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High performance exciplex-based fluorescence-phosphorescence white organic light-emitting device with highly simplified structure Zhan Chen, Xiao-Ke Liu, Cai-Jun Zheng, Jun Ye, Chuan-Lin Liu, Fan Li, Xue-Mei Ou, Chun-Sing Lee, and Xiao-Hong Zhang Chem. Mater., Just Accepted Manuscript • Publication Date (Web): 10 Jul 2015 Downloaded from http://pubs.acs.org on July 10, 2015

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Chemistry of Materials

High performance exciplex-based fluorescence-phosphorescence white organic light-emitting device with highly simplified structure Zhan Chen,1,2,∥ Xiao-Ke Liu,1,3,∥ Cai-Jun Zheng,1* Jun Ye,4 Chuan-Lin Liu,1 Fan Li,1 Xue-Mei Ou,1 Chun-Sing Lee,3* and Xiao-Hong Zhang1,5* 1

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, P.R. China. 2 University of Chinese Academy of Sciences, Beijing 100039, P.R. China. 3

Center of Super-Diamond and Advanced Films (COSDAF) and Department of Physics and Materials Science, City University of Hong Kong, Hong Kong SAR, P.R. China.

4

China–Australia Joint Research Center for Functional Molecular Materials, School of Chemical and Material Engineering, Jiangnan University, Wuxi 214122, P.R. China.

5

Functional Nano & Soft Materials Laboratory (FUNSOM) and Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou, Jiangsu 215123, P.R. China.

KEYWORDS: exciplex systems, device simplification, fluorescence and phosphorescence hybrid WOLEDs

ABSTRACT: Exciplex systems have become increasingly important due to their good performance in the devices. Considering their bipolar transporting properties, exciplex systems with strong blue fluorescence and high triplet energies could potentially be ideal hosts for fluorescence-phosphorescence hybrid (F-P) WOLEDs. Here we got an efficient blue exciplex system formed with a new material TPAPB and TPBi, which exhibited a high photoluminescence quantum yield of 44.1%. Using only these two organic materials, a high performance blue-emitting OLED was fabricated with a maximum external quantum efficiency (EQE) of 7.0±0.4%. Such high efficiency is not only among the highest-results of blue fluorescent OLEDs, but also indicates that high χ (fraction of excitons that can potentially radiatively decay) can be as well achieved via a TTA triplet up-conversion process. By simply doping an orange phosphor, the first F-P WOLED using an exciplex host system was realized with a single-emission-layer structure and merely three organic materials. The WOLED exhibits a maximum power efficiency, current efficiency and EQE of 29.6±0.2 lm W−1, 42.5±0.3 cd A−1 and 15.7±0.3%, respectively. To the best of our knowledge, this highly efficient and structurally-simplified exciplex-based F-P WOLED is truly unprecedented.

INTRODUCTION Organic light-emitting diodes (OLEDs) have attracted much attention for their applications in flat-panel displays and solid-state lighting.1-6 Recently, interest in exciplex systems formed via intermolecular interaction between electron donors and electron acceptors has expanded rapidly for their good performance.7-16 In 2012, Adachi et al showed that fluorescent OLEDs using exciplex systems as emitters can break the theoretical efficiency limit of conventional fluorescent devices by upconversion of triplet to singlet excitons.17 Moreover, due to the combination of donor and acceptor molecules, the exciplex systems usually exhibit bipolar property making them ideal host materials for phosphorescent dopants. In the past couple of years, several exciplex-based phospho-

rescent devices with external quantum efficiencies (EQEs) over 20% have been reported.18-20 Furthermore, by balancing the hole and the electron mobilities of the exciplexbased host systems, the OLEDs can be formed with a highly simplified device structure without using additional carrier transporting materials, which can remarkably reduce production cost. In 2013, Jankus et al and Kim et al respectively reported fluorescent and phosphorescent exciplex-based OLEDs based on such simple-structure devices, further demonstrating the superiority of exciplex systems.18-21 However, till now, high performance white OLEDs (WOLEDs) based on exciplex systems are still rarely reported.22-25

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In recent years, fluorescence-phosphorescence hybrid (F-P) WOLEDs with blue fluorescence and orange/red phosphorescence have been considered as one of the best candidates for commercial applications.3,26-28 Bipolar host materials which can give efficient blue fluorescence and also sensitize orange/red phosphors, have been shown to be useful for fabricating high-performance F-P WOLEDs with simple structures.29-31 Considering their bipolar transporting properties, exciplex systems with strong blue fluorescence and high triplet energies could potentially be used as ideal hosts for F-P WOLEDs. However, in fact, there is so far no report on F-P WOLEDs using an exciplex system as host. The challenge of implementing this scheme is the need of a high efficiency blue emitting exciplex system and a suitable matching phosphorescent dopant to allow simultaneous radiative decay of both singlet and triplet excitons. To address these, here we got an efficient blue exciplex system formed with a new material (4dimesitylboryl)phenyltriphenylamine (TPAPB) and 1,3,5tri(1-phenyl-1H-benzo[d]imidazol-2-yl)phenyl (TPBi). A 50 mol% TPAPB:TPBi mixed film was found to give a strong blue emission (peaked at 471 nm) with a high photoluminescence quantum yield (PLQY) of 44.1%. To the best of our knowledge, this is the highest PLQY value reported in blue exciplex emission.21 At the same time, the good hole- and electron- transporting properties of TPAPB and TPBi respectively endow their mixed film with good bipolar transporting properties. Using only these two organic materials, we fabricated a high performance blue-emitting OLED with a maximum external quantum efficiency (EQE) of 7.0±0.4% and a maximum power efficiency (PE) of 7.2±0.5 lm W-1. By simply doping an orange Ir phosphorescent complex into the TPAPB:TPBi layer, we fabricated the first F-P WOLED using an exciplex host system. The WOLED has a very simple single-emissionlayer structure and uses only three organic materials. Without the use of any out-coupling enhancement techniques, the maximum PE, current efficiency (CE) and EQE of the WOLED in the forward direction are 29.6±0.2 lm W−1, 42.5±0.3 cd A−1 and 15.7±0.3%, respectively. To the best of our knowledge, this performance is among the best reported in F-P WOLEDs. This work not only successfully proves the capability of exciplex systems for achieving high performance F-P WOLEDs, but also provides a novel simple approach for achieving highefficiency, simple-structured and low-cost WOLEDs.

RESULTS AND DISCUSSION The new material TPAPB was newly developed here as a hole-transporting material (HTM); while purchased TPBi and tris(2-phenylquinoline)iridium (Ir(2-phq)3) were used respectively as an electron-transporting material (ETM) and a phosphorescent dopant. TPAPB was synthesized by reacting 4'-bromo-N, N-diphenylbiphenyl-4-amine with dimesitylboron fluoride in the existence of n-butyllithium. The synthetic route and the molecular structure of TPAPB

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are depicted in Scheme 1. The structure of TPAPB was fully characterized and confirmed with 1H, 13C NMR spectra and mass spectrum. Key physical properties of TPAPB are summarized in Table 1. As shown in Table 1, the PL spectrum of TPAPB in dilute ethyl acetate solution peaks at 478 nm, while that is 444 nm in solid films. The difference may be explained by the intramolecular chargetransfer character of TPAPB.31 The highest occupied molecular orbital (HOMO) energy level of TPAPB was estimated to be -5.36 eV from the onset of its oxidation curve with reference to ferrocene, whose Fermi level is known as 5.1 eV.32 As no clear reduction curve was observed, its lowest unoccupied molecular orbital (LUMO) energy level was estimated to be -2.35 eV from the difference between its HOMO energy and optical energy gap (Eg) determined from the film’s optical absorption edge. Scheme 1. Synthetic Route and Chemical Structure of TPAPB F B H N

Br

Br

N

N

Br

B

TPAPB

Table 1. Physical Properties of TPAPB λabs/λem (nm)a

376/478

λabs/λem (nm)b

376/444

Φfc

0.69

λfluo/λphos (nm)d

447/523

T1 (eV)e

2.37

Eg (eV)f

3.01

HOMO (eV)g

-5.36

LUMO (eV)

-2.35

a

Measured in dilute ethyl acetate solution at room temperab c ture. Measured in film state. Measured in dilute cyclohexd ane solution by using an integrated sphere method. Mease ured in 2-MeTHF glass matrix at 77 K. Calculated from the f phosphorescence peak at 77 K using 1240/λphos. The energy g bandgap as determined from the film absorption edge. Determined from the onset oxidation potential of the cyclic voltammetry curve in DMF.

Density functional theory (DFT) calculation was employed at the B3LYP/6-31G (d) level to gain insight of the electronic structures of TPAPB. Figure S3 shows the molecular frontier orbital distributions in TPAPB. The HOMO is dominantly located on the electron-rich diphenylamine unit and the biphenyl π bridge; whereas, the LUMO is localized predominantly on the electrondeficient dimesitylboron unit and the biphenyl π bridge. The separation between the HOMO and LUMO orbitals maintains the individual electronic properties of the functional groups as well as their own electron donating and accepting properties.33 To further investigate the carrier transporting properties of TPAPB, hole- and electron-only devices were fabricated. The configuration of the hole-

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only device is ITO/MoO3 (12 nm)/TPAPB (60 nm)/MoO3 (12 nm)/Al, while the electron-only device has the structure of ITO/Al (60 nm)/TPAPB (60 nm)/LiF (1 nm)/Al. It can be assumed that only single carriers can be injected and transported through the devices on account of the work function of MoO3 (or Al) being high (or low) enough to block electron (or hole) injection.34,35 In order to give a qualitative assessment of the charge mobility of TPAPB, a corresponding hole-only device of NPB and electron-only device of TPBi were also fabricated as a reference. As shown in Figure S4, TPAPB is capable of transporting hole efficiently.

Figure 1. PL spectra of TPAPB, TPBi and 50 mol% TPAPB:TPBi in solid films. The excitation wavelength for the pristine TPAPB film and 50 mol% TPAPB:TPBi co-deposited film was 375 nm, while for the pristine TPBi film was 270 nm.

As shown in Figure 1, the photoluminescence (PL) of a 50 mol% TPAPB:TPBi co-deposited film peaked at 471 nm which is obviously red-shifted comparing to those of the two constituting materials. The emission of TPAPB:TPBi corresponds to an energy of 2.66 eV which matches well to the difference between the HOMO level of TPAPB and the LUMO level of TPBi suggesting that the TPAPB:TPBi system does form a blue-emitting exciplex. To further confirm the exciplex formation in the mixed film, transient PL decay characteristics of the deposited films were investigated at 300K. As shown in Figure S5, a film of pristine TPAPB shows a single exponential decay of 1.05 ns; while a pristine film of TPBi shows two lifetimes of respectively 2.35 and 5.03 ns. In contrast, as shown in Figure 2, the 50 mol% TPAPB:TPBi mixed film shows prompt decays of 2.39 ns and 4.88 ns, and delayed decays of 1.89 μs and 12.01 μs, which are due to the upconversion of triplet to singlet excitons in the exciplex systems as discussed in previous reports.14,17,21 Measured with the integrating sphere method under ambient atmosphere, the exciplex system exhibits a high photoluminescence quantum yield (PLQY) of 44.1%, while the pristine TPAPB and TPBi films possess much lower PLQY of 25.1 and 23.2% respectively. The high PLQY suggests the capability of TPAPB:TPBi exciplex system as a blue emitter.

Figure 2. The transient PL decay characteristics of 50 mol% TPAPB:TPBi co-deposited film with different ranges of time. The excitation wavelength was 375 nm, and the emission 2 2 wavelength was 471 nm. (a) 50 ns, χ = 1.191. (b) 200 μs, χ = 4.995.

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We then analyze the efficiency of the device with the equation: EQE = γχηPLηoc

(1)

where, γ is the recombination efficiency of injected holes and electrons; χ is the fraction of excitons that can potentially radiatively decay; ηPL is the intrinsic PL efficiency of the emitter (ηPL) and ηoc is the light out-coupling factor. For the TPAPB:TPBi mixed film, ηPL was measured to be 44.1%. Assuming a γ of 1 and a ηoc of around 20%, χ is estimated to be 83.9%. This is much higher than the theoretical singlet population of 25% and suggests that triplet excitons have also been harvested as in other exciplex systems.14,17,21 As shown in Figure S7, the lowest triplet energy level in the exciplex system is TTPAPB, thus, the up-conversion of triplet to singlet excitons is mainly via triplet-triplet annihilation (TTA).21 The two power-law exponents delayed decay components in TPAPB:TPBi film (Figure 2b) are also consistent with such TTA process.21 We can estimate χ in an ideal condition from the following: In first circle, direct singlet contribution is 0.25, and TTA contribution from triplet is 0.75/2. In second circle, assuming no other non-radiative loss, the exciton contribution is (0.25+0.75/2)*(1-ηPL)/2. Sequentially, in the nth circle, the exciton contribution is (0.25+0.75/2)*[(1-ηPL)/2]n-1. Thus, the total exciton contribution χ should be χ = (0.25+0.75/2)+(0.25+0.75/2)*(1ηPL)/2+…+(0.25+0.75/2)*[(1-ηPL)/2]n-1 = (0.25+0.75/2){1[(1-ηPL)/2]n}/[1-(1-ηPL)/2] = 0.625*{1-[(1ηPL)/2]n}/(0.5+0.5ηPL) (2) Considering the limitation, χ∞ = 0.625/(0.5+0.5ηPL) Figure 3. (a) PE-EQE-luminance plots of blue OLED. (b) The EL spectra recorded at various luminances of blue OLED. (c) PE-EQE-luminance plots of orange OLED. (d) PE-EQEluminance plots of white OLED. (e) The EL spectra recorded at various luminances of white OLED.

A blue-emitting device was fabricated with the TPAPB:TPBi mixed film as the emitting layer with a configuration of ITO/TPAPB (30 nm)/50 mol% TPAPB:TPBi (30 nm)/TPBi (40 nm)/LiF (1 nm)/Al. As undoped TPAPB and TPBi are used respectively as the HTM and the ETM, the device has a highly simplified structure and uses only two organic materials. As the device does not use any other organic materials as the carrier transporting or injection layers, unnecessary injection barriers can be avoided and resulting in a low turn-on voltage of 3.2 V.1820,36,37 As shown in Figure 3(b), the device exhibits similar blue emission spectra at different luminances with peaks at 468 nm and CIE coordinates of (0.14, 0.18). The maximum PE, CE and EQE of the device are 7.2±0.5 lm W−1, 9.1±0.7 cd A−1 and 7.0±0.4%, respectively. To the best of our knowledge, these high efficiencies are among the best results of all reported blue fluorescent OLEDs.21,30,38

(3)

From Equation 3, we can get the ideal χ of the TPAPB:TPBi film is 86.7%, which is only slightly higher than our experimental result of 83.9%, indicating our exciplex-based blue fluorescent device can harvest triplet energy with efficiency close to the theoretical limit for the TTA process. We further prepared an orange phosphorescent OLED by simply doping 4 wt% of Ir(2-phq)3 into the TPAPB:TPBi layer of the above blue exciplex device (i.e configuration is: ITO/TPAPB (30 nm)/TPAPB:TPBi:Ir(2phq)3 (30 nm)/TPBi (40 nm)/LiF (1 nm)/Al. EL spectra, JV-L characteristics and efficiencies of the orange device are shown in Figures S9a, S9b and 3c respectively. The device has a very low turn-on voltage of 2.9V and high maximum CE, PE and EQE of 49.5±0.2 cd A-1, 44.3±0.3 lm W-1, 18.5±0.2%.

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Figure 4. The energy diagram of the exciplex-based WOLED.

Table 2. Electroluminescence Properties of the Devices CE @ 100 cd m−2

PE @ 100 cd m−2

EQE @ 100 cd m−2

(cd A−1)

(lm W−1)

(%)

7.0±0.4

7.4±0.1

5.0±0.1

5.6±0.1

(0.14, 0.18)

44.3±0.3

18.5±0.2

45.6±0.1

35.7±0.2

17.0±0.2

(0.53, 0.47)

29.6±0.2

15.7±0.3

37.8±0.2

25.1±0.1

14.0±0.2

(0.46, 0.43)

Vona

max CEb

max PEc

max EQEd

(V)

(cd A−1)

(lm W−1)

(%)

Blue OLED

3.2

9.1±0.7

7.2±0.5

Orange OLED

2.9

49.5±0.2

White OLED

3.2

42.5±0.3

a

b

c

d

e

CIE(x, y)e

−2

Turn-on voltage. Current efficiency. Power efficiency. External quantum efficiency. Estimated at 1000 cd m .

Finally, using the three organic materials (the exciplex pair + Ir complex), we fabricated a simple structured F-P WOLED with only one emitting layer. The WOLED in fact used the same device structure as the orange device (i.e. anode/TPAPB/mixture of the three materials/TPBi/cathode). The only difference is that we doped a very low concentration of Ir complex (which is optimized to 0.5 wt%) into the TPAPB:TPBi layer to balance the exciplex’s singlet and the Ir complex’s triplet emissions.29 Figure 4 illustrates the device structure and energy diagram of the device. In this WOLED device, the exciplex system serves as the carrier-transporting materials, blue fluorescent emitter and the host for the orange phosphorescent dye simultaneously. Thus, the device uses merely three organic layers and three organic materials, which not only endows the device with low operating voltages, but also effectively reduces the production cost. Forward-viewing efficiencies, and EL spectra of the WOLED are illustrated in respectively Figures 3d and 3e, and the device performances are summarized in Table 2. As shown in Figure 3e, the EL spectra of the WOLED exhibit warm white light emission comprising distinct blue exciplex emission and orange phosphorescent emission bands covering the range from 400 to 700 nm. The CIE coordinates show a moderate blue shift from (0.46, 0.43) at 1,000 cd m−2 to (0.41, 0.39) at 10,000 cd m−2. The change in the CIE coordinates and corresponding spectra at dif-

ferent luminances can be attributed to the increasing blue exciplex emission caused by the delayed fluorescence at high current densities. Without the use of any outcoupling enhancement techniques, the maximum PE, CE and EQE of device W in the forward direction are 29.6±0.2 lm W−1, 42.5±0.3 cd A−1 and 15.7±0.3%, respectively. To the best of our knowledge, this highly efficient exciplex-based WOLED is unprecedented. Such excellent efficiency performance is not only the best result for WOLEDs with simple configurations,29-31 but also comparable with the highest efficiencies of WOLEDs reported in literatures.1-3,39-43

CONCLUSIONS In summary, we have presented an extremely simple strategy to realize a highly simplified WOLED based on multifunctional exciplex systems. By using a novel electron donor molecule TPAPB with a conventional electron acceptor TPBi, a highly efficient blue exciplex system has been obtained. Without any another charge transporting materials, a highly efficient blue fluorescent OLED has been fabricated by employing this exciplex system as the emitter, realizing a high EQE of 7.0±0.4%. Furthermore, the TPAPB:TPBi exciplex system also performs well as a phosphorescent host. An orange OLED using the TPAPB:TPBi exciplex host gives a high EQE of 18.5±0.2%. On the basis of the superior applications of the blue exci-

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plex system serving as a blue fluorescent emitter and sensitizer for orange phosphor simultaneously, we then demonstrate an exciplex-based F-P WOLED with only three layers and three organic materials. The WOLED exhibits excellent efficiency performance with a maximum EQE of 15.7±0.3%, a maximum CE of 42.5±0.3 cd A−1 and a maximum PE of 29.6±0.2 lm W-1. These outstanding results successfully show the feasibility of exciplex systems for achieving high-efficiency, simple-structure and low-cost WOLEDs.

ASSOCIATED CONTENT Supporting Information Cyclic voltammograms, EL spectra, energy level diagrams, current density-luminance-voltage characteristics, PE-EQEluminance plots and CIE coordinates. This information is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (X.-H. Z.), [email protected] (C.-J. Z.), [email protected] (C.-S. L.).

Author Contributions ∥Z.

Chen and Dr. X. K. Liu contributed equally to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Grant No. 51373190, 51103169), the Beijing Nova Program (Grant No. Z14110001814067) and the Instrument Developing Project of the Chinese Academy of Sciences (Grant No. YE201133), P. R. China. C.S. Lee would like to acknowledge support from the Research Grants Council of the Hong Kong SAR, China (Project No. T23-713/11).

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