New Organic Thin-Film Encapsulation for Organic Light Emitting Diodes

doi:10.4236/jeas.2011.12003

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Journal of Encapsulation and Adsorption Sciences, 2011, 1, 23-28

doi:10.4236/jeas.2011.11003 Published Online June 2011 (http://www.scirp.org/journal/jeas)

New Organic Thin-Film Encapsulation for Organic Light

Emitting Diodes

Rakhi Grover1,2, Ritu Srivastava1, Omwati Rana1, D. S. Mehta2, M. N. Kamalasanan1

1Center for Organic Electronics, Physics of Energy Harvesting Division, National Physical Laboratory (Council of

Scientific and Industrial Research), New Delhi, India

2Instrument Design Development Center, Indian Institute of Technology Delhi, New Delhi, India

E-mail: [email protected]

Received March 11, 2011; revised April 12, 2011; accepted April 20, 2011

Abstract

Organic Light-Emitting diodes (OLEDs) are extremely sensitive to water vapour and oxygen, which causes

rapid degradation. Epoxy and cover glass with large amount of desiccant are commonly applied to encapsu-

late bottom emitting OLEDs which is not a viable option for flexible as well as top emitting OLEDs. This

paper reports a completely organic encapsulating layer consisting of four periods of alternate stacks of two

organic materials with different morphologies deposited by simple vacuum thermal evaporation technique.

Standard green OLED structures with and without encapsulation were fabricated and investigated using

structural, optical and electrical studies. Moreover, the encapsulation presented being organic is safe for un-

derlying organic layers in OLEDs and is ultrathin, transparent and without any cover glass and desiccant,

ensuring its application in flexible and top emitting OLEDs.

Keywords: OLED, Thin Film Encapsulation, Diffusion Barrier, Atomic Force Microscopy

1. Introduction

Organic Light-Emitting diodes (OLEDs) are considered

as one of the most potential display technology today due

to their low power consumption, low cost, and superior

viewing ability [1] especially for their possibility to build

flexible displays [2-6] as they are ultra-thin and light

weight. Top emitting OLED devices are also becoming

increasingly important because of the increase in aper-

ture ratio (the ratio of actual emitting area to the total

area of the pixel) obtained as compared to bottom emitter

approach. However one obstacle to these developments

is the susceptibility of these devices to water vapor and

oxygen, which causes rapid degradation.

Device reliability issues, in part, arise due to the envi-

ronmental instability of both the active materials and low

work function electrode in the devices [7-11]. Low work

function metals are highly reactive with oxygen and wa-

ter vapor and, thus, oxidize very quickly. This results in

the formation of insulating oxide barriers, making the

injection and collection of charge carriers less efficient.

Exposure to water vapor and oxygen in the environment

may also result in the formation of black spots in OLEDs,

reducing their light output and lifetimes [12]. Another

detrimental degradation mechanism which can arise from

environmental-induced oxidation is the delamination

within the device. When water vapor permeates through

defects into the interface formed by the cathode and an

active layer, it may cause chemical reactions which in-

duce out gassing or volumetric expansion leading to de-

lamination [13,14].

A number of approaches to encapsulation have been

developed including the use of metal lids, glass and the

sealing of devices between two glass substrates or plastic

substrates treated with barrier films [15,16]. Additionally,

getter materials such as calcium and barium are used to

remove any residual water in the encapsulated volume or

water vapour which diffuses through the epoxy sealant

[17]. However, these rigid materials are not amenable for

use in flexible OLEDs which require flexible covering as

well as for top emitting OLEDs in which the emitted

light has to pass through the cover glass and the opaque

desiccant.

Therefore Thin Film Encapsulation (TFE) technology

is the key technique to meet the requirements of

small-size flexible and top emitting electronic display

devices. For active layers and substrates with low glass-

transition temperatures and thermal stability, the proc-

R. GROVER ET AL.

essing temperatures at which the barrier layers can be

deposited should be low. However, processing at low

temperature may lead to more defects in the films, limit-

ing the overall barrier performance. Therefore, inorganic

films are limited in their performance mainly due to the

presence of defects in the films which provide pathways

for water vapour and oxygen to permeate through the

barrier layers.

By applying multilayer films with alternating stacks,

defects which span the entire thickness of the individual

inorganic layers are interrupted and do not channel con-

tinuously through the film structure. This structure cre-

ates a tortuous path resulting in very long effective diffu-

sion pathways, increasing the barrier performance (“de-

fect interruption layer”) [18]. These results suggest that

developing ultra-high barrier performance encapsulation

layer as well as more stable materials for organic device

is important for extended lifetime of organic devices.

Also, the deposition techniques of these inorganic mate-

rials can damage the underlying sensitive organic layers

used in OLEDs. Thus, the development and integration

of high-barrier encapsulation films with organic elec-

tronics remains a challenging endeavour.

The focus of the present work is to prepare completely

organic thin film encapsulation for OLEDs which has been

demonstrated by combining alternate stacks of two differ-

ent organic materials having different morphologies and

different thin film forming properties. One of the organic

material chosen is TPD (TPD-N, N’-diphenyl-N, N’-bis-3

-methylphenyl [1, 1’-bipheny]-4, 4’-diamine which is well

known as hole transporting material in OLEDs but also

known to get easily crystallized because of its low glass

transition temperature (Tg = 64˚C). Another organic mate-

rial chosen is newly synthesized material XP (2.2.6. 5, 5′-(4,

4′-(2,6-di-tert-butylanthracene-9,10-diyl)bis(4,1-phenyle

ne))bis(2-(4-hexylphenyl)-1, 3, 4-oxadiazole) [19] having

highly amorphous and stable thin film forming ability with

a quite high glass transition temperature of 108˚C. Alter-

nate stacks of thin layers of these materials have been fab-

ricated and studied using Atomic force microscopy and

Calcium corrosion test involving UV-Visible absorption

spectroscopy. Finally the OLED devices with the opti-

mized encapsulation showed enhanced lifetime as com-

pared to the devices with no encapsulation layers.

2. Experimental

Plain glass plates were used as the starting substrates and

cleaned sequentially using de ionized water, acetone, tri-

chloroethylene and isopropyl alcohol for 20 minutes each

using an ultrasonic bath and then dried in vacuum oven.

Thin films of Ca (Calcium), TPD and XP were deposited

with thicknesses of 250 nm, 20 nm and 10 nm respectively

under a high vacuum (10−5 - 10−6 Torr).The correspond-

ing device structures being (also shown in Figure 2):

(a) Glass/Ca

(b) Glass/Ca/TPD

The thicknesses of these films were measured in situ

by a quartz crystal thickness monitor. Morphological

properties were examined using Atomic force micros-

copy (AFM) (NT-MDT). UV-Vis spectra were taken

using a high resolution UV-Vis spectrophotometer (Shi-

madzu 2401 PC) in the range of 200 - 800 nm.

Then OLEDs were fabricated incorporating the opti-

mized encapsulation layer. Indium-tin oxide (ITO) (thi-

ckness of 120 nm) coated glass plates with a sheet resis-

tance of 20 Ω/□ (Vin Karola, USA) were used as starting

substrates and were patterned and cleaned using deion-

ised water, acetone, trichloroethylene and isopropyl al-

cohol sequentially for 20 min each using an ultrasonic

bath and dried in vacuum.The device structure of OLEDs

grown by vacuum thermal deposition was:

ITO/m-MTDATA (20 nm)/α-NPD (10 nm)/CBP +5%

Ir (ppy)3 (35 nm)/TPBi (30 nm) /LiF (1 nm)/Al (200 nm).

4, 4’, 4”-tris(3-methyl-phenylphenylamino) triphenyl-

amine (m-MTDATA) and N, N’-di-1-naphthalenyl-N,

N’- diphenyl-1.1’ biphenyl-4, 4’-diamine (α-NPD) were

used as hole injection and hole transporting layers re-

spectively. 5% (Ir (ppy)3) doped (CBP) was used as the

emissive layer. 2, 2’, 2’’-(l, 3, 5-benzenetriyl)-tris (L-ph-

enyl-l-H-benzimidazole (TPBi) and LiF were used as

electron transporting and electron injection layers re-

spectively. The size of each pixel was 5 mm  5 mm.

Then four periods of TPD (20 nm)/XP (10 nm) layers

were deposited. The current density-voltage-luminesc-

ence (J-V-L) characteristics have been measured with a

luminance meter (LMT-1009) interfaced with a Keithley

2400 programmable current-voltage digital source meter.

All the measurements were carried out at room tempera-

ture under ambient conditions.

3. Results and Discussion

3.1. Structural Studies

Figure 1 shows the morphology of the thin films of XP

as recorded from AFM with time .The films were

found to be highly amorphous and more significantly

stable also. As explained in Section 1, densely packed,

continuous, and highly conformal coatings are desir-

able for excellent barrier properties, so that all small

particles are well encapsulated. Table 1 shows the av-

erage roughness values of the thin film of XP with time

and corresponding AFM images are presented in Fig-

ure 1.

R. GROVER ET AL.25

Table 1 Average roughness values of thin film of XP with

time.

S. No. Time Average Roughness value (nm)

1 After deposition 1.011

2 After 480 hours (20 days)1.937

3 After 960 hours (40 days)2.235

(a)

(b)

(c)

Figure 1. AFM images of 10nm thick films of XP. (a) after

deposition (b) after 20 days (c) after 40 days of storage.

Highly amorphous thin films of XP exhibited an av-

erage roughness of 2.235 nm even after 960 hours of

storage at room temperature (45% RH).

TPD is well known in literature to crystallize easily

due to its low glass transition temperature (64˚C) [20]

and hence the alternate arrangement of such a crystal-

line layer and highly amorphous layer of XP can prove

to be an efficient encapsulating layer which increases

the effective diffusion length for ambient water vapors

or oxygen molecules as explained in Section 1.

3.2. Optical Studies

Calcium corrosion test is a widely known method for

measuring ultra-low permeation rate of barrier films.

Calcium is a conducting and opaque metal which be-

comes transparent after oxidation and is very sensitive

for detecting the presence of oxygen and water vapour.

Hence, the measurement of Ca transparency with time

provides an indirect method to determine the oxidation

and corrosion rates of Ca. Thus well encapsulated films

of Ca show relatively lower values of transmittance as

compared to the Ca film without encapsulation. Figure 2

shows the device structures fabricated for the calcium

corrosion test. Figure 3(a) shows the Transmission spec-

tra against wavelength for the as deposited structures

2(a), 2(b) and 2(c). The films were found to have quite

good optical transparency. Figure 3(b) shows the

Transmission spectra with time for the structures 2(a),

2(b) and 2(c). The more rapid increase in optical trans-

parency of the structure with bare Ca in Figure 3(b)

shows that it rapidly oxidizes in air as compared to rela-

tively slow oxidation of structures 2(b) and 2(c) (slopes

of increment in oxidation with time being 0.13, 0.09 and

0.04 for the structures 2(a), 2(b) and 2(c) respectively).

Moreover, the device structure 2(c) exhibited greatest

resistance against moisture. This is an indication of im-

proved barrier properties of the alternate stack of the

structures.

(a) (b)

(c)

Figure 2. Device structure fabricated for Calcium corrosion

R. GROVER ET AL.

test.

(a)

(b)

Figure 3. (a) Transmission spectra for the as deposited films

and (b) Transmission values for the films degraded with

time.

Figure 4. Schematic device structure of the fabricated

OLED.

3.3. Electrical Studies / OLED Fabrication

Figure 4 shows the schematic device structure of the

OLED fabricated by vacuum thermal evaporation tech-

nique. Phosphorescent molecule Ir (ppy)3 was used as the

green emitter. On the top of the cathode layer, the opti-

mized encapsulating structure was deposited which con-

sisted of four periods of TPD (20 nm)/XP (10 nm) layers.

Figure 5 shows the Luminance-Voltage characteristics

of the devices with and without thin film encapsulation

layers. Both the devices with and without encapsulation

showed relatively similar initial L-V characteristics.

To further confirm the encapsulating properties of al-

ternate stacks of proposed TFE layers, the luminous de-

cay of the devices under a constant current operation

were investigated. The luminance decay curve of the

devices with and without TFE is shown in Figure 6. We

Figure 5. Luminance-Voltage characteristics of the OLEDs

fabricated with and without TFE.

Figure 6. Luminous decay of the devices with and without

TFE under a constant current density of 0.4 mA/cm2.

R. GROVER ET AL.27

have measured luminance of the devices at a continuous

constant current of 0.4 mA/cm2, which gave an initial

luminance of 140 Cd/m2 .As shown in Figure 6, the en-

capsulated device, could stay longer (12 hours) as com-

pared to the device without encapsulation layer (1.1

hours). The enhancement in lifetime of encapsulated

device being around ten times higher than that of the

device without encapsulation. Further studies on the

work are in progress. Although the obtained device life-

times are quite low in terms of efficient organic displays

but can be enhanced due to fact that encapsulation layers

were deposited without any further capping of the device

as reported in literature with additional aluminium foil

capping [21]. It is worth noting that the alternate stacking

type encapsulation is ultrathin, optically transparent and

flexible, suiting the encapsulation of flexible OLEDs as

well as top emitting OLEDs.

The barrier properties of the proposed TFE have been

attributed to the defect interruption due to different types

of film forming abilities of TPD and XP layers. TPD is

well known to have a glass transition temperature as low

as 64˚C whereas the newly synthesized material XP has a

high glass transition temperature of 108˚C. The alternate

stacks of these different types of films one highly amor-

phous and other crystalline basically increase the diffu-

sion lengths of ambient oxygen or water vapours thereby

protecting the device. Additionally the TFE being or-

ganic in nature can be deposited at much lower tempera-

tures as compared to earlier reported inorganic encapsu-

lation layers [22,23] and is therefore more compatible

with the organic transport and emissive layers used in the

organic based devices. Thus use of ultra-thin vacuum

thermally-deposited alternate organic thin films is effec-

tive as a barrier layer for significantly increasing the life-

time of an OLED device.

4. Conclusions

The encapsulation of an OLED device with the alternate

stacks of two organic thin films TPD and XP with dif-

ferent morphologies and different thin film forming

properties using the vacuum thermal evaporation method

has been carried out for the first time. Vacuum thermally

deposited organic films showed excellent barrier proper-

ties. The TFE was found to significantly slow down the

oxygen and moisture diffusion into the device leading to

a longer operational lifetime.

5. Acknowledgement

The authors wish to acknowledge the Director, NPL,

Delhi and Dr. S. S. Bawa for suggestions and discussions.

The authors would like to acknowledge Department of

Science and Technology DST and Council of Scientific

and Industrial Research CSIR, New Delhi, India for fi-

nancial support.

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