Solution-processable fluorene derivative for organic thin-film transistors

Organic Electronics 76 (2020) 105464

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Solution-processable fluorene derivative for organic thin-film transistors Dongkyu Kim a, 1, M. Rajeshkumar Reddy b, 1, Kwanghee Cho a, Dongil Ho a, Choongik Kim a, *, SungYong Seo b, ** a b

Department of Chemical and Biomolecular Engineering, Sogang University, Seoul, 04107, Republic of Korea Department of Chemistry, Pukyong National University, Busan, 48513, Republic of Korea

A R T I C L E I N F O

A B S T R A C T

Keywords: Fluorene Organic semiconductor Organic thin-film transistors Donor-acceptor-donor Solution-process

New fluorene derivatives with donor-acceptor-donor (D-A-D) structure were synthesized and characterized as solution-processable organic semiconductors for top-contact/bottom-gate organic thin-film transistors (OTFTs). Physicochemical properties of the new compounds were characterized by cyclic voltammetry (CV), UV–vis ab­ sorption spectroscopy, thermogravimetric analysis (TGA), differential scanning calorimeter (DSC), and density functional theory (DFT) calculation. All compounds were TFT active and compounds with strong electron withdrawing groups showed ambipolar semiconductor performance with electron mobility of up to 0.1 cm2 V-1 s1 and current on/off ratio of > 106.

1. Introduction In recent years, development of low-cost, large-area organic elec­ tronic devices has been underway in various applications such as organic thin-film transistors (OTFTs), organic light-emitting diodes (OLEDs), and organic photovoltaics (OPVs) [1–9]. Especially, the design and synthesis of π-conjugated polymeric or small molecular organic semiconductors is driving the development of new organic electronic devices [10–16]. Of these, small molecular organic semiconductors have gained much interest since they can be synthesized relatively easily and purified at a high purity with excellent reproducibility [17–24]. In recent semiconductor studies, organic semiconducting materials containing both electron-accepting (acceptor) and electron-donating (donor) moieties in their molecular structure have been studied exten­ sively because of their high electrical performance [25–31]. Tuning the highest occupied/lowest unoccupied molecular orbital (HOMO/LUMO) energy to improve the charge transport of both holes and electrons through the π-conjugated backbone can be easily accomplished by designing appropriate donor-acceptor structures [32–35]. Among many acceptor moieties, fluorene moieties have been studied extensively due to their air stability with strong solid state interactions [36–38]. For donor structure, thiophenes are commonly employed since they exbihit sufficiently low ionization potentials to ensure ohmic contact to holes [39].

In this regard, we have synthesized four new organic semiconductors with donor-acceptor-donor (D-A-D) structure comprising fluorene deri­ vate and thiophenes as electron-accepting and electron-donating moi­ eties, respectively (Fig. 1). Furthermore, terminal alkyl chains (ethyl hexyl or octyl) were employed to ensure the solution-processability of the corresponding compounds in common organic solvents. Thermal, optical, and electrochemical properties of all new compounds were confirmed by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), UV–vis spectroscopy, and cyclic voltammetry (CV). The synthesized molecules were tested as solution-processed organic semiconductors for top-contact/bottom-gate OTFTs. The resulting de­ vice showed electron mobility as high as 0.1 cm2 V-1 s-1 and current on/ off ratio of > 106. 2. Experiment details 2.1. General methods Air and/or moisture sensitive reactions were carried out under an argon atmosphere in oven-dried glassware and with anhydrous solvents. Unless otherwise noted, all compounds were purchased from commer­ cial sources and used without further purification. Tributyl(5-(2-ethyl­ hexyl)thiophen-2-yl)stannane (7a) [40], tributyl(5-octylthiophen-2-yl) stannane (7b) [40], and 2,7-diiodo-9H-fluoren-9-one [41] were

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (C. Kim), [email protected] (S. Seo). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.orgel.2019.105464 Received 24 July 2019; Received in revised form 26 September 2019; Accepted 26 September 2019 Available online 30 September 2019 1566-1199/© 2019 Elsevier B.V. All rights reserved.

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mixture, and the precipitate was filtered precipitate washing with excess of acetonitrile. Compound 2 was obtained as grayish green powder (151.5 mg, 95.1%). 1H NMR (600 MHz, CDCl3): δ 8.51 (d, 1.38 Hz, 2H), 7.61–7.60 (m, 2H), 7.45 (d, 8.22 Hz, 2H), 7.21 (d, 3.42 Hz, 2H), 7.00–6.99 (m, 4H), 6.64 (d, 3.48 Hz, 2H), 2.70 (d, 6.68 Hz, 4H), 1.56–1.54 (m, 2H), 1.4–1.29 (m, 16H), 0.89–0.87 (m, 12H). 13C NMR (150 MHz CDCl3): δ 160.8, 144.5, 140.2, 140.0, 138.4, 135.0, 134.9, 134.4, 125.9, 124.6, 123.8, 123.7, 123.2, 76.7, 41.3, 34.1, 32.3, 28.8, 25.4, 22.9, 14.1, 10.8. HRMS-EI(m/z): [Mþ] calcd. for C48H48N2S4, 780.2700; found, 780.2616. 2.2.5. Synthesis of 2,7-bis(50 -octyl-[2,20 -bithiophen]-5-yl)-9H-fluoren-9one (3) [47] Compound 6 (200 mg, 0.40 mmol), tributyl(5-octylthiophen-2-yl) stannane 7b (579.8 mg, 1.2 mmol), and Pd(PPh3)4 (48 mg, 0.041 mmol) were dissolved in toluene (20 mL) and then increased temperature to 110 � C for 12 h under N2 atmosphere. After cooling down to room temperature, the mixture was washed with water and brine using CH2Cl2 and dried over anhydrous MgSO4. The solvent was evap­ orated in vacuo and the residue was purified by flash column chroma­ tography on silica gel to afford compound 3 as red powder (581 mg, 71.3%). 1H NMR (400 MHz, CDCl3): δ 7.88 (2H), 7.69–7.67 (b, 2H), 7.50–7.48 (b, 2H), 7.27–7.26 (b, 2H), 7.06 (d, 3.6 Hz, 2H), 7.01 (d, 3.6 Hz, 2H), 6.68 (d, 3.2 Hz, 2H), 2.78 (t, 15.6 Hz, 8 Hz, 4H), 1.69–1.63 (b, 4H), 1.37–1.26 (b, 22H), 0.88–0.84 (b, 6H). HRMS-EI(m/z): [Mþ] calcd. for C45H48OS4, 732.2588; found, 732.2526.

Fig. 1. Schematic of top-contact/bottom-gate organic thin-film transistor and chemical structures of organic semiconductors employed in this study.

synthesized according to methods in the literatures. Solvents were freshly distilled or dried by passing through an alumina column. General analysis method of the synthesized compounds was already described in previous work [42].

2.2.6. Synthesis of 2-(2,7-bis(50 -octyl-[2,20 -bithiophen]-5-yl)-9H-fluoren9-ylidene)malononitrile (4) [46] Compound 3 (200 mg, 0.27 mmol) and malanonitrile (0.5 mL) were dissolved in DMSO and then increased reaction temperature to 150 � C for 12 h under N2 atmosphere. Acetonitrile was added to the reaction mixture, and the precipitate was filtered washing with excess of aceto­ nitrile. Compound 4 was obtained as grayish green powder (175 mg, 79.9%). 1H NMR (600 MHz, CDCl3): δ 8.34 (s, 2H), 7.54–7.52 (b, 2H), 7.39–7.37 (b, 2H), 7.10 (s, 2H), 6.92–6.90 (b, 4H), 6.61 (s, 2H), 2.74–2.73 (b, 4H), 1.64 (b, 4H), 1.40–1.24 (b, 22H), 0.92–0.82 (b, 6H). 13 C NMR (150 MHz, CDCl3): δ 207.0, 160.5, 145.8, 140.0, 139.7, 138.2, 134.6, 134.1, 130.3, 124.7, 124.4, 123.6, 122.6, 120.7, 113.1, 76.6, 31.8, 31.4, 30.9, 30.1, 29.3, 29.2, 22.6, 14.1. HRMS-EI(m/z): [Mþ] calcd. for C48H48N2S4, 780.2700; found, 780.2612.

2.2. Synthesis 2.2.1. Synthesis of 2,7-di(thiophen-2-yl)-9H-fluoren-9-one (5) Compound 5 was synthesized according to a published procedure [43]. 1H NMR (400 MHz, CDCl3) δ 7.90 (s, 2H), 7.71 (d, 5.04 Hz, 2H), 7.49 (d, 5.04 Hz, 2H), 7.37 (d, 2.28 Hz, 2H), 7.30 (d, 3.2 Hz, 2H), 7.09 (t, 2.72, 5.48 Hz, 2H). 2.2.2. Synthesis of 2,7-bis(5-bromothiophen-2-yl)-9H-fluoren-9-one (6) Compound 6 was synthesized employing a published procedure [44]. 1H NMR (400 MHz, CDCl3): δ 7.80 (d, 1.16 Hz, 2H), 7.60–7.62 (m, 2H), 7.50 (d, 4.96 Hz, 2H), 7.11 (d, 2.2 Hz, 2H), 7.04 (d, 2.2 Hz, 2H). 2.2.3. Synthesis of 2,7-bis(5’-(2-ethylhexyl)-[2,20 -bithiophen]-5-yl)-9Hfluoren-9-one (1) [45] Compound 6 (200 mg, 0.40 mmol), tributyl(5-(2-ethylhexyl)thio­ phen-2-yl)stannane 7a (579.8 mg, 1.2 mmol), and Pd(PPh3)4 (48 mg, 0.041 mmol) were dissolved in toluene (20 mL) and then increased temperature to 110 � C for 12 h under N2 atmosphere. After cooling down to room temperature, the mixture was washed with water and brine using CH2Cl2 and dried over anhydrous MgSO4. The solvent was evap­ orated in vacuo and the residue was purified by flash column chroma­ tography on silica gel to afford compound 1 as red powder (140 mg, 48.0%). 1H NMR (600 MHz, CDCl3): δ 7.84 (d, 1.68 Hz, 2H), 7.65–7.63 (m, 2H), 7.45 (d, 8.22 Hz 2H), 7.24–7.23 (m, 2H), 7.05–7.04 (m, 2H), 7.00 (d, 4.08 Hz, 2H), 6.66 (d, 3.3 Hz, 1H), 2.72 (d, 4.6 Hz, 4H), 1.55–1.60 (m, 2H), 1.33–1.40 (m, 4H), 1.26–1.33 (m, 12H), 0.86–0.92 (m, 12H). 13C NMR (150 MHz, CDCl3): δ 193.2, 144.4, 142.5, 140.7, 138.0, 135.0, 134.6, 131.0, 125.9, 124.2, 123.8, 120.9, 120.7, 41.3, 34.1, 32.3, 28.8, 25.4, 22.9, 14.1, 10.8. HRMS-EI(m/z): [Mþ] calcd. for C45H48OS4, 732.2588; found, 732.2548.

2.3. Device fabrication

2.2.4. Synthesis of 2-(2,7-bis(5’-(2-ethylhexyl)-[2,20 -bithiophen]-5-yl)9H-fluoren-9-ylidene)malononitrile (2) [46] Compound 1 (150 mg, 0.20 mmol) and malanonitrile (0.5 mL) were dissolved in DMSO and then increased reaction temperature to 150 � C for 12 h under N2 atmosphere. Acetonitrile was added to the reaction

2.4. Device characterization

The OTFTs were fabricated with the top contact/bottom gate (TC/ BG) structure. The highly n-doped silicon wafers with thermally grown 300 nm oxide (areal capacitance; Ci ¼ 11.4 nF cm 2) were cleaned via sonication in acetone for 10 min and treated by air plasma for 5 min (Harrick plasma, PDC-32G, 18 W). Organic semiconductor layers were formed via solution-shearing on PS-brush-treated substrates. The PSbrush (Mw ¼ 1.7–28.0 kg mol 1) treatments were implemented following the general recipe. The concentration of semiconductor solu­ tion, solvent type, substrate temperature, and shearing speed were optimized. The solution-sheared substrates were annealed in a vacuum oven at various temperature for 10 h to remove the residual solvent. The thicknesses of semiconductor layers for a single fluorenone layer (60–100 nm) were measured using a profilometer (DEKTAK-XT, Brucker). The Au layers (50 nm) with various channel widths (W; 500, 1000 μm) and lengths (L; 50 μm) were thermally evaporated to define the source and drain electrodes.

The electrical performances of OTFTs were characterized in vacuum conditions at room temperature with a semiconductor parameter analyzer (Keithley 4200-SCS) equipped with a probe station. Carrier 2

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Organic Electronics 76 (2020) 105464

Scheme 1. Synthetic scheme of compounds 1–4.

mobilities (μ) were determined in the saturation regime by the standard relationship, μsat ¼ (2IDSL)/[WCi(VG-VT)2], (IDS; source-drain current, L; the channel length, W; channel width, Ci; areal capacitance of the gate dielectric, VG; gate voltage, VT; threshold voltage). The microstructure and surface morphology of the semiconductor thin-films were measured by atomic force microscopy (AFM, NX10, Park systems) and wide-angle X-ray diffraction (XRD, Smartlab, Rigaku).

3. Results and discussion 3.1. Synthesis 2,7-Di(thiophen-2-yl)-9H-fluoren-9-one (5) and 2,7-bis(5-bromo­ thiophen-2-yl)-9H-fluoren-9-one (6) were synthesized employing refer­ ence methods [43,44]. Stille coupling of compound 6 with suitable tin compounds 7a and 7b afforded 2,7-bis(5’-(2-ethylhexyl)-[2,20 -bithio­ phen]-5-yl)-9H-fluoren-9-one (1) and 2,7-bis(50 -octyl-[2,20 -bithiophe­ n]-5-yl)-9H-fluoren-9-one (3), as shown in Scheme 1 [47]. Also, 2-(2, 7-bis(5’-(2-ethylhexyl)-[2,20 -bithiophen]-5-yl)-9H-fluoren-9-ylidene) 3

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Organic Electronics 76 (2020) 105464

Table 1 Optical, electrochemical, and thermal properties of fluorene derivatives. Cmpd.

o Ta) m [ C]

o Tb) d [ C]

λc) max [nm]

Band gap [eV]d)

Potential [V]e)

1 2 3 4

154 174 190 119

414 425 387 434

394 406 393 405

2.09 1.58 2.11 1.57

0.71 0.75 0.74 0.65

Eox

HOMO [eV]f)

LUMO [eV]f)

Band gap [eV]e)

Ered 1.04 0.80 1.06 0.82

5.31 5.35 5.34 5.25

3.56 3.80 3.54 3.78

1.75 1.55 1.80 1.47

a)

Melting temperature, b) Decomposition temperature (5%), c) Measured by UV–vis spectroscopy, d)Determined from the onset wavelength of UV–vis spectra, e) Measured by cyclic voltammetry in o-C6H4Cl2 at 25 � C (using ferrocene/ferrocenium as internal standard, Eox ¼ Oxidative potential, Ered ¼ Reductive potential, f) HOMO ¼ -(Eox-EFc/Fcþ) – 4.80 eV; LUMO ¼ ¼ -(Ered-EFc/Fcþ) – 4.80 eV.

Fig. 3. UV–Vis spectra of compounds 1–4 in chloroform.

Fig. 2. Cyclic voltammograms of compounds 1–4 in dichloromethane (0.1 M Bu4NþPF6 , scan rate ¼ 100 mV s 1).

afforded smaller energy gap than the fluorenone group due to its stronger electron-accepting ability. The optical absorption spectra of the compounds in chloroform are shown in Fig. 3. Strong absorption peaks corresponding to π → π* transition of the fluorene derivative were observed for all compounds at ~400 nm [51]. Furthermore, low absorption peaks occurred at ~490 nm (π → π* transition of carbonyl group; compounds 1 and 3) and ~680 nm (π → π* transition of fluorenedicyanovinylene group; compounds 2 and 4), respectively. As described above, strong electron accepting ability of the fluorenedicyanovinylene group as well as extended pi-conjugation upon dicyanovinylene functionalization afforded red shifts of absorption peaks for compounds 2 and 4. The energy band gaps of compounds 1–4 determined from the onset wavelength were 2.09, 1.58, 2.11, and 1.57 eV, respectively, exhibiting similar trend to that observed for the electrochemical characterization (vide supra).

malononitrile (2) and 2-(2,7-bis(50 -octyl-[2,20 -bithiophen]-5-yl)-9H-­ fluoren-9-ylidene)malononitrile (4) were obtained using malonitrile, as shown in Scheme 1 [46]. 3.2. Thermal, optical, and electrochemical properties Thermal properties of the fluorene derivatives were investigated using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). Compounds 1–4 showed good thermal stability with a loss of 5% by weight at 387–434 � C (Table 1). The melting point of compounds 1–4 were observed at 119–190 � C (Table 1), ensuring the validity of annealing temp (100 � C) after solution process (vide infra). For compounds 1 and 2, going from carbonyl to dicyanovinylene, melting point was increased as reported in the literatures [48,49], possibly due to the increased local/molecular dipoles and pi-extension contributing to intermolecular interactions. For the other set of mole­ cules which showed the opposite trend, other intermolecular cohesive forces originating from using different type of alkyl chains (branched vs linear) could be effective in determining the intermolecular interactions, not just the functional groups. Cyclic voltammetry (CV) measurements of the new compounds were performed in o-C6H4Cl2 solution at room temperature. The measured electrochemical potentials were calibrated to a ferrocene/ferrocenium cation (Fc/Fcþ) redox couple [50]. As shown in Fig. 2, all compounds exhibited oxidation and reduction potentials at 0.65–0.75 V and 0.80 to 1.06 V, respectively. Hence, the HOMO and LUMO energy levels of the corresponding compounds were calculated using ferrocene/ferro­ cenium as internal standard (Table 1). The band gaps of compounds 1 and 3 were 1.75 and 1.80 eV, while those of compounds 2 and 4 were 1.55 and 1.47 eV, respectively. The fluorenedicyanovinylene group

3.3. Theoretical calculation The molecular structure and HOMO/LUMO energy distribution of fluorene derivatives were determined by DFT (B3LYP) calculation using 6-31G(d) basis sets (Fig. 4). As expected, the HOMO orbitals of all compounds are dispersed on the main backbone moiety and the LUMO orbitals are restricted to fluorene derivatives. The theoretical HOMO/ LUMO energies obtained by DFT calculation were 5.06/-2.36, 5.14/3.20, 5.05/-2.35, and 5.14/-3.19 eV, respectively. Similarly to the experimental data, compounds 2 and 4 exhibit lower LUMO levels compared to compounds 1 and 3, due to the stronger electron accepting ability of dicyanovinylene compared to the carbonyl group. 3.4. Thin-film transistor characterization Semiconductor properties of the fluorene derivatives 4

were

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Fig. 4. Molecular orbital surfaces of HOMO and LUMO by DFT calculation of compounds (a) 1, (b) 2, (c) 3, and (d) 4. Table 2 TFT device performance parameters based on the thin films of fluorene derivatives.a Compound

Method

Surface treatment

Type

μmax (μavg) (cm2 V 1s 1)

1 2

SS

PS-brush

P P N P P N

1.06 � 10 1.44 � 10 1.00 � 10 0.64 � 10 0.14 � 10 0.10 � 10

3 4 a

2

(0.87 � 0.17) � 10 2 (1.33 � 0.08) � 10 1 (0.85 � 0.11) � 10 2 (0.48 � 0.11) � 10 3 (0.12 � 0.01) � 10 2 (0.07 � 0.01) � 10-

2 2 1 2 3

Ion/Ioff

VT (V)

(24.6 � 2.8) � 103 (19.2 � 0.6) � 101 (43.0 � 3.4) � 105 (22.3 � 3.9) � 105 (13.8 � 0.2) � 101 (12.2 � 0.5) � 102

57.8 � 5.8 72.6 � 4.5 57.5 � 8.6 39.3 � 7.4 107.3 � 8.9 29.7 � 5.2

μ: carrier mobility, Ion/Ioff: current on/off ratio, VT; threshold voltage. Measured in vacuum. The average values obtained from six different devices.

determined by fabricating top-contact/bottom-gate (TC/BG) OTFTs. The organic semiconductor layer was formed by shearing the semi­ conductor solution to create a highly-ordered anisotropic crystal struc­ ture [52–54]. Electrical performance of fluorene derivatives as active layer are summarized in Table 2 and Fig. 5. Thin films of compounds 1 and 3 showed p-type characteristic with relatively poor electrical per­ formance (carrier mobility ~ 10-2 cm2 V-1 s-1). On the other hand, thin films of compounds 2 and 4 with fluorenedicyanovinylene groups afforded ambipolar behaviour. Relatively low LUMO levels (~ 3.8 eV) as well as small band gaps (~1.5 eV) of compounds 2 and 4 can be ascribed to the ambipolar charge transport of the corresponding com­ pounds [55,56]. Note that the LUMO levels of compounds 2 and 4 are not low enough to ensure ambient stability; both compounds exhibited 1–2 orders of magnitude lower electron mobility in ambient. Relatively large threshold voltages of ambipolar semiconductors might be due to charge transport of both carriers (holes and electrons) inside the active layer [57].

diffraction peaks of compounds 1–4 were observed at 3.20, 4.22, 2.44, and 2.64� , respectively. The d-spacings calculated for these primary peaks correspond to 2.74, 2.17, 3.62, and 3.44 nm, respectively. The calculated d-spacing values are less than the computed molecular length (compound 1 and 2: 3.12 nm, compounds 3 and 4: 3.80 nm), indicating that molecules are tilted relative to the substrate normal. Thin films of compound 3 showed two distinct polymorphs, which might indicate poor device performance of the corresponding compounds (vide supra) [58,59]. The surface morphology of thin films of compounds 1–4 was investigated by AFM. Thin films of compounds 2 and 4 show relatively smooth surface with terrace-like layered structure (Fig. 5 and Fig. S3), while those of compounds 1 and 2 exhibit large islands with more voids. 4. Conclusion In this study, new fluorene derivatives were synthesized as organic semiconductors for organic thin-film transistors. By employing donoracceptor-donor structure, HOMO/LUMO energy levels of the corre­ sponding compounds were modulated. Especially, compounds with strong electron-accepting moiety exhibited ambipolar charge transport behaviour with electron mobility as high as 0.1 cm2 V-1 s-1. Our study provides the attractions of using fluorene moiety for designing new solution-processable organic semiconducting materials.

3.5. Thin-film microstructure and morphology Thin film microstructure and surface morphology of the solutionprocessed semiconductor thin films have been characterized using wide-angle θ-2θ X-ray diffraction and atomic force microscopy (Fig. 5, Fig. S2-S3). Overall, no clear correlation between device performance and film microstructure was observed. Thin films of fluorine compounds showed multiple (2–4) Bragg reflections in XRD spectra. The primary 5

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Organic Electronics 76 (2020) 105464

Fig. 5. Thin-film transistor characterization, microstructure, and morphology of compounds 2. a) transfer curve; b) output characteristics; c) XRD data; d) AFM data. Channel widths and lengths of 500 μm and 50 μm were used, respectively.

Declaration of competing interest [3]

The authors declare no conflict of interest.

[4]

Acknowledgements This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2017R1A2B4001955 and NRF2018R1D1A1B07045299) and was supported by Busan Institute of sci­ ence and technology evaluation and planning (BISTEP) grant (2019 Busan Open Lab. Program) funded by the Korea government(Ministry of Trade, Industry and Energy) and Busan metropolitan city.

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Appendix A. Supplementary data

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Supplementary data to this article can be found online at https://doi. org/10.1016/j.orgel.2019.105464.

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