Solution-Process-Feasible Deep-Red Phosphorescent Emitter

Article pubs.acs.org/JPCC

Solution-Process-Feasible Deep-Red Phosphorescent Emitter Jwo-Huei Jou,*,† Yu-Ting Su,† Ming-Ting Hsiao,† Hui-Huan Yu,† Zhe-Kai He,† Shen-Chin Fu,† Chi-Heng Chiang,† Chien-Tien Chen,*,‡ Chia-hung Chou,‡ and Jing-Jong Shyue‡ †

Department of Materials Science and Engineering and ‡Department of Chemistry, National Tsing Hua University, Hsinchu, Taiwan 30013, Republic of China § Research Center for Applied Sciences, Academia Sinica, Taipei, Taiwan 11529, Republic of China ABSTRACT: Solution processing enables organic devices to be fabricated cost-effectively, while deep-red emitters enable displays with high color saturation and lighting with high light quality. However, limited deep-red emitters were reported with solutionprocess feasibility and high efficiency. We demonstrate here a high-efficiency, solution-process-feasible deep-red emitter by coupling the highly efficient phosphorescent complex with a highly thermally stable fluorescent compound. The device exhibits a maximum external quantum efficiency of 11.2% at 10 cd m−2 and 8.4% at 100 cd m−2. The latter is coupled with an ultradeep red emission (0.70, 0.27) and a potential color saturation of 108%. Besides the intrinsically high-efficiency nature of the phosphorescent complex, the record high efficiency may be attributable to the spirally configured fluorene moiety in the fluorescent compound to prevent concentration-quenching effect, a proper host to enable an effective host-to-guest energy transfer, and the employed cohost with electron-trapping character to enable a balanced carrier injection.

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efficient and extremely stable P−I−N architecture.22 Hwang’s group reported an EQE of 4.3% with (0.69, 0.32) by incorporating a deep-red iridium complex.23 Wang’s group proposed an EQE of 3.0% with (0.70, 0.29) by incorporating a thermally activated delayed fluorescence (TADF) emitter.24 By using a wet process, Lin et al. reported an EQE of 17.7% with (0.58, 0.41) for a small-molecule-based orange-red device.25 Lee and Jin’s group published an EQE of 18.0% with (0.64, 0.34) by employing a mixed-host system.26 Burn and Samuel’s group proposed an EQE of 3.8% with (0.67, 0.33) by incorporating a phosphorescent dendrimer.27 Wang’s group obtained an EQE of 5.0% with (0.67, 0.33) by incorporating an iridium-dendrimer-based emitter.28 Zhang et al. published an EQE of 5.8% with (0.68, 0.31) by using a heteroleptic iridium dye.29 Lee’s group proposed an EQE of 3.0% with (0.69, 0.29) by introducing an o-carborane to an iridium complex.30 Cho’s group published an EQE of 6.1% with (0.70, 0.30) with a carbazole-branched dendrimer having an iridium core.31 Bryce’s group proposed an EQE of 2.5% with (0.70, 0.27) with a porphyrin-fluorene dye.32 As noted above, the efficiencies of red devices are comparatively high, but much limited in the orange-red or red region, while deep-red emission can be observed in some devices but with relatively low efficiency. To achieve high device efficiency, three major approaches can be applied. They include the design and synthesis of

INTRODUCTION Organic light-emitting diode (OLED) has become a mainstream technology in the applications of display as well as lighting.1−8 The fabrication of OLEDs is either dry-process or solution-process. The latter is advantageous in enabling large area size, flexibility, roll-to-roll manufacturing, and costeffectiveness.9 To make OLED displays more competitive, a better color saturation is necessary. To enhance lighting quality of OLEDs, a spectrum with an extremely high color rendering index or even an extremely high natural light spectrum resemblance index is highly desirable.10,11 In addition, a blue-hazard free lighting source is demanded in order to safeguard human health.12−15 All of these necessitate a deep-red emission with Commission International de L’E-clairage x (CIEx) coordinate greater than 0.67.16,17 Moreover, a lighting source with a deepred or near-infrared emission is needed in bioimaging and biomedical applications.18 Over the past years, some efficient red OLEDs had indeed been published. Taking the devices using dry-process for example, Jou’s group proposed, at 100 cd m−2, an external quantum efficiency (EQE) of 20.3% with CIE coordinates of (0.57, 0.42) by incorporating stepwise energy-level architecture with double emissive layers.19 Chi and Shu’s groups reported an EQE of 19.9% with (0.64, 0.36) by incorporating a fluorinebased bipolar host material with an osmium phosphor.20 Fukagawa’s group proposed an EQE of 18.5% with (0.66, 0.34) by incorporating a novel platinum complex.21 Walzer’s group reported an EQE of 12.4% with (0.68, 0.32) by using a highly © 2016 American Chemical Society

Received: August 1, 2016 Published: August 2, 2016 18794

DOI: 10.1021/acs.jpcc.6b07740 J. Phys. Chem. C 2016, 120, 18794−18802

Article

The Journal of Physical Chemistry C electroluminescent (EL) active materials, the design and employment of efficiency-effective device architectures, and the incorporation of internal and external light out-coupling technologies. From material perspective, phosphorescent as well as TADF emitters are attracting increasing attention due to their high quantum efficiency. From a device structure perspective, they include the employment of low interfacial resistance P−I−N structures,33−35 low carrier-injection barriers,36−39 balanced carrier injection,40−45 carrier and exciton confinement,46−50 stepwise emissive layers,19 carrier modulation layers,51,52 structures enabling exciton to generate on host or on both host and guest,53 structures facilitating host-toguest energy transfer,54and co-host structures.55−57 From a light out-coupling perspective, the efficiency enhanced techniques include the employment of macroextractors, microlens thin-film arrays, photonic crystals, structured substrate surfaces, high refractive index glass, and low refractive index layers, as well as reduction of metal surface plasmons and orientation control of the molecular dipoles.58 Nevertheless, employing a highly efficient phosphorescent emitter in wellengineered devices is the most determinant factor in achieving high efficiency. Having a robust emitter is crucial to film integrity and device reliability. We propose here a new approach to design a high-efficiency emitter with robustness by merging a highly thermally stable seven-member-ring-based fluorophore with highly efficient iridium complex based phosphor. The resultant deep-red emitter [bis(spirofluorenedibenzosuberene[d]quinoxaline -C2,N)-monoacetylacetonate]iridium(III) Ir(SQ)2(acac) has shown good solution-process feasibility as well as high device efficiency as a proper mixedhost system, such as a N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)-benzidine (NPB) host mixed with a cohost of bis(10hydroxybenzo[h]quinolinato)beryllium (Bebq2),59−61 is employed. The resulting device shows an EQE of 11.2% at 10 cd m−2 and 8.4% at 100 cd m−2 with (0.72, 0.27) and (0.70, 0.27) by doping 10 wt % of the red emitter into the mixed-host system. As the doping concentration increases to 15 and 20 wt %, for example, the chromaticity can shift to about ultradeep red (0.72, 0.27) with 7.4% and 6.9% EQE, respectively, at 100 cd m−2. The EQE is the highest among all proposed solutionprocessed deep-red devices (Figure 1).

Figure 1. Comparison of the CIE diagram of the EQE results of the red OLEDs with a solution-processed emissive layer containing the novel deep-red emitter Ir(SQ)2(acac), against those previously reported. All these data are compared at 100 cd m−2.

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EXPERIMENTAL SECTION Device Fabrication. Figure 2 shows the studied red OLED device structures and corresponding energy level diagrams. The device consists of a 125 nm indium tin oxide (ITO) anode layer, a 35 nm poly(3,4-ethylene-dioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) hole-injection layer (HIL), a 25 nm emissive layer with the red light-emitting dye, Ir(SQ)2(acac), doped in a NPB host, a 40 nm 1,3,5-tris(Nphenyl-benzimidazol-2-yl)benzene (TPBi) electron-transporting layer (ETL), a 1 nm lithium fluoride (LiF) layer, and a 150 nm aluminum (Al) cathode layer. Besides the NPB host, four other hosts were also investigated. They are 3,5-di(9H-carbazol9-yl)tetraphenylsilane (SimCP2), 4,4′,4″-tri(N-carbazolyl)triphenylamine (TCTA), 2,7-bis(carbazo-9-yl)-9,9-ditolylfluorene (Spiro-2CBP), and 4,4′-bis(carbazol-9-yl)biphenyl (CBP). Moreover, we have also fabricated a device by blending a cohost of Bebq2 in NPB host. The fabrication process included first spin coating an aqueous solution of PEDOT:PSS at 4000 rpm for 20 s to form a HIL on a precleaned ITO anode and then dried at 120 °C to remove residual solvent. Before

Figure 2. Schematic diagrams of the energy levels of the studied red OLEDs with (a) five different hosts: SimCP2, TCTA, Spiro-2CBP, NPB, and CBP and (b) a cohost system with an NBP host mixed with a bluish green Bebq2 cohost.

depositing the following emissive layer (EML), the solution was prepared by dissolving the host and guest molecule in tetrahydrofuran solvent and sonicated for 0.5 h at 50 °C. The resulting solution was then spin coated at 2500 rpm for 20 s. Followed by the TPBi, LiF and Al, were deposited by thermal evaporation under high vacuum of 5 × 10−6 Torr. Device Characterization. Photo Research PR-655 spectrascan was used to measure the luminance, CIExy, and EL spectra of the resultant red OLEDs. The characteristics of current−voltage (I−V) were measured by using a Keithley 2400 electrometer. The emission area of the devices was 9 mm2, and only the luminance in the forward direction was detected. Spectral Characterization. SpiroQ: MALDI-TOF-MS (m/z): 445.2 (M+H+, 100%). 1H NMR (400 MHz, CDCl3) 18795


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The Journal of Physical Chemistry C Scheme 1. Schematic Illustration of the Synthesis of the Red Dopant, Ir(SQ)2(acac)

δ 8.41 (dd, J = 7.2 Hz, 1.4 Hz, 2H), 8.32 (q, J = 3.4 Hz, 2H), 7.88 (q, J = 3.5 Hz, 2H), 7.75(d, J = 7.5 Hz, 2H), 7.49−7.45 (m, 2H), 7.32 (bs, 2H), 7.25−7.19 (m, 4H), 7.04 (bs, 2H), 6.72 (bs, 2H). 13C NMR (100 MHz, CDCl3) δ 153.17, 149.29, 145.70, 141.72,140.10, 138.01, 133.27, 130.21, 129.47, 129.38, 128.32, 127.86, 127.81, 127.06, 120.34, 66.42. Ir(SQ)2(acac): MALDI-TOF-MS (m/z): 1179.4 (M+H+, 3%). 1H NMR (400 MHz, CDCl3) δ 8.26 (dd, J = 8.3 Hz, 1.2 Hz, 2H), 8.21 (d, J = 8.8 Hz, 2H), 8.04 (dd, J = 7.7 Hz, 1.5 Hz, 2H), 7.83 (dd, J = 7.1 Hz, 1.3 Hz, 2H), 7.76−7.70 (m, 4H), 7.58 (d, J = 7.5 Hz, 2H), 7.51 (q, J = 7.4 Hz, 6H), 7.43 (td, J = 6.9 Hz, 1.1 Hz, 2H), 7.22 (td, J = 7.0 Hz, 1.6 Hz, 2H), 7.13− 7.07 (m, 4H), 6.74 (td, J = 7.3 Hz, 1.1 Hz, 2H), 6.69 (dd, J = 8.0 Hz, 1.0 Hz, 2H), 6.57 (d, J = 7.9 Hz, 2H), 6.39 (dd, J = 7.4 Hz, 1.0 Hz, 2H), 6.23 (t, J = 7.7 Hz, 2H), 4.60 (s, 1H), 1.48 (s, 6H). 13C NMR (100 MHz, CDCl3) δ 186.00, 164.23, 156.07, 153.50, 151.38, 150.47, 146.31, 144.11, 143.37, 141.48, 141.18, 140.27, 138.48, 137.33, 136.97, 133.55, 130.38, 130.28, 129.67, 129.25, 129.16, 128.15, 128.09, 127.82, 127.70, 127.32, 126.79, 125.80, 124.24, 123.20, 120.54, 120.19, 100.02, 66.48, 27.99. Quantum Chemical Calculation. The calculation of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbitals (LUMO) resulted from energyoptimized structure with Gaussian 09, Revision C.01 using the density functional theory (DFT) at the B3LYP level. The 631G basis set was employed for H, C, N, and O. For a heavy metal, iridium, pseudopotential and LANL2DZ basis set were employed. Visualizations of the molecular orbitals were performed with GaussView 5.08 with an isovalue of 0.02. Data Analysis. The error bars in Figure 7b were determined from the average efficiencies and their corresponding standard deviations based on testing four samples per device structure. Additionally, the different luminance values ranging from 10 to 500 were obtained by interpolation.

Figure 3. TGA thermogram for the Ir(SQ)2(acac) compound.

Figure 4. Ultraviolet−visible (UV−vis) and photoluminescence (PL) spectra of the deep-red emitter, Ir(SQ)2(acac), and its precursor, SpiroQ. The data were measured in dichloromethane at room temperature.

pentadione in a mixture of 0.636 g of sodium carbonate and 20 mL of 2-ethoxyethanol refluxing for 18 h. The final product, Ir(SQ)2 (acac), crude residue was purified by column chromatography (dichloromethane/hexanes, 1/3) on silica gel to give 212 mg (18%) of pure Ir(SQ)2(acac). Thermal Properties. The thermal properties of Ir(SQ)2(acac) were investigated by thermogravimetric analysis (TGA). Figure 3 shows a 5% weight loss decomposition temperature (Td) at 361 °C, indicative of its good thermal stability. The glass transition temperature was not found at temperature before reaching its decomposition temperature. Photophysical Properties. As shown in Figure 4, the UV−vis and PL spectra of the novel deep-red emitter Ir(SQ)2(acac) and its precursor SpiroQ were measured in

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RESULTS AND DISCUSSION Synthesis. Scheme 1 shows the synthesis route of the novel emitter, Ir(SQ)2(acac). The SpiroQ was synthesized by compound 3 and o-phenylenediamine with p-toluenesulfonic acid (p-TSA) in choloform refluxing for 24 h. In an inert atmosphere, the intermediate compound 4 was synthesized by 0.299 g of iridium(III) trichloride hydrate and 1.334 g of SpiroQ in a mixture of 0.636 g of sodium carbonate and 20 mL of 2-ethoxyethanol refluxing for 18 h. After cooling to room temperature, filtering, and washing with dichloromethane, the intermediate compound 3 was added with 0.30 g of 2,418796


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The Journal of Physical Chemistry C

calculations from DFT, the HOMO is highly localized at the vicinity of the fluorene moiety in the SpiroQ molecule (Figure 5). As the two SpiroQ molecules merged with Ir(acac), electrons move from the electron-rich fluorene to the electrondeficient Ir atom, making the state destabilized and leading to a −5.18 eV HOMO, more shallow than that of the SpiroQ (−5.84 eV). The reason why SpiroQ exhibits a deeper HOMO is because of the relatively large resonance coplanar structure on fluorene. Also due to the presence of the electron-deficient Ir atom, the LUMO state of the resultant Ir(SQ)2(acac) is stabilized. Hence, it yields a LUMO of −2.29 eV, which is lower than that of SpiroQ (−1.99 eV). The bandgap of the emitter is therefore much smaller for having a shallower HOMO and a lower LUMO. These explain why there is a marked bathochromic shift as Ir(acac) is incorporated into the SpiroQ molecules. Electrochemical Properties. Figure 6 shows the oxidation and reduction potential of Ir(SQ)2(acac) and SpiroQ measured by the cyclic voltammetry (CV) method. The energy level of HOMO was calculated from the oxidation potential of Ir(SQ)2(acac). The energy level of LUMO was obtained by adding the bandgap energy to the HOMO energy level. The resulting energy values are −5.4, −3.5, and 1.9 eV for HOMO, LUMO, and bandgap, respectively (Table 1). Neat Film Performance. Without a host, the devicecontaining neat film of pure Ir(SQ)2(acac) shows a nearinfrared emission peaking at 700 nm with CIExy of (0.72, 0.27) but with a very low maximum EQE, 0.01%. This indicates the necessity of the employment of a proper host or cohost system with an optimized doping concentration to boost up its efficiency performance. Host Incorporation. Table 2 summarizes the effects of host materials on the EL characteristics of the resultant red OLED devices. With the incorporation of a host, such as NPB, the Ir(SQ)2(acac)-containing device shows an EQE of 5.1% with CIExy (0.52, 0.21) at 100 cd m−2. Incorporating a host indeed enhances the efficiency very significantly, while it also causes an undesirable blue shift, resulting from an emission of the host NPB at 444 nm. The undesirable blue shift has been greatly

Figure 5. Effects of using the quinoxaline/diphenylfluorene hybrid as cyclometalating ligand on the electron distributions in the ground state (HOMO) and excited state (LUMO) of the present deep-red emitter, Ir(SQ)2(acac), and its precursor, SpiroQ, where the calculations were performed according to the density functional theory.

dichloromethane. In the precursor state, the peak of the UV− vis spectrum at 273 nm corresponds to the n−π* transitions and that at 352 nm to the charge-transfer transitions. The intense absorption bands of Ir(SQ)2(acac) below 450 nm can be matched to its ligand-centered π−π* transitions, while the long-wavelength absorption from 450 to 700 nm belongs to the admixture of spin-allowed singlet metal-to-ligand charge transfer (1MLCT) and spin-forbidden 3MLCT transitions. The reason why the emission exhibits an extremely large bathochromic phenomenon may be attributable to two folds, i.e., a much more shallow resultant HOMO and a lower resultant LUMO. On the basis of quantum chemical

Figure 6. Cyclic voltammograms of the deep-red emitter, Ir(SQ)2(acac), and its precursor, SpiroQ. 18797


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The Journal of Physical Chemistry C Table 1. Thermal, Photophysical, and Electrochemical Characteristics of SpiroQ and the Novel Deep-Red Emitter Ir(SQ)2(acac) compound

Tda (°C)

Mwb (g mol−1)

HOMOc (eV)

Egd (eV)

LUMOe (eV)

Eoxc (V)

Eredc (V)

Φf (%)

λPL (nm)

λabs (nm)

SpiroQ Ir(SQ)2(acac)

357 361

445 1,179

−5.98 (−5.84) −5.41 (−5.18)

3.22 (3.85) 1.89 (2.89)

−2.76 (−1.99) −3.52 (−2.29)

1.18 0.61

−2.04, −2.59 −1.78, −2.12

 51

407 686

273, 352 392, 485

a Decomposition temperature. bMolecular weight. cThe HOMO, Eox, and Ered values were measured by the CV method. dThe energy band gap was calculated from the onset of the absorption spectrum. eThe energy of LUMO was obtained by subtracting the bandgap from the HOMO energy level [ELUMO = EHOMO − Eg]. The data in parentheses were obtained by DFT calculations. fQuantum yield measured in dichloromethane at room temperature.

Table 2. Effects of Different Host Materials, and Doping Concentrations, on the Operation Voltage (OV), Power Efficiency (PE), Current Efficiency (CE), External Quantum Efficiency (EQE), CIE Coordinates, and Maximum Luminance of the Novel Deep-Red Emitter Ir(SQ)2(acac)-Containing OLED Devices Studied Herea OV (V)

PE (lm W )

−1

CE (cd A )

EQE (%)

CIE coordinates

dopant conc. (wt %)

cohost conc. (wt %)

100 15 15 5

   

6.5 /6.9/ /5.6/ /5.9/

0.002 /0.08/ /0.12/ /0.26/

0.001 /0.18/ /0.21/ /0.48/

0.01 /3.3/ /3.6/ /6.2/

(0.72, 0.27) /(0.70, 0.26)/ /(0.66, 0.25)/ /(0.58, 0.22)/ 

7 10 15 20 3 5 7 10 15 20 7 10 15 20 7

              10

/6.1/7.8 /5.8/7.5 /5.8/8.0 /5.5/7.4 /6.1/ /6.5/7.9 /6.9/8.4 /7.9/14 /6.1/7.8 /5.8/7.6 /4.4/6.4 /4.5/6.5 /4.8/6.7 /4.9/6.8 3.0/4.0/6.0

/0.21/0.08 /0.18/0.09 /0.14/0.07 /0.13/0.07 /0.26/ /0.24/0.10 /0.19/0.10 /0.15/0.05 /0.17/0.10 /0.15/0.09 /0.30/0.16 /0.28/0.15 /0.26/0.14 /0.21/0.11 0.77/0.52/0.27

/0.41/0.20 /0.33/0.21 /0.26/0.17 /0.23/0.16 /0.50/ /0.51/0.25 /0.40/0.27 /0.39/0.24 /0.32/0.26 /0.27/0.22 /0.41/0.32 /0.40/0.31 /0.40/0.30 /0.33/0.23 0.74/0.67/0.51

/6.4/ /5.7/2.6 /4.9/2.2 /4.5/ /6.2/ /7.1/ /6.7/ /6.5/ /6.4/4.0 /5.4/ /5.2/3.8 /5.9/4.1 /6.7/4.3 /5.6/3.4 11.1/9.7/6.3

VII-2

10

10

3.0/4.2/6.2

0.48/0.40/0.19

0.68/0.53/0.38

11.2/8.4/5.3

VII-3

15

10

3.1/4.4/6.7

0.51/0.31/0.15

0.49/0.43/0.31

8.7/7.4/4.8

VII-4

20

10

3.2/4.5/6.4

0.43/0.28/0.14

0.43/0.39/0.29

7.8/6.9/4.7

/(0.66, 0.25)/ /(0.70, 0.26)/(0.60, 0.26) /(0.71, 0.26)/(0.64, 0.27) /(0.71, 0.27)/ /(0.51, 0.20)/ /(0.60, 0.23)/ /(0.66, 0.25)/ /(0.68, 0.25)/ /(0.71, 0.27)/(0.67, 0.26) /(0.72, 0.27)/ /(0.52, 0.21)/(0.49, 0.20) /(0.61, 0.24)/(0.55, 0.22) /(0.65, 0.25)/(0.59, 0.23) /(0.68, 0.26)/(0.63, 0.24) (0.71, 0.27)/(0.67, 0.27)/(0.62, 0.27) (0.72, 0.27)/(0.70, 0.27)/(0.67, 0.26) (0.73, 0.27)/(0.72, 0.27)/(0.70, 0.27) (0.73, 0.27)/(0.73, 0.27)/(0.71, 0.27)

device I* II III IV-1 IV-2 IV-3 IV-4 IV-5 V-1 V-2 V-3 V-4 V-5 V-6 VI-1 VI-2 VI-3 VI-4 VII-1

a

−1

host  SimCP2 TCTA Spiro2CBP

CBP

NPB

NPBBebq2

maximum luminance (cd m−2)

@ 10/100/500 cd m−2 0.2 286 568 461 558 764 810 727 425 579 640 512 800 754 820 1286 1403 1083 1407 1218 1158 1242

The device marked with “*” is measured at maximum luminance.

predominantly form on the guest molecules due to its relatively deep electron trap (−0.8 eV) and relatively low hole injection barrier (0.5 eV) (Figure 2). By changing the host into Spiro-2CBP, the emission becomes redder as the red dye concentration increases from 5 wt % to 20 wt %, i.e. the corresponding CIExy red-shift from (0.58, 0.22) to (0.71, 0.27). Its EQE is increased from 6.2% to 6.4% as the doping concentration is increased from 5 wt % to 7 wt %. Above 7 wt %, the EQE starts dropping. The EQE rolls off at a concentration slightly higher (7 wt %) than that in the CBP host containing a counterpart (5 wt %). This can be attributed to the fact that some excitons can be formed on the host molecules because the Spiro-2CBP host has a hole injection barrier (0.1 eV) much smaller than that of the guest (0.5 eV), which would lead the majority of holes to enter into the Spiro-2CBP molecules. The holes formed thereon would in

diminished by increasing the original doping concentration from 7 wt % to 20 wt %; the CIExy consistently red-shifts from (0.52, 0.21) to (0.68, 0.26). Meanwhile, the EQE also increases from 5.1% to 6.6% with the increase of doping concentration from 7 wt % to 15 wt %. Above 15 wt %, the EQE starts to drop, plausibly resulting from concentration quenching. Host Effect. By further switching the host from NPB to CBP, the emission has similarly become redder as the red dye doping concentration increases from 3 wt % to 20%; the corresponding CIExy red-shifts from (0.51, 0.20) to (0.72, 0.27). The device shows a peak EQE of 7.1% at 5 wt %. Above 5 wt %, the EQE starts to drop. It is important to note that as compared with the NPB-containing counterparts the EQE rolls off at a comparatively lower doping concentration in the CBPcontaining devices. It may be attributed to exciton quenching occurring on the guest molecules. It is because excitons would 18798


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Figure 7. Host effects on the (a) EQE, (b) EQE with error bars, (c) luminance, and (d) current density results of the deep-red dye Ir(SQ)2(acac)containing OLED devices.

Figure 9. AFM image of the surfaces of the films that comprise a 10 wt % deep-red emitter doped into the hosts of (a) NPB and (b) NPB/ Bebq2. Both films were spin-coated on the PEDOT:PSS layer as employed in devices.

turn help attract the electrons from the ETL, which originally had an extremely high electron injection barrier (1.0 eV), unfavorable to the injection of electron. Since most of the holes prefer to form on the host, this leaves little space for a hole to form on the guest initially. However, with the deep electrontrapping character, the electrons that enter into the guest molecules would help attract holes, and as a result, not null but much less excitons would generate on the guest. These explain why exciton quenching and/or concentration quenching occurs at a concentration slightly higher in the Spiro-2CBP-containing devices than the CBP-containing counterparts. As the host is changed from CBP to TCTA, the EQE becomes much lower, which is 3.6% at 15 wt % and 6.4% for CBP. The much lower efficiency can be attributed to the fact that much fewer excitons could generate on the TCTA host molecule because it has a hole injection barrier (0.8 eV) higher than that of the guest (0.56 eV) and also has an unfavorable electron injection barrier (0.4 eV), contrary to the highly favorable electron-trapping character (−0.2 eV) possessed by the CBP host. Relatively, much less excitons, if not none at all, could hence be formed on the TCTA host, and likewise less host-to-guest energy transfer would take place. These explain

Figure 8. Cohost effect on the EL spectra at 10, 100, and 500 cd m−2. The undesirable blue emission from the NPB host is diminished upon the mixing of the cohost Bebq2.

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Figure 10. NTSC color saturation of the deep-red devices composing the NPB host and the Bebq2 cohost doped with a respective 7, 10, 15, and 20 wt % red dye, Ir(SQ)2(acac).

containing glass substrates. Both the single-host and mixed-host films possessed smooth surfaces with low roughness average (Ra), indicating a good miscibility between the hosts and dopant. Additionally, the mixed host NPB/Bebq2 film showed a smaller Ra (0.27 nm) than that of the single host counterpart (0.30 nm), implying a better uniformity to result upon mixing with the cohost. NTSC Color Gamut. The NTSC color gamut can be improved from 100% to 102, 108, 111, and 112% as the NTSC standard red is replaced by the deep-red devices composing the NPB host and the Bebq2 cohost doped with a respective 7, 10, 15, and 20 wt % red dye, Ir(SQ)2(acac), as shown in Figure 10.

why the TCTA host-containing device shows much poorer efficiency performance. For the same reasons, the SimCP2containing device also shows comparatively poor efficiency, whose EQE is 3.3% at 100 cd m−2. To note, the device shows a 1.2 eV hole injection barrier and a 0.2 eV electron injection barrier. Figure 7 summarizes the performance of the deep-red OLEDs with the five different hosts at their optimized conditions. From an EQE perspective, the CBP host-containing device shows a maximum EQE of 8.3% at 11 cd m−2. From a luminance perspective, the NPB host-containing counterpart shows the highest luminance, 1400 cd m−2, at 8.5 V. The device also shows the highest current density of 862 mA cm−2 at 9 V. Cohost Effect. As noted above, the employment of an electron-transporting host into the emissive layer can markedly enhance the efficiency performance of the red devices. Among, the host CBP shows the highest EQE due to a favorable electron-trapping character, while all the other hosts possess an electron barrier. Therefore, the device efficiency might be improved if an electron-transporting cohost is incorporated into the latter device systems. Taking the NBP host-containing device system for example, the EQE can be increased from 5.9% to 8.4%, an increment of 42%, as a 10 wt % bluish-green Bebq2 cohost is introduced into the emissive layer (Device VII2). Meanwhile, a red shift occurs with the CIExy changing from (0.61, 0.24) to (0.70, 0.27); i.e., its color changes from purplish red to deep-red. Notably, the NPB/Bebq2 mixed-host device shows a maximum EQE of 11.2% at 10 cd m−2 with a deep-red emission peaking at 688 nm. Figure 8 shows the EL spectra of the NPB and NPB/Bebq2 devices doped with a 10 wt % Ir(SQ)2(acac) at 10, 100, and 500 cd m−2. Throughout the investigated luminance, the unwanted blue emission from the host NPB can be totally diminished upon the mixing of the cohost Bebq2, enabling a desired deep-red emission. Figure 9 shows the images of the films of the host NPB and the mixed-host NPB/Bebq2 incorporated with 10 wt % deepred dye Ir(SQ)2(acac) by using atomic force microscopy (AFM). The films were spin-coated on the PEDOT:PSS layer

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CONCLUSIONS

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AUTHOR INFORMATION

We demonstrate here a new approach to design a highly thermally stable as well as highly efficient emitter with solutionprocess feasibility by merging a robust seven-membered-ring fluorophore with an iridium complex based phosphor. By using the resulting emitter, a deep-red device with the highest EQE has been fabricated. The device exhibits, for example, a maximum EQE of 11.2% at 10 cd m−2 and 8.4% at 100 cd m−2 with deep-red emission (0.70, 0.27) by spin-coating. The proposed red emitter could serve as a promising candidate in high-quality display fabrication or physiologically friendly and high efficiency lighting panels with an extremely low color temperature as well as a highly energy-saving character using solution-process.

Corresponding Authors

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

The authors declare no competing financial interest. 18800


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ACKNOWLEDGMENTS The support of this work via grants MEA 104-EC-17-A-07-S3012 and MOST 104-2119-M-007-012 and 103-2923-E-007003-MY3 is gratefully acknowledged.

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Solution-Process-Feasible Deep-Red Phosphorescent Emitter

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