Controlling Ion Distribution for High-Performance Organic Light

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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Controlling Ion Distribution for High-Performance Organic LightEmitting Diodes Based on Sublimable Cationic Iridium(III) Complexes Dongxin Ma,*,† Chen Zhang,† Ruihuan Liu,† Yong Qiu,† and Lian Duan*,†,‡ †

Key Lab of Organic Optoelectronics & Molecular Engineering of Ministry of Education, Department of Chemistry and ‡Center for Flexible Electronics Technology, Tsinghua University, Beijing 100084, P. R. China

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S Supporting Information *

ABSTRACT: Sublimable charged iridium(III) complexes are becoming an attractive family of new phosphors and making their way into vacuumevaporated-deposited organic light-emitting diodes, while it remains challenging to achieve high device performance. Here, we demonstrate a substantial mitigation of exciton quenching not only by reducing the dopant concentration, but also by controlling the ion distribution in the emissive material layers. We, therefore, achieved green luminescence with high brightness, superior efficiencies, and low driving voltages. Following this strategy, we further developed another six sublimable charged iridium(III) complexes and attained blue-green, yellow, and red-emitting devices with record-high performance. This study represents an important advance in the construction of bright electroluminescence from ionic transition metal complexes and shows their great promise in various optoelectronic applications. KEYWORDS: organic light-emitting diodes, electroluminescence, charged iridium(III) complexes, exciton quenching, ion distribution

1. INTRODUCTION Charged iridium(III) complexes have continued to attract considerable interest owing to their excellent photophysical properties, high redox stabilities, and superior luminescence efficiencies.1−3 Generally, a charged iridium(III) complex consists of a light-emitting cation with two major and one ancillary bidentate ligands chelating the iridium center and a negative counter-ion to maintain electric neutrality.4 The highest occupied molecular orbital (HOMO) usually resides on the major ligands and the iridium core, whereas the lowest unoccupied molecular orbital (LUMO) independently resides on the ancillary ligands, therefore, the molecular orbital (MO) energy levels can be conveniently controlled, enabling fullcolor emission.5−14 Cationic iridium(III) complexes are nowadays extensively used for phosphorescent light-emitting electrochemical cells (LEECs) with a spin-coated emissive material layer (EML) sandwiched between two inert electrodes.15−17 However, the simultaneous achievement of both high brightness and efficiency remains quite a tough problem for LEECs,18,19 not to mention the slow response or inferior stability.20−27 By contrast, organic light-emitting diodes (OLEDs) are another type of more popular optoelectronic devices with a faster response, higher efficiency, and greater brightness, and therefore, have already stepped into the large-scale industrial production.28,29 The working mechanism of OLEDs mainly relies on their multiple functional layers embedded between © XXXX American Chemical Society

the electrodes, which provide a better balanced injection of holes and electrons across, but require a more sophisticated preparation technique like vacuum evaporation deposition. Unfortunately, it was considered difficult to sublimate charged iridium(III) complexes, so their utilization in OLEDs was hindered.30−41 Recently, we devised a judicious approach to obtain sublimable cationic iridium(III) complexes by choosing tetraphenylborate-type anions with well-dispersed charge and bulky steric hindrance, which could significantly weaken the electrostatic attraction and improve the material sublimability, thus, allowing fabrication of vacuum-evaporated-deposited devices thereof.42 Nevertheless, the relevant mechanism has been poorly explored, and the mediocre device performance becomes a major roadblock to the eventual deployment of this new phosphorescent material system.43−46 In this study, we demonstrate that in the OLEDs of sublimable charged iridium(III) complexes, the exciton quenching could be effectively suppressed not only by reducing the dopant concentration, but also by controlling the ion distribution in the EMLs, thus, achieving remarkably enhanced device performance. Then, we further develop another six sublimable cationic iridium(III) complexes, discuss the Received: May 5, 2018 Accepted: August 9, 2018

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DOI: 10.1021/acsami.8b07382 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

Device I, ITO/NPB/DIC-TRZ: 6.0% [Ir(ppy)2(Phpyim)][B(5FPh)4]/TPBi/Mg: Ag/Ag; device II, ITO/NPB/DIC-TRZ: 4.0% [Ir(ppy)2(Phpyim)][B(dCF3Ph)4]/TPBi/Mg: Ag/Ag; device III, ITO/HATCN/NPB/TCTA/DIC-TRZ: 2.8% [Ir(ppy)2(Phpyim)][B(5FPh)4]/BPBiPA/LiF/Al; device IV, ITO/HATCN/NPB/TCTA/DIC-TRZ: 1.2% [Ir(ppy)2(Phpyim)][B(dCF3Ph)4]/BPBiPA/ LiF/Al. The data of devices I and II originated from our previous work.42 bλPL, the PL peak wavelength; φ, the PLQY; τ, the excited state lifetime. cVd, the driving voltage at 1 cd/m2; Lmax, the maximum luminance; λEL, the EL peak wavelength; CIE, Commission Internationale de I′Eclairage. For devices III and IV, both the maximum and average (in the parentheses, averaged from three devices) efficiency values are provided.

61 53 86 78 529 528 537 538 I II III IV

a

532 532 542 546

(0.35, (0.36, (0.37, (0.39,

λEL (nm)

5.0 5.3 16.4 (16.3 ± 0.1) 12.6 (12.5 ± 0.1) 9.2 7.1 45.4 26.1 13.4 13.1 49.4 36.1 15.3 16.3 51.9 (51.8 ± 0.1) 38.2 (37.9 ± 0.3) 4.9 4.3 2.3 2.4

EQE (%) CE at 10 000 cd/m2 (cd/A) φ (%) λPL (nm)

τ (ns)

Vd (V)

CE (cd/A)

CE at 1000 cd/m2 (cd/A)

performance of the OLEDc

8.3 10.5 48.8 (47.6 ± 1.2) 35.1 (34.8 ± 0.3)

>27.3 × 103

CIE (x, y) B

devicea

3. RESULTS AND DISCUSSION 3.1. Green OLEDs with Enhanced Performance. In the previous work, we reported green OLEDs of two sublimable cationic iridium(III) complexes, namely [Ir(ppy)2(Phpyim)][B(5FPh)4] and [Ir(ppy)2(Phpyim)][B(dCF3Ph)4]. In the vacuum-evaporated-deposited EMLs of the optimal devices I and II, the bipolar host material DIC-TRZ was doped with 6.0% [Ir(ppy) 2 (Phpyim)][B(5FPh) 4 ] or 4.0% [Ir(ppy)2(Phpyim)][B(dCF3Ph)4], respectively. The HTL material of NPB and the ETL material of TPBi, together with the EMLs, were embedded between the anode of indium tin oxide (ITO) and the cathode of Mg: Ag/Ag. As can be seen in Table 1, the optimal device I delivered green emission with a maximum external quantum efficiency (EQE) of 5.0%, current efficiency (CE) of 15.3 cd/A, power efficiency (PE) of 8.3 lm/ W, and a driving voltage of 4.9 V. In comparison, the optimal device II expressed similar electroluminescence (EL) spectra with a higher EQE of 5.3%, CE of 16.3 cd/A, PE of 10.5 lm/W but a lower driving voltage of 4.3 V.42 The above results were decent among OLEDs of cationic iridium(III) complexes,30−46 while still lagged behind those of the neutral ones.50−56 To enhance the device performance, herein, we inserted thin films of HATCN and TCTA near the HTL to facilitate the charge carrier balance and control the exciton generation, preparing devices III and IV in the structure of ITO/HATCN (HIL, 5 nm)/NPB (HTL, 40 nm)/TCTA (EBL, 10 nm)/ DIC-TRZ doped with x% [Ir(ppy)2(Phpyim)][B(5FPh)4] or [Ir(ppy)2(Phpyim)][B(dCF3Ph)4] (EML, 12 nm)/BPBiPA (ETL, 50 nm)/LiF/Al, respectively (Figure S1, Supporting Information). The energy level diagram in Figure 1a indicates a better balance between the positive and negative charge

PL of the EMLb

Table 1. PL Characteristics of the Optimal Green EMLs and Device Performance of the Corresponding OLEDs

PE (lm/W)

2.1. Synthesis. In this article, 2-(2,4-difluorophenyl)pyridine (dFppy) and 2-phenylpyridine (ppy) were purchased commercially, whereas 2,3-diphenylquinoxaline (dpq), 2-(1-phenyl-1H-imidazol-2yl)pyridine (Phpyim), 1-phenyl-2-(pyridin-2-yl)-1H-benzo[d]imidazole (Phpybi), [Ir(dFppy) 2 (Php yim)][BF 4 ], [Ir(ppy)2(Phpybi)][BF4], and [Ir(dpq)2(Phpybi)][BF4] were synthesized according to the previous literatures.8,38,47 Complexes 1, 3, and 5 were obtained through ion-exchange reactions between their tetrafluoroborate analogues and sodium tetrakis(2,3,4,5,6pentafluorophenyl)borate (Na[B(5FPh)4]), whereas complexes 2, 4, and 6 were similarly produced from sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (Na[B(dCF3Ph)4]).42 2.2. Materials. All the organic functional materials used in OLEDs were purchased commercially, including the host materials of 2,4diphenyl-6-bis(12-phenylindolo[2,3-α]carbazole-11-yl)-1,3,5-triazine (DIC-TRZ)48 and bis[4-(9,9-dimethyl-9,10-dihydroacridine)phenyl]sulfone (DMAC-DPS)49 in the EMLs, the hole-injection layer (HIL) material of dipyrazino[2,3-f:2′,3′-h]quinoxaline-2,3,6,7,10,11-hexacarbonitrile (HATCN), the hole-transport layer (HTL) material of N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)benzidine (NPB), the exciton-blocking layer (EBL) material of 4,4′,4″-tris(carbazol-9yl)triphenylamine (TCTA) and the electron-transport layer (ETL) materials of 2,2′,2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1H-benzimidazole) (TPBi), and 9,10-bis[4-(2-phenyl-1H-benzo[d]imidazol-1-yl)phenyl]anthracene (BPBiPA).

Lmax (cd/m2)

2. EXPERIMENTAL SECTION

884 903 861 881

physicochemical properties, and finally achieve bright OLEDs with high-efficiency blue-green to red emission and low driving voltages. Our results prominently outperform the everreported performance of analogous OLEDs, therefore, predict the promising prospect of sublimable cationic iridium(III) complexes in various optoelectronic applications.30−46

0.57) 0.57) 0.58) 0.55)

ACS Applied Materials & Interfaces


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Figure 1. (a) Energy level diagram, (b) luminance vs voltage curves, (c) CE vs luminance curves, (d) EL spectra of the optimal green devices, and (e) PL spectra of the corresponding EMLs.

Figure 2. Schematic illustration of TOF-SIMS depth-profiles of the optimal EMLs III and IV. The ion intensity was recorded vs that of the DICTRZ host material and measured across the EML from the surface (0 nm) to the ITO substrate (12 nm), adjacent to the ETL and EBL in the corresponding OLED, respectively.

Compared with devices I and II, devices III and IV feature superior performance. When x = 2.8, the optimal device III achieves a peak EQE of 16.4%, CE of 51.9 cd/A and PE of 48.8

carriers, so that more excitons would be electro-generated in the EMLs, resulting in higher efficiencies and lower driving voltages. C


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Figure 3. Schematic cations and chemical structures of complexes 1−6.

Figure 4. (a) Absorption spectra, (b) PL spectra in oxygen-free acetonitrile solutions, (c) PL spectra in spin-coated neat films, and (d) PL spectra at 77 K. D


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9.1 × 105 4.9 × 105 712g 641

5

4

3

6

240 (5.48), 336 (2.05)

75

644

35

1.2 × 106 7.7 × 105 521g 640 (1.80)

74

645

40

7.1 × 105 5.2 × 105 813 590 (2.74),

34

566

42

6.0 × 105 7.9 × 105 588 (2.53),

26

567

57

717

3.2 × 105 6.0 × 105 1079 65 496 59 496 (2.61), 2

a In acetonitrile solutions. bIn oxygen-free acetonitrile solutions. cIn spin-coated neat films, the lifetimes are depicted in Figure S14, while Kr and Knr denote the radiation and nonradiation rates, respectively. dIn acetonitrile, the percentage of each lifetime is labeled, the lifetimes are depicted in Figure S15. eIn degassed solutions with ferrocene serving as the internal standard. fIn a nitrogen atmosphere, ΔT5% denotes the 5% mass reduction temperature. gThe lifetime of complex 5 is a mixture of 261 ns (50.6%) and 787 ns (49.4%), whereas that of complex 6 is a mixture of 411 ns (42.4%) and 933 ns (57.6%).

313 2.44 −3.35 −1.45 −5.79 0.99

322 2.43 −3.36 −1.44 −5.79 0.99

341 2.61 −3.04 −1.76 −5.65 0.85

377 2.60 −3.04 −1.76 −5.64 0.84

332 3.08 −2.85 −1.95 −5.93 1.13

363 3.07 −2.85 −1.95 −5.92 1.12 2.4 × 105 496

248 (4.17), 296 362 (0.51) 243 (4.85), 296 361 (0.58) 251 (4.08), 318 334 (2.32) 248 (4.89), 318 334 (2.50) 242 (4.40), 336 1

(2.55),

55

497

75

1049

7.1 × 105

468 (3448, 5997 [25.7%]), 498 (2746, 5175 [55.2%]) 471 (3277, 5670 [49.0%]), 500 (2819, 5701 [53.5%]) 541 (2276, 4902 [73.0%]), 578 (2312, 4966 [69.7%]) 541 (1762, 4569 [74.1%]), 581 (2151, 4865 [71.0%]) 641 (2054, 5830 [65.4%]), 693 (2637, 6071 [67.4%]) 644 (1896, 5806 [71.0%]), 689 (1887, 5663 [71.0%])

ΔT5% (°C) ELUMO (eV) Ered (V) EHOMO (eV) λ (nm) (τ (ns)) Knr (s−1) Kr (s−1) τ (ns) φ (%) φ (%) λ (nm) λ (nm) (ε (104 M−1 cm−1))

PL in solutionsb absorptiona

Table 2. Physicochemical Characteristics

λ (nm)

PL in spin-coated neat filmsc

PL at 77 Kd

Eox (V)

electrochemical behaviors in solutionse

Egap (eV)

TGAf


Figure 5. MO surfaces of [Ir(dFppy) 2 (Phpyim)] + , [Ir(ppy)2(Phpybi)]+, and [Ir(dpq)2(Phpybi)]+ (|Ψ| = 0.02).

lm/W with a very low driving voltage of only 2.3 V, whereas the optimal device IV shows a lower EQE of 12.6%, CE of 38.2 cd/A and PE of 35.1 lm/W when x = 1.2 (Figure 1b−d and Table 1). The detailed data are displayed in Figures S2, S3, and Table S1. It is also worth mentioning that at high luminance, devices I and III doped with [Ir(ppy)2(Phpyim)][B(5FPh)4] deliver much slighter efficiency roll-off than devices II and IV based on [Ir(ppy)2(Phpyim)][B(dCF3Ph)4] (Figure 1c). For instance, in device III (x = 2.8%), the CE reaches 49.4 cd/A at 1000 cd/m2 and decreases to 45.4 cd/A at 10 000 cd/m2 with a small attenuation of 8%. By contrast, in device IV (x = 1.2), the CE is 36.1 cd/A at 1000 cd/m2, while drops to 26.1 cd/A at 10 000 cd/m2 with a large attenuation of over 27%. In addition, the optimal device III shows a longer lifetime than the optimal device IV in ambient atmosphere (Figure S4), however, still not desirable and requiring further investigation. 3.2. Investigation into the EMLs. To explain the different device characteristics, we first investigated the photophysical properties of the corresponding EMLs. In comparison with EMLs I and II, EMLs III and IV show redshifted photoluminescence (PL) spectra peaked at 537 and 538 nm (Figure 1e), respectively, together with higher photoluminescence quantum yields (PLQYs) up to 90% and shorter excited state lifetimes (Figure S5), indicating less exciton quenching along with the lower dopant concentration. Nevertheless, EML II or IV doped with 4.0 or 1.2% E


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[Ir(ppy)2(Phpyim)][B(dCF3Ph)4] show quite lower PLQYs and longer excited state lifetimes than EML I or III doped with 6.0 or 2.8% [Ir(ppy)2(Phpyim)][B(5FPh)4], although the volume of [B(dCF3Ph)4]− (425 cm3/mol) is larger than that of [B(5FPh)4]− (321 cm3/mol). Then we performed profiling measurements of EMLs III and IV by time-of-flight secondary ion mass spectrometry (TOF-SIMS). The surface-profiles confirmed the existence of [B(5FPh)4]− (C24BF20, [m/z] = 678.98−) in EML III and [B(dCF3Ph)4]− (C32H12BF24, [m/z] = 863.06−) in EML IV (Figure S6). In the depth-profiles, we mainly focused on C39H26N5 ([m/z] = 564.17+), C36H27IrN5 ([m/z] = 722.19+), and B+ ([m/z] = 11.01+) ions as the signal of the DIC-TRZ host material, [Ir(ppy)2(Phpyim)]+, and the anions, respectively. Figure 2 displays the depth-dependent normalized ion intensity of each EML, indicating that [Ir(ppy)2(Phpyim)]+ tends to distribute near the surface (adjacent to the ETL in the corresponding OLEDs), whereas the anions behave differently. In EML III, [B(5FPh)4]− stays near the surface, spaces out the emissive cations, leading to less exciton quenching, superior EQE, slighter efficiency roll-off, and higher stability, whereas in EML IV, [B(dCF3Ph)4]− is depleted near the surface and prefers to accumulate away (adjacent to the EBL in the corresponding OLEDs), resulting in severe triplet−triplet annihilation, inferior EQE, larger efficiency roll-off, and lower stability.57 Therefore, we proved that the exciton quenching could be effectively suppressed not only by reducing the dopant concentration, but also by controlling the ion distribution in the EMLs doped with sublimable cationic iridium(III) complexes, achieving more than twofold improved EQEs and fivefold enhanced PEs over the previously reported green OLEDs. 3.3. Material Design and Characteristics of BlueGreen to Red Emitters. Next, we sought to expand the luminescence to a full-color emission range and developed sublimable cationic iridium(III) complexes 1−6 containing three different emissive cations of [Ir(dFppy)2(Phpyim)]+, [Ir(ppy)2(Phpybi)]+, and [Ir(dpq)2(Phpybi)]+ (Figure 3). Here, dFppy, ppy, or dpq were employed as the major ligand, whereas Phpyim or Phpybi served as the ancillary ligand. Note that these new cations show a bulky volume of 402, 438, and 626 cm3/mol, respectively, quite larger than that of [Ir(ppy)2(Phpyim)]+ (367 cm3/mol), which, together with the large-sized anions, could lessen the self-quenching effect and enhance efficiency. Phpyim and Phpybi were synthesized via condensation of amines and aldehydes, then the active N−H bonds were passivated by introducing a phenyl pendant (Figure S7).8 Complexes 1−6 were produced through facile ion-exchange reactions from their tetrafluoroborate analogues (Figures S8−S10) and fully characterized (Experimental Section, Supporting Information). Some of the single-crystal structures have been already attained and confirmed by X-ray crystallography (Figures S11−S13). In acetonitrile solutions (10−5 mol/L) compounds with the same cation exhibit similar absorption spectra (Figure 4a). The intense band around 250 nm is possibly attributed to the singlet ligand-centered π−π* transitions, whereas the lower bands over 300 nm probably derive from the singlet metal-toligand and ligand-to-ligand charge-transfer transitions, for example, from dπ(Ir)-π(dFppy) to π*(Phpyim) for complexes 1 and 2, from dπ(Ir)-π(ppy) to π*(Phpybi) for complexes 3

a x, the dopant concentration. bλPL, the PL peak wavelength; φ, the PLQY; τ, the excited state lifetime. cVd, the driving voltage at 1 cd/m2; Lmax, the maximum luminance; λEL, the EL peak wavelength; CIE, Commission Internationale de I′Elcairage. Both the maximum and average (in the parentheses, averaged from three devices) efficiency values are provided.

0.45) 0.43) 0.54) 0.52) 0.34) 0.34) (0.24, (0.23, (0.42, (0.47, (0.65, (0.65, 33.4 26.0 3.2 2.5

λEL (nm]

500 500 556 566 626 626 103 103 103 103 103 103 × × × × × × 11.0 9.0 >27.3 >27.3 20.0 16.2 (27.6 ± 0.4) (20.3 ± 0.7) (41.1 ± 0.5) (29.0 ± 0.7) (7.0 ± 0.3) (6.2 ± 0.3) 28.0 21.0 41.6 29.7 7.3 6.5 (10.2 ± 0.2) (8.5 ± 0.2) (13.2 ± 0.2) (11.7 ± 0.2) (8.9 ± 0.2) (9.7 ± 0.1) 10.4 8.7 13.4 11.9 9.1 9.8 6.0

21.3 18.4 38.5 32.6 6.8 7.1 (26.0 ± 0.4) (20.5 ± 0.6) (40.3 ± 0.5) (35.1 ± 0.5) (7.8 ± 0.2) (8.2 ± 0.1) 26.4 21.1 40.8 35.6 8.0 8.3 2.5 2.7 2.3 2.4 2.4 2.6 999 1046 1106 1145 2060 2139 59 57 82 72 73 69 477 483 568 563 638 636 1.1 1.0 1.9 0.8 1.5 1.5

Lmax (cd/m2) PE (lm/W) EQE (%) CE at 1000 cd/m2 (cd/A) CE at 10 000 cd/m2 (cd/A) xa (%)

λPL (nm) φ (%)

τ (ns)

Vd (V)

CE (cd/A)

Performance of the OLEDc PL of the EMLb

Table 3. PL Characteristics of the Optimal EMLs Doped with Complexes 1−6 and Device Performance of the Corresponding OLEDs

1 2 3 4 5 6

CIE (x, y)


F


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Figure 6. (a) Current density vs voltage curves, (b) luminance vs voltage curves, (c) CE vs luminance curves, and (d) EL spectra of the optimal devices of complexes 1−6, (e) PL spectra and (f) excited state lifetimes of the corresponding EMLs.

and 4, or from dπ(Ir)-π(dpq) to π*(Phpybi) for complexes 5 and 6, respectively. Upon excitation, complexes 1−6 show blue-green to red emission in oxygen-free acetonitrile solutions (10−5 mol/L) and spin-coated neat films with featureless PL spectra (Figure 4b−d), indicating more triplet metal-to-ligand or ligand-toligand charge-transfer than ligand-centered π−π* character of the excited states, because the latter always results in vibronically structured spectra.58,59 In dilute acetonitrile solutions (10−5 mol/L), the peak wavelengths are about 496, 590, and 640 nm, respectively, nearly independent of the anions. In neat films, the peak wavelengths of complexes 3 and 4 are blue-shifted, whereas those of complexes 5 and 6 are redshifted, mainly due to the strong π−π stacking interaction in dpq. Besides, all these complexes show high PLQYs ranging from 35 to 75% in solid sates, thanks to the bulky steric hindrance of both the cations and anions (Table 2).60 Cyclic voltammetry (CV) reveals that complexes 1−6 exhibit reversible redox signals in degassed solutions (10−3

mol/L) (Figures S16−S18). The oxidation and reduction potentials of complexes 1 and 2 in N,N-dimethylformamide and acetonitrile solutions are 1.1 and −2.0 V, so the corresponding HOMO and LUMO energy levels are −5.9 and −2.8 eV, respectively, suggesting an energy level gap of 3.1 eV, quite larger than those of complexes 3 and 4 (2.6 eV) and complexes 5 and 6 (2.4 eV). Thermogravimetric analysis (TGA) and vacuum sublimation experiments then demonstrate the excellent thermal stability and sublimability of complexes 1−6, which allow fabricating OLEDs by vacuum evaporation deposition (Figure S19). Next, the ground-state and lowest-lying triplet excited state characteristics of [Ir(dFppy)2(Phpyim)]+, [Ir(ppy)2(Phpybi)]+, and [Ir(dpq)2(Phpybi)]+ were studied theoretically. Density functional theory (DFT) calculations suggest that all the HOMOs reside on the major ligands and the iridium core, whereas all the LUMOs reside on the ancillary ligands (Figure 5). Time-dependent DFT calculations suggest that for [Ir(dFppy)2(Phpyim)]+, the T1 state comes G



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from dπ(Ir)-π(dFppy) to π*(Phpyim), whereas the T2 state is mostly from dπ(Ir)-π(dFppy) to π*(dFppy). However, for [Ir(ppy)2(Phpybi)]+ and [Ir(dpq)2(Phpybi)]+, both the T1 and T2 states are principally derived from dπ(Ir)-π(ppy) to π*(Phpybi) or dπ(Ir)-π(dpq) to π*(dpq). The detailed results are illustrated in Table S2 and Figures S20−S22. 3.4. High-Performance Polychromic OLEDs. OLEDs of complexes 1−6 were finally fabricated by vacuum evaporation deposition in the following structure of ITO/HATCN (5 nm)/NPB (40 nm)/TCTA (10 nm)/EML (12 nm)/ETL (TPBi 30 nm or BPBiPA 50 nm)/LiF/Al. Similar to the green OLEDs, we also employed DIC-TRZ as the host material for complexes 3−6 matching with the ETL of BPBiPA, whereas chose another host material DMAC-DPS for the blue-greenemitting complexes 1 and 2 matching with the ETL of TPBi (Figure S23). By optimizing the dopant concentrations, we attained OLEDs with blue-green, yellow, and red emission peaked at 500, 556, and 626 nm, respectively. The optimal device characteristics and PL properties of the corresponding EMLs are presented in Table 3 and Figure 6, with detailed data in Table S3 and Figures S24−S29. We again found that the EMLs based on complexes 1, 3, and 5 with [B(5FPh)4]− showed higher PLQYs and shorter excited state lifetimes than those based on complexes 2, 4, and 6 with [B(dCF3Ph)4]−, leading to better performance and slighter efficiency roll-off, consistent with the green OLEDs. The optimal OLED of complex 1 achieves a peak EQE of 10.4%, CE of 26.4 cd/A, and PE of 28.0 lm/W, transcending that of complex 2 with an EQE of 8.7%, CE of 21.1 cd/A, and PE of 21.0 lm/W. Analogously, complex 3 exhibits superior device performance compared to complex 4, with a peak EQE of 13.4%, CE of 40.8 cd/A, and PE of 41.6 lm/W. For the optimal OLEDs of complexes 5 and 6, the former shows a higher brightness of 20.0 × 103 cd/m2, whereas the latter exhibits a superior EQE of 9.8%. As tabulated in Table S4, the device performance mentioned above is quite excellent for OLEDs of cationic iridium(III) complexes.30−46

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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.8b07382. Synthesis and structural characterization, experimental methods, material and structures, lifetimes, TOF-SIMS surface profiles, CV, TGA, MO surfaces, schematic functional materials structures, device architectures and detailed device characteristics (PDF) Crystallographic data (ZIP)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (D.M.). *E-mail: [email protected] (L.D.). ORCID

Lian Duan: 0000-0001-7095-2902 Author Contributions

D.M. and L.D. developed the concept. D.M., C.Z., R.L., and L.D. designed the experiments and carried out the measurements. D.M., C.Z., R.L., L.D., and Y.Q. co-wrote the manuscript. L.D. and Y.Q. supervised the project. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the National Key Basic Research and Development Program of China (Grant Nos. 2015CB655002, 2016YFB0400702, 2016YFB04001003) and the National Natural Science Foundation of China (Grant Nos. 51525304, U1601651) for financial support.

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ABBREVIATIONS HOMO, highest occupied molecular orbital; LUMO, lowest occupied molecular orbital; MO, molecular orbital; LEEC, light-emitting electrochemical cell; EML, emissive material layer; OLED, organic light-emitting diode; HIL, hole-injection layer; HTL, hole-transport layer; EBL, exciton-blocking layer; ETL, electron-transport layer; EQE, external quantum efficiency; CE, current efficiency; PE, power efficiency; EL, electroluminescence; PL, photoluminescence; PLQY, photoluminescence quantum yield; TOF-SIMS, time-of-flight secondary ion mass spectrometry; CV, cyclic voltammetry; TGA, thermogravimetric analysis; DFT, density functional theory

4. CONCLUSIONS In summary, we have achieved blue-green to red OLEDs of sublimable charged iridium(III) complexes with record-high device performance, particularly notable for two reasons. First, we have effectively suppressed the exciton quenching not only by reducing the dopant concentration but also by controlling the ion distribution in the EMLs. We also suggest that this strategy would lead to far greater luminescence, while further investigation is required, including rational material design and intensive study into the sublimation mechanism, for example, how the cations and anions ultimately separate, sublimate, and deposit during the device fabrication. Second, we have developed new kinds of sublimable charged iridium(III) complexes, discussed the physicochemical properties, and then successfully fabricated vacuum-evaporated-deposited OLEDs with blue-green to red emission, accomplishing excellent EQEs, superior brightness, low driving voltages, and slight efficiency roll-off. This work has realized best EL performance of charged iridium(III) complexes, pointing out their promising prospect for various optoelectronic applications.

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Controlling Ion Distribution for High-Performance Organic Light

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