to fairly balanced p- and n-channel.

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INTRODUCTION Conjugated polymers (CPs) have shown promise as candidates for future use in organic electronics because of advantages such as light weight, mechanical flexibility and excellent solution processability, which enables adopting more cost-effective manufacturing methods such as roll-to-roll printing.1,2 In addition, the tunability of optical and electrical properties of CPs is also a good strength for use in electronic applications such as organic light-emitting diodes (OLEDs),3 organic field-effect transistors (OFETs),4–7 inverters, logic circuits2,8 and organic solar cells (OSCs).9,10 In particular, alternately packed electron rich-electron deficient (D-A) types of polymers have shown strong intermolecular interactions as well as intramolecular interactions showing good charge-carrier transport property.11–13 Great improvements of the electronic properties and device performances have been achieved in several representative moieties such as diketopyrrolopyrrole (DPP), isoindigo (IID), benzothiadiazole(BT) units as accepting groups and thienothiophene(TT), thienylenevinylene (TV) as donor groups.14–18 The previously reported CPs with high performance were synthesized by following design approaches6: (i) D-A polymers with intrinsically different polarities of electron-rich and electron-deficient units could have strong intermolecular interactions, which often enhance charge-carrier mobilities,14 (ii) a coplanar polymeric backbone is not only beneficial in minimizing both steric hindrances and torsional angle between each monomeric unit, but it also increases the intracharge transport and interchain ordering,5,19 and (iii) alkyl side chains play important roles in intermolecular interactions that promote a highly ordered orientation of polymer chains as well as the solubility of the polymers in organic solvents during solution processes.20–23 To utilize these advantages and ensure polymer both solubility and planarity, we synthesized and characterized new D-A CPs, using alternating thienylenevinylene (TV) and benzotriazole (BTz) units, and explored their structural and electrical characteristics in this study. TV derivatives have been regarded as one of the promising candidates because of the coplanarity of vinylene group, extended π -conjugation length and facile alkylation at various positions.4,24 The 3



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vinylene linkage between two adjacent thiophene rings acts as a “conjugated spacer” to reduce steric hindrance and angular torsion on successive aromatic rings.25 As a result, the degree of coplanarity of the conjugated polymers backbone can be increased and its π-conjugation system can be extended, showing improved intra-charge transfer along the polymer backbone, when a TV unit is polymerized with other electron-accepting units.26 Furthermore, through alkylation at various positions of thiophene, the molecular packing between polymer side chains can be controlled and solubility in organic solvents can be improved as well.27 As an electron-accepting unit, we introduced BTz unit, a heteroaromatic compound with two electron withdrawing imine -C=N- nitrogens.28 The N-H bond of the BTz is easily modified, and an alkylatedBTz is particularly useful for the design of conjugated polymers with better solubility.29 The alkylated BTz monomers are also easily synthesized in two steps for polymerization.30,31 Moreover, incorporating solubilizing alkyl chains on the nitrogen position could promote close packing of the polymer chains and increase the charge-carrier mobilities of resulting polymers.32 Even though the BTz units are potential materials, their structural relationship has rarely reported especially in OFET application. To predict the structural planarity of the TV-BTz polymer backbone, density functional theory (DFT) calculations of simplified molecular structures were performed using the B3LYP/6-311G level (Gaussian 09). The detailed DFT data (.txt data) are provided in Supporting Information Table S1. The torsional angle between the TV and BTz units exhibited no significant twists, as shown in Figure 1, implying both very small steric hindrance between intramolecular units and an exceptionally high degree of planarity for the polymer backbone. Furthermore, the electron densities of HOMO and LUMO were well delocalized along the polymer backbones. The structural planarity and a delocalized electronic structure of polymer backbone as organic semiconductors are well known to exert significant influences on charge-carrier mobility.33,34 The DFT calculation results herein showed that TV-BTz polymers had a very large potential for efficient charge transport properties.35 4


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Figure 1. (a) Top view, (b) side view, (c) HOMO, and (d) LUMO of a dimer based on the Density functional theory (DFT) method using Gaussian 09 (B3LYP, 6-311G).

Here, we describe a synthesis of D-A type new conjugated polymers, poly[(E)-1,2-(3,3′didodecyl-2,2′-dithienyl)ethylene-alt-2-hexyl-2H-benzo[d][1,2,3]triazole] (PTV6B) with linear alkyl side chains and poly[(E)-1,2-(3,3′-didodecyl-2,2′-dithienyl)ethylene-alt-2-(2-ethylhexyl)2H-benzo[d][1,2,3]triazole] (PTVEhB) with branched alkyl side chains, using TV and BTz units as shown in scheme 1. We also experimentally confirmed that these new polymers formed a highly coplanar backbone structure that enabled an OFET device to exhibit a maximum fieldeffect mobility of 2.57 cm2/(V·s) by using alkyl side chain engineering without special processing treatments. In addition, complementary metal–oxide–semiconductor (CMOS)-like inverters showed fairly balanced p- and n-channel electrical property using ZnO-PTVEhB hybrid semiconducting bilayer.

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Scheme 1. Synthetic Routes for PTV6B and PTVEhB

RESULTS AND DISCUSSION Both polymers, PTV6B and PTVEhB, were synthesized via Stille polymerization by coupling (E)-1,2-bis(3-dodecyl-5-(trimethylstannyl)thiophen-2-yl)ethene and 4,7-dibromo-2-(2-alkyl)-2Hbenzo[d][1,2,3]triazole using Pd2(dba)3/P(o-tolyl)3 in a chlorobenzene solution, as shown in Scheme 1. As an acceptor moiety, the BTz units were synthesized in only 2 steps: (i) alkylation and (ii) bromination. Detailed descriptions of the synthesis and NMR spectra are summarized in the Supporting Information Figure S1-5. Both polymers are well soluble in common organic solvents

such

as

chloroform,

chlorobenzene,

tetrahydrofuran,

dichloromethane,

and 6


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dichlorobenzene (~10mg/ml) due to the presence of alkyl chains on the respective TV and BTz units. The number-average molecular weights (Mn) of PTV6B and PTVEhB were 18.6 kDa and 15.6 kDa with the PDI of 1.96 and 2.45, respectively (See Supporting Information Figure S6). The thermal properties of both polymers were measured via thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) (The detailed information is provided in Supporting Information Figure S7 and Table 1). The TGA thermograms of PTV6B and PTVEhB exhibited high decomposition temperatures of 401 and 405 °C at 5% weight loss, respectively, showing that the polymers had good thermal stabilities. DSC scans, therefore, were performed from room temperature up to a temperature that was below the decomposition temperature for each polymer. The DSC results revealed that both polymers did not show any melting temperature peaks by polymer backbone because of the planarity of their identical polymer backbone structure. However, PTV6B showed weak thermal induced transition peak, melting temperatures (Tm), at 110 °C during second heating, which was caused by the melting behaviors of its linear alkyl side chains, and a clear crystallization temperature (Tc) of 60 °C due to the recrystallization of alkyl side chains, which strongly supports that the transition at 110 °C is due to linear alkyl side chains. While PTVEhB exhibited no melting transition peak during both cooling and heating due to reduced intermolecular interaction between branched chains. These dissimilar results suggested that PTV6B and PTVEhB might have different packing structures caused by their different alkyl side chains.

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Table 1. Physical and optical properties of PTV6B and PTVEHB PDIa Tmc HOMOd LUMOe Tdb Egopt, f Mn/Mwa [kDa] [oC] [eV] (Mw/Mn) [oC] [eV] [eV] PTV6B 18.6 /36.5 1.96 401 110 −4.90eV −3.18eV 1.71 PTVEhB 15.6/38.2 2.45 405 −4.90eV −3.19eV 1.72 a b Determined by high-temperature GPC using TCB as an eluent. Determined by TGA. c Determined by DSC. d Spin-coated from 7 mg/ml DCB solution and determined by CV. e LUMO=HOMO + optical energy gap. f Optical energy gap is determined by onset of UV/Vis absorption spectrum in polymer film. Polymer

Optical and electrochemical properties. As shown in figure 2, the results from the UV-vis absorption spectra and from cyclic voltammetry (CV) supported the explanation that the optical and electrochemical characteristics of the PTV6B and PTVEhB polymers depend upon the polymer backbone, while their alkyl side chains might not provide a significant influence to their properties. In the UV-vis absorption spectra, the two polymer films had similar optical band gaps of 1.71 and 1.72 eV, respectively, and exhibited comparable absorption features with two, 0-0 and 0-1, absorption peaks both in solutions and in thin films due to strong inter-chain contact of conjugated backbone (ie. polymer packing arrangement or polymer backbone planarity). The absorption peaks of both polymer films were red-shifted as much as 50~60nm compared with those in solutions because of the strong intermolecular interaction of polymer backbone and reorganization of alkyl chains. For as-cast and annealed films at 200 °C, the absorption spectra showed two maximum absorption peaks at 612 and 673 nm for PTV6B, and at 611 and 669 nm for PTVEhB (See Supporting Information Table S2). These results demonstrate that replacement of the alkyl chain from a linear form to a branched form showed a slight blue-shift absorption, but it is considered that the alkyl side chains of these polymers had no significant impact on the optical properties. Also, both polymers showed the same electrochemical properties with HOMO levels of −4.9 eV. The 8 ACS Paragon Plus Environment

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oxidation onset potentials for the HOMO values were determined from the point of intersection between the baseline and the slope of signal current as shown in Figure S8 and Table S3. The calculated LUMO levels of PTV6B and PTVEhB were −3.18 eV and −3.19 eV, respectively, which were calculated from the optical band gaps and HOMO energy levels. The PTV-BTz polymers did not have a low-lying HOMO energy level compared with those of the previously reported benzotriazole (BTz)-based polymers. It seems possible that the benzotriazole (BTz) is not a strong acceptor to decrease the HOMO value compared to well-known strong acceptors such as benzothiadiazole (BT), diketopyrrolopyrrole (DPP) and isoindigo (IID). Consequently, the HOMO level could not be deep due to weak electron accepting properties. Another possible explanation is that the vinylene group with electron donating properties increased the HOMO energy level. Meanwhile, the introduction of a TV unit could also induce a planar structure and an extended π-conjugation length because of the reduced steric hindrance, leading to raising the HOMO level. Therefore, it have influenced the higher HOMO energy level of PTV-BTz by reducing the electron band gap.

Figure 2. UV/Vis absorption spectra of (a) PTV6B and (b) PTVEhB



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Polymer Structural Properties and Morphological Study. To understand the charge transport properties and thermal annealing effect, the morphologies and microstructures of both polymer thin films were characterized using tapping-mode atomic force microscopy (AFM) (Figure 3 and Supporting Information Figure S9) and grazingincidence wide-angle X-ray diffraction (2D-GIWAXD) measurements. As the phase images in figures 3a and 3b show, the fine isotropic “granular” features of as-cast PTV6B films were changed into larger fibrillars by the thermal annealing process at 200 °C. In a similar manner, the nano-fibrillar nanostructures of PTVEhB films formed remarkable clusters, as shown in figures 3c and 3d, when the polymer films were annealed. It will be discussed later in OFET performance, but it is clear that larger crystalline domains with conspicuous nano-fibrillars promoted more favorable charge transport in the annealed films of both polymers.

Figure 3. AFM phase images of (a, b) PTV6B and (c, d) PTVEhB. Scan size 1 µm x 1 µm.

We used 2D-GIWAXD to investigate the orientation in both as-cast and annealed films including the 2D-profile patterns of those films with linear alkyl side chains and with branched


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side chains. In the case of PTV6B, the as-cast film showed relatively broad (h00) peaks and distinguishable (010) peaks in the out-of-plane direction, as shown in figure 4a, indicating an mixed face-on and edge-on structure with relatively random orientation.36,37 However, the annealed film in figure 4b showed more sharp and narrow diffraction (h00) peaks in the out-ofplane and (010) peaks in the in-plane direction, while (010) peaks in the out-plane direction disappeared in the as-cast films. These results were caused when random molecular orientation of PTV6B changed the coexisted face-on and edge-on orientations to an edge-on orientation by annealing process.38 However, the lamellar packing of PTV6B showed a small change (22.78 Å to 22.44 Å) probably because PTV6B polymer chains have tightly packed structures that consisted of a planar backbone and linear side chains. The results of the aforementioned DSC study, which showed that PTV6B has a Tc, support these 2D-GIWAXD results.



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Figure 4. 2D GIWAXD pattern of PTV6B (a) As-cast and (b) 200 °C annealed film; PTVEhB (c) As-cast and (d) 200 °C annealed film.

On the other hand, compared with PTV6B films, PTVEhB films showed a relatively large change in lamellar packing (22.79 Å to 20.96 Å) via the thermal annealing process, while there was no differences in the molecular orientation with (h00) and (h01) reflections, as shown in figures 4c and 4d. Bulky branched side chains of PTVEhB might result in slightly tilted stacking structure due to the hindrance of branched alkyl side chains on BTz when they were organized by thermal annealing. Consequently, d-spacing of the branched side chains decreased due to the reorganization of adjacent alkyl side chains forming slightly tilted backbone structure by thermal annealing process, leading to a significant overlap of π -electron densities.39,40,41 The 2DGIWAXD patterns of both PTV6B and PTVEhB polymers showed more closely packed microstructures by thermal annealing process, providing an increased coherence length (Lc)36, which implied a growth of the ordered domain size in polymer films, as shown in Table 2. These results match well with the morphology analyses that showed large fibrillars in the AFM measurement.

Table 2. Lamellar spacing and π-π spacing of PTV6B and PTVEHB* π-π spacing

Lamellar spacing

[Å-1]

d(100 ) [Å]

FWH M [Å-1]

PTV6B As-cast

0.276

22.78

PTV6B 200 ℃

0.280

22.44

polymer

qz(100) *

Lc [Å ]

qz(001 ) [Å-1]

d(001 ) [Å]

qz(010 ) [Å-1]

d(010 ) [Å]

qxy(010 ) [Å-1]

d(010 ) [Å]

0.0385 146.6

0.252

24.93

1.755

3.58

1.708

3.68

0.0329 171.5

0.247

25.44

-

-

1.735

3.62


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PTVEh B Ascast

0.276

22.79

0.0388 145.6

0.249

25.23

-

-

-

-

PTVEh B 200 ℃

0.299

20.96

0.0334 169.2

0.250

25.13

-

-

-

-

*

Scattering data that are calculated as a function of the scattering vector q = 4π/ λ sin θ, where θ is half of the scattering angle and λ is the wavelength of the incident radiation. The respective qxy and qz indicate the scattering vector parallel and perpendicular to the substrate.

It is also interesting that in both PTV6B and PTVEhB strong reflection peaks were observed next to (h00) in the 2D-GIWAXD, which can be assigned to (h01) reflection along the directions of the polymer backbone. This unusual presence of (h01) reflections (~25 Å) in both as-cast and annealed films proved an improved crystalline order of those films resulting from the planar and rigid backbones of PTV6B and PTVEhB, which enables an effective intra-chain charge transport.36,42,43 The DFT calculations that showed highly coplanar polymer backbones support the 2D-GIWAXD analysis and ensuing OFET device results exhibiting high field-effect mobilities, even though the molecular packing of those polymers did not present extremely strong orientations. The corresponding 1D-diffractogram profiles of the polymer films were also provided in the Supporting Information Figure S10.44

Organic Field-Effect Transistor Performance and CMOS-like Inverter. To investigate the charge-carrier transport properties of the polymers, top-gate and bottomcontact (TG/BC) field-effect transistors (FETs) were fabricated on a glass substrate via simple spin coating method under a nitrogen (N2) atmosphere. This TG/BC device architecture is preferred due to the encapsulation effect of the top PMMA dielectric layer for the underlying semiconductor.45,46 A detailed description of the fabrication process was given in the 13 ACS Paragon Plus Environment


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experimental section. Figure 5 shows the typical transfer characteristics of PTV6B and PTVEhB (as-cast and thermally treated at 250 ℃ films, respectively). The field-effect mobilities were calculated over a much wider gate voltage (Vg) range of 10 V and up to a high gate voltage of Vg = −75 V from the slope of (Id)1/2-Vg curves in the saturation regime. The results were confirmed and measured using more than 50 devices. The detailed numerical values of device characteristics and output curves were listed in Table 3 and in the Supporting Information Figure S11, respectively. We fabricated field-effect transistors with semiconducting layers, which were treated at various thermal annealing temperatures (as-cast, 110, 150, 175, 200, 225, 250, and 300 ℃). All polymers exhibited typical p-type dominant OFET characteristics, 200 ℃ annealed devices presented the highest field-effect mobilities. The devices using thermally annealed films showed one order-of-magnitude better field-effect mobility than those using as-cast semiconductor films. As the temperature was increased, the average mobilities of devices using both types of polymers were increased from 0.55 cm2/(V·s) at 25 °C to 1.39 cm2/(V·s) at 200 °C for PTV6B and 0.39 cm2/(V·s) at 25 °C to 1.90 cm2/(V·s) at 200 °C for PTVEhB. In particular, the maximum hole mobility of PTVEhB was 2.58 cm2/Vs was gained in polymer film prepared simply by spin coating without the use of extra align processes such as solution shearing and offcenter method.47 The HOMO levels of the polymers from CV results match well with the work function of gold electrode (Au). This means that the energy barrier for charge injection between source/drain (S/D) electrode and the organic semiconductors in OFET devices could be minimized.48,49 In the result of standard deviation for the mobility, as-cast PTV6B, 200 ℃ thermal annealed PTV6B, and as-cast PTVEhB devices had small standard deviations of ±0.10, but the 200 ℃ thermal annealed PTV6B device had a deviation of ±0.63. The difference could have been related to packing behavior of the polymer backbone depending on the reorganization 14 ACS Paragon Plus Environment

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between adjacent alkyl side chains of the polymer during thermal annealing. The alkyl side chains can be interdigitated to be stable structure when thermal energy is applied. In case of PTV6B, linear alkyl side chain already formed tight packing structure when they films were deposited, so the morphology did not show a large change during thermal annealing. On the contrary, the branched side chains of PTVEhB might have formed slightly different packing behavior with tilted packing structure due to the steric hindrance of branched alkyl side chains when they were organized by thermal annealing. Therefore, the molecular ordering in PTVEhB films had a great influence on the thermal treatment forming closed lamellar d-spacing , while that of PTV6B was slightly changed by thermal annealing. GIWAXD analysis supported the relatively large change in lamellar packing for PTVEhB (22.79 Å to 20.96 Å) by annealing process compared with PTV6B (22.78 Å to 22.44 Å). The annealed PTVEhB films, therefore, showed standard deviation that was larger than that of PTV6B. Based on the results of previous UV-vis absorption, DFT calculation, AFM, and 2D-GIWAXD, coplanarity of polymer backbone as well as the molecular packing by the alkyl side chain motion induced by thermal annealing process resulted in enhanced intramolecular/π-π intermolecular interaction of the polymer films and effective charge transport behavior of OFETs.

Table 3. The summary of OFET performances of PTV6B and PTVEHB

Polymer PTV6B

PTVEhB

Processing Conditions As-cast

Maximum Mobility

Average Mobilitya

2

2

Threshold Voltage

[V] [cm /Vs] [cm /Vs] 0.64 0.55 (±0.10) -29 (±0.97)

On/Off

Subthreshold Swing

Ratio

[V/dec.]

102

40.32 (±0.71)

200 ℃

1.51

1.39 (±0.13) -33 (±1.59)

104

7.83 (±0.30)

As-cast

0.43

0.39 (±0.04) -36 (±1.08)

104

6.33 (±0.25)

1.90 (±0.63) -48 (±1.90)

4

7.85 (±0.24)

200 ℃

2.58

10



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a

Mobilities were calculated from the saturation regime at Vd = -80 V and the standard deviations were extracted in more than 50 devices.

Figure 5. Transfer characteristics of OFETs processed from the PTV6B (a) and PTVEhB (b) (as cast and thermal annealed at 200 °C for 30 min)(c) Voltage transfer characteristics of the ZnOPTVEhB hybrid bilayer CMOS-like inverter at 200 °C annealed films (Wp/ Lp =Wn/Ln= 1 mm/ 20 ㎛).

As the basic elements for complex integrated circuits, we fabricated CMOS-like inverter using inorganic-organic hybrid bilayer using as semiconducting active layer. The static characteristics of an inverter are described by a plot of voltage transfer characteristics (VTCs), which shows an output voltage as a function of input voltage, in figure 5(c). Due to fairly balanced p- and nchannel electrical property of the hybrid semiconducting bilayer of ZnO and PTVEhB, the inorganic-organic bilayer based CMOS-like inverter showed that the inverting voltage (Vinv) was positioned near of the ideal switching point at ½ VDD. Even though the device has some amount of Vout losses at static on and off states, but it is a typical phenomena in the ambipolar CMOSlike inverters. The schematic of the hybrid inverter and output voltage gains (Vg-Vin curve) are also shown in Supporting Information Figure S12. The CMOS-like inverters operated with gain


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around ~6 at a supplied voltage (VDD) of 30 V, showing promising candidates in organic electronic applications.

CONCLUSION In conclusion, D-A type of new polymers with coplanar and good crystalline structure (PTV6B and PTVEhB), composed of TV and BTz units, were successfully synthesized and were well soluble in common organic solvents. The optical and electrochemical properties of the polymers were influenced by the backbone structure of polymers rather than by the alkyl side chains, while the different types of alkyl side chains (linear and branched chains) affected the formation of both the microstructure and morphology. DFT calculation confirmed that the delocalized structure of the coplanar TV-BTz conjugated polymer induced effective charge transport properties. The morphological results from AFM and 2D-GIWAXD demonstrated how the fine change of alkyl side chain and the larger crystals of the polymer films that were caused by the thermal annealing process could improve the molecular ordering and facilitate charge transport. In addition, the energy levels of these polymers proved to be suitable for application in p-type OTFTs. Among the number of previously reported BTz-based semiconductors, PTVEhB exhibited high field-effect mobility at 2.58 cm2/Vs. Furthermore, inorganic-organic hybrid bilayer CMOS-like inverter using ZnO and PTVEhB showed ideal switching point at ½ VDD, which is basic elements of most of the logic circuits. Our work suggests that TV and BTz units as conjugated moieties are attractive building blocks for high charge-carrier mobility due to their facile design approaches and highly planar structural properties for organic electronics.



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EXPERIMENTAL SECTION Materials. All chemical materials in this study were purchased from Alfa aesar SigmaAldrich, Acros and Strem chemicals and they were used without extra purification. All reactions were synthesized by using anhydrous commercial solvents in a nitrogen atmosphere. As electron acceptor and electron donor units, detailed synthetic method of final monomers were described in Supporting Information Figure S1-3. 2-hexyl-2H-benzo[d][1,2,3]triazole, 2-ethylhexyl-2Hbenzo[d][1,2,3]triazole, 4,7-dibromo-2-hexyl-2H-benzo[d][1,2,3]triazole and 4,7-dibromo-2-(2ethylhexyl)-2H-benzo[d][1,2,3]triazole were synthesized by modifying the previously reported procedures and synthetic method.29,30 (E)-1,2-bis(3-dodecyl-5-(trimethylstannyl)thiophen-2yl)ethene, (E)-1,2-bis(3-dodecylthiophen-2-yl)ethene, 3-dodecylthiophene-2-carboaldehyde, 3dodecylthiophene, 2-bromo-3-dodecylthiophene were synthesized as reported previously.4,5 Polymerization Procedure. For the Stille polymerization, Final Monomer 6 (0.5 mmol) and 9 (0.5 mmol) in 15ml anhydrous chlorobenzene (CB) were dissolved in 50 ml two neck flask under a nitrogen atmosphere. The resultant materials mixed with P(o-tolyl)3 (0.08 mmol) as a ligand and Pd2(dba)3 (0.01 mmol) as a palladium catalyst and was then refluxed at a controlled temperature of 130 °C for 2 overnights. 2-bromothiopehe and 2-(tri-n-butylstannyl)thiophene were added in order as end cappers and stirred for 3hrs. Finally, the resulting mixture was slowly cooled down to room temperature for termination of the reaction and the concentrated resultant polymers were precipitated in 150 mL of methanol, and was then mixed with 30 mL of


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HCl solution for removing catalysts and ligands. The final polymers, PTV6B and PTVEhB, were purified for cutting the molecular weight via multiple Soxhlet extractions (methanol, hexane, and chloroform) and dried in 60 °C vacuum oven. The polymers stored at N2 atmosphere in a desiccator. PTV6B. 1H NMR (400 MHz, CDCl3): δ 7.91 (s, 2H), 7.60 (s, 2H), 7.15 (s, 2H), 3.73 (br, 2H), 2.78 (br, 4H), 1.87-1.25 (br, 48H), 0.86 (m, 9H). Anal. Calcd (%): C, 75.66; H, 9.80; N, 5.75; S, 8.78. Found (%): C, 75.40; H, 9.81; N, 5.71; S, 9.04. PTVEhB. 1H NMR (400 MHz, CDCl3): δ 7.96 (s, 2H), 7.58 (s, 2H), 7.14 (s, 2H), 3.74 (br, 2H), 2.76 (br, 4H), 2.01 (br, 1H), 1.85 (br, 4H), 1.71-1.25 (m, 44H), 0.85 (m, 12H). Anal. Calcd (%): C, 76.03; H, 9.97; N, 5.54; S, 8.46. Found (%): C, 75.87; H, 9.96; N, 5.35; S, 8.49. Characterization. Computational Methodology. The density functional theory (DFT) calculation was performed by Gaussian 09 program. Ground-state molecular geometry as a dimer was optimized for theoretical molecular simulation of the polymers with methyl group without alky side chains. The calculation used Becke’s three-parameter function (B3LYP, the Lee, Yang, and Parr correlation functional) with 6-311G(d,p). Synthesis. Elemental analysis (EA) was performed for the quantitative analysis of respective element (C, H, N and S) on an EA1112 instrument (CE Instrument) at Seoul University. 1H NMR spectra in experimental section, Supporting Information Figure S1-5 were recorded to confirm chemical structures of synthesized materials on a JEOL (JNM-ECX400, 400 MHz) spectrometer in CDCl3. An internal tetramethylsilane (TMS) standard were used as comparative material for recording the chemical shifts.



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Polymer property. Polydispersity index (PDI), the weight-average molecular weight (Mw) and the number-average molecular weight (Mn) were determined by high temperature (135 ℃) gel permeation chromatography (HT-GPC) at Korea Institute of Industrial Technology. Trichlorobenzene (TCB) as the eluent was used for the injection at flow rate of 1 mL/min (YOUNGLIN Acme 9000 instrument) and the standard materials for calibration were employed a narrow polydispersity polystyrene (PS) Thermal property. TA-2050 instrument were used for thermogravimetric analysis (TGA) measurement, the thermograms were conducted up to 800 °C at a heating rate of 10 °C/min under a nitrogen atmosphere. Differential scanning calorimetry (DSC) analysis was conducted on a TA2010 instrument under a nitrogen atmosphere (range: 30 °C to 320 °C at a heating scan rate of 10 °C/min). Electrical and optical property. The HOMO and LUMO energy level of polymers were determined by cyclic voltammetry (CV) measurements (AUTOLAB eco Chemie instrument). All polymer films were spin coated on indium tin oxide (ITO, working electrode), and a platinum wire (counter electrode) and a Ag/AgCl (the reference electrode) were used for electrochemical reaction. The measurement were performed at a scan rate of 50 mV/s in an anhydrous electrolyte with mixture of 0.1 M tetrabutylammonium perchlorate (Bu4NClO4 and acetonitrile solution (ACN)). The UV-vis absorption spectra were obtained using a Perkin-Elmer Lambda 750 instrument. Morphological study. The 2D-GIWAXD analysis was studied with spin-coated polymer films at the Pohang accelerator laboratory (PAL) on a beamline 3C. The topography and phase images of AFM (Atomic-force microscopy) were gathered in a tapping-mode at Korea basic science institute.


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OFET Fabrication and Characterization. Top-gate and bottom-contact (TG/BC) field-effect transistors (FETs) were fabricated on a glass substrate under a nitrogen (N2) atmosphere. The patterned corning glasses (Eagle 2000) as device substrates were cleaned in de-ionized water, acetone, and isopropanol (IPA) for 10 min each in an ultrasonic baths in sequence. On the glass substrates, the gold/nickel (15/3 nm) was used as the S/D electrodes and patterned using a conventional lift-off photolithography technique. The channel width (W)/channel length (L) was 1.0 mm/20 µm and Ni was the adhesion layer. As semiconductors, 7mg of PTV6B and PTVEhB polymers were respectively dissolved in 1ml of anhydrous dichlorobenzene solvent. And then they were heated up to 80 °C for untangling the aggregation between polymers. The cooled conjugated polymer solutions were filtered with a 0.45 µm PTFE syringe and spin-coated onto the as-cleaned substrates (spin speed: 2000 rpm for 120s after 3s acceleration), and the films were thermally annealed at 200 °C for 30 min in a nitrogen glove box. 80mg of PMMA (120 kDa) was dissolved in 1 ml of anhydrous n-butyl acetate solvent for polymer gate dielectric layers. The films were thermally annealed at 80 °C for 2 hrs under a N2 atmosphere. On the dielectric layer, 45 nm thick aluminum of gate electrode was thermally evaporated through a shadow mask. The characteristics of the respective polymer OFET devices were derived using a Keithley instrument (4200-SCS) in a N2 atmosphere glovebox. Inverters Characterization. ZnO nanopowder was purchased from Sigma-Aldrich and dissolved with in Ammoniumhydroxide (alfa aesar). After sequential cleaning the patterned electrode substrate (Ni (3nm)/Au (12nm) S/D patterned Corning Eagle XG glass) with deionized water, then acetone, then isopropanol in an ultrasonic bath, the ZnO solution was spin-coated onto the substrate and the films were thermally annealed at 150°C for 30 min under ambient condition. In order to form the organic-inorganic active bilayer, the PTVEHB solution in DCB



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was spin-coated onto ZnO films and annealed at 200 °C for 30 min in glove box under nitrogen inert condition. For a dielectric layer, PMMA (εr = 3.5, ~ 500 nm) solution was spin-coated and thermally baked to remove the residual solvent at 80 °C for 2h. The Al gate electrode (~40 nm) was thermally deposited using a shadow mask.

ASSOCIATED CONTENT Supporting Information. Supporting information of PTV6B and PTVEhB (DFT data, material synthesis and characterization of 1H-NMR, GPC, TGA, DSC, UV-vis spectra and CV) and additional morphological study (AFM, 1D-XRD profile) and device data (Output characteristics of OFETs and schematic structure and output voltage gains characteristics of CMOS-like inverter). This information is available free of charge via the Internet at http://pubs.acs.org/.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (D.-Y.K.). Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the National Research Foundation of Korea (NRF) Grant funded by the Ministry of Science (NRF-2015R1A2A1A10054466).

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