High-efficiency blue phosphorescent organic light-emitting devices

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Organic Electronic Devices

High-efficiency blue phosphorescent organic lightemitting devices with low efficiency roll-off at ultrahigh luminance by reduced the triplet-polaron quenching Ziwei Yu, Jiaxin Zhang, Shihao Liu, Letian Zhang, Yi Zhao, Hongyu Zhao, and Wenfa Xie ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b19280 • Publication Date (Web): 18 Jan 2019 Downloaded from http://pubs.acs.org on January 18, 2019

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ACS Applied Materials & Interfaces

High-efficiency

blue

phosphorescent

organic

light-emitting devices with low efficiency roll-off at ultrahigh luminance by reduced the triplet-polaron quenching Ziwei Yu1, Jiaxin Zhang1, Shihao Liu1, Letian Zhang1, Yi Zhao1, Hongyu Zhao2*,Wenfa Xie1* 1State

key Laboratory of Integrated Optoelectronics, College of Electronics Science

and Engineering, Jilin University, Changchun, 130012, People’s Republic of China. 2Beijing

Tuocai Optoelectronics Technology CO. LTD, Beijing, 100086, People’s

Republic of China. ABSTRACT: High performance phosphorescent organic light-emitting devices (PhOLEDs) at high luminance are still a remaining problem needing to be solved, especially blue PhOLEDs.

Here,

5-(5-9H-carbazol-9-yl)

pyridin-2-yl)-8-

(9H-carbazol-9-yl)

-5H-pyrido [3, 2-b] indole (p2PCB2CZ) with excellent characteristics as host is designed to realize a novel host-guest system without hole trapping effect in blue PhOLEDs. The device in which p2PCB2CZ and Bis (3, 5-difluoro-2-(2-pyridyl) phenyl-(2-carboxypyridyl)iridium(III) (FIrpic) is used as host and guest, respectively, is proposed to improve the performances of blue PhOLEDs at high luminance, especially ultrahigh luminance (>30000 cd/m2). The maximum external quantum efficiency (EQE) of this type blue PhOLEDs is 19.2%, while the maximum EQE of reference blue PhOLEDs is 18.7 %. Nevertheless, the p2PCB2CZ-based devices

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exhibit significant advantage at high luminance, because its EQE still attains to 10.8% even when the luminance increases to 30000 cd/m2, which is 1.67 times that of the reference device. From measurements based on steady-state and time-resolved spectroscopy, the reduction of triplet-polaron quenching in p2PCB2CZ-based devices is proved to the main reason for improving the performances of blue PhOLEDs at high luminance. Keywords: Blue PhOLED; Hole trapping; Ultrahigh luminance; Efficiency roll-off; Triplet-polaron quenching.


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INTRODUCTION In recent years, organic light-emitting devices (OLEDs) have been generally recognized as the next generation display and lighting technology. Nevertheless, high performance OLEDs at high luminance are required.1-6 For high-resolution display, their pixel size is always limited so that the luminance of each pixel should be over 5000 cd/m2 to ensure display quality. Moreover, for lighting applications, the brightness requirement might be even higher.7-9 Because of the ability to use both singlet excitons and triplet excitons, phosphorescent OLEDs (PhOLEDs) have potential to realize 100% internal quantum efficiency (IQE).10 However, in PhOLEDs, the efficiency roll-off at high luminance is serious due to the long lifetime of triplet excitons.11 The main reasons to cause the serious of efficiency roll-off of PhOLEDs at high luminance is Triplet-triplet annihilation (TTA) and triplet-polaron quenching (TPQ). TTA is a bimolecular interaction between two triplet excitons, increasing with the square of triplet exciton density. The triplet exciton density is governed by excitons lifetime and carrier balance, therefore, TTA can be alleviated by lower lifetime and better carrier balance.12, 13 In contrast to TTA, TPQ is generally considered to be caused by polaron accumulation in the guest, which is mainly due to the energy matching between the materials.14 Additionally, organic semiconductors typically show highly asymmetric hole and electron mobilities, with the hole mobility exceeding the electron mobility by orders of magnitude in most cases.15 Therefore, avoiding hole accumulation is of great importance to reduce TPQ.



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Many efficient efforts have been made to reduce TTA and TPQ of PhOLEDs by researchers.16-18 Especially, green and red PhOLEDs with low efficiency roll-off at high or ultrahigh luminance have been reported.19,

20

Employing a bipolar host

PPI-F-TPA, Tong et al. reported high-performances orange-red PhOLEDs with a bipolar host which has a low roll-off of external quantum efficiency (EQE) less than 13.6% at 10000 cd/m2 and 29.5% at 50000 cd/m2.11 However, compared to green and red PhOLEDs, high efficiency blue PhOLED with low efficiency roll-off is still challenging. Certainly, introducing an excellent host material is one of the main approaches to realize high performances PhOLEDs. For blue PhOLEDs, it is extremely important to choose a suitable host material.21 By using bipolar carbazole host, blue PhOLEDs with low efficiency roll-off were reported, but low efficiency roll-off can only be observed in these blue PhOLEDs at low or medium luminance.12, 17, 22

Bis(3,5-difluoro-2-(2-pyridyl)phenyl-(2-carboxypyridyl)iridium(III) (FIrpic) is the most commonly used blue phosphorescent material due to its stability and high efficiency. However, the higher highest occupied molecular orbital (HOMO) energy level of FIrpic in host-guest system always leads to the accumulation of holes, which can cause serious TPQ and serious reduction of performance at high luminance. Here, novel host-guest system in which the HOMO energy level of host is higher than that of the guest FIrpic is proposed to accomplish high-performance blue PhOLEDs with low efficiency roll-off at high luminance. This principle can effectively reduce the hole accumulation in phosphorescent molecules, resulting in lower TPQ in PhOLEDs


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at high luminance. Here, we designed carbazole material5-(5-9H-carbazol-9-yl) pyridin-2-yl)-8-(9H-carbazol-9-yl) -5H-pyrido[3, 2-b] indole (p2PCB2CZ). Except suitable HOMO energy level, p2PCB2CZ also shows excellent properties as host material such as high triplet energy, balanced carrier transport and better thermal stability. In order to verify the superiority of the novel host-guest system in reducing efficiency roll-off, blue PhOLEDs with the novel host-guest system are fabricated and their electroluminescent performances are studied by comparison to blue PhOLEDs with guest charge trapping emitting layer. RESULTS AND DISCUSSIONS The molecular structures of two carbazole hosts are shown in Fig. 4. From the molecular structures, both host materials exhibit bipolar properties because of the presence of both electron-donating unit and electron-withdrawing unit, which is used for hole and electron transporting, respectively.24 The electron-donating carbazole derivatives

are

typical

host

materials

developed

for

PhOLEDs.24

Many

carbazole-based materials have a low glass transition temperature (Tg), which is not beneficial to form a uniform amorphous film upon thermal evaporation. Compared with common carbazole-based materials, p2PCB2CZ has a much higher Tg of 152oC with the introduction of δ-Carboline, an electron-withdrawing unit which can raise the Tg effectively.24 The raise of Tg of host materials can slightly avoid the effect of the accumulative joule heating under device operation and improve the problem of efficiency roll-off at high luminance.26



Thermal Properties: We concluded the thermal properties of the new material p2PCB2CZ by thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) under a nitrogen atmosphere. From Fig. 1 (a), a high decomposition temperature (Td) of 475 ℃ which corresponds to 5% weight loss is observed. What's more, compared with 2,6-bis(3-(carbazol-9-yl)phenyl)pyridine (DCZppy, a common host material)25 (102oC), p2PCB2CZ has a much higher Tg of 152oC. All of these indicated that p2PCB2CZ has superior thermal properties to bear high temperature in the process of manufacturing the devices by thermal evaporation.

Figure 1. DSC (a) and TGA (b) thermograms of p2PCB2CZ

Photophysical and Electrochemical Properties: Fig. 2 shows the PL spectra of p2PCB2CZ and DCZppy, the room-temperature UV-Vis absorption spectra of p2PCB2CZ and FIrpic, as well as the low temperature (T=77 K) phosphorescence of p2PCB2CZ. Four absorption peaks can be seen from the absorption spectrum of p2PCB2CZ. A strong absorption peak at around 240 nm should be attributed to the π-π* transition of the Pyridine27 and the peaks at 293, 315 and 336 nm to the π-π* transitions of the carbazole chromophore. The T1 of p2PCB2CZ is estimated to be 2.72 eV according to the highest peak of low temperature absorption spectra at 448


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nm, higher than FIrpic (2.62 eV), a common blue phosphorescent material.30 It indicates that the energy could be efficiently transferred from the T1 of p2PCB2CZ to the T1 of FIrpic via Dexter transfer. Furthermore, the S1 (the lowest single energy level) of p2PCB2CZ can be energetically transferred to the lowest S1 in FIrpic through Förster energy transfer. Then the S1 of FIrpic can be transferred to the T1 of FIrpic.6, 27 From Fig. 2, a large overlap between the absorption spectrum of FIrpic and the emission spectrum at room temperature of p2PCB2CZ can be seen, which indicates that a relatively complete energy-transfer process occur from p2PCB2CZ to FIrpic.27 And the PL spectrum of DCZppy was also measured. There is also a large overlap between the emissions of DCZppy and the absorption band of FIrpic. Moreover, it is important to reveal the molecular energy levels of the host materials.31 To evaluate the electrochemical proper of p2PCB2CZ, cyclic voltammetry (CV) was performed in dichloromethane solvent. As shown in Fig. 2 (b), the HOMO energy levels of p2PCB2CZ could be obtained from the onset potential of the first oxidation onset wave (Eonset +4.4) as -5.57 eV.28, ox ) according to the Equation EHOMO = ―(Eox

29

And the LUMO energy level, which can be determined by the EHOMO and Eg, is -2.32 eV for p2PCB2CZ. The thermal properties of the both two host materials are shown in Table 1.



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Figure 2. (a) The UV-Vis absorption spectra of p2PCB2CZ and FIrpic, the phosphorescence spectra at 77 k of p2PCB2CZ and the PL spectra at room temperature of p2PCB2CZ and DCZppy; (b) Cyclic Voltammetry (CV) curve of p2PCB2CZ. Table 1. Photophysical characteristics and thermal properties of p2PCB2CZ and DCZppy. Material

Td (oC)

Tg (oC)

HOMOa (eV)

LUMOb (eV)

Egc (eV)

T1d (eV)

p2PCB2CZ

475

152

-5.57

-2.32

3.25

2.72

DCZppy

455 e

102 e

-6.05e

-2.56e

3.49e

2.71e

aHOMO: Confirmed by CV curve, bLUMO: Calculated by HOMO and E . cEnergy gap (E ): Estimated from the g g UV-vis absorption spectrum. dCalculated from the phosphorescence spectra spectrum at 77 K. eObtained from Ref. [30]

Carrier Transport Properties: Introducing a suitable host material which possesses bipolar charge transport property is important to achieve high performance PhOLEDs. Single-hole and single-electron devices with the structures of ITO/MoO3 (3 nm)/ 1,1-bis[(di-4-toly-lamino) phenyl] cyclohexane(TAPC: 40 nm)/p2PCB2CZ or DCZppy (20 nm)/TAPC (50 nm)/Mg:Ag (15:1 by weight, 150 nm) and ITO/1,3,5-tri[(3-pyridyl) -phen-3-yl] benzene (TmPyPB: 50 nm)/p2PCB2CZ or DCZppy (20 nm)/TmPyPB (50 nm)/LiF/Mg:Ag (15:1 by weight, 150 nm) were fabricated to explore the charge transport abilities of these host materials. In the single-hole devices, MoO3 and TAPC with mobility of 10-2 cm2 · V-1 · s-1 were used to inject and transport holes and TAPC/Mg: Ag interface was used to prevent the


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injection of electrons. Accordingly, in the single-electron devices, TmPyPB with mobility of 10-4 cm2 · V-1 · s-1 functions as the electron transportation layers and hole blocking layer.32 From Fig. 3 (a), both single-hole and single-electron exhibit high current densities in the voltage range typically suitable for OLEDs, indicating that all the two hosts can transport both hole and electron efficiently and possess a bipolar carrier transport characteristic.

Figure 3. The current density-voltage of (a) single carrier devices, (b) device B1 and corresponding probe devices and (c) device B2 and corresponding probe devices.

EL Performances of Blue PhOLEDs: The structure of the blue devices is ITO/MoO3 (3 nm)/TAPC (35 nm)/4,4',4''-tris (carbazole-9-yl)-triphenylamine(TcTa: 5 nm)/FIrpic doped p2PCB2CZ or DCZppy (10 wt.%, 20 nm)/TmPyPB (50 nm)/LiF/Mg: Ag (15:1 by weight, 150 nm). In these devices, TAPC works as the hole transportation layer (HTL),32 TcTa are the electron blocking layer (EBL) and TmPyPB

functions

as

the

electron

transportation

layers

(ETL).32

The

p2PCB2CZ-based device is marked as device B1, while the DCZppy-based device is marked as device B2. Fig. 4 reveals the energy level of function layers of blue PHOLEDs.



Figure 4. The energy level diagram of devices and the molecular structures of DCZppy and p2PCB2CZ.

Figure 5. (a) Normalized EL spectra at 5 V, inserts is current density-voltage-luminance (J-V-l) curves; (b) Current efficiency, power efficiency and EQE curves of device B1 and B2.

As shown in Fig. 5 (a), both devices show only typical FIrpic emission with main peak at around 470 nm and shoulder peak at 500 nm, indicating that the energy could transfer from the host to the guest effectively. The insert of Fig. 5 (a) reveals the current density-voltage-luminance (J-V-L) curves of blue devices. The turn-on voltages (driving voltage at a brightness of 1 cd/m2) of device B1 and B2 are 2.8 V and 3.5 V, respectively. The lower turn-on voltage of device with p2PCB2CZ as host


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could profit from an appropriate Eg of materials and a good carrier injection.25 And the current density of device B2 with both higher hole and electron mobility host material (DCZppy) is lower than that of device B1, which is due to the blocking effect of barrier at the interface of TCTA/DCZppy and traps existing in doped system.33 Thus, the device B2 shows a lower luminance than device B1, while device B1 and B2 realize maximum luminance of 45370 cd/m2 and 32330 cd/m2, respectively. The difference between device B1 and B2 in luminance could also be attributed to the transport balance and the recombination rates of carriers. The efficiency-luminance characteristics of the blue devices are shown in Fig. 5 (b). There is no significant difference in CEmax (maximum current efficiency) and EQEmax (maximum external quantum efficiency), and device B1 shows a higher PEmax (maximum power efficiency) of 36.8 lm/W due to the low driving voltage. In terms of efficiency roll-off, there is no significant difference in efficiency roll-off when the luminance is less than 5000cd/m2. As luminance increases, device B1 has a roll-off of EQE about 34% at 20000 cd/m2 and 44% at 30000 cd/m2. However, the roll-off of EQE in device B2 is 48.4% at 20000 cd/m2 and 65.5% at 30000 cd/m2. The efficiency roll-offs at different luminance are listed in Table 2. Besides, table 2 also shows efficiency roll-off at different luminance of other devices with guest charge trapping EML.



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Table 2. EL characteristics and annihilation rate constants of blue OLEDs Device

Von(V)

Lmax(cd/m2)

PEmax(lm/W)

CEmax(cd/A)

EQEmax(%)

CIE at 5V(x, y)

B1

2.8

45368

36.9

35.2

19.2

(0.15, 0.29)

B2

3.5

32333

26.3

35.7

18.7

(0.15, 0.30)

Efficiency roll-off under different luminance (%) Device

1000cd/m

5000cd/m2

10000cd/m2

20000cd/m2

30000cd/m2

B1

3.6± 0.9

12.5± 1.5

20.3± 3

34.4± 2.3

43.7± 2.0

B2

2.7± 1.6

13.5± 1.4

23.7± 2.0

48.4± 3.1

65.5± 2.9

BETC26

8

22.6

32.3

-

-

mCP34

3.9

13.7

25.9

-

-

2

Lmax: maximum luminance; CEmax, PEmax: maximum CE and maximum PE; EQEmax: maximum EQE of devices.

In order to study the reduction of efficiency roll-off of device B1, we analyse the energy level of devices as shown in Fig. 4. Efficiency roll-off in PhOLEDs is attributed to a number of quenching mechanisms including TTA and TPQ.16, 35 TTA is mainly related to the concentration of triplet exciton density in EML. Triplet exciton density is typically determined by electrons because the electron mobility is slower than hole mobility by orders of magnitude.36 Therefore, because of similar electron transport properties of DCZppy and p2PCB2CZ-based host-guest systems, the difference of triplet exciton density in these two type devices will be very small. Considering above, TTA will be not the main reason that device B1 and B2 exhibit absolutely different efficiency roll-off. On the contrary, the doping of FIrpic has different effects on the holes transport properties of device B1 and B2. Compared with p2PCB2CZ (-5.57 eV), FIrpic (-5.9 eV) has a lower HOMO energy level. Therefore, there is no hole trapping sites in p2PCB2CZ doped with FIrpic system and few holes can jump to FIrpic molecules. Excitons of device B1 will be mostly formed in host molecules and then transferred to


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FIrpic via energy transfer. However, in device B2, because the HOMO of DCZppy (-6.05 eV) is lower than that of FIrpic (-5.9 eV), a large number of holes will be trapped by FIrpic, resulting in higher holes concentration in the FIrpic molecules. High concentration of holes in FIrpic will lead to a serious TPQ, therefore, device B2 has a more serious roll-off at high luminance. To prove the analysis, a series of probe devices, having a 0.5 nm-thick red phosphorescent sensitizer bis(2-methyl-dibenzo[f,h] quinoxaline) (acetylacetonate) iridium [Ir(MDQ)2acac] inserted in the HTL/EML interface or EML/ETL interface of devices B1 and B2, were fabricated. The structures are ITO/MoO3 (3 nm)/TAPC (35 nm)/TcTa (5 nm)/Ir(MDQ)2acac (0.5 nm)/FIrpic doped p2PCB2CZ or DCZppy (0 wt.% or 10wt.%, 20 nm)/TmPyPB (50 nm)/LiF/Mg:Ag (15:1 by weight, 150 nm) and ITO/MoO3 (3 nm)/TAPC (35 nm)/TcTa (5 nm)/FIrpic doped p2PCB2CZ or DCZppy (0% or 10%, 20 nm)/Ir(MDQ)2acac (0.5 nm)/TmPyPB (50 nm)/LiF/Mg:Ag (15:1 by weight, 150 nm). The energy levels of probe devices are shown in Fig. 6 (c) and (d). The probing mechanism is to investigate the exciton density formed in the target zone by inserting low bandgap Ir(MDQ)2acac and detecting its corresponding EL emission. If a red emission from Ir(MDQ)2acac in the spectra is observed, excitons formation must exist in the corresponding zone. The red emission intensity represents the excitons concentration generated in the corresponding zone. Additionally, as shown in Fig. 3 (b) and (c), it can be confirmed by the similar current density-voltage characteristics that the introduction of Ir(MDQ)2acac probe produce negligible effects



on the carrier transport properties of blue PHOLEDs. Besides, due to the lower T1 of Ir(MDQ)2acac (2.0 eV), excitons formed in the FIrpic which has higher T1 (2.62 eV), will finally transfer to the nearby Ir(MDQ)2acac. Therefore, the Ir(MDQ)2acac probe can almost detect the whole excitons in the target zone.

Figure 6. Spectra of devices with FIrpic doped (a) p2PCB2CZ (0% or 10%) and (b) DCZppy (0% or 10%) with Ir(MDQ)2acac inserted at HTL/EML or EML/ETL interface driven by 6 voltage; Detailed carriers transport and accumulation of the DCZppy-based probe device (c) and p2PCB2CZ-based probe device (d) are described in energy level diagram.

The EL spectra of these probe devices are shown in Fig. 6 (a) and (b). When the Ir(MDQ)2acac probe is inserted at the interface of EML/ETL, the stronger red emission indicates that the main recombination regions of these two type devices are mostly located near the interface of EML/ETL. Therefore, the hole transport properties of emitting layers will play the major role on the devices performances. Nevertheless, it can be also seen from Fig. 6 (b) that the intensity of blue emission at


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475 nm is nearly identical to 630 nm in device with Ir(MDQ)2acac/FIrpic doped DCZppy as EML. It indicates that some holes are also accumulated at the HTL/EML interface in this device. However, this phenomenon cannot be observed in the p2PCB2CZ-based devices, noting that there are no barriers between p2PCB2CZand TCTA. Insets in the Fig. 6 (a) and (b) are the red emissions (550 nm to 750 nm) of these spectra. In DCZppy-based device, red emission is partly enhanced after doping FIrpic when the Ir(MDQ)2acac thin layer is on the EML/ETL interface. It should be attributed to the hole trapping effect of FIrpic in DCZppy. After FIrpic is doped in DCZppy, a large number of holes are trapped in FIrpic due to its lower HOMO. Near the EML/ETL interface, the energy recombined by traps on the FIrpic can be effectively transferred to the probe Ir(MDQ)2acac because of the lower T1 of Ir(MDQ)2acac (2.0 eV) than that of FIrpic (2.62 eV) and its red emission is enhanced.37 However, the case in p2PCB2CZ-based devices is different since only small difference in red emission can be observed after introducing FIrpic. It indicates that there is no hole trapping effect in the p2PCB2CZ doped with FIrpic system. According to the above analyses about the transport and accumulation of carriers in p2PCB2CZ doped with FIrpic system and DCZppy doped with FIrpic system, the detailed carriers transport properties are shown in Fig. 6 (c) and (d), respectively.



Figure 7. Transient EL of devices with FIrpic doped (a) p2PCB2CZ (0% or 10%) and (b) DCZppy (0% or 10%) with Ir(MDQ)2acac inserted at HTL/EML or EML/ETL interface driven by 6 voltage pulse; (c) Transient EL of devices B1 and B2 at 6 V, and B2 with negative turn-off voltage; The decay part of the transient EL of the device B1 (d) and B2 (e) with turn-off voltages are 0 V, -3 V, -7 V; The rise part of the transient EL of the device B1 (f) and B2 (g) with turn-off voltages are 0 V, -1 V, -3 V, -5 V, -7 V, -9 V.


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To further verify our speculation, the transient EL performances of these devices were also measured, and the results are shown in Fig. 7 (a) and (b). It can be seen that the doping of FIrpic has a great influence on the hole transport in DCZppy-based device. When the Ir(MDQ)2acac probe is inserted at the EML/ETL interface, the delay time of DCZppy-based devices is greatly increased after doping FIrpic, while the delay time of p2PCB2CZ-based devices doesn’t change obviously. It indicates that when holes transport through DCZppy doped with FIrpic system, a large number of holes are trapped by FIrpic. In contrast, the hole trapping effect in p2PCB2CZ-based device is very small. When the Ir(MDQ)2acac probe is inserted at the HTL/EML interface, the delay time is raised slightly in DCZppy-based devices and p2PCB2CZ-based devices, which indicated that two host-guest systems have similar effect on electron traps. It confirms our previous inference that both two devices have same electron transport properties. The transient analysis of device B1 and B2 is carried out to study the main recombination mechanism of the blue devices. Fig. 7 (c), (d), (e), (f) and (g) show the transient EL characteristics of devices B1 and B2. When the pulsed voltage is turned on, the EL will rise for a period of time we call rise time and when the pulsed voltage is switched off, the EL will decay for a period of time we call decay time, all of which can reflect the carrier recombination ability in EML.37 The transient EL of device B1 and B2 at different negative bias voltages was measured, exhibited in Fig. 7 (d), (e), (f) and (g). And the rise time and decay time of two devices all show different characteristics after acceding to negative bias. When the switching



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voltage is changed from 0 V to -7 V, DCZppy-based devices show a significantly decreased rise time and decay time with the increase of negative bias voltage. Because the direction of electric field is opposite to the built-in electric field when the turn-off voltage is negative, it can accelerate the attenuation of the built-in field and help to release the trap charges. With the change of negative bias voltage from 0 V to -7 V, the traps release rate becomes faster and the decay time becomes shorter. Therefore, trap-assisted recombination mechanism plays a major role in the DCZppy-based devices.39 On the contrary, no recombination rate change is observed in p2PCB2CZ-based devices, indicating that there is almost no trap-assisted recombination

process,

and

energy

transfer

is

the

main

mechanism

in

p2PCB2CZ-based device. From Fig. 7 (c), when the turn-off voltage is zero, device B1 has much faster carrier recombination ratio than device B2. The decay time of device B2 with negative turn-off voltage become significantly shorter than device B1. It also explains the trapping action in the device B2.

Figure 8. The EQE/EQEmax-luminance curves of (a) p2PCB2CZ-based and (b) DCZppy-based blue PhOLEDs with different dopant concentrations (6 wt.%, 10 wt.% and 20 wt.%); Inset tables: the EQE roll-off under different luminance.


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We prepared a series of blue devices with different dopant concentrations (6 wt.%, 10 wt.% and 20 wt.%) to confirm that the p2PCB2CZ-based device is subjected to a less

degree of charge trapping. In order to observe efficiency roll-off more convenient, the EQE/EQEmax-luminance curves of p2PCB2CZ-based devices and DCZppy-based devices are shown in Fig. 8 (a) and (b), respectively, and the EQE roll-off values are detailed in the inset tables. From Fig. 8, in p2PCB2CZ-based devices, the effect of different doping concentrations on the efficiency roll-off is negligible, indicating that the main reason of the roll-off is not due to the trap effect of the guest. In contrast, different doping concentrations have a great impact on the efficiency roll-off of DCZppy-based devices. When the doping concentration is low, the efficiency roll-off is very serious. Due to the dominance of trap-assisted recombination in DCZppy-based devices, the lower doping concentration (6 wt.%) makes fewer holes captured for recombination, thus increasing the accumulation of holes leads to more serious TPQ. And the EQE-current density and current density- voltage curves of blue PhOLEDs are shown in Fig. S1. (Supporting Information) Finally, to analyze the effect of hole trapping on efficiency roll-off quantitatively, mathematical modeling of TTA and TPQ rate in blue PhOLEDs is used. In phosphorescent systems, the singlet density is not necessarily considered because all singlet excitons are quickly transferred to the T1 via ISC. Therefore, only two significant quenching mechanisms TTA and TPQ need to be considered in the phosphorescent system, and the rate equations are given as:9



d𝑛𝑇 dt

1

= 𝑘𝐿𝑛2𝑝 ― 𝑘𝑇𝑛𝑇 ― 2𝑘𝑇𝑇𝑛2𝑇 ― 𝑘𝑇𝑃𝑛𝑇𝑛𝑃(1) d𝑛𝑃 dt

𝐽

= 𝑞𝑤 ― 𝑘𝐿𝑛2𝑝

(2)

Here, the J, q and w are current density of device, the electron charge, and the thickness of the exciton recombination zone which approximates to the EML thickness, nT and nP are triplet exciton density and polaron density, kT is triplet radiation rate constant, kTT and kTP are TTA and TPQ rate constants respectively and the kL is Langevin complex rate constant obtained by simulation. Fig. 9 (a) and (b) show the relative contribution of TTA and TPQ to the overall quenching for the device B1 and B2, respectively. The simulated annihilation rate constants kTT and kTP and the other parameters in the Equations (1) and (2) are shown in Table 3. It can be seen that our hypothesis is confirmed. The largest difference between the two type devices is the TPQ annihilation mechanism and the proportion of TPQ in device B2 is much larger than that in B1. Therefore, the TPQ is the main annihilation mechanism leading to aserious efficiency roll-off of device B2.


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Figure 9. EQE (dashed line) as a function of the current density for blue PhOLEDs based on: (a) p2PCB2CZ: FIrpic and (b) DCZppy: FIrpic as EML; Triplet density and polaron density (lines) are calculated according to Equations 1-2 using the annihilation rates from Table 3; Hatched areas indicate the relative contribution of TTA and TPQ as well as of the emission to the overall exciton decay.

Table 3. The parameters in Equations 1 and 2. KL (10-11 cm3s-1)

τ (10-6

KT s)

(106 s-1)

w

KTT

KTP

(nm)

(10-12 cm3s-1)

(10-12 cm3s-1)

B1

1.13

1.23

0.813

20

3.82

0.107

B2

3.12

1.32

0.758

20

9.05

3.64

KL, kTT and kTP are simulated based on the Equations 1-2 and the current density-EQE curves of the devices, τ is triplet exciton life, kT is triplet radiation rate constant, and wis the thickness of the exciton recombination zone.

Conclusions In summary, an excellent host material 5-(5-9H-carbazol-9-yl) pyridin-2-yl) -8-(9H-carbazol-9-yl) -5H-pyrido[3,2-b] indole for blue PHOLEDs was designed. By using this material, we realize a novel host-guest system in which the HOMO energy level of host is higher than that of the guest FIrpic. And blue PhOLEDs with the novel host-guest system (p2PCB2CZ-FIrpic) are fabricated and their electroluminescent



performances are studied by comparison to blue PhOLEDs with guest charge trapping emitting layer (DCZppy-FIrpic). The p2PCB2CZ-based device achieved higher maximum luminance of 45368 cd/m2 and EQEmax of 19.2%. Moreover, the device exhibited lower efficiency roll-off at high luminance, its EQE still attains to 10.8% (56.3% of EQEmax), even when the luminance increases to 30000 cd/m2, which is 1.67 times that of the DCZppy-based device. Overall, PhOLEDs with novel host-guest system show higher luminance and lower efficiency roll-off at high luminance due to lower carrier accumulation in HTL/EML interface and the reduction of TPQ ratio revealed by transient PL, transient EL and kinetics of exciton. EXPERIMENTAL SECTION Material Synthesis: The synthetic route and molecular structure of p2PCB2CZ are shown in Scheme 1. The compound A was synthesized according to the general Ullmann coupling reaction conditions.23 Under the protection of nitrogen atmosphere, a mixture of compound A (24.7 g, 100 mmol), 2 ， 5-bromo pyridine (28.3 g, 100 mmol), and N,N-dimethylformamide (DMF, 1L) were heated under reflux for 24 hours. The solution was cooled to room temperature, filtered under vacuum. The crude product was isolated with column chromatography on silica gel, using CH2Cl2 as the eluent. The product was further purified by recrystallization using CH2Cl2, pale yellow powder (compound C) was collected (35.2 g, 74%). Under the protection of nitrogen atmosphere, a mixture of compound C (40.3 g, 100 mmol), carbazole (33.5 g, 210 mmol), and DMF (1.5 L) were heated under reflux for 24 hours. The solution was cooled to room temperature, filtered under vacuum. The product was isolated


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with column chromatography on silica gel, using CH2Cl2: MeOH=10:1 as the eluent. The product was further purified by recrystallization using CH2Cl2 and MeOH, pale yellow powder p2PCB2CZ was collected(43.7 g, 76%). 1H NMR (600 MHz, CDCl3, ppm): δ9.048 (d, J=1.8 Hz, 1H), 8.721-8.736 (m, 2H), 8.417 (d, J=8.4 Hz, 1H), 8.237 (d, J =8.4 Hz, 1H), 8.195-8.231 (m, 5H), 7.936 (d, J=8.4 Hz, 1H), 7.790 (d, J=8.4 Hz, 1H), 7.481-7.543 (m, 7H), 7.392-7.454 (m, 4H), 7.316-7.341 (m, 2H). 13C NMR (151 MHz, CDCl3, ppm): 149.53, 148.19, 144.30, 142.79, 141.46, 140.64, 138.80, 137.14, 133.85, 132.58, 132.18, 127.59, 126.53, 126.04, 125.08, 123.95, 123.40, 121.28, 120.98, 120.76, 120.36, 119.98, 119.75, 119.35, 118.87, 112.96, 109.89, 109.34.MS (ESI): calcd for [C40H25N5] ([M+H]+) m/z 576.2188, found 460.2189.

Scheme 1. Synthesis route of p2PCB2CZ.

Devices preparation and measurement: In this paper, the devices are fabricated on indium tin oxide (ITO) substrates. Firstly, deionized water and Decon 90 are used to clean the substrates. Then the substrates were put into a drying oven for about 10 minutes, and finally treating in a plasma cleaner chamber for 5 minutes. The organic layers and electrodes are deposited by thermal evaporation on the substrates under



vacuum (~ 5.0×10-4 Pa). The anode and cathode are regulated by a shadow mask which fabricates four identical devices on substrate. The spectra and current density-voltage-luminance curves are measured by Goniophotometric Measurement System based on spectrometer (GP-500, Otsuka Electronics Co. Osaka, Japan) in air. And the characteristic of transient EL is detected and collected by using HOLITA Fluorescence Spectrum Analyzer (FSA) System (HOLITA Co. Beijing, China). Corresponding Author *E-mail

(H. Y. Zhao): [email protected].

*E-mail

(W. F. Xie): [email protected].

Author Contributions Z. W. Y designed and prepared all the devices in this paper and conducted most of the experiments. J. X. Z assisted in part of the preparation of devices. S. H. L performed the simulation of the kinetics of exciton. L. T. Z mainly measured and analyzed the transient EL experiments. Y. Z made some meaningful comments and suggestions for the experiments. H. Y. Z. carried out the synthesis of p2PCB2CZ. W. F. X initiated the study, designed the experiments and prepared the manuscript. All authors discussed the results and commented on the manuscript.

Acknowledgments


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This work was supported by the National Natural Science Foundation of China (Grant Nos. 61774074, 61475060, 61474054), and Science and Technology Development Planning of Jilin Province (No. 20190101024JH).

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TOC


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High-efficiency blue phosphorescent organic light-emitting devices

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