Encapsulation issues in organic light emitting diodes

The materials used in small organic molecule and conjugated polymer OLED are vulnerable to degradation by oxygen and water vapor, which can trigger early failure.. Reducing the surface r

The key to OLED technology is the development of organic semiconductor materials，which is also a crucial starting point for so-called "plastic electronics" During the 1980s, Kodak and UK-based start-up Cambridge Display Technology (CDT) developed displays that formed luminescent images when electric currents were passed through thin layers of organic materials based on small organic molecules and conjugated polymers to generate light In effect, each pixel in the display behaved in the same way as

a miniature light emitting diode (LED)

1.1.2 Advantages of OLED over LCD display technology

An OLED is an electronic device made by placing a series of organic thin films between two conductors When an electrical current is applied, light is emitted by a process called electroluminescence as electrons and holes injected from the cathode and anode respectively, recombine in the organic light-emitting layer

Figure 1.1 OLED structure showing injection of electrons from the cathode and holes from the ITO anode, and their recombination in the electroluminescent (EL) layer

OLED displays are lightweight, durable, power efficient and ideal for portable applications OLED display fabrication has fewer process steps and also uses lower-cost materials compared to liquid crystal display (LCD) It is believed that OLEDs can replace

hν

the current LCD technology in many display applications due to the following performance advantages over LCD:

• Greater brightness

• Faster response time for full motion video

• Wider viewing angles

• Lighter weight

• Higher power efficiency

• Broader operating temperature ranges

• Greater cost-effectiveness

1.1.3 Lifetime challenges

The lifetime of a display is the number of hours that the display is functional [2] It can be classified into storage lifetime and operational lifetime Storage lifetime denotes how long it would be possible to store the display in the absence of current Operational lifetime is usually defined as the time for the electroluminescence of the OLED to degrade to half its initial brightness (typically 100 cd/m2)

Although some of the properties of OLED match, and in some cases, surpass those of current LED’s, the fact remains, however, that the lifetime in these devices needs to be further improved for long-term consumer applications [3] Currently, the only products

having displays based on OLEDs are “short-term” commodity items such as cell phones, car stereo systems, and digital camera displays Most recently, Seiko Epson has reported

a 40-inch TV screen prototype made from conjugated polymer OLED

Considerable research had been done to identify the causes of degradation, which shortened the lifetime of the OLEDs [2] The most prominent morphological change observed in some degrading devices was the de-lamination of the cathode material [4] The de-lamination appeared in electroluminescence micrographs as non-emissive spots These non-emissive areas were sometimes referred to as ‘‘dark spots’’ or ‘‘black spots’’ [5] (although they do not look ‘‘black’’ under external illumination, see below) The non-emissive spots were attributed initially to local heating caused by short circuits which led

to the formation of pinholes and local ablation [6] or local fusion of the metallic cathode [7] Recent studies [8] revealed that the non-emissive spots had domelike structures termed ‘‘bubbles’’ filled with gases (mostly oxygen) presumably evolved in the course of electrochemical and photo-electrochemical processes in the presence of water vapor [9, 10] According to some recent reports [5, 11], the bubbles originated from pinholes in the metallic electrode when the device was powered up in the presence of atmospheric humidity In a recent report on state-of-the-art devices, the hole-injecting conducting polymer layer was found to be locally doped in the black spot, which pointed to an electrochemical degradation process driven by electrical current in the presence of

1.2 OLED device fabrication process

by the shadow mask that is used to define the micron sized pixels of the display

2) Conjugated polymers -The advantage of conjugated polymer light-emitting display (PLED) materials is their solubility in organic solvents, allowing them to be deposited onto a glass or flexible plastic substrate using spin-coating or ink jet printing Conjugated polymer technology enables the fabrication of larger displays as compared to small-molecule OLED, as there is no need for shadow masking or vacuum deposition processing PLED displays can also be operated at lower voltages and are more power-efficient than those based on small organic molecules

1.2.2 Display fabrication

OLED displays can be made using four simple steps:

1) Substrate preparation The first step is the preparation of the substrate, also

known as the backplane Indium tin oxide (ITO) coated glass is the substrate used for passive matrix display fabrication ITO sputter deposition is carried out in a vacuum chamber and film patterning in a class 100 yellow room

2) OLED fabrication The next step is the fabrication of the OLED part of the

display This step involves the spin-coating or printing, deposition of the hole transport layer (HTL), electroluminescence (EL) layer and electron transportation layer (ETL) Finally, the cathode electrode is deposited by a thermal evaporation process

3) Encapsulation To protect the OLEDs from being exposed to water vapor and

oxygen, the display is hermetically sealed in a protective package This is essential to maximize the display's performance and lifetime

4) Assembly Finally, the driving circuits that drive each pixel are wired up to the

display

An example of such a display fabricated in IMRE in our work using a blue-emitting polymer is shown in Fig 1.2

Figure 1.2 IMRE matrix blue OLED display (100×32 pixels)

1.3 Objective of the thesis

OLED technology offers the prospect of realizing flexible displays on plastic substrates using roll-to-roll manufacturing processes One of the biggest challenges to the OLED display industry is not competition from the incumbent LCD industry but from water vapor and oxygen The materials used in small organic molecule and conjugated polymer OLED are vulnerable to degradation by oxygen and water vapor, which can trigger early failure Sealing of the OLEDs from atmospheric oxygen and water vapor is typically accomplished with a glass or metal lid attached to the display substrate using a

bead of ultraviolet (UV) cured epoxy Such an encapsulation technique is not viable for a flexible display, since low-moisture permeability epoxies are rigid The obvious solution,

a plastic or thin film encapsulation is non trivial Plastics are permeable materials, often with holes many microns in diameter that allow water vapor and oxygen to permeate through The barrier specification required for OLED displays is unclear, since the mechanism of long-term degradation is still a subject of debate [13] However, it is clear that a stable OLEDs requires a moisture barrier which transmits < 10-5 g/m2/day of water vapor [14] All conventional vapor deposited barrier films are therefore inadequate for OLED applications by several orders of magnitude

Normally an organic and inorganic composite structure is used in OLED encapsulation The quality of encapsulation depends on the materials, surface topography and the adhesion between the organic and inorganic layers This thesis focuses on issues

in encapsulating organic light-emitting diodes with thin films

Other than the quality of the encapsulation film, the surface roughness is also a factor limiting the stability and the efficiency of the OLEDs [13] Reducing the surface roughness of the ITO reduces the area of adsorption sites for water vapor and thereby extends the lifetime of the OLED device Better encapsulation could also be obtained with reduced surface roughness Reactive ion etch (RIE) planarization of the indium tin oxide (ITO) substrate, and associated changes in electrical, optical properties and

improvements in the manufacturing process to be made to achieve better stability of the organic layers and the cathode With improvements in encapsulation and efficiency of OLED devices, mass commercial OLED production would be possible

1.4 Organization of the thesis

Chapter 2 reviews current research interest in lifetime and encapsulation of OLED Oxygen and water vapor are the two main known agents leading to degradation of OLED devices

Chapter 3 describes the experimental techniques we used to fabricate the OLED devices, measure the ITO surface parameters after RIE treatment and evaluate the various fabrication processes relating to encapsulation of OLEDs

Chapter 4 investigates the planarization effect of ITO surface using RIE Reducing the surface roughness improves OLED electrical performance and also reduces the water vapor permeation through the interface between ITO and the encapsulation film

Chapter 5 describes laser ablation as a method to realize 300-micron sized pixels This method avoids the formation of photo-resist mushroom structures which traps water vapor The micron sized spikes along the cathode runners formed during laser ablation however poses a challenge to thin film encapsulation Some methods to address these post ablation spikes are discussed in this chapter

Chapter 6 describes the use of multi-layer encapsulation to improve the barrier properties to oxygen and water vapor Subsequently the barrier film was used to

encapsulate conjugated polymer and small organic molecule OLED devices to investigate the effect of barrier film encapsulation on device lifetime

Chapter 7 summarizes the thesis and briefly describes possible future work

Chapter 2 Literature Review

Contributing factors to the degradation of device performance over time include reaction of organic materials with the cathode and its de-lamination, indium diffusion into the organic layer, inter diffusion, chemical degradation, and crystallization of the same Careful engineering of device fabrication by manipulating different parameters such as the purity and choice of materials, the rate of film growth during organic layer deposition, the vacuum under which devices are fabricated and encapsulation have improved device performance [15] This thesis however is focused on the issues in encapsulating OLED device, namely the effect of surface roughness and topography on encapsulation and developing a barrier stack to improve the lifetime performance

Oxygen and humidity are now believed to be the most important factors for degradation of the OLED device It is very important to understand how oxygen and humidity penetrates the encapsulation layer and react with the organic film and cathode layers Firstly techniques important to OLED fabrication and encapsulation are reviewed This is followed by a review of OLED degradation and lifetime Lastly experimental methods used to study water vapor permeation in encapsulation films are reviewed

2.1 OLED fabrication process

There are two ways to fabricate an OLED display One way uses solution based conjugated polymers The other way employs vacuum deposition of small organic

molecules The current method involved in making passive matrix display uses photo resist ribs which are traps for water as described by Nguyen [16]:

Starting with ITO coated glass substrates, rows are obtained by etching the lithographically patterned ITO layer Since the sensitivity of the organic layers to chemicals and humidity does not allow patterning of the metallic top contact after deposition, a different approach has to be used to pattern the organic and cathode layers

In situ photo resist shadow mask can be used to separate adjacent cathode rows But the photo resist barriers have a mushroom structure with high aspect ratio (Fig 2.2), which could seriously impact the backend thin film encapsulation process Such photo resist barrier ribs are also sources of traps for solvent and water vapor, which would contribute

to degradation of the OLED

2 µm

Recess sites

Two obvious recess sites are showed under the mushroom structure These sites can act as trap sites for water vapor or solvent

2.2 Oxygen plasma treatment of ITO surface

ITO has been by far the common choice for OLED anode materials, due to the availability, good transparency and low resistance, and also to the ease with which it can

be patterned [17] Among the many surface treatments of ITO, oxygen plasma treatment

is one of the most common techniques It has been reported that oxygen plasma treatment could effectively remove the surface carbon contamination, increase the work function of ITO, which may lead to a lower energetic barrier for hole injection at the ITO/polymer interface [18]

Cacialli reported that oxygen-plasma yielded the highest work function for a 10 minutes treatment, the smoothest surface, and the lowest surface resistance Higher work function was particularly attractive for improving injection of holes into the organic semiconductors [19] In conjugated polymer LED, conducting PEDOT [Poly (styrene sulfonate)-doped poly (3, 4-ethylene dioxythiophene)] are directly spin-coated or printed from water dispersion onto the surface-treated ITO In this regard Cacialli noted that oxygen-plasma induced the most polar surface leading to most uniform spreading of thin films of polar polymer solutions in order to achieve a better adhesion at the ITO/polymer interface [17]

Zhu demonstrated the enhancement in hole injection in device made with oxygen

treated ITO anode The best EL performance was found in the PLED made with an ITO anode treated with an oxygen flow rate of 60 sccm used in their study [18]

Since oxygen plasma treatment may improve the adhesion between ITO and polymer, which led to better electrical performance of OLED device, oxygen reactive ion etch (RIE) was explored for the pre-treatment of ITO anode in this thesis

2.3 Current encapsulation techniques

Normally thick glass and epoxy were used for OLED encapsulation [20] Fig 2.3 shows the structure of the OLED device encapsulated using a glass lid and perimeter epoxy seal The function of the desiccant (usually, calcium oxide or barium oxide) is to absorb the possible water vapor trapped in the metal can encapsulation

Glass

OLED layers and cathode

Epoxy adhesive Stainless

steel can

desiccant

Membrane

The flexibility of both small organic molecule and conjugated polymer OLED encouraged their use of these materials in flexible displays [21] OLED displays on flexible plastic substrates have been reported [22] However lifetime was short Development of a long-lived flexible OLED display on plastic substrate would therefore have to overcome two major challenges: preventing moisture and oxygen diffusion into the display area through the bottom/substrate and also through the top cathode

Chwang described encapsulated passive matrix OLED displays on flexible plastic substrates using a multilayer barrier encapsulation technology [23] The flexible OLED displays were based on highly efficient electro-phosphorescent OLED technology deposited on barrier coated plastic substrate and were hermetically sealed with an optically transmissive multilayer barrier coating (Barix encapsulation) (Fig 2.3) A lifetime of 2500 hours was achieved with a 5 mm2 encapsulated flexible OLED test diode

Figure 2.3 Fracture cross-section SEM of generic Barix film （Cited from Vitex Company）

2.4 OLED degradation

The degradation of a non-encapsulated conjugated polymer-based LED was accompanied by the appearance of strong fluctuations, in the radiance and in the film resistance [20] There was a correlation between the morphological changes which occurred during the degradation process and the strong fluctuation

Morphological changes observed during OLED degradation showed two distinct phases First, there was formation of ‘‘bubbles’’ at the metal–polymer interface due to delamination of the polymer film from the metal surface Second, carbonized areas in the

the form of ‘‘dark spots’’ can be observed

Figure 2.4 Micrographs of the device under external illumination: viewed through the ITO glass displaying dark spots

The inhibition of bubble formation when the device was operated in the presence of

an inert atmosphere supported the idea [5, 8] that humidity and/or air played a very important role in this initial delamination Recent mass-spectrometric study of the bubble contents [24] revealed the presence of mostly oxygen and some hydrogen beneath the delaminated aluminum film It was suggested that the electrode swelling was due to gaseous products produced in electrochemical or photo-electrochemical reactions A thermally activated reaction between metal electrodes and the emitting polymer was

contemplated by Salaneck et al [25]

After the initial bubble formation, the injected current was significantly higher in the

100 µm

perimeter area surrounding the bubble due to local electric field amplification at the edges The large currents flowing through these areas carbonized the polymer, leading to fusion of the electrodes [7] Indeed, it was well known that some degraded devices show

a direct short

2.5 The lifetime of OLED

The lifetime of OLED device is defined as the time that OLED device luminance decays to half initial point (L0 ~ 100 cd/m2) [2] Fig 2.5 indicates lifetime curves for OLED encapsulated in different ways

Figure 2.5 Normalized lifetime data under dc drive for an OLED [25]

It is observed from Fig 2.5 that the lifetime for the OLED device with glass encapsulation is about 10000 hours, which would be applicable in small screen products such as mobile phones It is obvious that thin film encapsulated OLED on plastic substrate have the poorest lifetime performance of OLED devices Rigid glass is expected

to be the best barrier material except that it could not be used for flexible displays

2.6 Calcium corrosion test for ultra-low permeation rates

Highly sensitive permeation measurements have been developed for the characterization and development of polymeric substrates for flexible display applications In particular, organic light-emitting devices require substrates with extremely low permeation rates for water vapor and oxygen Here Paetzold demonstrated

a technique (Fig 2.6) for measuring ultra-low permeation rates (< 10-6 g/m2/day) [26] The amount of oxidative degradation in a thin film calcium sensor was monitored by

in situ resistance measurements The improvement using this technique was demonstrated for polyester foils with single-sided and double-sided barrier coatings The sensitivity of the water vapor transmission rate in this measurement was limited by the quality of the encapsulation

V I

Water vapor and oxygen

Figure 2.6 Layout of the calcium corrosion permeation sensor

The benefits of this approach were high sample throughput, continuous and easy point acquisition and the possibility of measuring permeation rates over long periods of time

data-Chapter 3 Experimental methods

3.1 Fabrication of OLED passive matrix device

a Wet cleaning the substrate

ITO transparent glass is widely used in many active and passive electronic and electronic devices ranging from aircraft window heaters to charge-coupled imaging devices, as it exhibits high transmittance in the visible spectral region, high reflectance in the infrared (IR) region and high electrical conductivity [27] Glass coated with 120-nm thick ITO having a sheet resistance of 20 Ω /□ (ohms per square) was used as substrates Pre-cleaning comprised a photo-resist stripper soak for 60 minutes, followed by Isopropanol alcohol (IPA) soak for 60 minutes, and a final ultrasonic de-ionized (DI) water wash for 30 minutes

opto-b Patterning the anode

Photo resist was spin-coated onto the ITO film and baked at 100oC for 60 minutes After spin-coating, then resist film was exposed to UV light from a mask aligner, and then developed in photo resist developer An HBr etch removed the exposed ITO layer After ITO etching, the photo resist was stripped off to yield a patterned ITO anode

c Spin-coating the polymer onto the ITO electrode

Spin-coating is the preferred method for application of thin, uniform polymer films

on substrates with low surface roughness The polymer was dissolved in a suitable solvent and dispensed onto the substrate The substrate was then rotated at a high speed in

order to spread out the solution by centrifugal force Excess solution was run off the edge

of the substrate while the solvent was continually evaporated away to give a uniform polymer thin film In the spin-coating process the initial amount of solution placed on the substrate had little effect on the final thickness of the film, which largely depended on the speed at which the polymer solution is spin-coated [28]

Oxygen RIE was used to clean the ITO substrate before polymer spin-coat In this process, the surface roughness of ITO was reduced, which was helpful to reduce the adsorption sites for water vapor

d Cathode layer deposition

A vacuum thermal evaporator (ULVAC, Japan) was used to deposit the cathode layer (4Å LiF, 200 Å Ca, 1000 Å Ag) onto the conjugated electroluminescent polymer All the films were deposited under high vacuum (less than 6×10− 4Pa) After cathode deposition, the devices were either transferred to a nitrogen-filled glove box and characterized without encapsulation (Fig 3.1), or encapsulated as described in this thesis

Figure 3.1 A PLED display fabricated at IMRE showing the A*STAR logo

e Laser ablation

Laser ablation was used instead of the in-situ photo-resist shadow mask to define the

pixels in the display A 355 nm laser (Nd:YAG; width: 30 ns; frequency: 40 kHz; scan speed: 180 mm/s) was used to pattern the blanket cathode layer into cathode runners Avoiding the use of photo resist removed a source of water vapor traps in the display and reduced also the aspect ratio of the pixels for better integration with thin film encapsulation

3.2 RIE treatment of ITO surface and techniques used for characterizations of treated surface

3.2.1 Reactive ion etching

ITO has been widely used as anti-static coatings, heat mirrors, transparent electrode

in solar cells [29, 30], flat panel displays, sensors [31] and anode contact in OLED [32] because of its transparency and high conductivity The surface properties of ITO are also expected to affect the characteristics of the devices Abnormal device behaviors such as shorting, unstable current-voltage (I-V) characteristics and damage to the surface of the cathode contact have been observed in OLED built on bare cleaned ITO surfaces [33, 34]

It is found that the ITO/polymer interface influences the local current distribution during device operation The intense local current at sharp points degrades the polymer causing the formation of the dark center Further current stress causes a dark central core to carbonize which may lead to shorts and/or open circuits accompanied by fluctuations in the device current [34] Furthermore, as-received ITO films have been found to be less efficient for hole injection than low work function metal cathodes for electron injection, resulting in hole-limited devices [33, 35, 36]

A Trion Sirus RIE Etch System was used to modify the surface of ITO with oxygen plasma A detailed investigation of the changes in ITO surface properties such as

morphology, sheet resistance, and contact angle was carried out Previous studies have also shown that oxygen plasmas had considerable effect on the properties of ITO [37]

After acetone, methanol and DI-water cleaning, ITO substrates (based on polycarbonate) were exposed to oxygen RIE for different time The samples were a commercially available LCD substrate, with a polycarbonate/SiOx/organic film/ITO structure The parameters for the RIE are shown as follows:

Process time: 1, 2 or 5 min

After the RIE process, the substrate surface was studied using atomic force microscopy (AFM), contact angle, and electrical resistance measurements

3.2.2 Atomic force microscope

AFM has sufficient resolution to resolve roughness at the nm level Therefore AFM is broadly used in the fields of chemistry, physics, microbiology and material science

Fig 3.2 shows the Multi-Mode AFM that was used to measure the surface roughness

of the ITO The contact between a scanning sample and the stationery tip are monitored using a laser and a detector to track the cantilever motion The negative feedback loop moves the sample up and down via a piezoelectric tube (PZT) so as to maintain the interaction force at a pre-selected level called the reference force level A three dimension image can be constructed by recording the cantilever motion in the Z direction as a function of the X and Y position of the tip

Tapping mode AFM (TM-AFM) was used to measure the surface morphology of the ITO surface In the tapping mode high quality images could be obtained without damaging the sample [38] The phase shift between the driving force and the tapping movement of the cantilever gave additional surface information [39] AFM was used to collect the data for surface roughness of each substrate

3.2.3 Contact angle measurements

When a liquid comes into contact with a surface, the surface energy determines the contact angleθ Surface with higher energies exhibits a smaller contact angle, and has a higher hydrophilicity with water [40]

Figure 3.3 Definition of contact angle γL is the liquid-air surface energy, γS is the solid-air surface energy, and

γLS is the liquid-solid surface energy

Thirteen samples (10 mm×10 mm) from the same batch of as-received ITO substrate were prepared for contact angle measurement These samples had the same surface roughness as measured by AFM before exposure to RIE One piece was used as a reference sample The contact angle was determined by placing a drop of water (0.5 µL) onto the surface and focusing the video camera until the contact angle image was sharp as shown below (Fig 3.4) The contact angle was obtained from the software by measuring the tangent angle that the liquid hemisphere makes with the surface

Figure 3.4 Frame capture showing how to determine contact angle (Rame-hart INC,)

500µm

3.2.4 Sheet resistance measurement

The most commonly used method for measuring sheet resistance RS is the four-point probe technique (Fig 3.5) When the probes are placed on a material of semi-infinite extent, the resistance is given by [41]:

1)(

11

1

2

3 2 2

1 2

D I

V

+

−+

−+

In the measurement, the distance between adjacent two probes is usually kept at a constant value of d, or D1 = D2 = D3 = d, this gives:

V R

as infinite and Eq (3.3) could be used to determine sheet resistance RS

3.2.5 X-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy (XPS) is an electron spectroscopic method that uses X-rays to eject electrons from inner-shell orbital and analyses the kinetic energies of those electrons The kinetic energy, E , of these photoelectrons is determined by the

energy of the X-ray radiation (hν ), the electron binding energy(Eb) and the spectrometer work function (φ) as given by [42]:

Ek = hν - Eb -φ (3.4)

Figure 3.6 X-ray photoelectron process

The XPS technique is highly surface specific due to the short mean free path of the photoelectrons that are emitted in the solid The binding energy of the peaks is characteristic of each element The peak areas can be used to determine the atomic composition of the surface The shape of each peak and its binding energy can be slightly altered by the chemical state of the emitting atom Hence XPS can provide chemical bonding information as well XPS is not sensitive to hydrogen or helium, but can detect all other elements A schematic diagram of the photoemission is shown in Fig 3.6

3.3 Encapsulation films

3.3.1 Parylene film deposition

Deposition of thin films of parylene and its derivatives is a process that has been used for over 35 years to form protective and insulating coatings on electronic and other devices and to generate extremely thin, pinhole-free films [43]

The process consisted of three distinct steps [44] as shown in Fig 3.7 Parylene powders were put into vacuum chamber (~10-2 Torr) and chamber was heated up to

700oC to decompose the monomer The vapor was then cooled down in the deposition chamber to form a parylene film layer

C)

Sample (25oC)

The thickness of the deposited film was proportional to the amount of parylene used and the deposition time The thickness of the parylene film could be controlled using a pre-weighed amount Normally, about 1.2 g parylene powder could yield a 1 µm thick film The process time was about two and half hours

Parylene could be used as a first encapsulation and insulation layer This increased the process window when transferring the devices to the next encapsulation step

3.3.2 Spin-on-glass planarization

Spin-on-glass (SOG) material was a polysiloxane-based spin-on dielectric material, which was useful in planarization of surface topography [45] Also it had also been applied as a permanent dielectric layer In passive matrix display made with laser ablation process, SOG spin-coating process was used to planarize the post beam ablation ridgelines

SOG liquid was spin-coated at 2000 rpm speed and bake in 100oC for 1 min to give a dielectric layer of 1 µm thickness

3.3.3 Visual inspection of calcium degradation to monitor quality of encapsulation

In order to investigate the effect of surface roughness on the quality of the encapsulation film, visual inspection under the microscope was used to monitor the degradation of calcium covered by the encapsulation film Accelerated testing was done using a humidity chamber

A 2000 Å calcium film was deposited onto different kinds of surface with different surface roughness A 100 Å LiF transparent film was coated on top of the calcium film -

to protect it from degradation during sample transfer A freshly deposited undegraded calcium film had a mirror outlook A SiNx film encapsulated the calcium and was transparent to allow visual observation of calcium corrosion

The humidity chamber (WEISS WK1) was set to 60oC and 85% RH The calcium test samples were loaded into it The samples were removed at fixed time intervals for visual inspection and image captured with a digital camera

Chapter 4 Results & Discussion (I):

Reactive ion etched ITO surface for better

OLED performance and display encapsulation

4.1 Results

Table 4.1 shows the contact angle with water (θw) and AFM surface root square roughness (Rrms) for different surface treatments It is noted that oxygen RIE treatment decreases the θw values compared with the as-received sample This indicates that the treated surfaces are highly hydrophilic (Fig 4.1)

mean-AFM results of various treated ITO surfaces in comparison with as-received ITO (substrate 1) measures the improvement in surface roughness The most dramatic change

in surface morphology is found for 200 W and an oxygen flow of 60 sccm oxygen RIE treatment The surface roughness decreases within a short treatment time but increases after 5 min, even higher than the as-received ITO sample From Table 4.1, it is noted that

100 W oxygen RIE process with an oxygen flow of 60 sccm yields an ITO surface with a low surface roughness of 0.4 nm Further increases in process time damages the surface

of ITO as a result of prolonged oxygen ion bombardment

Table 4.1 Summary of the H 2 O contact angle and AFM root-mean-square roughness for ITO glass substrates resulting from different oxygen RIE treatments

Substrate RIE time(min) Contact angle ( o )

(±1 o )

surface roughness (nm) (±10%)

Flow: 60 sccm; Power: 200 W; Process pressure: 430 mbar

Flow: 20 sccm; Power: 200W; Process pressure: 250 mbar

Flow: 20 sccm; Power: 100 W; Process pressure: 250 mbar

The contact angle is very important for the wetting of ITO by water-based polymer, especially for the inkjet printing of the water-based PEDOT hole transport layer For

the contact angle is too large, the flow of polymer solution on the surface is not uniform, resulting in imperfect coverage

The quality of the encapsulation film deposited may be influenced by the underlying surface roughness of the ITO film When deposited onto a rough substrate, the encapsulation film has a high density of defects and pinholes, which are possible water vapor and oxygen permeation pathways into OLED devices This will be further discussed in Sections 4.1.7 and 6.1

4.1.1 Surface roughness

Root mean square roughness (Rrms) of the ITO films was obtained from AFM measurements over an area of 2 ×2 mµ 2 as shown in Table 4.1 The as-received ITO was commercial ITO with Rrms of 1.5 nm During RIE treatment, the chamber pressure was adjusted through the oxygen flow rate, with 430 mbar at 60 sccm, and 250 mbar at

20 sccm respectively Another significant parameter controlling the process was process power It was found that at low RIE power (100 W), oxygen RIE treatment had significant planarization effect on the surface roughness of ITO with increasing time But the planarization effect may be reversed at higher power (200 W) particularly for long times From Table 4.1, the surface roughness decreased initially (1-2 min) and increased again if exposed at high power RIE (200 W) for longer time (5 min) with an oxygen flow

of 60 sccm

4.1.2 Sheet resistance

Four point probe method was used to measure the sheet resistance of ITO film ITO samples were commercially obtained and have a 120 nm coating on 0.5 mm thick glass The sample size was 5 cm × 5cm 3 tests for each sample around the central part was obtained and the result is shown in Table 4.1 From Table 4.1, it is clear that after RIE process, the sheet resistance is slightly increased by 2%-4% This increase is not considered to be technologically significant

Table 4.2 Sheet resistance of ITO for RIE process (oxygen flow: 60 sccm; power: 100 W; process pressure: 430 mbar; ITO thickness: 120 nm) Results are given in ohms per square (Ω/□)

RIE time(min)

Sheet resistance 1 (Ω/□) (±0.1)

Sheet resistance 2 (Ω/□) (±0.1)

Sheet resistance 3 (Ω/□) (±0.1)

Average (Ω/□) (±0.1)

RIE process Time (min)

Figure 4.1 Contact angle measurement of ITO sample for oxygen RIE treatment

As-received ITO surface was hydrophobic with high contact angle to DI water (75o) Regardless of RIE process time and pressure, the oxygen RIE process changed the surface energy of ITO film and turned it to a hydrophilic surface The contact angle could

be reduced to as low as 7o (RIE conditions: flow: 20 sccm; pressure: 250 mbar; power:

100 W; RIE time: 1 min) At RIE process of 200 W, the contact angle increased slightly with RIE process time The surface of ITO after oxygen RIE was highly active so that it changed from hydrophilic to hydrophobic quickly when exposed to air

4.1.4 X-ray photoelectron spectroscopy (XPS) study of ITO surface

XPS was used to understand the key chemical changes occurring on the ITO surface during RIE treatment (Fig 4.2) All the calculations were based on O 1s, In 3d5/2 and Sn 3d5/2 core-level spectra The atomic ratios were obtained for an electron take-off angle of

90o, and corrected with empirical sensitivity factors The surface stoichiometry of ITO of the surface was considerably different before and after RIE treatment The oxygen atomic concentration ratio increased after RIE treatment The oxygen atomic ratio concentration (O+In+Sn=100%) increased from 34% to 57% At the same time indium concentration decreased from 60% to 38% and tin from 6% to 3% This suggested that the surfaces studied were oxidized after the O2 RIE treatment

Figure 4.2 XPS atomic concentration for O, In and Sn in ITO samples after oxygen RIE treatment (flow: 60 sccm;