Diamine-cored tetrastilbene compounds as solution-processable hole transport materials for stable organic light emitting diodes

Accepted Manuscript Diamine-cored tetrastilbene compounds as solution-processable hole transport materials for stable organic light emitting diodes Min Jy Cho, Kyu Min Sim, Sa-Rang Bae, Hye Ok Choi, Soo Young Kim, Dae Sung Chung, Kwangyong Park PII:

S0143-7208(17)31706-0

DOI:

10.1016/j.dyepig.2017.10.016

Reference:

DYPI 6314

To appear in:

Dyes and Pigments

Received Date: 9 August 2017 Revised Date:

11 October 2017

Accepted Date: 11 October 2017

Please cite this article as: Cho MJ, Sim KM, Bae S-R, Choi HO, Kim SY, Chung DS, Park K, Diaminecored tetrastilbene compounds as solution-processable hole transport materials for stable organic light emitting diodes, Dyes and Pigments (2017), doi: 10.1016/j.dyepig.2017.10.016. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

Diamine-cored tetrastilbene compounds as solution-processable hole transport materials for stable organic light emitting diodes Min Jy Cho,‡,a Kyu Min Sim,‡, b Sa-Rang Bae,a Hye Ok Choi,a Soo Young Kim,*,a Dae Sung Chung, *,b Kwangyong Park*,a a

School of Chemical Engineering and Materials Science, Integrative research center for two-dimensional functional materials, Chung-Ang University, 84 Heukseok-ro, Dongjak-gu, Seoul, 06974, Republic of Korea

b

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Department of Energy Systems Engineering, Daegu Gyeongbuk Institute of Science and Technology (DGIST), Daegu 42988, Republic of Korea

based on a thermal evaporation method, which has drawbacks in terms of manufacturing costs and large area production.[6,7] Consequently, studies have focused on solution-based processes that are expected to allow low-cost plastic electronics by large area production and high-throughput methods.[8] However, in the case of solution-processed OLEDs, the efficiency is still very low compared to that of vacuum-processed OLEDs.[9-12]

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ABSTRACT: A series of diamine-cored tetrastilbene (DTS) derivatives bearing various aliphatic and aromatic substituents was designed and synthesized for use as solution-processed hole transport layers (HTLs) in organic light emitting diodes (OLEDs). The chemical structures of the DTS derivatives were strategically designed to increase solubility in organic solvents as well as to avoid self-crystallization, and thus ensure a stable morphology under Joule heating while maintaining efficient hole transport capabilities. The five DTS derivatives, composed of different conjugation structures, yielded reasonably good hole transport behavior with a marginal charge carrier mobility of ~10−5 cm2V-1s-1, which is similar to that of vacuum-deposited N,N′-bis(1naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB). Due to the high glass transition temperatures of the DTS derivatives, this satisfactory charge transport behavior and smooth surface morphology were conserved up to 180 °C. Green OLEDs were prepared using tris-(8-hydroxyquinoline) aluminum (Alq3):C545T as the emission layer, and the OLED performances of the solution-processed DTS HTLs and the vacuum-deposited NPB HTL were compared. A high luminance efficiency of 11.5 cd A-1 was obtained for one solution-processed DTS HTL, which exceeds that of the NPB HTL (10.01 cd A-1). Furthermore, the DTS HTLs enabled a stable OLED operation, with double the accelerated half-life of the NPB-based device. Key words: Diamine-cored tetrastilbene, hole transport layer, organic light emitting diodes, solution process 1. Introduction Due to the rapid development of materials synthesis and lighting technologies, organic light emitting diodes (OLEDs) are considered as promising candidates for flat panel displays and solid-state lighting applications.[1-3] Recently, highly efficient emitters and out-coupling techniques yielded OLED current efficiencies exceeding 200 cd A-1, thus meeting the requirements for commercial application.[4,5] However, these achievements are

Due to the structural complexity of multilayer (injecting, transporting, and emitting layers along with their interfacial layers) stacked OLEDs, determination of a decisive factor to enhance the efficiency of solution-processed OLEDs was desirable. Among the known layers and interfaces, the hole injection layer (HIL) and hole transport layer (HTL) lower the energy barrier between the anode and the emitting layer to allow efficient charge delivery and lighting.[13] In addition to favorable energy barrier characteristics, the HIL/HTL must also exhibit anode planarization, high transparency, sufficient conductivity, good electron blocking ability, and good exciton blocking ability. Thus, for developing highly efficient solution-processed OLEDs, optimization of the HIL/HTL is a priority. For the HIL, poly(3,4ethylenedioxythiophene:polystyrene sulfonate) (PEDOT:PSS), metal oxides, and metal sulfides are the preferred materials due to their superior electrical/optical characteristics.[14-19] Among them, the majority of recent reports on solution-processed OLEDs have employed PEDOT:PSS as the HIL and reported their reasonably good performance. However, in the case of the solution processed HTL, there are very limited numbers of materials that have been known to allow a sufficient decrease of the energy barrier and satisfactory electrical and thermal stability.[20-22]

Ideally, in addition to the above-mentioned characteristics, HTLs must possess the following properties: i) The HTL must protect the emission layer (EML) from the acidity of PEDOT:PSS; ii) It must be deposited from a solvent orthogonal to that used for the HIL; and iii) It should be thermally stable to ensure the operational sta-

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ACCEPTED MANUSCRIPT analyses (C, H, N) were obtained using a FLASH 2000 (Thermo Fisher Scientific, Germany) elemental analyzer. 2.2. Synthesis of DTS compounds

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Synthetic details are presented in scheme 1. Palladium-catalyzed coupling reactions of ethylenediamine (1) with 4-bromobenzophenone in the presence of sodium tert-butoxide gave N,N,N′,N′-tetrakis(4benzoylphenyl) ethylenediamine (2) in good yield. Horner-Wadsworth-Emmons reactions of 2 with various benzylphosphonates in the presence of potassium tert-butoxide produced the corresponding DTSs 3 as pale yellow powders in good isolated yields. Synthetic details are written in below.

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bility of the resulting OLEDs under Joule heating. With these factors in mind, we have tried to find HTL materials with both good solubility in organic solvents and high thermal stability for extended lifetime. We found that diamine-coredtetrastilbene (DTS) compounds with a central ethylene diamine moiety could possess i) reasonably good solubility in common organic solvents due to the aliphatic moiety, ii) increased thermal stability with Tg higher than NPB due to the formation of threedimensional conformation instead of typical planar structure of conjugated compounds, and iii) marginal charge carrier mobility of 10-5 cm2 V-1 s-1 due to charge delocalization within the stilbenoids, as confirmed by space charge limited current (SCLC) and field effect transistor (FET) studies.[23] We also found that the introduction of bulky substituent groups inhibited intermolecular agglomeration, thus maintaining high hole transport characteristics of DTS even at elevated temperatures. As a result, we could demonstrate the preparation of highly stable and highly efficient green OLEDs containing a DTS derivative as the solution-processed HTL.

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NH 2 1

2.1. Materials preparation

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All reactions were carried out under a nitrogen atmosphere. Solvents were distilled from the appropriate drying agents prior to use, i.e., tetrahydrofuran (THF) was distilled from sodium-benzophenone ketyl, and 1,4dioxane was distilled from CaH2. 1H NMR (600 MHz) and 13C NMR (150 MHz) spectra were acquired using CDCl3 as the solvent and tetramethylsilane (TMS) as the internal standard. Chemical shifts are reported in δ units (ppm) based on a TMS 1H NMR shift of 0.00 ppm and a CDCl3 13C NMR shift of 77.2 ppm. All coupling constants, J, are reported in hertz (Hz). Column chromatography was performed using silica gel 60 Å, 70−230 mesh. Analytical thin layer chromatography (TLC) was performed using Merck Kieselgel 60 F254 precoated plates (0.25 mm) with a fluorescent indicator, which were visualized using UV light (254 and 365 nm). GC analyses were performed on a bonded 5% phenyl polysiloxane BPX-5 capillary column (SGE, 30 m, 0.32 mm i.d.), and mass spectrometry was carried out using a SYNAPT G2 HDMS quadrupole time-of-flight (TOF) spectrometer equipped with an electrospray ion source (Waters, Milford, MA, USA). The instrument was calibrated using NaF solution. Samples were dissolved in 1% (v/v) dichloromethane/MeOH and introduced into the ion source operating in positive mode by direct infusion at a flow rate of 20 µLmin-1. All spectra were acquired at m/z 50–1500 and all data are reported in mass units (m/z). Leucine enkephalin was used as the lock mass for exact mass measurement corrections. Elemental

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H 2N

2. Materials and Methods

O O

Ar

O O

2 Ar

Ar

OEt P OEt O

N

N

a: Ph b : 4-MePh c: 4- tBuPh d : 4-biphenyl e: 1-naphthyl

Ar

Ar 3

Scheme 1. Synthetic route for diamine-cored stilbenoid derivatives bearing various aromatic substituents. The products (3a–e) were designated as DTS-1, DTS-2, DTS-3, DTS-4, and DTS-5 according to their endaromatic groups (A = phenyl, B = 4-methylphenyl, C = 4-tert-butylphenyl, D = 4-biphenyl, and E = 1-naphthyl).

2.2.1 Preparation of N,N,N′,N′-tetrakis(4-benzoylphenyl) ethylenediamine (2) To a mixture of bis(dibenzylideneacetone)palladium(0) (250 mg, 0.435 mmol), (±)-2,2′-bis(diphenylphosphino)1,1′-binaphthalene ((±)-BINAP) (540 mg, 0.871 mmol), sodium tert-butoxide (8.44 g, 87.93 mmol), and 4bromobenzophenone (22.73 g, 87.06 mmol) in a twonecked round bottom flask under a nitrogen atmosphere was added 1,4-dioxane (100.0 mL). After stirring for 1 h at rt, ethylenediamine 1 (1.00 mL, 14.5 mmol) was add-

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2.2.2 General procedure for the preparation of 3(a–e)

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N,N,N′,N′-Tetrakis(4-(1-phenyl-2-(4-tert-butylphenyl) vinyl)phenyl) ethylenediamine (3c) was prepared by the reaction of tert-BuOK (1290 mg, 11.5 mmol), diethyl benzyl phosphonate (876 mg, 3.84 mmol), and 2 (500 mg, 0.64 mmol). The crude product was purified by column chromatography to afford 3c (617 mg, 74.0%) as pale yellow powders. Mp = 132–134 °C (uncorrected); TLC Rf 0.82 (CHCl3:n-hexane = 2:1); UV–Vis (chloroform): λmax (ε) = 302, 358 nm; IR (KBr): ν (cm−1) = 3026 (w), 2961 (m), 2865 (w), 1597 (m), 1508 (s), 1362 (w), 1269 (w), 1193 (w), 828 (w), 699 (w); 1H NMR (600 MHz, CDCl3, δ) 7.37–7.21 (m, 20H), 7.20–7.05 (m, 16H), 7.04–6.84 (m, 20H), 4.11–3.98 (m, 4H), 1.25–1.19 (m, 36H); Anal. Calcd for C98H96N2: C, 90.42; H, 7.43; N, 2.15. Found: C, 90.45; H, 7.42; N, 2.15.

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To tert-BuOK (1290 mg, 11.5 mmol) in a two-necked round bottom flask under nitrogen was added THF (100.0 mL) and diethyl-4-methylbenzyl phosphonate (1.18 g, 3.84 mmol). After stirring the resulting solution for at rt for 1 h, 2 (500 mg, 0.64 mmol) was added, and the reaction mixture heated at reflux for 72 h. After this time, the mixture was cooled to rt, diluted with chloroform (300.0 mL), washed with 1% aqueous HCl, water, and brine, and dried over MgSO4. Following filtration, the resulting solution was concentrated under reduced pressure to afford the crude product. Purification was by column chromatography.

chromatography to afford 3b (392 mg, 54.0%) as pale yellow powders. Mp 107–109 °C; TLC Rf 0.62 (CHCl3:n-hexane = 2:1); UV–Vis (chloroform): λmax (ε) = 298, 354 nm; IR (KBr): ν (cm−1) = 3022 (w), 2921 (w), 2853 (w), 1653 (m), 1597 (m), 1509 (s), 1369 (m), 1318 (w), 1195 (m), 824 (w), 812 (w), 700 (m); 1H NMR (600 MHz, CDCl3, δ) 2.17–2.27 (m, 12H), 4.01–4.13 (m, 4H), 6.84–7.00 (m, 28H), 7.05–7.09 (m, 4H), 7.15–7.23 (m, 8H), 7.23–7.36 (m, 16H); Anal. Calcd for C86H72N2: C, 91.13; H, 6.40; N, 2.47. Found: C, 91.20; H, 6.32; N, 2.48; HRMS (ESI) m/z: [M+H]+ calcd for C86H72N2, 1133.5774; found, 1133.5746.

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ed, and the reaction mixture heated at reflux for 72 h. The mixture was then cooled to rt, diluted with chloroform (400.0 mL), washed with 1% aqueous HCl, water, and brine, and dried over MgSO4. Following filtration, the resulting solution was concentrated under reduced pressure to afford the crude product. Purification was by recrystallization from isopropyl alcohol to give 2 (9.54 g, 84.3%) as a pale brown powder. Mp 204–205 °C; TLC Rf 0.68 (Et2O); UV–Vis (chloroform): λmax (ε) = 374 nm; IR (KBr): ν (cm−1) = 3078 (w), 3059 (w), 2925 (w), 1645 (s), 1587 (s), 1316 (s), 1283 (s), 1150 (m), 700 (m); 1 H NMR (600 MHz, CDCl3, δ) 4.26 (s, 4H), 7.07 (d, J = 8.86 Hz, 8H), 7.47 (t, J = 7.68 Hz, 8H), 7.57 (tt, J = 7.43, 1.30 Hz, 4H), 7.76 (dd, J = 8.33, 1.30 Hz, 8H), 7.78 (d, J = 8.86 Hz, 8H); 13C NMR (150 MHz, CDCl3, δ) 49.90 (2C), 119.93 (8C), 128.31 (8C), 129.71 (8C), 131.53 (4C), 132.14 (4C), 132.30 (8C), 137.90 (4C), 150.01 (4C), 195.08 (4C). Anal. calcd for C54H40N2O4: C 83.05, H 5.16, N 3.59; found: C 83.08, H 5.2, N 3.85.

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N,N,N′,N′-Tetrakis(4-(1,2-diphenylvinyl)phenyl) ethylenediamine (3a) was prepared by the reaction of tertBuOK (1290 mg, 11.5 mmol), diethyl benzyl phosphonate (876 mg, 3.84 mmol), and 2 (500 mg, 0.64 mmol). The crude product was purified by column chromatography to afford 3a (552 mg, 80.0%) as pale yellow powders. Mp = 105–107 °C (uncorrected); TLC Rf 0.70 (CHCl3:n-hexane = 2:1); UV–Vis (chloroform): λmax (ε) = 292, 362 nm; IR (KBr): ν (cm−1) = 3023 (w), 2922 (s), 2853 (m), 1590 (m), 1505 (s), 1374 (m), 1220 (m), 1181 (m), 812 (m), 695 (s); 1H NMR (600 MHz, CDCl3, δ) δ 7.37–7.25 (m, 16H), 7.23–7.16 (m, 8H), 7.14–7.04 (m, 20H), 7.01–6.88 (m, 16H), 4.11–4.02 (m, 4H); Anal. Calcd for C82H64N2: C, 91.41; H, 5.99; N, 2.60. Found: C, 91.39; H, 6.06; N, 2.60. N,N,N′,N′-Tetrakis(4-(1-phenyl-2-(4-tolyl)vinyl)phenyl) ethylenediamine (3b) was prepared by the reaction of tert-BuOK (1290 mg, 11.5 mmol), diethyl benzyl phosphonate (876 mg, 3.84 mmol), and 2 (500 mg, 0.64 mmol). The crude product was purified by column

N,N,N′,N′-Tetrakis(4-(1-phenyl-2-(4-biphenyl)vinyl) phenyl)ethylenediamine (3d) was prepared by the reaction of tert-BuOK (1290 mg, 11.5 mmol), diethyl benzyl phosphonate (876 mg, 3.84 mmol), and 2 (500 mg, 0.64 mmol). The crude product was purified by column chromatography to afford 3d (522 mg, 59.0%) as pale yellow powders. Mp = 136–138 °C (uncorrected); TLC Rf 0.68 (CHCl3:n-hexane = 2:1); UV–Vis (chloroform): λmax (ε) = 315, 356 nm; IR (KBr): ν (cm−1) = 3026 (w), 2926 (w), 2853 (w), 1596 (m), 1507 (s), 1366 (m), 1247 (w), 1194 (w), 764 (m), 697 (m); 1H NMR (600 MHz, CDCl3, δ) 7.55–7.44 (m, 8H), 7.43–7.25 (m, 32H), 7.24– 6.86 (m, 28H), 4.11–3.98 (m, 4H); Anal. Calcd for C106H80N2: C, 92.14; H, 5.84; N, 2.03. Found: C, 92.07; H, 5.83; N, 2.18.

N,N,N′,N′-Tetrakis(4-(1-phenyl-2-(1naphthyl)vinyl)phenyl) ethylenediamine (3e) was prepared by the reaction of tert-BuOK (1290 mg, 11.5 mmol), diethyl benzyl phosphonate (876 mg, 3.84 mmol), and 2 (500 mg, 0.64 mmol). The crude product was purified by column chromatography to afford 3e (229 mg, 28.0%) as pale yellow powders. Mp = 134– 136 °C (uncorrected); TLC Rf 0.69 (CHCl3:n-hexane = 2:1); UV–Vis (chloroform): λmax (ε) = 315, 360 nm; IR (KBr): ν (cm−1) = 3054 (w), 2927 (m), 2855 (w), 1598 (m), 1507 (s), 1366 (m), 1244 (w), 1194 (m), 776 (m),

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2.6. SCLC diode preparation The hole-only device was prepared as follows: Each DTS derivative (8 mg mL-1 in chloroform) was spin coated on the pre-cleaned ITO substrate at 2000 rpm. An aluminum electrode (Al, 100 nm) was then vacuumdeposited on each layer as the cathode.

3. Results and discussion

3.1. Physical/electrochemical characterization

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Table 1. Optical and electrochemical properties of the synthesized materials. HOMO (eV)a

Eg (eV)b

Tg (°C)c

10% mass loss temp. (°C)d

292, 362

5.31

3.05

97.2

368.82

298, 354

5.3

3.02

96.9

370.11

DTS-3

302, 358

5.3

3.02

120.3

368.81

DTS-4

315, 356

5.25

2.93

126.3

419.57

DTS-5

315, 360

5.24

2.93

125.4

400.45

Material DTS-1 DTS-2

UVmax (nm)

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Cyclic voltammetry (CV) was carried out using a CH Instruments electrochemical analyzer. A three electrode system was employed, consisting of a Ag/Ag+ reference electrode, a Ti wire counter electrode, and a working electrode. The working electrode was prepared using glassy carbon spin coated with each small molecule compound. All measurements were performed using a tetrabutylammonium hexafluorophosphate (C4H9)4N(PF6) solution in acetonitrile as the electrolyte, using a scan rate of 50 mV s-1. Thermogravimetric analysis (TGA) thermograms were obtained using a TGA-2050 (TA Instruments). Differential scanning calorimetry (DSC) measurements were carried out using a DSC Q20 instrument (Waters) at a heating rate of 20 °C min-1. All UV–Vis absorption spectra of the DTS derivatives were measured using a V-670 UV–Vis spectrometer (JASCO).

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2.3. Characterization

a

Measured by CV, bOptical bandgap measured by UV,

2.4. OLED Preparation

c

Measured by DSC, dMeasured by TGA

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The device structure employed herein was ITO/ PEDOT:PSS(40 nm)/DTS derivative/Alq3:C545T(30 nm)/Alq3(30 nm)/LiF(30 nm)/Al(100 nm). The ITO glass substrate (10 Ω -1) was cleaned using detergent, deionized water, acetone, and isopropyl alcohol. Each cleaning step was carried out in an ultrasonication bath over 10 min, and the cleaned ITO glass was treated using O2 plasma. PEDOT:PSS was spin coated at 4000 rpm for 60 s, and then annealed at 150 °C for 10 s. DTS derivatives were spin coated at 4000 rpm and 6000 rpm to achieve thickness control. The Alq3:C545T, Alq3, LiF, and Al layers were formed by thermal evaporation.

Tg of α−NPB

95ºC

Tg : 125.4ºC

DAS-5 ) . u . a ( l a m r e h t o d n E

Tg : 126.3ºC

DAS-4 Tg : 120.3ºC

DAS-3 DAS-2

Tg : 96.9ºC

DAS-1

Tg : 97.2ºC

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Temperature( ºC)

2.5. FET preparation

An n-doped silicon wafer with a 100 nm SiO2 dielectric layer was used as the substrate, which was cleaned using piranha solution. The Si substrate was then modified using octyltrichlorosilane (OTS), and the DTS derivatives (10 mg mL-1 in chloroform) were spin coated on the OTS-modified Si substrate at 2000 rpm for 60 s. The source and drain electrodes (80 nm) were then thermally evaporated onto the semiconductor layer.

Figure 1. DSC scans of DAS derivatives at a scan rate of 20℃/min To test the feasibility of our DTS derivatives for use as HTLs in OLEDs, their electrochemical properties were investigated using cyclic voltammetry (CV) (Figure S1). The HOMO energies of the DTS derivatives were determined from their oxidation potential onsets and are summarized in Table 1 together with additional opti-

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3.2. Charge transport

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Interestingly, as shown in Figure 1 and Table 1, the Tg values for all DTS derivatives were higher than that of NPB (~ 95 °C), a representative HTL material.[23] As expected, as the size of the substituent side group increased, Tg also tended to increase, reaching >120 °C in the case of DTS-3, DTS-4, and DTS-5. This implies that regardless of the conjugation length, larger stilbenoids can yield higher heat resistance in the DTS derivatives. When applied as OLEDs, HTL materials should possess sufficiently high Tg values to ensure that the upper light emitting layers maintain their morphology under continuous Joule heating. Therefore, the observed high Tg values for the DTS derivatives were promising in terms of HTL applications. The thermal degradation behavior of the DTS derivatives was also examined by TGA analyses, as shown in Figure S3 and Table 1. Overall, all DTS derivatives exhibited high robustness to thermal degradation, with a mass loss of 10% at >368 °C.

are summarized in Figure 2(b) as a function of the thermal annealing temperature. Notably, the DTS derivatives bearing smaller substituent groups (i.e., DTS-1 and DTS-2) exhibited a rapid decrease in mobility at annealing temperatures exceeding 100 °C, while those bearing larger stilbenes (DTS-3, DTS-4, and DTS-5) revealed improved thermal resistance.

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cal/physical characteristics. The DTS derivatives bearing various aromatic substituents yielded adequate HOMO levels for the HTL, with values of 5.2–5.3 eV, which are comparable to the value of NPB.[24] The absolute value of HOMO levels of DTS-4 and DTS-5, which possessed extended π-conjugated structures were slightly lower, presumably due to intramolecular charge delocalization. UV-Vis absorption spectra (Figure S2) also suggested the presence of extended charge delocalization in DTS-4 and DTS-5, with red-shifted optical bandgaps of 2.92 and 2.93 eV, respectively. In comparison, other DTS derivatives exhibited wider bandgaps of >3.02 eV. Besides the conjugated structure, the substituent group size increased upon moving from DTS-1 to DTS-5. Therefore, one can immediately expect a higher heat resistance for the larger DTS derivatives in the solid state, due to the existence of steric hindrance. To confirm these assumptions, the glass transition characteristics of all DTS derivatives were determined by DSC.

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To test the feasibility of the solution-processed DTS derivative layers for use as HTLs, SCLC measurements were conducted by preparing a hole-only device structure consisting of a sandwiched indium tin oxide (ITO)/DTS-derivative/Al structure. As SCLC occurs when the injected charge carrier density exceeds the intrinsic free-carrier density, the J-V characteristics of organic materials are governed by the SCLCs as a result of their low intrinsic free carrier densities.[24] In the SCLC regime, the J-V characteristics are described by J ~ V2, which is known as the Mott–Gurney equation.[25] Figure 2(a) shows the representative SCLC J-V characteristics of the hole-only devices prepared using various DTS derivatives of ~80 nm thickness. Using the Mott–Gurney equation, the SCLC mobility values were extracted and

To cross-check the validity of SCLC analyses, fieldeffect transistors (FETs) were prepared using DTS derivatives as the active channel layer. Note that FET enables the estimation of the in-plane charge transport ability whilst SCLC examines the out-of-plane charge transport. Therefore a study of both SCLC and FET structure can shed light on three dimensional charge transport nature of the system. In the case of FET, when operated at saturation, the channel current is determined by the following equation.[26] =

2

−

where W is the channel width, L is the channel length, µ is the charge carrier mobility, Ci is the dielectric capacitance (per unit area), Vg is the gate-source voltage, and Vt is the threshold voltage. This equation corresponds to saturation mode measurements, such as those carried out herein. Therefore, by fitting the obtained I-V characteristics with the above equation, the charge carrier mobility values can be obtained. The resulting transfer curves of the FET based on the solution-processed DTS layers are summarized in Figure 3(a). All DTS derivatives exhibited typical p-type transport behavior with Vt ~ −7 V, indicating the presence of large amounts of trapped states in the semiconductor layer. The FET charge carrier mobility values for all DTS derivatives were ~10–5 cm2 V-1 s-1, which are comparable to those obtained using the SCLC methods and also to the value obtained for NPB.[27] We then investigated the temperaturedependent FET behavior of all DTS derivatives, and the

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0.08 0.06 0.04

IDS

1/2

1/2

(1000 X A )

0.10

0.02 0.00

DAS-1 DAS-2 DAS-3 DAS-4 DAS-5

-30 -25 -20 -15 -10

-5

0

5

10

15

(b) -5

10

-6

10

DAS-1 DAS-2 DAS-3 DAS-4 DAS-5

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10

-8

10

0

50

VGS (V)

100

150

Temperature( ºC)

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Figure 3. (a) Representative transfer characteristics of DAS derivative transistors. (b) Calculated FET mobility as a function of various annealing temperature.

To demonstrate the application of DTS derivatives as HTLs in OLEDs, green emitting OLEDs were prepared based on the ITO/PEDOT:PSS/HTLs/Alq3:C545T/Alq3/ LiF/Al structure. DTS-3 and DTS-4 were employed via solution process (spin-coating), while NPB was used as the reference (vacuum deposited) to demonstrate the superior thermal/electrical characteristics of the novel materials. Despite the high thermal stability of DTS-5, its thin film morphology was not suitable for use as the OLED interlayer, likely due to the steric bulk imparted by the substituent. Thus, we initially varied the thickness of the DTS-3 HTL by controlling the solution concentration and the spin-coating speed. As a result, the film thickness was varied from 37 to 60 nm. The current density–voltage, luminance–voltage, luminance efficiency– current density, and power efficiency–current density characteristics of the OLED devices are summarized in Figure 5 for DTS-3 HTL thicknesses of 37, 55, and 60 nm. The 55 nm-thick DTS-3 HTL was optimal for application in OLEDs, yielding the highest luminance value of all samples investigated (~9000 cd m-2). In addition, the maximum luminance efficiency and power efficiency (9.68 cd A-1 and 3.81 lm W-1, respectively) were measured for the same material. As shown in Figure 6, DTS4 exhibited further improved performance, even comparable to the reference OLED based on the NPB HTL. The NPB-based reference device was prepared following the generally accepted optimal device structure with a vacuum-deposited NPB layer thickness of 40 nm. The maximum luminance efficiency and power efficiency values of the DTS-4 based device were 11.49 cd A-1 and 5.08 lm W-1, respectively. This is comparable to the vacuum-deposited NPB-based device, which exhibited a maximum luminance efficiency and power efficiency of 9.88 cd A-1 and 5.45 lm W-1, respectively.

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Filed Effect Mobility (cm V s )

(a)

3.3. HTL application in OLEDs

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results are summarized in Figure 3(b). DTS-1 and DTS2 demonstrated the poorest thermal stabilities, with a dramatic decrease in mobility being observed at annealing temperatures >100 °C. In contrast, DTS-3, DTS-4, and DTS-5 exhibited significantly improved thermal stabilities, maintaining reasonable hole mobility values of 10-6 cm2 V-1 s-1 up to 150 °C. These robust charge transport characteristics of the DTS derivatives under high-temperature exposure are encouraging, especially considering that these systems can be solutionprocessed.

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To examine the local crystallization of the DTS derivatives at elevated temperatures, the films were examined by optical microscopy (see Figure 4). The results showed good agreement with the charge transport studies and the Tg information, with DTS-3, DTS-4, and DTS-5 maintaining a stable morphology at elevated temperature annealing. Up to 150 °C, the formation of crystalline defects was not observed for the larger DTS derivatives, while local crystallization was observed for both DTS-1 and DTS-2. It is likely that improved heat resistance was observed with the larger stilbene derivatives due to the increase in steric hindrance preventing agglomeration.

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Figure 5. (a) J-V, (b) L-V, (c) luminance efficiency, and (d) luminance power efficiency characteristics of devices containing DTS-3 as a HTL with thickness of 37 nm, 55 nm, and 60 nm.

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As a final step, the acceleration lifetimes of the OLEDs containing the solution-processed DTS-4 HTL and the vacuum-deposited NPB HTL were compared. As shown in Figure 7, the measured acceleration half-lifetime of the DTS-4-based OLED was significantly longer than the NPB-based OLED (i.e., ~500 h cf. ~1200 h). We therefore argue that DTS derivatives with large substituent groups can be efficient HTL materials for OLEDs due to their high charge carrier mobility, well-matched energy levels, and their high thermal resistance under device operation.

PEDOT:PSS HIL layer. The highest occupied molecular orbital (HOMO) levels of the DTS derivatives measured by cyclic voltammetry ranged from 5.2 to 5.3 eV, which is comparable to the value obtained for NPB, a state-ofthe-art HTL material used in vacuum-processed OLEDs, and thus confirmed that our system is adequate for hole transport in a general OLED structure. Considering the relatively low glass transition temperature (Tg) of NPB (i.e., ~95 °C), the DTS derivatives synthesized in this study were strategically designed to have a high Tg to prevent self-crystallization under Joule heating while preserving a high charge carrier mobility, high transparency, and solution processability. The obtained DTS derivatives exhibited Tg values of 97–126 °C, rendering them suitable for stable OLED operation. We also observed that DTS derivatives containing a larger stilbene volume (and thus a higher Tg) exhibit higher heat resistance. Despite the introduction of large substituent groups, all DTS derivatives exhibited a reasonably good charge transport nature. Under both a space-charge limited current (SCLC) and a field-effect transistor (FET), the solution-processed DTS derivatives yielded a hole mobility of ~10−5 cm2 V-1 s-1, which is similar to that of vacuum-deposited NPB. Moreover, this satisfactory charge transport ability was maintained at elevated annealing temperatures in excess of 100 °C due to the large planar substituents, which suppressed intermolecular agglomeration. To demonstrate the feasibility of DTSderivative HTLs for application in OLEDs, green emitting OLEDs adopting solution-processed DTS derivatives as HTLs are fabricated and compared to the devices with vacuum-deposited NPB. A high luminance efficiency of 11.5 cd A-1 was obtained using the DTS HTL, which was higher than that obtained from the NPB system. More importantly, the DTS HTL enabled stable operation of OLEDs, doubling the accelerated half-life of the NPB-based OLED. We believe that our discovery of novel HTL materials opens up the possibility of using three-dimensional stilbenoids in the thermally stable functional layer of OLEDs.

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Figure 7. Comparison of the accelerated lifetime test between OLEDs containing solution-processed DTS-4 and vacuum-deposited NPB as HTLs.

4. Conclusion A series of DTS compounds for HTL application has been synthesized with high solubility in common organic solvents so that it can be solution-deposited onto

ASSOCIATED CONTENT Supporting Information. AUTHOR INFORMATION Corresponding Author

Soo Young Kim ([email protected]), Tel: 82-2820-5875, Fax: 82-2-824-3495. Dae Sung Chung ([email protected]), Tel: 82-53-7856426 Kwangyong Park ([email protected]), Tel: 82-2-820-5330, Fax: 82-2-824-3495

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ACCEPTED MANUSCRIPT organic light-emitting diodes by solution process. Chem Mater 2012;24:4581-7.

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This research was supported in part by a National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIP) (Nos. 2015K1A3A1A59073839, 2017H1D8A1030599) and in part by Korea Agency for Infrastructure Technology Advancement grant funded by Ministry of Land, Infrastructure and Transport (17IFIP-B133622-01).

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ACKNOWLEDGMENT

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Highlights

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A series of diamine-cored tetrastilbene (DTS) derivatives was designed and synthesized.

The DTS derivatives are used as solution-processed hole transport layers (HTLs) in

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OLEDs.

A high luminance efficiency of 11.5 cd/A was obtained for one solution-processed

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DTS HTL.

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The DTS HTLs enabled a stable OLED operation.