Synthesis, Structural Characterization, and Field-Effect Transistor

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Synthesis, Structural Characterization, and Field-Effect Transistor Properties of n-Channel Semiconducting Polymers Containing Five-Membered Heterocyclic Acceptors: Superiority of Thiadiazole Compared with Oxadiazole Hua-Jie Chen, Zhaoxia Liu, Zhiyuan Zhao, Liping Zheng, Songting Tan, Zhihong Yin, Chunguang Zhu, and Yunqi Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b12540 • Publication Date (Web): 15 Nov 2016 Downloaded from http://pubs.acs.org on November 21, 2016

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ACS Applied Materials & Interfaces

Synthesis, Structural Characterization, and Field-Effect Transistor Properties of n-Channel Semiconducting Polymers Containing FiveMembered Heterocyclic Acceptors: Superiority of Thiadiazole Compared with Oxadiazole Huajie Chen†,*, Zhaoxia Liu†,⊥, Zhiyuan Zhao‡,⊥, Liping Zheng†, Songting Tan†,*, Zhihong Yin†, Chunguang Zhu†, Yunqi Liu‡,* †

Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of

Education, and Key Laboratory of Advanced Functional Polymeric Materials of College of Hunan Province, College of Chemistry, Xiangtan University, Xiangtan 411105, P. R. China ‡

Beijing National Laboratory for Molecular Sciences, Chinese Academy of Sciences, Beijing

100190, P. R. China

ABSTRACT: Five-membered 1,3,4-oxadiazole (OZ) and 1,3,4-thiadiazole (TZ) heterocyclebased copolymers as active layer have long been ignored in solution-processible n-channel polymer field-effect transistors (PFETs) despite the long history of using OZ or TZ derivatives as the electron-injecting materials in organic light-emitting devices and their favorable electron affinities. Herein, we firstly report the synthesis and PFETs performance of two n-channel conjugated polymers bearing OZ- or TZ-based acceptor moieties, i.e., PNOZ and PNTZ, where simple thiophene units are utilized as the weak donors and additional alkylatednaphthalenediimides units are used as the second acceptors. A comparative study has been performed to reveal the effect of different heterocyclic acceptors on thermal properties, electronic properties, ordering structures, and carrier transport performance of the target polymers. It is found that both polymers posses low-lying LUMO values below – 4.0 eV, indicating high electron affinity for both heterocycle-based polymers. Because of strong polarizable ability of sulfur atom in TZ heterocycle, PNTZ exhibits a red shift in maximal absorption and stronger molecular aggregation even in the diluted chlorobenzene solution as compared to the OZ-containing PNOZ. Surface morphological study reveals that a nodule-like surface with a rough surface morphology is observed clearly for PNOZ films, whereas PNTZ films display highly uniform surface morphology with well interconnected fiber-like polycrystalline grains. Investigation of PFETs performance indicates that both polymers afford air-stable n-channel transport characteristics. The uniform morphological structure and compact 1 ACS Paragon Plus Environment


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π−π stacking endow PNTZ with a high electron mobility of 0.36 cm2 V–1 s–1, much higher than that of PNOZ (0.026 cm2 V–1 s–1). These results manifest the feasibility in improving electrontransporting property simply by tuning heteroatom substitutes in n-channel polymers; further demostrate that TZ derivatives possess much superior potential for developing highperformance n-channel polymers compared to OZ derivatives. KEYWORDS: 1,3,4-oxadiazole, 1,3,4-thiadiazole, n-channel field-effect transistors, air-stable, high performance

INTRODUCTION Recently, polymer field-effect transistors (PFETs) have attracted great attention owing to their facile solution-processability and good mechanical flexibility, which enables to fabricate low-cost, flexible, and low-environmental-impact productions.1–5 Over the past few decades, various polymeric semiconductors, including hole (p-channel),6–11 electron (n-channel),12–21 and electron-hole (ambipolar) transporting polymers,22–29 have been extensively developed. Among them, p-channel polymeric semiconductors, comprised of relatively electron-rich π-conjugated backbones, are studied most often, and there are now a great number of p-channel polymeric semiconductors with ultrahigh hole mobility and excellent air-stability for their PFETs.1–5 Hole mobilities above 8.0 cm2 V−1 s−1 have been demonstrated for many p-channel or ambipolar PFETs.6–12 It is well-known that n-channel polymers are the key part for the fabrication of complementary logic circuits and all-polymer solar cells.1–5 Although the number of n-channel polymers has increased recently, the electron mobility and devices stability of n-channel PFETs is still lower compared with p-channel counterparts.13–20 The reason is that electron transport is generally unstable because it can be readily quenched by H2O or O2 in air during transport.30,31 To solve this challenge, the most effective strategy is energy level control, namely, is to incorporate strongly electron-deficient building blocks, such as naphthalenediimides (NDI),17–20 perylenediimides (PDI),32,33 and benzodifurandione-based PPV derivatives (BDPPV)34,35, into the polymer backbones so that the electron-deficient groups could pull electrons out of the polymer backbones and therefore reduce LUMO energy levels. As a result, the electron injection and electron transport might be stabilized against H2O or O2 by energy level control.30,31 However, there is the scarcity of good building-blocks for the design of n-channel conjugated polymers. To further develop novel n-channel polymeric semiconductors and understand the fundamentals of their structure–property relationships, therefore development of novel electronwithdrawing building blocks is worthy of being studied.

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Five-membered 1,3,4-oxadiazole (OZ) and 1,3,4-thiadiazole (TZ) heterocycles, containing electron-deficient imine nitrogen (C=N), possess high electron affinity, good coplanarity, and multiple heteroatom characteristics.36 Therefore the use of these electron-deficient heterocycles in organic semiconductors can not only lower their frontier energy levels, but also can facilitate the crystallization, molecular organization, and interchain π–π packing of molecular backbones due to strong heteroatom coupling effect, which would be helpful for carrier transport.36 Recently on the basis of OZ and TZ heterocyclic acceptors, various π-conjugated oligomers and copolymers have been prepared and applied into the field of light-emitting devices and organic photovoltaics.37–42 However, very few researches successfully attempt to use OZ- or TZcontaining organic semiconductors (Figure 1) as a part of active layer in solution-processible field-effect transistors (FETs) applications.42–45 In 2005, Yamamoto group reported that a novel D–A type conjugated copolymer (PThdzTh), constructed from unsubstituted thiophene donor and TZ acceptor units, exhibited obvious ambipolar characteristic and relatively low hole and electron mobilities approach 3.4×10–4 and 5.4×10–3 cm2 V−1 s−1, respectively.43 However, PThdzTh was only soluble in trifluoroacetic acid due to the lack of solubilizing side groups. By copolymerization with alkylated-thiophene donors, Chen group prepared a highly soluble TZbased copolymer (PTZ).42 Moreover, an excellent hole-transporting characteristic (hole = 0.12 cm2 V−1 s−1) had been demonstrated for solution-processible p-channel PTZ-based PFETs, which is much higher than that of OZ-based copolymer (POZ, hole = 1.62×10–4 cm2 V−1 s−1). Additionally, by introducing second electron-deficient groups into the OZ-containing oligomers, Katz group reported the first n-channel OZ-based oligomer (OZ-11) for vacuum-deposited FETs, but affording a relatively low electron mobility value up to 2×10–3 cm2 V−1 s−1.44 To date, there is no n-channel OZ- and TZ-based copolymers reported which can be applied in solutionprocessible n-channel PFETs. Considering that OZ and TZ have unique properties including strong electron affinities, rigid coplanarity, multiple heteroatom as well as facile synthesis, OZ and TZ derivatives might be versatile electron-deficient building blocks for developing a wide range of high-performance n-channel polymeric semiconductors for PFETs applications. Based on the considerations mentioned above, we reported the synthesis and PFET devices performance of the two novel unipolar n-channel polymer semiconductors bearing OZ- or TZbased acceptor moieties, i.e., PNOZ and PNTZ (Figure 1), where simple thiophene units were used as the weak electron donor units. Herein, the additional alkylated-NDI moieties, using as the second electron acceptors, were also introduced into the polymer repeating units of both OZor TZ-based polymers, thus forming a regioregular A1–D–A2–D configuration. We expect that this facile synthetic strategy will not only can ensure good solution-processibility but also can 3 ACS Paragon Plus Environment


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further improve electron affinity of both OZ- or TZ-based polymers, because the electrondeficient imine and imide group in both heterocycles (OZ and TZ) and NDI moieties, respectively, would pull electrons out of the polymer backbones and in turn further lower polymeric LUMO levels effectively. In this contribution, a comparative study would be carried out to investigate the heterocyclic effect on electronic properties, film microstructures, and carrier transport performance of the polymers by performing systematic measurements. For topgate bottom-contact (TGBC) PFET devices, a peak electron mobility up to 0.36 cm2 V−1 s−1 and an excellent device stability had been demonstrated for PNTZ thin films under ambient conditions. This electron mobility value is the highest one observed to date among the OZ- or TZ-containing organic semiconductors. Furthermore, this study demostrates that, for the first time, high-performance n-channel transport characteristics are observed for both OZ- and TZcontaining polymeric semiconductors. C6H13 C6H13 C6H13 C6H13 O S

N N

S S

n

p-Channel, POZ h = 1.41 10-3 cm2 V-1 s-1

F F F F F O F FF N F N

O N N

vs

C6H13

S S

N N

S

p-Channel, PTZ h = 0.12 cm2 V-1 s-1

n

F F

FF

F

F

F F F

n-Channel oligomer, OZ-11 e = 2 10-3 cm2 V-1 s-1

S O

R N

R = 2-octyldodecyl

O S

O

S

S

NN O

NC

TZ N N

S

S

S

N N

OZ C6H13 C6H13 C6H13

CN

n

Ambipolar, P(ThdzTh) h = 3.4 10-4 cm2 V-1 s-1 e = 5.4 10-3 cm2 V-1 s-1

N R

O

X N N

S

n

This work n-Channel polymers

X=O, PNOZ, e = 0.026 cm2 V-1 s-1 X=S, PNTZ, e = 0.36 cm2 V-1 s-1 First report on OZ- and TZ-based polymers for n-channel PFETs

Figure 1. Chemical structures of π-conjugated oligomers and polymers containing OZ and TZ heterocycles.


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S COOH

SOCl2

. S COCl NH2NH2 H2O

quantitative

NMP, 70%

S

NHNH

1

2TOZ

1) n-BuLi, THF 2) Sn(CH3)3Cl, 18%

O

O

S

2

Sn S

NN O

S Sn

1) n-BuLi, THF

2TTZ

2) Sn(CH3)3Cl, 19%

Sn S

S Sn

S

o

O N O C8H17 C10H21 NDI-2Br

OZ-2Sn

C8H17 O N O

C8H17 O N O Br Pd2(dba)3, P(o-tol)3

Br

Chlorobenzene, 115 C, 93%

NN S TZ-2Sn

S Sn

O N O C8H17 C10H21 NDI-2Br

S n

PNOZ

C8H17 O N O

C8H17 O N O Br

Br

O N O C8H17 C10H21

O NN

C10H21

C10H21

Sn S

NN S Sn S TZ-2Sn

C10H21

C10H21

NN O

NN S S 2TTZ

S

OZ-2Sn

Sn S

NN S O 2TOZ

S

, 83% SOCl 2 Law reag esson ent, 85%

Pd2(dba)3, P(o-tol)3

S

o

Chlorobenzene, 115 C, 92%

O N O C8H17 C10H21

S NN

S n

PNTZ

Scheme 1. Synthetic route for five-membered heterocycle-containing monomers (OZ-2Sn and TZ-2Sn) and copolymers (PNOZ and PNTZ). RESULTS AND DISCUSSION Synthesis and Thermal Properties. Scheme 1 provides the synthetic routes for monomers (OZ-2Sn and TZ-2Sn) and polymers (PNOZ and PNTZ). The synthetic details and characterizations can be found in the Supporting Information. Based on 2-thiophenecarboxylic acid as the starting regent, two key intermediates (2TOZ and 2TTZ) were readily prepared via three steps reactions in a high yield.42 Then 2TOZ and 2TTZ were converted to the corresponding distannylated monomers (OZ-2Sn and TZ-2Sn) by the present of n-butyllithium and trimethyltin chloride. Because the purity of monomers can significantly affects comprehensive prosperities of the resulting polymers, herein two monomers (OZ-2Sn and TZ2Sn) were carefully recrystallized from methyl alcohol for three times. Another important monomer (NDI-2Br) was prepared following the reported procedure.22 Finally, the Stille crosscoupling reaction between OZ-2Sn (or TZ-2Sn) and NDI-2Br was carried out, affording PNOZ or PNTZ in a high yield above 90%. Then the solid product was collected and further purified by Soxhlet extraction technique. All the molecular structure of the as-synthesized monomers and polymers was confirmed by 1H NMR spectra (Figures S1–S5). The number-average molecular 5 ACS Paragon Plus Environment


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weight (Mn) of the two polymers are 24.0 kDa for PNOZ and 34.6 kDa for PNTZ. The polydispersity indexs of PNOZ and PNTZ are 2.05 and 2.39, respectively. (Figures S6~S7). At room temperature, the solubility of both polymers is good in common organic solvents (>15 mg mL−1), such as chloroform, xylene, chlorobenzene, and o-dichlorobenzene. Additionally, polymer thermal stability was evaluated by TGA under a N2 atmosphere (Figure S8). The results indicated that the onset decomposition temperatures (determined as 5% loss on weight) of PNOZ and PNTZ were 405 and 422 °C, respectively, exhibiting excellent thermal stability for the fabrication of organic optoelectronic devices. Photophysical and Electrochemical Properties. Optical properties were investigated by measuring the solution absorption spectra and thin film spectra of both polymers. As shown Figure 2a, both polymers exhibit typical dual-band absorption characteristics. One band located at 300–450 nm mainly arises from the π–π transitions. Another band located at 400−700 nm is attributed to the strong intramolecular charge transfer (ICT).27 Compared with their solution spectra, the λmax peaks of both polymers bathochromically shift to 565 nm for PNOZ and 590 nm for PNTZ, respectively, suggesting that strong backbone aggregation exists in their solidstate films.22 To further study this aggregation characteristic, the temperature-controlled situ absorption spectra of both polymer solution were measured at different temperatures (Figures 2b and 2c).46 Upon increasing solution temperature, a remarkable blue-shift of λmax at ICT band was observed for PNTZ solution, while almost no obvious change could be found for PNOZ solution. The results indicate that, in the diluted o-DCB solution, the TZ-containing PNTZ exhibits much stronger aggregation ability than that of the OZ-containing PNOZ. The reason is that the strong aggregation effect is probably caused by the sulfur atom in TZ heterocycles. Compared with oxygen atom in OZ heterocyles, sulfur atom possesses larger atomic radius as well as stronger polarizable ability, and therefore facilitating stronger intermolecular interactions for PNTZ.42 Generally, strong intermolecular interactions will be helpful for high-performance charge transport.22 Additionally the optical bandgaps (Egoptical) of PNOZ and PNTZ are 1.77 and 1.65 eV, respectively, which is deterimined from the onset absorption of the polymer films (700 nm for PNOZ and 750 nm for PNTZ, see Figure 2a).


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Table1. Molecular Weights, Thermal Properties, Physicochemical Properties, and Energy Level of the Polymers

Polymer

Mn/Mw a

Td b

(kDa)

(°C)

λmax (nm) Solution

Film

CV Data HOMO

c

DFT Data

LUMO

HOMO

LUMO

(eV)

(eV)

(eV)

(eV)

Egoptical d (eV)

PNOZ

24.0/49.2

405

350, 510

360, 565

–5.82

–4.05

–5.84

–3.60

1.77

PNTZ

34.6/82.7

422

356, 550

380, 590

–5.70

–4.02

–5.75

–3.60

1.65

a

Molecular weights evaluated by high temperature GPC. b5% weight loss temperature. cHOMO

energy levels determined by the following equation: HOMO = – (Egoptical– LUMO) eV.

d

Determined by the following equation: Egoptical = 1240/ λedge.

Figure 2. (a) Absorption spectra in chloroform solution and thin film on quartz; The temperature-controlled situ absorption spectra of PNOZ (b) and PNTZ (c) in o-DCB solution. To assess molecular energy levels of the two polymers, cyclic voltammetry (CV) were performed. Typical CV profiles and data for PNOZ and PNTZ are provided in Figure 3 and Table 1, respectively. Because of strongly electron-withdrawing characteristics for the whole polymer backbones, both polymers exhibit inconspicuous oxidation processes but strong reduction processes with clearly reduction peaks, indicating that both polymers can be operated as n-channel electron-transporting semiconductors.27 To calculate the reduction potentials of both polymer films, the ferrocene electrode was used as the external standard and calibrated via Fc/Fc+ couple (4.8 eV under vacuum).27 The CV measurement indicated the potential of the Fc/Fc+ couple was 0.38 eV under the same conditions (Figure S9). Thus the LUMO values can be calculated from the following equation: LUMO = – (Eredonset + 4.42 eV).27 As shown in Figure 3a, the LUMO values of PNOZ and PNTZ, calculated from the reduction potentials (Ered) of PNOZ (–0.37 V vs Ag/AgCl) and PNTZ (–0.40 V vs Ag/AgCl), are determined to –4.05 and –4.02 eV, respectively. Because the electronegativity of oxygen 7 ACS Paragon Plus Environment


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atom in OZ heterocycles is much stronger than that of sulfur atom in TZ heterocycles, the LUMO value of PNOZ is slightly lower as compared to PNTZ. We find that both polymers exhibit low-lying LUMO values below –4.0 eV, which are favorable for electron injection as well as improving devices’ air-stability against H2O or O2 in air eventually.30,31 To obtain high electron-injection

efficiency,

Au

source/drain

electrodes

were

treated

by

pentafluorobenzenethiol (PFBT).47 As a result, the function of PFBT-Au electrodes (4.77 eV, see Figure 3b) would match better with polymeric LUMO energy levels as compared to bare Au electrodes (5.13 eV),22,47 which might be helpfull for electron injection from PFBT-Au electrode to active layer. Calculated from the empirical equation of HOMO = – (Egoptical – LUMO) eV, the HOMO values of bothe polymers are the deep-lying values of –5.82 eV for PNOZ and –5.70 eV for PNTZ, respectively. The low-lying HOMO values mainly resulted from highly electrondeficient nature of NDI and five-membered heterocycles in polymer backbones. The fact is that too low HOMO values could mismatch with the function of PFBT-Au electrodes, and therefore going against hole injection.27

Figure 3. (a) Cyclic voltammetry traces of PNOZ and PNT; (b) A compared system about Energy levels of semiconductors and PFBT-Au electrodes. Computational Properties and Energy Levels. To understand the structural and electrochemical trends observed for the two polymers, we performed density functional theory (DFT, B3LYP/6-31G*) calculations on trimeric systems by using Gaussian 09 program.48 Herein, 2-octyldodecyl side chains were replaced by methyl groups to simplify calculation. The optimized model structures and molecular orbitals are provided in Figure 4. Note that both trimers exhibit a similar backbone configuration, and display very small dihedral angles for thiophene-OZ or thiophene-TZ segments (θ2 = 0.8° and θ3 = 1.1°), indicating two types of πconjugated linkages, including thiophene-OZ-thiphene and thiophene-TZ-thiphene, possess a good coplanarity and backbone conjugation. The fact is that the coplanar π-conjugated linkages 8 ACS Paragon Plus Environment

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would enhance molecular arrangement and in turn facilitate carrier transport.49 For both trimer (NDI-TZ and NDI-OZ), however, the dihedral angles for NDI-thiophene segments are relatively large because of strong steric hindrance effect.22 Generally, a large backbone torsion would break conjugation system and limit intermolecular charge transfer along the polymer main chain.27 Because of the backbone torsions in both trimers, the HOMO orbitals mainly distribute in π-conjugated linkages, while the LUMO orbitals mainly distribute in highly electron-deficient NDI units. Moreover, the calculated LUMO energy levels for both trimers are deep-lying values of –3.60 eV. The results imply that both polymers might be operated as n-channel semiconductors. It is found that the calculated HOMO and LUMO values for both trimers are incongruous with the results of CV measurements (Table 1), mainly caused by the exciton bonding energy of polymeric semiconductors.50 Because of stronger electronegativtiy for oxygen atom, the OZ-containing trimer NDI-OZ exhibits much low-lying HOMO value of – 5.84 eV as compared to the TZ-containing trimer NDI-TZ (–5.75 eV, Table 1 and Figure 3b).

Figure 4. Calculated model structures and molecular orbitals of methyl-substituted NDI-OZ and NDI-TZ trimers. Field-Effect Transistor Performance. To investigate heterocyclic effect on carrier transport performance of both polymer, solution-processbile TGBC PFET devices (Figure 5a) were fabricated by simple solution spin-coating techniques. Figures 5c–5f and Table 2 show the representative J–V curves and characteristic data of PFETs, respectively. At VDS = 100 V, all the PFET devices exhibited air-stable n-channel characteristics under ambient conditions, and afforded moderate current on/off ratios (Ion/Ioff >103). Interestingly we find that the electron mobilitis of the polymers are highly dependent on heterocylce structures; and a little atom 9 ACS Paragon Plus Environment


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change in polymer backbones leads to significant influence on carrier transport. For the OZcontaining PNOZ, the maximum electron mobility was demonstrated to be 0.026 cm2 V−1 s−1. Compared with PNOZ, the TZ-containing PNTZ exhibited excellent electron transporting characteristics, affording the maximum electron mobility of 0.36 cm2 V−1 s−1, an order of magnitude higher as compared to the OZ-containing PNOZ. The significantly enhanced electron mobilities for PNTZ can be well explained by its superior film morphology and compact backbone stacking demonstrated by AFM and GIXRD data. To investigate PFETs air-stability, TGBC PFETs fabricated from two polymer thin films were stored in ambient conditions. And their electron mobilities were monitored for 60 days. The results showed that the electron mobilities of both polymers-based PFETs exhibited an excellent air-stability with a negligible decay in more than 60 days (Figure 5b). The observed electron mobilities of PNOZ and PNTZ still maintained at 0.02 and 0.33 cm2 V−1s−1, respectively. Judged from the criteria reported by Marks group, the LUMO values (–4.05 and – 4.02 eV, Table 1) for PNOZ and PNTZ are still not deep enough for thermodynamic stability of electron transport because electron can be oxidized by H2O or O2 from air enviroment.51 Therefore, the remarkable air-stability for these TGBC PFET devices mainly arises from the encapsulation effect of thick PMMA (1350 nm) dielectric layer.22,52,53 Table 2. Summary of PFET Device Performance and GIXRD Results Polymers

µaver

µmax

2

2

–1 –1

–1 –1

Vth

Ion/Ioff

d–d

π–π

(Å)

(Å)

(cm V s )

(cm V s )

（V）

PNOZ

0.02 ± 0.002

0.026

7±3

>104

22.06

3.86

PNTZ

0.32 ± 0.03

0.36

7±3

>103

22.06

3.80


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Figure 5. (a) Device structure of TGBC PFETs; (b) Air-stability of PNOZ- and PNBTZ-based FETs; Typical Transfer (c) and output (d) curves for PNOZ-based FETs (0.026 cm2 V–1 s–1); typical Transfer (e) and output (f) curves for PNTZ-based FETs (0.36 cm2 V–1 s–1). Morphology and Microstructure. Since the film morphology, crystallinity, and ordering structure of the polymers can be well associated with the difference of their carrier transport performance,1–5 the surface morphology and microstructures of both polymer thin films were further studied by AFM and GIXRD measurements in order to investigate why PNTZ had higher electron mobilities as compared to PNOZ. Figure 6 shows the surface topography of thin films annealed at 180 °C. Note that the film surface morphology can be influenced by tuning heterocyclic acceptors significantly. The OZ-containing PNOZ thin films exhibit a root mean square roughness (RMS) of 1.35 nm; and a nodule-like surface morphology with some small “holes” can be observed clearly. As a result, a roughness surface morphology with a poor film interconnectivity was revealed for PNOZ thin films. The fact is that too much roughness surface structure will lead to inferior carrier transport.27 On the contrary, the TZ-containing PNTZ thin films exhibited more smooth surface with a surface roughness of 0.72 nm and afforded more uniform fiber-like morphologies with the interconnected polycrystalline grains. The results indicate that PNTZ thin films form a fairly homogeneous surface structure with a good film interconnectivity. Such smooth surface morphology would be helpful for forming a good interface contact between semiconductor layer and PMMA dielectric layer, and therefore facilitating carrier transport of TGBC PFETs.27 All these images show the evidences that, compared to PNOZ thin films, the TZ-containing PNTZ thin films provide higher order bulk



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organization, especially at the surface of films. Therefore this is an important reason to explain why PNTZ exhibits much higher electron mobilities than that of PNOZ.

Figure 6. AFM images (3 µm × 3 µm) of the polymer thin films annealed at 180 °C: (a–b) topography images; (c–d) phase images. To investigate crystallinity and stacking structure of the polymer thin films, GIXRD measurements were also performed. Figure 7 shows the typical GIXRD diffraction curves including out-of-plane and in-plane pattern. As shown in out-of-plane patterns (Figure 7a), both polymer thin films show sharp, multiple (h00) diffraction peaks, where (100) peaks appear at 2θ = 4.0° for PNOZ and PNTZ, with the same interlayer distance of 22.06 Å. Note that the observed interlayer distances are obviously shorten as compared to the fully extended 2octyldodecyl side chains (~36 Å). The results suggest that all the 2-octyldodecyl side chains can be well self-assembled with each other in their solid state films.10 As shown in Figure 7, the (100) diffraction peaks appear in both diffraction patterns simultaneously. Therefore the results imply that both polymers thin films adopt a random stacking model, mainly including face-on and edge-on models.22 However, we find that the (010) diffraction peaks that assigned to the π–π stacking can be detected from out-of-plane patterns clearly (Figure 7a), demonstrating that both polymers take mainly face-on stacking model. This kind of stacking model is similar with many reported high-performance polymer semiconductors.22 On the basis of the out-of-plane pattern data, both polymers thin films exhibit the compact (010) features at 2θ = 23.4 o for PNOZ and 23.0 o for PNTZ. Thus the calculated π–π stacking distances of PNOZ and PNTZ are 3.86 and PNOZ, respectively. Such compact π–π stacking will enhance devices stability because of the effective inhibition of H2O or O2 penetration into active layer.52,53 Consistent with the data of 12 ACS Paragon Plus Environment

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absorption spectra discussed above, strongly polarizable ability for sulfur atom in TZ heterocycles might provide an effective driving force for intermolecular interaction of PNTZ as compared to the OZ-containing PNOZ. As a result, PNTZ afforded slightly shorter π–π stacking distance as compared to PNOZ. More important, the advantageous including compact π–π stacking and uniform film morphology ensure more efficient intermolecular carrier hopping for PNTZ.

Figure 7. GIXRD diffraction patterns of the two polymer thin films annealed at 180 °C: Out-ofplane (a) and in-plane (b).

CONCLUSIONS We have demonstrated that two well-known five-membered heterocyclic acceptors, including 1,3,4-oxadiazole (OZ) and 1,3,4-thiadiazole (TZ) heterocycles, can be ultilized to design and synthesize novel D–A type polymeric semiconductors for their applications in solution-processible n-channel PFETs sucessfully. The resulting polymers (PNOZ and PNTZ) exhibited high electron affinity and deep-lying LUMO values of ca. –4.0 eV owing to the incorporation of electron-deficient heterocycles (OZ and TZ) and NDI acceptors into the polymer backbones. It was found that clear evidences for polymer backbone aggregations were observed in both solution and thin-film absorption spectra; and the TZ-containing PNTZ exhibited much stronger aggregation ability than that of the OZ-containing PNOZ. Under ambient conditions, air-stable n-channel transport characteristics had been observed for TGBC PFETs. Particularly, the TZ-containing PNTZ obtained the peak electron mobility up to 0.36 cm2 V−1 s−1, an order of magnitude higher compared to the OZ-containing PNOZ. Moreover, the observed electron mobility value are the highest one reported to date among the OZ- or TZ13 ACS Paragon Plus Environment


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containing organic semiconductors. These results have demonstrated that TZ unit provides more promising potential in the development of high-performance n-channel polymers as compared to OZ unit.

ASSOCIATED CONTENT Supporting Information Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Experimental and characterization method, PFETs fabrication and characterization, synthesis and characterization of the monomers and polymers, NMR spectra, GPC curves, TGA curves of the polymers, CV curves for the Fc/Fc+ couple.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (H. C.) *E-mail: [email protected] (S. T.) *E-mail: [email protected] (Y. L.) Author Contributions ⊥

Zhaoxia Liu and Zhiyuan Zhao equally contributed to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The GIXRD data was obtained at 1W1A, Beijing Synchrotron Radiation Facility. The authors gratefully acknowledge the assistance of scientists of Diffuse X-ray Scattering Station during the experiments. This work was supported by the National Natural Science Foundation of China (51403177, 51233006, and 21474081), the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB12030100), the Natural Science Foundation of Hunan Province (2015JJ3122), China Postdoctoral Science Foundation (2015T80877 and 2014M552141), the Science and Technology Planning Project of Hunan Province (2015RS4025), and the Research Foundation of Education College of Hunan Province (15C1314).

REFERENCES 14 ACS Paragon Plus Environment

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