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Enhanced Electron Affinity and Exciton Confinement in Exciplex-Type Host#Power Efficient Solution-Processed Blue Phosphorescent OLEDs with Low Turn-on Voltage Xinxin Ban, Kaiyong Sun, Yueming Sun, Bin Huang, and Wei Jiang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b10335 • Publication Date (Web): 04 Jan 2016 Downloaded from http://pubs.acs.org on January 10, 2016

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ACS Applied Materials & Interfaces

Enhanced Electron Affinity and Exciton Confinement in Exciplex-Type Host：：Power Efficient Solution-Processed Blue Phosphorescent OLEDs with Low Turn-on Voltage Xinxin Ban, Kaiyong Sun, Yueming Sun*, Bin Huang, and Wei Jiang* School of Chemistry and Chemical Engineering, Southeast University, Nanjing, Jiangsu, P. R. China 211189

Abstract:

A

benzimidazole/phosphine

oxide

hybrid

1,3,5-tris(1-(4-(diphenylphosphoryl)

phenyl)-1H-benzo[d]imidazol-2-yl)benzene (TPOB) was newly designed and synthesized as the electron-transporting

component

to

form

an

exciplex-type

host

with

the

conventional

hole-transporting material tris(4-carbazoyl-9-ylphenyl)amine (TCTA). Due to the enhanced triplet energy and electron affinity of TPOB, the energy leakage from exciplex-state to the constituting molecule was eliminated. Using energy transfer from exciplex-state, solution-processed blue phosphorescent organic light-emitting diodes (PHOLEDs) achieved an extremely low turn-on voltage of 2.8 V and impressively high power efficiency of 22 lm W-1. In addition, the efficiency roll-off was very small even at luminance up to 10000 cd m-2, which suggested the balanced charge transfer in the emission layer. This study demonstrated that molecular modulation was an effective way to develop efficient exciplex-type host for high performanced PHOLEDs.

Keywords: exciplex, solution-process, blue emission, turn-on voltage, organic light emitting diode

ACS Paragon Plus Environment


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■ Introduction Solution-processed organic light-emitting diodes (OLEDs) have attracted much scientific and industrial attention due to the low cost manufacturing technology, the processability over large area size, and compatibility with flexible sustrates.1-6 Although phosphorescent OLEDs (PHOLEDs) can approach 100% internal quantum efficiency by harvesting both singlet and triplet excitons, the excited state quenching induced by the triplet-triplet annihilation should be minimized to achieve highly efficient electroluminance.7 Thus, polymer,8-11 dendrimer,12-14 and bipolar host materials were elaborately designed to improve the external quantum efficiency of the solution-processed PHOLEDs over the past decade.15-19 Nevertheless, the low operating voltage is still a main challenge in solution-processed OLEDs due to the electron traps induced by the loose molecular accumulation and solvent impurity.20 In most cases, the turn-on voltages of blue phosphorescent OLEDs based on spin-coating are above 5 V, which simultaneously lead to the low power efficiency in large area application.21-25 Furthermore, the serious efficiency roll-off induced by the unbalanced charge transfer at high luminance also restricts its commercial prospects. Recently, developing host material with small triplet-singlet split (∆EST) was demonstrated to be an effective way to reduce the operating voltage of PHOLEDs.26-28 However, the exploitation of bipolar host with sufficiently small ∆EST, especially for solution-processed PHOLEDs, is a rather difficult task because of the severe limitation of molecular design.29 Fortunately, a new class of efficient small molecular co-host, which forms exciplex-state through the intermolecular interaction between donor and acceptor components, was developed to be an alternative strategy to achieve host material with intrinsically small ∆EST due to the complete frontier orbital separation.30-32 Importantly, the exciplex-type co-host possesses an inherent bipolar characteristic, which would facilitate the charge balance in the emission layer (EML) and minimize the efficiency roll-off at high luminance.33-36 Using phosphorescence dopant as emitter and exciplex as host, Kim et al. have reported that the vacuum-deposited blue and green PHOLEDs could achieve impressively high efficiencies and low operating voltages.37 Therefore, it is native to expect that using exciplex host system would be a promising approach for getting high power efficient solution-processed PHOLEDs with low operating


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voltage. However, there is few reports on exciplex-type host used for solution-processed PHOLEDs due to the lack of solution-processable electron-transporting material, which can efficiently forming exciplex-state with the conventional hole-transporting molecule. In this work, we designed and synthesised a new electron-transporting material TPOB based on 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBI) core and diphenyl phosphine oxide (DPPO) peripheral moieties, which could efficiently form an exciplex with tris(4-carbazoyl-9-ylphenyl)amine (TCTA) in the 1:1 molar blend film. DPPO groups were introduced to increase the solubility and to enhace the electron affinity of TPOB. In additon, the star shaped molecules and highly twisted configurations of the two components endow the exciplex good film forming ability through wet process. By using energy transfer from the exciplex, solution-processed PHOLEDs achieved a very high power efficiency of 22 lm W-1, and an extremly low turn-on voltage of 2.8 V for blue electroluminescent device. To the best of our knowledge, these performances are comparable with the state-of-the-art blue PHOLEDs fabricated by solution-porcess.

■ Experimental section The synthetic procedure for TPOB was illustrated in Scheme 1. The dry nitrogen atmosphere was used in the reactions involving air-sensitive reagents. 1,3,5-tris(1-(4-iodophenyl)-1H-benzo[d]imidazol-2-yl)benzene (TPBI-I3): N-(4-iodophenyl)-1,2phenylenediamine (2.0 g, 6.5 mmol) was dissolved in 10 mL 1-methyl-2-pyrrolidinone. Then, 1,3,5-benzenetricarbonyl trichloride (0.55 g, 2.0 mmol) was added to the solution. The reaction mixture was stirred at 50 °C for 2 h under nitrogen. After cooling, 50 mL water was added and the resulting precipitates were filtered. The crude tribenzamide was refluxed in 400 mL acetic acid for 20 h under nitrogen. After completion of the reaction, the acetic acid was removed and the crude residue was purified by column chromatography on silica gel with ethyl acetate/petroleum (1:5) as the eluent to give the white product (1.5g, 75 %). 1H NMR (300 MHz, CDCl3, δ): 7.89 (s, 3H), 7.87 (d, J = 7.5 Hz, 3H), 7.81 (d, J = 8.3 Hz, 6H), 7.33 (ddd, J = 18.2, 12.8, 5.2 Hz, 6H), 7.18 (d, J = 7.9 Hz, 3H), 6.95 (d, J = 8.2 Hz, 6H).

13

C NMR (75 MHz, CDCl3, δ): 150.8, 142.2, 137.6, 135.3, 133.5, 131.3,



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131.5, 129.7, 124.5, 123.6, 123.2, 120.5, 110.1. HRMS [m/z]: calcd for C45H27I3N6, 1031.9431; found, 1032.9492 [M+H+]. Anal. Calcd. for C45H27I3N6: C, 52.35; H, 2.64; N, 8.14. Found: C, 52.38; H, 2.66; N, 8.13. 1,3,5-tris(1-(4-(diphenylphosphoryl)phenyl)-1H-benzo[d]imidazol-2-yl)benzene

(TPOB):

A

mixture of TPBI-I3 (1.0 g, 1.0 mmol), diphenylphosphine oxide (0.73 g, 3.6 mmol), CuI (0.06 g, 0.30 mmol), Cs2CO3 (1.5 g, 4.5 mmol) and L-proline (0.04 g, 0.30 mmol) were added to 30 mL toluene. The mixture was refluxed under nitrogen for 48 h. After cooling, the reaction was quenched with water and extracted with CH2Cl2. The organic layer was dried by anhydrous MgSO4 and the solvent was removed in vacuum. Then, the product was purified by column chromatography using ethyl acetate/methanol (5:1) as eluent to afford a white solid (0.64, 52%). 1H NMR (300 MHz, CDCl3, δ): 7.93 (d, J = 7.8 Hz, 3H), 7.86 (s, 3H), 7.80 - 7.67 (m, 6H), 7.48 (dd, J = 12.0, 7.7 Hz, 16H), 7.35 (d, J = 7.3 Hz, 9H), 7.24 - 7.10 (m, 20H). 13C NMR (75 MHz, CDCl3, δ): 149.78, 142.71, 139.20, 136.53, 134.70, 133.94, 133.80, 133.29, 132.25, 131.75, 131.61, 131.06, 130.76, 130.37, 128.67, 128.51, 127.33, 127.17, 124.39, 123.83, 120.38, 110.54. HRMS [m/z]: calcd for C81H57N6O3P3, 1254.3705; found, 1255.3759 [M+H+]. Anal. Calcd. for C81H57N6O3P3: C, 77.50; H, 4.58; N, 6.69. Found: C, 77.55; H, 4.56; N, 6.70.

Scheme 1 Synthetic route and chemical structure of TPOB.

■ Results and Discussion TPOB

was

easily

synthesized

by

the

Ullmann

coupling

reaction

of

1,3,5-tris(1-

(4-iodophenyl)-1H-benzo[d]imidazol-2-yl)benzene with excess diphenyl phosphine oxide using


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CuI/L-proline/Cs2CO3 catalyst system in 69% yield. Prior to device fabrication, the product was purified by silica gel column chromatography and recrystallization to give white powders. 1H NMR, 13

C NMR, HRMS, and elemental analysis were used to confirm the chemical structure of the new

compounds.38 The density functional theory (DFT) calculations were performed to predict the electronic properties of the compounds. In general, the molecular orbital distributions play an important role in charge transporting and a more delocalised LUMO (lowest unoccupied molecular obital) allows better intermolecular orbital overlap, which would lead to easier electron transport by hopping.39, 40 Figure 1 shows the sketch of the LUMO and LUMO+1 of TPOB and TPBI, the similar electronic delocalisation of TPOB related to TPBI indicates the excellent charge transporting property of TPOB. Due to the strong electron withdrawing nature of diphenyl phosphine oxide moieties, the LUMO energy level of TPOB (-1.7 eV) was much lower than that of TPBI (-1.2 eV), which suggests that TPOB could serve as a better electron-transproting material owing to the enhanced electron injection ability. In additon, the triplet energy (T1) of TPOB was calculated to be 2.87 eV (Table 1), which was 0.09 eV higher than that of TPBI (2.78 eV) because of the more twisted molecular configuration induced π-conjugation interruption. Furthermore, the electron transfer rate (ket) was also estimated by semiclassical Marcus theory as following equation,41, 42

k =

exp −

(1)

where HAB is the charge transfer integral, λ is the reorganization energy and T is the absolute temperature. The h and kB are the Planck and Boltzmann constant, respectively. Due to the less variable quantity of HAB in the amorphous organic solids, the mobility of electron should be mainly dominated by its respective reorganization energy λ(e) in the exponential term.43 By directly computing from the adiabatic potential-energy surfaces of neutral and anion species at the B3LYP functional using 6–31G basis set, the λ(e) of TPOB is 0.40 eV, which was similar to that of TPBI (0.39 eV). This suggest that the electron transfer rate of TPOB was not affected by the peripheral non-conjugating DPPO groups. As for the theoretical electron affinity (EA), the attached DPPO units exert a positive influence on increasing the EA value (0.73 for TPOB and 0.14 for TPBI) and



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enhancing the electron injection ability of TPOB, which was consistent with the frontier orbital simulation aforementioned.

Figure 1. Optimized geometries and calculated LUMO and LUMO+1 density maps for TPBI and TPOB.

Table 1. Calculated electronic properties of TPBI and TPOB. λ1

λ2

λ(e)

EA

HOMO

LUMO

T1

S1

∆EST

[eV]

[eV]

[eV]

[eV]

[eV]

[eV]

[eV]

[eV]

[eV]

TPBI

0.19

0.20

0.39

0.14

-5.6

-1.2

2.78

4.03

1.25

TPOB

0.19

0.21

0.40

0.73

-6.1

-1.7

2.87

3.84

0.97

Compound

The good thermal properties of TPOB were characterized by differential scanning calorimetry (DSC) and thermal gravimetric analyses (TGA). Figure 2 shows that a high decomposition temperature (Td, corresponding to 5% weight loss) was detected at 525 °C and a high glass transition temperature (Tg) of 188 °C was obtained after the second scan of the DSC curve. Such high Tg would benefit the morphological stability of the uniform amorphous film through spin-coating technique. Indeed, according to the AFM images in Figure S2, the root-mean-square (RMS) roughness of TPOB film was only 0.30 nm, which was smaller than the value of TPBI (0.82 nm). The LUMO energy level of TPOB was measured to be -3.0 eV by cyclic voltammetry during the cathodic sweeping (Figure S1), while


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the optical energy band gap (Eg) of TPOB was determined to be 3.7 eV from the onset of UV-Vis absorption. As a result, the HOMO energy level was calculated to be -6.7 eV by substracting of the Eg from the LUMO (HOMO = LUMO – Eg). Then, the low temperature phosphorescent (PH) spectra of TPOB were measured at 77 K in 2-MeTHF solution. Based on the highest energy vibronic band, the T1 of TPOB was estimated to be 2.76 eV, which is higher than that of typical blue phosphor iridium (III) bis(4,6-difluorophenylpyridinato) picolinate [FIrpic].

Figure 2. a) TGA curve of TPOB recorded at a heating rate of 10 °C min-1. Inset: DSC trace recorded at a heating rate of 10 °C min-1; b) UV-Vis absorption and fluorescence spectra of TPOB in CH2Cl2 and phosphorescence spectra in 2-MeTHF.

Figure 3 shows the photoluminescence (PL) spectra of TCTA, TPBI, TPOB, TCTA:TPBI and TCTA: TPOB blend films. The emission peaks of TCTA, TPBI and TPOB are located at 392, 379 and 384 nm, while the TCTA:TPBI and TCTA:TPOB blend films exhibits significantly red-shifted PL peaks at 443 and 464 nm, respectively. In addition, the full width at half maximum (FWHM) of the TCTA:TPBI and TCTA:TPOB emission spectra are 86 and 91 nm, which are 32~44 nm wider than those of the pristine films (Table 2). All these results suggest that the newly formed excited states in the blend films are attributed to the bimolecular charge transfer exciplex formation.



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Figure 3. Photoluminescence (PL) spectra of spin-coated films. a) TCTA, TPBI and TCTA:TPBI. b) TCTA, TPOB and TCTA:TPOB.

Transient fluorescence decays of the constituting pristine molecules and the mixed exciplex emitters are also investigated at 300 K. As shown in Figure 4, pristine TCTA, TPBI, and TPOB films exhibit decays with single time constants of 2.2, 4.5, and 4.8 ns, respectively. On the other hand, blend films of TCTA:TPBI and TCTA:TPOB not only have prompt fluorescence decays of 19 and 24 ns, but also delayed fluorescence decays of 0.10 and 0.13 µs for exciplex emission, respectively. The delayed components can be attributed to the up-conversion of the triplet excitons, which was often observed in the emitters with sufficiently small ∆EST.

Figure 4. Transient fluorescence decays of the films at 300 K. a) TCTA, TPBI and TPOB. b) TCTA:TPBI and TCTA:TPOB.


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Table 2. Photophysical properties of the exciplex emitters and their constituting molecules. FWHM

λmax [nm] TCTA

392

TPBI

379

TPOB

a

384

a

[nm]

b

S1

∆EST

T1

[eV]

c

[eV]

c

[eV]

0.55 1.03

47

3.40e

2.85e

52

e

e

59

3.70

3.70

f

2.67

2.76

d

HOMO

LUMO

τ

[eV]

[ns]a

5.8e

2.4e

2.2 (100%)

e

e

4.5 (100%)

h

4.8 (100%)

[eV]

6.4

0.94

6.7

g

g

2.7 3.0

k

-

-

19 (33%), 100 (67%)

-

-

24 (28%), 130 (72%)

TCTA:TPBI

443

86

2.85

2.80

0.068

TCTA:TPOB

464

91

2.69

2.67

0.042k

Measured in deposited films at 300 K. b The full width at half maximum (FWHM) of the emission spectra. c Estimated

from high-energy peaks of film-state fluorescence and phosphorescence spectra at 77 K.

d

The difference between S1

and T1. e Obtained from the reference. f Estimated from the absorption edges in the films. g Calculated from the energy gap and LUMO.

h

Determined by the CV measurement.

k

Estimated from the difference between the onsets of

fluorescence and phosphorescence spectra.

The fluorescence and phosphorescence spectra of the TCTA: TPBI and TCTA:TPOB blend films were measured at a low temperature of 77 K (Figure S3). Based on the first phosphorescent peak, the triplet energies of the two exciplexes are estimated to be 2.80 and 2.67 eV for TCTA:TPBI and TCTA:TPOB, respectively. In addition, the ∆EST of TCTA: TPBI and TCTA:TPOB, which were determined by the difference between the onsets of the fluorescence and phosphorescence spectra, are 0.068 and 0.042 eV, respectively. It is noticed that the TCTA: TPBI film shows structured phosphorescence emission with features from the exciplex and constituting molecule TPBI, while the TCTA:TPOB film exhibits single emission peak, which is almost identical to its fluorescence spectra. Figure 5 shows the schematic diagram of the relative energy levels of triplet states, and the possible cause for TPBI emission in TCTA:TPBI film is that the T1 of TPBI is lower than that of exciplex, which would induce a serious energy leakage form the exciplex state to the triplet state of TPBI.31, 37 In contrast, the T1 of TCTA and TPOB are both higher than that of TCTA:TPOB exciplex, which guarantees the efficient exciton confinement in the exciplex state and consequently no energy leakage to the constituting molecules. The fluorescence quantum yields (∅f) for TCTA: TPBI and TCTA:TPOB films are measured by an integrating sphere at 300K, and the values are 9.7 and 19.6 %,



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respectively. The higher ∅f of TCTA:TPOB exciplex can be attributed to its smaller ∆EST, which facilitates a higher reverse intersystem crossing (RISC) of the triplet excitons. To investigate the energy transfer from the exciplex to the dopant, the PL spectra of FIrpic doped TCTA:TPBI and TCTA:TPOB films at various doping concentrations were measured at 300K. As shown in Figure S4, the exciplex emission was gradually eliminated with the increased doping concentration, which indicates the efficient energy transfer under photoexcitation. Furthermore, the ∅f of 10 wt% FIrpic-doped TCTA:TPBI and TCTA:TPOB films were 61.5 % and 75.2 %, respectively, which may be attribute to the high fluorescence efficiency of TCTA:TPOB. Therefore, all these results suggest that the TCTA:TPOB system was a much efficient exciplex, which would serve as a good host material to sensitize the phosphor emitter in the PHOLEDs.

Figure 5. Triplet energy transfer diagrams with exciplex formation.

In order to evaluate the performance of the newly designed exciplex as the universal host material for solution-processed PHOLEDs, blue devices were fabricated with the configuration of ITO/PEDOT:PSS/ Exciplex: 10 wt% FIrpic/TmPyPB/Cs2CO3/Al. Here, PEDOT:PSS and Cs2CO3 served as the hole- and

electron-injection layers, respectively. 1,3,5-tri(m-pyrid-3-yl-phenyl)

benzene (TmPyPB), which has higher triplet energy than that of FIrpic, was used as the electron-transporting layer (ETL) to minimized the triplet exciton quenching at the EML/ETL interface. Figure 6 depicts the relative energy levels of the materials employed in the devices. Table 3 summarized the electroluminescent data of the solution-processed blue PHOLEDs. As shown by the


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electroluminescence (EL) spectra in Figure S5, both the devices exhibit typical emissions originating from the corresponding guest FIrpic, which demonstrate the complete energy transfer from the exciplex-type

hosts

to

the

iridium

dopant

under

electro-excitation.

The

current

density-voltage-luminance (J-V-L) characteristics of these devices were presented in Figure 7. To further demonstrate the carrier transporting property, electron-only devices were also fabricated by solution-process (Figure S6). It is worthy to note that the current density of TPOB-based device is higher than that of TPBI-based one at the same operating voltage, which is consistent well with the higher electron injection ability of TPOB descripted in theoretical calculation. Although the ∆EST of the two exciplex hosts are sufficiently small, the turn-on voltages of these devices, corresponding to the voltage at the luminance of 1 cd m-2, are different. In TCTA:TPOB system, the turn-on voltage was 2.8 V, which was 1.2 V lower than that of TCTA:TPBI-based PHOLED and even approaching the corresponding photon energy (2.65 V, hv/e) of FIrpic. Since the exciton dynamics of exciplex-based device was determined by the intermolecular charge transfer from the LUMO of acceptor to the HOMO of donor component, the decreased operating voltages of TCTA:TPOB-based device can be attributed to the minimized electron injection barrier of TPOB. To the best of our knowledge, few hosts can use solution-process to achieve the turn-on voltage of blue PHOLED bellow 3 V, which fully reflects the superiority of exciplex-type host in low operating devices. The current efficiencies of TCTA:TPOB and TCTA:TPBI-based PHOLEDs are 18.1 and 28.2 cd A-1, respectively. One possible reason for the high efficiency of TCTA: TPOB-based device may be that the charge transfer in the EML was more balanced due to the enhanced film forming ability and electron-injection property of TPOB comparing to TPBI, as explained earlier. Another main reason can be attributed to the efficient exciplex-state excitons confinement and excluded energy leakage to the constituting molecules due to the higher T1 of TPOB and TCTA than that of TCTA:TPOB exciplex. Given the low operating voltage and high current efficiency, PHOLED based on TCTA:TPOB also realized the high power efficiency of 22.0 lm W-1, which was among the highest values of solution-processed blue devices with the same structure. More importantly, TCTA:TPOB-based device displays a low efficiency roll-off with a current efficiency of 28.1 cd A-1 at 1000 cd m-2, 26.2 cd A-1 at



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5000 cd m-2 and 23.8 cd A-1 at 10000 cd m-2, respectively, which indicates the stable charge carrier balance and broad recombination zone in the exciplex hosted emission layer. All these results demonstrate the huge potential of exciplex-type hosts for power efficient solution-processed PHOLEDs with low operating voltage.

Figure 6. Schematic diagram of the device with the energy levels and molecular structures of the organic compounds.

Figure 7. a) Current density−voltage−luminance (J−V−L) characteristics; b) Calibrated current and power efficiencies of the solution-processed blue PhOLEDs as a function of luminance.

Table 3. Electroluminescent data of the devices. Vona

CEmaxb

PEmaxc

EQEdmax

CEe

EQEe

[V]

[cd A-1]

[lm W-1]

[%]

[cd A-1]

[%]

TCTA:TPBI

3.9

18.1

9.2

8.9

16.7/17.5/16.1

8.2/8.5/7.9

(0.16, 0.34)

TCTA:TPOB

2.8

28.2

22.0

13.8

28.1/26.2/23.8

13.7/12.8/11.8

(0.15, 0.33)

Host


CIE(x, y)

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a d

Turn-on voltages estimated at the luminance of 1 cd m-2. b Maximum current efficiency. c Maximum power efficiency. Maximum external quantum efficiency.

e

The efficiencies at the luminance of 1000, 5000, and 10000 cd m-2,

respectively.

■ Conclusion In summary, we have developed a novel solution-processable electron transporting material TPOB, which can efficiently form an exciplex with the common used hole transporting molecule TCTA. By introducing the DPPO groups at the edge of the TPBI core, triplet energy leakage from the exciplex state of TCTA:TPOB to the constituting molecules can be successfully eliminated, while maintaining the excellent charge transporting property and enhancing the film forming ability through spin-coating. As a result, solution-processed blue PHOLED based on TCTA:TPOB exciplex-type host has realized the extremely low turn-on voltage of 2.8 V, which was comparable with the values of vacuum deposited devices. In view of the high performance and small efficiency roll-off, this investigation not only demonstrated the concept of exciplex-type host used for power efficient solution-processed PHOLEDs, but also established a useful approach of melecular modulation for efficent exciplex-formation.

■ Supporting Information Device fabrication and measurement; DFT calculations; Cyclic voltammograms; photophysical and electrochemical spectra; photophysical data; solution-processed PHOLEDs data. This material is available free of charge via the Internet at http://pubs.acs.org.

■ Author Information Corresponding Authors *

E-mail: [email protected] (W. Jiang).

*

E-mail: [email protected] (Y.M. Sun)

■ Notes



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The authors declares no competing financial interest.

■ Acknowledgements We are grateful for the Grants from the National Basic Research Program of China (2013CB932902), National Natural Science Foundation of China (51103023, 21173042). We also thank for the support of the Research Fund for Graduate Innovation Project of Jiangsu Province (KYLX15_0123), the Scientific Research Foundation of Graduate School of Southeast University and the Fundamental Research Funds for the Central Universities.

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