The locally twisted thiophene bridged phenanthroimidazole derivatives as dual-functional emitters for efficient non-doped electroluminescent devices

Organic Electronics 18 (2015) 61–69

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Organic Electronics journal homepage: www.elsevier.com/locate/orgel

The locally twisted thiophene bridged phenanthroimidazole derivatives as dual-functional emitters for efficient non-doped electroluminescent devices Yi Yuan a,b, Jia-Xiong Chen a, Wen-Cheng Chen a,b, Shao-Fei Ni c, Huai-Xin Wei b, Jun Ye b, Fu-Lung Wong b, Zhong-Wei Zhou d, Qing-Xiao Tong a,⇑, Chun-Sing Lee b,⇑ a

Department of Chemistry, Shantou University, Guangdong 515063, China Center of Super-Diamond and Advanced Films (COSDAF) and Department of Physics and Materials Science, City University of Hong Kong, Hong Kong Special Administrative Region c Department of Chemistry, South University of Science and Technology of China, Shenzhen 518055, China d 4/F R&D Building, Skyworth Science Park, Tangtou Industrial Zone, Shiyan Town, Baoan District, Shenzhen, China b

a r t i c l e

i n f o

Article history: Received 4 December 2014 Received in revised form 2 January 2015 Accepted 5 January 2015 Available online 12 January 2015 Keywords: Phenanthroimidazole Thiophene Electroluminescence Dual-functional emitter

a b s t r a c t A series of locally twisted dual-functional materials namely PIPT, PITT and PIFT have been designed and synthesized by introducing different polyaromatic hydrocarbon groups to a phenanthroimidazole backbone through a thiophene bridge. In these molecules, the thiophene bridge and phenanthroimidazole platform are nearly coplanar and this endows these materials with relatively shallow HOMO levels (5.35 to 5.21 eV). On the other hand, the bulky polyaromatic hydrocarbon units introduce non-planar twisty structures which reduce molecular aggregations. These three materials show color-tunable emission (emission peak from 468 to 532 nm in film) and high thermal stability (Tg > 160 °C). Simple trilayer devices using these three phenanthroimidazole derivatives as non-doped emitting layers exhibit low turn-on voltages (2.3–2.7 V) and high maximum efficiencies of 3.74, 6.15 and 6.89 cd/A for PIPT, PITT and PIFT, respectively. Above all, owing to their shallow HOMO levels for enabling efficient hole-injection, even simpler bilayer devices employing these materials as hole-transporting emitters show low turn-on voltages (2.6–2.8 V) and high efficiencies of 5.77 cd/A for PIPT, 6.03 cd/A for PITT and 6.04 cd/A for PIFT, respectively. These comparable performances with those of the trilayer configurations show the efficient hole-injection/transport ability of these three newly developed emitters. Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction Since the pioneering work on multilayered organic light-emitting devices (OLEDs) by Tang and Van Slyke [1], research on OLEDs have been intensively pursued for their applications in full-color display and solid-state lighting [2,3]. The past decades have witnessed significant advance ⇑ Corresponding authors. E-mail addresses: [email protected] (Q.-X. Tong), [email protected]. hk (C.-S. Lee). http://dx.doi.org/10.1016/j.orgel.2015.01.009 1566-1199/Ó 2015 Elsevier B.V. All rights reserved.

of OLEDs through developing novel efficient electroluminescent (EL) materials and device structures [4–10], resulting in wide commercial applications. However, cost reduction in OLED manufacturing remains an important challenge. In addition to the capital investment, the manufacturing cost of OLED mainly includes the costs for materials and device fabrication. A high performance OLED normally requires a sophisticated device configuration containing several organic layers with different functions including light-emission, carrier injection, transportation, blocking,

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etc. [11,12]. While this allows individual optimization of each material for its specific function, the device complexity and thus fabrication cost will be inevitably increased. Hence, simplifying the device configurations by using multifunctional materials is an attractive approach for cost saving [13–17]. Thiophene is an electron-rich moiety widely incorporated in various organic semiconductors for applications in organic photovoltaic cells (OPVs) [18–22], organic field-effect transistors (OFETs), etc. [23–25]. Thiopheneembedded conjugated systems normally exhibit high electrical conductivity as a result of their planar structure and strong intermolecular interactions (S. . .S and S. . .p) induced by the high polarizability of their sulfur atoms. However, these intermolecular interactions often cause fluorescence quenching [26]. For this reason, thiophene derivatives typically do not give efficient fluorescence [27–30]. Hence, the key for developing efficient thiophene-based emitters is to reduce the intermolecular interactions in their solid states. Phenanthroimidazole is a thermally stable and highly emissive heterocyclic aromatic compound and a number of phenanthroimidazole derivatives have been developed as bipolar and high efficiency emitters for OLEDs [31–34]. In our previous works, Zhang et al. showed that introduction of a thiophene ring on the C2-position of phenanthroimidazole can effectively decrease the ionization potentials (Ip) of the compounds to enable efficient hole-injection from the hole-transporting layer (HTL) [28]. However, the added thiophene group also increases intermolecular interaction and reduced EL performance. Most recently, we reported three highly efficient bipolar blue phenanthroimidazole derivatives, TTP-TPI, DPT-TPI and DPF-TPI by incorporating polyaromatic hydrocarbon groups to the phenanthroimidazole backbone through a phenyl bridge [31]. It was shown that introduction of the bulky polyaromatic groups can effectively reduce intermolecular interactions and enhance EL performance. However, the Ip of these materials are far from the work functions of the ITO electrode and thus do not allow these materials to directly accepting holes from the anode efficiently for simplifying the device configurations. In this work, we designed and synthesized a series of new materials, PIPT, PITT and PIFT, by combining phenanthroimidazole and different bulky polyaromatic hydrocarbon groups through a thiophene ring. It is anticipated that the electron-rich thiophene bridge would endow these materials with high-lying HOMO levels and thus reduce the hole-injection barrier into these materials. At the same time, intermolecular interactions of these compounds are expected to be effectively reduced by the highly twisted substituents on the 1-imidazole position as well as the end-capping polyaromatic hydrocarbon groups. We also expect that these three materials possess good thermal and morphological stabilities as a result of the bulky polyaromatic hydrocarbon groups. Thermal, photophysical, and electroluminescent properties of the compounds were comprehensively investigated. These three new phenanthroimidazole derivatives have been demonstrated to be efficient host emitter with good holetransporting properties.

2. Experimental section Starting materials 5-ethynylthiophene-2-carbaldehyde [35], 1,3-diphenyl-2H-cyclopenta[l]phenanthren-2-one [36], and 7,9-diphenyl-8H-cyclopenta-[a]acenaphthylen-8-one [37] were synthesized according to the literature procedures. All other chemicals and solvents were used as received from commercial suppliers without further purification.

2.1. Syntheses 5-(30 ,60 -Diphenyl-[1,10 :20 ,100 -terphenyl]-40 -yl)thiophene2-carbaldehyde (1). 5-Ethynylthiophene-2-carbaldehyde (0.36 g, 2.64 mmol) and 2,3,4,5-tetraphenylcyclopenta2,4-dienone (0.92 g, 2.40 mmol) were dissolved in o-xylene (70 mL) under an argon atmosphere, and the resultant mixture was heated for 24 h at 150 °C. After the mixture was cooled to room temperature, ethanol (100 mL) was added. The precipitate was filtered, washed with ethanol (150 mL), and dried in vacuum. Following column chromatography (petroleum ether: CH2Cl2 = 1:1) on silica gel, 1 was obtained as a white solid. Yield: 0.81 g (68.6%). 1H NMR (400 MHz, CDCl3) d 9.92–9.68 (m, 1H), 8.43 (dd, J = 8.2, 1.5 Hz, 2H), 7.85–7.63 (m, 2H), 7.56 (d, J = 8.5 Hz, 1H), 7.53–7.47 (m, 4H), 7.47–7.37 (m, 5H), 7.37–7.26 (m, 4H), 7.22 (dd, J = 8.0, 1.5 Hz, 2H), 7.15–6.98 (m, 2H), 6.61 (d, J = 3.9 Hz, 1H). MS (ESI+): m/z 493.0 (MH+). Calcd for C35H24OS: 492.63. 5-(1,4-Diphenyltriphenylen-2-yl)thiophene-2-carbaldehyde (2) and 5-(7,10-diphenylfluoranthen-8-yl)thiophene-2-carbaldehyde (3) were synthesized with the similar procedure as for 1 except that different ketones were used as starting materials as shown in Scheme 1. (2) Yield: 0.76 g (65.4%). 1H NMR (400 MHz, CDCl3) d 9.92–9.68 (m, 1H), 8.43 (dd, J = 8.2, 1.5 Hz, 2H), 7.85–7.63 (m, 2H), 7.56 (d, J = 8.5 Hz, 1H), 7.53–7.47 (m, 3H), 7.47– 7.37 (m, 5H), 7.37–7.26 (m, 3H), 7.22 (dd, J = 8.0, 1.5 Hz, 2H), 7.15–6.98 (m, 2H), 6.61 (d, J = 3.9 Hz, 1H). MS (ESI+): m/z 491.4 (MH+). Calcd for C35H22OS: 490.61. (3) Yield: 0.90 g (62.8%). 1H NMR (400 MHz, CDCl3) d 9.76 (s, 1H), 7.76 (dd, J = 11.0, 8.2 Hz, 2H), 7.72–7.66 (m, 2H), 7.62–7.48 (m, 8H), 7.48–7.42 (m, 2H), 7.39 (t, J = 7.6 Hz, 1H), 7.34–7.27 (m, 2H), 6.87 (d, J = 3.9 Hz, 1H), 6.57 (d, J = 7.2 Hz, 1H). MS (ESI+): m/z 465.5 (MH+). Calcd for C33H20OS: 464.58. 1-(4-(Tert-butyl)phenyl)-2-(5-(30 ,60 -diphenyl-[1,10 :20 ,100 terphenyl]-4 0 -yl)thiophen-2-yl)-1H-phenanthro[9,10-d] imidazole (PIPT). 9,10-Phenanthrenequinone (0.31 g, 1.50 mmol), 5-(30 ,60 -diphenyl-[1,10 :20 ,100 -terphenyl]-40 -yl) thiophene-2-carbaldehyde (1) (0.74 g, 1.50 mmol), 4-tert-butylbenzenamine (0.27 g, 1.80 mmol), and ammonium acetate (1.16 g, 15.0 mmol) were added into glacial acetic acid (30 mL) and the mixture was refluxed for 24 h under an argon atmosphere. After cooling to room temperature, a dark yellow mixture was obtained and poured into a methanol solution under stirring. The separated solid was filtered off, washed with methanol, and dried to give a yellow solid. The solid was purified by column chromatography (petroleum ether: CH2Cl2 = 2:1) on silica gel, PIPT

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Scheme 1. Synthetic routes for PIPT, PITT and PIFT.

was obtained as a pale yellow powder. Yield: 1.02 g (83.6%). 1H NMR (400 MHz, CD2Cl2) d 8.71 (dd, J = 18.6, 8.3 Hz, 3H), 7.67 (d, J = 8.3 Hz, 4H), 7.58 (s, 1H), 7.49– 7.42 (m, 3H), 7.24 (s, 1H), 7.17–7.11 (m, 6H), 7.09–7.05 (m, 4H), 6.97 (s, 2H), 6.92–6.89 (m, 3H), 6.88–6.81 (m, 6H), 6.79 (dd, J = 6.8, 2.8 Hz, 2H), 1.40 (s, 9H). MS (ESI+): m/z 814.2 (MH+). Calcd for C59H44N2S: 813.06. Yellow powder of 1-(4-(tert-butyl)phenyl)-2-(5-(1,4diphenyltriphenylen-2-yl)thiophen-2-yl)-1H-phenanthro [9,10-d]imidazole (PITT) and orange–yellow powder of 1-(4-(tert-butyl)phenyl)-2-(5-(7,10-diphenylfluoranthen8-yl)thiophen-2-yl)-1H-phenanthro[9,10-d]imidazole (PIFT) were obtained with the similar procedures as for PIPT except that (2) and (3) were respectively used as the starting materials instead of (1). PITT: yield: 0.84 g (85.4%). 1H NMR (400 MHz, CD2Cl2) d 8.72 (dd, J = 18.3, 8.3 Hz, 3H), 7.80–7.63 (m, 10H), 7.62– 7.53 (m, 7H), 7.50–7.43 (m, 6H), 7.39 (t, J = 7.6 Hz, 1H), 7.33–7.25 (m, 3H), 7.14 (d, J = 7.9 Hz, 1H), 6.53 (d, J = 7.1 Hz, 2H), 1.45 (s, 9H). MS (ESI+): m/z 811.6 (MH+). Calcd for C59H42N2S: 811.04. PIFT: yield: 0.95 g (82.1%). 1H NMR (400 MHz, CD2Cl2) d 8.71 (dd, J = 18.5, 8.2 Hz, 3H), 8.42 (d, J = 8.2 Hz, 2H), 7.70– 7.64 (m, 6H), 7.51 (d, J = 8.4 Hz, 2H), 7.47–7.39 (m, 10H), 7.32 (dt, J = 14.2, 6.9 Hz, 3H), 7.25–7.17 (m, 3H), 7.09– 7.04 (m, 1H), 6.99 (s, 1H), 1.38 (s, 9H). MS (ESI+): m/z 785.7 (MH+). Calcd for C57H40N2S: 785.01. 2.2. Characterization Nuclear magnetic resonance (1NMR) spectra were measured on a Varian Gemin-400 Varian spectrometer. Mass

spectra were recorded on a PE SCIEX API-MS system. Absorption and photoluminescence (PL) spectra of the materials were recorded with a PerkinElmer Lambda 2S UV/Vis spectrophotometer and a Perkin-Elmer LS50B luminescence spectrophotometer, respectively. Thermogravimetric analysis (TGA) measurements were performed on a TA Instrument TGAQ50 at a heating rate of 10 °C/min under a nitrogen atmosphere. Differential scanning calorimetric (DSC) measurements were performed on a TA Instrument DSC2910. The samples were first heated at a rate of 10 °C/min to melt and then quenched. Glass transition temperature (Tg) and crystallization temperature (Tc) were recorded by reheating the quenched samples at a heating rate of 10 °C/min. The highest occupied molecular orbital (HOMO) energies of the materials were measured directly with ultraviolet photoelectron spectroscopy (UPS); whereas, the lowest unoccupied molecular orbital (LUMO) energy was estimated by subtracting the HOMO energy with the optical band gap determined from the lowest energy absorption edge of the absorption spectrum. 2.3. Device fabrication and measurement Patterned indium tin oxide (ITO) coated glass with a sheet resistance of 15 X per square was used as the substrate. Before device fabrication, the ITO glass substrates were cleaned with isopropanol, Decon 90 and deionized water, dried in an oven at 120 °C, treated with UV-ozone, and finally transferred to a vacuum deposition system with a base vacuum better than 107 Torr for organic and metal deposition. Devices were fabricated by thermal deposition of organic layers at a deposition rate of 1–2 Å/s. The

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cathode was completed through thermal deposition of LiF at a rate of 0.1 Å/s, followed by Al deposition at a rate of 5 Å/s. EL spectra and CIE color coordinates were measured with a Spectrascan PR650 photometer and the current– voltage–luminance characteristics were measured with a computer-controlled Keitheley 2400 sourcemeter under ambient atmosphere.

3. Results and discussions 3.1. Synthesis and characterization Synthetic routes and molecular structures of the three compounds, PIPT, PITT and PIFT are depicted in Scheme 1. Key intermediates 5-(30 ,60 -diphenyl-[1,10 :20 ,100 -terphenyl]40 -yl)thiophene-2-carbaldehyde (1), 5-(1,4-diphenyltriphenylen-2-yl)thiophene-2-carbaldehyde (2) and 5-(7, 10-diphenylfluoranthen-8-yl)thiophene-2-carbaldehyde (3) were prepared via Diels–Alder reactions of 5-ethynylthiophene-2-carbaldehyde with respectively 2,3,4,5-tetraphenylcyclopenta-2,4-dienone, 1,3-diphenyl-2H-cyclopenta[l]phenanthren-2-one and 7,9-diphenyl-8H-cyclopenta[a]acenaphthylen-8-one. PIPT, PITT and PIFT were then prepared by ‘‘one pot’’ reactions of 9,10-phenanthrenequinone and 4-tert-butylbenzenamine with respectively 1, 2 and 3. Chemical structures of all compounds were fully characterized with 1H NMR spectroscopy and mass spectrometry (see Section 2).

3.2. Thermal properties Thermal properties of the three phenanthroimidazole derivatives were investigated by TGA and DSC under a nitrogen atmosphere. All the compounds exhibited good thermal stability. As shown in Fig. 1a, decomposition temperatures (Td), defined as the temperature at which the material shows a 5% weight loss, were measured to be 421, 456, and 457 °C for PIPT, PITT, and PIFT, respectively. DSC measurements were performed, from 20 to 350 °C for all of the materials. During the first heating process, PITT and PIFT only show an endothermic peak at their melting points (melting temperature, Tm) of 326 and 335 °C, respectively. When the samples were heated again, a glass-transition temperature was revealed at 186 °C for PITT and 174 °C for PIFT. Thermal properties of PIPT are similar to those of PITT and PIFT except for an additional phase transformation temperature (T1) at 282 °C in the first heating process and crystallization temperature (Tc) at 268 °C in the second heating process, and it exhibited a Tg and Tm of 163 °C and 298 °C, respectively (Fig. 1). Compared to the Tg value of PIPT, the Tgs of PIFT and PITT were improved gradually which could be attributed to the increase in p conjugation core of the end-capping polyaromatic hydrocarbon groups. Key thermal data of the phenanthroimidazole derivatives are summarized in Table 1. Their high Tgs should be attributed to the highly twisted and bulky polyaromatic hydrocarbon moieties which are beneficial to their morphological stability.

Fig. 1. TGA (a) and DSC (b–c) curves for PIPT, PITT and PIFT.

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Y. Yuan et al. / Organic Electronics 18 (2015) 61–69 Table 1 Key physical properties of the three compounds. Compound

PIPT PITT PIFT

kPL max (nm)

kabs max (nm) THF

Film

THF

Film

258, 378 262, 382 378

269, 388 268, 392 386

453 471 508

468 485 532

3.3. Optical properties Fig. 2 depicts normalized ultraviolet–visible (UV–vis) and photoluminescence (PL) spectra of the three materials measured in both diluted tetrahydrofuran (THF) solutions and solid-films prepared by thermal evaporation on quartz plates. Key optical parameters are summarized in Table 1. The three compounds have two similar absorption bands in solutions and show small bathochromic shifts (10 nm) in solid state. Strong absorption bands around 260 nm are probably originated from the p–p⁄ local electron transition of their common thiophene and benzene rings. The lower intensity bands between 370 and 430 nm can be assigned to the delocalized p–p⁄ electron transition of the phenanthroimidazole [28]. As shown in Fig. 2, the three compounds show sky-blue to green emissions with emission peaks ranging from 453 to 508 nm in diluted THF solutions and 468–532 nm in thin-films. It should be noted that for all compounds the absorption and PL spectra show relatively small differences in THF solutions and thin-film states. This indicates that the highly twisted substituent on the 1-imidazole position as well as the non-coplanar end-capped polyaromatic hydrocarbon moieties can effectively restrain intermolecular aggregation. Optical band gaps (Eg) determined from the threshold of the optical absorption are 2.92, 2.84 and 2.64 eV for PIPT, PITT and PIFT, respectively. The HOMO levels as measured with UPS are 5.35, 5.31 and 5.27 eV, respectively, which are similar to that of the widely used hole-transporting material N,N0 -bis(1naphthyl)-N,N0 -diphenyl-1,10 -biphenyl-4,40 -diamine (NPB, 5.4 eV). These levels are also about 0.3 eV shallower than that of their counterparts which using a phenyl bridge [31]. The LUMO levels of these compounds are estimated to be

Tg/Tm/Td (°C)

HOMO (eV)

Eg (eV)

LUMO (eV)

163/298/421 186/326/456 174/335/457

5.35 5.31 5.27

2.92 2.84 2.64

2.43 2.47 2.63

2.43, 2.47 and 2.63 eV respectively by adding the corresponding Eg to the HOMO values.

3.4. Theoretical calculation To better understand the correlation between molecular structures and physical properties of the three phenanthroimidazole derivatives at molecular level, density functional theory (DFT) calculations were carried out with geometry optimized at the B3LYP/6-31G(d) level. The optimized geometry and calculated frontier molecular orbital electron density are shown in Fig. 3. All the three molecular show locally twisted conformation. The phenanthroimidazole skeletons and thiophene rings are nearly coplanar and the dihedral angles are 4.10°, 2.84° and 2.37° (Fig. S1) for PIPT, PITT and PIFT respectively. On the other hand, the t-Bu-benzene ring at the 1N-position and the end-capping bulky polyaromatic hydrocarbon groups are highly twisted, for example, the dihedral angles between t-Bu-benzene ring and the phenanthroimidazole backbone are 87.6°, 89.5° and 89.2° (Fig. S1), respectively. Compared with other nearly coplanar thiophene-based emitters, this kind of locally twisted conformation could keep the advantages of thiophene units, while also effectively reduce the intermolecular interaction in solid states. According to the DFT calculations, the shapes of the HOMO orbitals are similar, mainly located on the phenanthroimidazole moiety, thiophene bridges and cores of the polyaromatic hydrocarbon groups. Their LUMOs show more difference, and with increased conjugated length of polyaromatic hydrocarbon groups from PIPT to PIFT, the LUMO orbitals are gradually localized on the thiophene bridges and the core of the polyaromatic hydrocarbon groups, which also

Fig. 2. Normalized UV–vis absorption (left) and PL spectra (right) of PIPT, PITT and PIFT in THF solution (solid symbols) and in thin film (open symbols).

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Fig. 3. The optimized molecular geometries and calculated spatial distributions of the HOMOs and LUMOs of PIPT, PITT and PIFT.

Fig. 4. Energy level diagrams of the three phenanthroimidazole derivatives based trilayer devices.

explains the perturbations observed in the higher wavelength absorption in these compounds. 3.5. Electroluminescence properties To investigate EL properties of the three phenanthroimidazole derivatives, simple trilayer non-doped devices with a configuration of ITO/NPB (70 nm)/PIPT or PITT or PIFT (30 nm)/1,3,5-tri(phenyl-2-benzimidazolyl)-benzene (TPBI, 30 nm)/LiF (1 nm)/Al (150 nm) were fabricated. In these devices, ITO and LiF/Al were used as the anode and the cathode, respectively; NPB was used as the HTL; TPBI

was used as the electron-transporting layer (ETL) and hole-blocking layer; and the three thiophene-bridged phenanthroimidazole derivatives were used as the emitting layer (EML). Energy levels of the correlative materials used in the devices are depicted in Fig. 4. Key device performance parameters are summarized in Table 2. Fig. 5 depicts normalized EL spectra (solid lines) of the PIPT, PITT and PIFT based trilayer devices with emission peaks at 472, 496 and 544 nm, respectively. The emission color ranged from sky-blue to green with CIE 1931 chromaticity coordinates of (0.17, 0.24) for PIPT, (0.22, 0.45) for PITT, and (0.37, 0.58) for PIFT. The EL spectra of the devices are almost the same as the corresponding PL spectra of solid films of the three compounds. This suggests that the EL is indeed from the three thiophene-bridged phenanthroimidazole emitters. Moreover, no excimer or exciplex emission can be observed. It is attributed to the locally twisted structure due to the substituent on the 1imidazole position and the bulky polyaromatic hydrocarbon moieties, which results in large steric hindrance. Current density–voltage–brightness (J–V–L) characteristics and plots of efficiency versus current density or brightness of the three devices are show in Fig. 6. All devices show low turn-on voltage (Von, defined as voltage required to give a luminance of 1 cd/m2) from 2.3 to 2.7 V, which should be attributed to the high-lying HOMO levels of these thiophene-bridged phenanthroimidazole

Table 2 Electroluminescence characteristics of the devices.

a

Device

Vona (V)

gCb (cd A1)

gPb (lm W1)

gextb (%)

kELc (nm)

CIEc (x, y)

PIPT trilayer PITT trilayer PIFT trilayer PIPI bilayer PITT bilayer PIFT bilayer

2.7 2.6 2.3 2.8 2.6 2.7

3.74/3.13 6.15/5.33 6.89/6.43 5.57/5.08 6.03/5.94 6.04/5.84

3.91/1.87 6.88/4.75 8.22/5.36 5.83/3.0 6.10/4.75 6.06/4.87

2.12/1.80 2.28/2.06 2.07/1.93 2.82/2.59 2.16/2.13 1.78/1.72

472 496 544 484 508 544

0.17, 0.22, 0.37, 0.17, 0.22, 0.37,

0.24 0.45 0.58 0.28 0.50 0.58

Von is the turn-on voltage at 1 cd/m2. gC, max, gP, max and gext, max the maximum current, power and external quantum efficiency, respectively. In the order of maximum, then values at 1000 cd/m2. c Measured at 1000 cd/m2. b

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Fig. 5. Normalized EL spectra of the trilayer (solid lines) and bilayer (dashed lines) devices at a brightness of 1000 cd/m2.

emitters, which are well-matched with the HOMO level of the adjacent NPB and thus enable efficient holes injection into the EML. Among them, the PITT-based device exhibits the best performance, with a maximum current efficiency (gC, max) of 6.15 cd/A, a maximum power efficiency (gP, max) of 6.88 lm/W and a maximum external quantum efficiency (gext, max) of 2.28%. The performances of two other devices are slightly lower than that of the PITT-based device, with gC, max, gP, max and gext, max of 3.74 cd/A, 3.91 lm/W and 2.12% for PIPT and 6.89 cd/A, 8.22 lm/W and 2.07% for PIFT. To our best knowledge, these results are actually much better than the performance of many other thiophene-based emitters in the literature (Table S1) [27–30,38,39]. In addition, these devices show low efficiency roll-off at high brightness, for example, the efficiencies are maintained at 3.13 cd/A (1.80%) for PIPT, 5.33 cd/A (2.06%) for PITT and 6.43 cd/A (1.93%) for PIFT based devices when the brightness reaches 1000 cd/m2. We note that the three materials have negligible holeinjection barriers at the NPB/EML junction for their highlying HOMO levels, this merit suggests that they may potentially serve as hole-transporting emitters to simplify device structures. Bilayer OLEDs with the structure of ITO/PIPT or PITT or PIFT (80 nm)/TPBI (30 nm)/LiF (1 nm)/Al (150 nm) were thus fabricated. Key device performance parameters are summarized in Table 2. As show in Fig. 5, the EL spectra (dashed lines) of simple bilayer devices are similar to that of the corresponding trilayer devices (solid lines), with emission peak of 484 nm for PIPT, 508 nm for PITT and 544 nm for PIFT, and the corresponding CIE coordinates are (0.17, 0.28), (0.22, 0.50) and (0.37, 0.58), respectively. The small difference in wavelength might be due to optical microcavity effects arising from the different device structures and/or a shift in the emission region upon removal of barrier at the NPB/EML interface from the trilayer device [17]. J–V–L characteristics and plots of efficiency versus current density or brightness of the bilayer devices are show in Fig. 7. Owing to the high-lying HOMO levels of the three materials which facilitate efficient hole-injection from the anode, these three devices also exhibit low Von from 2.6 to 2.8 V. The PIPTbased bilayer device exhibits the best performance, with gC, max, gP, max and gext, max of 5.57 cd/A, 5.83 lm/W and

Fig. 6. (a) Current density–voltage–luminance (J–V–L) characteristics, (b) current efficiency and power efficiency versus current density and (c) external quantum efficiency versus luminance curves for the trilayer devices.

2.82%, respectively. Moreover, the efficiency of this device still remains at 5.08 cd/A (2.59%) at a brightness of 1000 cd/m2. It is worth noting that this performance is even much better than that of the corresponding trilayer device, and the enhancement in the efficiency of the bilayer device might be attributed to a better balance of charge carriers within the emissive layer [40]. The PITTand the PIFT-based bilayer devices also show good performance, with gC, max, gP, max and gext, max of 6.03 cd/A, 6.10 lm/W and 2.16%; and 6.04 cd/A, 6.06 lm/W and 1.78%, respectively. When the brightness increases to 1000 cd/m2 the efficiencies are still as high as 2.13% (5.94 cd/A) for PITT and 1.72% (5.84 cd/A) for PIFT. Though

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thiophene bridge imparts these materials with shallow HOMO levels, and the bulky polyaromatic hydrocarbon groups are able to reduce intermolecular interactions in solid states. Simple trilayer devices prepared using these three phenanthroimidazole derivatives as fluorescent emitters exhibited tunable emission color from sky blue to green. The PIPT, PITT and PIFT based non-doped devices achieve good performances with gC, max (gext, max) of 3.74 cd/A (2.12%), 6.15 cd/A (2.28%) and 6.89 cd/A (2.07%), respectively. The shallow HOMO levels of these three materials also enable their use as hole-transporting layer to directly receive holes from the anode. Their corresponding simplified bilayer devices also show good performances with gC, max of 5.77 cd/A for PIPT, 6.03 cd/A for PITT and 6.04 cd/A for PIFT, which are comparable with those of their corresponding trilayer devices. In addition, these three bilayer devices also show mild efficiency rolloff at high brightness as compared to the trilayer devices. Performances of the present emitters are comparable to that of the best thiophene-containing emitter in the literature. Our study here presents a viable molecular design strategy of thiophene-based semiconductor for EL applications. Acknowledgements The work described in this paper was supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. T23713/11), National Natural Science Foundation of China (Nos. 21303150, 51273108 and 91027041), and the National Basic Research Program of China (973 Program 2013CB834803). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.orgel.2015.01.009. Fig. 7. (a) Current density–voltage–luminance (J–V–L) characteristics, (b) current efficiency and power efficiency versus current density and (c) external quantum efficiency versus luminance curves for the bilayer devices.

these two devices exhibited slightly lower performances than the PIPT-based bilayer device, but they are also comparable to their corresponding trilayer devices. These results reveal that these three thiophene-based semiconductors have great potential to be dual-functional holetransporting and emitting materials for low cost OLED applications. 4. Conclusions In summary, we have designed and synthesized three dual-functional emitters, PIPT, PITT and PIFT, by incorporating different polyaromatic hydrocarbon moieties to phenanthroimidazole through a thiophene bridge. The

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