Highly Efficient Deep-Blue Electroluminescence from a Charge

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Highly Efficient Deep-Blue Electroluminescence from a Charge-Transfer Emitter with Stable Donor Skeleton Wencheng Chen, Yi Yuan, Shao-Fei Ni, Zelin Zhu, Jinfeng Zhang, ZuoQuan Jiang, Liang-Sheng Liao, Fu-Lung Wong, and Chun-Sing Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b14638 • Publication Date (Web): 10 Feb 2017 Downloaded from http://pubs.acs.org on February 12, 2017

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ACS Applied Materials & Interfaces

Highly Efficient Deep-Blue Electroluminescence from a Charge-Transfer Emitter with Stable Donor Skeleton Wen-Cheng Chen,† Yi Yuan,‡ Shao-Fei Ni,§ Ze-Lin Zhu,† Jinfeng Zhang,† Zuo-Quan Jiang,‡ Liang-Sheng Liao,‡,* Fu-Lung Wong,† and Chun-Sing Lee†,* †

Center of Super-Diamond and Advanced Films (COSDAF) and Department of Biology and Chemistry, City University of Hong Kong, Hong Kong SAR, PR China E-mail: [email protected] ‡ Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Institute of Functional Nano & Soft Materials (FUNSOM) & Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou, 215123, PR China E-mail: [email protected] § Department of Chemistry, Southern University of Science and Technology, Shenzhen, 518055, PR China KEYWORDS: phenanthro[9,10-d]imidazole, donor-acceptor emitter, high efficiency, high stability, deep-blue OLED ABSTRACT Organic materials containing arylamines have been widely used as hole-transporting materials as well as emitters in organic light-emitting devices (OLEDs). However, it has been pointed out that the C-N bonds in these arylamines can easily suffer from degradation in excited states, especially in deep-blue OLEDs. In this work, phenanthro[9,10-d]imidazole (PI) is proposed as a potential donor with higher stability than those of arylamines. Using PI as the donor, a donor-acceptor type deep-blue

fluorophore

1-phenyl-2-(4''-(1-phenyl-1H-benzo[d]imidazol-2-yl)-

[1,1':4',1''-terphenyl]-4-yl)-1H-phenanthro[9,10-d]imidazole (BITPI) is designed and synthesized. Results from UV-aging test on neat films of BITPI and other three arylamine compounds demonstrate that PI is indeed a more stable donor comparing to

ACS Paragon Plus Environment


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common arylamines. An OLED using BITPI as an emitter exhibits good device performances (EQE over 7%) with stable deep-blue emission (color index: (0.15, 0.13)) and longer operation lifetime than the similarly-structured device using arylamine-based emitter. Single-organic layer device based on BITPI also show superior performances, which are comparable to the best results from the arylamine-based donor-acceptor emitters, suggesting that PI is a stable donor with good hole transport/injection capability.

1. INTRODUCTION Since the invention of multi-layer organic light-emitting devices (OLEDs) by Tang et al.,1 representative efficient devices are designed to confine charges in a recombination zone sandwiched between a hole- and an electron-transporting layers. In this sense, arylamines are ideal materials to be positioned between the emissive layers (EMLs) and the anode due to their excellent abilities to inject/transport hole and block electron stemming from their electron-rich nitrogen atoms.2,3 Apart from functioning as hole transporters, arylamines are also widely used as building blocks for emitters as well as host materials.4–8 Significantly, arylamines can also be paired with various electron-withdrawing groups for designing donor-acceptor (D-A) type fluorescent emitters with potential 100% internal quantum efficiencies in OLEDs.9–12 Adachi et al. recently used an arylamine as the donor for developing an efficient D-A emitter based on thermally activated delayed fluorescence (TADF) with external quantum efficiency (EQE) of 29.6% without optical out-coupling enhancement.13 Nowadays, numerous literatures reporting OLEDs using arylamines with EQEs


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approaching or exceeding theoretical limit show that the adoption of arylamines is generally competent for delivering high efficiency. However, recent studies demonstrated that arylamines may easily suffer from molecular degradation upon device operation.14–16 The degraded products could act as deep charge traps and non-radiative recombination centers, leading to performance degradation. It was discovered that degradation mainly arises around the hole transporting layer/EML interface, while little decomposition was observed in single-carrier aged devices, indicating that arylamine degradation mainly results from molecular excited states rather than charge transport.15,16 Kondakov et al. pointed out that the low dissociation energy of the C-N bond in arylamines is responsible for their vulnerability because this energy is comparable to those of the first singlet excited state (S1).16 Considering this issue, blue-emitting arylamine emitters with large S1 energies are more vulnerable with higher possibility of C-N bond breakdown in operating OLEDs. Nonetheless, to date, majority of the state-of-the-art fluorescence OLED emitters are D-A type molecules and a large number of them rely on arylamines as electron donors. Although some groups have suggested new approaches to improve blue device lifetimes, such as using bulky peripheral substituents to protect the emitting cores17 and re-designing the device structure with graded dopant concentration profile, etc.,18 it is still highly desirable to explore for alternative stable donors for application in D-A type emitters. In this study, we propose to use phenanthro[9,10-d]imidazole (PI) as a stable donor

for

constructing

D-A

type

emitters

with


high

S1

energies.


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1-phenyl-2-(4''-(1-phenyl-1H-benzo[d]imidazol-2-yl)-[1,1':4',1''-terphenyl]-4-yl)-1Hphenanthro[9,10-d]imidazole (BITPI, see Scheme 1) was designed and synthesized as a deep-blue emitter. BITPI displays a mild charge transfer (CT) feature as evidenced by theoretical simulation and photophysical analysis. UV-aging experiment was performed on thin films of BITPI and arylamine derivatives with similar S1 energies in inert atmosphere. The results demonstrated that BITPI is much more stable than the arylamine-based hole-transporter and emitters. Deep-blue emissions and high device efficiencies were achieved in BITPI-based OLEDs with both multi-layer and double-layer device structures as compared to those using the state-of-the-art deep-blue arylamine-based emitters.

2. EXPERIMENTAL SECTION 2.1. General Methods. NMR spectra were recorded on a Bruker 400 spectrometer at room temperature. Electrospray ionization mass spectrometry (ESI/MS) was carried out on a PE SCIEX API 365 mass spectrometer. Elemental analysis (C, H, N) was recorded using a Vario EL III CHNS elemental analyzer. Decomposition temperature (Td, 5% weight loss) was measured on a TA Instrument TGAQ50 at a heating rate of 10 °C/min under N2 atmosphere, while glass transition temperature (Tg) was determined on a Perkin-Elmer DSC 7 differential scanning calorimetric. UV-vis absorption and photoluminescence (PL) spectra were measured on a Perkin-Elmer Lambda 950 UV/vis spectrometer and a Perkin-Elmer LS50B spectrophotometer, respectively. Absolute photoluminescence quantum yield (PLQY) was measured with


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a LabsphereTM integrating sphere excited (at 360 nm) with a monochromatized Xe lamp

(NewportTM).

PLQYs

of

solutions

were

measured

by

using

9,10-diphenylanthracene (0.90) as the reference standard.19 Cyclic voltammetry was measured on a CHI600 voltammetric analyzer equipped with a three-electrode system (a platinum disk as working electrode, a platinum wire as the auxiliary electrode, a silver wire as the pseudo-reference electrode with Fc/Fc+ as internal standard, which has an absolute highest occupied molecular orbital (HOMO) level of -4.80 eV). Nitrogen-saturated CH2Cl2 was used as solvent with 0.1 mol/L tetrabutylammonium hexafluorophosphate as the supporting electrolyte. PL lifetime was tested on a Horiba Jobin Yvon FL-TCSPC fluorescence spectrophotometer. UV-aging experiment was carried out in a glove-box using a UV source powered by a DYMAX Light Curing System (Model 5000 Flood, output intensity at UVA is 225 mW/cm2). Six individual films with thickness of 50 nm for each material were prepared on quartz substrates via thermal vacuum evaporation, and then transferred to a glove-box without exposing to atmosphere before UV aging. All films were deposited at the same conditions. All chemicals and reagents were used as received from commercial suppliers without further treatment. BPI-pinB was synthesized as reported.20 Synthesis of BBI-Br: 0.35 g N1-phenylbenzene-1,2-diamine (1.90 mmol) and 0.60 g 4'-bromo-[1,1'-biphenyl]-4-carbaldehyde (2.30 mmol) were dissolved in 30 mL ethanol under ambient condition. 0.44 g AcONH4 (5.70 mmol) and 0.02 g Co(OH)2 (0.19 mmol) were added. The mixture was stirred and refluxed overnight. After cooling down to room temperature, 50 mL water was added to the resulting mixture



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and extracted with CH2Cl2 for three times. The organic phase was dried over anhydrous MgSO4, and then the solvent was removed via rotary evaporator. The raw product was purified by chromatography using CH2Cl2/petroleum ether as eluent to yield a white solid (82.0%). 1H NMR (400 MHz, CDCl3, δ): 7.93 (d, J = 8.0 Hz, 1H), 7.70-7.65 (m, 2H), 7.61-7.45 (m, 9H), 7.42-7.35 (m, 3H), 7.31 (m, 2H). Synthesis of 1-phenyl-2-(4''-(1-phenyl-1H-benzo[d]imidazol-2-yl)-[1,1':4',1''terphenyl]-4-yl)-1H-phenanthro[9,10-d]imidazole (BITPI): 30 mL toluene, 10 mL ethanol and 15 mL 2 M Na2CO3 aq. were added to a mixture of 1.49 g 2-(4'-bromo-[1,1'-biphenyl]-4-yl)-1-phenyl-1H-benzo[d]imidazole mmol),

2

g

(BBI-Br,

3.50

1-phenyl-2-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)

-1H-phenanthro[9,10-d]imidazole (BPI-pinB, 4.00 mmol) and 0.20 g Pd(PPh3)4 (0.18 mmol) in a degased three three-necked flask. Then the suspension was heated to 90 °C with stirring under an argon atmosphere. After 24 h, the mixture was allowed to cool to room temperature, extracted with CH2Cl2 and dried over anhydrous MgSO4 before removing the solvent. Finally, the raw product was purified by column chromatography on silica gel using CH2Cl2 as eluent to give a white solid, with a yield of 81.3% (2.03 g). 1H NMR (400 MHz, CDCl3, δ): 8.92 (d, J = 7.9 Hz, 1H), 8.81 (d, J = 8.3 Hz, 1H), 8.74 (d, J = 8.3 Hz, 1H), 7.94 (d, J = 8.0 Hz, 1H), 7.78 (t, J = 7.1 Hz, 1H), 7.69 (d, J = 3.2 Hz, 12H), 7.58 (ddd, J = 12.5, 5.9, 3.4 Hz, 11H), 7.44-7.36 (m, 3H), 7.30 (t, J = 3.2 Hz, 2H), 7.22 (d, J = 7.5 Hz, 1H); 13C NMR (100 MHz, CDCl3, δ): 130.26, 129.89, 129.88, 129.75, 129.16, 128.69, 128.29, 127.51, 127.43, 126.78, 126.71, 126.29, 125.65, 124.9, 124.2, 124.13, 123.33, 123.09, 122.77


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122.56, 120.87, 119.81, 110.47. MS (ESI) m/z [M + H]+ calcd for C52H34N4, 714.9; found, 715.9. Anal. Calcd for C52H34N4: C, 87.37; H, 4.79; N, 7.84; found: C, 87.56; H, 4.65; N, 7.73. 2.2. Device Fabrication and Measurement. Devices were fabricated on ITO-coated glass substrates with a sheet resistance of 15 Ω/square. Before device fabrication, the ITO substrates were swabbed with detergent (Decon-90) in de-ionized water, followed by 15-min ultrasonic baths in acetone and de-ionized water respectively, before rinsing with isopropanol. Finally, the substrates were dried under N2 flow to remove isopropanol on the surface, and then stored in an oven at 120 °C before use. After a 20-minute UV-ozone treatment, the clean ITO substrates were immediately loaded into a deposition chamber with a vacuum better than 5 × 10-6 torr. Deposition rates of organic films were monitored with a quartz oscillating crystal and controlled at ~ 1 Å/s. After deposition of the organic layers, a thin LiF layer (1 nm) was deposited at a rate of 0.1-0.2 Å/s. Finally, an Al cathode (5 Å/s, 150 nm) was capped on the top via thermal evaporation through a shadow mask. Current density-voltage (J-V) characteristics and electroluminescence (EL) luminescence were tested with a programmable Keithley 237 power source and a Spectrascan PR650 photometer respectively. General device measurements were performed at room temperature under ambient conditions without encapsulation. Devices for lifetime testing were measured after encapsulation in a glove box immediately after cathode deposition.

3. RESULTS AND DISCUSSION



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3.1. Synthesis and Characterization The present blue-emitting fluorophore BITPI employs two types of imidazole derivative as electron donor and acceptor, i.e. PI and benzimidazole (BI) respectively. The synthetic route of BITPI is shown in Scheme 1. BITPI was prepared by a Suzuki coupling reaction between borated PI derivative (BPI-pinB)20 and bromized BI compound (BBI-Br), and then separated by silica column chromatography (dichloromethane as eluent) with a good yield. Chemical structure of BITPI was determined with 1H/13C NMR, mass and elemental analysis. Thermal properties of BITPI were determined in nitrogen atmosphere using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). Figure S1 indicates that BITPI is stable upon heating, showing a Td of 470 °C. Additionally, BITPI has a high Tg of 130 °C, indicating that BITPI is thermally stable in general OLED fabrication and operation.

Scheme 1. Synthesis route of BITPI. a: N1-phenylbenzene-1,2-diamine, AcONH4, Co(OH)2, reflux in EtOH; b: Pd(PPh3)4, in toluene/ EtOH/ 2 M Na2CO3 aq. at 90 °C under argon.


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Table 1. Summary of the physical data of BITPI. Tda (°C)

Tgb (°C)

Egc (eV)

470

130

3.12

a

HOMOd (eV)

LUMOe (eV)

λabsf (nm)

λplf (nm)

PLQYf (%)

-5.46

-2.34

343 / 344

435 / 450

91.5 / 81.9

b

c

5% weight loss temperature. Glass transition temperature. Optical energy gap estimated from absorption onset. d Measured by cyclic voltammetry. d Calculated from LUMO = HOMO + Eg, f Measured in THF solution and solid thin film, respectively.

3.2. Theoretical Calculations Imidazole is a five-membered azo-heteroaromatic ring with a sextet of π-electrons that consists of two lone-pair electrons from the pyrrol-like nitrogen atom with electron-donating nature. Meanwhile, imidazole also contains an electron-deficient pyridine-like nitrogen atom (−N=) with two non-conjugated lone-pair electrons.21 This bipolar property endows imidazole with flexibility for tuning their electronic properties via molecular tailoring. In this work, we obtained a CT emitter by regulating electronic structures of two linked imidazoles. By fusing aromatic units of different sizes to the imidazole skeleton, a mild electron push-pull system is achieved in the BITPI molecule. In an electrostatic potential (ESP) map of BITPI (Figure 1a), the most negative site is evidently delocalized on the −N= moieties (red on the ESP map), while the positive potential mainly distributes around the pyrrol-like nitrogens (blue on the ESP map). In contrast, the arene fused on 4,5-positions of imidazole ring displays neutral ESP (yellow/green on the ESP map) over the two terminals of the molecule. In PI, fused phenanthrene ring has a larger volume than benzene ring in BI, resulting in decrease of electron pulling strength of −N= in PI. This is also supported by the fact that BI (pKα= 5.5) is a much weaker base than imidazole (pKα = 7.0), since the fused



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benzene ring in BI tends to decrease the proton affinity of the pyridine-like nitrogen by changing its electron density;21 thus larger influence can be expected in the case of PI. Frontier molecular orbital analysis was used to further understand the electronic properties of BITPI at the B3LYP/6-31G(d, p) level. HOMO is mainly concentrated on PI unit (Figure 1b, lower), with partial contribution from the central triphenyl section. In contrast, the lowest unoccupied molecular orbital (LUMO) is distributed on the triphenyl and BI segments, with little contribution on the PI unit (Figure 1b, upper). With mild difference in electron push-pull nature of the BI and the PI units, the partial separation of HOMO/LUMO distribution suggests intramolecular charge transfer (ICT) can happen from the PI to the BI moieties. Actually, BI is often used as electron acceptor in D-A molecules,5,6,22–26 while some literatures reported that PI can both donate and receive electrons in push-pull systems.12,20,27–29 These suggest that PI has stronger electron donating property over BI. On the other hand, the long π-linker employed in the present molecule plays a key role in achieving a high PLQY, since it can provide appropriate overlap between HOMO and LUMO. The energy levels of HOMO and LUMO are calculated to be -5.16 and -1.53 eV, respectively, resulting in a wide energy gap of 3.63 eV.


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Figure 1. a) Molecular electrostatic potential surface; b) frontier molecular orbital distribution; c) absorption and photoluminescence spectra in dilute THF solution and solid film; d) photoluminescence spectra of BITPI in solutions with different polarities.

3.3. Photophysical Properties UV-vis absorption and PL spectra of BITPI in THF solution (~ 10-6 mol/L) and film are shown in Figure 1c. Both absorption spectra peak at around 340 nm, which can be attributed to PI’s π-π* transition. However, although the characteristics of emission spectra are analogous, 15-nm redshift can be found in the BBTPI film comparing with its solution. Nevertheless, the influence of π-π stacking on emission in solid is not significant, as evidenced by the high PLQY in neat film (81.9%), which is comparable to those of solutions (Table S1). We further investigated photophysical properties of BITPI in solvents of different polarities; detailed parameters are listed in Table S1. Figure 1d shows



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emission spectra of BITPI in hexane, butyl ether, THF and acetonitrile, which have increasing orientation polarization (f: ~ 0, 0.10, 0.21 and 0.30, respectively). ICT feature of BITPI’s excited state is evidenced by its bathochromic emission upon increasing solvent polarity. For example, the emission peak in non-polar hexane is 404 nm, while an emission maximum at 447 nm is observed in polar acetonitrile. Furthermore, vibrational fine structure of the emission disappears progressively, associated with boarder spectra from hexane to acetonitrile, which are also signs of ICT in excited state.30 We rule out the existence of twisted intramolecular charge transfer (TICT) excited state in BITPI. Because it is found that low temperature can lead to an enhanced fluorescence and slightly blue shifted emission (Figure S2), which are quite different from the characteristics of typical TICT emitters.31,32 In contrast, there is little variation in the absorption spectra (see Figure S3 and Table S1), indicating that contribution of ICT in the ground state is negligible. We further plotted Stokes shift versus f curve, as shown in Figure S4. The result displays high linearity (r = 0.97) with a slope estimated to be 8438 cm-1, which it is similar to the case of TBPMCN.12 The dipole moment of excited state (µe) was measured to 15.9 D according to Lippet-Mataga model, which is in between the values of typical local emissive (LE) and CT compounds.30 Based on the studies by Ma et al., hybridized local and charge-transfer (HLCT) excited state may exist in BITPI.11,12 As shown in Table S1, BITPI shows high PLQYs in different solutions (80-100%, except in triethylamine solution). Generally, CT excited state is a dark state in solutions with high polarity. But the PLQYs in BITPI polar solutions, such as in DMF and


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acetonitrile, can maintain at high levels. This is another evidence that highly mixed CT and LE excited state may exist in BITPI. Results from natural transition orbital analysis also support the existence of HLCT, as discussed in Supporting Information (Figure S5-S7).

3.4. Stability of Molecular Skeleton

Figure 2. Time-dependent photoluminescence spectra of UV-aged films of a) BITPI, b) NPB, c) TPABBBI and d) DMAC-DPS (inset: normalized spectra); e) time-dependent PL deterioration of films.

Because of the bulky and rigid skeleton of PI with larger π system, PI is expected to have high molecular stability than general arylamines. To evaluate the molecular stability, UV treatments were performed on 50-nm films of BITPI and three arylamine derivatives:

common

hole

N,N'-bis-(1-naphthalenyl)-N,N'-bis-phenyl-(1,1'-biphenyl)-4,4'-diamine

transporter (NPB),

TPABBBI,26 a homologous deep-blue emitter similar to BITPI but using diphenylamine as donor, and a high-performance TADF blue emitter DMAC-DPS.9,10



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All of them emit blue fluorescence with similar S1 energies. Samples were aged in a glove-box under strong UV illumination. Figure 2a-d show time-dependent PL spectra of the films prepared by thermal evaporation. All samples experienced sharp PL decrease in the first two minutes, which may be due to reaction between residual oxygen in the glove-box and surface molecules on the films.33 Since then, different PL reduction were observed in the samples, as shown in Figure 2e. Because of the vulnerable C-N and C-S bonds,34 DMAC-DPS films showed the most serious PL deterioration upon UV aging. After experiencing 10-minute UV irradiation, PL of the DMAC-DPS film was almost completely quenched. Moreover, continuous emission hypochromatic shifts were detected (Figure 2d inset), implying gradual fragmentation of the DMAC-DPS π-conjugation system. NPB is regarded as a stable hole-transporting material, yet its PL intensity steadily decreased with UV exposure time. Similarly, the D-π-A emitter TPABBBI26 bearing a diphenylamine as donor underwent a comparable PL drop with NPB. By contrast, BITPI film showed gentlest PL roll-off, ~ 80% of PL was maintained after 10-minute UV aging. These results suggest that BITPI has a more stable molecular skeleton comparing to common arylamine derivatives.

3.5. Electroluminescence Performances Non-doped OLEDs were fabricated to assess EL performances of BITPI as an emitter. First, we studied a multi-layer device with a configuration of indium-tin-oxide (ITO)/HAT-CN (1,4,5,8,9,11-hexaazatriphenylenehexacarbonitrile, 5 nm)/NPB (70


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nm)/TCTA

(tris(4-carbazoyl-9-ylphenyl)amine,

5

nm)/BITPI

(25

nm)/TPBI

(1,3,5-tri(1-phenyl-1H-benzo[d]imidazol-2-yl)phenyl, 40 nm)/LiF (1 nm)/Al (150 nm). Figure 3 shows performance of the BITPI-based device, and key data are listed in Table 2. The device exhibits deep-blue EL emission with Commission Internationale de L’Éclairage (CIE) coordinate of (0.15, 0.13) and EL spectra constantly peaked at 452 nm from 10 to 5,000 cd/m2 (Figure 3 inset). Moreover, the device exhibits excellent performance with a maximum current efficiency of 6.78 cd/A and an EQE of 7.11%. The bipolar properties of BITPI endow its device with a low turn-on voltage (Von = 2.6 V, detected at 1 cd/m2) and a superior power efficiency up to 7.10 lm/W. Notably, the device also shows slow efficiency roll-off. Figure S8 displays a linear relationship between current density and EL intensity, suggesting that there is little current-induced fluorescence quenching nor contribution from long-lived excitons in the device.35,36 Furthermore, transient PL of a BITPI neat film shows no delayed fluorescence (Figure S9).



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Figure 3. Performances of the BITPI-based OLED with a device structure of ITO/HAT-CN (5 nm)/NPB (70 nm)/TCTA (5 nm)/BITPI (25 nm)/TPBI (40 nm)/LiF (1 nm)/Al (150 nm). Moreover, operation lifetime of the multi-layer device was also evaluated. Meanwhile, TPABBBI was used as a reference emitter with the same device structure. In this comparison, we controlled the device fabricating process, device structure and other factor in the same conditions, the only difference in the devices is their EMLs. The HOMO (measured by cyclic voltammetry, Figure S10) and LUMO (calculated from LUMO = HOMO + Eg) of BITPI are estimated to be -5.46 and -2.34 eV, respectively. The energy level alignment of the materials used in the device is shown in Figure S11. The energy barriers among the neighboring layers in the device are not large, ensuring efficient charge injection and transport. Figure S12 shows the device performance of TPIBBBI-based device, which is comparable to the reported data by Ouyang et al.26 The EL spectra of the BITPI and the TPABBBI based devices are similar, implying that the two emitters have comparable S1 energy levels. Figure 4


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shows lifetime curves of the two non-doped devices with initial luminance of 500 cd/m2. The device based on BITPI exhibit a longer lifetime (T50) of 55 h, which is about two times longer than that in the TPABBBI based control device (T50 = 27 h). This indicates that the PI emitter has a better EL stability than that of its arylamine counterpart.

Figure 4. Lifetime of OLEDs based on BITPI and TPABBBI with a device structure of ITO/HAT-CN (5 nm)/NPB (70 nm)/TCTA (5 nm)/emitter (25 nm)/TPBI (40 nm)/LiF (1 nm)/Al (150 nm).

To verify PI’s hole transporting/injecting capability, we used BITPI as a multifunctional emitter to fabricate double-layer OLEDs. Since BI is reported to be a good electron-transporting group,5,6 efficient electron transport/injection is expected. HOMO level was measured to be -5.46 eV, which is similar to that of NPB (~ -5.40 eV), implying a good hole-injection capacity. Two types of device were prepared, employing configurations with pristine ITO and HAT-CN modified ITO as anodes: ITO/BITPI (70 nm)/LiF (1 nm)/Al (150 nm) and ITO/HAT-CN (5 nm)/BITPI (70 nm)/LiF (1 nm)/Al (150 nm), respectively. Figure 5a shows J-V characteristics of the



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two devices. As expected, electrical property is better in the device with HAT-CN. At low driving voltages, luminance of the device with HAT-CN was higher, due to the efficient hole injection via HAC-CN, showing a lower Von (2.8 V versus 3.1 V). However, as the injection was enhanced by higher bias, luminance increases much faster in the device without HAT-CN after 5 V. This result demonstrates that BITPI does have hole-injection and -conduction capabilities. Figure 5b illustrates the efficiencies of the two devices. Although the device with HAT-CN exhibited higher peak efficiency than the device without HAT-CN, with current efficiency (EQE) up to 6.29 cd/A (4.12%), efficiency roll-offs were much slower in the latter. For example, maximum EQE of 4.12% was acquired at low luminance in the device with HAT-CN, but this figure dropped to only 1.65% at 1000 cd/m2; while the EQE of the device without HAT-CN showed mild roll-off from 2.98 to 2.55%. Difference in BITPI’s electron and hole transport capabilities may be responsible for this phenomenon. Since the hole current in BITPI-based hole-only device is larger than electron current in corresponding electron-only device, hole mobility is higher than electron mobility in BITPI (Figure S13). Because of the help of HAT-CN, at low driving voltage hole can inject in EML efficiently in the device with HAT-CN to meet the electron from the opposite pole, and then quickly recombine to produce photon. In this case, a better hole-electron balance is achieved to give maximum efficiency. At high voltages, because of more holes injection and hole is transported faster than electron in BITPI, the hole-electron current become more unbalanced and the recombination zone begins to shift towards the cathode side where exciton can be quenched, leading to serious


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efficiency roll-off. On the other hand, hole injection is milder in the device without HAT-CN. This leads to more balanced electron/hole current and thus a better performance at high voltages. In this work, for the first time, D-A emitter based on PI as donor was used in efficient single-organic layer OLEDs. The double-layer devices based on BITPI exhibit record high performances among non-arylamine-based devices, and are comparable to the state-of-the-art devices using arylamine-based D-A emitters.5,6,37–40

Figure 5. Performances of the BITPI-based double-layer OLEDs with and without HAT-CN: a) J-V characteristics; b) J-L-EQE curves. Table 2. EL performance of non-doped OLEDs based on BITPI as emitter. λmaxa (nm)

CIEb

Vonc (V)

CEd (cd/A)

PEe (lm/W)

EQEf (%)

Multi-layer

452

(0.15, 0.13)

2.6

6.78/ 6.31

7.10/ 4.33

7.11/ 6.59

Double-layer without HAT-CN

452

(0.15, 0.14)

3.1

3.77/ 3.65

3.40/ 1.71

2.98/ 2.55

460

(0.15, 0.17)

2.8

6.29/ 2.35

6.59/ 1.12

4.12/ 1.65

Device

Double-layer with HAT-CN a

2 b

2 c

2 d

EL peak at 1000 cd/m . Measured at 1000 cd/m . Voltage at 1 cd/m . Current efficiency, power efficiency and f external quantum efficiency at maximum and 1000 cd/m2, respectively.

e

4. CONCLUSION We propose PI as a stable donor for constructing D-A type fluorescent emitters with high S1 energies. Based on PI as a donor, a D-A type emitter BITPI was designed and



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synthesized for deep-blue OLEDs. Theoretical calculation indicates that there exists moderate electron push-pull property between BI and PI moieties in BITPI. A mild charge transfer feature was found in BITPI, as evidenced by the photophysical analysis. UV-aging experiment was performed on films of BITPI and common arylamines. The results demonstrate that the PI-based D-A emitter is much more stable than arylamine-based hole transporter and emitters with high S1 energies. Multi-layer BITPI-based OLEDs with deep-blue emissions (CIE: (0.15, 0.13) and high efficiencies (EQE up to 7.11%) were demonstrated. Due to the good hole-transport/injection properties of PI, BITPI also showed excellent performance in single-organic layer OLEDs. This is the best performance to date for non-doped OLEDs with only one organic layer based on non-arylamine emitters, which are also comparable to the state-of-the-art D-A type arylamine compounds. These results indicate that PI can serve as a stable donor with good hole-transport/injection property, providing a good molecular design strategy for high-performance stable deep-blue OLED emitters.

ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org. Analysis of thermal stability, additional photophysical properties of BITPI, TD-DFT calculation, current density-luminance characteristic of BITPI-based OLED, transient PL measurement, cyclic voltammetry scan, discussion of device efficiency and lifetime, current density-voltage characteristics of single type carrier devices.


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AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] * E-mail: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors would like to acknowledge the supports from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. T23-713/11), Guangdong Innovative and Entrepreneurial Research Team Program (Nos. 2013C090 and KYPT20141013150545116) and Natural Science Foundation of China (Nos. 61575136 and 21572152).

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Highly Efficient Deep-Blue Electroluminescence from a Charge

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