Highly efficient and stable white organic light emitting diode with triply doped structure

Displays 27 (2006) 161–165 www.elsevier.com/locate/displa

Highly efficient and stable white organic light emitting diode with triply doped structure Xue-Yin Jiang b

a,*

, Zhi-Lin Zhang

a,b

, Wen-Qing Zhu

a,b

, Shao-Hong Xu

a

a Department of Materials Science, Shanghai University, Jiading, Shanghai 201800, China Key Laboratory of Advanced Display and System Application, Ministry of Education, Shanghai University, Shanghai 200072, China

Received 11 August 2005; received in revised form 6 May 2006; accepted 17 May 2006 Available online 9 June 2006

Abstract A triply doped white organic light emitting diode with red and blue dyes in the light emitting layer and a green dye in another layer is proposed. The device structure was CuPc(12 nm)/NPB(40 nm)/ADN:DCJTB(0.2%):TBPe(1%)(50 nm)/Alq:C545(0.5%)(12 nm)/ LiF(4 nm)/Al. Here copper phthalocyanine (CuPc) is a buffer layer, N,N 0 -di(naphthalene-1-y1)-N,N 0 -dipheyl-benzidine (NPB) is a hole transporting layer, 9,10-di-(2-naphthyl) anthracene (ADN) is blue emitting layer, tris (8-quinolinolato)aluminium complex (Alq) is an electron transporting layer, 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidy1-9-enyl)-4H- pyran (DCJTB), 2,5,8,11-tetra-butylperylene (TBPe), Coumarin6 and deveriative (C545) are red, blue and green dyes, respectively. This device shows a luminance of 21200 cd/ m2 at driving current of 400 mA/cm2 and 1026 cd/m2 at 20 mA/cm2. Its efficiency is 6 cd/A and 3.11 Lm/W. It also shows a higher operating stability: the half lifetime is 22,245 h at an initial luminance of 100 cd/m2, while the driving voltage increased only 0.3 V.  2006 Elsevier B.V. All rights reserved. Keywords: White OLED; Triple-doping; Efficiency stability

1. Introduction White organic light emitting devices (WOLED) have drawn particular attention due to their use in various fields: such as full color display, backlight for liquid crystal displays and other illumination applications. A variety of methods have been proposed to realize WOLED. Many reports were concentrated on conventional multi-layered devices based on small molecules. A WOLED with 0.5 Lm/W efficiency has been reported by Jordan [1], a 0.41 Lm/W WOLED with quantum well structure and a 1.11 Lm/W one were reported by Xie and Liu [2,3], a 2.88 Lm/W one reported by Zhang [4], a 1.93 Lm/W one by C.W. Ko [5], and a 5 Lm/W WOLED with dual doping structure by Y.S. Huang [6]. Recently, phosphorescent WOLEDs with 11 Lm/W and as high as 38 cd/A (18 Lm/W) were reported by B.W. D’Andrade

*

Corresponding author. Tel.: +862169982586; fax: +862139988216. E-mail address: [email protected] (X.-Y. Jiang).

0141-9382/$ - see front matter  2006 Elsevier B.V. All rights reserved. doi:10.1016/j.displa.2006.05.002

and Yeh-Jiun Tung, respectively [7,8]. A WOLED based on Si Chip with efficiency of 8 cd/A was reported by T.A. Ali [9]. M. Kashiwabara reported a full color display with WOLED + microcavity + color filter, in which the WOLED showed an efficiency of 7.9 cd/A [10]. To the best of our knowledge, few reports on the stability of the WOLED have been presented so far. We constructed some kinds of WOLEDs before: (1) The WOLED with TPBI as a hole blocking layer and NPB as blue emitter showed an efficiency of 1.39 Lm/W and CIE coordinates 0.31, 0.32 [11]. (2) The WOLED with blue emitter JBEM doped with red dye DCJT as emitting layer showed an efficiency of 2.88 Lm/W, CIE coordinates 0.32, 0.38, and a half lifetime of 2860 h at the initial luminance of 100 cd/m2 [4]. (3) The WOLED with JBEM as a blue emitting layer and Alq doped with DCJTB as red emitting layer showed an efficiency of 1.07 Lm/W, CIE coordinates 0.32, 0.31, the half lifetime of 1072 h at the initial luminance of 100 cd/m2 [12]. In order to improve the efficiency and stability of WOLEDs, in this paper, we proposed a new

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structure, using blue emitter doped with both red dye and blue dye as one layer and Alq doped with a green dye as a separated layer. The new device shows a greatly improved efficiency and stability. 2. Experimental details As mentioned above, we constructed some kinds of WOLEDs before. In order to improve further the efficiency and stability of WOLEDs, we report, in this paper, using the different host materials and triply doped structure to construct a new device, The structure of this device is follows: CuPc(12 nm)/NPB(40 nm)/ ADN:DCJTB(0.2%):TBPe(1%)(50 nm)/ Alq:C545(0.5%)(12 nm)/LiF(4 nm)/Al (Cell-T). Here copper phthalocyanine (CuPc) is a buffer layer, NPB is a hole transporting layer (HTL), 9,10-di-(2-naphthyl) anthracene (ADN) is the blue emitter, tris(8-quinolinolato)aluminium complex (Alq) is the electron transporting layer (ETL). 4-(Dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidy1-9-enyl)-4H- pyran (DCJTB), 2,5,8,11-tetra-butylperylene (TBPe), Coumarin6 deveriative (C545) are red, blue and green dyes, respectively. For comparison, we also constructed two kinds of devices: single doped and dual doped devices. The structures were CuPc(12 nm)/NPB(40 nm)/ADN:DCJTB(50 nm)/ Alq(12 nm)/LiF(4 nm)/Al (Cell-S). CuPc(12 nm)/NPB(40 nm)/ ADN:DCJTB:TBPe(50 nm)/Alq(12 nm)/LiF(4 nm)/Al (Cell-D).

R

Fig. 1 shows the molecular structures of the used organic materials and their energy diagram. Some energy values of the materials are quoted from other paper or our work. The Alq and NPB used were synthesized and purified by vacuum train sublimation in our laboratory. ADN (>99%), TBPE (>99%), DCJTB (>99%) and C545 (>99%) were obtained from E-RAY Optoelectronics Technology CO. The devices were fabricated as follows: Indium Tin Oxide (ITO) coated glass plates (with a sheet resistance of 20 X/square) were cleaned by sonication, then in a detergent solution and deionized water, followed by air plasma treatment. The organic layers and the cathode LiF/Al were sequentially deposited by conventional vacuum vapor deposition in the same chamber without breaking the vacuum. The composite film was prepared from simultaneous co-deposition of host and dopants from two separate sources. The pressure of the chamber was 1 · 106 Torr. The thickness of the organic layers was measured by using quartz-crystal monitors. Typically, the deposition rates were 4 A/s for organic layers and 25 A/s for Al metal cathode. The active area of the OLED was 25 mm2. The devices were encapsulated by using glass covers and the measurements were carried out at room temperature. The electroluminescent (EL) spectrum and the CIE color coordinates were measured by Pro-650 Spectra Scan, the current–voltage–luminance measurements were performed using a Keithley 2400 Source Meter and a Minolta LS110 luminance meter. The lifetime measurements were performed at room temperature at a constant current of 12 mA/cm2. 3. Results and discussion Fig. 2 shows the dependences of current density and luminance on voltage.

NC CN S N

o N

ADN

DCJTB

TBPe

o

o

C545

Fig. 1. Molecular structures of organic materials used and the configurations of the devices (The energy level value of Alq/LiF/Al is quoted from Ref. [13], the energy level values of ADN, TBPe and DCJTB are quoted from Ref. [4,14].).

500

4

10

400

3

10 300

2

6

200

8 10 12 14

10

Voltage(V)

Cell-S Cell-D Cell-T

100 0 5

6

7

8

9

10 11 12 13 14 15

Voltage(V) Fig. 2. Current density versus voltage and luminance versus voltage curves for Cell-S, Cell-D and Cell-T.

It can be seen from Fig. 2 that in comparison with Cell-S, I–V curves of Cell-D and Cell-T shift to the higher voltage, indicating no enhancement of carrier injections due to introducing the dopants TBPe in Cell-D and Cell-T. However, Cell-T and Cell-D shows higher luminance than Cell-S, as shown in the inset of Fig. 2. This implied that the Cell-T and Cell-D had higher efficiency. Fig. 3 gives the dependences of efficiency on the current density of three devices. It can be seen that Cell-T shows the highest efficiency and a flat dependence of efficiency on the current density, e.g., a weak current induced fluorescent quenching. It reached a current efficiency of 6 cd/A, and a power efficiency of 3.11 Lm/W, which was twice that of Cell-S and 1.2 times than that of Cell-D. It is well known that the efficiency U can be expressed as U = gex gLm gout, here gex is the fraction of injected carriers that form excitons, gLm is the photoluminescence efficiency and gout is the fraction of photons that emit out from the device. It can be known from the above analysis that the increase of efficiency of Cell-T was not resulted from the increase of gex and gout. Therefore, gLm should be the key factor for the enhancement of the efficiency of Cell-T. Since the Cell-T and Cell-D both included DCJTB and co-dopant TBPe while Cell-S only contained DCJTB, a cascade energy transfer model is proposed to explain the increase

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of efficiency. As shown in Fig. 1, the energy levels of TBPe located between that of ADN and DCJTB, its absorption spectrum overlays with the emission spectrum of ADN as well as its emission spectrum overlays with the absorption of DCJTB, the cascade energy transfer from ADN though TBPE to DCJTB can occur. The cascade energy transfer in co-doping system was proved to have higher energy transfer efficiency and reported in other papers [15–17]. We attributed the enhancement of efficiency in Cell-T and Cell-D to the cascade energy transfer due to co-dopant TBPe. The reason why Cell-T is superior to Cell-D in efficiency is attributed to the higher fluorescent efficiency of C545. Injected holes from the anode go into the ADN layer and recombine with injected electrons from cathode, produce light emission, and extra holes can further enter Alq layer and recombine with injected electrons and emit green light. In general, the green component was not obvious due to the low fluorescent efficiency of Alq. When Alq doped with C545, the green component can be greatly enhanced due to the rather high fluorescent efficiency of C545, resulting in the enhancement of green component as well as the efficiency of the whole device. Fig. 4 shows the EL spectra of the three devices at 4 mA/cm2 (a) and the EL spectra of Cell-T at 4 mA/cm2 and 400 mA/cm2 (b). It can be seen that for Cell-T there is an obvious increase of the green composition indicating the emission zone extended to the Alq layer and the C545 with high fluorescent efficiency contributed to the emission. a

Cell-S (0.324,0.381) Cell-D (0.336,0.405) Cell-T (0.326,0.420)

1.0

Intensity(a.u.)

2

Luminance(cd/m )

2

Current Density(mA/cm )

X.-Y. Jiang et al. / Displays 27 (2006) 161–165

0.5

0.0 300

400

500

600

700

800

8 7 6 5 4 3 2 1 0

Cell-S

b

Cell-D Cell-T

2

4mA/cm 0.326,0.420

1.0 Intensity (A.U.)

Efficiency(cd/A)

Wavelength(nm)

2

400mA/cm 0.322,0.416

Cell-T 0.5

0.0 300

0

100 200 300 400 Current density(mA/cm2)

Fig. 3. Efficiency versus current density curves for Cell-S, Cell-D and Cell-T.

400

500

600

700

800

Wavelength(nm) Fig. 4. EL spectra of Cell-S, Cell-D and Cell-T at the driving current density of 4 mA/cm2 (a) and the EL spectra of Cell-T at the driving current density of 4 mA/cm2 and 400 mA/cm2 (b).

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It also can be seen from Fig. 4(b) that, for Cell-T, there is no obvious change of spectra with the increase of driving current from 4 mA/cm2 to 400 mA/cm2, although the CIE values change from (0.326, 0.420) to (0.321, 0.416). The spectra data of the three devices are listed in Table 1. Fig. 5 shows the degradation properties of Cell-T under driving current of 12 mA/cm2. It can be seen that the CellT shows superior stability. The measured half lifetime was 3862 h at an initial luminance of 575 cd/m2. Renormalizing to a starting luminance of 100 cd/m2, the half lifetime (t1/2) was estimated to be 22,245 h, while the raise of voltage was only 0.74 V during ageing period. In order to investigate the origin of the stability of this device, let us compare the stability of Cell-T with Cell-D and Cell-S. The lifetimes of Cell-D and Cell-S were measured to be 17,000 h and 15,000 h, respectively. There was no great difference for lifetime values of the three devices. But when we compare the degradations of the Cell-T and a blue OLED with the same structure only removing 0.2% red dye DCJTB, a great difference in the life times was observed. The blue device only showed a half lifetime of 100 h at an initial luminance of 354 cd/m2. Renormalizing to a starting luminance of 100 cd/m2, the half lifetime was 354 h, simultaneously, the raise of driving voltage was as high as 4 V. It is obvious that the improvement of stability of Cell-T (or Cell-D and Cell-S) did not result from TBPe or C545 but from the red dye DCJTB. Two possibilities are considered: one is that the hole barrier at the interface NPB/ADN decreased due

Table 1 gmax is the maximum efficiency, L20 and L400 are the luminance at 20 mA/cm2 and 400 mA/cm2, respectively Device gmax gmax L20 L400 CIE4 (cd/A) (Lm/W) (cd/m2) (cd/m2) (x, y) Cell-S 2.4 Cell-D 5.0 Cell-T 6.0

1.6 2.68 3.11

440 970 1026

8600 13,000 21,200

CIE400 (x, y)

0.324, 0.381 0.310, 0.368 0.336, 0.405 0.332, 0.396 0.326, 0.420 0.321, 0.414

CIE4 and CIE400 are the chromaticity coordinates at 4 mA/cm2 and 400 mA/cm2, respectively.

voltage(v)

700

2

Intensity(cd/m )

600 500

9 8

0

1000 2000 3000 4000

Time(hour)

400 300

100

Cell-T 0

1000

4. Conclusions A triply doped white OLED (Cell-T) shows a better performance than the cells with single dopant (Cell-S) and double dopants (Cell-D). A current efficiency of 6 cd/A and 3.11 Lm/W was obtained, which was twice than that of Cell-S and 1.2 times than that of Cell-D. The enhanced efficiency is attributed to a cascade energy transfer from host through co-dopant TBPe to DCJTB as well as the high fluorescent efficiency of C545. Cell-T also showed a higher operating stability. A half lifetime of 22,245 h at an initial luminance of 100 cd/m2 with a small raise of voltage during ageing was reached. Comparing the stability of Cell-T with that of Cell-D and Cell-S as well as a based blue OLED, the high stability of Cell-T is attributed to red dye DCJTB. The possible reasons are either that DCJTB with low ionization potential lowers the barrier for hole injection in the interface of NPB/ADN, thus decreases the production of Joule heat, or that DCJTB improved morphological stability of blue emitter ADN, i.e., it reduces the crystallization of molecules in ADN layer. Acknowledgements The authors would like to acknowledge the financial supports of the National Natural Science Foundation of China (90201034, 60477014 60577041), 973 project (2002CB13400), Shanghai Science and Technology Committee (012261055).

7 6

200

to the introduction of DCJTB with low HOMO level, resulting in the decrease of production of Joule heat, consequently improving the durability of the devices. C. Adachi [18] pointed out that the ionization potential of the hole transporting material is the key factor that affects the durability of an OLED. A high barrier for injection of hole or electron at the interface may produce large Joule heat, which causes a local aggregation of the molecules at the interface. Another possibility is that DCJTB improves morphological stability of the blue emitter ADN, i.e., it reduces the crystallization of molecules in ADN layer, the mechanism about the improvement of stability by dopant DCJTB will be studied further.

2000

3000

4000

Time(hour) Fig. 5. The degradation of luminance as well as the driving voltage change of Cell-T at an operation of constant current of 12 mA/cm2.

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