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Highly Efficient Deep Blue Organic Light-Emitting Diodes Based on Imidazole: Significantly Enhanced Performance by Effective Energy Transfer with Negligible Efficiency Roll-off Tong Shan, Yulong Liu, Xiangyang Tang, Qing Bai, Yu Gao, Zhao Gao, Jinyu Li, Jian Deng, Bing Yang, Ping Lu, and Yuguang Ma ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b10004 • Publication Date (Web): 10 Oct 2016 Downloaded from http://pubs.acs.org on October 11, 2016

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ACS Applied Materials & Interfaces

Highly Efficient Deep Blue Organic Light-Emitting Diodes Based on Imidazole: Significantly Enhanced Performance by Effective Energy Transfer with Negligible Efficiency Roll-off Tong Shan, † Yulong Liu, † Xiangyang Tang, † Qing Bai, † Yu Gao, † Zhao Gao, † Jinyu Li, † Jian Deng, † Bing Yang, † Ping Lu, †,* and Yuguang Ma‡ †

State Key Lab of Supramolecular Structure and Materials, Jilin University, 2699 Qianjin

Avenue, Changchun, 130012, P. R. China ‡

Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of

Luminescent Materials and Devices, South China University of Technology, Guangzhou, 510640, P. R. China KEYWORDS. OLEDs • deep blue • ambipolar • efficiency roll-off • energy transfer

ABSTRACT

Great efforts have been devoted to develop efficient deep blue organic light-emitting diodes (OLEDs) materials meeting the standards of European Broadcasting Union (EBU) standard with

ACS Paragon Plus Environment

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Commission International de L’Eclairage (CIE) coordinates of (0.15, 0.06) for flat-panel displays and solid-state lightings. However, high-performanced deep blue OLEDs are still rare for applications. Herein, two efficient deep blue emitters, PIMNA and PyINA, are designed and synthesized by coupling naphthalene with phenanthreneimidazole and pyreneimidazole, respectively. The balanced ambipolar transporting natures of them are demonstrated by singlecarrier devices. Their non-doped OLEDs show deep blue emissions with extremely small CIEy of 0.034 for PIMNA and 0.084 for PyINA, with negligible efficiency roll-off. To take advantage of high photoluminescence quantum efficiency of PIMNA and large fraction of singlet exciton formation of PyINA, doped devices are fabricated by dispersing PyINA into PIMNA. A significantly improved maximum external quantum efficiency (EQE) of 5.05% is obtained through very effective energy transfer with CIE coordinates of (0.156, 0.060), and the EQE remains 4.67% at 1000 cd m-2, which is among the best of deep blue OLEDs reported matching stringent EBU standard well.

INTRODUCTION Organic light-emitting diodes (OLEDs) have been widely investigated and commercially tested in recent years and have already been applied in some flat-panel display applications such as mobile phones and televisions.1-4 Nevertheless, there is still much restriction in commercialization for flat-panel displays and general-purpose lightings, one of the factors is the scarcity of efficient emitters with deep blue emission.5,6 Extensive efficient phosphorescent materials and thermally activated delayed fluorescence (TADF) materials are developed, and the devices based on which can harvest 100% internal quantum efficiency, meaning fully conversion from turning electricity to light.7-9 However, both these two kinds of materials hardly gains


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breakthrough in deep blue ones with Commission International de L’Eclairage (CIE) coordinates of y < 0.08, which matches the National Television System Committee (NTSC) standard blue CIE of (0.14, 0.08).10-13 It is even harder to develop high efficient OLEDs with stringent EBU standard CIE of (0.15, 0.06). When regulating the emissive metal-ligand charge-transfer (MLCT) band into deep blue region, the addition nonradiative pathway via the metal d-orbitals make it difficult to synthesize high efficient phosphors.14,15 TADF materials exhibit broad emission bands ascribed to the intramolecular charge transfer (ICT) transition because of their dipolar nature, generally debasing the color purity.16 As a consequence, developing efficient deep blue materials based on novel mechanism is still of great significance. Another issue that severely hinders the commercialization of OLEDs is the sharp efficiency roll-off at high current density. This effect is generally demonstrated in both phosphorescent and TADF materials owing to the TTA process.17-20 It is notable that most traditional efficient fluorophores also performs unnegligible efficiency roll-off, which is supposed to be the relatively unbalanced mobility of hole and electron at high current density, in addition to the reduction in carrier confinement at high bias.18,21-23 Therefore, to single out an efficient deep blue chromophore with balanced ambipolar property is promising to depress the efficiency roll-off. Imidazole, a classical asymmetric aromatic heterocycle, containing two nitrogen atoms with different bonding modes, is endowed with ambipolar nature.24,25 Imidazole-based derivatives have shown great potentials in optoelectronic fields.26,27 For example, 1,3,5-tris(Nphenylbenzimidazol-2-yl)-benzene (TPBi), a benzimidazole derivative, represents one of the mostly used traditional electron transporting materials.28,29 Recently, Su et al. reported a series of naphthoimidazole derivatives through topology of benzimidazole with excellent performance.30 Phenanthroimidazole (PIM) has been widely investigated to construct numerous efficient deep


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blue emitters with fine color purities.31-34 Noticeably, most of PIM derivatives show quite low roll-off, which further confirmed the ambipolar nature of PIM. As seen above, extending the structure of acene attached to imidazole could effectively enlarge the π-electronic delocalization as well as rigidity of plane, leading to more balanced carrier motilities. Up to date, pyrenoimidazole (PyI), possessing a more extended acene structure, is attracting ever increasing attentions.35-36 However, PyI derivatives with deep blue emission are rather rare, due to the enhanced π conjugation, propensity to form excimers between pyrene in solid state and strong ICT.37-38 Herein, to construct efficient deep blue emitters, PIM and PyI are selected owing to their high photoluminescence quantum efficiency (ΦPL) from local excited (LE) states. Instead of adopting conventional donor-acceptor (D-A) structure, a neutral group naphthalene (NA) is introduced to 2-position of imidazole in PIM and PyI to avoid the potential reduction efficiency from strong charge transfer (CT) states.39 NA could also enhance the steric hindrance from the strong repulsion between the neighboring hydrogen atoms of naphthyl and phenyl, which could suppress intermolecular interaction and maintain the high efficiency in solid-state simultaneously.40,41 Althouth NA possesses neither strong electron-donating ability nor electronaccepting ability, feeble ICT would be generated because of the twisty structure between PIM (or PyI) and NA. Appropriate component of CT in excited state would lead to a larger fraction of singlet exciton formation under electrical injection without decreasing ΦPL, which is proved by previous work of our group.42-45 The resultant molecule PIMNA and PyINA exhibit deep blue emission with high ΦPL as expected. Their non-doped OLEDs show negligible efficiency roll-off, and both of them exhibit balanced carriers mobility from single-carrier devices, which suggest the great ambipolar property of PIM and PyI. To take advantage of high ΦPL of PIMNA in sold-


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state and large fraction of singlet exciton formation of PyINA, we fabricate a doped device by dispersing PyINA into PIMNA, a significantly improved EQE of 5.05% is obtained through very effective energy transfer with CIE coordinates of (0.156, 0.060). Besides, the doped device display a quite lagre maximum luminance of 13600 cd m-2 and relatively low efficiency roll-off, which is among the best of deep blue OLEDs matching the requirement of European Broadcasting Union standard of (0.15, 0.06) well. RESULTS AND DISCUSSION Synthesis and Characterization Scheme 1 describes the synthesis route of PyINA and PIMNA. To begin with, pyrene-4,5dione was prepared by the oxidation of pyrene in the presence of ruthenium trichloride and sodium periodate. The dark brown suspension was stirred at room temperature overnight and was extracted by chloroform. Then the solvent was removed under reduced pressure to afford a dark orange solid which purified through chromatography to give red powder later. Subsequently, PyINA and PIMNA were synthesized through "one-pot" reaction. The mixtures of pyrene-4,5dione (or phenanthrene-9,10-dione), 1-naphthaldehyde, phenylamine and ammonium acetate were reacted in acetic acid at 120 °C under N2 atmosphere for 2 hours. The crude products were washed with acetic acid and then purified through chromatography to give target compounds as white solid with good yields. PyINA and PIMNA were fully characterized by NMR, MS and elemental analysis and corresponded well with their expected structures respectively. Scheme 1. Synthesis Routes of PIMNA and PyINA


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Thermal Properties Thermal properties were studied by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) under nitrogen atmosphere. Despite the small molecular weights of the two compounds, they exhibited quite high decomposition temperature (Td) of around 400°C, high glass transition temperature (Tg) of about 100°C, and no crystallization (Figure S7). The Td and Tg of PyINA were little higher as a result of relatively larger rigid plane. The good thermal properties are important to obtain appreciable device performance with favorable stability.46 Electrochemical properties Cyclic voltammetry (CV) was employed with a BAS 100W Bioanalytical Systems to calculate the HOMO/LUMO energy levels of the target materials, and detail information of the measurement was given in supporting information. Using ferrocenium/ferrocene (Fc+/Fc) redox couple as the internal standard, the HOMO/LUMO levels are calculated according to the following formula: E E

Fc

4.8 eV; E E Fc 4.8 eV.

where 4.8 eV was the absolute energy level of Fc below vacuum. Calculated by this method, PIMNA and PyINA exerted the HOMO level of -5.65 eV and -5.21 eV respectively, and gave the same LUMO level of -2.21 eV (Figure S8).


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Theoretical Calculations For the purpose of examining their nature of excited states, natural transition orbitals (NTOs) were calculated and analyzed on the basis of time-dependent DFT (TD-DFT) results with M062X/6-31G(d,p). As shown in Figure 1, for the S1 state of PIMNA, the hole and particle of NTOs were mainly delocalized over the backbone. Such orbital overlap and spatial separation denoted a powerful radiative transition rate, ensuring a high ΦPL. However, the transition of S1→S0 showed weak CT feature since only little change of electron density in NA, indicating a more LE-like transition, which could be ascribed to the almost perpendicular structure between PIM and NA with a large twist angle of 83°.47 On the other hand, the S1 state of PyINA clearly revealed a hybrid local and charge transfer excited state transition character: the hole was mostly distributed on pyrene and imidazole with sparse distribution on NA, while the particle was spread over whole molecule except phenyl ring. The calculated results demonstrated the coexistence of CT and LE components in the excited state of PyINA.

Figure 1. Natural transition orbitals (NTOs) of S1→S0. Photophysical Properties The solvation effect on photoluminescence (PL) was measured in several solvents with ranged polarity. As shown in Figure 2, the PL spectrum spectra of PIMNA in low-polarity solvents such


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as n-hexane and toluene exhibited fine vibrational structure originating from LE state, whereas PyINA only displayed fine vibrational structure in n-hexane. Furthermore, the PL spectra continuously broadened and became structureless gradually with increasing solvent polarities. The increased solvent polarity produced a total bathochromic shift of 39 nm (384 nm in n-hexane to 423 nm in dimethylsulfoxide (DMSO)), while PyINA displayed a larger bathochromic shift of 52 nm (384 nm in n-hexane to 436 nm in DMSO), indicating that PyINA had a more obvious CT emitting character. What is more, PyINA performed larger increment of full width at half maximum (FWHM) from low-polarity n-hexane to high-polarity DMSO solution, comparing to PIMNA. The relatively obvious CT character of PyINA might come from the stronger donating ability of PyI comparing to PIM, because the oxidation potential of PyINA is about 0.4 eV lower than that of PIMNA from the analyzation of cyclic voltammogram. Results of solvation effect demonstrate that the two compounds manifested hybrid local and charge transfer excited state characters, and also approve PyINA possesses a higher proportion of CT in excited state than PIMNA, which is consistent with theoretical calculations of NTOs.

Figure 2. PL spectra of PIMNA (A) and PyINA (B) in different solvents (Concentration: 10-5 mol L-1). The further investigation of their photophysical properties were carried out by the absorption and PL spectra measured in tetrahydrofuran (THF) dilute solution and thin films on quartz


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substrates. Key photophysical information are summarized in Table 1. Both two compounds exhibited deep blue emission in dilute solutions and solid-state films. PIMNA showed a maximum emission at 410 nm and a shoulder at around 400 nm in THF which weakened to nearly invisible in solid-state. Oppositely, the PL spectra of PyINA maintained structureless in THF and solid-state (Figure 3). PyINA also exhibited more pronounced Stokes-shift, sting larger reorganization in the excited state, and a bigger component of CT in excited state.48 Remarkable ΦPL of 77% and ~100% were estimated for PIMNA and PyINA, respectively. Consistently, a practically unchanged ΦPL of 82% was observed in PIMNA neat film. However, PyINA only showed a half ΦPL of 51% in neat film comparing to in solution. The undesirable decrease might be inferred that the polarity of PyINA film is relatively high, leading to a bigger component of CT in radiative transition. To confirm the reduction of ΦPL in solid-state was not from the intermolecular aggregation, ΦPL of 64% for PyINA were observed in doped PMMA films (5 wt. %), which was consistent with neat film. The results meanwhile approve the successful suppression of intermolecular aggregation by introducing NA substituent.

Figure 3. Absorption and PL spectra of PIMNA and PyINA in THF dilute solutions (Concentration: 10-5 mol L-1) (a) and spin-coating film (b).


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Table 1. Key Physical Properties of PIMNA and PyINA λabs [nm]

λabs [nm]

λPL [nm]

ΦPL [%]c

HOMO/LUMOe

E gf

Tg/ Tm/ Td

sola

filmb

sola/filmb

sola/filmb/dfilmd

[eV]

[eV]

[℃]

PIMNA

284,307,356

292,361

410/424

79/82/87

-5.62/-2.21

3.37

99/297/371

PyINA

337,350,380

356,383

420/447

~100/51/64

-5.21/-2.21

3.19

107/166/406

materials

a

Measured in dilute toluene solution (10-5 mol L-1) at room temperature. bMeasured neat film by coating. cAbsolute PL quantum yield evaluated using an integrating sphere. dDoped PMMA film (5 wt. %). eMeasured by cyclic voltammetry and calculated by comparing with ferrocene (Fc). fOptical band gap were estimated from onset wavelength of UV/Vis absorption spectra of the coating thin film. Crystal Structure The colorless and transparent crystals of tow compounds are both obtained by slow diffusion of chloroform into petroleum ether solution. The data of single crystal X-ray diffraction analysis are summarized in Table S1. As shown in Figure 4, the dihedral angles between NA and imidazole of PIMNA and PyINA are 83° and 65°, respectively. The twisty structure make it hard for the occurence of intermolecular aggregation. In consequence, though the distance bewteen adjacent molecular π conjugation planes are not very big (3-3.5 Å), there are barely π-π stacking interaction in solid-state stemming from the stacking mode with displacement.


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Figure 4. X-ray crystal structure and stacking mode of PIMNA (a) and PyINA (b). Electroluminescence Properties To verify the applicable potential of the two synthesized compounds as deep blue emitters for electroluminescence (EL) devices, non-doped multi-layer devices were initially fabricated with the optimized configuration of ITO/HATCN (6 nm)/TAPC (40 nm)/TCTA (10 nm)/PIMNA (20 nm)/TPBi (30 nm)/LiF (1.2 nm)/Al (120 nm), and ITO/HATCN (6 nm)/TAPC (50 nm)/PyINA (20 nm)/TPBi (30 nm)/LiF (1.2 nm)/Al (120 nm) (devices MA and MB), respectively, in which LiF and HATCN (hexaazatriphenylenehexacabonitrile) were used as buffer layers for the cathode and anode, respectively, TAPC (1,1'-bis(di-4-tolyl-aminophenyl)cyclohexane) and TCTA (4,4',4''-tri(N-carbazolyl)-triphenylamine) served as the hole-transporting layers (HTL), and TPBi served as the electron-transporting (ETL) and hole-blocking layer. TCTA with HOMO energy level of -5.7 eV in device MA was applied to match the HOMO energy level of PIMNA (-5.62 eV) to avoid the formation of exciplex between HTL and emissive layer (EML) interface with debased colour purity. The EL spectra of devices MA and MB was consistent with their PL spectra with CIEy coordinates smaller than 0.1. Especially for PIMNA, device MA exhibited an


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emission band maximum at 412 nm with a CIE coordinates of (0.160, 0.034) whose CIEy is the smallest reported in non-doped OLEDs to our best knowledge, owing to its narrow FWHM of 51 nm. Importantly, the EL spectra of MA device were stable at various voltages from 4 to 8 V, maintaining CIEy values smaller than 0.04. Oppositely, the EL spectra of device MB narrowed in long wavelength with unaltered emission peak at 440 nm as voltages ranging from 3 to 10 V, leading to higher color purity at higher voltage. Device of PyINA with TCTA showed a higher turn-on voltage (Von) of 3.1 V comparing to only 2.9 V of device MB with the same EL spectra and equal EQE (Table S2). As anticipated, both devices MA and MB displayed negligible efficiency roll-off (Figure 5 and Table 2), indicating good charge balance thanks to the ambipolar nature of PIM and PyI.

Figure 5. Characteristics of non-doped multi-layer devices MA and MB: (a) Current density (J– V) and luminance (L-V) plots; (b) Luminance efficiency-voltage and EQE-voltage characteristics; (c) EL spectra at 100 cd m-2. The thinner ETL would lead to a higher EQE with hypochromic shifted EL spectra (Table S2), which might be attributed to several possible factors: 1) As HTL and ETL were same in thickness, on account of the higher hole mobility of TAPC than electron mobility of TPBi (1×102

versus 3.3×10-5 cm2 V-1 s-1), the recombination profile of carriers would be near EML and ETL

interface resulting in more self-absorption. Reducing ETL thickness, the recombination zone


12

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would shift to ETL and EML interface with weaker self-absorption, exhibiting hypochromic shifted EL spectra. 2) The LUMO energy level of TPBi is -2.7 eV, which is much shallower than PIMNA and PyINA of -2.21 eV. Exciplex emission is possible to take place as the recombination profile was near EML and ETL interface, leading to bathochromic shifted EL spectra as well as reduced EQEs. 3) The microcavity modifies the photonic mode density within OLEDs, tuning color purity at the microcavity resonance.49 Table 2. Key Performance Parameters of Non-doped Multi-layer Devices of PIMNA (device MA) and PyINA (device MB) Vona

Luminancemax

LEmaxb

EQEcmax

ELλmax

CIEd

at 100/1000/10000cd·m-2

[V]

[cd m-2]

[cd A-1]

[%]

[nm]

[x, y]

EQE [%]c

MA

3.8

6580

0.513

2.43

412

(0.160, 0.034)

2.42/2.21/——

MB

2.9

14700

2.62

3.68

440

(0.157, 0.084)

3.61/3.66/3.39

device

a

Turn-on voltage at a luminance of 1 cd m-2. bMaximum Luminance efficiency. cExternal quantum efficiency. dMeasured at a current density of 100 mA cm-2. Unfortunately, device MA only reached the maximum EQE of 2.43% at a luminance of 170 cd m-2, which might be ascribed to the LE-like excited state of PIMNA. In addition, the outcoupling efficiency (OCE) is wavelength-dependent and less than 20% in deep blue region, and our limited testing instrument (PR-650 Spectroscan spectrometer) could not detect the EL luminance signal less than 380 nm, which are possible to obtain decreased EQE.50,51 OCE is even about 10% at wavelength of 380 nm, owing to much substrate guide and absorption loss, which is an obstacle to get high EQE of deep blue and near-ultraviolet OLEDs.6 On the other hand, a reasonable maximum EQE of 3.68% was obtained for device MB, and the EQEs were 3.61% at 100 cd m-2, 3.66% at 1000 cd m-2 and 3.39% at 10000 cd m-2, respectively. The radiative exciton yield could be calculated according to the following equation:


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EQE γη Φ! η " where γ is the carrier recombination efficiency which is supposed to be 100% only if holes and electrons are fully balanced and completely recombined to form excitons in EML, ηr is the radiative excitons ratio, ηout is the light out-coupling efficiency (ca. 20%), and ΦPL of PyINA thin film was 51%. Therefore, the ηr of PyINA in device MB is calculated to be 38%. However, the ηr of PIMNA in device MA is only 15%, indicating appropriate proportion of CT excited state can promote a larger ratio of singlet exciton formation in this system. To investigate whether PIMNA and PyINA possess ambipolar nature which is responsible for the negligible efficiency roll-off, hole- and electron-only devices were fabricated with respective configurations of ITO/HATCN (6 nm)/NPB (10 nm)/PIMNA or PyINA (70 nm)/NPB (20 nm)/Al (120 nm) (devices HA and HB) and ITO/TPBi (20 nm)/PIMNA or PyINA (70 nm)/TPBi (10 nm)/LiF (1.2 nm)/Al (120 nm) (devices EA and EB). Herein, NPB (N,N'-di-1-naphthyl-N,N'diphenylbenzidine) and TPBi functioned as electron- and hole-blocking materials, respectively. As shown in Figure 6, current density-voltage (J-V) curves illustrated their ambipolar transport capacity. The hole mobility of PIMNA is little better than electron unlike benzimidazole and naphthoimidazole, as Wang group reported. 25,52-54 And the difference between hole and electron mobility kept unchanged over current density of 0.1 mA cm-2, which is beneficial to a constant γ, resulting in low efficiency roll-off and stable EL spectra. Amazingly, PyINA exhibited an extremely balanced carriers mobility when current density was larger than 10 mA cm-2. Its property of carriers mobility would explain the EL spectra behavior of device MB, and also support our inference to ETL-thickness dependence for EL spectra and EQE. Overall, the carriers mobility of PyINA is much better than PIMNA (approximately 100 times). On account of the lager π conjugated plane of PyI, more delocalized electron distribution makes it easier to


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give and accept electron simultaneously. For their outstanding balanced carriers mobility, singlelayer devices were fabricated to probe their comprehensive electroluminescent properties with a configuration of ITO/PEDOT-PSS (40 nm)/PIMNA or PyINA (80 nm)/LiF (1.2 nm)/Al (120 nm) (device SA and SB). As shown in Figure S11 and Table S3, a maximum EQE of 1.75% was obtained from device SB, which was about half of device MB. The Von was only 3.6 V, demonstrating PyINA also has good carriers injection capability. However, device SA showed a very high Von of 9 V and relatively low EQE, which might be caused by hardly hole injection owing to its too deep HOMO energy level.55

Figure 6. Electron and hole current densities versus applied voltage of the single carrier devices on PIMNA and PyINA. To take advantage of high ΦPL of PIMNA and large fraction of singlet exciton formation of PyINA, we then fabricated doped devices using PIMNA with wider bandgap as host and PyINA as dopant. The optimized configuration is: ITO/HATCN (6 nm)/TAPC (40 nm)/TCTA (10 nm)/PIMNA: PyINA (25 nm)/TPBi (30 nm)/LiF (1.2 nm)/Al (120 nm) (device PP) with optimum doping concentration of 30% (Table S4). As shown in Figure 7 and Table 3, device PP exhibited an improved maximum EQE of 5.05% at a luminance of 100 cd m-2 with relatively slight roll-off comparing to reported high efficient OLEDs with similar CIE coordinates of


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(0.156, 0.060).56-59 The CIE coordinates of OLEDs are summarized in Figure 8. Some recently reported deep blue OLEDs with similar CIE coordinates to ours were also listed in Table S5. As parallel control, we also prepared a series of doped devices using typical hosts with the same configuration of device PP, DPEPO (bis(2-(diphenylphosphino)phenyl)ether oxide) had better performance among them (Table 3). However, the device DP using DPEPO as host only reached the maximum EQE of 3.67%, equal to MB. What is more, device PP showed a folder maximum luminance of 13600 cd m-2 comparing to device DP with identical EL spectra. Table 3. Key Electroluminescent Characteristics of the Doped Devices with the Structure of ITO/HATCN (6 nm)/TAPC (40 nm)/TCTA (10 nm)/host: PyINA 30wt. % (25 nm)/TPBi (30 nm)/LiF (1.2 nm)/Al (120 nm) Vona

Luminancemax

EQEbmax

ELλmax

CIEc

at 100/1000cd·m-2

ΦPL [%]

[V]

[cd m-2]

[%]

[nm]

[x, y]

EQEc [%]

dfilmd

PIMNA

3.4

13600

5.05

432

(0.156, 0.060)

5.05/4.67

66.4

DPEPO

3.2

7600

3.67

432

(0.155, 0.063)

3.62/3.49

62.3

26DCzPPy

4.4

10200

2.57

436

(0.155, 0.070)

2.41/2.06

57.9

DMPPP

3.3

13300

2.66

440

(0.159, 0.095)

1.54/2.15

64.7

host

a

Turn-on voltage at a luminance of 1 cd m-2. bExternal quantum efficiency. cMeasured at a current density of 100 mA cm-2. dAbsolute PL quantum yield of PyINA doped film (30 wt. %) evaluated using an integrating sphere.


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Figure 7. (a) The EQE-current density characteristics of PyINA based doped devices PP and DP, and (b) EL spectra at 100 cd m-2. (c) Device structure and energy diagram of PyINA based doped devices. To find out the reason of nearly 40% improvement of EQE from device DP to device PP, we measured four films with the same structures of emission layers (Table 3). There were little difference, suggesting Förster energy transfer is not the main factor leading to such large increment. Subsequently, we fabricated devices by doping a classical green-emitting phosphorescent dye, tris(phenylpyridine)iridium (Ir(ppy)3) into PIMNA, as well.60 The maximum of optimized device was only 10.4%, and was much inferior to most reported host materials based on PIM,52,61 indicating PIMNA is not a good host material of universality, probably imputing to its small molecular weight like mCP (1,3-bis(carbazol-9-yl)benzene).62 Therefore, the most possible reason is that very effective Dexter energy transfer occurs from PIMNA to PyINA in EL process. However, it is well understood that Förster energy transfer is rather effective than Dexter energy transfer generally, and one of the reasons is that only if there is sufficient relevant orbitals overlap between host and guest to ensure strong spin–orbit coupling and short interaction distance, Dexter energy transfer would be effective.63,64 It has also been


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reported that doped OLEDs utilizing similar structure of host and guest could display excellent performance.65,66 Coincidently, PIMNA and PyINA showing resemblant chemical structure assume similar electronic and excited state properties, which is possible to perform more effective energy transfer process in EL devices with high doping concentration.

Figure 8. CIE chromaticity coordinates of NTSC, EBU standard blue and the three devices. CONCLUSIONS In summary, two imidazole-based derivatives, PIMNA and PyINA have been designed and synthesized. It has proved that PIM and PyI have great potentials in building deep blue emitters with ambipolar nature. PIMNA exhibited deep blue emission with the smallest CIEy of 0.034 ever reported in non-doped OLEDs. PyINA also displays deep-blue emission (0.157, 0.084) and great EL performance (EQE = 3.68%) with negligible efficiency roll-off. By doping PyINA into PIMNA, a significantly improved maximum EQE of 5.05% is obtained through very effective energy transfer with CIE coordinates of (0.156, 0.060), and the EQE remains 4.67% at 1000 cd m-2, which is among the best of deep blue OLEDs matching EBU standard. Our study should


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provide novel ideas for designing high efficiency deep blue emitters and fabricating OLEDs with excellent performance. ASSOCIATED CONTENT Supporting Information. Experimental details, 1HNMR and 13CNMR identification, TGA and DSC curves, cyclic voltammogram, NTOs of singlet and triplet excited states, crystal stacking mode and structure refinements, detail electroluminescent characteristics. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author Ping Lu. E-mail: [email protected]. ACKNOWLEDGMENT This work was supported financially by the Ministry of Science and Technology of China (2013CB834801, 2016YFB0401001), the National Science Foundation of China (21374038) and the Jilin Provincial Science and Technology Department (20160101302JC). REFERENCES (1) D'Andrade, B. W.; Forrest, S. R. White Organic Light-Emitting Devices for Solid-State Lighting. Adv. Mater. 2004, 16, 1585-1595. (2) Sasabe, H.; Kido, J. Recent Progress in Phosphorescent Organic Light-Emitting Devices. Eur. J. Org. Chem. 2013, 7653-7663.


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(3) Reineke, S. Phosphorescence meets its match. Nat. Photon. 2014, 8, 269-270. (4) Nair. G. B.; Dhoble, S. J. A Perspective Perception on the Ppplications of Light-Emitting Diodes. Lumin. 2015, 30, 1167-1175. (5) Chien, C.-H.; Chen, C.-K.; Hsu, F.-M.; Shu, C.-F.; Chou, P.-T.; Lai, C.-H. Multifunctional Deep-Blue Emitter Comprising an Anthracene Core and Terminal Triphenylphosphine Oxide Groups. Adv. Funct. Mater. 2009, 19, 560-566. (6) Kaji, H.; Suzuki, H.; Fukushima, T.; Shizu, K.; Suzuki, K.; Kubo S.; Komino, T.; Oiwa, H.; Suzuki, F.; Wakamiya, A.; Murata, Y.; Adachi, C. Purely Organic Electroluminescent Material Realizing 100% Conversion from Electricity to Light. Nat. Commun. 2015, 6, 8476. (7) Yersin, H. Highly efficient OLEDs with phosphorescent materials, John Wiley & Sons, 2008. (8) Uoyama, H.; Goushi, K.; Shizu, K.; Nomura, H.; Adachi, C. Highly Efficient Organic Light-Emitting Diodes from Delayed Fluorescence. Nature 2012, 492, 234-238. (9) Cui, L. S.; Xie, Y. M.; Wang, Y. K.; Zhong, C.; Deng, Y. L.; Liu, X. Y.; Jiang, Z. Q.; Liao, L. S. Pure Hydrocarbon Hosts for ≈100% Exciton Harvesting in Both Phosphorescent and Fluorescent Light-Emitting Devices. Adv. Mater. 2015, 27, 4213-4217. (10) Zhang, Q.; Shizu, J.; Li, K.; Huang, S.; Hirata, S.; Miyazaki, H.; Adachi, C. Design of Efficient Thermally Activated Delayed Fluorescence Materials for Pure Blue Organic Light Emitting Diodes. J. Am. Chem. Soc. 2012, 134, 14706-14709.


20

Page 21 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(11) Liang, H.; Wang, X.; Zhang, X.; Ge, Z.; Ouyang, X.; Wang, S. Efficient Tuning of Electroluminescence from Sky-Blue to Deep-Blue by Changing the Constitution of Spirobenzofluorene Derivatives. Dyes Pigments 2014, 108, 57-63. (12) Zhang, Q.; Li, B.; Huang, S.; Nomura, H.; Tanaka, H.; Adachi, C. Efficient Blue Organic Light-Emitting Diodes Employing Thermally Activated Delayed Fluorescence. Nat. Photon. 2014, 8, 326-332. (13) Lee, J.; Chen, H. F.; Batagoda, T.; Coburn, C.; Djurovich, P. I.; Thompson, M. E.; Forrest, S. R. Deep Blue Phosphorescent Organic Light-Emitting Diodes with Very High Brightness and Efficiency. Nat. Mater. 2016, 15, 92-98. (14) Chou, P.-T.; Chi, Y.; Chung, M.-W.; Lin, C.-C. Harvesting Luminescence via Harnessing the Photophysical Properties of Transition Metal Complexes. Coord. Chem. Rev. 2011, 255, 2653-2665. (15) Xu, H.; Chen, R.; Sun, Q.; Lai, W.; Su, Q.; Huang, W.; Liu, X. Recent Progress in Metal– Organic Complexes for Optoelectronic Applications. Chem. Soc. Rev. 2014, 43, 3259-3302. (16) Zhang, J.; Ding, D.; Wei, Y.; Xu, H. Extremely Condensing Triplet States of DPEPOType Hosts Through Constitutional Isomerization for High-Efficiency Deep-Blue Thermally Activated Delayed Fluorescence Diodes. Chem. Sci. 2016, 7, 2870-2882. (17) Staroske, W.; Pfeiffer, M.; Leo, K.; Hoffmann, M. Single-Step Triplet-TripletAnnihilation: an Intrinsic Limit for the High-Brightness Efficiency of Phosphorescent Organic Light Emitting Diodes. Phys. Rev. Lett. 2007, 98, 197402.


21


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 30

(18) Giebink, N. C.; Forrest, S. R. Quantum Efficiency Roll-off at High Brightness in Fluorescent and Phosphorescent Organic Light Emitting Diodes. Phys. Rev. B 2008, 77, 235215. (19) Lee, J.; Lee, J. I.; Lee, J. Y.; Chu, H. Y. Stable Efficiency Roll-off in Blue Phosphorescent Organic Light-Emitting Diodes by Host Layer Engineering. Org. Electron. 2009, 10, 1529-1533. (20) Han, C.; Zhao, F.; Zhang, Z.; Zhu, L.; Xu, H.; Li, J.; Ma, D.; Yan, P. Constructing LowTriplet-Energy Hosts for Highly Efficient Blue PHOLEDs: Controlling Charge and Exciton Capture in Doping Systems. Chem. Mater. 2013, 25, 4966-4976. (21) Pu, Y.-J.; Nakata, G.; Satoh, F.; Sasabe, H.; Yokoyama, D.; Kido, J. Optimizing the Charge Balance of Fluorescent Organic Light-Emitting Devices to Achieve High External Quantum Efficiency Beyond the Conventional Upper Limit. Adv. Mater. 2012, 24, 1765-1770. (22) Jou, J. H.; Kumar, S.; Singh, M.; Chen, Y. H.; Chen, C. C.; Lee, M. T. Carrier Modulation Layer-Enhanced Organic Light-Emitting Diodes. Molecules 2015, 20, 13005-13030. (23) Sun, M.; Zhu, L.; Kan, W.; Wei, Y.; Ma, D.; Fan, X.; Huang, W.; Xu, H. Ternary Donor– Acceptor Phosphine Oxide Hosts with Peculiar High Energy Gap for Efficient Blue Electroluminescence. J. Mater. Chem. C 2015, 3, 9469-9478. (24) Yuan, Y.; Chen, J.; Lu, F.; Tong, Q.; Yang, Q.; Mo, H.; Ng, T.-W.; Wong, F.; Guo, Z.; Ye, J.; Chen, Z.; Zhang, X.-H.; Lee, C.-S. Bipolar Phenanthroimidazole Derivatives Containing Bulky Polyaromatic Hydrocarbons for Nondoped Blue Electroluminescence Devices with High Efficiency and Low Efficiency Roll-Off. Chem. Mater. 2013, 25, 4957-4965.


22

Page 23 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(25) Wang, K.; Wang, S.; Wei, J.; Miao, Y.; Liu, Y.; Wang, Y. Novel Diarylborane– Phenanthroimidazole Hybrid Bipolar Host Materials for High-Performance Red, Yellow and Green Electrophosphorescent Devices. Org. Electron. 2014, 15, 3211-3220. (26) Su, N.; Meng, F.; Chen, J.; Wang, Y.; Tan, H.; Su, S.; Zhu, W. Near-Infrared Emitting Pyrazole-Bridged

Binuclear

Platinum

Complexes:

Synthesis,

Photophysical

and

Electroluminescent Properties in PLEDs. Dyes Pigments 2016, 128, 68-74. (27) Liu, H.; Chen, P.; Hu, D.; Tang, X.; Pan, Y.; Zhang, H.; Zhang, W.; Han, X.; Bai, Q.; Lu, P.; Ma, Y. Separation of Electrical and Optical Energy Gaps: Selectively Adjusting the Electrical and Optical Properties for a Highly Efficient Blue Emitter. Chem. Eur. J. 2014, 20, 2149-2153. (28) Shi, J.; Tang, C. Metal Chelates as Emitting Materials for Organic Electroluminescence. Coord. Chem. Rev. 1998, 171, 161-174. (29) Kan, W.; Zhu, L.; Wei, Y.; Ma, D.; Sun, M.; Wu, Z.; Huang, W.; Xu, H. Phosphine Oxide-Jointed Electron Transporters for the Reduction of Interfacial Quenching in Highly Efficient Blue PHOLEDs. J. Mater. Chem. C 2015, 3, 5430-5439. (30) Liu, M.; Li, X.-L.; Chen, D.; Xie, Z.; Cai, X.; Xie, G.; Liu, K.; Tang, J.; Su, S.-J.; Cao, Y. Study of Configuration Differentia and Highly Efficient, Deep-Blue, Organic Light-Emitting Diodes Based on Novel Naphtho[1,2-d]imidazole Derivatives. Adv. Funct. Mater. 2015, 25, 5190-5198. (31) Zhang, Y.; Lai, S.-L.; Tong, Q.-X.; Lo, M.-F.; Ng, T.-W.; Chan, M.-Y.; Wen, Z.-C.; He, J.; Jeff, K.-S.; Tang, X.-L.; Liu, W.-M.; Ko, C.-C.; Wang, P.-F.; Lee, C.-S. High Efficiency


23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 30

Nondoped Deep-Blue Organic Light Emitting Devices Based on Imidazole-π-Triphenylamine Derivatives. Chem. Mater. 2012, 24, 61-70. (32) Gao, Z.; Cheng, G.; Shen, F.; Zhang, S.; Zhang, Y.; Lu, P.; Ma, Y. Highly Efficient Deep Blue Light Emitting Devices Based on Triphenylsilane Modified Phenanthro[9, 10-d]imidazole. Laser Photonics Rev. 2014, 8, L6-L10. (33) Qin, W.; Yang, Z.; Jiang, Y.; Lam, J. W. Y.; Liang, G.; Kwok, H. S.; Tang, B. Z. Construction of Efficient Deep Blue Aggregation-Induced Emission Luminogen from Triphenylethene for Nondoped Organic Light-Emitting Diodes. Chem. Mater. 2015, 27, 38923901. (34) Ouyang, X.; Li, X.; Ai, L.; Mi, D.; Ge, Z.; Su, S.-J. Novel “Hot Exciton” Blue Fluorophores for High Performance Fluorescent/Phosphorescent Hybrid White Organic LightEmitting Diodes with Superhigh Phosphorescent Dopant Concentration and Improved Efficiency Roll-Off. Appl. Mater. Interfaces 2015, 7, 7869−7877. (35) Kumar, D.; Thomas, K. R.; Lee, C. C.; Ho, K. C. Novel Pyrenoimidazole-Based Organic Dyes for Dye-Sensitized Solar Cells. Org. Lett. 2011, 13, 2622–2625. (36) Kumar, D.; Thomas, K.; Lee, C. P.; Ho, K. C. Pyrenoimidazole-Based Deep-BlueEmitting Materials: Optical, Electrochemical, and Electroluminescent Characteristics. Chem. Asian J. 2015, 4, 164-172. (37) Ouyang, X.; Li, X.-L.; Zhang, X.; Islam, A.; Ge, Z.; Su, S.-J. Effective Management of Intramolecular Charge Transfer to Obtain from Blue to Violet-Blue OLEDs Based on a Couple of Phenanthrene Isomers. Dyes Pigments 2015, 122, 264-271.


24

Page 25 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(38) Liu, Y.; Shan, T.; Yao, L.; Bai, Q.; Guo, Y.; Li, J.; Han, X.; Li, W.; Wang, Z.; Yang, B.; Lu, P.; Ma, Y. Isomers of Pyrene−Imidazole Compounds: Synthesis and Configuration Effect on Optical Properties. Org. Lett. 2015, 17, 6138-6141. (39) Kim, R.; Yoo, S.-J.; Kim, E.-K.; Yu, H. S.; Shin, S. C.; Lee, S.-K.; Kwon, S.-K.; Kim, Y.H. New Limb Structured Blue Light Emitting Materials for OLEDs. Dyes Pigments 2012, 95, 384-391. (40) Cho, I.; Kim, S. H.; Kim, J. H.; Park, S.; Park, S. Y. Highly Efficient and Stable DeepBlue Emitting Anthracene-Derived Molecular Glass for Versatile Types of Non-Doped OLED Applications. J. Mater. Chem. 2012, 22, 123-129. (41) Chen, W.-C.; Yuan, Y.; Wu, G.-F.; Wei, H.-X.; Ye, J.; Chen, M.; Lu, F.; Tong, Q.-X.; Wong, F.-L.; Lee, C.-S. Molecular Modification on Bisphenanthroimidazole Derivative for Deep-Blue Organic Electroluminescent Material with Ambipolar Property and High Performance. Org. Electron. 2015, 17, 159-166. (42) Li, W.; Pan, Y.; Xiao, R.; Peng, Q.; Zhang, S.; Ma, D.; Li, F.; Shen, F.; Wang, Y.; Yang, B.; Ma, Y. Employing ∼100% Excitons in OLEDs by Utilizing a Fluorescent Molecule with Hybridized Local and Charge Transfer Excited State. Adv. Funct. Mater. 2014, 24, 1609-1614. (43) Zhang, S.; Yao, L.; Peng, Q.; Li, W.; Pan, Y.; Xiao, R.; Gao, Y.; Gu, C.; Wang, Z.; Lu, P.; Li, F.; Su, S.; Yang, B.; Ma, Y. Achieving a Significantly Increased Efficiency in Nondoped Pure Blue Fluorescent OLED: a Quasi-Equivalent Hybridized Excited State. Adv. Funct. Mater. 2015, 25, 1755-1762.


25


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 30

(44) Han, X.; Bai, Q.; Yao, L.; Liu, H.; Gao, Y.; Li, J.; Liu, L.; Liu, Y.; Li, X.; Lu, P.; Yang, B. Highly Efficient Solid-State Near-Infrared Emitting Material Based on Triphenylamine and Diphenylfumaronitrile with an EQE of 2.58% in Nondoped Organic Light-Emitting Diode. Adv. Funct. Mater. 2015, 25, 7521-7529. (45) Tnag, X.; Bai, Q.; Peng, Q.; Gao, Y.; Li J.; Liu, Y.; Yao, L.; Lu, P.; Yang, B.; Ma, Y. Efficient Deep Blue Electroluminescence with an External Quantum Efficiency of 6.8% and CIEy < 0.08 Based on a Phenanthroimidazole−Sulfone Hybrid Donor−Acceptor Molecule. Chem. Mater. 2015, 27, 7050−7057. (46) Tang, S.; Liu, M.; Lu, P.; Xia, H.; Li, M.; Xie, Z.; Shen, F.; Gu, C.; Wang, H.; Yang, B.; Ma, Y. A Molecular Glass for Deep-Blue Organic Light-Emitting Diodes Comprising a 9,9'Spirobifluorene Core and Peripheral Carbazole Groups. Adv. Funct. Mater. 2007, 17, 2869-2877. (47) Moorthy, J. N.; Venkatakrishnan, P.; Natarajan, P.; Lin, Z.; Chow, T. J. Nondoped PureBlue OLEDs Based on Amorphous Phenylenevinylene-Functionalized Twisted Bimesitylenes. J. Org. Chem. 2010, 75, 2599-2609. (48) Birks, J. B.; Christophorou, L. G. Excimer Fluorescence Spectra of Pyrene Derivatives. Spectrochim. Acta 1963, 19, 401-410. (49) Kim, H. K.; Cho, S.-H.; Oh, J. R.; Lee, Y.-H.; Lee, J.-H.; Lee, J.-G.; Kim, S.-K.; Park, Y.I.; Park, J.-W.; Do, R. Y. Deep Blue, Efficient, Moderate Microcavity Organic Light-Emitting Diodes. Org. Electron. 2010, 11, 137-145.


26

Page 27 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(50) Greenham, N. C.; Friend, R. H.; Bradley, D. D. Angular Dependence of the Emission from a Conjugated Polymer Light-Emitting Diode: Implications for Efficiency Calculations. Adv. Mater. 1994, 6, 491-494. (51) Hong, K.; Lee, J.-L. Review Paper: Recent Developments in Light Extraction Technologies of Organic Light Emitting Diodes Electron. Mater. Lett. 2011, 7, 77-91. (52) Wang, K.; Zhao, F.; Wang, C.; Chen, S.; Chen, D.; Zhang, H.; Liu, Y.; Ma, D.; Wang, Y. High-Performance Red, Green, and Blue Electroluminescent Devices Based on Blue Emitters with Small Singlet-Triplet Splitting and Ambipolar Transport Property. Adv. Funct. Mater. 2013, 23, 2672-2680. (53) Wang, K.; Wang, S.; Wei, J.; Chen, S.; Liu, D.; Liu, Y.; Wang, Y. New Multifunctional Phenanthroimidazole–Phosphine Oxide Hybrids for High-Performance Red, Green and Blue Electroluminescent Devices. J. Mater. Chem. C 2014, 2, 6817-6826. (54) Liu, D.; Du, M.; Chen, D.; Ye, K.; Zhang, Z.; Liu, Y.; Wang, Y. A Novel Tetraphenylsilane–Phenanthroimidazole Hybrid Host Material for Highly Efficient Blue Fluorescent, Green and Red Phosphorescent OLEDs. J. Mater. Chem. C 2015, 3, 4394-4401. (55) Seo, J. H.; Lee, K. H.; Seo, B. M.; Koo, J. R.; Moon, S. J.; Park, J. K.; Yoon, S. S.; Kim, Y. K. High-Efficiency Deep-Blue Organic Light-Emitting Diodes Using Dual-Emitting Layer. Org. Electron. 2010, 11, 1605-1612. (56) Yu, D.; Zhao, F.; Zhang, Z.; Han, C.; Xu, H.; Li, J.; Ma, D.; Yan, P. Insulated Donor-PAcceptor Systems Based on Fluorene-Phosphine Oxide Hybrids for Non-Doped Deep-Blue Electroluminescent Devices. Chem. Commun. 2012, 48, 6157-6159.


27


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 30

(57) Kim, R.; Lee, S.; Kim, K.-H.; Lee, Y.-J.; Kwon, S.-K.; Kim, J.-J.; Kim., Y.-H. Extremely Deep Blue and Bighly Efficient Non-Doped Organic Light Emitting Diodes Using an Asymmetric Anthracene Derivative with a Xylene Unit. Chem. Commun, 2013, 49, 4664-4666. (58) Ye, J.; Chen, Z.; Fung, M.-K.; Zheng, C.; Ou, X.; Zhang, X.; Yuan, Y.; Lee, C.-S. Carbazole/Sulfone Hybrid D-π-A-Structured Bipolar Fluorophores for High-Efficiency BlueViolet Electroluminescence. Chem. Mater. 2013, 25, 2630-2637. (59) Hu, J.-Y.; Pu, Y.-J.; Satoh, F.; Kawata, S.; Katagiri, H.; Sasabe, H.; Kido, J. Bisanthracene-Based Donor-Acceptor-type Light-Emitting Dopants: Highly Efficient Deep-Blue Emission in Organic Light-Emitting Devices. Adv. Funct. Mater. 2014, 24, 2064-2071. (60) Baldo, M. A.; Lamansky, S.; Burrows, P. E.; Thompson, M. E.; Forrest, S. R. Very HighEfficiency Green Organic Light-Emitting Devices Based on Electrophosphorescence. Appl. Phys. Lett. 1999, 75, 4. (61) Huang, H.; Wang, Y.; Wang, B.; Zhuang, S.; Pan, B.; Yang, X.; Wang, L.; Yang, C. Controllably Tunable Phenanthroimidazole-Carbazole Hybrid Bipolar Host Materials for Efficient Green Electrophosphorescent Devices. J. Mater. Chem. C 2013, 1, 5899-5908. (62) Wu, M. F.; Yeh, S. J.; Chen, C. T.; Murayama, H.; Tsuboi, T.; Li, W. S.; Chao, I.; Liu, S. W.; Wang, J. K. The Quest for High-Performance Host Materials for Electrophosphorescent Blue Dopants. Adv. Funct. Mater. 2007, 17, 1887-1895. (63) Főrster, T. Transfer Mechanisms of Electronic Excitation. Discuss. Faraday Soc. 1959, 27, 7-17.


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Page 29 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(64) Dexter, D. L. A Theory of Sensitized Luminescence in Solids. J. Chem. Phys. 1953, 21, 836-850. (65) Volyniuk, D.; Cherpak, V.; Stakhira, P.; Minaev, B.; Baryshnikov, G.; Chapran, M.; Tomkeviciene, A.; Keruckas, J.; Grazulevicius, J. V. Highly Efficient Blue Organic LightEmitting Diodes Based on Intermolecular Triplet-Singlet Energy Transfer. J. Phys. Chem. C 2013, 117, 22538-22544. (66) Hirai, H.; Nakajima, K.; Nakatsuka, S.; Shiren, K.; Ni, J.; Nomura, S.; Ikuta, T.; Hatakeyama, T. One-Step Borylation of 1,3-Diaryloxybenzenes towards Efficient Materials for Organic Light-Emitting Diodes. Angew. Chem. Int. Ed. 2015, 54, 13581-13585.


29


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SYNOPSIS Efficient deep blue non-doped organic light-emitting diodes (OLEDs) based on PIMNA and PyINA show negligible efficiency roll-off owing to their ambipolar nature, with extremely small CIEy of 0.034 and 0.084, respectively. By doping PyINA into PIMNA, a significantly improved maximum EQE of 5.05% is obtained through very effective energy transfer with CIE coordinates of (0.156, 0.060), and the EQE remains 4.67% at 1000 cd m-2, which is among the best of deep blue OLEDs reported matching stringent European Broadcasting Union standard well.


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Highly Efficient Deep Blue Organic Light-Emitting ... - ACS Publications

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