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Electrohydrodynamic Inkjet Printing of OLED

Pixel Patterns with High Resolutions

Gyeomuk Kim

Chemical engineering program

Graduate school of UNIST

2013

1



Gyeomuk Kim

Chemical engineering program

Graduate school of UNIST

2



A thesis

Submitted to the graduate school of UNIST

in partial fulfillment of the

requirements for the degree of

master of science

Gyeomuk Kim

05. 31. 2013

Approved by

Major advisor

Jang-Ung Park

3



Gyeomuk Kim

This certifies that the thesis of Gyeomuk Kim is approved

05. 30. 2013 of submission

Signature

Jang-Ung Park

Signature

Myoung Hoon Song

Signature

Chang Young Lee

4



Abstract

We fabricated the OLED devices with high-resolution pixels with the minimum dimension of 5

30 μm by electrohydrodynamic inkjet (e-jet) printing technique combined to the standard micro-

fabrication method. An organic small molecule, 2-(t-butyl)-9, 10-bis (20-naphthyl) anthracene

(TBADN) was printed as the emitting layer with controlled thicknesses and dimensions. The

advantage of e-jet, the possibility of ejection of a jet with nanoscales makes possible to print the

solution-based functional material in the smallest pixel which other printing methods are difficult due

to their modest printing resolutions. This high-resolution printing capability can resolve the current

limitation on reducing pixel dimensions of the vacuum-processed OLED displays, and the low-cost,

solution-processibility is advantageous for macroelectronics, including large-area displays.

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Contents

Ⅰ. Introduction ------------------------------------------------------------------------------------------1

Ⅱ. Theoretical Background --------------------------------------------------------------------------------- 3

2.1 Absorption and emission ----------------------------------------------------------------------- 4

2.2 Energy transfer ----------------------------------------------------------------------------------------- 4

2.1.1 Förster transfer ------------------------------------------------------------------------ 4

2.1.2 Dexter transfer ------------------------------------------------------------------------------- 5

2.3 Charge transfer ---------------------------------------------------------------------------------- 7

2.4 Electroluminescence and charge injection ------------------------------------------------------- 7

2.5 Structures of OLED ----------------------------------------------------------------------------- 7

2.5.1 Anode materials ----------------------------------------------------------------------------- 8

2.5.2 Cathode materials --------------------------------------------------------------------- 9

2.5.3 Electron and hole transport materials ------------------------------------------------- 9

2.6 Active layer deposition methods -------------------------------------------------------------- 9

2.6.1 Thermal evaporation with shadow mask --------------------------------------------- 9

2.6.2 Spin coating --------------------------------------------------------------------------- 10

2.6.3 Laser induced transfer ------------------------------------------------------------------- 11

2.6.4 Ink jet printing ----------------------------------------------------------------------- 12

2.7 E-jet printing technique ---------------------------------------------------------------------------- 13

2.7.1 Printing system ----------------------------------------------------------------------- 15

7

2.8 Fluid Dynamics and Solidification --------------------------------------------------------------- 17

2.8.1 Fluid Dynamics ----------------------------------------------------------------------------- 18

2.8.2 Solidification --------------------------------------------------------------------------- 20

Ⅲ. Experimental Details --------------------------------------------------------------------------------- 24

3.1 Fabrication details --------------------------------------------------------------------------- 24

3.1.1 Fabrication process of the nozzles ------------------------------------------------------- 24

3.1.2 Fabrication process of the spin-coated sample ---------------------------------------- 26

3.1.3 Fabrication process of the printed sample --------------------------------------------- 27

3.2 E-jet set-up for OLED printing -------------------------------------------------------------- 27

Ⅳ. Results ---------------------------------------------------------------------------------------------------- 29

Ⅴ. Conclusion ------------------------------------------------------------------------------------------ 55

Ⅵ. Reference ------------------------------------------------------------------------------------------------ 56

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List of Figures

Figure 1 The structure of 2-(t-butyl)-9,10-bis (20-naphthyl) anthracene (TBADN)

Figure 2 Försters transfer.

Figure 3 Dexter transfer.

Figure 4 Structure of OLED devices.

Figure 5 Patterning of the emissive layer using a shadow mask.

Figure 6 The scheme summarizes the LIFT process for tri-color OLED pixels.

Figure 7 The schematic image of ink-jet printing for OLED device. Figure 8 Schematic image of thermal inkjet

Figure 9 Schematic image of piezoelectric inkjet.

Figure 10 the forces related to the e jet printing.

Figure 11 Illustration of the change in the ink meniscus due to an increase in electric field.

Figure12 Time-lapse images of electrohydrodynamic cone-jet transition and a printed droplet on a

substrate.

Figure 13 Fluid dynamics of drop impact

Figure 14 Optical micro scope image of coffee ring effect.

Figure 15 Confocal microscope images of deposit patterns composed of the silica microspheres

produced by ink-jet printing a single ink-jet droplet of water/FA-based ink. The ink concentrations

varied from 0.5 to 4 vol %. The substrate was a hydrophilic Si wafer.

Figure 16 Schematic cross-section view of the droplet profiles for three different ink compositions to

compare the different stages of droplet evolution. The contact angles are exaggerated to show

different angle values associated with each droplet. The non-uniform evaporation flux distribution

along the droplet surface is represented by the length of the blue arrows above the droplets. The

number of the black arrows drawn inside the evaporating droplets represents the relative magnitude of

the hydrodynamic flows.

Figure 17 Fabrication process of structured substrate

Figure 18 OLED device structures with PDL and without PDL

Figure 19 Schematic image of e-jet set-up for large OLED substrate

Figure 20 Line and grid pattern of anthracene ink.

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Figure 21 pixel patterns of anthracene ink on Si wafer a. optical microscope image b. fluorescence

microscope image.

Figure 22 Schematic image of printing sequence for making film - like pixel pattern

Figure 23 AFM data of anthracene ink printing a. 3D image of line printing using an anthracene ink b.

AFM image of pixel printing using an antheracene ink.

Figure 24 Complex pattern of anthracene ink. a. original JPG file image b. fluorescence microscope

image of anthraene ink printing c. optical microscope image of anthracene ink printing. 2μm of inner

diameter of the nozzle was used and printing velocity is 350μm/s and original ink was used.

Figure 25 The Characteristics of spin-coated OLED devices. a. J-V curve b. Luminance curve. c.

photo image of emitting OLED device. The thickness of dielectric layer and active layer are 200nm

and 130nm respectively. The dielectric layer has patterns which size is 20х30μm.

Figure 26 The schematic image of OLED printing.

Figure 27 5 x 5 size pixel printing of anthracene ink. a. optical microscope image b. fluorescence

microscope image. 2μm of inner diameter of the nozzle was used. Anthracene ink was diluted with

oDCB solvent and dilution ratio of anthracene : solvent is 1:5 (wt%).

Figure 28 AFM image of 5x5 size patterns. a The surface of patterned dielectric layer, b the surface

of patterned dielectric layer (a) spin-coated with PEDOT/PSS and c the surface of printed active layer

on b.

Figure 29 The thickness changes dependent on each process black line dielectric layer patterning, red

line hole injection layer spin coating blue line active layer printing.

Figure 30 The graph of thickness control of anthracene ink printing.

Figure 31 SEM cross-section image of OLED device.

Figure 32 Optical microscope images of the process of each fabrication step; a. PDL structure, b.

PEDOT/PSS spin coating, OLED ink printing, d. Lif/ Al deposition.

Figure 33 Optical microscope images of two different types of deposition technique. a spin-

coated 20х30 pixels b. e-jet printed 20х30 pixels.

Figure 34 Optical microscope images of two different types of deposition technique. a spin-coated 5

х 5 pixels b. e-jet printed 5 х 5 pixels.

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wavelength d. photo image of emitting OLED device. The thickness of dielectric layer and active

layer are 200 nm and 100 nm respectively. The dielectric layer has patterns which size is 20 x 30μm.





photo image of emitting OLED device. The thickness of dielectric layer and active layer are 200nm

and 100nm respectively. The dielectric layer has patterns which size is 5x30μm.

Figure 38 the efficiency of OLED devices dependent on size difference of pixels

Figure 39 Optical microscope image of OLED light emission

1

Ⅰ. Introduction

After observation of organic electroluminescence (EL) in the 1960s, a novel electroluminescent

device was constructed using organic materials as the emitting elements by Tang et.al. in 1987(1).

The diode has a double-layer structure of organic thin films, prepared by vapor deposition. This

method is still useful for low molecular weight (MW) materials due to the simplicity of thickness

control and the achievement of uniformity without pinholes(2). Since then, according to the

development of polymer organic light-emitting diode (OLED) materials, fabrication methods such as

roll-to-roll printing process(3), spin coating(4, 5), the photolithographic method, laser-induced

thermal imaging (6, 7), and ink-jet printing(8, 9) have been investigated(3- 11).

In those fabrication methods in OLED, significant parameters include patterning resolution,

uniformity, material utilization yield, and fabrication costs. And each method has a few problems

respectively. In the case of the thermal-evaporation method, a temperature may cause thermal

deformation of the stencil mask and substrate. This technique also shows low material yield and the

requirement of high-vacuum systems which may cause low cost efficiency. Moreover, this method

cannot achieve high resolution because of using stencil mask. In the fabrication methods for

polymeric material, spin coating can achieve high resolution. However, these deposition methods

needs a former structured substrate and may lead to a ring stain (or coffee stain) effect through

nonlinear evaporation, which makes it difficult to achieve high uniformity (12). In order to make a

uniform film, the spin casting method was proposed by Steve Blair et. al. in 2011(13). They

succeeded in making a uniform film which has 10μm sized pixel patterns. However, spin coating

method does not have advantages such as low fabrication cost, or high material-utilization yield as

liquid based methods used to have. Ink-jet printing method has the same difficulties to achieve a high

resolution and to remove ring stain effects.

The electrohydrodynamic (EHD) jet, so called e-jet, which is a one of the technique of ink jet, is a

method to fabricate thin film consisting of nanoparticle directly from solution on various substrates.

With this technique, we can print various materials and express complex patterns. Depending on the

nozzle size, printing speed and applied voltage, thickness control is simply available.

This method can achieve extremely high resolution due to the possibility of ejecting submicron sized

droplet using electrostatic force (14). The advantages of this method are that printing is performed

under atmospheric pressure and room temperature, which can make huge sized plastic substrates

possible to be used. Even though Yanchun Han et al. in 2011 showed the patterning of pin hole free

film by blending solvents which have high boiling temperature and high viscosity, and by controlling

surface energy, this method doesn’t require these processes to get rid of ring-stain effects(15), due to

2

deposition of almost dried nanoparticles by the electrostatic force. These nanoparticle-based printing

methods using EHD is a promising candidate to deal with the problems of the OLED fabrication

process, such as material damage, yield, thickness uniformity and fabrication cost.

In this paper, we demonstrated a pixel patterning method using e-jet technique. OLED structures is

based upon small MW material, 2-(t-butyl)-9,10-bis (20-naphthyl) anthracene (TBADN) producing

blue emission. A glass coated with ITO, was used as substrate and SiO2 was used as dielectric layer.

After printing, we measured their characteristics and compared with the data of spin coating method.

Finally, this technique is able to achieve an extremely high resolutio

3

Ⅱ. Theoretical background.

In 1990, the paper pertaining to conjugated polymers for light-emitting devices was published by J.

H. Burroughes et al. at Cambridge University (3). In 1991, the polymers which have a high

conductivity were reported by Alan Heeger and he presented a conclusive theory. For this research, he

was awarded the Nobel Prize in chemistry in 2000 (18, 19).

In the case of normal polymer which has bonds formed in the sp3-hybridization, all electrons are

localized in the states linked with the σ-orbitals and all these states are filled. Therefore, no electron is

free to move. However, considering a bond formed in the sp2-hybridization, the overlap of the pz

orbitals forms a π-bond which extends perpendicular to the σ–bond.

The interaction of orbitals causes a symmetric and asymmetric state (π and π*). The symmetric state

of the orbitals is the highest occupied molecular orbital (HOMO) of the structure. In a molecule in

ground state, all electrons are localized in states linked with the σ and π-orbitals. If the two atoms are

bound in the π* state, the geometrical configuration of the molecule can be changed due to the orbitals

extending out of the plane of the molecule so the orbital can interact with another π-orbitals on a

different molecules and electrical transport becomes possible, causing a high electrical conductivity.

If the polymer chain contains π-bonds, the electrical transport occurs along the polymer chain or

between different strands.

In a non-excited molecule which contains double bond, all the low energy states are occupied. So the

highest occupied molecular orbital (HOMO) is a π-orbital, acting as a valance band of semiconducting

material and the electron from the HOMO can be promoted to the lowest unoccupied molecular

orbital (LUMO), acting as a conduction band of semiconducting material when an energy gap Eg

between two energy state is overcome.

4

Figure 1 The structure of 2-(t-butyl)-9,10-bis (20-naphthyl) anthracene (TBADN)

The polymer which we used is an anthracene. The structure of anthracene is shown in figure 1. The

anthracene is a conjugated polymer whose backbone consists of alternating single and double bonds.

2.1 Absorption and Emission

A molecule can absorb energy by excitation of electron in ground state. It can occur by photon or a

phonon, by dipole-dipole interactions with neighboring molecules or by injecting electrical carriers

and when the excited electron relaxes to the ground state and recombine with a hole, the molecule can

emit energy as a photon or a phonon. An exciton is a state of an electron and a hole pair, considered as

quasi-particles which has capability of diffusion.

A polymer has small orbital overlap in polymeric system so the excitons are localized in their radius

which is less than 5 Å.

If the emission is occurred by absorbed photon energy, the process is called photoluminescence (PL);

emission occurred by injected charge is called electroluminescence.

5

2.1.1 Photoluminescence and Franck-Condon-Shift

Photon energy can promote the electron in ground state to the corresponding vibronic sub-level of

excited state (S1) by absorption of the photon energy. The nuclei are still located in unchanged

position due to the short time of this process. After promotion of electron, the electron relaxes down

to the lowest vibronic sub-level of the excited state as the nuclei are rearranged. For this

rearrangement, the electron loses the part of the absorbed energy. The energy shift between the

absorbed and emitted photon occurs referred as the Franck-Condon-Shift. The vibrational structure of

the ground state is analogous to the structure of excited state. When photon is absorbed, in most case,

each peak in the absorption spectrum is at the transit position of the certain vibronic state

corresponding to the absorption spectrum. These absorption and emission of the photon are reversible

process.

2.2 Energy transfer

Excited molecules can transfer their energy and their electrical charges to the molecule which linked

with orbitals

2.2.1 Förster transfer

Förster transfer is induced by the resonance coupling event of the electrical dipoles from donor to

acceptor. Because of its electrostatic (Coulombic) properties, energy migration can happen relatively

long distances (30 to 100 Å ).

A spin in ground state has to be opposite direction of a spin in excited state. Therefore an electron in

excited state forms a singlet exciton with a hole in ground state and the electron can relax from the

excited state. For this reason, only singlet excitons possibly contribute to this transfer. This transfer is

relatively fast compared with Dexter transfer.

In order to get a high efficiency of Förster transfer, the emission spectrum of the donor and the

absorption spectrum of the acceptor have to overlap sufficiently. (20)

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Figure 2 Försters transfer

2.2.2 Dexter transfer

Comparing with Förster transfer, dexter transfer is based on only the carrier transport induced by an

external electric field. Therefore the transfer rate is relative to the overlap of molecular orbitals which

means that Dexter transfer occurs in only short distance(~10 Å). For conservation of spin, only

singlet-singlet or triplet-triplet transfer is possible.

Figure 3 Dexter transfer

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Likewise Förster transfer, the emission spectrum of the donor and the absorption spectrum of the

acceptor also have to be overlapped sufficiently to achieve high efficiency. (21)

2.3 Charge transfer

The electrical conductivity is based on the carrier transport which means, in the case of polymers,

the charge carrier move from one molecular orbital to another. Therefore the molecule has to be

overlapped to another and the transfer probability is affected by the square of the overlap integral of

the electronic wave function of these orbitals. If the materials has large energy gap between two

molecular orbitals, the carrier would be trapped in the lower energy state. In the case of the energy

gap between the states on the order of KT, the carrier has enough energy to overcome the energy gap

and be transferred from one molecule to the neighboring molecule.

Sufficient overlap between orbitals can successively lead to transfer electrical charge.

2.4 Electroluminescence and charge injection

The most important mechanism of electroluminescence is that the excitation is occurred by injection

of electric charge. If an electron is injected into polymers by external electric field, the electron is

supposed to occupy LUMO state because all HOMO states are occupied. However, in this case, no

emission can occur because each state of HOMO level is occupied. Therefore, no excited electron can

relax to the ground state. In order to obtain emission, not only electron but also hole has to be injected

at the same time. The electron and hole injected by external electric field travel through the material

in opposite direction and form an exciton in emitting material by Coulombic interation. Then, the

excition can be radiatively decayed to the molecular ground state and emission of photon occurs. In

this case, the spin direction of electron in excited state has to be opposite of hole in ground state and

same number of electron and hole has to be injected. (22)

2.5 Structures of OLED

The device structure of an OLED is surprisingly simple (figure 4). The conducting polymer formed

as a film is located between two electrodes. If an electric field is applied across the electrodes,

electron and hole is injected into the states of the LUMO (lowest unoccupied molecular orbital) and

HOMO (highest occupied molecular orbital) respectively. Light emission from the organic materials

occurs when the electron relaxes from the excited state to the ground state and emits of a photon.

8

Single-layer devices tend to produce unbalanced charge injection and lower efficiency. To prevent

this and to increase the performance of emission, a charge transport or injection layer (for either holes

or injection or both) can be located in between emission layer and electrode. This may lower the

barrier at the electrode which can help the carrier not to be trapped in between layers and block the

transmission of the majority carrier, creating a space charge that reduces its current and balances the

charge transport under the suitable choice of orbital offsets between layers. (22) At least, one

electrode should be transparent for photon to escape from the device.

Figure 4 Structure of OLED devices

2.5.1 Anode Materials

For the bottom emission devices, Anode has to be transparent so the indium-tin- oxide (ITO) is

commonly used as hole injecting contact. ITO is transparent, conductive and high work function

ranged from 4.5 to 5eV. Indium oxide (In2O3) is semiconducting material which has wide band-gap.

However, to be used as an electrode, the conductivity is modified by doping with tin. In2O3 and SnO2

are deposited by sputter onto transparent substrates such as glass in a molar ratio of 92% : 9% to

obtain films. Preparation and cleaning processes is imperative due to the non-stoichiometric

characteristics causing electronic properties differences. Before deposition of hole injection layer, An

oxygen plasma treatment is implemented to increase the work function by as much as 0.5 eV.

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2.5.2 Cathode Materials

Material used for cathode should have good electron- injecting properties like alkali metals which

has a low work function. However those kinds of metals can be easily degraded by reacting with

either emitting material or with the surrounding atmosphere. In order to increase the device stability,

alloys containing these metals in a matrix of more stable metals like silver and aluminum are used.

2.5.3 Electron and Hole transport materials

After an oxygen plasma treatment, a buffer layer has been widely used to improve HI and to

decrease the injection barrier. The material mostly widely used as a buffer layer is Poly(3,4-ethylene-

dioxythiophene): poly(styrene sulfonic acid) (PEDOT:PSS) in an aqueous solution. However, post-

baking process is required to remove water in the film. Poly(arylene ether sulfone)-containing

tetraphenylbenzidine doped with 10% of tri(4-bromophenyl ammonium hexachloroantimonate)

(TPDPES: TBPAH) as a HI layer is reported that it is soluble in organic solvent with low boiling

point (23) so the baking process is not necessary after deposition and also the materials, ClCH2CH2Cl,

the use of TPDPES: TBPAH is not required post baking, bring more convenient for the device

fabrication. Copper phthalocyanine (CuPc) (24) has also been used as buffer layer, but it has a low

solubility in conventional organic solvents thus needs thermal evaporation.

Generally, the hole has a much higher mobility than the electron has in organic materials and thus

there are far more holes than electrons in OLEDs. To increase the efficiency of OLEDs, The

development of efficient ET materials is crucial. N -type doping, for example active metals Li, (31) Cs,

(32) and oxides like Cs2O (25) (or Cs2CO3 (26)), Al2O3, (27) TiOx, (28) MnO, (29) as well as

fluorides such as LiF (30) is effective to improve the EI and ET of the device.

2.6 Active layer deposition method

The most straightforward approach to form an active layer is direct deposition. However, the

efficiencies for commercialization are almost dependent on the method. In order to pattern pixels,

selective deposition has to be possible for red, blue and green color.

2.6.1 Thermal evaporation with shadow mask

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Figure 5 Patterning of the emissive layer using a shadow mask

Thermal evaporation is the simplest way to deposit emitting material on devices substrate. The

vaporized material which passes through shadow mask is patterned as the pattern the shadow mask

has. So the mask should contact closely with the substrate in order to reproduce accurately small size

which may cause mechanical damage of the devices. In addition the pattern size is limited to the

mask pattern for the stability of utensil mask. This method needs vacuum process and alignment in

the vacuum environment which causes high fabrication cost. After evaporation of first emitting color,

the shadow mask or substrate is shifted to the position where the second pixels are supposed to

deposit. This process is repeated until full colors are done. Buffer layer and cathode material can be

deposited directly after deposition of emissive layer without breaking the vacuum environment.

2.6.2 Spin coating

Emissive material can be deposited to substrate by spin coating method. Normally, polymers have

good properties to form uniform film. However, the material covers every area of substrate therefore

this method is only available for emitting a single color and relatively small size substrates. In

addition, in order to pattern pixels, pixel defining layer (PDL) is required to open channels.

11

2.6.3 Laser induced transfer

Figure 6 The scheme summarizes the LIFT process for tri-color OLED pixels

Laser-induced forward transfer (LIFT) as a direct- write method has been developed for OLED

devices. Transfer material like fused silica is coated on transparent donor substrate. Laser pulse

12

coming though the transparent substrate induces irradiation causing an evaporative forward ablation

of the target material. The acceptor substrate in the immediate vicinity of donor receives the material

to be transferred. However, polymer which is sensitive from heat can be damaged by high thermal

load induced by direct light absorption. In order to refrain from direct laser damaging, dynamic

release layer (DRL) is required for light- to - heat conversion. So the material for intermediate layer

has to be intrinsically high thermal load on the sensitive materials. (33)

Substrate of donor and acceptor are washed in common cleaning way and oxygen plasma is

implemented to the substrates. PEDOT/PSS is spin-coated on the receiver substrate which is pre-

patterned with ITO and donor substrate is spin-coated with triazene polymer (TP). The aluminum is

then evaporated on the TP layer. Buffer layer or emitting layer selectively deposited on each substrate.

For the LIFT experiment, single pulse laser is used (XeCl excimer laser λ=308nmτ=30nm ). The

shape of the laser become rectangular when it passes through a rectangular aperture and is focused

using an achromatic lens to expose uniform energy to donor substrate.

For the laser experiment, the donor and the receiver substrate are placed with a spacer between them

in a vacuum chamber. Photo triazene DRL absorbed the laser beam and get decomposed to gaseous

products which create pressure force on the overlying layers. This pressure force can transfer the layer

on donor substrate to the receiver substrate. (34)

2.6.4 Ink jet printing

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Figure 7 The schematic image of ink-jet printing for OLED device

Ink-jet printing provides an opportunity to use large-area substrate with full color displays of OLEDs

in high efficiency and low cost due to the possibility to deposit only desired area. Due to the simple

forming process and high performance of devices, small molecular organic light-emitting diodes

(SMOLEDs) in solution based process have increasingly gained attention. However, the drying

properties of drop are mainly concerned due to the non-linear evaporation rate leading to a non-

uniform emission pattern and ending up with early degradation of the devices. In addition liquid films

deposited on substrates can be dewetted from the surface of substrate. Therefore the ways to improve

the film forming properties such as patterned substrate localizing the dewetting chemically or

physically, are necessary. The use of pre-patterned bank or PDL as former mentioned in spin coating

method could help to confine the solution and help to form a uniform film. (35)

2.7 E-jet printing technique

E-jet is a kind of inkjet which can produce an image directly on to the substrate without impact. So

E-jet goes with the advantages which inkjet has.

Inkjet technique is cost effective due to its simple process compared with other patterning method.

Also, diverse substrate, not only rigid but also flexible substrate such as paper can be used size-

independently and the geometry of substrate has less effect for printing. Moreover, inkjet is the

technique using digital system which means that computational process is conveniently adoptable.

Commercially available ink jet, thermal / piezoelectric inkjet uses a pressure change in the ink

chamber. Figure 8 shows the schematic image of thermal inkjet. A heat source in an ink chamber is

controlled by short voltage pulse. When voltage is applied, the temperature on the surface of heater

becomes about 300ºC, then the fluid begins abruptly to boil and the bubble is formed and put the fluid

in motion. After disactivation of the heat source, bubble collapses. Figure 9 shows schematic image of

piezoelectric inkjet. Pressure difference is produced by the motion of the piezoelectric materials

which can change shape or volume in the electric field.

Even though these ink jet techniques have various advantages, they also have limitation of resolution

because of their relatively big size of droplet for the industrial needs. The resolution can be improved

by decreasing the size of the nozzle. However, as the size of the nozzle decreases, the forces, surface

tension and viscosity, opposed to the direction of the jet increases. Increasing chamber pressure is

required to overcome the increased forces and decreasing fluid resistance is incompatible with

14

decreasing nozzle size. (37-41)

Figure 8 Schematic image of thermal inkjet

15

Figure 9 Schematic image of piezoelectric inkjet

Electrohydrodynamic inkjet technique uses not thermal or acoustic energy but an electric field.

electrostatic force increased by applying voltage can make a smaller nozzle possible to be used. Thus

e-jet can eject far smaller droplet by using a smaller nozzle. The recent works showing capability of

ejection of smaller droplet can covet new application. (42-45)

2.7.1 Printing system

A grounded substrate or a conducting plates, is positioned on high resolution moving stage

below the nozzle which is coated with metal thin film to serve as an electrode to apply

16

electric field. An ink is supplied by pneumatic pressure regulator from a syringe to the nozzle

tip connected with the syringe. The nozzles with various sizes of internal diameters are used

in accordance with application and the applied voltage is varied depending on the nozzle size

and the distance between nozzle and substrate. Moving stage, voltage and pressure regulator

is controlled accurately by computers.

Compared with commercial inkjet which uses a cartridge, e-jet uses a sharp pointed nozzle which

can help to focus electric field on the nozzle tip to eject smaller droplet with low voltage. When

voltage is applied to the nozzle, the mobile charges in ink are accumulated at surface of the meniscus

of the ink at the nozzle tip and induced electrical stress which turns the hemispherical meniscus to

cone shape so called taylor cone (figure 11). If the supplied electric field sufficiently high thus the

electrical stress with gravity and applied pressure overcomes the net force of capillary pressure and

viscosity, a small droplet which contains charge is ejected to the substrate (figure 10). After losing

charges, the morphology of the cone-shaped meniscus is reverted back to the hemispherical geometry.

The sequence of charge accumulation, cone formation, ejection, and relaxation repeatedly occur by

applied voltage as shown in figure 12 then, the ink can be printed as a pulse type signals are applied.

17

Figure 10 the forces related to the e-jet printing. (46)

Figure 11 Illustration of the change in the ink meniscus due to an increase in electric field.

Figure 12 Time-lapse images of electrohydrodynamic cone-jet transition and a printed droplet on a

substrate. (47)

2.8 Fluid Dynamics and Solidification

The liquid drop leaving from the nozzle faces two steps which are Fluid dynamics and solidification.

Fluid dynamic phenomena contain spreading, splashing and bouncing on surface of the substrate and

some of them and coalescence can occur on liquid surfaces. After that, solvent is evaporated

(solidification) depending on the properties of the ink. These phenomena can affect uniformity of the

film forming on a substrate. (49, 50)

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2.8.1 Fluid dynamics

Figure 13 Fluid dynamics of drop impact

19

When the droplet traveling from the nozzle tip arrives on the surface of the substrates, the

phenomena for drop impact are divided into three kinds: spreading, splashing and bouncing. At lower

impact velocity, the process of spreading occurs because the kinetic energy of the droplet is small so

the spreading is the dominated force in a molecule. The process of spreading has two steps. Contact

line is formed at initial phase and a thin film is formed at impact phase. Before a droplet arrives on the

surface, a pressure wave has to put the air below the droplet. If the contact edge velocity is higher than

the pressure wave velocity and contact angle is not high like pined, the droplet can take in air bubble

which can cause a void or a defect inside a pattern. At impact phase, the fluid spreads very fast and

reaches a maximum radius. The surface energy and kinetic energy of the drop are dissipated by

viscous process in the thin sheet of fluid and transformed to additional surface energy.

At higher impact velocities, the jetting motion causes a disintegration of the droplet and splashing

can be shown.

20

Figure 14 Optical micro scope image of coffee ring effect. (16)

2.8.2 Solidification

When a drop is dried, various phenomena are shown such as coffee ring effect and periodic budging

which can lead to form a non-uniform film on a substrate depending on evaporation rate. Normally,

when the droplet arrives at the surface, the capillary flow is induced due to the non-linear evaporation

rate. This fluid flow to replenish the solvent evaporated in edge area carries solutes in solvent then the

solute is precipitated at the edge line seeming rim. (12, 16, 51) This effect is induced by temperature

gradient or concentration gradient. So this can be relieved by controlling non-linear evaporation rate

by mixing the solvent which has different vapor pressure and different surface tension or

21

manipulating the concentration of solute. Figure 15 demonstrates that a monolayer of Si sphere can be

achieved by controlling concentration. The inks concentration varied from 0.5 to 4 vol % were tested.

Figure 16 describes that uniformity can be improved by mixing solvent. In early stage on drying, the

solvent with higher evaporation rate is preferentially evaporated at the contact line and this causes

convective flow from center to the edge. If the solvent with higher evaporation rate and lower surface

tension is mixed, counterclockwise recirculation will occur as shown in figure 16c and this flow

prohibit to segregate the particles at the contact line and allow to form homogenous 2D colloidal film.

Figure 15 Confocal microscope images of deposit patterns composed of the silica microspheres

produced by ink-jet printing a single ink-jet droplet of water/FA-based ink. The ink concentrations

varied from 0.5 to 4 vol %. The substrate was a hydrophilic Si wafer. (16)

22

Figure 16 Schematic cross-section view of the droplet profiles for three different ink compositions

to compare the different stages of droplet evolution. The contact angles are exaggerated to show

different angle values associated with each droplet. The non-uniform evaporation flux distribution

along the droplet surface is represented by the length of the blue arrows above the droplets. The

number of the black arrows drawn inside the evaporating droplets represents the relative magnitude of

the hydrodynamic flows. (16)

Periodic bulging occurs if line pattern is printed on low surface energy, or high contact angle

substrates. The fluid drops such as printed by drop on demand method on the substrate can build the

periodic structure due to hydrodynamic instability.

23

Fast evaporation rate also can affect to form a uniform film. When droplets are printed continuously

on a substrate and the evaporation rate of the solvent is high, next droplet is fallen on dried droplet

printed formerly. The mixing between dried and fluid drop is easily broken. All these effects lead to

the uniformity of the film formation.

24

Ⅲ. Experimental

Used materials: ITO glass (glass slide with indium-tin-oxide) purchased from AMG Co., Ltd, and

Photoresist purchased from Shipley Co., Ltd were used for preparing substrates. Ortho

dichlorobenzene (Sigma- Aldrich) was used for diluting an OLED ink. The hole injection materials of

poly (styrene sulfonic acid)-doped poly (3,4-ethylenedioxy-thiophene) (PEDOT, Clevios P

VP.AI4083) were purchased from GmbH & Co. KG.

3.1 Fabrication details

3.1.1 Fabrication process of nozzles

To fabricate the nozzles for e-jet printing experiment, glass capillary is purchased from World

Precision Instruments. A glass capillary is set up on the pulling machine which can pull both ends of it

in same rate during heating at the middle of the capillary where a filament is. Dependent on the

pulling rate and force, the inner diameter of the nozzle is varied. Pulling without heat can break the

middle of the capillary where its diameter is tapered. Then, the nozzle is bonded to a lure which can

be connected to a syringe. After that metal such as copper (100nm) is deposited on them to function as

an electrode to apply electric field and chromium (10nm) is also deposited on them to increase

adhesion between the surface of the nozzle and copper film. A metal evaporator is used to deposit

metals. To avoid nozzle wetting with an inks, hydrophobic treatment is implemented to the tip of the

nozzle. A nozzle is dipped in the 1H, 1H, 2H, 2H-perfluorodecane-1-thiol solution which is diluted

with DMF for 5 minutes and then is rinsed with DMF solvent for 2 minutes. After rinsing, the DMF

left in the nozzle is removed by blowing air into the nozzle.

25

Figure 17 Fabrication process of structured substrate

26

3.1.2 Fabrication process of the spin-coated sample

In order to fabricate the spin-coated sample, ITO coated glass (thickness 200nm, average resistance

~20Ω/square) was used as a substrate. The fabrication process is shown in figure 17. ITO glass is

washed with detergent and then is rinsed by water, acetone and IPA with 20 minute of sonication in

each process and the cleaned substrate is dried in oven perfectly. 200 nm thick SiO2 was deposited on

the substrate using sputter as a dielectric layer and then pixel arrays ranged from 30 to 5μm size were

patterned using photolithography. After patterning, a reactive ion etching (RIE) was used for SiO2

etching. Then, the photoresist (PR) on the substrate was removed. PEDOT/PSS was filtered, using a

450 nm pore size hydrophilic filter and was spin-coated (4000RPM for 30 second) on the substrate

which was exposed to UVO plasma before spin coating. Then, the substrate was baked at 140 ºC for

10 minute on a hot plate. After perfect dying of the hole injection layer, an active layer was spin-

coated on the PEDOT/PSS film. We used the 2-(t-butyl)-9, 10-bis (20-naphthyl) anthracene (TBADN;

commercial OLED materials) as an active layer. Finally, a 1nm thickness of Lif and a 150nm

thickness of Al were deposited on the substrate using a thermal evaporator.

27

Figure 18 OLED device structures with PDL and without PDL

3.1.3 Fabrication process of the printed sample

To fabricate the printing sample, ITO coated glass (AMG, thickness 200nm, average resistance

~20Ωn was used as a substrate. 200nm thick SiO2 was deposited on the substrate using sputter as a

dielectric layer and then then pixel arrays ranged from 30 to 5μm size were patterned using

photolithography. After patterning, a reactive ion etching (RIE) was used for SiO2 etching. Then, the

photoresist (PR) on the substrate was removed. PEDOT/PSS was also coated on the printing

substrate in the same manner of the spin-coated sample. After perfect dying of the hole injection

layer, the active layer was printed using e-jet printing technique. Finally, electron injection layer and

cathode metal were deposited in same manner of spin-coated sample

3.2 E-jet setup for OLED printing

A high-resolution electrohydrodynamic inkjet (e-jet) system was used to print an emitting material

ink. The system was set up by our own as fore-mentioned in e-jet setup. The ITO surface of the

substrate is grounded with Cu tapes while the voltage was applied to the nozzle for printing. The inner

diameter of the nozzle used was 2μm. Figure 19 shows the schematic image of e-jet setup for OLED

printing on large substrate. During the printing, from 0 to 0.2psi of pressure was applied as voltage

which ranged from 220V to 360V was applied. The distance between nozzle and substrate was fixed

at 15μm.

28

Figure 19 Schematic image of e-jet set-up for large OLED substrate

29

Ⅳ. Results and discussion

First, we printed line patterns and grid patterns on a silicon wafer to learn how it can be printed. An

original ink and 2μm size of nozzle were used for printing. As shown in figure 20, smooth edge of a

printed pattern was achieved. Moreover, no remarkable edge change was found even on intersection

regions of two lines which is the important point to make a uniform film.

Figure 20 Line and grid pattern of anthracene ink.

30

Figure 21 Pixel patterns of anthracene ink on Si wafer a. optical microscope image b. fluorescence

microscope image.

31

Figure 22 Schematic image of printing sequence for making film - like pixel pattern

In order to print film-like patterns, printed lines are supposed to be overlapped with the part of

neighboring printed line and next printing or electric field should not affect former printed line. The

narrowest line width we could print is near 1μm shown in AFM data of figure 23 a. 1μm size nozzle

and ink diluted 1:20 wt% were used. In figure 21, 2μm nozzle was used for printing the patterns. If we

use 2μm size nozzle, we can print a near 2μm-width line. Therefore we set the space between the lines

as 1μm during printing. With overlapped-near 1μm line printings, 30ⅹ100 size pixel patterns were

easily made as shown in figure 21 a and b. Printing sequence is demonstrated in figure 22. The

moving stage has a motion along the red line then the overlapped drops become lines and surfaces.

The AFM data of figure 23b support that there are no remarkable problem to imitate a current pixel

size. We also print a complex pattern with anthreacene ink. Figure 24c shows optical microscope

image of printed anthracene ink describing a face and figure 24b shows fluorescence microscope

image of the printed anthracene ink and figure 24a is original JPG file of same image. We used an

original ink without dilution through a nozzle whose inner diameter was 2μm and printing velocity

was 300μm/s.

32

Figure 23 AFM data of anthracene ink printing a. 3D image of line printing using an anthracene

ink b. AFM image of pixel printing using an antheracene ink.

33

Figure 24 Complex pattern of anthracene ink. a. original JPG file image b. fluorescence

microscope image of anthraene ink printing c. optical microscope image of anthracene ink printing.

2μm of inner diameter of the nozzle was used and printing velocity is 300 μm/s and original ink was

used.

Before using e-jet printing system, we fabricated OLED devices following common structure to

understand the characteristics of anthracene. A 1.1mm thick glass plate was used as a substrate. ITO

and Aluminum were used as an anode and a cathode electrode respectively. PEDOT/PSS was spin-

coated on ITO electrode as a hole injection layer. Anthracene ink was spin-coated as an emitting layer

after hole injection layer’s coating. A Lif as an electron injection layer and an aluminum were

deposited on an emitting layer using Thermal evaporator.

The spin coating method was also implemented to a structured substrate. To pattern the structure of

substrate, standard micro-fabrication technique was used. 200 nm thick SiO2 layer was deposited on

the ITO glass substrate and the pixel arrays ranged from 20 x 30μm to 5 x 5μm are patterned by

34

photolithography using Shipley 1805 positive photoresist as a dry etching mask. Then the bare area of

SiO2 is etched away by RIE to open the channels. After removing PR, PEDOT/PSS and emitting layer

were spin-coated in same manner of non-structured spin-coated sample. However, as the size of

pattern decreases, the surface of the substrate is hardly coated uniformly due to the surface energy.

Figure 33a shows the Optical microscope image of the spin coated sample with 5 x 5 patterns. As

shown that the around of the pixels is uncovered with OLED ink. Additionally, even though the pixel

size is small, color gradient is also seen likewise bigger size pattern in figure 32a. Therefore, the

device of 20 x 30 μm size only works appropriately. Its characteristics, 16872 cd/m2 of maximum

luminance and 4V of turn-on voltage are shown in Figure 25. Pixel size is 20 x 30 μm. The space

between the pixels is 40 μm and the number of pixel is 30. The characteristics of the OLED devices

are measured using IVL measurement system (IVL-2000). The diameter of the detector is 1cm and the

values of the performances, related with area has to be calculated depending on the area covered with

pixel arrays.

Figure 26c shows the photo image of emission area which was covered with arrayed pixels. The

emitting light from OLEDs offers wide viewing angle. Therefore the light from each pixel was

interfered with each other and seemed a large pixel.

36



layer are 200nm and 130nm respectively. The dielectric layer has patterns which size is 20х30μm.

Based on the characteristics, we fabricated OLED devices using e-jet printing technique. Figure 27a

is the optical microscope image of printed pixels of anthracene ink and figure 27b is the fluorescence

microscope images of same pattern. Before printing OLED ink, we made pixel arrays as a pixel

defining layer (PDL) of dielectric layer in the same manner of spin-coated sample. In fact, PDL

structure has important roles in this paper. This PDL not only open the channel where ink was printed

but also help to confine the ink to the inside of PDL structures. In addition, this structure prohibits

current leakage. IF there is no PDL structure, PEDOT/PSS layer directly contacts Lif layer as shown

in figure 18. Both layers can transfer electrons or holes due to their properties so most injected current

will pass through the region where no active layer and this will result in no emission.

After patterning the PDL structures, PEDOT/PSS was spin-coated on the sample after then the

sample was dried at 140 ºC for 10 minute on a hot plate. The printing was implemented right after

drying.

Printing was implemented in atmospheric environment due to the situation of our printing system. In

fact, OLED material has weak properties from oxygen and water. During printing, the OLED material

was exposed to atmospheric environment for an hour inevitably, Therefore, this condition might affect

37

the characteristics of OLED devices such as luminance and efficiencies.

Figure 26 The schematic image of OLED printing.

38

Figure 27 5 x 5 size pixel printing of anthracene ink. a. optical microscope image b. fluorescence

microscope image. 2μm of inner diameter of the nozzle was used. Anthracene ink was diluted with

oDCB solvent and dilution ratio of anthracene : solvent is 1:5 (wt%).

39

Figure 28 AFM image of 5x5 size patterns. a a surface of patterned dielectric layer, b the surface of

patterned dielectric layer (a) spin-coated with PEDOT/PSS and c the surface of printed active layer on

b.

40

Figure 29 The thickness changes dependent on each process black line dielectric layer patterning,

red line hole injection layer spin coating blue line active layer printing.

In e-jet printing, depending on the applied voltage and pressure, the thickness of OLED printing can

be simply controlled. Voltage is not the substantive reason which ejects ink but electric field. The

electric field is dependent on the standoff height. So we fixed height value and changed voltage.

Thickness can be also controlled by the dilution ratio as shown in the graph of figure 30. We used a

ortho-dichlorobenzene(oDCB) to dilute inks. We printed original ink and 1:2, 1:5 and 1:20 ratio of

diluted inks. The thickness of printed pattern ranged from 40nm up to 2um. When the dilution ratio of

solvent increases, the thickness of printing decreases. However, as learned from the graph, as the

dilution ratio of solvent increases, printing voltage also increases. Because when we add more solvent

into the ink, concentration of ion which can be charged in ink solution are also changed. Therefore,

the voltage when first drop ejected from the end of the nozzle became changed as well. Moreover,

thickness can be controlled by controlling printing velocity. As implied in graph of figure 30, as the

velocity of stage which the substrate mounts on, increases, the thickness of printing decreases.

Because the number of droplet or the parts of droplet overlapped with the next droplet also decreases

dependent on velocity increasing.

41

Figure 30 The graph of thickness control of anthracene ink printing.

42

Figure 31 SEM cross-section image of OLED device.

In OLED display, thickness control is one of important factors, because it can affect turn-on voltage

(17), and another is the uniformity of printing which is related to the uniformity of brightness. Figure

28a, b and c shows the atomic force microscope (AFM) data which shows the surface changes of each

process. Figure 28a is AFM data of 5 x 5 size patterned dielectric layer. Figure 28b is AFM data of

patterned dielectric layer (figure 28a) spin-coated with PEDOT/PSS which indicates that PEDOT/PSS

can be smoothly spin-coated even though pixel patterns are formed on a desired substrate except edge

area. After that, printing was conducted on the sample of figure 28b as shown in figure 28c. These

data indicate a good uniformity of printing, and no pinhole and unprinted area were found. Figure 31

re-prove that these AFM data are exactly consistent with the cross-section SEM image. Moreover, the

phenomena known as coffee ring effect, which is frequently shown in solution process, was not found.

Normally, coffee ring effect can be recognized by color gradient as shown in figure 32a. The color in

the middle of the PDL is reddish. However, the color of the edge is vague. Different color means

different thickness between middle and edge and this might cause unstable emission. There are two

possible explanations of no phenomena of coffee ring effect. First, the coffee ring effect is caused by

the convective flow from center of droplet when a droplet falls on a substrate. This convective flow

can be relieved by mixing solvent which has higher evaporation rate and lower surface tension than

the solvent of original ink has. Then counterclockwise Marangoni flow in the direction opposite to

convective flow is established (16). Likewise, we mixed a solvent to dilute inks to prevent nozzle

clogging caused by high evaporation rate of solvent at the nozzle hole during the printing. Therefore,

this mixing solvent delivers counterclockwise Marangoni flow. Another reason is that solvent at the

tip of the nozzle is easily vaporized which means that during the printing, even though ink is highly

diluted, almost dried solute could be printed thus there are no enough time for solute moving by

convective flow.

43

Figure 32 Optical microscope images of two different types of deposition technique. a spin-coated

20х30 pixels b. e-jet printed 20х30 pixels.

44

Figure 33 Optical microscope images of two different types of deposition technique. a spin-coated 5

х 5 pixels b. e-jet printed 5 х 5 pixels.

46

Figure 34 Optical microscope images of the process of each fabrication step; a. PDL structure, b.

PEDOT/PSS spin coating, OLED ink printing, d. Lif/ Al deposition; the size of the pixel is 10 x 30.

Figure 34 shows the image of each step according to the fabrication progress. OLED pixels were

printed bigger than the PDL structure for making better contact with Al electrode. The reason is that

the thickness of OLED layer is around 100nm and this thickness has to overcome the thickness

difference of the PDL structure. Otherwise current injection into inside the pixel is impossible. The

thickness of PEDOT/PSS layer in PDL structure is 50 nm and outside 70nm.

48




Figure 35 shows the performance of printed OLED device. Pixel size is 20 x 30 μm. The space

between the pixels is 40 μm and the number of pixel is 30. The characteristics of the OLED devices

are measured using IVL measurement system (IVL-2000). The diameter of the detector is 1cm and the

values are calculated depending on the area covered with pixel arrays. Maximum luminance, 311.8

cd/m2 , far smaller than those of spin-coated sample is shown.

50




Figure 36 shows the performance of printed OLED device. Pixel size is 10 x 30 μm. The space

between the pixels is 30 μm and the number of pixel is 35. The values are calculated depending on the

area covered with pixel arrays. Maximum luminance is 1332.3 cd/m2 and the turn-on voltage is 4V.

52




53

Figure 38 the efficiency of OLED devices dependent on size difference of pixels

Finally, we fabricated a 5x30 μm size pattern OLED device. Its characteristics, 6867cd/m2 of

maximum luminance and 4V of turn-on voltage are shown in Figure 37. The values are calculated

depending on the area covered with pixel arrays as did. The space between the pixels is 5 μm and 10

μm. The number of the pixels is 45. The photo image of emission area which was covered with

54

arrayed pixels, also seemed one large pixel. Figure 33d shows that the devices fabricated by e-jet

printing method display much weaker emission compared with those of OLED devices fabricated by

spin coating method. As the pixel size of device decreases, the efficiency of the devices also tended to

decreases as shown in figure 38. As the size of the pattern decrease, total area covered with pixel

arrays also decrease. In other words, the area where current is injected is far bigger than the area

where emission occurs. Therefore the electric field is highly focused on local area and might lead to

early corruption. This can be proved by high current density and low efficiency depending on the

pixel size.

Figure 39 Optical microscope image of OLED light emission

The emission image of 5 x 30 pixels is shown in figure 39. As shown that, the edge seems tortuous

because of the spin-coated hole injection layer which tends to show gradient surface inducing

different emission efficiencies.

55

Ⅴ. Conclusion

In summary, we have demonstrated solution-based patterning technique for fabricating high-

resolution anthracene OLED pixels based upon e-jet printing combined with a structured surface

consisting of etched channels. This method achieves extremely small size of pixels and the critical

parameter, thickness and uniformity control is successfully implemented. This high-resolution

printing capability can resolve the current limitation on reducing pixel dimensions of the vacuum-

processed OLED displays. In addition, the low-cost, solution-processibility is advantageous for

macroelectronics, including large-area displays. However, we need to better understand the small size

effect which shows the lower efficiency of the device when the pixel size of the device becomes

smaller. Besides, further efforts to improve device performance are needed.

56

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62

Acknowledgements

First of all, I want to thank thy name who has always tagged with me.

I also thank my Prof. Jang-Ung Park who has kindly guided me. Now I can understand how well he

cared me. The things he taught me would be always kept in my mind. I believe that they must be

informative though my entire life. Furthermore, I am really appreciated with Prof. Myoung Hoon

Song and his student, Bo Ram Lee for helping me to finish my course.

The lab members, studying and working together, Mi-Sun, Sohyun, Dr. Lee, all of undergraduate

students and people who pass by, I was very lucky because I had chance to share the part of my life

with them. What I want to say is that this time was definitely like the password of our microscope

computer. I hope so.

Looking back, there were many occasions which I could hardly forget even if I felt difficulties at the

moment. Time would make them all become far more precious as much as the time passes. I believe.

Ⅰ. IntroductionⅡ. Theoretical Background2.1 Absorption and emission2.2 Energy transfer2.1.1 F?rster transfer2.1.2 Dexter transfer

2.3 Charge transfer2.4 Electroluminescence and charge injection2.5 Structures of OLED2.5.1 Anode materials2.5.2 Cathode materials2.5.3 Electron and hole transport materials

2.6 Active layer deposition methods2.6.1 Thermal evaporation with shadow mask2.6.2 Spin coating2.6.3 Laser induced transfer2.6.4 Ink jet printing

2.7 E-jet printing technique2.7.1 Printing system

2.8 Fluid Dynamics and Solidification2.8.1 Fluid Dynamics2.8.2 Solidification

Ⅲ. Experimental Details3.1 Fabrication details3.1.1 Fabrication process of the nozzles3.1.2 Fabrication process of the spin-coated sample3.1.3 Fabrication process of the printed sample

3.2 E-jet set-up for OLED printing

Ⅳ. ResultsⅤ. ConclusionⅥ. Reference

13¥°. Introduction 1¥±. Theoretical Background 3 2.1 Absorption and emission 4 2.2 Energy transfer 4 2.1.1 F?rster transfer 4 2.1.2 Dexter transfer 5 2.3 Charge transfer 7 2.4 Electroluminescence and charge injection 7 2.5 Structures of OLED 7 2.5.1 Anode materials 8 2.5.2 Cathode materials 9 2.5.3 Electron and hole transport materials 9 2.6 Active layer deposition methods 9 2.6.1 Thermal evaporation with shadow mask 9 2.6.2 Spin coating 10 2.6.3 Laser induced transfer 11 2.6.4 Ink jet printing 12 2.7 E-jet printing technique 13 2.7.1 Printing system 15 2.8 Fluid Dynamics and Solidification 17 2.8.1 Fluid Dynamics 18 2.8.2 Solidification 20¥². Experimental Details 24 3.1 Fabrication details 24 3.1.1 Fabrication process of the nozzles 24 3.1.2 Fabrication process of the spin-coated sample 26 3.1.3 Fabrication process of the printed sample 27 3.2 E-jet set-up for OLED printing 27¥³. Results 29¥´. Conclusion 55¥µ. Reference 56

Documents

Disclaimerscholarworks.unist.ac.kr/bitstream/201301/1044/1...development of polymer organic light-emitting diode (OLED) materials, fabrication methods such as roll-to-roll printing