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Journal of Luminescence 122123 (2007) 639641

Study of the characteristics of new red dopants in OLED

Jing Xiaoa, Yishan Yaob, Zhenbo Denga,, Xuesong Wangb, Chun-jun Lianga

aInstitute of Optoelectronics Technology, Beijing Jiaotong University, Beijing 100044, ChinabTechnical Institute of Physics and Chemistry of CAS, Beijing 100101, China

Available online 20 March 2006

Abstract

New red dopants TDCJTB and TDIN have been synthesized for using as red fluorescent dye molecules in organic light-emitting diodes

(OLEDs). Red-light-emitting devices have been fabricated with a structure of ITO/PVK: dopant/2, 9-dimethyl-4, 7 diphenyl-1, 10-phenanthroline (BCP)/Alq3/LiF/Al. The Commission International dEclairage (CIE) coordinates and PL peaks of the devices are [0.61,

0.39], [0.62, 0.38] and 579, 597 nm, respectively. The maximum EL brightness of TDCJTB and TDIN reached 1311 cd/m2 (18V) and

2497 cd/m2 (16 V), respectively. Compared with traditional DCM ([0.58, 0.41], 1032 cd/m2 (18V)) the two devices have better CIE

coordinates and brightness. When TBQ is doped in these materials, the EL efficiency can both be improved at different level. Present

research work shows that when TBQs concentration is 30%, the maximum EL efficiency TDCJTB and TDIN is 2.57 and 2.54 cd/A at

40 mA/cm2, respectively.

r 2006 Elsevier B.V. All rights reserved.

Keywords: Oled; Red color; TDCJTB; TDIN; Efficient

1. Introduction

Organic light-emitting devices (OLEDs) have attracted

much attention owing to their advantages of low-power

consumption, high brightness, high contrast and potential

applications to full color flat panel displays [1]. But the red

color OLED due to its low efficiency, poor color purity,

remains to be the weakest part in realizing the full potential

of full color OLED. One of the key enablers in the

development of OLED technology can be attributed to the

discovery of the guesthost doped emitter system [2]. By

dispersing 5% of rubrene as a red emitting assist dopant

with 2% DCJTB in Alq3, Hamada and coworkers at Sanyo

[3] were able to achieve a luminance efficiency of 2.1 cd/A

with CIEx,y [0.64, 0.35]. Subsequently, the Sanyo/Kodakteam discovered [4] that by adding 6% of NPB as hole-

trapping dopant to the above emitting system, its efficiency

could be further improved to 2.8 cd/A at 20 mA/cm2 and a

red chromaticity coordinate of CIEx,y [0.65, 0.34] was

also obtained.

In this work, we synthesized two new red fluorescent

dopants and applied it in the fabrication of OLEDs. ThePL and EL properties of devices utilizing red dopants

dispersed PVK as light-emitting layer were studied. At the

same time, we doped the dopants in co-host emitter (CHE)

[5] systems of TBQ/PVK. It showed that by adjusting the

ratio of these two host emitters, the EL efficiency,

luminance as well as color can be dramatically improved.

2. Experimental

New red dopants TDCJTB and TDIN have been

synthesized for using as red fluorescent dye molecules inOLED. They were designed and synthesized via piperidine

-catalyzed condensation of aldehyde with 4-(dicyanomethy-

lene)-2-(t-butyl)-6-methyl-4H-pyran or 4-(10, 30-indandione)

-2-(t-butyl)-6-methyl-4H-pyran in good yields. The molecule

structures of these are shown in Fig. 1.

We used PVK as host material in order to get the film-

forming possibility and conductivity of their mixture. The

emitting layer was made by spin coating, which can

avoid the thermal decomposition caused by the thermal

evaporation.

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www.elsevier.com/locate/jlumin

0022-2313/$ - see front matterr 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.jlumin.2006.01.247

Corresponding author. Tel.: +86 10 51688675; fax: +86 10 51683933.

E-mail address: zbdeng@center.njtu.edu.cn (Z. Deng).

http://www.elsevier.com/locate/jluminhttp://localhost/var/www/apps/conversion/tmp/scratch_4/dx.doi.org/10.1016/j.jlumin.2006.01.247mailto:zbdeng@center.njtu.edu.cnmailto:zbdeng@center.njtu.edu.cnhttp://localhost/var/www/apps/conversion/tmp/scratch_4/dx.doi.org/10.1016/j.jlumin.2006.01.247http://www.elsevier.com/locate/jlumin

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In this work two kinds of devices were fabricated: (1)

Glass/ITO/PVK: dopant/BCP/Alq3/LiF/Al; (2) ITO/CHE

(70 nm)/BCP (20 nm)/Alq3 (15 nm)/LiF (0.3nm)/Al

(150 nm). The CHE consists of a mixture of TBQ/PVK

doped with a series of red fluorescent dyes (10%). The

light-emitting film of CHE was prepared by spin coating

their chloroform solution onto ITO glass substrate (with a

sheet resistance of 20O/&). And then hole-blocking

material (BCP), electron-transporting material(Alq3), LiF,

Al were deposited onto the emitting layer by a conven-

tional thermal evaporation method at a chamber pressure

of about 3 103 Pa. Prior to organic deposition, the ITO-

coated glass plate was thoroughly cleaned by scrubbing,

sonication, vapor degreasing and oxygen plasma treat-

ment. The deposition rates were maintained to be 0.05, 0.05

and 0.02 nm s1 for BCP, Alq3 and LiF, respectively. The

thickness of each layer was monitored by a quartz

oscillating thickness monitor (IL-400). LiF was deposited

onto an electron transport layer (ETL) as an electron

injection material [6]. The PL and EL spectra were

measured by the Fluolog-3 fluorescent spectrometer, and

the brightness was measured by PR-650. All the measure-

ments were carried out in ambient atmosphere at room

temperature.

3. Results and discussion

The results are shown in Fig. 2. The PL peaks of

TDCJTB, TDIN and TBQ were 579, 597 and 459 nm,

respectively. The properties of monolayer device(ITO/

PVK: dopant /LiF/Al) with the swing film rate of

1000 r/min(70 nm) and the weight ratio of 1:10 (dopant:

PVK) were best. Bright red emission could be obtained

from this device and the blue light of PVK could not be

seen. This indicated that emission of the red dopants

originate from the excitation of PVK. The EL peaks of

TDCJTB, TDIN and DCM are 603, 607 and 598 nm,

respectively.

Bright red emission could be obtained from the

optimized double-layer device ((1) ITO/PVK: dopant /

BCP/Alq3/LiF/Al). The devices with 20 nm-thick blocking

layer (BCP) showed only the light emission from the red

ARTICLE IN PRESS

N

OO

NC

O

CN

CN

NC CN CN

TDCJTB

N

OO

O O OO

O O

O

TDIN

N

TBQ

Fig. 1. Molecular structures of TDCJTB, TDIN, TBQ.

20 0 2 50 30 0 3 50 40 0 4 50 50 0 5 50 6 00 6 50 7 00 7 50

0.0

0.2

0.4

0.6

0.8

1.0

PLIntensity(a.u.)

Wavelength (nm)

TDCJTBTDINTBQ

(a)

300 350 400 450 500 550 600 650 700 750

0.0

0.2

0.4

0.6

0.8

1.0

Intensity(a.u.)

Wavelength (nm)

TDCJTBTDINDCM

(b)

Fig. 2. PL spectra of TDCJTB, TDIN and TBQ and EL spectra of TDCJTB, TDIN and DCM.

J. Xiao et al. / Journal of Luminescence 122123 (2007) 639641640

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dopants. This indicates that the BCP layer with 20 nm

thickness could completely block the holes flowing to the

Alq3 layer, and the Alq3 layer in this device only functions

as electron-transporting layer. The Commission Interna-

tional dEclairage (CIE) coordinates was [0.61, 0.39], [0.62,

0.38]. Compared with traditional DCM ([0.58, 0.41]) the

two devices have better CIE coordinates.

EL spectra at different voltages of the double-layer

device for TDCJTB and TDIN showed that with increasing

driven voltage the emission that came from the red dopants

were intensified, yet the locations of the peaks were not

modified.

Fig. 3 (a) showed the characteristics of brightness versus

voltage of device (2). From this figure we could see thehighest EL brightness of TDCJTB and TDIN reached

1311 cd/m2 at the applied voltage of 18 V and 2497 cd/m2 at

the applied voltage of 16 V, respectively. Compared with

traditional DCM (1032 cd/m2 at applied voltage of 18 V)

the two devices have higher brightness.

In order to enhance the properties of the device, we add

TBQ as co-host to the emitting layer. Experiments showed

that when TBQs concentration is 30%, the properties of

the device were best. The highest EL brightness of

TDCJTB and TDIN reached 2080 cd/m2 at the voltage of

18 V and 4676 cd/m2 at the applied voltage of 17V,

respectively. The maximum EL efficiency TDCJTB and

TDIN is 2.57 and 2.54 cd/A at 40 mA/cm2, respectively

(Fig. 3(b)). From EL spectra of TDCJTB and TDIN with

30% TBQ in PVK of 10% red dopants doped CHE device,

we could not see the emitting light from TBQ, but the

intensity of red light was increased. So TBQ might act as a

bridge in the process of energy transfer. This might indicate

that some emission of red dopants originate from the

excitation of PVK and some emission of red dopants

originated from the excitation of TBQ.

4. Conclusions

In summary, new red dopants TDCJTB and TDIN have

been synthesized for using as red fluorescent dye molecule

in OLEDs. By using BCP as holes block layer the OLEDs

has been fabricated. The CIE coordinates and PL peaks of

the devices are [0.61, 0.39], [0.62, 0.38] and 579, 597 nm,

respectively. The highest EL brightness of TDCJTB and

TDIN reached 1311 cd/m2 (18 V) and 2497 cd/m2 (16 V),

respectively. Compared with traditional DCM ([0.58, 0.41],

1032 cd/m2 (18 V)) the two devices have better CIE

coordinates and brightness. When TBQ is doped in these

materials the EL efficiency can both be improved at

different level. Present research work shows that whenTBQs concentration is 30%, the maximum EL efficiency

TDCJTB and TDIN is 2.57 and 2.54 cd/A at 40 mA/cm2,

respectively.

Acknowledgements

This work was supported by National Natural Science

Foundation of China (under contract no.90201004), and

Beijing Science & Technology Foundation ( under contract

no. H030430020410).

References

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[2] C.W. Tang, S.A. VanSlyke, C.H. Chen, Appl. Phys. 65 (1989) 3610.

[3] Y. Hamada, H. Kanno, T. Tsujioka, H. Takahashi, T. Usuki, Appl.

Phys. Lett. 75 (1999) 682.

[4] T.K. Hatwar, G. Rajeswaran, J. Shi, Y. Hamada, H. Kano, H.

Takahashi, in: Proceeding of EL00, Hamamatsu, Japan, p. 31,

December 2000.

[5] T.H. Liu, C.Y. Iou, C.H. Chen, Curr. Appl. Phys. 5 (2005) 218221.

[6] L.S. Hung, C.W. Tang, M.G. Mason, Appl. Phys. Lett. 70 (1997) 152.

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0 2 4 6 8 10 12 14 16 18 20

0

500

1000

1500

2000

2500

3000

ELBrightne

ss(cd/m2)

Voltage (V)

TDCJTB

TDIN

DCM

(a)

4 6 8 10 12 14 16 18 20

0.5

1.0

1.5

2.0

2.5

ELEfficie

ncy(cd/A)

Voltage (V)

TDCJTB

TDIN

(b)

Fig. 3. Brightness (a) and EL efficiency (b) curves vs. applied voltage of red OLED doped.

J. Xiao et al. / Journal of Luminescence 122123 (2007) 639641 641