전도성 고분자의 이해 및 응용

Conducting/semiconducting

polymers in organic electronics

Organic Electronic Devices

김 철 암한국전자통신연구원 기반 기술연구소

유기전자소자팀

D i g i t a l - p a p e r d i s p l a y contents

1.1. Fundamentals of Conducting PolymersFundamentals of Conducting Polymers

2. Specific Materials

3. Application of CP in organic electronics

- CP in Flat Panel Display Industries

4.Problem and the Future

Conjugated polymers

• 1977 Sirikawa, Heeger & MacDiarmid , to dope polyacetylene with arsenic pentafluoride (AsF5): conducting/semiconducting nature in polymer before/after doping

• Two reasons for good conductivity of doped conjugated polymers

: Increasing the number of free charge carriers: Increasing the charge carrier mobility

σ = neμ

σ : 전도도 (conductivity, S/cm)n : charge carrier 농도

e : electron의 chargeμ: 이동도 )charge carrier mobility, cm2/Vs)

Conducting polymer의 특성발현을 위한 기본 구조

Conjugated double bond의 존재에서 conductivity발현

Conjugated polymers

• Degenerate ground state (축퇴된 기저 상태) : To able to interchange single and double bonds without changing the ground state energy

• Non-degenerate ground state

: An interchange single and double bonds is associated with two states of energy.

Conjugated polymer 의 기하학적 특성

Trans - polyacetylene Cis - polyacetylene

aromatic

quinoid

Conjugated polymersConducting polymer

CH CHX

S X

S

O O

X

N XH

S X

CH CH

N

X

H

polyacetylene

polythiophene

poly(3,4-ethylenedioxythiophene)

polypyrrole

poly(thienylene vinylene)

polyaniline

Doping of polymersNeutral state

• To span a conductivity range from close to an insulator to semiconductors

106

104

102

100

10-2

10-4

10-6

10-8

10-10

10-12

10-14

10-16

Co

nd

uct

or

Ag, CuFe

Mg

DopedDoped trans-(CH)x[105 S/cm]

DopedDoped PEDOT[103 S/cm]In, Sn

Incr

easi

ng d

opin

g le

vels

Ge

Si

AgBr

Glass

Diamond

Nylon

Quartz

trans-(CH)x[10-5 S/cm]

DopedDoped polyaniline[100 S/cm]

Sem

ico

nd

uct

or

PEDOT, polyaniline[10-10 S/cm]

Insu

lato

r

Doping of Semiconductors

Doping of crystalline semiconductors (Si or Ge)

As+

e–

n-type Arsenic doped Si crystal

The four valence electrons of As allow it to bond just like Si but the fifth electron is left orbiting the As site. The energy required to release to free fifth-electron into the CB is very small.

x

As+ As+ As+ As+

EcEd

EvAsatomsitesevery106 Si atoms

Distanceintocrystal

~0.03eV

CB

Electron

Energy band diagram for an n-type Sidoped with 1 ppm As. There are donor energy levels just below Ec around As+

sites

Doping of Semiconductors

Doping of crystalline semiconductors (Si or Ge)

p-type Boron doped Si crystal

B–h+

Free

B–

B has only three valence electrons. When it substitutes for a Si atom one of its bonds has an electron missing and therefore a hole as shown in (a). The hole orbits around the B–site by the tunneling of electrons from neighboring bonds as shown in (b). Eventually, thermally vibrating Si atoms provides enough energy to free the hole from the B– site into the VB as shown.

(a) (b)

Doping of Semiconductors

Doping of crystalline semiconductors (Si or Ge)

p-type Boron doped Si crystal

x

B–

Ev

Ea

B atom sites every 106 Si atoms

Distanceinto crystal

~0.05 eV

B– B– B–

h+

VB

Ec

Electron energy

Energy band diagram for a p-type Si doped with 1 ppm B. There are acceptor energy levels just above Ev around B-sites. These acceptor levels accept electrons from the valence band and therefore create holes in the valence band.

Doping of polymersDoping of conducting polymers

• To require much higher doping fraction ( ~ % level)

• Dopant forms an ionic complex with polymer chain

• Several methods for the doping of polymersi) Chemical dopingii) Electrochemical dopingiii) photo-dopingiv) charge-injection doping

Doping of polymers

−−+− +⇔+ yeAPyAP yy

Chemical doping

• P-doping → oxidation of the polymer chain: to withdraw electrons from the π-system of polymer backbone: to result in a positively charged unit

• n-doping → to introduce electrons into the π-system of polymer backbone: to form a negatively charged unit

−++− ⇔++ yy APyAyeP

P = the polymer chainA = the charge-compensating counter ione- = electrony = the number of counter ions

n-doping : to react quickly with oxygen in airnormally unstable in ambient atmosphere

Free charge carriers in conjugated polymers

Charge carriers: solitons

Conduction band

-

Neutral soliton Negative solitonPositive soliton

Valence band

Negative solitonCharge: zeroSpin: 1/2

Positive solitonCharge: zeroSpin: 1/2

Neural solitonCharge: zeroSpin: 1/2

Free charge carriers in conjugated polymersCharge carriers: polarons or bipolarons

S

OO

S

O O

S

OO

S

O O

S

OO

S

O O

S

OO

S

O O

S

OO

S

O O

S

OO

S

O O

n

-e-

-e-

S

OO

S

O O

S

OO

S

O O

S

OO

S

O O

Neural polymer

Bipolaron

Polaron

Bipolaronbands

Doping of conducting polymers

• The highest conductivity is associated with “mixed valence”states of fractional charges per repeat unit of polymers.

• To easily move non-charged sites to charged sites → High σ

• Measurements of potential dependence of polythiophenes, polypyrroles and polyaniline on conductivity

: For polyaniline (PANI)level of more than 0.5 electrons per repeat unit oxidised state→ to start to show a decrease in conductivity

Free charge carriers in conjugated polymers

Free charge carriers in conjugated polymers

N

H

N

H

y

N N1-y x

Doping of PANI

N

H

N

H

HN NReduced repeat unit Oxidized repeat unit

Average oxidation state (1-y) variation

N

H

N

H

N N

H H

x(1-y) = 0, leucoemeraldine

N

H

N

H

N Nx

(1-y) = 0.5, emeraldine

N N N Nx

(1-y) = 1, pernigraniline

Free charge carriers in conjugated polymers

Optical state change by doping level

• Doping introduces new state in the energy band gap.– To cause the absorption to shift towards lower energies

• For typical conjugated polymers, to have a band gap of 1.7 – 3 eV– To absorb visible wavelengths of light (in neutral form)– To switch to the infrared absorption by doping

Electrochromism

• The band gap of the polymer is dependent on the structure of polymer. (to determine the conjugation length).

• A long conjugation length (large electron delocalization)– To yield a small band gap

• By introducing different side groups along the polymer– To tune the band gap– To influence solubility, thermal stability, oxidation potential….

Doping of polymersDopants of conducting polymers

CH 3H3CCH 2SO3

-

O

CH3

SO3-

SO 3- SO 3

- SO 3- SO 3

- SO 3- SO 3

-

O

O

O

O3

- OS

O

O

O

O3

- OS

camphorsulfonate

p-toluenesulfonate polystyrenesulfonate (Bayer)

di(2-ethylhexyl)sulfosuccinate

dioleylsulfosuccinate

OS3-

dodecylbenzenesulfonate

Specific materialsPEDOT:PSS

• One of the most studied & characterized conjugated polymers

PSS : polystyrenesulfonate

S

OO

S

O O

S

OO

S

O O

S

OO

S

O O

SO3- SO3H SO3H SO3H SO3

- SO3H

n

nPEDOT : poly(3,4-ethylenedioxythiophene)

Mean Particle Size : 50nm

D i g i t a l - p a p e r d i s p l a y contents

1.1. Fundamentals of Conducting PolymersFundamentals of Conducting Polymers

2.2. Specific MaterialsSpecific Materials

3. Application of CP in organic electronics

- CP in Flat Panel Display Industries

4.Problem and the Future

Specific materialsDifferent applications PEDOT as electrode or conductors

PEDOT

Solid state applications

Electrochemical applications

Transistor LED Memory

Transistor Smart window

Display Batteries Super capacitors

Coatings

Anti-static

Corrosion

Structure of Structure of Flat Panel DisplayFlat Panel DisplayLCDLCD PDPPDP

ELEL FEDFED

Electrode Fabrication

Deposition Deposition -- SputteringSputtering glassITO

photoresist

optical mask

developed resist

etched ITO

resist removed

Etchant : HCl, HNO3

ProblemsProblems of ITO Electrodeof ITO Electrode

originated from processing conditions

High manufacturing cost ♦ Vacuum process

(sputtering, vacuum deposition)♦ Photoresist process

(etchant disposal)

ProblemsProblems of ITO Electrodeof ITO Electrode

originated from processing conditions

High manufacturing cost ♦ Vacuum process

(sputtering, vacuum deposition)♦ Photoresist process

(etchant disposal)

originated from plastic substrate

High processing temperatureLow mechanical property

♦ Low flexibility(200W/ z 50X106W/ z)

♦ Low durability for flexible applicationThermal expansion mismatching

Folding

RequirementsRequirements of Flexible Electrodeof Flexible Electrode

High Transparency (>80%)High Conductivity (<100Ω/ )Low CostFlexible Mechanical PropertyAppropriate Thermal ExpansionPrinting & Patterning CapabilityEnvironmental Stability

Transmittance of PEDOT/chloroform coating

400 600 800 1000 12000

20

40

60

80

100

3%

BlueGreenRed

450 500 550 600

80

85

C

B

A

layer ThicknessA 200 nmB 350 nmC 450 nm

Tra

nsm

itta

nce (

%)

Wavelength (nm)

Metallic conductivity of PEDOT film doped with PF6 : 300 S/cmMetallic conductivity of PEDOT film doped with PF6 : 300 S/cm

particle size: 800 nm

Surface resistance: > 500 (Ω/)

ProtonationProtonation in PANIin PANI

N

H

N

H

y

N N1-y x

N

H

N

H

y

N N1-y x

H+H+

Reaction with protonicacid adds 2H+

Geometrical (bond length Relaxation (Quinoid to Benzenoid)

N

H

N N N

H

x

H+ H+

N

H

N

H+

n

Redistribution of charge & spin; having of unit cell

ProtonationProtonation in PANIin PANI

N

H

N

H

y

N N1-y x

N

H

N N

H

x

H H

Cl-Cl-N

2 x HCl

Protonation

EmeraldineEmeraldine salt typesalt type: conductor: conductor

Protonation (doping) with organo sulfonic acid

N

H

N N

H

x

H H

N

CH 3

H 3C CH 2SO 3

-

OCH 3

H 3C CH 2SO 3

-

O

To increase the conductivity & solubility

PANI-CSA salt

PANI-CSA in cresol: σ ~ 100 S/cm

ProtonationProtonation in PANIin PANI

Through a mask exposed and developed circuit board based on PANI-CSA. The lines are green (conducting) and background blue (insulating).

UV lithographic process steps to pattern electrically conducting PANI-CSA film.

Ref. T. Makela et. al., Synthetic Metals 101 (1999) p.705-706

Soluble polypyrrole

N XH

SO

O

O

O(H2C)8(H2C)7(CH2)8 (CH2)7

O

O

O- Na+

di(2-ethylhexyl)sulfosuccinate

dioleylsulfosuccinate

O

O

O

O3

- OS

DEHSNa

DOSNa

Soluble Polypyrrole(NH4)2S2O8(0.05 mol ) in 100 mL distilled water

+ Pyrrole(0.2 mol)/DEHSNa (0.1 mol) in 900 mL distilled water

Polypyrrole Precipitate

Filter cake

Polypyrole Powder

Solution

Polypyrrole Film

Filter & Washing

Drying

Dissolve in Organic Solvents

Solution Casting

UV-Vis/NIR, GPC

σ, σ vs 1/T, XRD etc.

FT-IR, Solubility test

Gel permeation chromatogram of Ppy-DEHS

Mw 62,296 ; PD 1.87No. of neutral pyrrole ring : 303

Mw 174,756 ; PD 1.06No. of neutral pyrrole ring : 850

Synthetic Metals, 125 (2002) 267, E. J. Oh, K. S. Jang, A.G. MacDiarmid

Solubility and electrical conductivity of Ppy-DEHS

solvent solubility

(wt/vol%)

σ (S/cm)

DMSO ~8 9

DMF ~9 7

NMP ~9 7.2 x 10-1

m-cresol ~7 2.5 x 10-1

o-chlorophenol ~6 1.2 x 10-1

dichloromethane ~5 1.7 x 10-1

chloroform ~5 2.2 x 10-2

benzene ~2 8.2 x 10-3

trifluoroacetic acid ~3 1.5 x 10-2

acetic acid ~2 7.5 x 10-3

formic acid ~2 4.3 x 10-3

D i g i t a l - p a p e r d i s p l a y contents

1.1. Fundamentals of Conducting PolymersFundamentals of Conducting Polymers

2. Specific Materials

3. Application of CP in organic electronics3. Application of CP in organic electronics

-- CP in Flat Panel Display IndustriesCP in Flat Panel Display Industries

4.Problem and the Future

Flexible DisplayFlexible Display

Ch-LCD

E-ink

Toner

LCD

EL

Gyricon media

Flexible Organic Displays

Flexible display using light weight & thin film– Display materials : EPD*, LC, Organic EL etc. – Driving device : Organic TFT– Substrate : Glass ⇒ Plastic

[Substrate]

Driving part

[Display layer]

[Passivationlayer]

[Structure][Structure]

*EPD : Electro-phoretic Display

Hierarchy of active matrix display

Hierarchy of active matrix display

Source

Drain

Gate

Organicsemiconductor

Organic ThinOrganic Thin--Film Transistor (OTFT)Film Transistor (OTFT)

Top View

Relationship to SiliconRelationship to Silicon--based Transistorsbased Transistors

• Also studied for more than 50 years

• Huge variety of choices for organic molecules for use in semiconductors

• Charge transport significantly different (to be discussed)

• Usually comprised of many individual molecules held together by Van der Waalsforces

• Many different fabrication techniques on different substrates

What and why an What and why an organic transistororganic transistor??• Organic transistors are transistors

that use organic molecules rather than silicon for their active material. This active material can be composed of a wide variety of molecules.

• The Pros– compatibility with plastic substances– lower temperature manufacturing

(60-120° C)– lower-cost deposition processes such

as spin-coating, printing, evaporation– less need to worry about dangling

bonds makes for simpler processing• The Cons

– lower mobility and switching speeds compared to silicon wafers

– usually do not operate under inversion mode (more on this later)

Organic Thin Film Transistor (OTFT)Organic Transistor?- Organic semiconductor similar to Si-Semiconductor- Easy processible

Why Organic Transistor?- Lightweight, Low-cost, Flexibility, Roll to Roll

TFT Structure

Conjugated Organicmaterials

Si semiconductor

Organic GateInsulator

SiO2, SiNx

Organic TFTSi TFTCVD process Printing process

Conducting polymerOrg. Inorg. Hybrid

MetalGlass

Glass, Plastic

The Manufacture of OTFT

All Polymeric OTFT

All Polymeric OTFT

PEDOT FET 구조

Mobility : 1.7 x 10-2 cm2/Vs

Ion/Ioff : 4.1 x 103

Threshold voltage : 1.5 VH. Okuzaki, Synth. Met., 137(2003)947-948

Low-cost all-polymer integrated circuits

PANi

Photochemical patterning of PANI-CSA thin films. A 0.2 μm thin PANI film is exposed in a N2 ambient to deep UV through a mask. The photoinitiator in the nonexposed parts is then removed by sublimation through heating the film at 110 OC. Topography in the final films is less than 50 nm. The sheet resistance increases upon exposure from 1 kΩ/sq. to more than 1014 Ω/sq.

Large area, high resolution, dry printing of PANIsfor organic electronics

Laser beamDNNSA-PANI/SWNT layer : source & drain electrodes

H(CH2)9

H(CH2)9 SO- H+O

O

SO- H+

O

O

μ = 0.3 cm2/V s, Ion/off ~ >103

ElectrophoreticElectrophoretic Display (Display (전자종이전자종이))

E-Ink사의 D-Paper 시제품Electrophoretic (Microcapsule 형)

Applications (E-Ink)

SID 2002 Conference & Exhibition

SID 2004 Conference & Exhibition

e-Book(SONY+E-ink)

Video speed E-ink panel

D i g i t a l - p a p e r d i s p l a y contents

1.1. Fundamentals of Conducting PolymersFundamentals of Conducting Polymers

2. Specific Materials

3. Application of CP in organic electronics

- CP in Flat Panel Display Industries

4. Problem and the Future4. Problem and the Future

D i g i t a l - p a p e r d i s p l a y Microelectronics

Stone Age

Bronze Age

Iron Age

Age of Plastics

Electronic Polymer