Self-Host Blue-Emitting Iridium Dendrimer Containing Bipolar

Oct 13, 2016 - A novel self-host blue-emitting iridium dendrimer, namely, B-CzPO, has been designed and synthesized via a postdendronization route, wh...
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Self-Host Blue-Emitting Iridium Dendrimer Containing Bipolar Dendrons for Nondoped Electrophosphorescent Devices with Superior High-Brightness Performance Yang Wang,†,‡ Yaoming Lu,§ Baoxiang Gao,§ Shumeng Wang,†,‡ Junqiao Ding,*,† Lixiang Wang,*,† Xiabin Jing,† and Fosong Wang† †

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China ‡ University of the Chinese Academy of Sciences, Beijing 100049, PR China § College of Chemistry and Environmental Science, Hebei University, Baoding 071002, PR China S Supporting Information *

ABSTRACT: A novel self-host blue-emitting iridium dendrimer, namely, B-CzPO, has been designed and synthesized via a postdendronization route, where a bipolar carbazole/triphenylphosphine oxide hybrid is selected as the peripheral dendron instead of the p-type oligocarbazole used in unipolar analogue B-CzG2. This structural modification can render B-CzPO with more balanced charge transportation relative to that of B-CzG2. As a result of the significantly reduced efficiency roll-off, the nondoped phosphorescent organic light-emitting diodes (PhOLEDs) of B-CzPO show a superior high-brightness performance, revealing a luminous efficiency of 21.2, 16.1, and 10.5 cd/A at 1000, 5000, and 10 000 cd/m2, respectively. Compared with that of B-CzG2 (i.e., 7.8 cd/A @5000 cd/m2), more than doubled high-brightness performance is achieved for B-CzPO. The results indicate that the design of self-host phosphorescent dendrimers with a bipolar feature will be a promising strategy to develop efficient nondoped PhOLEDs suitable for high-brightness applications including general illumination and micro displays. KEYWORDS: bipolar dendrons, self-host, iridium dendrimers, nondoped PhOLEDs, high-brightness performance

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INTRODUCTION

encapsulation effect from the peripheral dendrons can reduce or eliminate the intermolecular interactions between the emitting cores. Meanwhile, a dendritic molecule possesses not only the well-defined structures of small molecules but also the superior solution processing property of polymers. These favorable characteristics urge many groups to design efficient phosphorescent dendrimers.19−27 For example, based on twisted biphenyl dendrons with high triplet energy, Burn et al. reported blue-emitting Ir dendrimers with an external quantum efficiency (EQE) of 3.9%.21 The performance seemed to be relatively low due to the inferior charge transporting capacity of the electrically insulating biphenyl dendrons. To

Recently, transition metal complexes have attracted great attention in phosphorescent organic light-emitting diodes (PhOLEDs) since they can harvest both singlet and triplet excitons to realize 100% theoretical internal quantum efficiency.1−7 Owing to the strong concentration quenching, they must be doped into an appropriate host material to realize high efficiency.8−10 As for this doping technology, phase segregation may occur and ultimately deteriorate the stability of PhOLEDs over a long-term operation.11,12 Therefore, nondoped PhOLEDs, where a phosphor is utilized as the emitting layer (EML) by itself, should be developed to avoid the intrinsic phase segregation existing in doped PhOLEDs.13−18 Phosphorescent dendrimer composed of a phosphorescent core, dendritic wedge, and surface group is believed to be a promising candidate for nondoped PhOLEDs because the © 2016 American Chemical Society

Received: August 8, 2016 Accepted: October 13, 2016 Published: October 13, 2016 29600

DOI: 10.1021/acsami.6b09732 ACS Appl. Mater. Interfaces 2016, 8, 29600−29607

Research Article

ACS Applied Materials & Interfaces

Figure 1. Molecular design of self-host blue-emitting Ir dendrimers by modifying the peripheral dendron from p-type oligocarbazole to bipolar carbazole/triphenylphosphine oxide hybrid.

(BCPO) is selected as the peripheral dendron in B-CzPO because BCPO has been proved to be a highly efficient universal bipolar host for blue, green, and red PhOLEDs.33 This structural modification can render B-CzPO with excellent bipolar transporting capability. Consequently, the nondoped device of B-CzPO gives a considerably high luminous efficiency of 23.1 cd/A (EQE = 10.8%), which slightly decays to 21.2, 16.1, and 10.5 cd/A at 1000, 5000, and 10 000 cd/m2, respectively. Compared with that of unipolar analogue B-CzG2 (i.e., 7.8 cd/A @5000 cd/m2), more than doubled highbrightness performance is achieved for B-CzPO, indicative of its great potential in general lightings.

solve this problem, the hole-transporting oligocarbazole was then attached to a blue-emitting Ir core through a nonconjugated linkage.24 In this case, an efficient self-host system was formed in one dendritic molecule, where the surrounding carbazole dendrons could function as the host for the central core. Consequently, an improved EQE of 15.3% was realized for solution-processed nondoped PhOLEDs. However, the developed self-host dendrimers still suffer from a severe efficiency roll-off of about 30% at 1000 cd/m2, resulting in poor high-brightness performance. Such limitation prevents them from being further applied in white-emitting PhOLEDs because the general illumination requires high luminance above 1000 cd/m2. In contrast, bipolar molecules, which consist of an electrondonating moiety capable of mediating hole injection/transporting and an electron-withdrawing moiety capable of mediating electron injection/transporting, have been widely adopted to host phosphors in doped PhOLEDs.28−37 As a result of balanced charge flux, the exciton recombination zone is broadened to avoid the accumulation of triplet excitons in the EML. Thereby the related triplet−triplet annihilation (TTA) and triplet−polaron annihilation (TPA) could be weakened to improve high-brightness performance. For instance, by designing a silicon-bridged bipolar host containing triphenylamine and benzimidazole units (p-BISiTPA), Yang et al. prepared blue-emitting doped PhOLEDs with a small efficiency roll-off of 11% at a brightness of 1000 cd/m2.36 In this article, by introducing the bipolar concept into the self-host dendritic platform, we develop a novel blue-emitting Ir dendrimer B-CzPO for nondoped electrophosphorescent devices with superior high-brightness performance. As depicted in Figure 1, instead of the p-type carbazole dendron used in BCzG2, a bipolar carbazole/triphenylphosphine oxide hybrid

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EXPERIMENTAL SECTION

Synthesis. All reagents and chemicals were purchased from commercial suppliers and used without further purification. According to a standard procedure, solvents were distilled before usage. p-HOdfppyIr was synthesized following a previous route reported by our group.38 Compound 1. A mixture of carbazole (16.7 g, 0.1 mol), 1,4dibromobenzene (47.2 g, 0.2 mol), L-proline (3.45 g, 30 mmol), K2CO3 (27.2 g, 0.2 mol), and CuI (2.91 g, 17 mmol) was dissolved in DMSO (350 mL) and heated to 150 °C under argon atmosphere for 18 h. Then, the crude mixture was filtered and extracted with water and ethyl acetate twice. The combined organic layer was washed with saturated brine, dried over Na2SO4, and evaporated under vacuum to remove the organic solvent. The resulting residue was purified with column chromatography using petroleum ether as eluant to give 1 as a white solid (18.5 g, 58%). 1H NMR (400 MHz, CDCl3, ppm, δ) 8.13 (d, J = 7.7 Hz, 2H), 7.72 (d, J = 8.4 Hz, 2H), 7.45 (d, J = 8.5 Hz, 2H), 7.43−7.35 (m, 4H), 7.29 (t, J = 7.3, 7.3 Hz, 2H). Compound 2. A mixture of 1 (1.5 g, 4.6 mmol) and magnesium (134 mg, 5.6 mmol) was heated to 60 °C in THF (15 mL) for 2 h to form Grignard reagent and then cooled to room temperature. Diethylphosphite (0.18 mL, 1.41 mmol) was added dropwise to the 29601

DOI: 10.1021/acsami.6b09732 ACS Appl. Mater. Interfaces 2016, 8, 29600−29607

Research Article

ACS Applied Materials & Interfaces

7.42−7.37 (m, 2H), 7.33−7.27 (m, 4H), 7.11 (d, J = 8.7 Hz, 2H), 3.91 (s, 3H). Compound 7. Compound 6 (13.6 g, 50 mmol) was dissolved in acetic acid (150 mL) at 90 °C. Then, KI (12.4 g, 75 mmol) and KIO3 (8.0 g, 37.5 mmol) were added, and the system was maintained at 80 °C for 8 h. After that, the mixture was poured into water and filtered. The filter cake was recrystallized from ethanol to give 16 g (62%) of a nude solid. 1H NMR (400 MHz, CDCl3, ppm, δ) 8.38 (s, 2H), 7.65 (d, J = 8.6 Hz, 2H), 7.36 (d, J = 8.7 Hz, 2H), 7.10 (d, J = 8.7 Hz, 2H), 7.07 (d, J = 8.6 Hz, 2H), 3.91 (s, 3H). Compound 8. Carbazole (1.83 g, 11 mmol), compound 7 (2.6 g, 5 mmol), CuI (190 mg, 1 mmol), K3PO4 (5.21 g, 25 mmol), and trans1,2-cyclohexanediamine (80 μL, 0.5 mmol) were added to 30 mL of degassed 1,4-dioxane under argon atmosphere. The reaction mixture was heated to reflux for 24 h. Then, the mixture was filtered; the filtrate washed by dilute ammonia−water, diluted hydrochloric acid, and saturated brine successively, and dried over Na2SO4. Pure product 8 (1.75 g, 58%) was obtained by column chromatography using petroleum ether/THF = 2:1 as eluent. 1H NMR (400 MHz, CDCl3, ppm, δ) 8.30 (s, 2H), 8.19 (d, J = 7.7 Hz, 4H), 7.65 (d, J = 8.7 Hz, 2H), 7.62−7.58 (m, 4H), 7.45−7.40 (m, 8H), 7.34−7.27 (m, 4H), 7.24 (d, J = 8.8 Hz, 2H), 3.99 (s, 3H). Compound 9. Compound 9 (1.6 g, 97%) was prepared according to a similar procedure described for 4. 1H NMR (400 MHz, CDCl3, ppm, δ) 8.30 (s, 2H), 8.19 (d, J = 7.7 Hz, 4H), 7.64−7.58 (m, 6H), 7.45−7.40 (m, 8H), 7.34−7.27 (m, 4H), 7.17 (d, J = 8.6 Hz, 2H). Compound 10. Compound 10 (1.5 g, 83%) was prepared according to a similar procedure described for 5. 1H NMR (400 MHz, CDCl3, ppm, δ) 8.29 (s, 2H), 8.19 (d, J = 7.7 Hz, 4H), 7.65 (d, J = 8.7 Hz, 2H), 7.62−7.58 (m, 4H), 7.44−7.40 (m, 8H), 7.33−7.27 (m, 4H), 7.22 (d, J = 8.8 Hz, 2H), 4.18 (t, J = 5.9, 5.9 Hz, 2H), 3.58 (t, J = 6.5, 6.5 Hz, 2H), 2.23−2.15 (m, 2H), 2.12−2.05 (m, 2H). B-CzG2. Dendrimer B-CzG2 (500 mg, 73%) was prepared according to a procedure similar to that described for B-CzPO. 1H NMR (400 MHz, CDCl3, ppm, δ) 8.26 (s, 6H), 8.15 (d, J = 7.7 Hz, 12H), 7.82 (br, 3H), 7.59−7.52 (m, 18H), 7.41−7.35 (m, 24H), 7.32 (d, J = 6.3 Hz, 3H), 7.29−7.26 (m, 12H), 7.14 (d, J = 8.6 Hz, 6H), 6.46 (d, J = 5.5 Hz, 3H), 6.40−6.31 (m, 3H), 6.26 (d, J = 8.8 Hz, 3H), 4.15−4.13 (m, 12H), 2.05 (br, 12H). 19F NMR (376 MHz, CDCl3, δ (vs fluorobenzene)) 4.17 (d, J = 9.8 Hz, 3F), 2.05 (d, J = 9.8 Hz, 3F). Anal. Calcd for C171H117F6IrN12O6: C 74.90, H 4.30, N 6.13 Found: C 74.90, H 4.37, N 5.83. MALDI-TOF MS: 2741.9 [M+] Methods and Instrumentation. 1H NMR, 19F NMR, and 31P NMR spectra were recorded by Bruker Avance NMR spectrometer. The elemental analysis was measured with a Bio-Rad elemental analysis system. Matrix-assisted laser desorption ionization/time-offlight (MALDI/TOF) mass spectra were performed on an AXIMA CFR MS apparatus (COMPACT). 2-[(2E)-3-(4-tert-Butylphenyl)-2methylprop-2-enylidene] malononitrile (DCTB) was used as the matrix. Thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) were performed under a flow of nitrogen with PerkinElmer-TGA 7 and PerkinElmer-DSC 7 system, respectively. The UV−vis absorption and photoluminescence (PL) spectra were recorded using a PerkinElmer Lambda 35 UV/vis spectrometer and a PerkinElmer LS 50B spectrofluorometer, respectively. The PL quantum yield was measured in argon-saturated toluene using facIr(ppy)3 (Φp = 0.40) as the reference. The transient PL spectra were measured under argon atmosphere excited at 355 nm with ca. 3 ns pulse width from a Quanty-Ray DCR-2 pulsed Nd:YAG laser. Moreover, the average lifetimes were calculated according to the equation: τav = (A1τ12 + A2τ22)/(A1τ1 + A2τ2). Cyclic voltammetry (CV) was carried out on an EG&G 283 (Princeton Applied Research) potentiostat/galvanostat system using a platinum working electrode, a platinum counter electrode, and an Ag/AgCl reference electrode. The solvents CH2Cl2 and DMF are used for anodic and cathodic sweeping, respectively, and 0.1 M tetrabutylammonium perchlorate (nBu4NClO4) is used as the supporting electrolyte. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels were calculated according to the equations HOMO = −e[Eox + 4.8 V] and LUMO = −e[Ered +

mixture and stirred at room temperature for 2 h. After that, aqueous NH4Cl was added slowly, and ethyl acetate was used to extract the mixture. Then, the organic phase was washed with aqueous NaHCO3, brine, and dried over Na2SO4. After removing the solvent, the residue was purified by column chromatography with ethyl acetate/petroleum ether = 2:3 as eluent to give 2 (525 mg, 70%). 1H NMR (400 MHz, CDCl3, ppm, δ) 8.93 (s, 0.5H), 8.15 (d, J = 7.7 Hz, 4H), 8.06 (dd, J = 13.2, 8.0 Hz, 4H), 7.83 (d, J = 7.8 Hz, 4H), 7.72 (s, 0.5H), 7.50 (d, J = 8.2 Hz, 4H), 7.44 (t, J = 7.6, 7.6 Hz, 4H), 7.33 (t, J = 7.4, 7.4 Hz, 4H). Compound 3. 4-Iodoanisole (280 mg, 1.2 mmol), 2 (426 mg, 0.8 mmol), Pd(OAc)2 (17.9 mg, 0.08 mmol), 1,4-bis(diphenylphosphino)butane (34 mg, 0.08 mol), and (i-Pr)2NEt (413 mg, 3.2 mmol) were added to DMSO (5 mL) under argon atmosphere. The mixture was heated to 110 °C for 6 h and poured into water. After being filtered and dried in vacuum, the crude product was purified by column chromatography with ethyl acetate/petroleum ether = 2:3 as the eluent to give 3 (367 mg, 72%). 1H NMR (400 MHz, CDCl3, ppm, δ) 8.15 (d, J = 7.7 Hz, 4H), 8.03 (s, J = 8.3 Hz, 2H), 7.99 (d, J = 8.3 Hz, 2H), 7.83−7.76 (m, 6H), 7.52 (d, J = 8.2 Hz, 4H), 7.43 (t, J = 7.7, 7.7 Hz, 4H), 7.32 (t, J = 7.4, 7.4 Hz, 4H), 7.10 (d, J = 7.1 Hz, 2H), 3.91 (s, 3H). Compound 4. A solution of 3 (1.66 g, 2.6 mmol) in dry CH2Cl2 (12 mL) was cooled to 0 °C, and BBr3 (0.52 mL 1 M solution in CH2Cl2, 5.2 mmol) was added dropwise. After stirring for 2 h 0 °C, the reaction was carefully quenched with methanol, followed by adding a saturated solution of NaHCO3. Then, the mixture was extracted with CH2Cl2, washed with water, and dried with Na2SO4. The product (1.3 g) was obtained after removing all solvent and drying in vacuum in a yield of 82%. 1H NMR (400 MHz, CDCl3, ppm, δ) 8.13 (d, J = 7.7 Hz, 4H), 8.03 (d, J = 8.2 Hz, 2H), 8.00 (d, J = 8.2 Hz, 2H), 7.78 (d, J = 7.6 Hz, 4H), 7.67−7.62 (m, 2H), 7.51 (d, J = 8.2 Hz, 4H), 7.41 (t, J = 7.7, 7.7 Hz, 4H), 7.30 (t, J = 7.4, 7.4 Hz, 4H), 7.16 (d, J = 7.6 Hz, 2H). Compound 5. A mixture of 4 (1.31 g, 2.1 mmol), 1,4dibromobutane (1.27 mL, 10.5 mmol), and K2CO3 (1.45 g, 10.5 mmol) was added to DMF (20 mL) and heated at 70 °C for 10 h. After cooling to room temperature, the mixture was poured into water, extracted with CH2Cl2, and dried with Na2SO4. Pure product 5 (630 mg, 55%) was obtained by column chromatography with ethyl acetate/petroleum ether = 2:3 as eluent. 1H NMR (400 MHz, CDCl3, ppm, δ) 8.15 (d, J = 7.7 Hz, 4H), 8.02 (d, J = 8.2 Hz, 2H), 7.99 (d, J = 8.3 Hz, 2H), 7.82−7.77 (m, 6H), 7.52 (d, J = 8.2 Hz, 4H), 7.43 (t, J = 7.7, 7.7 Hz, 4H), 7.32 (t, J = 7.4, 7.4 Hz, 4H), 7.09 (d, J = 7.7 Hz, 2H), 4.10 (t, J = 5.9, 5.9 Hz, 2H), 3.50 (t, J = 6.5, 6.5 Hz, 2H), 2.13−2.06 (m, 2H), 2.04−1.97 (m, 2H). B-CzPO. A mixture of 5 (502 mg, 0.66 mmol), p-HO-dfppyIr (162 mg, 0.2 mmol), and Cs2CO3 (215 mg, 0.66 mmol) was added to DMF (10 mL) and heated at 80 °C for 6 h. After poured into water, the mixture was filtered and then dried in vacuum. Pure product B-CzPO (440 mg, 77%) was obtained by column chromatography with CH2Cl2/methanol = 50:1 as eluent. 1H NMR (400 MHz, CDCl3, ppm, δ) 8.13 (d, J = 7.7 Hz, 12H), 8.01 (d, J = 8.3 Hz, 12H), 7.81 (br, 3H), 7.78−7.75 (m, 18H), 7.50 (d, J = 8.2 Hz, 12H), 7.41 (t, J = 7.6, 7.6 Hz, 12H), 7.33−7.28 (m, 15H), 7.07 (d, J = 7.3 Hz, 6H), 6.47 (dd, J = 6.2, 1.7 Hz, 3H), 6.38−6.28 (m, 3H), 6.24 (dd, J = 9.1, 1.9 Hz, 3H), 4.12−4.10 (m, 12H), 2.00 (br, 12H). 19F NMR (376 MHz, CDCl3, δ (vs fluorobenzene)) 4.09 (d, J = 9.8 Hz, 3F), 2.01 (d, J = 9.8 Hz, 3F). 31P NMR (161 MHz, CDCl3, δ) 27.82. Anal. Calcd for C171H123F6IrN9O9P3: C 72.14, H 4.35, N 4.43 Found: C 71.57, H 4.40, N 4.16. MALDI-TOF MS: 2845.8 [M+]. Compound 6. A mixture of carbazole (16.7 g, 0.1 mol), 4iodoanisole (28 g, 0.12 mol), trans-1,2-cyclohexanediamine (0.8 mL, 5 mmol), K3PO4 (50 g, 0.24 mol), and CuI (1.9 g, 0.01 mol) was dissolved in 1,4-dioxane (300 mL) and heated to reflux under argon atmosphere for 24 h. Then, the crude mixture was filtered, CH2Cl2 added, and washed with diluted aqua ammonia, 1 M hydrochloric acid, saturated salt water, and water successively. The extracted organic layer was combined and evaporated under vacuum to remove most of organic solvent. The pure product can be obtained by recrystallization from ethyl acetate as a white solid (22.8 g, 84%). 1H NMR (400 MHz, CDCl3, ppm, δ) 8.14 (d, J = 7.7 Hz, 2H), 7.45 (d, J = 8.6 Hz, 2H), 29602

DOI: 10.1021/acsami.6b09732 ACS Appl. Mater. Interfaces 2016, 8, 29600−29607

Research Article

ACS Applied Materials & Interfaces Scheme 1. Synthesis of Dendrimers B-CzPO and B-CzG2a

a Reagents and conditions: (i) 1,4-dibromobenzene, CuI, L-proline, K2CO3, DMSO, 150 °C; (ii) Mg, THF, reflux, then diethylphosphite, room temperature; (iii) 4-iodoanisole, Pd(OAc)2, 1,4-bis(diphenylphosphino)butane, (i-Pr)2NEt, DMSO, 110 °C; (iv) BBr3, CHCl3, 0 °C; (v) 1,4dibromobutane, K2CO3, DMF, 70 °C; (vi) Cs2CO3, DMF, 70 °C; (vii) 4-iodoanisole, CuI, K3PO4, trans-1,2-cyclohexanediamine, 1,4-dioxane, reflux; (viii) KI, KIO3, CH3COOH, 90 °C; (ix) carbazole, CuI, K3PO4, trans-1,2-cyclohexanediamine, 1,4-dioxane, reflux.

4.8 V], where Eox and Ered were the onset oxidation peak value and reduction peak value, respectively. Device Fabrication and Characterization. To fabricate nondoped PhOLEDs, a film of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) with a thickness of 45 nm was first deposited on the precleaned ITO-glass substrates (20 Ω per square) and baked at 120 °C for 30 min. Subsequently, self-host blue Ir dendrimers from chlorobenzene solutions were spin-coated onto the PEDOT:PSS layer to form a 80 nm thick EML and annealed at 100 °C for 30 min to remove the residual solvent in argon atmosphere. Finally, a 0.5 nm thick film of LiF and a 100 nm thick film of Al were evaporated successively on the EML at a base pressure less than 8.0 × 10−4 Pa. According to the same method, the double-layer devices were also prepared by adding another 55 nm thick film of SPPO13 as the electron transporting layer (ETL). In addition, the hole-only devices with a structure of ITO/PEDOT:PSS (45 nm)/Ir dendrimers (80 nm)/Au (75 nm) and electron-only devices with a structure of ITO/ PEDOT:PSS (45 nm)/Al (50 nm)/Ir dendrimers (80 nm)/LiF (0.5 nm)/Al (100 nm) were constructed to characterize the transporting properties of dendrimers. For these devices, the active area was 14 mm2. A PR650 spectra colorimeter and a Keithley 2400/2000 source meter calibrated with a silicon photodiode were used to measure the EL spectra with Commission International de L’Eclairege (CIE 1931) coordinates and the current density−voltage−brightness character-

istics, respectively. All the measurements were performed under ambient conditions at room temperature. The EQE was calculated from the brightness, current density, and EL spectrum assuming a Lambertian distribution.

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RESULTS AND DISCUSSION Similar to our previous work,39,40 dendrimer B-CzPO could be conveniently synthesized via a postdendronization method (Scheme 1). The carbazole/triphenylphosphine oxide hybridized dendron was first functionalized with alkyl bromide (denoted as 5) by the reaction between 1,4-dibromobutane and corresponding aryl phenol intermediate 4 in a moderate yield of 55%. Subsequently, Williamson reaction between 5 and trihydroxyl Ir-complex core (p-HO-dfppyIr)40 was performed with Cs2CO3 as the base and DMF as the solvent, affording desired dendrimer B-CzPO in a high yield of 77%. For comparison, reference dendrimer B-CzG2 with carbazole-based dendrons was also prepared according to the same procedure. Both B-CzPO and B-CzG2 are well-soluble in common organic solvents including CH2Cl2, THF, toluene and chlorobenzene, which could guarantee that their high-quality films formed through wet processes. Meanwhile, they display 29603

DOI: 10.1021/acsami.6b09732 ACS Appl. Mater. Interfaces 2016, 8, 29600−29607

Research Article

ACS Applied Materials & Interfaces Table 1. Photophysical, Electrochemical, and Thermal Data of Dendrimers B-CzPO and B-CzG2 B-CzG2 B-CzPO

λabs (nm)a

λem (nm)b

ΦPLc

τav (μs)d

HOMO (eV)e

LUMO (eV)e

Td (°C)

Tg (°C)

264, 343, 385, 418, 449 292, 316, 381, 417, 448

467 467

0.64 0.60

0.44 0.51

−5.28 −5.28

−2.26 −2.26

422 423

243 206

Measured in 10−5 M dichloromethane solution. bMeasured in 10−5 M toluene solution. cMeasured in N2-saturated toluene solution with Ir(ppy)3 (ΦPL = 0.40) as the reference. dMeasured in neat films under N2 excited at 355 nm; the lifetimes are obtained as an average value by a biexponential fit of emission decay curves. eHOMO = −e(Eoxonset + 4.8 V), LUMO = −e(Eredonset + 4.8 V). a

HOMO and LUMO energy levels are determined to be −5.28 and −2.26 eV for B-CzG2 and B-CzPO, respectively. Noticeably, it is found that their HOMO/LUMO levels measured from CV match well with those of B-G0 without any dendron,24 for the inner Ir core is more easily oxidized and reduced than the outer dendron. In spite of this, the outer dendron itself may still contribute to the charge injection and transportation in a self-host dendrimer and finally influence its electrical behavior as well as device performance in PhOLEDs, which will be discussed later. To explore the bipolar transporting capability of B-CzPO, first, hole- and electron-only devices were fabricated with a configuration of ITO/PEDOT:PSS (45 nm)/B-CzPO (80 nm)/Au (75 nm) and ITO/PEDOT:PSS (45 nm)/Al (50 nm)/B-CzPO (80 nm)/LiF (0.5 nm)/Al (100 nm), respectively. At the same time, the control single-carrier devices using B-CzG2 as the active layer were also produced for comparison. As shown in Figure 3, the hole current of B-CzG2

glass transition temperatures higher than 200 °C and decomposition temperatures higher than 420 °C (Table 1 and Figure S5). The obtained excellent thermal stability is believed to be favorable for the long-term reliability of PhOLEDs. The UV−vis absorption and PL spectra of B-CzPO and BCzG2 are shown in Figure 2. As one can see, the weak

Figure 2. Absorption spectra in CH2Cl2 and PL spectra in toluene for dendrimers B-CzPO and B-CzG2.

absorption bands in the region of 350−450 nm are attributed to the metal-to-ligand charge-transfer (MLCT) transitions from the inner Ir core, while the strong absorptions below 350 nm are assigned to the ligand-centered (LC) transitions together with the 1π−π* transitions of the outer dendrons. In addition, B-CzG2 and B-CzPO exhibit nearly the same PL spectral profile with an emission peak at 467 nm as well as similar PL quantum yields of 0.60−0.64 and close lifetimes of 0.44−0.51 μs (Table 1 and Figure S6). These observations suggest that on going from B-CzG2 to B-CzPO the structural alteration of peripheral dendron does not affect their optical properties obviously. Their electrochemical properties were then investigated using CV measurements (Figure S7a). Upon the anodic scan, B-CzG2 and B-CzPO show multiple reversible oxidation signals. The first wave located at 0.48 V is from the central Ir core, and the more positive waves appeared at 0.60/0.75 V in B-CzG2 and 0.79 V in B-CzPO are from the oxidation of the corresponding dendrons. This assignment is reasonable when taking into account the electrochemical behavior of each functional fragment independently (Figure S7b). During the cathodic scan, two distinct waves at about −2.54 and −2.67 V are observed, which could be ascribed to the reduction from the central Ir core and the outer dendrons, respectively. This suggests that both the Ir core and dendrons are able to participate in the n-doping processes. With regard to the energy level of ferrocene/ferrocenium (4.8 eV under vacuum), the

Figure 3. Current density−voltage characteristics of hole- and electron-only devices for dendrimers B-CzPO and B-CzG2.

is several orders higher than the electron current at a voltage ranging from 0 to 10 V, indicative of its unipolar transporting behavior. By contrast, for B-CzPO, the electron current is distinctly enhanced relative to B-CzG2 and becomes comparable with the reduced hole current at the whole voltage range. It can be rationally anticipated that owing to the bipolar nature a more balanced charge flow would be realized in BCzPO than in B-CzG2. This point is further confirmed by their single-layer device performance (Figure S8 and Table S2). In comparison to B-CzG2 (0.003 cd/A), the peak luminous efficiency of B-CzPO is drastically increased to 2.2 cd/A. As mentioned above, the hole flux is dominant in B-CzG2 so that excitons are mainly generated near the cathode. Nevertheless, in B-CzPO with comparable hole and electron fluxes, the exciton recombination zone could be shifted away from the cathode to prohibit the related cathode-induced exciton quenching, leading to improved efficiency. 29604

DOI: 10.1021/acsami.6b09732 ACS Appl. Mater. Interfaces 2016, 8, 29600−29607

Research Article

ACS Applied Materials & Interfaces

Meanwhile, the excess holes in EML could also quench the generated triplet excitons through TPA process, which would further aggravate the efficiency roll-off.42 However, this is not the case any more for the B-CzPO device. As discussed before, the injected holes and electrons seems to be more balanced in the EML, so the exciton recombination region is expectedly broadened to lower the density of triplet excitons. In such a case, the quenching from both TTA and TPA could be alleviated efficiently, thereby resulting in the gentle efficiency roll-off of B-CzPO. We note that the performance of B-CzG2 is inferior to that of the analogue B-G2.24 The lack of t-butyl groups at the surface may be responsible for the luminescence quenching induced by the intermolecular interactions,43,44 as further evidenced by the shorter film lifetime (0.44 μs) relative to that of B-G2 (0.59 μs). In addition, despite its promising highbrightness device performance, the current density−voltage curve of B-CzPO moves toward a higher driving voltage compared with B-CzG2 (Figure S10b). Correspondingly, the turn-on voltage is increased by 0.8 V, and the maximum power efficiency at low brightness is decreased by about 17% for BCzPO. Given the same emissive Ir core in both B-CzPO and BCzG2, the only difference lies in the used dendrons with different HOMO/LUMO levels (Table S1). Therefore, we can tentatively suppose that apart from the Ir core the energy band gap of the dendron also plays an important role on the device driving voltage and power efficiency. Work is now under way on how to realize power-efficient nondoped PhOLEDs by tuning the HOMO levels of the peripheral dendron.

Subsequently, double-layer nondoped PhOLEDs of B-CzPO compared with B-CzG2 (Figure S9) were prepared by introducing 2,7-bis(diphenylphosphoryl)-9,9′-spirobifluorene (SPPO13)41 as an additional ETL. Similar to the PL counterpart, the electroluminescence (EL) spectrum of BCzPO is almost the same as that of B-CzG2, showing characteristic peaks from the emissive Ir core with CIE coordinates of (0.16, 0.32) (Figure S10a). No residual emission from outer dendrons is observed, implying that the exciton energy is completely transferred from dendron to Ir core. Figure 4 shows the luminous efficiency as a function of luminance, and the related data are tabulated in Table 2. As one

Figure 4. Luminous efficiency as a function of luminance for the double-layer devices of dendrimers B-CzPO and B-CzG2. From left to right, the data pointed by arrows represent the efficiency roll-off at a high luminance of 1000, 5000, and 10 000 cd/m2, respectively.

4. CONCLUSIONS A bipolar self-host blue-emitting Ir dendrimer, namely, BCzPO, has been designed and synthesized by replacing a carbazole-based dendron with a carbazole/triphenylphosphine oxide hybrid. In terms of the bipolar character, the broadened exciton recombination region and thus the reduced TTA and TPA can be within our expectation in B-CzPO relative to unipolar analogue B-CzG2. As a result, the nondoped electrophosphorescent device of B-CzPO achieves a superior high-brightness performance with a state-of-art current efficiency as high as 16.1 cd/A at 5000 cd/m2, which is about two times that of B-CzG2 (7.8 cd/A). This work demonstrates that designing bipolar self-host phosphorescent dendrimers is a promising strategy to develop nondoped PhOLEDs used for high-brightness applications including general illumination and micro displays.

can see, with respective to B-CzG2 (19.5 cd/A and 6288 cd/ m2), B-CzPO possesses an improved maximum luminous efficiency of 23.1 cd/A accompanied by a brighter emission of 11872 cd/m2. Furthermore, a much smaller efficiency roll-off is simultaneously realized for B-CzPO. For example, even at a luminance of 1000, 5000, and 10 000 cd/m2, the luminous efficiency of B-CzPO still remains at 21.2, 16.1, and 10.5 cd/A, corresponding to a roll-off of 8, 30, and 54%, respectively. But for B-CzG2, the luminous efficiency significantly drops to 15.1 cd/A at 1000 cd/m2 (roll-off: 23%) and 7.8 cd/A at 5000 cd/ m2 (roll-off: 60%), and the device is indeed destroyed before the luminance reaches 10 000 cd/m2. The obtained superior high-brightness performance of B-CzPO to B-CzG2 can be rationalized by the bipolar nature of B-CzPO. According to the literature,8,9 the efficiency roll-off in PhOLEDs is mainly caused by TTA and/or TPA. In the B-CzG2 device, the triplet excitons are formed and then accumulated near the EML/ETL interface because the hole-transporting capability of B-CzG2 is much higher than its electron-transporting capability. Thus, the resultant large triplet exciton density, originating from the narrow exciton recombination region, may inevitably lead to severe TTA especially at high current density or luminance.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b09732. Experimental details, thermal properties, optical and electrochemical properties and EL performance data of single-layer devices (PDF)

Table 2. Comparison of the Double-Layer Device Performance between Dendrimers B-CzPO and B-CzG2 B-CzG2 B-CzPO a

L (cd/m2)a

ηc (cd/A)b

ηp (lm/W)b

EQE (%)b

CIE [x, y]c

6288 11872

19.5, 15.1, 7.8, n.d. 23.1, 21.2, 16.1, 10.5

13.1, 7.6, 2.8, n.d. 14.0, 10.4, 6.0, 3.3

9.1, 7.1, 3.7, n.d. 10.8, 9.9, 7.5, 4.9

(0.16, 0.32) (0.16, 0.32)

Maximum brightness (L). bMaximum values and those at 1000, 5000, and 10 000 cd/m2, respectively. cCIE coordinates at 10 V. 29605

DOI: 10.1021/acsami.6b09732 ACS Appl. Mater. Interfaces 2016, 8, 29600−29607

Research Article

ACS Applied Materials & Interfaces

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the 973 Project (No. 2015CB655001) and Natural Science Foundation of China (no. 51322308, 51573183, 91333205, 21474106 and 21174144) for financial support of this research.

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DOI: 10.1021/acsami.6b09732 ACS Appl. Mater. Interfaces 2016, 8, 29600−29607