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Highly efficient simplified single-emitting-layer hybrid WOLEDs with low roll-off and good color stability through enhanced Förster energy transfer Dongdong Zhang, Minghan Cai, Yunge Zhang, Deqiang Zhang, and Lian Duan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b10783 • Publication Date (Web): 08 Dec 2015 Downloaded from http://pubs.acs.org on December 11, 2015

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ACS Applied Materials & Interfaces

Highly efficient simplified single-emitting-layer hybrid WOLEDs with low roll-off and good color stability

through

enhanced

Förster

energy

transfer Dongdong Zhang,▽ Minghan Cai,▽ Yunge Zhang, Deqiang Zhang, Lian Duan*

Key Lab of Organic Optoelectronics and Molecular Engineering of Ministry of Education, Department of Chemistry, Tsinghua University, Beijing 100084, China. ▽These authors contributed equally to this work.

ABSTRACT Hybrid white organic light-emitting diodes with single-emitting layer (SELhybrid-WOLEDs) usually suffer from low efficiency, significant roll-off and poor color stability, attributed to the incomplete energy transfer from the triplet states of the blue fluorophors to the phosphors. Here we demonstrate highly efficient SEL-hybrid-WOLEDs with low roll-off and good color-stability utilizing blue thermally activated delayed fluorescence (TADF) materials as the host emitters. The triplet states of the blue TADF host emitter can be up-converted into its singlet ones, and then the energy is transferred to the complementary phosphors through the long-range Förster energy transfer, enhancing the energy transfer from the host to the dopant. Simplified SEL-hybrid-WOLEDs achieve a highest forward-viewing external quantum

ACS Paragon Plus Environment

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efficiency (EQE) of 20.8% and power efficiency of 51.2 lm/W with a CIE coordinates of (0.398, 0.456) at luminance of 500 cd/m2. The device EQE only slightly drops to 19.6% at a practical luminance of 1000 cd/m2 with power efficiency of 38.7 lm/W. Besides, the spectra of the device are rather stable with the raising voltage. The reason can be assigned to the enhanced Förster energy transfer, wide charge recombination zone as well as the bipolar charge transporting ability of the host emitter. We believe that our work may shed light on future development of highly efficient SEL-hybrid-WOLEDs with low roll-off and good color-stability simultaneously.

KEYWORDS (hybrid white organic light-emitting diodes, blue thermally activated delayed fluorescence, high efficiency, low efficiency roll-off, good color-stability)

INTRODUCTION White organic light-emitting diodes (WOLEDs) show great promise to have a major share in future solid-state lighting sources and backlights for full-color displays due to their favorable properties such as high resolution, homogenous large-area emission and potential realization on flexible. Recent conceptual advancements have led to many exciting approaches to highefficiency WOLEDs,1-3 among which hybrid WOLEDs incorporating blue fluorescence with green/red or orange phosphorescence are considered as an ideal candidates.4-8 Such strategy is intrinsically superior to the fully fluorescent or phosphorescent approach since it combines the advantages of long-term stability and high efficiency of the fluorophors and phosphors, respectively.9 To achieve high efficiency, triplet loss through the non-emissive triplet states of the blue fluorophors should be prevented.4,5 Therefore, complicated structures are usually required to


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separate the singlet and the triplet excitons,4,9 which is unfavorable for the reproducibility of the mass production and increasing the cost of the devices.8,10 Leo et al proposed that significant simplified WOLEDs with a single-emitting-layer (SEL-hybrid-WOLEDs) can be realized unitizing a bulk blue fluorophor simultaneously acting as host for green and orange phosphors, which may be taken as the simplified SEL-hybrid-WOLEDs.9 The key point to realize such concept is the blue host emitter materials. Zhang and Lee et al have developed a series of blue fluorophors with high triplet energies to be utilized as the blue host emitters in simplified SELhybrid-WOLEDs, achieving high efficiency at low luminance.6,11-14 However, the efficiencies are still not satisfied and the efficiency roll-off is significant under high current density. What is worse, the device color stabilities are usually poor owing to the enhanced blue emission intensity with the raising voltage. In the simplified SEL-hybrid-WOLEDs, the concentrations of the phosphors are usually controlled less than 1 wt% to obtain efficient blue emission.6,11-14 The energy transfer from the host triplet to the dopant is through the Dexter transfer mechanism, which is a short-range interaction. Therefore, to achieve complete energy transfer, the triplet excitons of the blue host emitters should diffuse to the host molecules nearby the dopants and then the energy can be transferred to the dopants.4,6 During the diffusion process, parts of the triplet excitons are inevitable to deactivate through other process, such as triplet-triplet annihilation (TTA) or tripletpolaron annihilation (TPA) processes, especially under high current density. The TTA process has been demonstrated to be the main reason for the significant efficiency roll-off as well as the increased intensity of the blue emission with the increasing voltage.5,6 To solve those problems, efficient energy transfer should be realized to promote device efficiencies and stabilize their emission.


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Recently reported thermally activated delayed fluorescence (TADF) may provide an alternation to achieve highly efficient simplified SEL-hybrid-WOLEDs.15-18 It has been demonstrated that in phosphorescent OLEDs utilizing TADF materials as the hosts, the triplet excitons of hosts can be thermally up-converted into their singlet, and then the energy of the host singlet can be transferred to the dopant through the Förster transfer.19 Since the Förster energy transfer is a long-range interaction, complete energy transfer from the host triplet to the dopant can be achieve even at dopant concentration as low as 1 wt%. Therefore, hybrid WOLEDs using blue TADF host emitters may achieve more efficient energy transfer. Besides, the triplet can also be utilized by the blue host emitters, further reducing the triplet loss. Therefore, blue TADF host emitter incorporating orange phosphors may be an ideal system to achieve high performance simplified SEL-hybrid-WOLEDs. We have demonstrated single-emitting layer hybrid WOLEDs with EQEs as high as 19% by doping blue TADF emitters and orange phosphor into a host material.17 But till now, the efficiency roll-off of the TADF-based hybrid WOLEDs are still large owing to the inefficient blue TADF emitters. And simplified SEL-hybrid-WOLEDs with TADF materials as the blue emitters as well as the hosts for complementary phosphors have not been reported to our knowledge. The key prerequisite to realize such simplified SEL-hybrid-WOLEDs is the blue TADF materials, which possess high photoluminance (PL) efficiencies in the form of pure films and be adequate to be the hosts for phosphors. In this manuscript, simplified SEL-hybrid-WOLEDs incorporating blue TADF host emitters with orange phosphor were reported, achieving high efficiencies, low efficiency roll-off as well as good stable-stabilities. A blue TADF material, 10,10′-(sulfonylbis(4,1-phenylene))bis(9,9dimethyl- 9,10-dihydroacridine) (DMAC-DPS),20 which possesses high PL efficiency in pure neat film, is chosen to be the blue emitter as well as host for orange phosphor,


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

(PO-01).21

It

was

demonstrated that the triplet of DMAC-DPS can be up-converted into its singlet and then transferred to PO-01 through the long-range Förster energy transfer, enhancing the energy transfer from the host to the dopant. Fabricated simplified SEL-hybrid-WOLEDs achieve a highest EQE of 20.8% and power efficiency of 52 lm/W with a CIE coordinates of (0.398, 0.456) at luminance of 500 cd/m2, and the device EQE only slightly drops to 19.6% at a practical luminance of 1000 cd/m2. Besides, the spectra of the device are rather stable with the increasing voltage. The reason for the small efficiency roll-off and the good color-stability is assigned to the enhanced Förster energy transfer, wide charge recombination zone as well as the bipolar charge transporting ability of the host emitter. These performances are among the best ones among the simplified SEL-hybrid-WOLEDs, even comparable to the complicated multi-emitting layers devices. EXPERIMENTAL SECTION Materials: All the materials used to fabricate the devices were purchased and used as received. All the purities of the compounds are higher than 99.9%, detected using high performance liquid chromatography

(HPLC).

Dipyrazino[2,3-f:2',3'-h]quinoxaline-2,3,6,7,10,11-hexacarbonitrile

(HATCN) is purchased from LG Chem. Co. Ltd. while N,N'-bis(1-naphthalenyl)-N,N'-diphenyl[1,1'-biphenyl]-4,4'-diamine (NPB) is from Beijing Visionox Technology Co. Ltd. N,N,N-Tris(4(9-carbazolyl)phenyl)amine

(TCTA),

1,3-bis(9H-carbazol-9-yl)benzene

(mCP)

and

4,7-

diphenyl-1,10-phenanthroline (Bphen) are purchased from Jilin Optical and Electronic Materials Co. Ltd. Besides, the DMAC-DPS and bis(2-(diphenylphosphino)phenyl)ether oxide (DPEPO) is purchased from Xi’an Polymer Light Technology Co. Ltd.


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PL characterization: UV-vis absorption spectra were recorded by an Agilent 8453 spectrophotometer. The films are spin coated on the quartz substrate using dichloromethane as the solvent 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: The device structures are ITO/ HATCN (5 nm)/ NPB (30 nm)/ TCTA (10 nm)/ mCP (10 nm)/ EMLs (30 nm)/ DPEPO (10 nm)/ Bphen (30 nm)/ LiF (0.5 nm)/ Al (150 nm). Where the EMLs are DMAS-DPS and DMAC-DPS: 6 wt% PO-01 for blue and orange devices while DMAC-DPS: 0.3 wt%, 0.5wt% and 0.8 wt% PO-01 for white devices, respectively. Before device fabrication, the ITO glass substrates were pre-cleaned 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 while the total ones are measured using an integrating sphere (Bluefly illumia 610). 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. Current–voltage characteristics of singlecarrier devices were measured using the same semiconductor parameter analyzer as for white devices. 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.


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RESULTS AND DISCUSSION The devices and the molecule structures are shown in Figure 1a. The device structures are simplified one as discussed above since only one emitting layer (EML) is adopted with a blue TADF material, DMAC-DPS, as the blue emitter and the host for the orange phosphor, PO-01. DMAC-DPS was adopted here since it has been demonstrated to be efficient blue TADF emitters, possessing PL efficiency as high as 0.88 in pure neat film.22 What is more, the triplet energy of DMAC-DPS is 2.7 eV, much higher than that of the PO-01(2.2 eV), indicating that DMAC-DPS is adequate to be the host for PO-01. To confine the excitons in the EML, mCP and DPEPO layers with high triplet energies were adopted. HATCN and LiF were used as the hole and electron injection layer while NPB and Bphen were used as hole and electron transporting layers, respectively. The energy transfer diagram from the host emitter to the guest is shown in Figure 1b. For singlet excitons of the host, aside from converted into its triplet through intersystem crossing (ISC), they can also be used to give out blue emission or transferred to the dopant, which can be tuned by the dopant concentration. For the triplet states of the host, since the dopant concentration is usually low, direct Dexter energy transfer can be neglected. Like the conventional fluorophors, the triplet can diffuse to the host nearby the dopant and then transferred through Dexter transfer. Here, another route can be anticipated, that is the host triplet can be up-converted into its singlet, from where the energy can be transferred through the longrange interaction. Such long-range direct transfer can eliminate the triplet loss during the diffusion process, enhancing energy transfer from the host triplet to the dopant, which is superior to the ones using conventional fluorophors with high triplet. The singlet excitons up-converted from the triplet of the host, either utilized by the blue emitters or transferred to the dopant, can


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minimize the triplet density under high current density, which is helpful to reduce the efficiency roll-off. First, the energy transfer mechanism should be demonstrated. As can be seen from Figure 2a, the pure film of DMAC-DPS show emission peak at 470 nm while that of PO-01 is 560 nm. The absorption spectra of PO-01 show significant overlap with the emission spectrum of DMAC-DPS, guaranteeing efficient energy transfer. The PL spectra of the DMAC-DPS: PO-01 doped films were also measured. As the dopant concentration increase, the emission intensity of DMAC-DPS is gradually reduced, which is invisible at dopant concentration of 5 wt%, indicating efficient energy transfer. As can be seen from the PL transient decay curves of DMAC-DPS of the doped films (Figure 2c), the lifetime of the prompt parts and the ratio of the delayed parts are gradually reduced with the increasing dopant concentration. Under PL excitation, only singlet excitons can be formed first. The triplet can only be formed through ISC process. The energy transfer and the ISC are competitive processes. With the increasing dopant concentration, the energy transfer is enhanced, indicating that more singlet excitons are directly transferred to the dopant. Therefore, the lifetime of the prompt parts and the ratio of the delayed parts are reduced. The reduced lifetimes of the delayed parts with the increasing dopant concentration can be attributed to the more efficient energy transfer from the host singlet excitons to the dopant, reducing the numbers of excitons cycled between the triplet and the singlet of the hosts, which was also demonstrated for fluorescent devices using TADF materials as the hosts.23 The longer lifetime of the PO-01 at low dopant concentration compared with the one with dopant concentration of 5 wt%, as can be seen from Figure 2d, can also be attributed to the energy transferred from the singlet excitons up-converted from the triplet excitons. Therefore, the energy transfer mechanism can be demonstrated.


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To demonstrate that DMAC-DPS can be efficient host, its electron and hole transporting abilities are investigated. Hole-only device with structure of ITO/ NPB (10 nm)/ DMAC-DPS (100 nm)/ NPB (10 nm)/ Ag (100 nm) and electron-only device with structure of ITO/ Bphen (10 nm)/ DMAC-DPS (100 nm)/ Bphen (10 nm)/ Mg/ Ag (100 nm) are fabricated. As can be seen from Figure 3a, both carrier-only devices show high current density, indicating that DMAC-DPS is a bipolar host. The electron current density is higher than that of the hole, suggesting that the electron mobility is higher than the holes. To evaluate the performance of DMAC-DPS as a host, orange devices with DMAC-DPS as the host and PO-01 as the dopant are fabricated adopting the same structures as the white ones while the dopant concentration is as high as 5 wt%. Besides, blue device without dopant was also fabricated to evaluate the performance of DMAC-DPS as an emitter. As can be seen from Figure 2b, the emission peaks of the blue and the orange devices are 476 nm and 561 nm, respectively, which match the PL spectra of the films under PL excitation. The current density of the orange and the blue devices are almost the same (Figure 3c), suggesting that the dopant does not influence the carrier transporting abilities in the devices. The energy level of the device structure (Figure 1a) shows that the LUMO energy of PO-01 is lower than that of DMAC-DPS, indicating that the electrons mainly transport through the host. On the other hand, although the HOMO energy level of PO-01 is shallower than that of DMACDPS, the holes directly injected into the host is also easy since the HOMO energy level of the mCP is the same with that of the host. Therefore, both the charges transfer and recombination takes place on the host and then the energy is transferred to the dopant. The maximum EQE of the orange OLEDs is as high as 23.1%, which is one of the highest values among the orange PHPLEDs.21 Besides, the efficiency roll-off is low with EQE as high as 19.5% obtained at 10000 cd/m2. Besides from the bipolar transporting ability of the host, the low efficiency roll-off has


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also been demonstrated to get benefit from the reverse ISC (RISC) process, which suppresses the TTA process on the host.24 The high performances of the orange OLEDs demonstrate that the DMAC-DPS is suitable to be the host for PO-01. For the blue device, the maximum EQE of 11% is obtained, which is much higher than the theoretical values (5%) of devices based on conventional fluorophor with high triplet energies. At a brightness of 1000 cd/m2, the EQE of the blue device is still as high as 9.0%, demonstrating that DMAC-DPS as an emitter is efficient. Since DMAC-DPS has been demonstrated adequate to be the host for PO-01 and the energy transfer from the host to the dopant was also observed, the performances of the white devices were thereof evaluated, as shown in Figure 4 and Table 1. On the basis of structural engineering to enhance the efficiency and to tune spectral quality of the WOLEDs, the concentration of PO01 is optimized from 0.3 wt% to 0.8 wt%. All three devices show white emission with their spectra covers the whole region from 400 nm to 700 nm. The emission peak at 460 nm can be attributed to DMAC-DPS emission while the emission peak at 560 nm is due to the emission of PO-01. The CIE coordinates are (0.348, 0.422), (0.358, 0.430) and (0.398, 0.456) for devices with dopant concentration of 0.3 wt%, 0.5 wt% and 0.8 wt%, respectively at the operational voltage of 5V. With the dopant concentration increasing, the blue emission is gradually reduced, indicating more efficient energy transfer from DMAC-DPS to PO-01. Besides, the emission spectra of the devices are rather stable within the investigated voltage range. As can be seen from Table 1, the changes of the CIE coordinate are only to be (0.015, 0.016), (0.014, 0.015) and (0.011, 0.012) for devices with dopant concentration of 0.3 wt%, 0.5 wt% and 0.8 wt%, respectively with the voltage increased from 4 V to 7V. As we know, for hybrid WOLEDs especially the ones with single-emitting layers, their color-stabilities are poor with blue emission intensity enhanced under high current density. The reason can be assigned to the inefficient


10

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energy transfer from the triplet host to the dopant, resulting in enhanced singlet excitons of the blue emitters from TTA process and thus increasing the blue emission.5,6 Different to the previous devices, the energy transfer in devices here is much efficient and thus reducing the TTA process. Besides, since the triplet can also be utilized by the blue TADF material itself, the density of the triplet excitons can be further reduced. The stable spectra of the devices may be also benefit from the stable recombination zone in the devices. As we discussed above, the recombination is mainly on the host, demonstrated by the current density of the devices which are independent on the dopant concentration as can be seen from Figure S2a. The energy is then transferred from the DMAC-DPS to the dopant. The excitons distribution ratio between the two emitters is mainly determined by the dopant concentration. Since the dopant concentration is fixed, the ratio of blue and the orange emission is also unchanged, resulting in stable white emission. A most remarkable observation from Figure 4d is that forward-viewing EQE as high as 20.8% and PE as high as 51.2 lm/W were achieved for WOLEDs with the dopant concentration of 0.8 wt%. As the dopant concentration reduced, the EQE of the devices is gradually reduced, with EQE of 19.1% and 15.0% for devices with dopant concentration of 0.3 wt% and 0.5 wt%, respectively. The reduction in EQE with enhanced blue emission can be attributed to the inefficient performance of the blue emitters. As we discussed above, EQE of only 11% was obtained for the blue devices. For hybrid WOLEDs adopted blue TADF emitters with orange phosphors, the excitons loss can be eliminated if both emitters can achieve unity efficiency. Under such ideal conditions, the EQE of the device should also be independent on the spectra of the device. We believe that with the development of the blue TADF emitters, higher efficiency hybrid WOLEDs with cool emission can be achieved. More important, the device efficiency roll-


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off is quite small. For device with dopant concentration of 0.8 wt%, EQE of 19.8% and power efficiency of 38.7 lm/W were remained at a practical luminance of 1000 cd/m2. Even at a luminance of 5000 cd/m2, the EQE is still higher than 15%. Besides form the bipolar transporting ability of the host as we discussed above, the small roll-off can be attributed to, on one hand, the wide recombination zone since charges are recombined on the host through the entire EML. On the other hand, the efficient utilization of the host triplet, either by TADF process or being transferred to the dopant, greatly reduces the density of the host triplet, suppressing the TTA or TPA process. Other two white devices also show small roll-off with about 90% of the highest EQE remained at luminance of 1000 cd/m2. All the above performances of WOLEDs measured above only calculate the light from the forward direction of the device. For light emitting device, another frequently reported parameter is the total efficiency measured in an integrating sphere without any covering, which also collects emission from the edges and from the back of the device.5 For the device with dopant concentration of 0.8 wt%, the total EQE achieves a maximum value of 35.4% and it is still as high as 34.2% at 1000 cd/m2. Table 2 lists the forwardviewing and total EQE of the representative hybrid devices with single-emitting layers. The performance of the hybrid WOLEDs in this work are among the highest ones though the device structures are much simpler. Besides, the small efficiency roll-off of our devices may be superior to the WOLEDs with conventional fluorophors, which show relatively significant roll-off as can be seen from Table 2. The transient decay curves of the devices were also measured to reveal the energy transfer in device under electrical excitation. As can be seen from Figure 5, compared to the decay curves of the orange devices, those of the WOLEDs show long tails reducing with increasing dopant concentration. This is similar to the transient decay curves of the films under PL excitation. The


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long tails can be attributed to the energy transferred from the host singlet up-converted from their triplet as we discussed above. With the dopant concentration increasing, the excitons cycling numbers between the host singlet and triplet will be reduced, and thus the long tails. CONCLUSIONS In conclusion, we have demonstrated highly efficient simplified SEL-hybrid-WOLEDs incorporating blue TADF host emitter with orange phosphor. The energy transfer from the host to the dopant is enhanced through the long-range Förster energy transfer. Device spectra are stabilized by the fixed recombination zone and the efficient energy transfer. EQE as high as 20.9% was achieved with a CIE coordinate of (0.398, 0.456). Besides, due to the wide recombination zone, the efficiency roll-off is small, with EQE of 19.6% is remained at a practical luminance of 1000 cd/m2. We believe that with the development of the blue TADF emitters, more efficient hybrid WOLEDs with simple structures can be achieved. FIGURES 2.3 eV

a

2.6 eV

2.4 eV 2.8 eV

2.9 eV LiF/Al

2.9 eV TCTA

NPB

mCP

ITO 4.8 eV

2.9 eV

5.6 eV

5.7 eV

25%

PO-01

75%

DPEPO Bphen

5.4 eV HATCN

Charges Recombination

b

2.6 eV

S1

DMACDPS 5.7 eV 5.9 eV 5.9 eV

T1

6.3 eV

T1

6.5 eV O

N

N

N

O S

O N

Ir O

N S

mCP

DMAC-DPS

O P O

O P

S0

2

PO-01

DPEPO

Blue TADF host emitter

S0 Orange phosphor

Figure 1. a) The energy diagram of the devices and the molecule structures. b) The energy transfer diagram from the blue TADF host emitter to the orange phosphor.


13


a

1.0

DMAC-DPS: 0.3% PO-01 DMAC-DPS: 0.5% PO-01 DMAC-DPS: 0.8% PO-01 DMAC-DPS: 5% PO-01

b

1.2

1.0

Intensity (a.u.)

Intensity (a.u.)

1.4

PO-01 PO-01 DMAC-DPS

0.8

0.6

0.4

0.8

0.6

0.4 0.2 0.2

0.0

0.0 300

350

400

450

500

550

600

650

400

700

450

500

c

d

1

DMAC-DPS DMAC-DPS: 0.3% PO-01 DMAC-DPS: 0.6% PO-01 DMAC-DPS: 0.7% PO-01

Intensity

1000

10

100

600

650

700

DMAC-DPS: 0.3% PO-01 DMAC-DPS: 0.5% PO-01 DMAC-DPS: 0.8% PO-01 DMAC-DPS: 5% PO-01

100

Intensity (a.u.)

Intensity

1000

550

Wavelength (nm)

Wavelength (nm)

1 0

200

Time(ns)

0.1

10

0.01

1 0

2000

4000

6000

0

8000

2000

4000

6000

8000

10000

Time (ns)

Time(ns)

Figure 2. a) The absorption spectrum of PO-01 and the emission spectra of PO-01 and DMACDPS. b) The emission spectra of the DMAC-DPS: PO-01 doped films. c) The PL transient decay curves of the DMAC-DPS: PO-01 doped films observed at 470 nm. d) The PL transient decay curves of the DMAC-DPS: PO-01 doped films observed at 560 nm.

a

1000

800

blue device orange device

b

1.0

electron only device hole only device 0.8

Normalized PL (a.u.)

-2

Current density (mA cm )

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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600

400

200

0.6

0.4

0.2

0

0.0 0

5

10

Voltage (V)

15

20

400

450

500

550

600

650

700

750

Wavelengthe (nm)


14

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1600

100

c

1400

d

Blue device Orange device

-2

Current density (mA cm )

1200

Blue device Orange device EQE (%)

1000

800

10

600

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200 1 2

3

4

5

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8

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10000

-2

Brightness (cd m )

Voltage (V)

Figure 3. a) The current density curves of the hole-only and electron-only devices. b) The emission spectra of the blue and the orange devices. c) The current density-voltage characteristics of the blue and the orange devices. d) The EQE-brightness characteristics of the blue and the orange devices.

a

1.0

0.8

Intensity (a.u.)


4V 5V 6V 7V 0.5 wt%

b

1.0

4V 5V 6V 7V 0.3 wt%

0.8

0.6

0.4

0.2

0.6

0.4

0.2

0.0 400

450

500

550

600

650

700

0.0

750

400

Wavelength (nm)

500

600

700

Wavelength (nm) 100

d

4V 5V 6V 7V 0.8 wt%

0.8

10

EQE

0.6

10 1

0.4

0.3 wt% 0.5 wt% 0.8 wt%

0.2

0.3 wt% 0.5 wt% 0.8 wt%

Power efficiency

c

1.0


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


0.1 1

0.0 400

450

500

550

600

650

700

750

100

1000

10000 2

Brightness (cd/m )

Wavelength (nm)


15


Figure 4. a) The emission spectra of the WOLEDs with dopant concentration of 0.3 wt%. b) The emission spectra of the WOLEDs with dopant concentration of 0.5 wt%. c) The emission spectra of the WOLEDs with dopant concentration of 0.8 wt%. d) The EQE-brightness-power efficiency characteristics of the WOLEDs. 1000

0.3 wt% 0.5 wt% 0.8 wt% 5 wt%

100

Intensity

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 16 of 22

10

1 15

20

Time(µs)

25

30

Figure 5. The transient decay curves of the devices observed at 560 nm.

Table 1 Summary of blue, orange and white devices performance EQE1000cd/m2 PE1000cd/m2 (%) (lm/W)

Doping concentration

Von (V)a

EQEmax (%)

PEmax (lm/W)

0%

3.43

10.8

17.5

8.8

10.7

(0.199,0.302)

5%

2.94

23.1

61.2

22.8

45.9

(0.491,0.502)

0.3%

3.19

15.0

29.4

13.2

21.4

(0.348,0.422)

(0.015,0.016) 63/5990

0.5%

3.11

19.1

41.5

17.4

30.4

(0.358,0.430)

(0.014,0.015) 54/4155

0.8%

3.06

20.8

51.2

19.6

38.7

(0.398,0.456)

(0.011,0.012) 48/3671

CIE (x,y)

∆CIE(x,y)b

CRI/CCT

Notes: a Turn-on voltage, estimated at the brightness of 1cd m-2. b Measured from 4V to 7V.


16

Page 17 of 22

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


Table 2. The performances of the representative hybrid devices with single-emitting layers. Ref

EQEmax

total EQEmax

EQE1000 cd/m2

total EQE1000 cd/m2

CIE

Ref. (6)a

15.6

26.6

12.5

21.2

(0.46, 0.44)d

Ref. (11) a

12.8

21.8

8.6

14.6

(0.48, 0.44) d

Ref. (14) a

14.5

24.7

10.8

18.3

(0.41, 0.46)e

Ref. (13) a

15.4

10.8

(0.45, 0.44) e

Ref. (17)b

19.6

13.3%

(0.42, 0.48)d

Ref. (25)c

22.9

Ref. (26) b

25.5

white3b

20.8

17.3

(0.40,0.43) d

14.8 35.4

19.6

(0.47, 0.40) d

34.2

(0.40,0.45)f

a) hybrid WOLEDs with conventional fluorophores. b) hybrid WOLEDs with TADF blue emitters. c) hybrid WOLEDs with “hot exciton” blue fluorophores. d) masured at 100 cd/m2 e) measured at 1000 cd/m2. f) measured at 500 cd/m2.

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


17


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Page 18 of 22

The brightness-voltage and the power efficiency-brightness characteristics of the monochrome devices. The current density-voltage and the brightness-voltage characteristics of the WOLEDs. This material is available free of charge via the Internet at http://pubs.acs.org. REFERENCES (1) Farinola, G. M.; Ragni, R. Electroluminescent Materials for White Organic Light Emitting Diodes. Chem. Soc. Rev. 2011, 40, 3467–3482. (2) Wang, Q.; Ma, D. G. Management of Charges and Excitons for High-performance White Organic Light Emitting Diodes. Chem. Soc. Rev. 2010, 39, 2387–2398. (3) Gather, M. C.; Köhnen, A.; Meerholz, K. White Organic Light-Emitting Diodes. Adv. Mater. 2011, 23, 233–248. (4) Sun, Y.; Giebink, N. C.; Kanno, H.; Ma, B.; Thompson, M. E.; Forrest, S. R. Management of Singlet and Triplet Excitons for Efficient White Organic Light-emitting Devices. Nature. 2006, 440, 908-912. (5) Schwartz, G.; Pfeiffer, M.; Reineke, S.; Walzer, K.; Leo, K. Harvesting Triplet Excitons from Fluorescent Blue Emitters in White Organic Light-emitting Diodes. Adv. Mater. 2007, 19, 3672–3676. (6) Ye, J.; Zheng, C. J.; Ou, X. M.; Zhang, X. H.; Fung, M. K.; Lee, C. S. Management of Singlet and Triplet Excitons in a Single Emission Layer: a Simple Approach for a Highefficiency Fluorescence/Phosphorescence Hybrid White Organic Light-emitting Device. Adv. Mater. 2012, 24, 3410–3414.


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TOC Charges Recombination

S1

75%

Power efficiency

25%

100

10

T1

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

Page 22 of 22

10 1

T1

0.3 wt% (0.348,0.422) 0.5 wt% (0.358,0.430) 0.8 wt% (0.398,0.456)

0.1

1

S0 S0 Blue TADF host emitter Orange phosphor

Enhanced Förster energy transfer

100

1000

Brightness (cd/m2)

10000

High efficiency with low rollroll-off, good colorcolor-stability


22

Highly Efficient Simplified Single-Emitting-Layer ... - ACS Publications

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