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Acceptor-donor-acceptor small molecules based on fuse ring and 2-(2-oxindolin-3-ylidene)malononitrile derivatives for solution-processed n-type organic field-effect transistors
Synthetic Metals 256 (2019) 116143
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Acceptor-donor-acceptor small molecules based on fuse ring and 2-(2oxindolin-3-ylidene)malononitrile derivatives for solution-processed n-type organic field-effect transistors ⁎
Guobing Zhanga,b, , Mingxiang Suna,b, Weiwei Wanga,b, Longzhen Qiua,b,
T
⁎
a National Engineering Laboratory of Special Display Technology, State Key Laboratory of Advanced Display Technology, Academy of Photo-Electronic Technology, Hefei University of Technology, Hefei, 230009, PR China b School of Chemistry and Chemical Engineering, Hefei University of Technology, Key Laboratory of Advance Functional Materials and Devices of Anhui Province, Hefei, 23009, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Organic field-effect transistors n-type 2-(2-oxindolin-3-ylidene)malononitrile derivatives Acceptor-donor-acceptor
The di-cyanovinyl-based small molecules were successfully used as non-fullerene acceptors in high-performance organic solar cells. However, the field-effect performances of these types of small molecules are still rarely studied. In this paper, two acceptor-donor-acceptor (A–D–A) small molecules with fuse ring as donor and strong electron-withdrawing 2-(2-oxindolin-3-ylidene)malononitrile derivatives as acceptors were synthesized and characterized for use in solution-processed organic field-effect transistors (OFETs). The new molecules had deep lowest unoccupied molecular orbital energy level (˜ –4.0 eV) for marching the electron transport and exhibited ntype charge transport characteristics. The OFETs based on the small molecules exhibited the highest electron mobility of over 3 × 10–3 cm2V–1s–1. This work indicates that di-cyanovinyl-based small molecules can also be applied in n-type OFETs.
1. Introduction Solution-processed organic field-effect transistors (OFETs) have received tremendous attention due to their potential applications in lightweight, cost-effective, and large-area flexible electronics. [1–3] Recent researches demonstrate that the conjugated polymers based on donor (D) and acceptor (A) units is a promising structures to develop highperformance OFETs. Indeed, the past few years have witnessed the rapid progress of solution-processed D–A polymers with the highest mobilities surpassing 10 cm2V–1s–1 for both holes and electron. [4–6] Conjugated polymers showed batch-to-batch variations and poor reproducibility resulted from molecular weight and polydispersity which have strong influence on the field-effect performances [7,8]. In comparison, solution-processed small molecules can be synthesized with well-defined structure and precise molecular weight that endow the semiconductors with excellent reproducibility. [9,10] In the past few years, the small molecules based on D and A units have attracted intensive interest and been used in non-fullerene acceptor-based organic solar cells. [11–13] Significant progresses in devices performances have been made, with the power conversion
efficiencies exceeding 14% for non-fullerene acceptor-based organic solar cells [14,15]. However, these D/A small molecules have been rarely study as the semiconductor layer for OFETs. The overall development of OFET based on D/A small molecules lags much behind their application in organic solar cells in terms of device performances, amount, and so on [16–18]. In this paper, two A–D–A small molecules (M1 and M2, Scheme 1) based on fused ring indacenodithieno[3,2-b]thiophene (IDTT) as donor and 2-(2-oxindolin-3-ylidene)malononitrile derivatives as acceptors were synthesized for application in n-type OFETs. IDTT is the laddertype arene which endows the A–D–A small molecules with a large-plane conjugated center and excellent molecule packing. 2-(2-oxindolin-3ylidene)malononitrile is a very strong acceptor because of containing lactam and cyan groups which can effectively lowered the lowest unoccupied molecular orbital energy (LUMO) levels of A–D–A small molecules. Consequently, the bottom-gate/top-contact (BG/TC) OFET devices based on the two small molecules as the semiconductors exhibited n-type transport with electron mobility of up 3.4 × 10–3 cm2V–1s–1.
⁎ Corresponding authors at: National Engineering Laboratory of Special Display Technology, State Key Laboratory of Advanced Display Technology, Academy of Photo-Electronic Technology, Hefei University of Technology, Hefei, 230009, PR China. E-mail addresses: [email protected] (G. Zhang), [email protected] (L. Qiu).
https://doi.org/10.1016/j.synthmet.2019.116143 Received 26 May 2019; Received in revised form 1 August 2019; Accepted 10 August 2019 Available online 17 August 2019 0379-6779/ © 2019 Elsevier B.V. All rights reserved.
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Scheme 1. Synthetic routes to monomers and small molecules.
(trimethylstannane) was supplied by Derthon Optoelectronics Materials Science Technology Co. LTD. Other chemicals used in this work were purchased from Sinopharm Chemical Reagent Co. LTD. and Shanghai Titan Scientific Co. LTD., China. 6-Bromo-1-hexyl-indoline-2,3-dione (1) and 6-bromo-1-(2-hexyl)-1H-pryyolo[2,3-b]pyridine-2,3-dione (3) were synthesized according to the literature. [9] 2.2. Measurements and characterization Nuclear magnetic resonance (NMR) spectra were record using an Agilent VNMRS (400 MHz). Thermogravimetric analysis (TGA) was conducted with a STA449F5 at a heating rate of 20 °C/min under nitrogen flow. UV–Vis absorption spectra were taken on an Agilent Cary 5000 model spectrophotometer. Cyclic voltammetry (CV) was conducted on a CHI660D electrochemical work station in anyhydrous acetonitrile solution containing 0.1 M tetra-n-butylammounium hexafluorophosphate with a scan rate of 100 mV/s. A platinum (Pt) disk electrode was used as the working electrode and a Pt wire was used as counter electrode. Small molecule thin film was coated on the platinum-disc electrode. The Ag/Ag+ electrode was used as the reference electrode. Grazing-incidence-X-ray diffraction (GIXD) measurements were performed using 9A beamlines at the Pohang Accelerator Laboratory (PAL) in Korea. The atomic force microscopy (AFM) images were obtained using a SPA300 HV instrument.
Fig. 1. Normalized UV–Vis–NIR absorption spectra of M1 and M2 in solution and in solid films. Table 1 Thermal, optical, and electrochemical properties of small molecules. Small molecule
M1 M2
Td
λmax [nm]
[oC]a
solution
417 425
687 725
λonset
Eopt g
ELUMO
EHOMO
Ecv g
film
[nm]
[eV]b
[eV]c
[eV]c
[eV]
713 745
870 897
1.43 1.38
–4.0 –4.10
–5.46 –5.35
1.46 1.25
2.3. Fabrication and characterization of OFET devices
The 5% weight loss temperatures. Eg = 1240/λonset (in film). LUMO = + ox −(4.75 +Ered onset ) and HOMO = −(4.75 + Eonset ); the redox Fc/Fc was located + at 0.05 V related Ag/Ag . a
b
opt
c
BG/TC devices were fabricated on a gate of n-doped Si with a 300nm-thick SiO2 dielectric layer. The substrates were subjected to a piranha solution followed by UV–ozone treatment. The surface of the wafer was modified with octadecyltrimethoxysilane (OTS) self-assembled monolayer (SAM) according to the previous procedures. [19] A chloroform solution (˜ 6 mg/mL) was dropped onto the treated Si/SiO2 and spin-coated at 3500 rpm for 40 s in a glove box. The small molecule films were annealed at different temperatures (150–210 °C) in a glove box. The Au source-drain electrodes were deposited by thermal evaporation. The OFET devices had a channel length (L) of 130 μm and a
2. Experimental section 2.1. Materials All the chemicals were used as received without further purification. Tetrakis(4-hexylphenyl)-indacenodithieno[3,2-b]thiophene-bis2
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Fig. 2. (a) Cyclic voltammograms of M1 and M2. (b) The calculated molecular orbital of small molecules using DFT calculations at the B3LYP/6–31 G(d) level.
24 h and then quench with water. The organic layer was extracted with dichloromethane and dried with anhydrous sodium sulfate. The residue was purified by flash chromatography on silica gel with ether/petroleum ether (1: 5) to give the compound 4 (0.2 g, 34.5%). 1H NMR (400 MHZ, CDCl3): δ = 8.10 (d, 1 H), 7.28 (d, 1 H), 3.81 (t, 2 H), 1.72 (m, 2H), 1.2–1.4 (m, 6H), 0.89 (t, 3 H).
Table 2 Field-effect performances of devices based on the small molecules. Annealing Temperature (oC)
n-channel μe,max (cm2V−1s−1)
μe,avg (cm2V−1s−1)
Ion/Ioff
Vth (V)
M1 180 210 240
2.4 × 10−4 1.0 × 10−3 1.0 × 10−4
2.3 × 10−4 7.0 × 10−4 8.5 × 10−5
2.4 × 103 1.7 × 103 7.2 × 103
25.0 14.6 15.6
M2 130 180 210 240
4.1 × 10−4 7.7 × 10−4 3.4 × 10−3 1.1 × 10−3
2.9 × 10−4 5.4 × 10−4 1.0 × 10−3 6.8 × 10−4
5.9 × 103 3.2 × 103 4.7 × 103 1.0 × 103
29.1 14.4 12.1 26.9
2.4.3. Synthesis of M1 A Schlenk tube was charge with compound 2 (0.096 g, 0.28 mmol), tetrakis(4-hexylphenyl)-indacenodithieno[3,2-b]thiophene-bis(trimethylstannane) (0.16 g, 0.12 mmol), and toluene (15 mL). The solution was degassed by nitrogen flow for 30 min. Tri(dibenzylideneacetone)dipalladium (Pd2(dba)3, 4 mg) and tri-o-tolylphosphine (P(otolyl)3, 5 mg) were added into the solution. The tube was capped and heated to 105 °C for overnight. After cooling to room temperature, the mixture was poured into a solution and extracted with dichloromethane. The combined organic layer was dried over anhydrous sodium sulfate. After removing the solvent, the residue was purified by silica gel column chromatography using a mixture of petroleum ether/ ether/dichloromethane (5:1:1) to afford a blue solid (100 mg, 54.6%). 1 H NMR (400 MHZ, CDCl3): δ = 8.08 (d, 2 H), 7.78 (s, 2 H), 7.56 (s, 2 H), 7.30 (d, 2 H), 7.20 (d, 8 H), 7.13 (d, 8 H), 3.75 (t, 4 H), 2.58 (t, 8 H), 1.70 (m, 4 H), 1.60 (m, 6 H), 1.25–1.45 (m, 38 H), 0.84-0.92 (m, 18 H). 13C NMR (100 MHZ, CDCl3): δ = 163.09, 154.13, 147.67, 147.35, 146.56, 146.23, 143.64, 143.36, 143.29, 142.26, 139.58, 136.28, 135.68, 128.67, 127.92, 127.48, 119.98, 117.45, 117.00, 111.16, 105.29, 79.66, 62.90, 40.47, 35.61, 31.70, 31.36, 31.32, 29.19, 27.43, 26.55, 22.61, 22.51, 14.12, 14.01. Anal. Calcd for C102H104N6O2S4: C, 77.82, H, 6.66, N, 5.34; found: C, 77.69, H, 6.49, N, 5.31.
channel width (W) of 760 μm. The devices were characterized under vacuum condition using a Keithley 4200 semiconductor parametric analyzer. The saturation-regime mobility (μ) was obtained using the following equation: Id = (W/2L)Ciμ(Vg-Vth)2, where Id is the drain current, Ci is the capacitance of the gate dielectric, Vg is the gate-source voltage, and Vth is the threshold voltage, Vd is the source-drain voltage. 2.4. Synthesis 2.4.1. Synthesis of 2-(6-bromo-1-(2-hexyl)-2-oxoindolin-3-ylidene) malononitrile (2) Toluenesulfonic acid (TsOH, 10 mg) was added to a mixture of compound 1 (1.0 g, 3.22 mmol), malononitrile (0.43 g, 6.44 mmol), and acetic acid (10 mL) in a round flask under nitrogen atmosphere. The reaction mixture was reflux for overnight. After cooled to room temperature, solvent was removed under reduced pressure and residue was purified by flash chromatography on silica gel with ether/petroleum ether (1: 5) as eluent to give the titled compound (1.1 g, 95.5%). 1H NMR (400 MHZ, CDCl3): δ = 8.0 (d, 1 H), 7.30 (d, 1 H), 7.05 (s, 1 H), 3.71 (t, 2 H), 1.70 (m, 2H), 1.2–1.4 (m, 6H), 0.91 (t, 3 H).
2.4.4. Synthesis of M2 A mixture of compound 4 (0.063 g, 0.18 mmol), tetrakis(4-hexylphenyl)-indacenodithieno[3,2-b]thiophene-bis(trimethylstannane) (0.11 g, 0.08 mmol), Pd2(dba)3 (4 mg), and P(o-tol)3 (5 mg) were taken in toluene (15 mL) to synthesize M2 according to the procedure of M1. A blue solid was collected (60 mg, 46.5%). 1H NMR (400 MHZ, CDCl3): δ = 8.08 (d, 2 H), 7.78 (s, 2 H), 7.56 (s, 2 H), 7.30 (d, 2 H), 7.20 (d, 8 H), 7.13 (d, 8 H), 3.75 (t, 4 H), 2.58 (t, 8 H), 1.70 (m, 4 H), 1.60 (m, 6 H), 1.25–1.45 (m, 38 H), 0.84-0.92 (m, 18 H). 13C NMR (100 MHZ, CDCl3): δ = 162.98, 159.65, 157.56, 154.55, 147.91, 146.95, 146.55, 144.50, 143.65, 142.30, 139.50, 138.55, 136.50, 133.46, 128.68, 127.94, 122.40, 117.55, 113.40, 112.80, 110.90, 110.32, 79.94, 63.09, 39.70, 35.60, 31.70, 31.30, 31.25, 29.70, 29.16, 27.36, 26.37, 22.57,
2.4.2. Synthesis of 2-(6-bromo-7-aza-1-(2-hexyl)-2-oxoindolin-3-ylidene)malononitrile (4) TsOH (10 mg) was added to a mixture of compound 3 (0.5 g, 1.61 mmol), malononitrile (0.21 g, 3.22 mmol), and acetic acid (10 mL) in a round flask under nitrogen atmosphere. The mixture was reflux for 3
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Fig. 3. The output and transfer curves of two small molecules, a) M1, and b) M2.
absorption of both films showed significant red-shift of about 32 and 38 nm for M1 and M2, respectively, indicating that the molecules may be well aggregated in the solid state resulted from the intermolecular and intramolecular interactions. The maximum absorption of M2 distinctly red shifted to ˜20 nm and the whole absorption spectra were much broader compared with the absorption spectra of M1. This can be attributed to the different electron-deficient power of acceptor units. The electron-withdrawing characteristics of compound 4 with N-substitution is stronger than that of compound 2 because of electronegative N atom, resulting in stronger interaction between D and A units compared with M1 without N substitution. The optical bandgaps of M1 and M2 were 1.43 and 1.38 eV calculated from the onset absorption in the solid state. The electrochemical properties of M1 and M2 were characterized by cyclic voltammetry (CV) (Fig. 2a). The corresponding data, such as LUMO, highest occupied molecular orbital (HOMO), and electrochemical bandgap (Ecv g ) were also summarized in Table 1. Deep LUMO/ HOMO energy levels were obtained for the two small molecules because of the strong electron-deficient di-cyanovinyl units. M1 exhibited LUMO/HOMO energy levels of about –4.0/–5.46 eV, indicating that the small molecule may be potential n-type semiconductor for electron injection and transport when a suitable mental (e.g., Au) is used as the electrode in OFET devices. M2 which had the electronegative N-substitution, exhibited the LUMO/HOMO energy levels of –4.10/–5.35 eV, indicating the N-substitution mainly lowered the LUMO energy level. Density functional theory (DFT) calculation of both small molecules were performed using Gaussian 09 at the B3LYP/6–31 G(d) level to investigate the electron structures. All the alkyl side chains were
22.43, 14.07, 14.00. Anal. Calcd for C100H102N8O2S4: C, 76.20, H, 6.52, N, 7.11; found: C, 76.08, H, 6.69, N, 7.06. 3. Results and discussion 3.1. Synthesis and characterization The synthetic routes for the small molecules M1 and M2 are shown in Scheme 1. The compound 1 and 3 were synthesized according to the reported procedures [9]. The di-cyanovinyl groups were introduced via Knoevenagel condensation reaction to endow compound 2 and 4 with strongly electron-withdrawn characteristics. The A–D–A small molecules (M1 and M2) were obtained via Stille coupling reactions of dicyanovinyl compounds and di-stannylated fused-monomer. The small molecules were examined by NMR (1H and 13C) and elemental analysis. The thermal properties of two small molecules were characterized by thermogravimetric (TGA) under nitrogen atmosphere (Fig. S1). The TGA showed excellent thermal stabilities for the two small molecules with 5% weight loss temperature (Td) of over 400 °C. 3.2. Optical and electrochemical properties The UV–Vis–NIR absorption spectra of two A–D–A molecules in chloroform solutions and on drop-cast thin films are present in Fig. 1. The corresponding data are also summarized in Table 1. Two small molecules exhibited dual absorption bands covering from ˜350–900 nm. The maximum absorption (λmax) of M1 and M2 were 687 and 713 nm, respectively. Compared with those of solution, the maximum 4
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Fig. 4. 2D-GIXD patterns and AFM height images (5 μm × 5 μm) of small molecule films spin-coated on OTS-treated SiO2/Si substrates.
3.4. Thin-film microstructural characterization
replaced with methyl groups in order to simplify the calculation (Fig. 2b). The LUMO energy levels are both concentrated on the 2-(2oxindolin-3-ylidene)malononitrile derivatives (A) unit, whereas the HOMO energy levels are located mainly on the fused ring (D). This may be interpreted why the N-substitution mainly lowered the LUMO energy level of M2. M1exhibited a dihedral angle of 18.5° between 2-(2-oxindolin-3-ylidene)malononitrile and fused ring units (Fig. S2). In comparison, M2 exhibited a relatively planar conjugated backbone and the dihedral angles decrease to 0.5°. This may be attributed to the N-substitution which can induce intramolecular noncovalent interactions (S···N or CH···N). [9]
The annealing films of the small molecules (M1 and M2) were investigated by two-dimensional grazing incidence X-ray diffraction (2DGIXD) and atomic force microscopy (AFM). The 2D-GIXD images are shown in Fig. 4a and b. The M1 film exhibited strong diffraction peak at 0.4 Å−1 corresponding to (001) along the out-of-plane qz direction. The corresponding d spacing was 15.7 Å for M1. The film of M2 showed strong first-order diffraction peak at 0.38 Å−1 ascribing to (001), and also showed weak second-order diffraction peaks at 0.75 Å−1 corresponding to (002). Therefore, the lamellar spacing was 16.5 Å for M2. The results indicate that small molecule M2 had ordered lamellar crystalline structure. The height images of the annealing films are shown in Fig. 4c and d. The AFM images of annealing films showed bulk crystal in the whole area for both small molecules. The AFM results are consistent with the GIXD results mention above. However, the crystals in the whole substrates exhibited disordered and loose packing, which result in obvious and deep brain boundaries and may be difficult to form highly efficient pathways for charge carrier transport. Moreover, both the films showed very rough surface with large root mean square roughness (RMS) of 8.96 and 8.72 nm for M1 and M2, respectively. The too rough surface may lead to poor interfacial adhesion and be also unfavorable for charge transport. Therefore, the OFET devices based on the small molecules exhibited moderate field-effect performances.
3.3. OFET properties The charge transport characteristics of the two small molecules were investigated using BG/TC device architectures. The small molecules were spin-coated from chloroform solution (6 mg/mL), followed by annealing at different temperature to optimize the performances. Table 2 summarized the corresponding data such as the field effect mobility (μ), threshold voltage (Vth), and the on/off current ratio (Ion/ Ioff) of the OFET devices. Fig. 3 shows the typical output and transfer curves of M1 and M2-based devices. The two small molecules exhibited unipolar electron transport characteristics under vacuum conditions. This may be relative to the LUMO energy levels which were low enough for stable electron transport under vacuum condition. The optimized temperatures were 210 °C for both small molecules. M1 exhibited the highest electron mobility of 1.0 × 10–3 cm2V–1s–1 with an average electron mobility of 7.0 × 10–4 cm2V–1s–1. M2 displayed the optimized electron mobility of 3.4 × 10–3 cm2V–1s–1 with an average mobility of 1.0 × 10–3 cm2V–1s–1. The two small molecules showed relatively low field-effect performances.
4. Conclusion Two A–D–A small molecules (M1 and M2) based on the fused ring (IDTT) as the donor and 2-(2-oxindolin-3-ylidene)malononitrile derivatives as acceptors were synthesized and characterized. The small molecules have low optical bandgaps of about 1.4 eV and deep LUMO energy levels of about –4.0 eV for electron injection and transport. OFET devices based on small molecules exhibited unipolar electron 5
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transport characteristics with moderate mobility of 3.4 × 10–3 cm2V–1s–1. This work indicates that the di-cyanovinyl-based small molecules can also be useful semiconductor for solution processed n-type OFETs.
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Acknowledgments This work was supported by the Open Foundation of Key Laboratory of Advanced Functional Materials and Devices of Anhui Province, and Double First Class Enhancing Independent Innovation and Social Service Capabilities of Hefei University of Technology (Grant No. 4500411104/011), the Fundamental Research Funds for the Central Universities (Grant No. JZ2018HGPB0276), and the National Natural Science Foundation of China (NSFC, Grant nos. 51573001, 51573036, 51703047). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.synthmet.2019. 116143. References [1] H. Sirringhaus, 25th anniversary article: organic field-effect transistors: the path beyond amorphous silicon, Adv Mater 26 (2014) 1319–1335. [2] Y. Liu, F. Wang, J. Chen, X. Wang, H. Lu, L. Qiu, G. Zhang, Improved transistor performance of isoindigo-based conjugated polymers by chemically blending strongly electron-deficient units with low content to optimize crystal structure, Macromolecules 51 (2018) 370–378. [3] J. Mei, Y. Diao, A.L. Appleton, L. Fang, Z. Bao, Integrated materials design of organic semiconductors for field-effect transistors, J. Am. Chem. Soc. 135 (2013) 6724–6746. [4] L. Shi, Y. Guo, W. Hu, Y. Liu, Design and effective synthesis methods for highperformance polymer semiconductors in organic field-effect transistors, Mater. Chem. Front. 1 (2017) 2423–2456. [5] A.F. Paterson, S. Singh, K.J. Fallon, T. Hodsden, Y. Han, B.C. Schroeder, et al., Recent progress in high-mobility organic transistors: a reality check, Adv Mater 30 (2018) 1801079.
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Acceptor-donor-acceptor small molecules based on fuse ring and 2-(2oxindolin-3-ylidene)malononitrile derivatives for solution-processed n-type organic field-effect transistors ⁎
Guobing Zhanga,b, , Mingxiang Suna,b, Weiwei Wanga,b, Longzhen Qiua,b,
T
⁎
a National Engineering Laboratory of Special Display Technology, State Key Laboratory of Advanced Display Technology, Academy of Photo-Electronic Technology, Hefei University of Technology, Hefei, 230009, PR China b School of Chemistry and Chemical Engineering, Hefei University of Technology, Key Laboratory of Advance Functional Materials and Devices of Anhui Province, Hefei, 23009, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Organic field-effect transistors n-type 2-(2-oxindolin-3-ylidene)malononitrile derivatives Acceptor-donor-acceptor
The di-cyanovinyl-based small molecules were successfully used as non-fullerene acceptors in high-performance organic solar cells. However, the field-effect performances of these types of small molecules are still rarely studied. In this paper, two acceptor-donor-acceptor (A–D–A) small molecules with fuse ring as donor and strong electron-withdrawing 2-(2-oxindolin-3-ylidene)malononitrile derivatives as acceptors were synthesized and characterized for use in solution-processed organic field-effect transistors (OFETs). The new molecules had deep lowest unoccupied molecular orbital energy level (˜ –4.0 eV) for marching the electron transport and exhibited ntype charge transport characteristics. The OFETs based on the small molecules exhibited the highest electron mobility of over 3 × 10–3 cm2V–1s–1. This work indicates that di-cyanovinyl-based small molecules can also be applied in n-type OFETs.
1. Introduction Solution-processed organic field-effect transistors (OFETs) have received tremendous attention due to their potential applications in lightweight, cost-effective, and large-area flexible electronics. [1–3] Recent researches demonstrate that the conjugated polymers based on donor (D) and acceptor (A) units is a promising structures to develop highperformance OFETs. Indeed, the past few years have witnessed the rapid progress of solution-processed D–A polymers with the highest mobilities surpassing 10 cm2V–1s–1 for both holes and electron. [4–6] Conjugated polymers showed batch-to-batch variations and poor reproducibility resulted from molecular weight and polydispersity which have strong influence on the field-effect performances [7,8]. In comparison, solution-processed small molecules can be synthesized with well-defined structure and precise molecular weight that endow the semiconductors with excellent reproducibility. [9,10] In the past few years, the small molecules based on D and A units have attracted intensive interest and been used in non-fullerene acceptor-based organic solar cells. [11–13] Significant progresses in devices performances have been made, with the power conversion
efficiencies exceeding 14% for non-fullerene acceptor-based organic solar cells [14,15]. However, these D/A small molecules have been rarely study as the semiconductor layer for OFETs. The overall development of OFET based on D/A small molecules lags much behind their application in organic solar cells in terms of device performances, amount, and so on [16–18]. In this paper, two A–D–A small molecules (M1 and M2, Scheme 1) based on fused ring indacenodithieno[3,2-b]thiophene (IDTT) as donor and 2-(2-oxindolin-3-ylidene)malononitrile derivatives as acceptors were synthesized for application in n-type OFETs. IDTT is the laddertype arene which endows the A–D–A small molecules with a large-plane conjugated center and excellent molecule packing. 2-(2-oxindolin-3ylidene)malononitrile is a very strong acceptor because of containing lactam and cyan groups which can effectively lowered the lowest unoccupied molecular orbital energy (LUMO) levels of A–D–A small molecules. Consequently, the bottom-gate/top-contact (BG/TC) OFET devices based on the two small molecules as the semiconductors exhibited n-type transport with electron mobility of up 3.4 × 10–3 cm2V–1s–1.
⁎ Corresponding authors at: National Engineering Laboratory of Special Display Technology, State Key Laboratory of Advanced Display Technology, Academy of Photo-Electronic Technology, Hefei University of Technology, Hefei, 230009, PR China. E-mail addresses: [email protected] (G. Zhang), [email protected] (L. Qiu).
https://doi.org/10.1016/j.synthmet.2019.116143 Received 26 May 2019; Received in revised form 1 August 2019; Accepted 10 August 2019 Available online 17 August 2019 0379-6779/ © 2019 Elsevier B.V. All rights reserved.
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Scheme 1. Synthetic routes to monomers and small molecules.
(trimethylstannane) was supplied by Derthon Optoelectronics Materials Science Technology Co. LTD. Other chemicals used in this work were purchased from Sinopharm Chemical Reagent Co. LTD. and Shanghai Titan Scientific Co. LTD., China. 6-Bromo-1-hexyl-indoline-2,3-dione (1) and 6-bromo-1-(2-hexyl)-1H-pryyolo[2,3-b]pyridine-2,3-dione (3) were synthesized according to the literature. [9] 2.2. Measurements and characterization Nuclear magnetic resonance (NMR) spectra were record using an Agilent VNMRS (400 MHz). Thermogravimetric analysis (TGA) was conducted with a STA449F5 at a heating rate of 20 °C/min under nitrogen flow. UV–Vis absorption spectra were taken on an Agilent Cary 5000 model spectrophotometer. Cyclic voltammetry (CV) was conducted on a CHI660D electrochemical work station in anyhydrous acetonitrile solution containing 0.1 M tetra-n-butylammounium hexafluorophosphate with a scan rate of 100 mV/s. A platinum (Pt) disk electrode was used as the working electrode and a Pt wire was used as counter electrode. Small molecule thin film was coated on the platinum-disc electrode. The Ag/Ag+ electrode was used as the reference electrode. Grazing-incidence-X-ray diffraction (GIXD) measurements were performed using 9A beamlines at the Pohang Accelerator Laboratory (PAL) in Korea. The atomic force microscopy (AFM) images were obtained using a SPA300 HV instrument.
Fig. 1. Normalized UV–Vis–NIR absorption spectra of M1 and M2 in solution and in solid films. Table 1 Thermal, optical, and electrochemical properties of small molecules. Small molecule
M1 M2
Td
λmax [nm]
[oC]a
solution
417 425
687 725
λonset
Eopt g
ELUMO
EHOMO
Ecv g
film
[nm]
[eV]b
[eV]c
[eV]c
[eV]
713 745
870 897
1.43 1.38
–4.0 –4.10
–5.46 –5.35
1.46 1.25
2.3. Fabrication and characterization of OFET devices
The 5% weight loss temperatures. Eg = 1240/λonset (in film). LUMO = + ox −(4.75 +Ered onset ) and HOMO = −(4.75 + Eonset ); the redox Fc/Fc was located + at 0.05 V related Ag/Ag . a
b
opt
c
BG/TC devices were fabricated on a gate of n-doped Si with a 300nm-thick SiO2 dielectric layer. The substrates were subjected to a piranha solution followed by UV–ozone treatment. The surface of the wafer was modified with octadecyltrimethoxysilane (OTS) self-assembled monolayer (SAM) according to the previous procedures. [19] A chloroform solution (˜ 6 mg/mL) was dropped onto the treated Si/SiO2 and spin-coated at 3500 rpm for 40 s in a glove box. The small molecule films were annealed at different temperatures (150–210 °C) in a glove box. The Au source-drain electrodes were deposited by thermal evaporation. The OFET devices had a channel length (L) of 130 μm and a
2. Experimental section 2.1. Materials All the chemicals were used as received without further purification. Tetrakis(4-hexylphenyl)-indacenodithieno[3,2-b]thiophene-bis2
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Fig. 2. (a) Cyclic voltammograms of M1 and M2. (b) The calculated molecular orbital of small molecules using DFT calculations at the B3LYP/6–31 G(d) level.
24 h and then quench with water. The organic layer was extracted with dichloromethane and dried with anhydrous sodium sulfate. The residue was purified by flash chromatography on silica gel with ether/petroleum ether (1: 5) to give the compound 4 (0.2 g, 34.5%). 1H NMR (400 MHZ, CDCl3): δ = 8.10 (d, 1 H), 7.28 (d, 1 H), 3.81 (t, 2 H), 1.72 (m, 2H), 1.2–1.4 (m, 6H), 0.89 (t, 3 H).
Table 2 Field-effect performances of devices based on the small molecules. Annealing Temperature (oC)
n-channel μe,max (cm2V−1s−1)
μe,avg (cm2V−1s−1)
Ion/Ioff
Vth (V)
M1 180 210 240
2.4 × 10−4 1.0 × 10−3 1.0 × 10−4
2.3 × 10−4 7.0 × 10−4 8.5 × 10−5
2.4 × 103 1.7 × 103 7.2 × 103
25.0 14.6 15.6
M2 130 180 210 240
4.1 × 10−4 7.7 × 10−4 3.4 × 10−3 1.1 × 10−3
2.9 × 10−4 5.4 × 10−4 1.0 × 10−3 6.8 × 10−4
5.9 × 103 3.2 × 103 4.7 × 103 1.0 × 103
29.1 14.4 12.1 26.9
2.4.3. Synthesis of M1 A Schlenk tube was charge with compound 2 (0.096 g, 0.28 mmol), tetrakis(4-hexylphenyl)-indacenodithieno[3,2-b]thiophene-bis(trimethylstannane) (0.16 g, 0.12 mmol), and toluene (15 mL). The solution was degassed by nitrogen flow for 30 min. Tri(dibenzylideneacetone)dipalladium (Pd2(dba)3, 4 mg) and tri-o-tolylphosphine (P(otolyl)3, 5 mg) were added into the solution. The tube was capped and heated to 105 °C for overnight. After cooling to room temperature, the mixture was poured into a solution and extracted with dichloromethane. The combined organic layer was dried over anhydrous sodium sulfate. After removing the solvent, the residue was purified by silica gel column chromatography using a mixture of petroleum ether/ ether/dichloromethane (5:1:1) to afford a blue solid (100 mg, 54.6%). 1 H NMR (400 MHZ, CDCl3): δ = 8.08 (d, 2 H), 7.78 (s, 2 H), 7.56 (s, 2 H), 7.30 (d, 2 H), 7.20 (d, 8 H), 7.13 (d, 8 H), 3.75 (t, 4 H), 2.58 (t, 8 H), 1.70 (m, 4 H), 1.60 (m, 6 H), 1.25–1.45 (m, 38 H), 0.84-0.92 (m, 18 H). 13C NMR (100 MHZ, CDCl3): δ = 163.09, 154.13, 147.67, 147.35, 146.56, 146.23, 143.64, 143.36, 143.29, 142.26, 139.58, 136.28, 135.68, 128.67, 127.92, 127.48, 119.98, 117.45, 117.00, 111.16, 105.29, 79.66, 62.90, 40.47, 35.61, 31.70, 31.36, 31.32, 29.19, 27.43, 26.55, 22.61, 22.51, 14.12, 14.01. Anal. Calcd for C102H104N6O2S4: C, 77.82, H, 6.66, N, 5.34; found: C, 77.69, H, 6.49, N, 5.31.
channel width (W) of 760 μm. The devices were characterized under vacuum condition using a Keithley 4200 semiconductor parametric analyzer. The saturation-regime mobility (μ) was obtained using the following equation: Id = (W/2L)Ciμ(Vg-Vth)2, where Id is the drain current, Ci is the capacitance of the gate dielectric, Vg is the gate-source voltage, and Vth is the threshold voltage, Vd is the source-drain voltage. 2.4. Synthesis 2.4.1. Synthesis of 2-(6-bromo-1-(2-hexyl)-2-oxoindolin-3-ylidene) malononitrile (2) Toluenesulfonic acid (TsOH, 10 mg) was added to a mixture of compound 1 (1.0 g, 3.22 mmol), malononitrile (0.43 g, 6.44 mmol), and acetic acid (10 mL) in a round flask under nitrogen atmosphere. The reaction mixture was reflux for overnight. After cooled to room temperature, solvent was removed under reduced pressure and residue was purified by flash chromatography on silica gel with ether/petroleum ether (1: 5) as eluent to give the titled compound (1.1 g, 95.5%). 1H NMR (400 MHZ, CDCl3): δ = 8.0 (d, 1 H), 7.30 (d, 1 H), 7.05 (s, 1 H), 3.71 (t, 2 H), 1.70 (m, 2H), 1.2–1.4 (m, 6H), 0.91 (t, 3 H).
2.4.4. Synthesis of M2 A mixture of compound 4 (0.063 g, 0.18 mmol), tetrakis(4-hexylphenyl)-indacenodithieno[3,2-b]thiophene-bis(trimethylstannane) (0.11 g, 0.08 mmol), Pd2(dba)3 (4 mg), and P(o-tol)3 (5 mg) were taken in toluene (15 mL) to synthesize M2 according to the procedure of M1. A blue solid was collected (60 mg, 46.5%). 1H NMR (400 MHZ, CDCl3): δ = 8.08 (d, 2 H), 7.78 (s, 2 H), 7.56 (s, 2 H), 7.30 (d, 2 H), 7.20 (d, 8 H), 7.13 (d, 8 H), 3.75 (t, 4 H), 2.58 (t, 8 H), 1.70 (m, 4 H), 1.60 (m, 6 H), 1.25–1.45 (m, 38 H), 0.84-0.92 (m, 18 H). 13C NMR (100 MHZ, CDCl3): δ = 162.98, 159.65, 157.56, 154.55, 147.91, 146.95, 146.55, 144.50, 143.65, 142.30, 139.50, 138.55, 136.50, 133.46, 128.68, 127.94, 122.40, 117.55, 113.40, 112.80, 110.90, 110.32, 79.94, 63.09, 39.70, 35.60, 31.70, 31.30, 31.25, 29.70, 29.16, 27.36, 26.37, 22.57,
2.4.2. Synthesis of 2-(6-bromo-7-aza-1-(2-hexyl)-2-oxoindolin-3-ylidene)malononitrile (4) TsOH (10 mg) was added to a mixture of compound 3 (0.5 g, 1.61 mmol), malononitrile (0.21 g, 3.22 mmol), and acetic acid (10 mL) in a round flask under nitrogen atmosphere. The mixture was reflux for 3
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Fig. 3. The output and transfer curves of two small molecules, a) M1, and b) M2.
absorption of both films showed significant red-shift of about 32 and 38 nm for M1 and M2, respectively, indicating that the molecules may be well aggregated in the solid state resulted from the intermolecular and intramolecular interactions. The maximum absorption of M2 distinctly red shifted to ˜20 nm and the whole absorption spectra were much broader compared with the absorption spectra of M1. This can be attributed to the different electron-deficient power of acceptor units. The electron-withdrawing characteristics of compound 4 with N-substitution is stronger than that of compound 2 because of electronegative N atom, resulting in stronger interaction between D and A units compared with M1 without N substitution. The optical bandgaps of M1 and M2 were 1.43 and 1.38 eV calculated from the onset absorption in the solid state. The electrochemical properties of M1 and M2 were characterized by cyclic voltammetry (CV) (Fig. 2a). The corresponding data, such as LUMO, highest occupied molecular orbital (HOMO), and electrochemical bandgap (Ecv g ) were also summarized in Table 1. Deep LUMO/ HOMO energy levels were obtained for the two small molecules because of the strong electron-deficient di-cyanovinyl units. M1 exhibited LUMO/HOMO energy levels of about –4.0/–5.46 eV, indicating that the small molecule may be potential n-type semiconductor for electron injection and transport when a suitable mental (e.g., Au) is used as the electrode in OFET devices. M2 which had the electronegative N-substitution, exhibited the LUMO/HOMO energy levels of –4.10/–5.35 eV, indicating the N-substitution mainly lowered the LUMO energy level. Density functional theory (DFT) calculation of both small molecules were performed using Gaussian 09 at the B3LYP/6–31 G(d) level to investigate the electron structures. All the alkyl side chains were
22.43, 14.07, 14.00. Anal. Calcd for C100H102N8O2S4: C, 76.20, H, 6.52, N, 7.11; found: C, 76.08, H, 6.69, N, 7.06. 3. Results and discussion 3.1. Synthesis and characterization The synthetic routes for the small molecules M1 and M2 are shown in Scheme 1. The compound 1 and 3 were synthesized according to the reported procedures [9]. The di-cyanovinyl groups were introduced via Knoevenagel condensation reaction to endow compound 2 and 4 with strongly electron-withdrawn characteristics. The A–D–A small molecules (M1 and M2) were obtained via Stille coupling reactions of dicyanovinyl compounds and di-stannylated fused-monomer. The small molecules were examined by NMR (1H and 13C) and elemental analysis. The thermal properties of two small molecules were characterized by thermogravimetric (TGA) under nitrogen atmosphere (Fig. S1). The TGA showed excellent thermal stabilities for the two small molecules with 5% weight loss temperature (Td) of over 400 °C. 3.2. Optical and electrochemical properties The UV–Vis–NIR absorption spectra of two A–D–A molecules in chloroform solutions and on drop-cast thin films are present in Fig. 1. The corresponding data are also summarized in Table 1. Two small molecules exhibited dual absorption bands covering from ˜350–900 nm. The maximum absorption (λmax) of M1 and M2 were 687 and 713 nm, respectively. Compared with those of solution, the maximum 4
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Fig. 4. 2D-GIXD patterns and AFM height images (5 μm × 5 μm) of small molecule films spin-coated on OTS-treated SiO2/Si substrates.
3.4. Thin-film microstructural characterization
replaced with methyl groups in order to simplify the calculation (Fig. 2b). The LUMO energy levels are both concentrated on the 2-(2oxindolin-3-ylidene)malononitrile derivatives (A) unit, whereas the HOMO energy levels are located mainly on the fused ring (D). This may be interpreted why the N-substitution mainly lowered the LUMO energy level of M2. M1exhibited a dihedral angle of 18.5° between 2-(2-oxindolin-3-ylidene)malononitrile and fused ring units (Fig. S2). In comparison, M2 exhibited a relatively planar conjugated backbone and the dihedral angles decrease to 0.5°. This may be attributed to the N-substitution which can induce intramolecular noncovalent interactions (S···N or CH···N). [9]
The annealing films of the small molecules (M1 and M2) were investigated by two-dimensional grazing incidence X-ray diffraction (2DGIXD) and atomic force microscopy (AFM). The 2D-GIXD images are shown in Fig. 4a and b. The M1 film exhibited strong diffraction peak at 0.4 Å−1 corresponding to (001) along the out-of-plane qz direction. The corresponding d spacing was 15.7 Å for M1. The film of M2 showed strong first-order diffraction peak at 0.38 Å−1 ascribing to (001), and also showed weak second-order diffraction peaks at 0.75 Å−1 corresponding to (002). Therefore, the lamellar spacing was 16.5 Å for M2. The results indicate that small molecule M2 had ordered lamellar crystalline structure. The height images of the annealing films are shown in Fig. 4c and d. The AFM images of annealing films showed bulk crystal in the whole area for both small molecules. The AFM results are consistent with the GIXD results mention above. However, the crystals in the whole substrates exhibited disordered and loose packing, which result in obvious and deep brain boundaries and may be difficult to form highly efficient pathways for charge carrier transport. Moreover, both the films showed very rough surface with large root mean square roughness (RMS) of 8.96 and 8.72 nm for M1 and M2, respectively. The too rough surface may lead to poor interfacial adhesion and be also unfavorable for charge transport. Therefore, the OFET devices based on the small molecules exhibited moderate field-effect performances.
3.3. OFET properties The charge transport characteristics of the two small molecules were investigated using BG/TC device architectures. The small molecules were spin-coated from chloroform solution (6 mg/mL), followed by annealing at different temperature to optimize the performances. Table 2 summarized the corresponding data such as the field effect mobility (μ), threshold voltage (Vth), and the on/off current ratio (Ion/ Ioff) of the OFET devices. Fig. 3 shows the typical output and transfer curves of M1 and M2-based devices. The two small molecules exhibited unipolar electron transport characteristics under vacuum conditions. This may be relative to the LUMO energy levels which were low enough for stable electron transport under vacuum condition. The optimized temperatures were 210 °C for both small molecules. M1 exhibited the highest electron mobility of 1.0 × 10–3 cm2V–1s–1 with an average electron mobility of 7.0 × 10–4 cm2V–1s–1. M2 displayed the optimized electron mobility of 3.4 × 10–3 cm2V–1s–1 with an average mobility of 1.0 × 10–3 cm2V–1s–1. The two small molecules showed relatively low field-effect performances.
4. Conclusion Two A–D–A small molecules (M1 and M2) based on the fused ring (IDTT) as the donor and 2-(2-oxindolin-3-ylidene)malononitrile derivatives as acceptors were synthesized and characterized. The small molecules have low optical bandgaps of about 1.4 eV and deep LUMO energy levels of about –4.0 eV for electron injection and transport. OFET devices based on small molecules exhibited unipolar electron 5
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transport characteristics with moderate mobility of 3.4 × 10–3 cm2V–1s–1. This work indicates that the di-cyanovinyl-based small molecules can also be useful semiconductor for solution processed n-type OFETs.
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Acknowledgments This work was supported by the Open Foundation of Key Laboratory of Advanced Functional Materials and Devices of Anhui Province, and Double First Class Enhancing Independent Innovation and Social Service Capabilities of Hefei University of Technology (Grant No. 4500411104/011), the Fundamental Research Funds for the Central Universities (Grant No. JZ2018HGPB0276), and the National Natural Science Foundation of China (NSFC, Grant nos. 51573001, 51573036, 51703047). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.synthmet.2019. 116143. References [1] H. Sirringhaus, 25th anniversary article: organic field-effect transistors: the path beyond amorphous silicon, Adv Mater 26 (2014) 1319–1335. [2] Y. Liu, F. Wang, J. Chen, X. Wang, H. Lu, L. Qiu, G. Zhang, Improved transistor performance of isoindigo-based conjugated polymers by chemically blending strongly electron-deficient units with low content to optimize crystal structure, Macromolecules 51 (2018) 370–378. [3] J. Mei, Y. Diao, A.L. Appleton, L. Fang, Z. Bao, Integrated materials design of organic semiconductors for field-effect transistors, J. Am. Chem. Soc. 135 (2013) 6724–6746. [4] L. Shi, Y. Guo, W. Hu, Y. Liu, Design and effective synthesis methods for highperformance polymer semiconductors in organic field-effect transistors, Mater. Chem. Front. 1 (2017) 2423–2456. [5] A.F. Paterson, S. Singh, K.J. Fallon, T. Hodsden, Y. Han, B.C. Schroeder, et al., Recent progress in high-mobility organic transistors: a reality check, Adv Mater 30 (2018) 1801079.
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