Iwamoto M., Kwon Y.-S., Lee T. (Eds.) Nanoscale Interface for Organic Electronics

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Metal Complexes for Nanoscale OLED Devices 79

The complexes of 8-hydroxyquinone with aluminum (Alq3) was first

applied for OLED in 1987 by Tang and Van Slyke but until 1993

the complexes with zinc metal (Znq2) was first reported to apply for

OLED by Hamada,

the studies of zinc complexes as active materials

for OLEDs have focused on improving electrons mobility or producing

a blue shift. Besides the Al and Zn, other metal complexes, the

8-hydroxyquinone were also investigated with tin or iridium complexes.

Benzo-compounds: Including 2-2(hydroxylphenyl)benzoxazole,

2-2(hydroxylphenyl)benzothiazole, 2-2(hydroxylphenyl)benzoimidazole

and their derivatives.

The general structures of the benzo-compounds (3) are shown in

Fig. 4, where X can be oxygen, sulfur, or hydronitrogen. Like the

8-hydroxyquinone, these ligands also can coordinate to metals, especially

Zn. The zinc complexes with these ligands usually have the EL and PL

of blue or green emission with the wavelength around 400-520 nm. For

examples, zinc 2-2(hydroxyphenyl) benzoxazole shows efficient blue

material, we have indicated that it is also good for the performance of

device if the material is applied as the hole blocking layer. While bluish-

green emitting layer of bis 2-2[(hydroxyphenyl)benzothiazolate] zinc

(Zn(BTZ)

), doped with other materials to make white light emitting

diode device.

2-Phenyl pyridine: This ligand was synthesized with iridium (III),

Ir(ppy)

, at the first time in 1985 by King.

It opened the new method

to synthesized and new materials which later successfully applied

to OLED as the emitting material. After the success of these researches,

a serial of reports based on the structure of pyridine and the (C^N)

ligands were investigated to support the good materials for the

OLED. Most of the iridium complexes in this field are based on

2-phenylpyridine and its derivative. In addition, in the last decade

2-phenylpyrine was known as to have several potent azapeptide Human

Immunodeficiency Virus (HIV) protease inhibitors, a moiety showing

anti-HIV activity such as BMS-232632a.

Strategies for the synthesis of

the 2-phenylpyridine nucleus has varied from Chichibabin condensation

reactions to Heck substitution reaction, and a new route for the synthesis

of 2-phenylpyridines over molecular sieve catalysts was report by

Radha Rani.

T. D. Hoanh and B.-J. Lee 80

3. Light Emitting Ir(III), Zn(II), and Sn(IV) Materials

3.1. Blue Emitting Materials

Blue emitting materials are the one of the main color for fabricating the

white OLED device, they are also one of the most challenges of OLED’s

drawback because of the lifetime limitation. Therefore, blue emitting

materials have been received much attention from researchers (Fig. 5). In

the previous section, Ir(ppy)

was introduced as the emitting materials for

OLED, then in 2001, Lamansky

et al.,

modified the structure to

Ir(ppy)

(acac) (acac = acetylacetonate) and Ir(dfppy)

(acac), compounds

(

4) and (5) where R

= R

= CH

, and they showed the PL at green

and blue emission at 519 and 543 nm. Later Liu et al.,

prepared the

compounds (

4’) and (5’) (where R

= -t-Bu, R

= 9-ethyl-9H- carbazole).

The PL of

4 = 4’ and 5 = 5’ but in the device the maximum luminence

efficiency of compound

4’ (4.54 lm/W) and 5’ (0.51 lm/W) are higher

than that of

4 (0.53 lm/W) and 5 (0.06 lm/W). Compounds (6) and

(

7) were synthesized by Ragni et al.,

with the presence of electron-

withdrawing fluorine substituents in the chelating ligands, which should

decrease the HOMO energy and, as the consequence, increase the

HOMO-LUMO energy gap. These materials showed the blue emitting

with emission quantum yields in solution as high as 53%. The other

serials of Iridium complexes (

8), (9), (10), (11), based on diazine were

prepared by Ge and Guo,

showed EL around blue-green region

and the best performance in the device of compound (10) with the

configuration of the device ITO/NPB (40 mn)/CBP: (

10) (30 nm)/TPBi

(15 nm)/Alq

(50 nm)/Mg:Ag (150 nm, 10:1)/Ag (10 nm). With turn-on

voltage of 3.2 V has its highest brightness (L

max, cd/m

2) of 120,450 cd/m

at 18 V, a maximum external quantum efficiency (

ext.max

) of 13.9%, a

highest power efficiency (

max, lm/W

) of 48.6 lm/W and a highest current

efficiency (η

max, cd/A

) of 58.2 cd/A.

Since zinc complexes are promised as blue emitting materials,

we also investigated the zinc complex with other ligands (

12), Zn(II) 2-

(2-hydroxyphenyl)benzoxazole,

which showed EL of blue emission at

447 nm when (

12) as the emitting layer, turn-on voltage at 4 V and the

max

of 2800 cd/m

Metal Complexes for Nanoscale OLED Devices 81

In this research also indicated that when (

12) was applied as the hole

blocking layer in the device structure of ITO/NPB (40 nm)/Alq

(60 nm)/

12 (5 nm)/ LiF/Al, the L

max

was improved immediately of 15,000 cd/m

when the voltage at 10 V. The (

12) complex was suggested to use as the

Fig. 5. The chemical structures of blue emitting materials based on the Zn- and Ir-containing

complexes.

T. D. Hoanh and B.-J. Lee 82

hole blocking layer due to its wide energy band gap, as well as a deep

HOMO (6.5 eV) and LUMO (2.8 eV) closely with that on ETL.

Recently, Su

et al.,

synthesized the suitable ligand to coordinate

with zinc metal (13), where the R

and R

are changed for investigation

, R

= H,

Bu, Br). These complexes showed the PL from 476 to 578

nm. An other new type of zinc complex was done by Yu to produce the

blue emitting complex with the wavelength (

max

) of 455 nm (14),

but

the L

max

= 5.26 cd/m

is not excellent.

A serial of important blue zinc complexes (

15), (16), (17), (18), (19),

(20), (21), were synthesized by Tokito,

the EL peaks from 447 to 508

nm. The best performance in the device of complexes (

19) is the

maximum luminance at 11,450 cd/m

when the voltage at 15 V, the

wavelength

max

= 485 nm and it exhibited photometric efficiency of

1.3 cd/A and a quantum efficiency of 1.2%.

3.2. Green Emitting Materials

If the wavelength λ

max

in the EL or PL arranges from 500 to 550 nm, it

shows the green emission. The green emission can be obtained from

organic, inorganic, and organometallic compounds. In this section we

present the green emitting materials which are based on the complexes of

Ir(III), Zn(II) and Sn(IV) (Fig. 6).

The well-known green complex, Ir(ppy)

, is the topic for researchers

who want to continue break out and renew this field by modifying

the chemical structure of ppy (2-phenylpyridine). Xu

et al.

made

(

22) by attaching the methoxy and methoxyphenyl group to the

2-phenylpyridine.

The EL of (

22) is red shift to 531 nm compared with the data of

Ir(ppy)

of 512 nm. Maximum luminance of 35,000 cd/m

, maximum

quantum efficiency of 9.0% and luminance efficiency of 36 cd/A were

achieved, which are higher than that of Ir(ppy)

. By adding trimethylsilyl

group to replace the hydrogen in the structure of 2-phenylpyridine,

Jung

et al.

got the compound (23).

Metal Complexes for Nanoscale OLED Devices 83

Fig. 6. The chemical structures of blue emitting materials based on the Zn- and Ir-containing

complexes.

22 23 24 25

26 27 28

29 30 31

T. D. Hoanh and B.-J. Lee 84

To compare with Ir(ppy)

the EL have changed not much but

significant improvement of the power efficiency (17.3%), which is

nearly 200% higher than that of Ir(ppy)

In the blue emitting material section, we have already mentioned

the benzoxazole complex with zinc (

12). Because benzoxazole and

the derivatives have the high quantum yield,

high non-linear

optical effectiveness, photostable properties.

Here several kinds of

benzoxazole derivatives were synthesized then prepared a series of

cyclometalated iridium (III) complexes (

24)-(32) by Chen.

Most

of them show the PL of green emission at 530 nm but the (

28) and (31)

show the yellow emission at 568 and 579 nm.

In the year 2009, Li

et al.

has synthesized the high-efficient

phosphorescent iridium (III) complexes with benzimidazole ligand,

compounds (

33) and (34). Both of them showed the promise

performance in the devices. Devices using (

33) and (34) as dopants were

fabricated, they emitted strong cyan light with an emission maximum at

498 and 492 nm, low turn-on voltage at 3.4 and 3.1 V, high maximum

brightness of 47,100 cd/m

and 41,150 cd/m

, current efficiency at 21.7

and 35.3 cd/A.

Zinc complexes are not only known as the blue emitting materials

but also the excellent green emitting ones (Fig. 7). The green emitting zinc

complexes (

35), (36), (37), and (38)

29-32

are the examples of that. Where

the (

35) complex was done by Sharma et al., (36) by Wang et al., (37) by

Chen

et al., (38) by Park et al. The complex (35) showed the green EL in

the OLED device at the wavelength of 545 nm, the turn-on voltage at

7 V (it is higher than normal) and the maximum brightness of 363 cd/m

at 15 V. The same (

35), the complex (36) also based on the derivative of

8-hydroxyquinone. The single layer OLED device was fabricated with

(

36), which exhibited the green-yellow emission with the peak at 557 nm

and the maximum luminance of 450 cd/m

at 20 V. In addition, (36) is

very heat stable compound that the thermal decomposition occurs at the

temperature of 435°C. The abundant complexes of (

37) are possible

when using the different groups of R in the structure. Most of them

showed the blue-green PL, and the best performance in the device

where R = carbazole, the maximum brightness at 2249 cd/m

, and EL at

552 nm.

Metal Complexes for Nanoscale OLED Devices 85

The complex (

38) was successfully synthesized in our laboratory and

applied to OLED. In the both PL and EL properties, it exhibited the green

emission at 530-540 nm, high thermal stability up to 400°C. When it was

used as the emitting layer the maximum brightness was 9500 cd/m

9.5 V, but when it played as the role of hole blocking layer the maximum

of brightness is twice higher than that of emitting layer at 18,700 cd/m

when applied the voltage of 10.5 V. And the same as in (

12), it also

suggested to be used as the hole blocking layer in the OLED devices.

Tin (IV) organometallic compounds have a lot of applications as

biocides like fungicides or antifouling agents,

but in the field of OLED

it seems to be new and there are not so many literatures about this topic.

Tin (IV) complexes of (

39) and (40)

were the first time synthesized

and applied for OLED in our laboratory. The EL of (

40) is blue emission,

and green emission in case of (

39). The (40) also showed the good

performance when it was applied in the hole blocking layer with the

maximum luminance of 17,600 cd/m

and maximum EL efficiency reached

1.4 lm/W at 34 mA/cm

, while the (39) showed L

max

= 720 cd/m

at 7.5 V.

From these results we could observe the good properties of benzoxazole

ligand when it can coordinate with various metals to form metal complexes

Fig. 7. The chemical structures of green emitting materials based on Zn- and Sn-

containing

complexes.

T. D. Hoanh and B.-J. Lee 86

having the heat stability, high luminance efficiency, high brightness, and

also good material for hole blocking layer in the multilayer structure devices.

3.3. Red Emitting Materials

Iridium complexes are most suitable for the red emitting materials,

although these materials are abundance for the application of OLED,

including organic and organometallic compounds.

In the visible light region, if the materials are the red emitting

materials, then their wavelength must be belong to the range of 600 to

720 nm. The red emitting complex (

41)

was synthesized by Shen

et al., the PL in the THF solution showed the red emission with 623 nm,

then it was used as an efficient dopant to fabricate the nearly saturated

red polymer light-emitting diode which gave high external quantum

efficiency of 8.5% and the maximum luminance is 2858 cd/m

at 16.5 V.

The series of compounds (42)

were obtained when changing the

different X

and X

(where X

and X

= H, F, CF

) by Park. The PL data

of them showed in the peaks around 600 nm. Gao

et al., synthesized

(

43)

which shows the PL of pure red light at 675 nm, when fabricated

the diode with blending 3% in CBP, the EL spectrum shows the

wavelength at 677 nm, L

max

= 2800 cd/m

at 22 V. The (44), (45), (46),

and (47) were synthesized by different authors, Chuang,

Kim,

Huang,

but they can be classified as the same group, because changing the

chemical structure a little, all of them show the emission in the red region.

It is rarely to see the zinc complex shows the red emission but in case

of (

48)

when Zn(II) in the center of porphyrins it exhibited the PL in

red emission of 720 nm, because of the PL of the ligand also in the red

region. This work was investigated by Paul-Roth

To modify the structure of 2-phenylpyridine (ppy), Yao et al.

designed and synthesized the crown ligand instead of ppy, (

49), (50),

(

51), and (52) when acac was used as the second ligand after the

synthesis of the chloro bridge dimmer. In summary of EL and device

characteristics the (

52) has the EL of red emission at 609 nm when the

rest have the yellow emission around 550 nm, and the (49) showed the

best LE

max

of 36.4 cd/A with the L

max

= 30,956 cd/m

. There are a lot of

other red emitting materials but in the restricted area of this review we

just introduced some of them which are shown in Fig. 8.

Metal Complexes for Nanoscale OLED Devices 87

Red emitting materials are very important when fabricating white

OLED, the efficiency and high luminance of the diode depend on the

blending, combination, and the nature of the monochromatic of red, blue

and green color. In the next section we will discuss the white emitting

Fig. 8. The chemical structures of red emitting materials based on the Ir- and Zn-

containing

complexes.

T. D. Hoanh and B.-J. Lee 88

diodes which based on main color emitting materials of Ir(III), Zn(II) and

Sn(IV) complexes.

3.4. White Emitting Diodes by Ir(III), Zn(II) and

Sn(IV) Complexes

White organic light-emitting diodes have drawn much attention due to

their potential applications in full color displays with the help of color

filters, in backlight for LCDs, and eventually in solid-state lighting

sources.

To obtain white emission from an OLED, two complementary

colors (ex., blue and orange) or three primary colors (red, green and

blue) from different emitting molecules are combined.

This can be done

either using a multilayer structure with two or more emitting layers or

doping an active host material with several fluorescent dyes (Fig. 9).

Starting with compound (

53) and (54),

which are the two main

emitting color materials to make the this white OLED, where (

53) is blue

emission, meanwhile (

54) is red one. This work was carried out by

Lim

et al. A device with structure of ITO/α-NPD (50 nm)/53 (4 nm)/

53:54 (10 nm, 0.8%)/Alq

(30 nm)/LiF (0.5 nm)/ Al shows an external

QE of about 0.5%, a luminous efficiency of 0.27 lm/W. The luminance

of about 4000 cd/m

was achieved at 17 V and 580 mA/cm

We have combined the compound (

55-blue) and (56-red) to obtain

the white OLED.

But we found that the white emission was only

achieved when the thickness of (

55) layer is 30 nm and that of (55):(56)

is 10 nm. That means the control of device structure is important

to get the white emission. The CIE coordinates are (0.304, 0.332) at

10.5 V. Before this work, we also characterized the white OLED

using Zn(phen)q (57)

as a yellowish green emitting layer and

10-phenanthroline (BCP) as the hole blocking layer, the (

58) NPB was

used as the HTL and the blue emitting layer. The result indicated that

when adjust the thickness of BCP layer from 0 to 10 nm, the white

emission was achieved at 5 nm of thickness. And the blue emission

increases in proportion to applied voltage. The CIE coordinates are (0.26,

0.33) at an applied voltage of 10 V.