High Performance All Fluorescence White Organic Light Emitting

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High Performance All Fluorescence White Organic Light Emitting Devices with a Highly Simplified Structure Based on Thermally Activated Delayed Fluorescence Dopants and Host Wei Liu,§,‡,# Cai-Jun Zheng,*,§,‡ Kai Wang,†,‡ Ming Zhang,§ Dong-Yang Chen,†,‡ Si-Lu Tao,§ Fan Li,‡ Yu-Ping Dong,# Chun-Sing Lee,*,∥ Xue-Mei Ou,†,‡ and Xiao-Hong Zhang*,†,‡ §

School of Optoelectronic Information, University of Electronic Science and Technology of China (UESTC), Chengdu 610054, P.R. China † Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, Suzhou, Jiangsu 215123, P.R. China ‡ Nano-organic Photoelectronic Laboratory and Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, P.R. China # College of Materials Science and Engineering, Beijing Institute of Technology, Beijing, 100081, P. R. China ∥ Center of Super-Diamond and Advanced Films (COSDAF) & Department of Physics and Materials Sciences, City University of Hong Kong, Hong Kong SAR, P.R. China S Supporting Information *

ABSTRACT: Thermally activated delayed fluorescence (TADF) emitters of different colors commonly need different hosts, which cause the complexed device structure and low efficiency of all fluorescence white organic light-emitting devices (F-WOLEDs). To solve this, novel concept of employing TADF exciplex as universal host of TADF emitters with different colors was proposed. All blue, green, and orange devices based on the TADF exciplex host show much lower turn-on voltages, and comparable and even higher efficiencies than corresponding devices based on conventional hosts. The two color F-WOLED with extremely simplified device structure was finally fabricated, achieving a white emission with the maximum current efficiency (CE), power efficiency (PE), and external quantum efficiency (EQE) respectively of 50.1 cd A−1, 63.0 lm W−1, and 19.0% in the forward-viewing direction without any light out-coupling technology, which is the best performance among reported F-WOLEDs, demonstrating the superiority of the novel concept. KEYWORDS: thermally activated delayed fluorescence, exciplex, all fluorescence, reverse intersystem crossing, WOLEDs generation all fluorescence WOLEDs (F-WOLEDs) based on pure organic TADF emitters have been in the spotlight.15−17 Because of the extremely small singlet and triplet splittings, TADF emitters enable efficient excitons up-conversion from dark triplet state to radiative singlet state.14,18−23 Thus, such TADF-based OLEDs present a promising approach for high performance WOLED. In 2014, Adachi et al. further reported a TADF-based F-WOLED by doping blue and yellow TADF emitters respectively into two different layers. The device showed white light emission with a CIE coordination of (0.32, 0.39), but the maximum EQE and power efficiency (PE) are only 6.7% and 16.4 lm W−1.24 They further improved the maximum EQE of the TADF-based F-WOLEDs to 17.1% by doping blue, green, and red TADF emitters into three different hosts. However, due to the high driving voltages in this three-

1. INTRODUCTION White organic light-emitting device (WOLED) has drawn great attention due to its applications in the areas of lighting and displays.1−3 In the past two decades, much efforts have been put on WOLEDs based on phosphorescence, because phosphorescent emitters can utilize electro-generated triplet excitons, realizing 100% exciton harvesting. However, up until now performances of blue phosphors are still much below those of the red and green phosphors.4,5 For this reason, hybrid WOLEDs employing blue florescent emitters and phosphors of other colors have been widely adapted.5−13 As most blue fluorescent emitters cannot harvest the triplet exciton, the hybrid WOLED is in fact a compromising approach which scarifies part of the efficiency for a better device stability. Since Adachi et al. reported the first fluorescent organic light emitting devices (F-OLEDs) that gave a high external quantum efficiency (EQE) of 19.3% by employing an emitter 1,2,3,5tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) with thermally activated delayed fluorescence (TADF),14 the new© XXXX American Chemical Society

Received: July 12, 2016 Accepted: November 10, 2016 Published: November 10, 2016 A

DOI: 10.1021/acsami.6b08546 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. (a) Energy transfer route of the TADF emitters with exciplex host (solid arrow represents Förster energy transfer and dash arrow represents Dexter energy transfer). (b) Device structures and the energy diagrams of the TADF devices with exciplex host CDBP:PO-T2T.

were finally fabricated by simultaneously doping blue and orange TADF emitters into the CDBP:PO-T2T host. The extremely simplified device exhibits a white emission with the maximum EQE, PE, current efficiency (CE), respectively of 19.0%, 63.0 lm W−1, and 50.1 cd A−1 in the forward-viewing direction without any light out-coupling technology, which is the best performance among reported F-WOLEDs,15−17,24−28 demonstrating the superiority of the present device concept.

host system, the maximum PE of this F-WOLED is only 33.4 lm W−1.25 Moreover, the complicated device structure would also hinder the practical use of F-WOLEDs. Some attempts have also been made to avoid the complicated emitting layers (EML) in F-WOLEDs.16,17,26−28 In 2015, Lee et al. reported an F-WOLEDs using blue TADF material simultaneously as a blue emitter and a host for a yellow conventional fluorescent emitter in a single emitting layer (EML). The single-EML device showed a low turn-on voltage of 2.5 V, but the maximum EQE and PE were just 15.5% and 39.3 lm W−1, which should be ascribed to the triplet exciton loss in conventional fluorescent emitter.28 Apparently, it is highly desirable to combine the simple structure and low operating voltages of single EML devices with high efficiency all-color TADF emission for designing F-WOLEDs. However, this is so far not achieved due to the lack of high performance bipolar materials for simultaneously hosting TADF emitters of different colors.14,18−23,29,30 Recently, novel exciplex emitters with TADF characteristic have drawn much attention for they can not only realize high exciton utilization ratio through efficient up-conversion of triplet exactions,31−37 but also exhibit excellent performance as the host of both conventional phosphorescent and fluorescent emitters.10,38−40 Herein, we propose a new concept of employing TADF exciplex as the host of TADF emitters. As shown in Figure 1a, the 100% energy transfer can be achieved through either Förster energy transfer or Dexter energy transfer from exciplex host to TADF emitter dopant; and the energy transfer circle between singlet and triplet energy levels (S1 and T1) can extend the lifetimes of singlet and triplet excitons on exciplex host, benefiting the energy transfer from host to dopant. Moreover, the exciplex hosts can easily realize balanced carrier transporting property and have no carrier injectionbarrier to EML.10,38−40 These merits suggest that TADF exciplex hosts should potentially be effective energy donors to a much wider range of dopants comparing to conventional hosts. In this work, a TADF exciplex CDBP (4,4′-bis(9-carbazolyl)2,2′-dimethylbiphenyl):PO-T2T ((1,3,5-triazine-2,4,6-triyl)tris(benzene-3,1-diyl)tris(diphenylphosphine oxide)) was chosen as the host for other TADF emitters.10 With good hole and electron transporting properties from CDBP and PO-T2T respectively, the devices based on CDBP:PO-T2T host have been constructed without using any other carrier transporting materials for blue, green and orange TADF emitters as shown in Figure 1b. All blue, green, and orange devices based on the CDBP:PO-T2T host show much lower turn-on voltages, and comparable and even higher efficiencies than corresponding devices based on conventional hosts. Two-color F-WOLED

2. RESULTS AND DISCUSSION In our previous report, CDBP:PO-T2T exciplex has been reported as an efficient blue TADF exciplex emitter with a high EQE of 13% and a CIE coordinate of (0.17, 0.29) in the device.10 To characterize the capacity of CDBP:PO-T2T exciplex as the host for other TADF emitters, three TADF emitters, 1,2-bis(carbazol-9-yl)-4,5-dicyanobenzene (2CzPN), 4CzIPN, and 2,6-di(9H-carbazol-9-yl)anthracene-9,10-dione (AnbCz), which have lower S 1 and T 1 values than CDBP:PO-T2T exciplex and emission colors with sky-blue, green and orange, have been chosen to fabricate OLEDs with the exciplex host. Among them, 2CzPN and 4CzIPN are widely used sky-blue and green TADF emitters;14,42−44 and AnbCz is a newly designed and synthesized orange TADF emitter, which has a similar structure with the orange TADF emitters reported recently.48 The devices based on the CDBP:PO-T2T host were manufactured with the structure of ITO/MoO3 (1 nm)/CDBP (45 nm)/CDBP:50 wt % PO-T2T:dopant (30 nm)/PO-T2T (45 nm)/LiF (1 nm)/Al (100 nm). The doping concentrations of 2CzPN, 4CzIPN and AnbCz are carefully optimized to 6, 6, and 4 wt % to achieve the best performance. In the devices, ITO/MoO3 and LiF/Al were the anode and the cathode, respectively. Benefited from the good hole and electron transporting capacities of CDBP and PO-T2T respectively, no additional carrier transporting material was needed in the devices. The undoped CDBP and PO-T2T are the hole transporting layer (HTL) and the electron transporting layer (ETL), respectively. The EML is the mixed layer of exciplex and dopant. Energy level diagrams of the devices are presented in the Figure 1b. All energy levels were newly measured in our laboratory. As shown, the holes and the electrons can be respectively injected from the HTL and the ETL to the EML with little barrier. Such “barrier-free” device structure would contribute to reduce driving voltages hence realize high PEs. Furthermore, the HOMO energy levels of all three dopants are lower than CDBP, which can not only prevent the exciplex forming between PO-T2T and dopants, but also effectively minimize the trapping effects on dopants, thus the excitons are mainly generated on exciplex. To better evaluate performance B


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Figure 2. (a) EL spectra, (b) current density−luminance−voltage characteristics, (c) PE−luminance plots, and (d) EQE−luminance plots of devices B-1, G-1 and O-1.

Table 1. Summary of Optimized OLEDs Performance device B-1 B-2 B-3 B-4 B-5 G-1

CDBP:POT2T DPEPO

G-2

mCP CDBP PO-T2T CDBP:POT2T mCP

G-3

CBP

G-4

DPEPO

G-5

CDBP

G-6

PO-T2T

O-1

CDBP:POT2T mCP CBP DPEPO CDBP PO-T2T

O-2 O-3 O-4 O-5 O-6

emitter

Vona (V)

max. CEb (cd A−1)/ PEc (lm W−1)/ EQEd (%)

EQE (%) at 100 cd m−2

EQE (%) at 1000 cd m−2

λELe (nm)

CIE(x, y)f

6 wt % 2CzPN

2.3

40.0/47.4/19.5

8.9

3.5

481

(0.18, 0.32)

15 wt % 2CzPN 8 wt % 2CzPN 7 wt % 2CzPN 7 wt % 2CzPN 6 wt % 4CzIPN 6 wt % 4CzIPN 7 wt % 4CzIPN 8 wt % 4CzIPN 7 wt % 4CzIPN 6 wt % 4CzIPN 4 wt % AnbCz

3.8

41.0/30.0/18.2

8.0

1.8

492

(0.18, 0.34)

3.3 3.4 3.5 2.3

25.1/23.2/14.2 23.9/21.7/12.7 17.5/14.5/7.3 58.5/70.6/21.4

8.3 5.9 3.5 20.5

3.0 2.4 1.8 16.5

472 465 491 503

(0.16, (0.17, (0.19, (0.22,

3.3

57.8/48.8/20.2

18.0

15.5

503

(0.24, 0.51)

3.4

57.1/53.2/19.4

18.3

15.6

506

(0.25, 0.53)

4.0

50.0/35.7/15.8

10.0

7.3

521

(0.28, 0.56)

3.4

53.6/48.4/18.7

16.8

14.0

503

(0.24, 0.52)

3.5

52.0/44.1/16.7

14.0

12.6

519

(0.29, 0.55)

2.4

34.9/34.2/13.2

8.8

4.1

582

(0.49, 0.49)

5 5 5 6 5

3.4 3.4 4.8 3.5 4.7

30.3/25.1/10.8 23.3/19.8/8.3 12.3/7.0/5.1 31.3/26.2/10.5 22.0/11.9/8.6

8.4 6.6 2.5 7.1 5.6

3.9 3.5

571 572 586 562 581

(0.48, (0.47, (0.52, (0.45, (0.48,

host

wt wt wt wt wt

% % % % %

AnbCz AnbCz AnbCz AnbCz AnbCz

2.6

0.24) 0.25) 0.29) 0.48)

0.51) 0.52) 0.47) 0.52) 0.52)

Turn-on voltage, estimated at the brightness of 1 cd m−2. bCurrent efficiency. cPower efficiency. dExternal quantum efficiency. eEL emission maximum. fEstimated at 100 cd m−2. a

of the present devices that employing the exciplex with TADF characteristic as the host for other TADF emitters, corresponding optimized devices with conventional hosts, DPEPO, mCP, CBP, and the pristine materials, CDBP, PO-T2T, were also

fabricated for comparison. (The molecular structures of all these materials are shown in Figure S1.) Figure 2 shows characteristics of the three devices based on the CDBP:PO-T2T host. Key performance parameters of all C


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Figure 3. EL spectra of the WOLEDs at different luminance: (a) devices W-1, (b) W-2, and (c) W-3. (d) Current density−luminance−voltage characteristics, (e) PE−luminance, and (f) EQE−luminance plots of the three F-WOLEDs.

TADF devices based on the CDBP:PO-T2T host and the conventional hosts are summarized in Table 1. As shown in Figure 2a, the three devices based on the exciplex host exhibit respectively blue, green, and orange emissions from the TADF dopants of 2CzPN, 4CzIPN, and AnbCz. To further prove the energy transfer process from the exciplex host to the dopants, the phosphorescence spectra of CDBP:PO-T2T and 6 wt % 2CzPN doped CDBP:PO-T2T films were measured at 77 K. As shown in Figure S2, although 2CzPN possess the highest energy levels in three emitters, the excitons formed on CDBP:PO-T2T can also fully transfer to 2CzPN. From Table 1, the emissions of TADF emitters are more obvious redshift in electron-accepting hosts (DPEPO and PO-T2T) than that in electron-donating hosts (mCP, CBP and CDBP), which might be ascribed to the larger intermolecular interactions between TADF emitters and electron-accepting hosts and the different polarities of the host materials.45−47 Accordingly, the emission peaks of TADF emitters in CDBP:PO-T2T exciplex hosts are between that in CDBP and PO-T2T. From the current density−luminance−voltage characteristics shown in Figure 2b, all the three devices based on the exciplex host have extremely

low turn-on voltages around 2.3 V (at the brightness of 1 cd m−2), which are even lower than the theoretical limit values defined by photon energies for sky-blue and green emissions. Moreover, such values are also much lower than those of the devices using the conventional hosts (>3.3 V), indicating the superiority of the exciplex host and also proving the exciton generated on exciplex in the three devices. Using the conventional hosts, the maximum EQEs of the 2CzPN-based devices are 18.2% for DPEPO (device B-2), 14.2% for mCP (device B-3), 12.7% for CDBP (device B-4), and 7.3% for PO-T2T (device B-5), respectively. The highest EQE is realized in the DPEPO with a complicated structure of ITO/TAPC (20 nm)/TCTA (20 nm)/CzSi (10 nm)/ DPEPO:15 wt % 2CzPN (20 nm)/DPEPO (10 nm)/TmPyPB (30 nm)/LiF (1 nm)/Al (100 nm), which is consistent with the results of the reported blue TADF devices.4,14,43,44 Correspondingly, the 2CzPN-based device employing the CDBP:PO-T2T host (device B-1) possesses a highly simplified structure, and gives a maximum EQE of 19.5%, showing the best performance among all OLEDs based on the TADF emitter 2CzPN.14,43,44 Such a high efficiency also demonstrate effective host-to-dopant D


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ACS Applied Materials & Interfaces Table 2. Electroluminescence Properties of the F-WOLEDs

PE/EQE/CRI/CIE device

a

host

W-1

CDBP:POT2T

W-2

mCP

W-3

CDBP

guest 7.5 wt % 2CzPN 0.6 wt % AnbCz 7.5 wt % 2CzPN 0.6 wt % AnbCz 7.5 wt % 2CzPN 0.6 wt % AnbCz

Von (V)a

max. CEb (cd A−1)

max. PEc (lm W−1)

max. EQEd (%)

2.3

50.1

63.0

19.0

17.8/9.0/62 (0.35, 0.45)

10.3/5.6/63 (0.34, 0.44)

3.2

31.7

31.1

11.0

7.4/3.9/71 (0.31, 0.38)

4.8/2.8/73 (0.29, 0.35)

3.2

37.2

33.3

13.1

8.4/5.2/69 (0.32, 0.40)

5.3/3.2/72 (0.30, 0.36)

at 500 cd m−2

at 1000 cd m−2

Turn-on voltage, estimated at the brightness of 1 cd m−2. bCurrent efficiency. cPower efficiency. dExternal quantum efficiency.

Table 3. Summary of EL Performance of TADF Based F-WOLEDs this work ref 26. ref 28. ref 17. ref 27. ref 15.c ref 16. ref 24. ref 25. ref 49. a

EML

Von (V)a

max. CE (cd A−1)b

max. PE (lm W−1)b

max. EQEb (%)b

single layer single layer single layer single layer double layer double layer double layer double layer triple layer triple layer

2.3 2.5

50.1 20.2 38.4 35.1 8.1 27.7

63.0 15.9 39.3 36.2 6.4 15.8 22.0 16.4 33.4 28.9

19.0 7.5 15.5 14.0 4.4 11.6 12.1 6.7 17.1 15.6

>4.0 4.0 4.0 >3.0 5.0 3.6 5.3

40.3 31.3

CIE 1000 cd m−2 (0.34, (0.36, (0.28, (0.31, (0.34, (0.29, (0.25, (0.32, (0.30, (0.36,

0.44) 0.44) 0.35) 0.37) 0.34) 0.35) 0.31) 0.39) 0.38) 0.38)

Turn-on voltage, estimated at the brightness of 1 cd m−2. bForward-viewing efficiencies. cTandem device.

devices, except the EML was optimized as CDBP:PO-T2T: 7.5 wt % 2CzPN: 0.6 wt % AnbCz. As shown in Figure 3a, this extremely simplified F-WOLED achieved a white emission with CIE coordinates slightly changed from the (0.35, 0.45) at practical luminance of 500 cd m−2 to (0.31, 0.42) at high luminance of 5000 cd m−2. To better reveal the performance of the F-WOLED based on the CDBP:PO-T2T exciplex, the comparison F-WOLEDs with 2CzPN and AnbCz were also fabricated employing conventional hosts of mCP and CDBP. Devices W-2 and W-3 were fabricated with the same doping concentrations for 2CzPN and AnbCz in Device W-1, and the device structures were optimized as ITO/TAPC (40 nm)/ TCTA (10 nm)/mCP or CDBP:7.5 wt % 2CzPN:0.6 wt % AnbCz (30 nm)/TmPyPB (45 nm)/LiF (1 nm)/Al (100 nm). White emission spectra of Device W-2 and W-3 are shown in Figure 3b and 3c, and the CIE coordinates of two devices are (0.31, 0.38) at practical luminance of 500 cd m−2 to (0.29, 0.35) at high luminance of 1000 cd m−2 for device W-2, and (0.32, 0.40) at practical luminance of 500 cd m−2 to (0.30,0.36) at high luminance of 1000 cd m−2 for device W-3. Similar to the observation with the monochromatic devices, the turn-on voltage of device W-1 (2.3 V) is much lower than those of the mCP and the CDBP-based devices (3.2 V). As shown in Figure 3e and Table 2, devices W-2 and W-3 only gave maximum efficiencies of 31.7 cd A−1/31.1 lm W−1/11.0% and 37.2 cd A−1/33.3 lm W−1/13.1% for CE/PE/EQE, respectively, indicating energy loss cannot be avoided in F-WOLEDs based on conventional hosts. In the contrary, device W-1 exhibits much higher maximum efficiencies of 50.1 cd A−1, 63.0 lm W−1, and 19.0%, respectively, for CE, PE, and EQE in forward-viewing direction without any light out-coupling technology and lower efficiency roll-off, which may be ascribed to the balanced carrier transporting property of CDBP:PO-T2T host, no carrier injection barrier to EML in the devices, and the better energy transfer from host to dopants caused by the

energy transfer, confirming that CDBP:PO-T2T is an excellent host for the sky-blue TADF emitter 2CzPN. Moreover, due to the low driving voltages, Device B-1 exhibits a much higher PE of 47.4 lm W−1 comparing to 30.0 lm W−1 of Device B-2. As shown in Table 1, performances of the green devices based on 4CzIPN in different hosts show less spread. The maximum EQEs of the devices are 21.4% for CDBP:PO-T2T (device G-1), 20.2% for mCP (device G-2), 19.4% for CBP (device G-3), 15.8% for DPEPO (device G-4), 18.7% for CDBP (device G-5), and 16.7% for PO-T2T (device G-6), respectively. The highest EQE is also realized with the CDPB:PO-T2T host, and the maximum PE even reaches a high value of 70.6 lm W−1 because of the low driving voltages. Since orange and red TADF emitters are less reported,14,18,24,48 a newly designed and synthesized orange TADF emitter AnbCz was used in this work. As shown in Table 1, The orange device employing the CDBP:PO-T2T host (device O-1) shows significantly higher EQE than those based on the conventional hosts. The maximum efficiencies of device O-1 are 34.9 cd A−1 for CE, 34.2 lm W−1 for PE, and 13.2% for EQE, respectively. As far as we know, these results are among the best reported orange TADF OLEDs.14,18,24,48,49 It is considered that the overlap between the absorption spectrum of AnbCz and the fluorescence spectrum of CDBP:PO-T2T exciplex (Figure S6) contribute to effective Förster energy transfer and thus the high device efficiency. On account of the results demonstrated above, the TADF exciplex CDBP:PO-T2T was demonstrated to be excellent host for TADF emitters of different emission colors from sky-blue to orange, which can be hardly realized with conventional hosts. Therefore, F-WOLEDs were finally constructed by simultaneously doping the blue and the orange TADF emitters (2CzPN and AnbCz) into the CDBP:PO-T2T host. The basic structure of the white device based on CDBP:PO-T2T host (device W-1) is the same with that of the monochromatic E


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reference electrode, working electrode, and the supporting electrolyte, and the scan rate of 10 mV/s. 4.3. OLEDs Fabrication. The ITO-coated glasses were cleaned and treated with UV-ozone for 15 min, and then organic materials were deposited in a vapor deposited instrument under the pressure of 4 × 10−4 Pa. The deposited rate of organic materials, LiF, and Al were controlled at 1−2 Å s−1, 0.1 Å s−1, and 10 Å s−1, respectively. The detailed structures of all devices are shown in the Supporting Information. The optical and electrical data of the devices were respectively recorded with a PR650 Spectrascan and Keithley 2400 SourceMeter under ambient atmosphere simultaneously. CE, PE, and EQE were calculated with the data of current, luminance, and emission spectrum, assuming a Lambertian distribution.

longer lifetimes of singlet and triplet excitons on TADF exciplex host.38,41 Interestingly, the peak EQE of device W-1 is even higher than the corresponding monochromatic devices (device B-1 and O-1), this may be benefited from the additional energy transfer process from the higher energy components to the lower ones effectively reducing the exciton concentration, and the low concentration doped of AnbCz avoiding the exciton annihilation. Moreover, as listed in Table 3, the performance of the present simple-structured WOLED is far better than those reported F-WOLEDs based on the newgeneration TADF emitters,15−17,24−28,50 and even closed to the all-phosphorescence WOLEDs.51,52 Although the efficiencies of device W-1 decrease fast with the increased luminances as shown in Figure 3e, such evident efficiency roll-off should be mainly ascribed to the poor efficiency-stability of corresponding TADF emitters caused by the exciton annihilation when the exciton concentration gets higher with the increased driving voltage, which can be also observed in the monochromatic devices based on 2CzPN and AnbCz. Thus, we believe such flaw can be solved by replacing 2CzPN and AnbCz with more efficiency-stable TADF emitters.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b08546. Synthesis process and physicochemical property of AnbCz, detailed structures of all tested devices, molecular structures of the materials used, and additional data and spectra (PDF)

3. CONCLUSION In a summary, to address the problems that hindering the practical use of F-WOLEDs: (a) low efficiency, (b) complexed device structure, and (c) high turn-on voltage, we proposed to use a TADF exciplex to simultaneously hosting TADF emitters of different color such that single EML WOLED can be realized with high efficiency TADF emissions. The TADF exciplex CDBP:PO-T2T is successfully demonstrated as an excellent host for sky-blue, green and orange TADF emitters, 2CzPN, 4CzIPN and AnbCz. The highly simplified F-WOLED based on this concept achieved an extremely low turn-on voltage of 2.3 V, maximum efficiencies of 50.1 cd A−1, 63.0 lm W−1, and 19.0%, respectively, for CE, PE, and EQE in the forwardviewing direction without any light out-coupling technology, demonstrating the superiority of the present device design for high-performance F-WOLEDs.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Chun-Sing Lee: 0000-0001-6557-453X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant No. 51533005, 51373190), the Beijing Nova Program (Grant No. Z14110001814067), Collaborative Innovation Center of Suzhou Nano Science & Technology, the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) and Qing Lan Project, P.R. China.

4. EXPERIMENTAL SECTION 4.1. Materials Sources. All materials used were directly purchased from Shanghai Han Feng Chemical Co., LTD, except AnbCz, which is newly designed and synthesized. Synthetic route, detailed synthesis, the information on 1H NMR, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), mass spectroscopies, elemental analysis, and TADF characteristic of AnbCz were given in the Supporting Information. 4.2. General Information. 1H NMR data, mass spectral data, and elemental analysis (C, H, N) were, respectively, measured with Bruker Advance-400 spectrometer, Finnigan 4021C gas chromatography−mass spectrometry instrument and an Elementar Vario ELIII element analyzer. DSC and TGA data were obtained respectively with a NETZSCH DSC204 instrument and TAQ 500 thermogravimeter in the N2 atmosphere. The measurements of fluorescence quantum yields (PLQY), elemental analysis, and PL temperature-dependent transient decay lifetime were carried out at The Analysis and Test Center of Beijing University of Chemical Technology. Cyclic voltammetry data were measured with CHI660E electrochemical analyzer, with a saturated calomel electrode (SCE), Pt disks, and 0.1 M Bu4NPF6, respectively, as the

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REFERENCES

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High Performance All Fluorescence White Organic Light Emitting

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