Secondary ion mass spectroscopy study of failure mechanism inorganic light emitting devices

Lin Ke, Keran Zhang, Nikolai Yakovlev, Soo-Jin Chua �, Peng Chen

Institute of Materials Research & Engineering, 3 Research Link, Singapore 117602, Singapore

Received 3 December 2001; received in revised form 25 February 2002; accepted 12 March 2002

Abstract

Secondary ion mass spectroscopy is used to examine the dark, non-emissive defects in organic light-emitting devices. Movements

in the interfaces between the electrodes and polymer originate from electrode imperfections. Due to the soft polymer layer, the

Indium and Calcium elements migrate under electrical stress and their profiles overlap to a large extent in the dark spot areas. The

proximity between the Indium tin oxide sharp spikes and cathode metal protrusions lead to the large current flow initiating the

formation of dark spots. We demonstrate that the presence of cathode imperfection and interface roughness of different layers

correlate well with the formation of device dark spots.

# 2002 Elsevier Science B.V. All rights reserved.

Keywords: Mass spectroscopy; Cathode imperfection; Polymer

After the first report of an organic electroluminescent

(EL) device by Tang and Vanslyke [1] in 1987, research

progress in organic light emitting diodes (OLEDs) has

been made since and the device performance has now

approached a level where it is commercially viable for

applications. Degradation mechanism of the organic EL

devices has also been studied extensively. Among all the

proposed degradation processes, degradation of the

metallic cathode is considered to be a more rapid failure

mechanism. The formation of non-emissive dark spot

defects has been reported by several researchers [2�/7],

but no conclusive theory has been accepted to explain

their formation or propagation.The electrical shorts [8] and metal diffusion [9] into

polymers have been proposed as the cause of device

failure. The device failure may also be due to the

electrochemical reactions taking place at polymer/metal

interfaces [10] and to the melting [11] of the polymer.

However, the degradation process of EL devices has not

been clarified in detail since most of the degradation

processes take place at the local micro-areas between

two electrodes. Secondary ion mass spectroscopy has

been adopted to study small molecular devices [12] and

the quality of polymer LED cathode deposition [13]. In

this paper, the OLED dark spot area was studied for the

first time using SIMS. The SIMS depth profile of the

dark spot area was compared with that in the emissive

area in both electrically stressed and non-stressed

devices. A possible mechanism is proposed to explain

the formation of the non-emissive dark areas.

OLEDs were prepared by first spin coating the hole

transport (HT) material, polyethylenedioxythiophene

(PEDOT), on the cleaned surface of the patterned ITO

glass substrate. After baking at 200 8C for 5 min to

remove moisture, an active layer of MEH-poly

(p -phenylene vinylene) (PPV) was spun coated. The

PEDOT and PPV-copolymer layers were 20 and 90 nm

thick, respectively. Following that, a 5 nm thick calcium

cathode was deposited on the PPV copolymer by

thermal evaporation. To protect the Ca cathode, a 200

nm thick silver layer was deposited. Thermal evapora-

tion was carried out in an Edwards Auto 306 thermal

evaporation system at a base pressure of 2.0�/10�6

Torr. Devices fabricated in accordance with the proce-

dure mentioned above, turned on at about 2.6 V. The

device started to emit light at about 2.8 V. The

luminance (L ) achieved a value of 100 cd m�2 at 3.6

V. Secondary ion mass spectrometry (SIMS) was carried� Corresponding author.

E-mail address: elecsj@nus.edu.sg (S.-J. Chua).

Materials Science and Engineering B97 (2003) 1�/4

www.elsevier.com/locate/mseb

0921-5107/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.

PII: S 0 9 2 1 - 5 1 0 7 ( 0 2 ) 0 0 1 2 9 - 0

out in a TOF-SIMS IV instrument with time-of-flight

secondary ion detection. Pulsed Ga 25 kV beam was

used for analysis. An Ar 3 kV beam was used for

sputtering during depth profiling.

SIMS depth profiling was performed for Ag, Ca, C,

In and O on both the bright and dark spot areas in

electrically stressed devices. Ag and Ca come from the

cathode, while C comes from the PPV polymer layer. In

and O come from the ITO anode layer. Devices were

taken out of the evaporator after cathode deposition

and electrically stressed at a constant voltage of 7 V in

room ambient. The position of the dark spots formed in

the active area were monitored and marked using an

optical microscope (Olympus BX-30) with an attached

CCD camera. The sample was then loaded into the

SIMS chamber for analysis. The total time the devices

were exposed to atmosphere was about half an hour.

SIMS analysis was also performed on a device prepared

together on the same ITO glass substrate, but without

subjecting it to electrical stress.

Three cases, viz. (1) non-stressed device, (2) bright

and (3) dark regions in the stressed devices, were

studied. Fig. 1 shows the results of Ca and In distribu-

tions. Obvious boundary movements for the element

profiles for both the bright and dark areas in stressed

device were observed. However, the shifts and widths of

the profiles were different for both the bright and dark

areas. The Ca profile is wider in the dark spot area

compared with that in the bright area. The origin of the

abscissa in all the figures is at the device surface.

However, due to material imperfection, such as porosity

of the metal film, the stressed device caved in towards

the glass side and the thickness of the whole device has

shrunk as shown in the Fig. 1. The indium profile

decreases in width in the bright area. It is suspected that

the movements of the polymer/electrode interfaces are

due to ITO surface roughness in the first place.

Although In atoms tend to move towards the cathode

under electrical stress, the ITO/glass interface provides

the reference since this boundary is rigid. It is thus seen

that the ITO layer has become thinner and smoother

than when it started with. The C and O profiles are

displayed in Fig. 2. Both the O and C profiles have

moved closer relative to one another after electrical

stress. The C profile is wider in the dark spot area.

Surprisingly, the intensity of the O signals are almost the

same in all three cases. In Fig. 3, where the Ca and C

signals are shown together, it is clear that Ca and C

moved in tandem and their front edges take the same

shape. The SIMS profiles show good repeatability

among the devices studied.

Fig. 1 shows that the Ca peak has shifted significantly

towards the surface under the electrical field. This is due

to the cathode layers being fabricated by thermal

evaporation. The porosity and other defects are almost

impossible to control by this evaporation technique.

Under electrical stress, metal ions move along the

electrical field direction and tend to densify. The

cathode region near the surface in the bright areas

become more solid during electrical stress. In the case of

non-stressed device, a severe Calcium profile tail has

already been observed entering into the PPV polymer

layer. This is attributed to the polymer porosity. The

solvent preparation and spin coating conditions are also

shown to influence the properties of PPV polymer.

There is a significant roughness of the polymer/Ca

interface during the cathode evaporation as reported

by Rasmusson et al. [14].

The narrower Ca and C profiles in the bright area

imply that they are more densely packed. Ca and In

profiles have less steep slopes in the overlap region in

non-emissive areas indicating that they have rougher

surfaces. This will lead to local variations in the distance

Fig. 1. SIMS depth profile for Ca and In.

L. Ke et al. / Materials Science and Engineering B97 (2003) 1�/42

between the two electrodes, creating strong electric field

intensity and large local current densities.

The SIMS observations introduced in this work reveal

that the distinct mechanism of device failure in OLED is

due to cathode imperfection and interfaces roughness.

The cathode imperfection causes the relative movements

of the polymer/electrode boundaries. The metal move-

ment will pack the pores, which originate from the

deposition process. The interface roughness results in

the variation of the relative positions of the different

layers. In certain localized areas, Ca protrusions and

ITO come close to one another. In the microscopic

protrusion point, a very large electric field intensity and

current density is created. The enhanced local lumines-

cence and local heating around it lead to instability and

further growth of local current densities until cata-

strophic failure results. A surrounding circular region

will appear as a growing dark spot.

One of the most popular conjectures on dark spot

formation in OLEDs is attributed to the moisture and

oxygen reaction at the cathode. However, from Fig. 2, it

is seen that the O signal does not reach the Ca surface.

SIMS results in Fig. 3 clearly show that Ca and C

moved synchronously in both the dark and bright areas

in the stressed device. Their front edges remained the

same shape. This strongly suggests that there are no Ca

and polymer delamination during the device degrada-

tion process.

Fig. 2. SIMS depth profile for O and C.

Fig. 3. SIMS depth profile for Ca and C.

L. Ke et al. / Materials Science and Engineering B97 (2003) 1�/4 3

From the discussion given above, it is suggested that

the device failure originate from the cathode porosity

and other defects, resulting in the movement of the

electrode/polymer interface under electrical stress. In

some localized areas, the Ca and ITO protrusions, as

shown schematically in Fig. 4, result in strong localized

current densities leading to polymer degradation.The experimental observations of the large localized

current density initiating the dark spot formation

observed by Vadim et al. [15] further supports the

proposed failure mechanism given here. Some other

researchers using luminance scope observed that the

precursor to the formation of the non-emissive, dark

spot shadow is the sudden increase in luminescence

followed by abrupt termination of emission.In summary, we have studied the elemental distribu-

tions in both electrically stressed and non-stressed

OLEDs. By comparing the cathode and anode element

profiles obtained by SIMS, it is found that the metals

migrate, with the polymer penetrating into the metal

pores. In areas where the metal protrusions result in

high electrical fields, a large localized current density

causes the degradation of the PPV polymer resulting indark spots.

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Fig. 4. Schematic illustration of device failure mechanism.

L. Ke et al. / Materials Science and Engineering B97 (2003) 1�/44