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Structure Optimization for High Efficiency WOLEDs 109

introduction of polymer binders resulted in good film formation

properties as well as better device performance than that of the PVK

conventionally used as a host material. When the 23% PS binder was

added, the device emits the white light of CIE

x,y

(0.33, 0.39) and has

an EQE and a current efficiency of 6.1% and 13 cd/A, respectively, at

100 cd/m

3.3. WOLEDs Based on Color Conversion Method

Generally, the emission color of WOLEDs is easily changed from white

to reddish-white as driving voltage is increased. The problem is very

serious in WOLEDs with blended red, green, and blue emitters, as well

as those with a white emissive single polymer. In order to overcome this

problem, the color conversion method was developed by GE (Fig. 11).

At that time, PLED was fabricated with a blue emitting PF derivative

from CDT, and then the color conversion materials — organic (perylene

orange, perylene red) and inorganic (Y(Gd)AG:Ce) fluorescent materials

— were sequentially deposited on the reverse side of a glass substrate.

Subsequently, OSRAM also fabricated a WOLED with a similar method,

in which a non-conjugated polymer host and Firpic are blended in the

emission layer and the blue emission color due to Firpic is converted by

nitridosilicate fluorescent material.

These color conversion methods are

also a core issue for the realization of highly efficient WOLEDs by

wet-process.

Fig. 11. Down conversion WOLED structure by GE.

J. H. Seo, J. H. Seo and Y. K. Kim 110

3.4. Wet-Coating Techniques for the Multilayer Formation

Polymers cannot be thermally evaporated in a vacuum chamber because

they generally crosslink or decompose upon heating. This limitation

imposes restrictions on the nature of the polymers and the sidegroups

attached to the polymer backbone since the polymer must be soluble.

For example, unsubstituted poly(p-phenylenevinylene (PPV) is insoluble,

thus, it is generally fabricated by coating a soluble precursor polymer

onto the desired anode substrate. The precursor polymer film is

then converted to PPV by annealing at a temperature 150 ≤ T ≤

250°C for up to ~ 24 h.

55,93,94

As this conversion process yields an

insoluble layer of PPV, additional layers may be deposited on it by

wet-process.

On the other hand, when soluble PPV derivatives such as 2,5-

dialkoxy PPVs are coated onto the substrate, only solvents which would

not redissolve the deposited film can be used to deposit additional

layers. For example, Gustafsson et al.

fabricated flexible PLEDs

by sequentially spin-coating an aqueous solution of water-soluble,

conducting transparent polyaniline onto a transparency, and a xylene

solution of poly(2-methoxy-5-(2'-ethyl)-hexoxy-1,4-phenylene vinylene)

(MEH-PPV).

In order to form the multilayer, S-OLED Co. from Korea developed

a self-assembled graded junction made by simple wet-process.

this case, it is necessary to prepare the predissolving organic charge-

transporting and light-emitting compounds in mixed organic solvents that

have different volatilities and solubilities for the compounds used. Upon

coating the solution on to a prepared substrate, the organic layers are

deposited sequentially according to the different evaporation speeds of

the solvents.

GE fabricated the multilayer OLEDs by solvent rinsing.

After the

PVK:Firpic film was formed, only Firpic existed on the top side of the

PVK:Firpic film. It was rinsed with p-xylene, which selectively removes

Firpic, as shown in Fig. 12. It has been reported that device efficiency

can be improved after the rinsing process.

Structure Optimization for High Efficiency WOLEDs 111

In wet-process, these multilayer formation techniques can be helpful

in improving the efficiency, lifetime, and stability of the device because

each functionalized layer can enhance the balance of hole and electron in

emission layer and suppress the exciton leakage out of the emission layer.

4. Conclusions and Outlook

Highlights from WOLED development over the past few years were

reviewed, and recent trends in dry- and wet-process and structure

optimization for high efficiency WOLEDs were reviewed.

Table 1 summarizes the device characteristics of some representative

results regarding the dry- and wet-process WOLEDs. It has been

established that WOLEDs fabricated by dry-process exhibit higher

device performance than those developed by wet-process. Because of the

simple process and low cost, WOLEDs will have to change from the

wet-process to the dry-process-type in future. WOLEDs were assessed as

a SSL source and backlight in liquid-crystal displays and full-color

OLEDs. WOLEDs will not be practical in most SSLs because they

are less efficient (~ 50 lm/W) than a fluorescent lamp (~ 70 lm/W). In

addition, WOLEDs currently have short lifetimes in comparison with

other displays or SSLs. Although WOLEDs have many weak points, this

technology will soon be practical in manufactured goods such as SSLs

and backlights in liquid-crystal displays and full-color OLEDs.

Fig. 12. Multilayer formation process through the solvent rinsing process.

J. H. Seo, J. H. Seo and Y. K. Kim 112

Table 1. Summarized results of the device characteristics from representative WOLEDs.

Process type

Power efficiency or

Current efficiency

EQE CIE

(x,y)

All fluorescent

WOLED

12.8 cd/A, 4.3 lm/W

at 20 mA/cm

- (0.31, 0.38)

All phosphorescent

WOLED

44 lm/W

at 1000 cd/m

25%

at 1000 cd/m

(0.34, 0.40)

Hybrid WOLED

28.75 lm/W

at 1000 cd/m

13.18%

at 1000 cd/m

(0.48, 0.43)

Excimer WOLED

37.8 cd/A,

12.6 lm/W

at 500 cd/m

15.9%

at 1000 cd/m

(0.46, 0.47)

Exciplex WOLED

12 cd/A, 2 lm/W

at 1000 cd/m

- (0.34, 0.34)

PIN WOLED

10 cd/A, 9.3 lm/W

at 1000 cd/m

- (0.32, 0.43)

Tandem (2 unit)

WOLED

19 lm/W

at 1000 cd/m

38%

at 1000 cd/m

(0.38, 0.44)

Sensitizer WOLED

20.2 lm/W

at 800 cd/m

13.1%

at 800 cd/m

(0.38, 0.42)

Microcavity

12.8 cd/A

at 300 cd/m

- (0.32, 0.40)

PF + PFD blending

Max. 0.26 cd/A - (0.33, 0.35)

PVK + Firpic +

Ir(mppy)

Ir(piq)

(acac) blending

Max. 9.5 lm/W,

Max. 24.3 cd/A

Max. 14.4% (0.34, 0.46)

CBP + PS + Firpic +

Ir(acac) blending

13 cd/A

at 100 cd/m

6.1%

at 100 cd/m

(0.33, 0.39)

Single-component RGB

white polymer

Max. 5.4 lm/W,

Max. 8.6 cd/A

- (0.33, 0.36)

Single-component RB

white polymer

Max. 8.51 lm/W,

Max. 12.8 cd/A

- (0.31, 0.36)

Single-component

white phosphorescent

polymer

Max. 7.21 lm/W,

Max. 8.2 cd/A

- (0.35, 0.38)

Structure Optimization for High Efficiency WOLEDs 113

Acknowledgments

This work was supported by Energy Resources Technology

Development program (2007-E-CM11-P-07), Strategy Technology

Development program (10030834) from Ministry of Knowledge

Economy (MKE) and the ERC program of the Korea Science and

Engineering Foundation (KOSEF) grant funded by the Korea Ministry of

Education, Science and Technology (MEST) (No. R11-2007-045-03001-0).

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