Constructing a novel single-layer white organic light-emitting device through a new sky-blue fluorescent bipolar host

Organic Electronics 15 (2014) 3514–3520

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

Constructing a novel single-layer white organic light-emitting device through a new sky-blue fluorescent bipolar host Jun Ye a,b, Kai Wang c, Zhan Chen c, Fei-Fei An c, Yi Yuan a, Chi Zhang b, Xiao-Hong Zhang c,⇑, Chun-Sing Lee a,⇑ a Center of Super-Diamond and Advanced Films (COSDAF) and Department of Physics and Materials Sciences, City University of Hong Kong, Hong Kong Special Administrative Region b China–Australia Joint Research Center for Functional Molecular Materials, School of Chemical and Material Engineering, Jiangnan University, Wuxi 214122, PR China c Nano-organic Photoelectronic Laboratory and Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China

a r t i c l e

i n f o

Article history: Received 10 September 2014 Received in revised form 30 September 2014 Accepted 4 October 2014 Available online 16 October 2014 Keywords: White organic light emitting device Single layer Bipolar host Blue fluorescence Fluorescence and phosphorescence hybrid

a b s t r a c t A novel device concept was realized for simple single-layer small-molecule white organic light emitting devices. The single organic active layer here is simply comprised of a newly synthesized sky-blue fluorescent bipolar host (TPASO) and a common orange phosphorescent dopant. Suppressed singlet Föster energy transfer induced by a low-concentration doping and spontaneous high- to low-lying triplet energy transfer, respectively, lead to sky-blue fluorescence from TPASO and orange phosphorescence from the dopant. The resulting two-organic-component device exhibits a low turn-on voltage of 2.4 V, maximum current/power efficiencies up to 11.27 ± 0.02 cd A1 and 14.15 ± 0.03 lm W1, and a warmwhite CIE coordinate of (0.42, 0.45) at 1000 cd m2. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction White organic light-emitting devices (WOLEDs) are of intensive interest nowadays because they show the great potential in solid-state lighting applications. The progressive inventions of novel device concepts and organic semiconducting materials have resulted in significant progress in this field [1–4]. To gain high efficiencies, the most popular device strategy is using a multilayer structure in which different functional layers, e.g. for charge carrier injection, transport, blocking and exciton confining, are employed in order to achieve carrier recombination balance [5–12]. These multilayer configurations may become more ⇑ Corresponding authors. E-mail addresses: [email protected] (X.-H. Zhang), apcslee@ cityu.edu.hk (C.-S. Lee). http://dx.doi.org/10.1016/j.orgel.2014.10.007 1566-1199/Ó 2014 Elsevier B.V. All rights reserved.

complicated in a white device, once it arranges independent emissive layers to individually emit at one of the primary RGB colors. With the aim to save the manufacturing cost which is highly dependent on the consumed materials and processing time, special attentions then have been put on single-layer (SL) devices in which all the carrier injection, transport, recombination processes occur within one single organic layer. Currently, SL polymer-WOLEDs have already experienced considerable progress [13–20]. Zou et al. [18] realized a high power efficiency (PE) of 20.7 lm W1 at 1000 cd m2 by co-doping blue and yellow phosphorescent complexes into a blend of poly(N-vinylcarbazole) (PVK) host and 1,3-bis[(4-tert-butylphenyl)-1,3,4oxadiazolyl] phenylene (OXD-7) electron-transport material. By contrast, the reports on the fabrication of SL small-molecule (SM) WOLEDs are still rare [21–27]. Approaches of blending blue/red fluorescent dopants [21],

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combining exciplex [22] or dimer emissions [23] have used to be demonstrated, however, with low efficiencies. Only very recently, remarkable improvement for SL SM-WOLEDs has been obtained in several fully-phosphorescence devices [24–27]. These devices share a common concept of using non-radiative hosts with high triplet energy levels (ET) to sensitize two or more phosphors from blue to red. For examples, Qiu’s group [24] realized a maximum power efficiency (PE) of 7.2 lm W1 in a RGB phosphorescent device, using a dual-host system of 9,9-bis[4-(3,6-di-tert-butylcarbazol-9-yl)phenyl]fluorene (TBCPF) and 3-Bis[(4-tertbutylphenyl)-1,3,4-oxadiazolyl]phenylene(OXD-7); Chen et al. [25] examined a series of oxadiazole-based materials mixed in a host of 3,5-di(carbazol-9-yl)tetraphenylsilane (SimCP2) to boost electron injection from the cathode and achieved a maximum PE of 8.86 lm W1 in a blue/orange phosphorescent SM-WOLED; Xie’s group [26] also fabricated a blue/orange two-phosphor device basing on a bipolar host, 2,7-bis(diphenylphosphoryl)-9-[4-(N,Ndiphenyl-amino) phenyl]-9-phenylfluorene (POAPF), showing a maximum PE of 21.9 lm W1. Therefore, using a blend of n- and p-type hosts or a bipolar host seems to be an efficient method to enhance the performance of SL SM-WOLEDs. However, in these fully-phosphorescence devices, high ETs (>2.65 eV) are indispensable for the hosts to confine the blue triplet excitons, and thus would inevitably enlarge their energy gaps (Eg), leading to high carrier injection energy barriers at the organic/electrode interfaces. On the other hand, the use of a non-radiative bipolar host or blended hosts still increases the complexity of device fabrication. In this paper, we present an alternative concept for designing efficient SL SM-WOLEDs, where the single active organic layer is simply comprised of a newly synthesized sky-blue fluorescent bipolar host and a common orange phosphorescent dopant. This fluorescence and phosphorescence (F/P) hybrid concept has the following advantages as compared to the aforementioned fully-phosphorescence concept: (i) narrower Eg in a blue fluorescent host with respect to that of a non-radiative host for sensitizing blue phosphors, thus endowing more efficient carrier injection from the electrodes; (ii) simpler fabrication process with the single organic layer readily formed by co-deposition of only two organic materials; (iii) easier way to obtain white light by solely controlling the Föster energy transfer from the fluorescent host to the orange phosphor. A new compound, 4-(dibenzothiophene-S,S-dioxide-2-yl)triphenylamine (TPASO), was characterized to have well-matched frontier molecular orbital levels, highest occupied molecular orbital (HOMO) level of 5.25 eV and lowest unoccupied

molecular orbital (LUMO) level of 2.69 eV, with the work functions of the MoO3-modified ITO anode and the LiF/Al composite cathode. It also possesses a unit fluorescent quantum yield (Uf) in toluene, a sky-blue EL light with a peak wavelength at 493 nm, as well as a ET above 2.24 eV which is sufficiently high to sensitize those orange triplet excitons (ET  2.1 eV). The SL F/P-hybrid SM-WOLED doping TPASO with 0.2 wt% tris(2-phenylquinoline)iridium(III) Ir(2-phq)3 exhibits a warm-white emission with a Commission Internationale de L’Eclairage (CIE) coordinate recorded at (0.42, 0.45) at 1000 cd m2, and a low turnon voltage of 2.4 eV which is significantly reduced as compared to those (>3.5 V) reported for SL fully-phosphorescence SM-WOLEDs [24–27]. In addition, impressive maximum current/power efficiencies (CE/PE) of 11.27 ± 0.02 cd A1 and 14.15 ± 0.03 lm W1 were also observed, suggesting that efficient WOLEDs with such an extremely simplified configuration could be realized. 2. Experimental 2.1. General information 1

H spectra was measured with a Varian Gemin-400 spectrometer. Mass spectra were recorded on a Finnigan 4021C GC–MS spectrometer. Elemental analysis was performed on a Vario EL III microanalyzer. The film samples (30 nm-thick) were prepared by vacuum deposition on quartz plate. The absorption and emission spectra were recorded on a Hitachi U-3010 UV–vis spectrophotometer and a Hitachi F-4500 fluorescence spectrophotometer, respectively. The film photoluminescence yield was measured by an integrating sphere method. Cyclic voltammetry was performed with a CHI600A analyzer with ascan rate of 100 mV s1 at room temperature. The electrolytic cell was a conventional three-electrode setup consisting of a glassy carbon working electrode, a Pt wise auxiliary electrode, and an aqueous saturated calomel electrode (SCE) as the reference. Tetra-n-butylammoniumhexafluorophosphate (TBAPF6, 0.10 M) was used as the supporting electrolyte and DMF or CH2Cl2 as the solvent, respectively. The ferrocene/ferrocenium couple was used as the internal standard. 2.2. Synthesis All solvents and materials were used as received from commercial suppliers without further purification. The synthetic route is shown in Scheme 1.

N

S

(HO)2B

Br

Pd(PPh3)4 / 2M Na2CO3 / Toluene / Ethanol

S

N

KMnO4 / MnSO4

O O S

N

CH2Cl 2

TPASO Scheme 1. Synthetic route of TPASO.

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Abs (Toluene, rt)

Normalized Intensity

1.0

Fluo (Toluene, rt) Phos (2-MeTHF, 77K)

0.8

Fluo (Film, rt) Phos (Film, 77K)

0.6 0.4 0.2 0.0 300

400

500

600

700

Wavelength (nm) Fig. 1. Room-temperature (rt) UV–vis and fluorescence spectra, and lowtemperature (77 K) phosphorescence spectra of TPASO in organic solvents (lines) and in neat film state (symbols).

4-(Dibenzothiophene-2-yl)triphenylamine: Toluene (8 ml), ethanol (3 ml), and 2 M aqueous sodium carbonate solution (6 ml) were added to a mixture of triphenylamine-4-boronic acid (580 mg, 2 mmol), 2-bromodibenzothiophene (395 mg, 1.5 mmol) and tetra-(triphenylphosphine)platinum (117 mg, 0.1 mmol). With stirring, the suspension was heated at 90 °C for 24 h under N2 protection. When cooled to room temperature, the mixture was extracted with CH2Cl2 and dried over Na2SO4. After the solvent had been removed, the residue was purified by column chromatography on silica gel using CH2Cl2/petroleum ether (1:6) as the eluent to give a white solid with a 63% yield (430 mg). 1H NMR (400 MHz, CDCl3) d [ppm]: 8.31 (s, 1H), 8.21–8.18 (m, 1H), 7.88–7.83 (m, 2H), 7.66 (dd, J1 = 8.3, J2 = 1.2, 1H), 7.58 (d, J = 8.5, 2H), 7.48–7.44 (m, 2H), 7.30–7.24 (m, 4H), 7.20–7.14 (m, 6H), 7.04 (t, J = 7.1, 2H). EI MS (m/z): 427.14. 4-(Dibenzothiophene-S,S-dioxide-2-yl)triphenylamine (TPASO): The mixture of 4-(dibenzothiophene-2-yl)triphenylamine (427 mg, 1 mmol), KMnO4 (500 mg, 3.12 mmol) and MnSO4H2O (500 mg, 2.97 mmol) were stirred continuously in CH2Cl2 (30 ml) for 10 h. The reaction mixture was then filtered and the residue was washed with additional CH2Cl2. The crude product was obtained by evaporation of the collected CH2Cl2 and was further purified via silica gel column chromatography using CH2Cl2/ petroleum ether (3:1) as the eluent to afford a light yellowish solid, with a 78% yield (360 mg). 1H NMR (400 MHz, CDCl3) d [ppm]: 7.94 (s, 1H), 7.87–7.84 (m, 3H), 7.70– 7.64 (m, 2H), 7.57–7.49 (m, 3H), 7.31 (t, J = 7.8, 4H), 7.18–7.14 (m, 6H), 7.09 (t, J = 7.3, 2H). EI MS (m/z):

Fig. 2. Cyclic voltammogram of TPASO with oxidation tested in CH2Cl2 and reduction tested in DMF. Inset: Calculated electronic HOMO and LUMO distributions of TPASO.

459.13. Elemental analysis calcd (%) for C30H21NO2S: C 78.41, H 4.61, N 3.05, S 6.98; found: C 78.39, H 4.62, N 3.07, S 6.96. 2.3. Device fabrication and measurement An ITO-coated glass with a sheet resistance of 15 X square1 was used as the substrate. Before device fabrication, the ITO glass substrates were cleaned with isopropyl alcohol and deionized water, dried in an oven at 120 °C, treated with UV–ozone, and transferred to a vacuum deposition system with a base pressure below 1  106 torr for organic and metal depositions. Devices were fabricated by evaporating the organic layers at a rate of 1–2 Å s1. The cathode was completed through the thermal deposition of LiF at a rate of 0.1 Å s1 and then Al metal at 10 Å s1. The overlap between ITO and Al electrodes was 3.3  3.3 mm2 as the active emissive area of devices. EL spectra and International Commission on illumination color coordinates were measured using a Spectrascan PR650 photometer. The current–voltage characteristics were measured using a computer-controlled Keithley 2400 Sourcemeter under ambient atmosphere. Carrier mobilities were evaluated using thespace-charge-limited current (SCLC) method with current density–voltage (J–V) measurements of their holeand electron-only devices. The hole- and electron-only devices were fabricated with respectively configurations of ITO/MoO3 (2 nm)/TPASO (100 nm)/MoO3 (10 nm)/Al

Table 1 The physical properties of TPASO.

TPASO a b c d

kabsa (nm)

kfa/b (nm)

Ufa/b

ETa/b (eV)

EOxc (V)

ERec (V)

HOMO (eV)

LUMO (eV)

ECVd (eV) g

307, 368

444/491

1/0.237

2.42/2.24

0.45

2.11

5.25

2.69

2.56

In toluene. In film. The onset potential of oxidation (EOx) or reduction (ERe) as referenced to ferrocene/ferrocenium (4.8 eV, relative to the vacuum level). Energy gap calculated from the energy difference between the HOMO and the LUMO.

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3

10

Hole-only Electron-only

2

10

1.0

Normalized Intensity

Current density (mA cm-2)

(a)

1

10

0

10

-1

10

-2

10

-3

10

Abs of neat Ir(2-phq)3 film PL of neat TPASO film

0.8 0.6 0.4 0.2

-4

10

0.0

-5

10

-1

0

10

300

1

10

10

(b) Mobility (cm2 V-1s-1)

500

600

700

Fig. 4. Absorption spectra of a 20-nm-thick neat film of Ir(2-phq)3 and PL spectra of TPASO film.

0

10

Hole Electron

-1

10

cm1) is the free space permittivity, E is the electric field, L is the thickness of active layer, and b is the field-activation factor.

-2

10

-3

10

-4

10

-5

10

3. Results and discussion

-6

10

TPASO was prepared by Suzuki cross-coupling of 2-bromodibenzothiophene and triphenylamine-4-boracic acid, followed by further oxidization by KMnO4 (Scheme 1). Ultraviolet–visible (UV–vis) and photoluminescence (PL) spectrometers were used to examine the photophysical properties of TPASO (Fig. 1 and Table 1). The two absorption bands peaked at 307 nm and 368 nm can be ascribed to the intrinsic p–p⁄ transition of triphenylamine and the intramolecular charge transfer transition from the electron-donor to the acceptor, respectively. Bright blue fluorescent emission with a peak wavelength at 444 nm and a high Uf of 1 was observed from TPASO in a lowpolarity toluene solution, using the quinine sulfate in 1 N H2SO4 (Uf = 0.56) as the calibration standard [28]. The 77 K phosphorescence spectra of TPASO in 2-MeTHF presents a sharp highest-energy peak at 512 nm. To be more relevant to the situation in devices, we also prepared and measured neat thin-film samples of TPASO. Compared to those in solution, both the fluorescence and phosphorescence spectra undergo obvious red shifts with peak wavelength at 491 nm and 554 nm, respectively. These changes could be reasonably attributed to the surrounding polar medium in solid state. On the other hand, the Uf of the thin film was also affected, having a distinct decrease to 23.7%. However, while considering the high ET of 2.24 eV (2.42 eV) determined from the highest-energy peak of the

-7

10

-8

10

0.0

5

2.0x10

5

4.0x10

5

6.0x10

5

8.0x10

6

1.0x10

Electric field (V cm-1) Fig. 3. (a) J–V plots measured in the hole- and electron-only devices with the solid lines indicating the SCLC fitting curves, and (b) Electric fielddependent hole and electron mobility of TPASO.

and ITO/1,3,5-tri[(3-pyridyl)-phen-3-yl] benzene (TmPyPB) (10 nm)/TPASO (100 nm)/TmPyPB (10 nm)/LiF (1.5 nm)/Al. The mobilities were extracted from the J–V curves according to the Mott–Gurney square law, which is described as

J¼

400

Wavelength (nm)

Voltage (V)

 pffiffiffi 9 E2 ee0 l0 exp b E 8 L

ð1Þ

and

 pffiffiffi

l ¼ l0 exp b E

ð2Þ

where J is the measured current density, l0 is the zero-field carrier mobility, l is the field-dependent carrier mobility, e (3.0) is the relative permittivity, e0 (8.85  1014 C V1 Table 2 Fitting parameters in the SCLC measurement for TPASO. Compound

Hole

l0 (10 TPASO

2.09

Electron 4

2

cm V

1

s

1

)

b (10 4.08

3

(V cm

1 1/2

)

)

l0 (107 cm2 V1 s1)

b (103 (V cm1)1/2)

5.09

9.00

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100 cd m-2

(a) Normalized Intensity

1.0

500 cd m-2 1000 cd m-2

0.5

0.0 400

500

600

700

800

Wavelength (nm)

(b)

(c)

Fig. 5. (a) EL spectra at different luminances, (b) J–V–L and (c) CE–L–PE plots of the single-layer F/P hybrid SM-WOLED.

film (solution) phosphorescence spectra, TPASO is supposed to be capable of sensitizing an orange phosphor. Cyclic voltammetry (CV) measurement was carried out to explore the electrochemical properties of TPASO. As clearly shown in Fig. 2, TPASO undergoes both reversible oxidation and reduction processes, indicating a bipolar nature as well as a good electrochemical stability. Relative to the ferrocene/ferrocenium (Fc/Fc+) couple [29], the HOMO and LUMO levels of TPASO are determined from the onset potentials for oxidation and reduction,

respectively. The HOMO level of 5.25 eV is close to the work function of a MoO3-modified ITO anode (5.3 eV) [30], while the LUMO level of 2.69 eV is only slightly higher than the work function of a LiF/Al composite cathode (2.9 eV) [31]. Therefore, the resulting small energy barriers at the two proposed TPASO/electrode interfaces should lead to effective injection of both hole and electron. The energy gap (Eg) is calculated to be 2.56 eV, which indeed is much less than that of those non-radiative hosts for blue phosphors. With the aim to have a deeper insight into the

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3V 4V 5V 6V

Normalized Intensity

(a)

400

450

500

550

600

7V 8V 9V 10 V

650

700

Wavelength (nm) 1.8

External Quantum Efficiency (%)

molecular electronic structure, a DFT calculation at the B3LYP/6-31G(d) level was then performed on TPASO. It can be seen in Fig. 2 that the HOMO and the LUMO are dominantly locating on the electron-rich moiety of triphenylamine and the electron-deficient moiety of dibenzothiophene-S,S-dioxide, respectively, suggesting an obvious orbital spatial separation. To a large extent, this separation may enable TPASO to inherit intrinsic electronic properties from its constituting units of triphenylamine and dibenzothiophene-S,S-dioxide. This hypothesis is in fact supported by the fact that the HOMO and LUMO levels of TPASO are respectively very close to those of triphenylamine and dibenzothiophene-S,S-dioxide derivatives in literatures [32,33]. The hole and electron mobility of TPASO were also estimated by the space-charge-limited current (SCLC) method in hole- and electron-only devices (Fig. 3 and Table 2), respectively to be 7.59  104 and 8.77  106 cm2 V1 s1 (at an electric field of 1.0  105 V cm1). These good bipolar carrier transporting properties are another important premise for constructing efficient simple single-layer electroluminescent devices. To realize hybrid white emission from a single layer, we employ a low-concentration doping strategy, in order to controllably suppress the singlet Föster energy transfer from the blue fluorescent host to lower-energy phosphors through suitably increasing the average distance between them [34]. For the case in this work, the absorption of the used dopant, Ir(2-phq)3, has a good overlap with the PL emission of TPASO (Fig. 4). As a result of low-level doping, the blue fluorescence can still be sufficiently maintained, whilst, the other excitons of triplet state can still be harvested by those phosphors with low-lying ET due to their typical long life-time and long diffusion distance. Recently, basing on this strategy, our group has successfully demonstrated several efficient multilayer F/P hybrid SM-WOLEDs [34–36]. Here, we adopt an optimized doping concentration of 0.2 wt%, and fabricated a single-layer F/P hybrid SM-WOLED of anode/TPASO: 0.2 wt% Ir(2-phq)3 (80 nm)/ cathode, where the anode and cathode stand for 2 nmMoO3-modified ITO and LiF (1.5 nm)/Al, respectively. Fig. 5(a) shows the warm-white EL spectra which comprise the sky-blue fluorescence at 493 nm from TPASO and the orange phosphorescence of 577 nm from Ir(2-phq)3. CIE coordinates are recorded at (0.42, 0.45) at a practical luminance of 1000 cd m2. Meanwhile, it is also notable that with increased brightness, the spectra reveal a gradual increase (decrease) in the contribution from the blue

1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2

(b)

0.0 0

1000

2000

3000

4000

5000

Luminance (cd m-2) Fig. 6. (a) EL spectra at different voltages, (b) EQE–L characteristic of the single-layer sky-blue fluorescent device. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

fluorescence (the orange phosphorescence). This spectral variation may mainly arise from the triplet loss caused by T–T annihilation at high current densities. Moreover, for the simple single-layer device, the electrically-generated triplet excitons, which decay much more slowly than the singlets do inherently, should have a rather bigger chance to migrate and approach the electrodes, then quenched non-radiatively. The current density–voltage–luminance

Table 3 Performance summary of the single-layer sky-blue and F/P hybrid white devices. Device

Vona (V)

kpeak (nm)

100 cd m2

1000 cd m2

CIEc

Sky-blue

2.5

493

3.73 ± 0.09, 4.46 ± 0.04, 1.64 ± 0.04

3.56 ± 0.10, 3.09 ± 0.09, 1.56 ± 0.05

3.64 ± 0.02, 2.24 ± 0.01, 1.59 ± 0.01

(0.18, 0.43)

White

2.4

493, 577

11.27 ± 0.02, 14.15 ± 0.03, 5.42 ± 0.02

9.65 ± 0.11, 8.49 ± 0.11, 4.63 ± 0.05

7.16 ± 0.08, 4.44 ± 0.05, 3.38 ± 0.04

(0.42, 0.45)

CEb (cd A1), PEb (lm W1), EQEb (%) Max

a b c

Turn-on voltage. CE, PE and EQE are current, power and external quantum efficiency, respectively, averaged from four devices. At 1000 cd m2.

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(J–V–L) characteristics of the device are depicted in Fig. 5(b). Impressively, the white device turns on (a luminance of 1 cd m2 is obtained) at a very low voltage of 2.4 V. While considering that Ir(2-phq)3 only accounts for a very low ratio, it is reasonable to judge that TPASO take the main responsibility for the injection and transport of both hole and electron. The low turn-on voltage therefore should be attributable to the small interface energy barriers for effective carrier injection, as well as to the bipolartransport ability of TPASO. Notably, those SL fully-phosphorescence SM-WOLEDs reported in literatures exclusively showed turn-on voltages above 3.5 V, presumably resulting from the use of high-Eg non-radiative hosts [24–27]. In this context, our novel device concept incorporating a blue fluorescent bipolar host might be more feasible in constructing low voltage-driving devices. The efficiencies are shown in Fig. 5(c) and Table 3. The maximum forward-viewing CE, PE and external quantum efficiency (EQE) are 11.27 ± 0.02 cd A1, 14.15 ± 0.03 lm W1 and 5.42 ± 0.02%, respectively. With the luminance increasing from 100 to 1000 cd m2, the CE gradually decreases from 9.65 ± 0.11 to 7.16 ± 0.08 cd A1. These efficiencies are actually still lower than those of complicated F/ P-WOLEDs that employ a series of organic functional layers to optimize carrier utilizations. Therefore, we then fabricated a non-doped single-layer fluorescent device of ITO/ MoO3 (2 nm)/TPASO (80 nm)/LiF (1.5 nm)/Al as the simplest prototype device. The device exhibits a stable skyblue emission with a CIE coordinate of (0.18, 0.43) (Fig. 6(a)). What is noteworthy is that the EQE reached a maximum value of only 1.68% (Fig. 6(b)), which means that there is still large room for further performance enhancement as EQE for traditional non-doped fluorescent devices is well recognized to have a limit up to 5%. This target might be realized by optimizing the mobility balance and filmstate quantum yield of the blue fluorescent hosts in the future. However, in terms of the extremely simplified device structure, the results of the single-layer devices in this work are still quite intriguing.

50825304, 51033007, 51103169, and 51128301), the Beijing Natural Science Foundation (No. 2111002), the National High-tech R&D Program of China (863 Program) (Grant No. 2011AA03A110) and the Instrument Developing Project of the Chinese Academy of Sciences (Grant No. YE201133), PR China.

4. Conclusions

[23]

In summary, basing on a new sky-blue fluorescent bipolar host (TPASO), we established a novel device concept (F/P hybrid) for SL SM-WOLEDs with only the new host and an orange phosphor coexistent in the single active organic layer between two electrodes. The device exhibited efficient warm-white emission with a CIE coordinate of (0.42, 0.45) at 1000 cd m2, a low turn-on voltage of 2.4 V as well as impressive maximum CE and PE of 11.27 ± 0.02 cd A1 and 14.15 ± 0.03 lm W1, respectively. The novel device concept for SL SM-WOLEDs may provide a new method to the fabrication of simple, low-cost and efficient white light-emitting devices.

[24]

Acknowledgements

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C.S. Lee would like to acknowledge support from the Research Grants Council of the Hong Kong (Project No. T23-713/11) and X.H. Zhang acknowledges support from the National Natural Science Foundation of China (Nos.

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