Hyperfluorescence-based full fluorescent white organic light-emitting diodes

Accepted Manuscript Title: Hyperfluorescence-based full fluorescent white organic light-emitting diodes Authors: Wook Song, Kyoung Soo Yook PII: DOI: Reference:

S1226-086X(17)30706-2 https://doi.org/10.1016/j.jiec.2017.12.044 JIEC 3800

To appear in: Received date: Revised date: Accepted date:

7-11-2017 10-12-2017 18-12-2017

Please cite this article as: Wook Song, Kyoung Soo Yook, Hyperfluorescence-based full fluorescent white organic light-emitting diodes, Journal of Industrial and Engineering Chemistry https://doi.org/10.1016/j.jiec.2017.12.044 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Hyperfluorescence-based full fluorescent white organic light-emitting diodes Wook Song and Kyoung Soo Yook*

2066, Seobu-ro, Jangan-gu, Suwon, Gyeonggi, 16419, Korea

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E-mail: [email protected]

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School of Chemical Engineering, Sungkyunkwan University

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Graphical abstract

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ABSTRACT In this study, we developed hyperfluorescence-based full fluorescent white organic lightemitting diodes by doping blue and yellow fluorescent emitters in the blue thermally activated delayed fluorescent emitting layer. The blue thermally activated delayed fluorescent emitter acted as sensitizer for energy transfer to the fluorescent blue and yellow fluorescent emitters. The hyperfluorescence emission of the fluorescent emitters produced two white color emissions with a color coordinate of (0.26,

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0.32). In addition, the fluorescent white organic light-emitting diodes exhibited a high external quantum efficiency of 13.0%.

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Keywords: white organic light-emitting diodes; fluorescent device; hyperfluorescence; delayed

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fluorescence

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Introduction

The development of white organic light-emitting diodes (WOLEDs) has become a key issue in the

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organic light-emitting diode (OLED) industrial field owing to their increasing demand for high

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efficiency WOLEDs for large size OLED televisions and high-resolution OLEDs for virtual display.

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The WOLEDs are integrated into an OLED panel with a color filter, easily producing full color OLEDs. The WOLEDs can emit white light in various ways by changing the device structure and emitting

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materials. In general, blue and yellow emitter combination [1–3] or blue, green and red emitter combination are used to produce the WOLEDs.[4–6] Several combinations of the emitting materials are

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used to emit white light, and they are all emitters [7–9], all fluorescent emitters [10,11], and a hybrid of fluorescent and phosphorescent emitters [12–15]. All phosphorescent WOLEDs have a high external quantum efficiency (EQE), but have poor stability. They are advantageous in terms of stability; however, they have intrinsically low EQE. The hybrid WOLEDs are in between all the fluorescent and all phosphorescent WOLEDs in terms of EQE and stability. In general, it is quite difficult to improve the 2

stability of organic emitters, and thus improving the EQE of all the stable fluorescent WOLEDs is highly desirable. Increasing the EQE of fluorescent WOLEDs has been very slow, but the advent of hyperfluorescence, thermally activated delayed fluorescent (TADF) emitter sensitized fluorescence, opened a way of upgrading the EQE of fluorescent OLEDs. Although the hyperfluorescence was mainly developed to improve the EQE of single color fluorescent OLEDs, it can be utilized to promote the EQE

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of WOLEDs.[16–18]

In this study, all the fluorescent WOLEDs operated by hyperfluorescence were developed to promote the EQE of two color fluorescent WOLEDs. Blue and yellow fluorescent emitters were co-doped in the blue TADF emitter doped emitting layer to induce white emission by hyperfluorescence. The optimized

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WOLEDs showed white color coordinate of (0.26, 0.32) and a high EQE of 13.0% for all the

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fluorescent WOLEDs.

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Result and discussion

In the design of the hyperfluorescence-based WOLEDs, a triple-doped emitting layer system with a blue

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TADF emitter, a blue fluorescent emitter, and a yellow fluorescent emitter was used. The blue TADF

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emitter, blue fluorescent emitter, and yellow fluorescent emitter were well-known bis[4-(9,9-dimethyl9,10-dihydroacridine)phenyl]sulfone (DMAC-DPS), 2,5,8,11-tetra-tert-butylperylene (TBPe), and 2,8-

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di-tert-butyl-5,11-bis(4-tert-butylphenyl)-6,12-diphenyltetracene (TBRb), respectively. The PL emission

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wavelength of DMAC-DPS overlapped with the absorption wavelength of TBPe and TBRb, inducing sensitizing effect in the emitting layer. As reported, the TADF sensitizer can harvest singlet excitons of

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the TBPe and TBRb fluorescent emitters by Forster energy transfer derived PL emission process rather than electroluminescence (EL) process.[19] Therefore, high EQE was anticipated from all the hyperfluorescence based fluorescent WOLEDs. The chemical structures and energy levels of the materials including the device structure are displayed in Figure 1. In the device design, the doping concentration of TBRb was varied to optimize the device performances

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based on the device test results of blue hyperfluorescence devices containing DMAC-DPS and TBPe in the bis-(2-(diphenylphosphino)phenyl)ether oxide (DPEPO) host. The EQE and EL spectra of the TBPe hyperfluorescence devices are shown in Figures 2(a) and (b), respectively. The DMAC-DPS sensitized TBPe devices showed a high EQE of 11.8% 1000 cd/m2 with fluorescence emission of TBPe. Similar results were reported for the hyperfluorescence device comprising 10-(4-((4-(9H-carbazol-9-

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yl)phenyl)sulfonyl)phenyl)-9,9-dimethyl-9,10-dihydroacridine (CzAcSF) and TBPe.[20]

Based on the efficient TADF sensitizing effect in the DPEPO:DMAC-DPS:TBPe devices, TBRb emitter was additionally doped in the DPEPO:DMAC-DPS emitting layer. The doping concentration of TBRb was in the range 0.5–0.9% to adjust the yellow intensity of the white emission. Figure 3(a) shows the

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current density (J)–voltage (V)–luminance (L) plots of the hyperfluorescence WOLEDs with TBRb

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doping concentration. A slight reduction in the current density and luminance was apparent in the device

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data by increased TBRb doping in the emitting layer, because of the carrier trapping effect by TBRb.

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TBRb is a yellow emitter with the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of 5.4 and 3.2 eV, respectively. The HOMO–LUMO gap of TBRb is

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narrower than that of the DPEPO host. Therefore, hole and electron carriers are trapped by TBRb,

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reducing the current density at a high TBRb doping concentration, which in turn decreased the luminance.

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The EQE of the hyperfluorescence WOLEDs was compared, as shown in Figure 3(b). The maximum

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EQE continuously decreased with increasing TBRb doping concentration from 12.9% (0.5% doping) to 10.8% (0.9% doping). This reduction of the EQE can be correlated to the emission mechanism of

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hyperfluorescence. In the hyperfluorescence, the emission from the fluorescent emitter follows energy transfer and charge trapping pathways. In the energy transfer process, Forster energy transfer process is favored because, Dexter energy transfer unfavoured. However, the Dexter energy transfer process would increase at a high fluorescent emitter doping concentration by the shortened intermolecular distance between DMAC-DPS and fluorescent emitters, and this is one reason for the decreased EQE at

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a high TBRb doping concentration. The other reason is carrier trapping by TBRb as projected from the J–V curves of the WOLEDs. The carrier trapping effect confirmed in the current density reduction at high TBRb doping concentration allows fluorescence emission by the EL process rather than the PL process, limiting the internal quantum efficiency of the TBRb emitter to 25% compared to 100% in the PL process assuming no loss process. The carrier trapping becomes significant at a high TBRb doping

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concentration, degrading the EQE of the hyperfluorescence WOLEDs. The best performing hyperfluorescence WOLEDs with a TBRb doping concentration of 0.5% showed the maximum EQE of 12.9% and EQE of 10.8% at 1,000 cd/m2. Considering that the EQE of all the fluorescent WOLEDs is 10%, the EQE of the current hyperfluorescence WOLEDs suprisngly increased.

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The TBRb doping concentration affected the EL spectra of the hyperfluorescence WOLEDs, as shown

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in Figure 3(c). A mixed emission of TBPe and TBRb was observed from the EL spectra by

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hyperfluorescence mechanism. The color coordinates of the hyperfluorescence WOLEDs red-shifted at

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high TBRb doping concentration, and they were (0.26, 0.32), (0.35, 0.40), and (0.35, 0.41) at 0.5, 0.7, and 0.9% TBRb doping concentration, respectively. The color coordinate change of hyperfluorescence

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WOLEDs are listed in Table 1.

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WOLEDs from 1,000 cd/m2 to 4,000 cd/m2 was 0.02(Δxy). The device performances of all the

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Conslusion

In conclusion, hyperfluorescence WOLEDs having blue and yellow fluorescent emitters doped in the

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blue TADF emitting layer achieved 12.9% EQE with a color coordinate of (0.26, 0.32). In addition they exhibited improved EQE relative to all traditional fluorescent WOLEDs, because of the sensitizing

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effect by the blue TADF emitter. Therefore, the approach to realize the white emission by hyperfluorescence process would be effective to enhance the EQE of all fluorescent WOLEDs.

Experimental The hyperfluorecence WOLEDs were designed to have triple-doped structure of a TADF blue emitter, a 5

blue fluorescent emitter, and a yellow fluorescent emitter with a high triplet host in the emitting layer. The

device

structure

of

OLEDs

has

indium

tin

oxide

(ITO,

120

nm)/poly(3,4-

ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS, 60 nm)/4,4'-cyclohexylidenebis[N, Nbis(4-methylphenyl)aniline]

(TAPC,

nm)/DPEPO:DMAC-DPS:fluorescent

20

nm)/1,

emitter

3-bis(N-carbazolyl)benzene (25

nm)/diphenylphosphine

(mCP,

10

oxide-4-

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(triphenylsilyl)phenyl (TSPO1, 5 nm)/1,3,5-tris(Nphenylbenzimidazole-2-yl)benzene (TPBI, 30 nm)/LiF (1 nm)/Al (200 nm). The doping concentrations of DMAC-DPS and TBPe blue fluorescent emitter were 50% and 1.0%, respectively, for blue OLEDs and WOLEDs. The doping concentration of TBRb red fluorescent emitter was 0.5%, 0.7%, and 0.9% for WOLEDs. Devices performances of the

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blue OLEDs and WOLEDs were characterized using a CS2000 spectroradiometer and a Keithley 2400

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source measurement unit after encapsulating the device using a calcium oxide getter and a glass lid.

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List of figures Figure 1. (a) Chemical structure of emitter materials used in the device fabrication and (b) energy level diagram of the hyperfluorescence based WOLEDs. Figure 2. (a) External quantum efficiency–luminance curves and (b) electroluminescence spectrum of

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the TBPe hyperfluorescence device.

Figure 3. (a) Current density–voltage–luminance, (b) external quantum efficiencies curves and (c)

electroluminescence spectra of the hyperfluorescence based WOLEDs with different TBRb doping

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concentrations.

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DMAC-DPS

TBPe

TBRb

(b) 2.1 2.5 2.4

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2.9

TPBi

TSPO1

TBPe

TBRb

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3.2

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DMAC-DPS

2.8

2.7

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mCP

2.5

DPEPO 3.0

TAPC

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(a)

6.1

5.3

5.4 6.1

5.9 6.8

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5.1

Figure 1. (a) Chemical structure of emitter materials used in the device fabrication and (b) energy level

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diagram of the hyperfluorescence based WOLEDs.

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a)

20

15

10

5

0 0.1

1

10

100

1000

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Quantum efficiency (%)

25

10000

2

Luminance (cd/m )

380

480

580

680

780

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Wavelength (nm)

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Intensity (arb. unit)

b)

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Figure 2. (a) External quantum efficiency–luminance curves and (b) electroluminescence spectrum of

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the TBPe hyperfluorescence device.

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a) 40

10000 TBRb 0.5% TBRb 0.7% TBRb 0.9%

1000 2

30

Luminance (cd/m )

2

100

25 20

10

15

1

10 0.1

5 0

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Current density (mA/cm )

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0.01 0

1

2

3

4

5

6

7

8

Voltage (V)

b) 12

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10 8

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6 TBRb 0.5% TBRb 0.7% TBRb 0.9%

2

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Quantum efficiency (%)

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1

10

100

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0 1000

10000

2

Luminance (cd/m )

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c)

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Intensity (arb. unit)

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TBRb 0.5% TBRb 0.7% TBRb 0.9% TBPe(PL) TBRb (PL)

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380

480

580

680

780

Wavelength (nm)

Figure 3. (a) Current density–voltage–luminance, (b) external quantum efficiencies and (c) electroluminescence curves of the hyperfluorescence-based WOLEDs with different TBRb doping concentrations.

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Table 1. Device performances of the hyperfluorescence-based WOLEDs. Doping concentration of TBRb (%)

External quantum efficiency (%)

Power efficiency

Current efficiency

(lm/W)

(cd/A)

0.5%

10.8a), 12.9b)

17.3a), 31.1b)

24.0a), 29.8b)

0.26, 0.32c) 0.25, 0.30d)

0.7%

9.7a), 11.2b)

17.8a), 31.6b)

26.1a), 31.1b)

0.35, 0.40c) 0.33, 0.39d)

0.9%

9.4a), 10.8b)

17.0 a), 30.6b)

25.4a), 30.5b)

0.35, 0.41c) 0.34, 0.39d)

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Color coordinate

a) efficiency at 1000 cd/m2, b) maximum efficiency, c) measured at 1000 cd/m2, d) measured at 4000

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cd/m2

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