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Simple-Structured Phosphorescent Warm White Organic LightEmitting Diodes with High Power Efficiency and Low Efficiency Roll-off Jiaxiu Wang, Jiangshan Chen, Xianfeng Qiao, Saad M. Alshehri, Tansir Ahamad, and Dongge Ma ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b00861 • Publication Date (Web): 08 Apr 2016 Downloaded from http://pubs.acs.org on April 9, 2016

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ACS Applied Materials & Interfaces

Simple-Structured Phosphorescent Warm White Organic Light-Emitting Diodes with High Power Efficiency and Low Efficiency Roll-off Jiaxiu Wang,† Jiangshan Chen,† Xianfeng Qiao,† Saad M. Alshehri,§ Tansir Ahamad,§ and Dongge Ma*,†,‡,§

†State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, University of Chinese Academy of Sciences, Changchun, 130022, People’s Republic of China. ‡State Key Laboratory of Luminescent Materials and Devices, Institute of Polymer Optoelectronic Materials and Devices, South China University of Technology, Guangzhou, 510640, People’s Republic of China. §Department of Chemistry, King Saud University, Riyadh, Kingdom of Saudi Arabia

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected];

1 ACS Paragon Plus Environment


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ABSTRACT We present phosphorescent WOLEDs fabricated simply by inserting an ultra-thin non-doped orange layer within the blue emissive zone, where an efficient exciplex system is applied as the host. The resulting WOLED shows maximum power efficiency of 75.3 lm/W and 63.1 lm/W at the luminance of 1000 cd/m2. The exciton density profile in the emitting layer, the operational mechanism and the quenching process at high luminance are systematically investigated by experimental and theoretical methods, from which it is concluded that the efficient utilization of excitons via exciplex host and the wide recombination zone are the key factors for the prominent achievement of high efficiency and greatly reduced efficiency roll-off.

KEYWORDS: exciplex host, ultra-thin layer, exciton density profile, recombination zone, quenching process


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White organic light-emitting devices (WOLEDs) have attracted worldwide attention in both research and industry owing to their promising applications in flat-panel displays and solid-state lighting.1,2 Recent studies mainly focus on how to obtain WOLEDs with high efficiency, low efficiency roll-off and high color rendering index (CRI).3,4 The principal strategy to realize high-efficiency WOLEDs is to achieve 100% internal quantum efficiency (IQE). Traditionally, fluorescence-phosphorescence hybrid and all-phosphorescent WOLEDs are mostly exploited to achieve this goal.5,6 Recently, an efficient kind of thermally activated delayed fluorescence (TADF) materials, including intra-molecular energy transferred and exciplex materials can similarly achieve 100% IQE via reverse intersystem crossing (RISC) process from triplet to singlet excited states.7-10 One necessary condition to facilitate this process is that the activation energy of RISC (∆EST) should be small. Generally, the ∆EST of exciplexes is expected to be much smaller than that of intramolecular excited states since the distance between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) in exciplexes is much larger.11 By using exciplex as host to fabricate OLEDs, the working voltage can be significantly reduced compared with the conventional host because the injection and transport of electrons and holes are via the LUMO of acceptors and the HOMO of donors, respectively. Moreover, the approximately equal singlet and triplet energy levels of exciplex makes it easy to adjust the energy level of the exciplex to that of a used guest material by selecting appropriate donors and acceptors, which makes the



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energy transfer from both singlet and triplet levels of exciplex to the triplet level of the phosphorescent guest more easily. This will further reduce the working voltage. As we know, lower operating voltage is very beneficial to high efficiency. Recently, some high performance monochromatic OLEDs based on exciplex host systems have been reported.12-15However, only limited efforts have been made to develop the exciplex-based WOLEDs.16,17 In this paper, by adopting an exciplex system as host for the blue phosphorescent emitter, and simply inserting an ultra-thin non-doped layer of highly efficient orange phosphorescent emitter within the blue layer, we successfully achieved WOLEDs with low working voltage, high efficiency and reduced efficiency roll-off. The maximum forward-viewing efficiencies reached 64.5 cd/A, 75.3 lm/W and 20.0 % without using light out-coupling technique and remained 62.8 cd/A, 63.1 lm/W and 19.5 % at the luminance of 1000 cd/m2. This should be among the best results reported so far for phosphorescent WOLEDs based on exciplex host with simple device structures. Figure S1 illustrates the device structure and the energy level diagram of the used materials. The corresponding chemical structures are demonstrated in Figure S2. The blue emission directly determines the performance of WOLEDs. Therefore, it is very important to first optimize the blue emission property. Here, we choose the most widely used iridium (III) bis[(4,6-difluorophenyl)-pyridinato-N,C2′]picolinate (FIrpic) with T1 of 2.62 eV as blue phosphorescent emitter, and N,N′-dicarbazolyl-3,5-benzene


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(mCP) : bis-4,6-(3,5-di-3-pyridyl-phenyl)-2-methylpyrimidine (B3PYMPM) exciplex with a higher T1 (2.97 eV) (Figure S1) than FIrpic as host.14 To eliminate the charge injection barrier between the charge-transporting layer and the emitting layer (EML), we exploit mCP as the HTL and B3PYMPM as the ETL, and fabricate device B1: ITO/MoO3 (10 nm)/mCP: MoO3 (10%, 50 nm)/mCP (20 nm)/mCP: B3PYMPM: FIrpic (1:1:0.4, 10 nm)/B3PYMPM (15 nm)/B3PYMPM: Li2CO3 (3%, 40 nm)/ Li2CO3 (1 nm)/Al. It is expected to realize a low turn-on voltage and a high power efficiency. However, the turn-on voltage is as high as 3.0 V and the maximum power efficiency reaches merely 29.4 lm/W (Table S1), which shows no improvement compared with traditional blue phosphorescent OLEDs.18 As is reported, the hole mobility of mCP is 5.0× 10-4 cm2 v-1 s-1, which is not fast enough to transport holes into the EML at a low voltage.19 Moreover, the T1 of mCP (2.90 eV) is lower than that of the exciplex, as a result, the triplet excitons formed on the exciplex can be transferred to the adjacent mCP layer and decay by a non-radiative process, leading to energy loss. To resolve these problems, we replaced the mCP by 1,1-bis-(4-bis(4-tolyl)-aminophenyl)cyclohexene (TAPC) as the HTL and electron/exciton blocking layer (EBL), thus fabricated device B2: ITO/MoO3 (10 nm)/TAPC: MoO3 (10%, 50 nm)/TAPC(20 nm)/mCP: B3PYMPM: FIrpic (1:1:0.8, 10 nm)/B3PYMPM (15 nm)/B3PYMPM: Li2CO3 (3%, 40 nm)/Li2CO3 (1 nm)/Al. Because TAPC has a higher hole mobility (1.0×10-2 cm2 v-1 s-1 ) and T1 (2.98 eV) than mCP,19,20 as expected, the turn-on voltage of device B2 is reduced to 2.4 V



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while the maximum power efficiency is improved to 47.1 lm/W (Table S1), which should be quite high for a blue OLED based on FIrpic emitter.18 The achievement of the high-efficiency blue OLEDs indicates the validity of mCP: B3PYMPM exciplex as the host and the significance of exploiting organic materials with high T1 as the EBL. Furthermore, we explored the exciton density profile in the EML by using the structure of ITO/MoO3 (10 nm)/TAPC: MoO3 (10%, 50 nm)/TAPC (20 nm) /mCP: B3PYMPM: FIrpic (1:1:0.08, x nm)/PO-01 (0.07 nm)/mCP: B3PYMPM: FIrpic (1:1:0.08, (10-x) nm)/B3PYMPM(15 nm)/B3PYMPM: Li2CO3(3%, 40 nm)/Li2CO3(1 nm)/Al, where x varies from 0 to 10 nm to change the position of the non-doped orange layer (Figure 1a). As shown in Figure 1b, the ratio of the orange emission peak intensity to the blue intensity as a function of the position of orange thin layer at the driving voltage of 4.0 V is given. It reveals that the orange emission is strongest at the position 3 nm from the interface between the EBL and the EML, and remains higher intensity than blue emitter at about 6 nm from this interface. This implies that the exciton recombination zone in device B2 is rather broad, which contributes to the high-efficiency blue emission.


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Figure 1. (a) Schematic diagram for the exploration on exciton density distribution in the EML. (b) Ratio of orange emission peak intensity to blue intensity as a function of the position of orange thin layer in the EML at a driving voltage of 4.0 V.

Based on the explored exciton density profile in the EML, we fabricated WOLEDs (device

W)

by

inserting

an

ultra-thin

non-doped

(acetylacetonato)bis[2-(thieno[3,2-c]pyridin-4-yl)phenyl]iridium(III)

layer

of

(PO-01)

orange at

the

interface between the EBL and the EML to make full use of the high exciton densityhere. The optimized device structure is as follows: ITO/MoO3 (10 nm)/TAPC: MoO3 (10%, 50 nm)/TAPC (20 nm) /PO-01 (0.06 nm)/mCP: B3PYMPM: FIrpic (1:1:0.4, 10 nm)/B3PYMPM (15 nm)/B3PYMPM: Li2CO3 (3%, 40 nm)/Li2CO3 (1 nm)/Al. Figure 2a



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plots the current density-luminance-voltage characteristics of device W. It can be seen that the turn-on voltage is 2.4 V. The current efficiency (CE), power efficiency (PE) and external quantum efficiency (EQE) versus luminance properties are shown in Figure 2b and c, respectively. The maximum forward-viewing CE, PE and EQE reach 64.5 cd/A, 75.3 lm/W and 20.0%, respectively, and remain 62.8 cd/A, 63.1 lm/W and 19.5% at the luminance of 1000 cd/m2 (Table S1) without using light out-coupling enhancement techniques, which should be among the highest values for leading-edge phosphorescent WOLEDs based on exciplex host with simple device structures.3,6,21The achievement of the high PE is obviously related to the ingenious design of the device structure. Besides the good exciton confinement in the EML by using wide band-gap electron-transporting and hole-transporting materials with high T1, as we know, the exciplex host can harvest 100% excitons and transfer energy from both the singlet and triplet excited states to the guests, leading to the high efficiencies. More importantly, the injected holes transport only along the HOMO of mCP while the injected electrons transport only along the LUMO of B3PYMPM, thus the electrical excitation energy required to achieve the excitation state of the exciplex host is much lower than that of conventional hosts. Hence, the driving voltage can be significantly reduced. Furthermore, the introduction of the ultra-thin orange emitting layer has a negligible influence on the charge transport property (Figure S3 c and d), which is also favorable to realize a low working voltage. Combining the utilization of p-doping and n-doping, respectively, in the HTL and


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ETL,22-24 we finally obtained WOLEDs with low working voltage and high efficiency.

Figure 2. EL characteristics of device W. (a) Current density–luminance–voltage characteristics, with luminance measured in forward direction without out-coupling enhancement techniques. (b) PE and CE versus luminance properties. (c) EQE versus luminance property. The inset of (c) shows the normalized EL spectra at different 9 ACS Paragon Plus Environment


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brightness levels.

As we know, efficiency roll-off is a common problem in phosphorescent OLEDs, which may limit their implementation at high brightness in lighting applications. Generally, triplet-triplet annihilation (TTA), triplet-polaron quenching (TPQ) and field-induced quenching are possible quenching processes in OLEDs.25-27 Baldo et al.26 reported that TTA is the main reason that cause the efficiency decline of OLEDs while Reineke et al.28hold the opinion that both TTA and TPQ should be considered for common OLEDs. As described by Baldo et al,26 for TTA model, the EQE () dependence of current density () can be expressed as follows: = 1 + 8 − 1 (1) 4 For TPQ model, the equation reads: 28

1 = (2) () 1 + ( )

where is the EQE without the triplet exciton quenching while and is the

EQE in the presence of TTA and TPQ, respectively. is the critical current density for

TTA model and for the TPQ model. In order to figure out the cause of efficiency roll-off in our devices, we applied the models of TTA and TPQ to device B2 and device W. Figure 3 illustrates the external quantum efficiency-current density (EQE-J) dependence of our devices together with TTA fitting. It is demonstated that our


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experimental data can be well fitted with TTA model, indicating that the TTA process is the main reason for the efficiency decline of our devices at high current density. The failure of TPQ model fitting implies the well-balanced charge injection and transportation in our devices. Moreover, the wide exciton recombination zone (Figure 1b) conduces to the decrease of triplet exciton concentration, thus reducing the TTA effect and finally resulting in the reduced efficiency roll-off.

Figure 3. EQE-J dependence of device B2 and device W. The solid black and red lines refer to the TTA fitting curves of device B2 and device W, respectively.

To probe into the working mechanism in Device W, we fabricated hole-only and electron-only devices. The current density-voltage (J-V) characteristics of all the devices are shown in Figure S3. We can see that the current density of both hole-only and electron-only devices do not show much variation with the increase of the FIrpic doping concentration, indicating that the charge trapping effect of FIrpic dopant is negligible. On



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the other hand, comparing the current density in the hole-only and electron-only devices with and without 0.07 nm PO-01, it is clear that the trapping effect of the ultra-thin non-doped layer is also negligible. Therefore, we conclude that the blue emission from FIrpic and the orange emission from PO-01 are mainly originate from the energy transfer processes. The injected electrons and holes from electrodes into EML firstly form singlet and triplet excitons on exciplex host, and then the energies of the formed excitons transfer to the FIrpic molecules, leading to the blue emission. The energies of the formed excitons on exciplex host and FIrpic molecules transfer to the PO-01 molecules, resulting in the orange emission. Duan et al. have investigated the enhanced energy transfer from TADF host to phosphor dopant through the long-range Förster energy transfer.10 This enhanced energy transfer process should also exist in our WOLEDs, considering the high efficiency of device W. The inset of Figure 2c shows the spectra of device W at different brightness levels. The variation of CIE coordinates was only (0.010, 0.002) when the brightness increases from 1000 to 5000 cd/m2. The excitons are used by PO-01 prior to FIrpic because PO-01 has lower T1 (2.20 eV) and smaller energy gap (HOMO=5.1 eV, LUMO=2.7 eV).29,30 With the increase of current density, the exciton density rises. However, the excitons utilized by PO-01 should saturate due to the limited quantity of PO-01 molecules, thus more excitons will then be utilized by FIrpic molecules, resulting in a slight increase in the relative intensity of blue emission with the increase of brightness.


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In conclusion, we have successfully achieved simple-structured phosphorescent warm white organic light-emitting diodes with high power efficiency and low efficiency roll-off. The resulting WOLED realizes a maximum forward-viewing PE of 75.3 lm/W and remains 63.1 lm/W at the luminance of 1000 cd/m2. The energy transfer mechanism has been determined to be the main emission processes in the fabricated WOLED. The utilization of exciplex as host of the blue dopant and the insertion of an ultra-thin non-doped orange emitting layer not only guarantee the high efficiency and low efficiency roll-off, but also greatly simplify the device structure, which is very valuable for the further design of high-performance WOLEDs in the future.

ASSOCIATED CONTENT Supporting Information Available: experimental methods, device structure, summary of device performance, energy level diagram and chemical structure of the used materials, J-V characteristics of the hole-only and electron-only devices. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes



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The authors declare no competing financial interests.

ACKNOWLEDGMENTS The authors gratefully acknowledge the National Natural Science Foundation of China (51333007, 91433201), Ministry of Science and Technology of China (973 program No. 2013CB834805), the International Cooperation Foundation of China (2015DFG12470) and Key Scientific and Technological Project of Jilin Province (20130206003GX) for the support of this research. Prof. D. G. Ma extends the appreciation to the Distinguished Scientist Fellowship Program (DSFP) at King Saud University, Riyadh, Kingdom of Saudi Arabia for financial support.

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Simple-Structured Phosphorescent Warm White ... - ACS Publications

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