Simultaneous Enhancement of Efficiency and Stability of

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Simultaneous Enhancement of Efficiency and Stability of Phosphorescent OLEDs Based on Efficient Förster Energy Transfer from Interface Exciplex Dongdong Zhang, Minghan Cai, Yunge Zhang, Zhengyang Bin, Deqiang Zhang, and Lian Duan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b10561 • Publication Date (Web): 22 Jan 2016 Downloaded from http://pubs.acs.org on January 24, 2016

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ACS Applied Materials & Interfaces

Simultaneous Enhancement of Efficiency and Stability of Phosphorescent OLEDs Based on Efficient Förster Energy Transfer from Interface Exciplex Dongdong Zhang, Minghan Cai, Yunge Zhang, Zhengyang Bin, Deqiang Zhang, Lian Duan* Key Lab of Organic Optoelectronics and Molecular Engineering of Ministry of Education, Department of Chemistry, Tsinghua University, Beijing 100084, China.

ABSTRACT Exciplex forming cohosts have been widely adopted in phosphorescent organic light-emitting diodes (PHOLEDs), achieving high efficiency with low roll-off and low driving voltage. However, the influence of the exciplex-forming hosts on the lifetimes of the devices, which is one of the essential characteristics, remains unclear. Here, we compare the influence of the bulk exciplex and interface exciplex on the performances of the devices, demonstrating highly efficient orange PHOLEDs with long lifetime at low dopant concentration by efficient Förster energy transfer from the interface exciplex. A bipolar host, (3′-(4,6-diphenyl-1,3,5triazin-2-yl)-(1,1′-biphenyl)-3-yl)-9- carbazole (CzTrz) was adopted to combine with a donor molecule, tris(4-(9H-carbazol-9-yl)phenyl)amine (TCTA) to form exciplex. Devices with energy transfer from the interface exciplex achieve lifetime almost two orders of magnitudes higher than

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the ones based on bulk exciplex as the host by avoiding the formation of the donor excited states. Moreover, a highest EQE of 27% was obtained at the dopant concentration as low as 3 wt% for a device with interface exciplex, which is favorable for reducing the cost of fabrication. We believe that our work may shed light on future development of ideal OLEDs with high efficiency, long-lifetime, low roll-off and low cost simultaneously.

KEYWORDS (phosphorescent organic light emitting diodes, interface exciplex, Förster energy transfer, high efficiency, long operational lifetime)

INTRODUCTION Since the pioneering work of Tang et al in 1987,1 organic light-emitting diodes (OLEDs) have attracted much attention due to their unique advantages in solid displays and lighting applications. At the early development stage of OLEDs, more attention has been focused on improving the device efficiency.2-4 Nowadays external quantum efficiencies (EQEs) over 30% have been achieved for red, green and blue (RGB) OLEDs.5,6 For further development considering practical applications, ideal OLEDs possessing high efficiencies with small efficiency roll-off, long lifetimes and low costs are highly desired.7 It has been reported that phosphorescent OLEDs (PHOLEDs) utilizing materials with thermally activiated delayed fluorescence (TADF) emission as the hosts,which were called TADF-sensitized PHOLEDs, may provide an effective approach to achieve ideal OLEDs.8 In the TADF-sensitized PHOLEDs, the host triplets can be thermally up-converted into singlets due to their small singlet-triplet splitting energies (∆ESTs), and then the energy of the host singlets can be transferred to the dopant through the long range Förster energy transfer (FET).7,9,10 Therefore,


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the energy transfer from the host to the guest is greatly enhanced compared with the devices using conventional hosts, in which the triplet excitons are transferred through the Dexter mechanism.7,11 The change of the energy transfer from the short-range Dexter transfer to the long-range Förster transfer could greatly reduce the triplet density in the host and gives rise to advantages such as long lifetime. Such conception has been widely demonstrated for PHOLEDs using single-molecule TADF materials as the hosts, achieving high efficiencies with low efficiency roll-off and long lifetimes simultaneously at low dopant concentrations. Besides single-molecule TADF,12-14 exciplexes formed between electron-donating and electron-accepting molecules, have also been demonstrated to give TADF emission.15 It has been reported that materials with exciplex emission may possess intrinsically smaller ∆ESTs and thus more efficient reverse intersystem crossing (RISC) process compared with the single-molecule TADF materials.16 And the Förster energy transfer from the exciplex to the phosphors has been demonstrated by Kim et al.17 Therefore, it is natural to anticipate that the hosts with exciplex emission employed in the TADF-sensitized PHOLEDs may lead to PHOLEDs with even better performance through the enhanced Förster transfer. Exciplex can be formed incorporating blend donor and acceptor materials which is called bulk exciplex, or formed at the interface of the donor and acceptor layers, namely interface exciplex. Kim and co-works have demonstrated a series of high performance PHOLEDs based on bulk exciplex as the host with EQEs above 30% achieved for RGB emission.5,18 Although the fabrication process was greatly reduced compared with the bulky ones, materials with interfacial exciplex were rarely used as the hosts for PHOLEDs. Wang et al demonstrated solutionprocessed high performance orange PHOLEDs with low operation voltage based on energy transfer from interface exciplex.19 Kido et al also developed blue PHOLEDs with low driving


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voltage adopting such strategy.20 Despite all the efforts made in scientific literatures, the influence of the exciplex-forming hosts on the lifetimes of the devices has not been studied until now, which is one of the essential characteristics of the devices. Here, a bipolar host, (3′-(4,6-diphenyl-1,3,5-triazin-2-yl)-(1,1′-biphenyl)-3-yl)-9-carbazole (CzTrz)21 was chosen to combine with a well-known donor molecule, tris(4-(9H-carbazol-9yl)phenyl)amine (TCTA) to form exciplex. Devices based on energy transfer from both bulk exciplex and interface exciplex have been studied. The highest EQE of 27% was obtained at the dopant concentration as low as 3 wt% for a device with interface exciplex, which is favorable for reducing the cost of fabrication. What is more, devices with energy transfer from the interface exciplex achieve lifetimes almost two orders of magnitudes higher than the ones based on bulk exciplex as the host by avoiding the formation of the donor excitate states. We believe that our work may shed light on future development of ideal OLEDs with high efficiency, long-lifetime, low roll-off and low cost as well. EXPERIMENTAL SECTION PL characterization: UV-vis absorption spectra were recorded by an Agilent 8453 spectrophotometer. The films are spin coated on the quartz substrate using the solvent of dichloromethane and the solvent is volatilized before measurement. The PL spectra of the films were recorded with a fluorospectrophotometer (Jobin Yvon, FluoroMax-3). While the PL transient decay curves of the films were measured using a transient spectrometer (Edinburg FL920P). Device characterization: All of the organic materials used were purified by a vacuum sublimation approach. Before device fabrication, the ITO glass substrates were pre-cleaned


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carefully. Then the sample was transferred to the deposition system. The OLED devices were fabricated by thermal evaporation under high vacuum (~7×10-4 Pa) onto clean ITO-coated glass substrates. The forward-viewing electrical characteristics of the devices were measured with a Keithley 2400 source meter. The electroluminescence spectra and luminance of the devices were obtained on a PR650 spectrometer. All the device fabrication and characterization steps were carried out at room temperature under ambient laboratory conditions. For measurement of the transient electroluminescence characteristics, short-pulse excitation with a pulse width of 15 µs was generated using Agilent 8114A. The amplitude of the pulse is 9V, and the baseline is -3V. The period is 50 µs, and delayed time is 25 µs while the duty cycle is 30%. The decay curves of devices were detected using the Edinburg FL920P transient spectrometer. Lifetime analysis of the devices was done at a constant current mode at an initial luminance of 1000 cd/m2. RESULTS AND DISCUSSION


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Figure 1. a) The absorption spectrum of the CzTrz and emission spectra of CzTrz, TCTA and CzTrz: TCTA films. b) The energy diagram and molecule structures of CzTrz, TCTA and exciplex. c) The transient decay curves of the exciplex. d) The absorption spectrum of PO-01 and the emission spectra of exciplex: x% PO-01 films. To achieve TADF-sensitized phosphorescence system with interface exciplex, efficient RISC as well as FET should be achieved simultaneously. On one hand, to achieve efficient RISC process, besides from the small ∆EST of the exciplex, the triplet energies of both the donor and the acceptor should be higher than that of the exciplex to avoid quenching the exciplex energy. Based on such concept, CzTrz was chosen.20 First, the exciplex nature between TCTA and CzTrz was studied. The normalized photoluminance (PL) spectra of the TCTA, CzTrz and co-mixed


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TCTA: CzTrz (molar ratio of 1:1) films and the absorption spectrum of CzTrz were shown in Figure 1a. The PL emission peaks of TCTA and CzTrz are located at 400 nm and 430 nm, respectively, while the emission peak of the TCTA: CzTrz film showed a broad and featureless PL spectrum at 502 nm, which is red-shifted compared with the TCTA and CzTrz emissions. The corresponding photon energy of the emission peak is evaluated to be 2.48 eV. The highest occupied molecule orbital (HOMO) and the lowest unoccupied molecule orbital (LUMO) of CzTrz have been reported to be 3.28 eV and 6.08 eV.21 The photon energy is almost equivalent to the energy difference between HOMO (5.70 eV) of TCTA and LUMO (3.28 eV) of CzTrz, confirming that the emission was originated from the exciplex emission. The triplet of the exciplex was also reported to be energetically equivalent to its singlet,21 which was 2.48 eV. Therfore, the triplet of the exciplex is lower than triplets of both CzTrz (2.71 eV) (Figure S1) and TCTA (2.76 eV), as can be seen from Figure 1b. The small ∆EST of the exciplex guarantees the efficient RISC process. The PL transient decay of the exciplex was shown in Figure 1c. The prompt and the delayed parts were clearly seen from the transient decay curve, indicating that the light emission of the exciplex partly originate from TADF emission. The ratio of the delayed part was calculated to be almost 90% (see supporting information 2),22 indicating that the RISC process is efficient, which can be attributed to the small ∆EST and the higher triplet energy of CzTrz and TCTA, confining the triplet excitons on the exciplex. On the other hand, efficient Förster transfer from the exciplex to the dopant should be realized.

An

orange

phosphor,

(acetylacetonato)bis[2-(thieno[3,2-c]pyridin-4-

yl)phenyl]iridium(III) (PO-01), was chosen as the dopant.23 Since the energies of both the singlet and triplet of the exciplex are higher than the triplet of PO-01, it is adequate to be the host for PO-01. As can be seen from Figure 1d, the overlap between the absorption of PO-01 and the


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emission of the exciplex is large enough, indicating efficient energy transfer. The PL spectra of the exciplex: dopant was measured. At the dopant concentration of 2wt%, the exciplex emission is almost invisible, indicating that the energy transfer is efficient.


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Figure 2. a) The energy diagram of the device structures. b) The emission spectra of the bulk exciplex and interface exciplex with CzTrz thickness of 30 nm and 10 nm. c) The emission spectra of devices with CzTrz thickness of 30 nm. d) The current density-voltage curves of devices with CzTrz thickness of 30 nm. e) The EQE-brightness curves of devices with CzTrz thickness of 30 nm. f) The EQE-brightness curves of devices with CzTrz thickness of 10 nm.

Since efficient RISC and FET processes have been demonstrated, a key prerequisite in the realization of the present concept is a novel stack design rendering it possible to form exciplex at the interface of the donor and the acceptor layers. A basis of the design is a profound understanding of the transport properties of the donor and the acceptor. The donor, TCTA, has been widely taken as a hole-transporting type material. To investigate the transport ability of CzTrz, single-carrier device was fabricated. The current density of the single-electron device of CzTrz is much higher than that of the single-hole device (Figure S2), indicating that the CzTrz is an electron-transport dominating material. The devices were fabricated with the structures of ITO/ HATCN (5 nm)/ NPB (40nm)/ TCTA (20 nm)/ CzTrz (30 nm): PO-01/ BPhen (40 nm)/ LiF (0.5 nm)/ Al (150 nm). HATCN, NPB, TCTA and Bphen are Dipyrazino[2,3-f:2',3'h]quinoxaline-2,3,6,7,10,11-hexacarbonitrile, biphenyl]-4,4'-diamine,

N,N'-bis(1-naphthalenyl)-

N,N,N-Tris(4-(9-carbazolyl)phenyl)amine

and

N,N'-diphenyl-[1,1'4,7-Diphenyl-1,10-

phenanthroline, respectively. As can be seen from Figure 2a, the LUMO of CzTrz is deep than that of PO-01, indicating that the electrons will transport through the host rather than trapped by the dopant. Furthermore, the energy barriers for the hole and the electron to cross the TCTA: CzTrz interface are 0.38 eV and 0.88 eV, respectively. Therefore, taking account of the transport abilities and the energy barriers, it can be well assumed that the recombination zone is mainly situated at the interface of TCTA and CzTrz layers, where the interface exciplex is formed. The


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formation of the interface exciplex was identified by the spectra of the device without dopant as shown in Figure 2b with an emission peak at 510 nm, demonstrating our assumption above. The spectrum of the device with bulk exciplex was also shown. Compared with the bulk one, the emission of the interface exciplex show little blue emission at 440 nm. By reducing the thickness of EML from 30 nm to 10 nm, the blue emissionis is significantly reduced. The emission spectrum at 440 nm can be attributed to the CzTrz emission. Since the energy barrier of the HOMOs between TCTA and CzTrz is not large enough, holes will be inevitably injected into the CzTrz layer, resulting in the emission of CzTrz. Devices with dopant concentration of 1~5 wt% were fabricated (Figure S3) and their emission were shown in Figure 2c. The exciplex emission can only be seen for device with 1 wt% PO-01. When the dopant concentration is increased to 2 wt%, the exciplex emission is hardly visible, indicating that the energy transfer is efficient. The current density of the devices are also shown in Figure 2d and no significant change of the current density is observed with the increasing dopant concentration under the same voltage, suggesting that the dopant show negligible trapping effect on the electron transporting of the device. The highest EQE of 26.0% is achieved at dopant concentration as low as 3 wt% (Figure 2e), among one of the highest values for orange PHOLEDs.23,24 Besides, the efficiency roll-off of the device is also small, with EQEs of 24.3% at 5000 cd/m2 and 22.7% at 10000 cd/m2. The small roll-off can be partly attributed to the efficient energy transfer from the exciplex to the dopant, significantly reducing the triplet concentration, which has been demonstrated in PHOLEDs using single-molecule TADF materials as the hosts. The maximum EQEs of devices with 1 wt% and 2 wt% PO-01 are 24.3% and 25.4%, respectively, which are quite close to the highest one obtained at 3 wt%. The efficient energy transfer at low dopant can be attributed to the efficient Förster energy transfer


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process as we discussed above. When the dopant concentration further increases to 5 wt%, the maximum EQE is reduced, possibly resulted from the efficiency quenching effect at high dopant concentration. Devices with thickness of EMLs reduced to 10 nm were also fabricated (Figure S4) and the highest EQE of 27% was also obtained at 3 wt% as can be seen from Figure 2f. Besides, EQEs as high as 25.0% and 26.1% were achieved at dopant concentrations of 1 wt% and 2 wt% (Table 1), respectively. The relatively higher EQEs of the devices with EML thickness of 10 nm than the ones with 30 nm can be attributed to the more efficient energy transfer from the interface exciplex since the direct recombination on the CzTrz are minimized by reducing the thickness of CzTrz layer. To compare the device perfromances with or without energy transfer from the interface exciplex, 1,3-bis(N-carbazolyl)benzen (mCP) was inserted between the TCTA and the CzTrz layers to prevent the formation of the interface exciplex. As can be seen from Figure S5, although the highest EQE of 23.5% was also obtained at dopant concentration of 3%, only 18.2% is obtained at low dopant concentration of 1%, indicating that the energy transfer is less efficient compared with the one with interface exciplex. Since no exciplex is formed, the recombination zone is suited in the CzTrz: PO-01 layer, which can be demonstrated by the current density of the device. As can be seen from Figure S5b, with the dopant concentration increased form 1wt% to 3 wt%, the current density is reduced, indicating that the charges are trapped by the dopant. When the dopant concentration further increase to 5%, the current density increases, meaning that the at high dopant concentration the charges are transported through the dopant. The formed CzTrz triplets can only be transferred to the dopant through the short range Dexter energy transfer, thus requiring higher dopant concentration to achieve efficient energy transfer.


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Figure 3. a) The energy diagram of the devices with inserted CzTrz layers. b) The transient decay curves of the devices with inserted CzTrz layers measured at 500 nm. c) The transient decay curves of the devices with inserted CzTrz layers measured at 570 nm. d) The diagram of the energy transfer mechanism in the devices with interface exciplex. The direct evidence of the Förster transfer can be identified using the transient decay curves of the devices with pure CzTrz inserted between the TCTA and CzTrz: PO-01 layers (Figure 3a). As can be seen from Figure S6, the exciplex emission is gradually increased with the increasing thickness of the inserted pure CzTrz layer, suggesting that the energy transfer is less


12

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efficient with the increasing distance of the dopant from the recombination zone. The transient decay curves of the exciplex and the PO-01 emission were measured. As can be seen from Figure 3b, the lifetimes of the decay curves of exciplex emission are gradually increased with the thickness of the undoped CzTrz layer increased from 2 nm to 12 nm. The same trend was also observed for the decay curves of the PO-01 emission (Figure 3c), indicating that the dopant energy was transferred from the singlet energy of the interface exciplex. In those devices, the singlet excitons of the interface exciplex can decay to the ground state, or to the triplet states through intersystem crossing, or to the dopant through energy transfer, which are competing processes (Figure 3d). With the increasing thickness of the inserted layers, the energy transfer rate is reduced, and thus more excitons will decay to the ground states of the molecules and the emission of the exciplex is gradually increased. Besides, exciotns will cycle more between the singlet and triplet of the exciplex before they finally decay to the ground states or being transferred to the dopant, resulting in the long tail of the transient decay curves of both the exciplex and the PO-01 emission. As can be seen from the transient decay curves of the exciplex device without dopant (Figure 3b), the ratio of the delayed part is about 73%, approaching the theoretical value of 75%. This means that the triplet up-conversion process is efficient, guaranteeing the efficient Förster transfer between the exciplex and the dopant.


13


30

a

b

100 90

20

80 I/I0

EQE

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1% PO-01 2% PO-01 3% PO-01 5% PO-01 bulk exciplex

10

70 60

0 0

2500

5000 2 Brightness (cd/m )

7500

10000

50 0.01

TCTA/ CzTrz: 3% PO-01 TCTA/ CzTrz: 3% PO-01 TCTA/ CzTrz: 5% PO-01 41.7% CzTrz: 55.3% TCTA: 3% PO-01 58.0% CzTrz: 39.0% TCTA: 3% PO-01 TCTA/ mCP/ CzTrz: 3% PO-01

0.1

1 10 Lifetime (h)

100

1000

Figure 4. a) The EQE-brightness curves of devices with bulk exciplex. b) The lifetimes of the devices. To compare the device performances with interface exciplex or bulk exciplex as the host, the devices with mixed TCTA: CzTrz (molar ratio of 1:1) as cohosts were also fabricated (Figure S7). As can be seen from Figure 4a, the highest EQE of the devices with bulk exciplex is 23.1%, lower than the one with the interface exciplex. Besides, the 85% of the maximum EQE was remained at 10000 cd/m2, which was also little lower than the device with interface exciplex (87%). Furthermore, the lifetimes of the devices with interface exciplex and bulk exciplex were measured at an initial luminance of 1000 cd/m2. As can be seen from Figure 4b, the T85 of the device with interface exciplex is nearly 190 h regardless of the dopant concentration. It has been demonstrated that the degradation of the PHOLEDs is mainly attributed to the biomolecule annihilation, especially the triplet-polaron annihilation (TPA). The TPA process has been demonstrated to be the dominant mechanism for the degradation of the PHOLEDs, which leads to the formation of defect sites acting as luminance quenchers, nonradioactive recombination centers and deep charge traps. For PHOLEDs with interface exciplex, the long lifetime may be attributed to the reduced annihilation process. On one hand, the efficient long-range Förster energy transfer can significantly reduce the concentration of excitons at the donor-acceptor


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interface, facilitating to reduce the TPA process. On the other hand, compoared with the excited states of the donor and acceptor, the energy of the exciplex is significantly reduced. The low energy of the excited states reduce the risk of the molecule cleverage, benificial to promote the device lifetime. The lifetime of the devices with bulk exciplex (55.3 wt% TCTA: 41.7 wt% CzTrz: 3 wt% PO-01), surprisingly, only show T85 less than 1h. The lifetime of the device with interface exciplex is almost two orders of magnitudes higher than the one based on conventional bulk exciplex as the host. The reason may be attributed to the instability of TCTA as the host, which can be demonstrated by the longer lifetime of devices (3.6 h) with lower ratio of TCTA (39.0 wt% TCTA: 58.0 wt% CzTrz: 3 wt% PO-01). The influence of TCTA on the device lifetimes can be revealed by the recombination mechnisim of the devices. As can be seen from Figure S8a and S8b, although the main recombination strategy of both the two type devices are through the exciplex mechnisim, other recombination mechnismes are also involved. For devices with interface exciplex, the recombnation on the CzTrz also takes place, demonstrated by the emission of CzTrz in devices without dopant (Figure 2b). Due to the large energy barrier between the LUMOs of CzTrz and TCTA (0.88 eV), the electrons are hardly injected into the TCTA layers. Therefore, the chance to form TCTA excitons is poor. On the contrary, for devices with bulk exciplex, the electrons are directly injected into the TCTA layers from the Bphen layers. The enengy barriers of LUMOs between TCTA and Bphen (0.6 eV) is relatively lower than the one between CzTrz and TCTA (0.88 eV), indicating that the charges recombine on TCTA molecules should also be involved besides from the recombination on the exciplex and CzTrz molecules. The stability of TCTA have been widely studied,25,26 and found that the bond dissociation reactions of arylamines occur by the hemolysis of the lowest singlet exited states


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formed by recombining charge carriers in the operating OLED devices. Therefore, the TCTA is unstable in bulk exciplex owing to the formation of the TCTA excitons, which will lead to the short lifetime of the devices. Contrarily, the TCTA is stable in the devices with interface exciplex since the TCTA is mianly responsible for hole transporting without excitons formed. Therefore the devices with energy transfer from the interface exciplex can achieve longer lifetimes than the ones based on bulk exciplex. Notably, the lifetimes of devices with interface exciplex are even longer than the ones based on single-molecule TADF materials as the hosts in similar devices configuration,22 demonstrating the superior of the concept demonstrated here. By adopting more stable electron-transporting materials, the lifetimes of the devices can be further promoted. The trend of the lifetime we demonstrated here may be general for the devices with exciplex cohosts. For the mixed hosts reported until now, the most commonly used donor molecules are usually

aromatic

(TAPC),16,20 or

amines,

like

4,4’-cyclohexylidenebis[N,N-bis-(4-methylphenyl)aniline

carbazole derivatives, such as TCTA5,18 and 4, 4′, 4″ -tris[3-

methylphenyl(phenyl)amino]triphenylamine (m-MTDATA).13,19 For those materials, the singlet state energy and the carbon-nitragen bond strength are comparable and thus homolytic bond dissociations are likely to take place, resulting in low device stability. For improving the lifetimes of OLEDs, it is highly suggested to consider the relative bond strengths in charged states or excited states of OLED materials.27 Here we demonstrated that utilizing interficial exciplex rather than the bulk one may be another solution for the further imporvement of OLEDs lifetimes by avoid the formation of the donor excited states. Without doubt that the prerequiste to obtain long lifetime from the PHOLEDs based on interface exciplex is adopt stable acceptor materials. Compared with the limited types of donor moeties, the acceptors are widely studied


16

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and many kinds have been designed. The acceptor CzTrz, chosen here is a bipolar molecule possessing smaller energy gap compared with the commonly unipolar ones, which is beneficial for the device lifetime. To demonstrate that CzTrz is stable, the lifetime of devices without interface exciplex is also measured as can be seen from Figure 4b. Although unstable mCP molecule is adopted, the lifetime is larger than that of the one with bulk exciplex, demonstarting that CzTrz is stable. We believe that by choosing more adequte acceptors, the PHOLEDs with interface exciplex may achieve better performances. Table 1. The summary of the device performances. Maximum Concentration (Thickness)

V (v) (100cd/m2)

Interface exciplex

1% (30 nm)

Interface exciplex

5000 cd/m2

10000 cd/m2

EQE (%)

PE (lm/W)

EQE (%)

PE (lm/W)

EQE (%)

PE (lm/W)

3.82

24.3

57.5

20.2

33.0

17.8

25.8

2% (30 nm)

3.84

25.4

57.8

22.3

38.4

21.2

32.5

Interface exciplex

3% (30 nm)

3.86

26.0

58.0

24.3

39.0

22.7

33.8

Interface exciplex

5% (30 nm)

3.84

23.5

53.1

22.4

36.8

21.2

31.3

Interface exciplex

3% (10 nm)

3.06

27.0

73.1

25.6

52.1

24.0

44.6

Bulk exciplex

3% (30 nm)

3.36

23.5

58.5

21.5

41.1

19.5

33.2

Exciplex free

3% (30 nm)

4.94

23.2

44.4

21.2

28.7

20.1

24.6

CONCLUSIONS In conclusion, we report high performance orange PHOLEDs with low dopant concentration by efficient Förster energy transfer from the interface exciplex between CzTrz and TCTA layers. High EQE as high as 27% with low efficiency roll-off was achieved at low dopant concentration of 3 wt%. What is more, devices with interface exciplex as the hosts achieve lifetime almost two orders of magnitudes higher than the one based on bulk exciplex as the host. It is believed that the strategy proposed here is a general way to fabricate stable PHOLEDs with


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high efficiency, low efficiency roll-off and long lifetime at low dopant concentration achieved simultaneously. AUTHOR INFORMATION Corresponding Author *Correspondence to: E-mail address: [email protected]: +86 10 62795137; Tel: +86 10 62782197 Notes The authors declare no competing financial interests. ACKNOWLEDGMENT We would like to thank the National Natural Science Foundation of China (Grant No. 51173096) and the National Key Basic Research and Development Program of China (Grant No. 2015CB655002) for financial support. SUPPORTING INFORMATION Photoluminance spectra of CzTrz. Calculation of the ratio of the prompt and delayed parts of exciplex emission. The current density of the single-carrier devices. The performance of devices with interface exciplex (EML of 30 nm). The performance of devices with interface exciplex (EML of 10 nm). The performance of devices with exciplex free. The emission spectra of devices with inserted CzTrz layer. The performance of devices with bulk exciplex. The recombination mechnisims of devices with exciplex. This material is available free of charge via the Internet at http://pubs.acs.org.


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REFERENCES (1) Tang, C. W.; Vanslyke, S. A. Organic Electroluminescent Diodes. Appl. Phys. Lett. 1987, 51, 913. (2) Baldo, M. A.; O’Brien, D. F.; You, Y.; Shoustikov, A.; Sibley, S.; Thompson, M. E.; Forrest, S. R. Highly Efficient Phosphorescent Emission from Organic Electroluminescent Devices. Nature. 1998, 395, 151-154. (3) Adachi, C.; Baldo, M. A.; Thompson, M. E.; Forrest, S. R. Nearly 100% Internal Phosphorescence Efficiency in an Organic Light-emitting Device. J. Appl. Phys. 2001, 90, 5048. (4) Yook, K. S.; Lee, J. Y. Organic Materials for Deep Blue Phosphorescent Organic Lightemitting Diodes. Adv. Mater. 2012, 24, 3169–3190. (5) Kim, K. H.; Lee, S.; Moon. C. K.; Kim, S. Y.; Park, Y. S.; Lee, J. H.; Lee, J. W.; Huh, J.; You, Y.; Kim, J. J. Phosphorescent Dye-based Supramolecules for High-efficiency Organic Light-emitting Diodes. Nat. Commun. 2013, 5, 4769. (6) Xiang, C. Y.; Koo, W.; So, F.; Sasabe, H.; Kido, J. A Systematic Study on Efficiency Enhancements in Phosphorescent Green, Red and Blue Microcavity Organic Light Emitting Devices. Light-Sci. App. 2013, 2, e74. (7) Zhang, D. D.; Duan, L.; Zhang, D. Q.; Qiu, Y. Towards Ideal Electrophosphorescent Devices with Low Dopant Concentrations: the Key Role of Triplet Up-conversion. J. Mater. Chem. C. 2014, 2, 8983-8989.


19


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 23

(8) Li, C.; Duan, L.; Zhang, D. D.; Qiu, Y. Thermally Activated Delayed Fluorescence Sensitized Phosphorescence: A Strategy To Break the Trade-Off between Efficiency and Efficiency Roll-Off. ACS Appl. Mater. Interfaces. 2015, 7, 15154–15159. (9) Fukagawa, H.; Shimizu, T.; Kamada, T.; Yui, S.; Hasegawa, M.; Morii, K.; Yamamoto, T. Highly Efficient and Stable Organic Light-emitting Diodes with a Greatly Reduced Amount of Phosphorescent Emitter. Sci. Rep. 2015, 5, 9855. (10)

Zhang, D. D.; Duan, L.; Li, C.; Li, Y. L.; Li, H. Y.; Zhang, D. Q.; Qiu, Y. High-efficient Fluorescent Organic Light-emitting Devices Using Sensitizing Hosts with a Small Singlettriplet Exchange Energy. Adv. Mater. 2014, 26, 5050–5055.

(11)

Jeon, W. S.; Park, T. J.; Kim, S. Y.; Pode, R.; Jang, J.; Kwon, J. H. Ideal Host and Guest System in Phosphorescent OLEDs. Org. Electron. 2009, 10, 240-246.

(12)

Uoyama, H.; Goushi, K.; Shizu, K.; Nomura, H.; Adachi, C. Highly Efficient Organic Light-emitting Diodes from Delayed Fluorescence. Nature. 2012, 492, 234-238.

(13)

Zhang, Q. S.; Li, B.; Huang, S. P.; Nomura, H.; Tanaka, H.; Adachi, C. Efficient Blue Organic Light-emitting Diodes Employing Thermally Activated Delayed Fluorescence. Nat. Photonics. 2014, 8, 326-332.

(14)

Lee, D. R.; Kim, M.; Jeon, S. K.; Hwang, S. H.; Lee, C. W.; Lee, J. Y. Design Strategy for 25% External Quantum Efficiency in Green and Blue Thermally Activated Delayed Fluorescent Devices. Adv. Mater. 2015, 27, 5861–5867.


20

Page 21 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(15)

Goushi, K.; Yoshida, K.; Sato, K.; Adachi, C. Organic Light-emitting Diodes Employing Efficient Reverse Intersystem Crossing for Triplet-to-singlet State Conversion. Nat. Photonics. 2012, 6, 253-258.

(16)

Liu, X. K.; Chen, Z.; Zheng, C. J.; Chen, M.; Liu, W.; Zhang, X. H.; Lee, C. S. Nearly 100% Triplet Harvesting in Conventional Fluorescent Dopant-based Organic Light-emitting Devices Through Energy Transfer from Exciplex. Adv. Mater. 2015, 27, 2025–2030.

(17)

Park, Y. S.; Jeong, W. I.; Kim, J. J. Energy Transfer from Exciplexes to Dopants and its Effect on Efficiency of Organic Light-emitting Diodes. J. Appl. Phys. 2011, 110, 124519.

(18)

Kim, S. Y.; Jeong, W. I.; Mayr, C.; Park, Y. S.; Kim, K. H.; Lee, J. H.; Moon, C. K.; Brütting, W.; Kim, J. J. Organic Light-emitting Diodes with 30% External Quantum Efficiency Based on a Horizontally Oriented Emitter. Adv. Funct. Mater. 2013, 23, 3896– 3900.

(19)

Wang, S. M.; Wang, X. D.; Yao, B.; Zhang, B. H.; Ding, J. Q.; Xie, Z. Y.; Wang, L. X. Solution-processed Phosphorescent Organic Light-emitting Diodes with Ultralow Driving Voltage and very High Power Efficiency. Sci. Rep. 2015, 5, 12487.

(20)

Seino, Y.; Sasabe, H.; Pu, Y. J.; Kido, J. High-performance Blue Phosphorescent OLEDs Using Energy Transfer from Exciplex. Adv. Mater. 2014, 26, 1612–1616.

(21)

Oh, C. S.; Kang, Y. J.; Jeon, S. K.; Lee, J. Y. High Efficiency Exciplex Emitters using Donor-acceptor Type Acceptor Materials. J. Phys. Chem. C. 2015, 119, 22618−22624.


21


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(22)

Page 22 of 23

Nakagawa, T.; Ku, S. Y.; Wong, K. T.; Adachi, C. Electroluminescence Based on Thermally Activated Delayed Fluorescence Generated by a Spirobifluorene Donor–acceptor Structure. Chem. Commun. 2012, 48, 9580-9582.

(23)

Zhang, D. D.; Duan, L.; Li, Y. L.; Li, H. Y.; Bin, Z. Y.; Zhang, D. Q.; Qiao, J.; Dong, G. F.; Wang, L. D.; Qiu, Y. Towards High Efficiency and Low Roll-off Orange Electrophosphorescent Devices by Fine Tuning Singlet and Triplet Energies of Bipolar Hosts Based on Indolocarbazole/1,3,5-triazine Hybrids. Adv. Funct. Mater. 2014, 24, 3551– 3561

(24)

Lee, S.; Kim, K. H.; Limbach, D.; Park, Y. S.; Kim, J. J. Low Roll-off and High Efficiency Orange Organic Light Emitting Diodes with Controlled Co-doping of Green and Red Phosphorescent Dopants in an Exciplex Forming Co-host. Adv. Funct. Mater. 2013, 23, 4105–4110.

(25)

Scholz, S.; Kondakov, Denis.; Lüssem, B.; Leo, K. Degradation Mechanisms and Reactions in Organic Light-emitting Devices. Chem. Rev. 2015, 115, 8449–8503.

(26)

So, F.; Kondakov, D. Degradation Mechanisms in Small-molecule and Polymer Organic Light-emitting Diodes. Adv. Mater. 2010, 22, 3762–3777.

(27)

Lin, N.; Qiao, J.; Duan, L.; Wang, L. D.; Qiu, Y. Molecular Understanding of the Chemical Stability of Organic Materials for OLEDs: a Comparative Study on Sulfonyl, Phosphine-oxide, and Carbonyl-containing Host Materials. J. Phys. Chem. C. 2014, 118, 7569–7578.

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Simultaneous Enhancement of Efficiency and Stability of

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