physics and applications of conjugated polymers semiconductors

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Physics and applications of conjugated polymers semiconductors

孟心飛交通大學物理所

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感謝洪勝富清大電機系施宙聰清大物理系許千樹交大應化系陳壽安清大化工系翰立光電研發部

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Conjugated polymer:organic semiconductor with direct bandgap of 2-3 eV

4

Outline Overview

Triplet exciton formation

Field-effect transistor

Multi-color LED

5

Technologies of conjugated polymers 1970-80, metallic conductivity reached by mo

lecular doping 1990, first polymer LED was made 1998-99, polymer flat-panel-display was dem

onstrated, other opto-electronic devices are underway

Solution processing, large area, light-weight, high-brightness, flexible

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Science of conjugated polymers 1D semiconductor Electron-electron and electron-phonon

enhanced in 1D Quasi-particle: solitons, polarons .. Complicated recombination Spin-triplet exciton formation Transport in disordered materials

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PPV semiconductor band structure

One -electron for each carbon atom

E(k)

C : 1s2 2s2 2p2

2s,2px,2py sp2 hybridization -bond 2pz -bond

xy

11

LED Device Operation

Conduction

Valence

12

Triplet exciton formation in polymer LED

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+ _ Exciton (large binding energy)

+ _ Electron-hole pair

photon

Coulomb capture

Radiative decay

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Electron spin = 1/2 , Hole spin = 1/2

Exciton spin =

0 (Singlet)1 (Triplet)

Total spin of exciton (electron-hole bound state)

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Singlet Triplet

ST

G 3 G

Free electron-hole pair

Ground State

Spin-independent recombination γ= 3

Radiative:light

Nonradiative:heat

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Not so simpleT.-M. Hong and H.-F. Meng, Phy. Rev. B, 63, 075206 (2001)

Ground state

S1

T2

T1

sg

tt

tg

2

st1

t1s

Free carrier continuum

s 1-s

Conjugated polymer: S/T splitting EL < ¼ ??

S2

s2t2

s1t2

RadiativeDecay

Non-radiativeDecay

Bottleneck

17

S

T

sT

G γG

Ground State


Induced absorptionat near IR (1.3-1.6 eV)

Visible lightemission

Detection of singlet and triplet excitonsNo quantitative relation available!

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How do we measure γ ?Compare EL and PL rate equations

EL : electric excitationPL : optical excitation

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G: generation rate for singlet exciton

τ s: singlet exciton lifetime

τ t: triplet exciton lifetime

EL ELs

s

EL

s NGN1

ELT

T

EL

T NGN

1

ST

sT

G γG

Ground State

1. EL Rate equation

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2. PL Rate equation

:intersystem crossing lifetime.T

S

Tisc

PL

pump


Ground State

PLT

T

PLS

isc

PL

T NNN11

isc

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Steady-state Ns

EL = NtPL

01

ELs

s

NG

01

ELT

T

NG

011

PLT

T

PLS

isc

NN

PLT

ELT

isc

S

N

N

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MEH-PPV LED

Glass

PEDOT 40nmITO 80nm

Al 100nm

Ca 10nm

MEH-PPV (100nm or 50nm)

ITOAl

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Experiment setup

sample holder

PumpLaser

Beamexpender

Attenuator

lens

lens

Triplet detector

Singlet detector

lens

850nm probe

laser

Function generator

Lock-in

Preamplifier

EL

PL

24Optical table

25Infrared semiconductor probe lasers

26Cooling system (under construction)

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EL-induced absorption (EA) spectrum due to the triplet exciton

1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 2.1

2

3

4

5

6

7

8

9

10

R/R

x

10

5 (T

rip

let

exc

iton

ind

uce

d-a

bso

rptio

n)

Probe photon energy (eV)

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Triplet and singlet exciton density

-20 0 20 40 60 80 100 120 140 1600

5

10

15

20

25

30

35

6

5.554.543.53

2.5

21.5

1

1

1.5

2

2.5

3

3.5 76.565.55

4.54

Tri

ple

t (

mV

)

Singlet (mV)

100nm EA vs EL 50nm EA vs EL 100nm PA vs PL 50nm PA vs PL

linear

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0 2 4 6 8 10 121E-4

1E-3

0.01

0.1

1

0.64 ns (592nm)

Lu

min

esc

en

ce in

ten

sity

(a

.u.)

Time (ns)

Time-resolved PL, s=0.64 ns

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1x105 2x105 3x105 4x105 5x105 6x105 7x105 8x1050

2

4

6

8

10

12

14

16

18

20

Tri

ple

t/S

ing

let

ra

tio

E (V/cm)

50nm 100nm

Phys. Rev. Lett., 90, 036601 (2003)

d : thickness of MEH-PPV.

Vbi : built-in voltage

dVVE bi /)(

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1S

2S

1T

Free carrier continuum

Ground state

0.1ev

1ev

0.3ev2T1/4 3/4

Phonon bottleneck

1. Field dissociation

Two possible explanations

2. Quenching by polarons

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Conclusion

γ is not a constant but a strong universal function of the electric field

γ is much larger than 3 for intermediate bias and smaller than 3 for high bias

Triplet exciton formation is no longer the main limit for the efficiency of a LED operated under high bias

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Parallel transport and field effect transistors

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Light emitting polymers have very low carrier mobility

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Motivation for horizontal structure

Carriers transport by hopping in the sandwich structure – low mobility Carriers transport along the backbone mostly in a horizontal device

structure –high mobility

j

Perpendicular transport(high mobility)

Glass substrate

j

Parallel transport(low mobility)

Glass substrate

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Yi-Shiou Chen and Hsin-Fei Meng, Phys. Rev. B, 66, 035191 (2002)

Theoretical basis:High intrachain mobility can be achieved even with many conjugation defects

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Parallel hole transport

polymer

glass

Au Au

d

hd = 2 micronh = 100 nm

39Thermal coater

40Mask aligner for photo-lithography

41Spinner

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1μm gold source/drain channel on glass or SiO2/ITO

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Interdigited1 μm channel

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ITO/PPV/Au sandwich device

3

2

8

9

L

V

Hole-only device T=307K SCLC model J= p=510-11m2/Vs

=510-7cm2/Vs

PRB55,R656(1997)

R1=CH3, R2=C10H21

r 0

3

2

8

9

L

V

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Space charge limited current

Steady state: J=nqE Poisson’s eq.:

……Mott-Gurney law

ndx

dE

q

dx

dEJE

212

1

2)(0)0( x

JXEE

232

1

9

8,)( L

JVVLV

3

2

8

9

L

VJ

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fixed T, variable SD distance d

Ohmic: J=n0 q p E There is little depen

dence between p and d.

0

2000

4000

6000

8000

10000

curr

ent d

ensi

ty J

(A/m

2 )

bias(V)

d=2.5micron d=4.5micron

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J-E plot

106 107

102

103

104

curr

ent

den

sity

J(A

/m2 )

field E(V/m)

d=2.5micron d=4.5micron

The slope of J-E curve = n0 q p

n0 :extrinsic carrier density q:electron charge p: hole mobility 由 n0 倒推回 p

p=3.810-3 cm2/Vs

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sandwich device:ITO/MEH-PPV/Ca/Al

0.01 0.1 1 10

10-4

10-3

10-2

SCLC

Ohmic

curr

ent d

ensi

ty(A

/m2 )

bias voltage(V)

bias>3V: SCLC J=

=3 L =1200Å p =1.44×10-5cm2V-1s-1

bias<3V: Ohmic J=n0 q p E n0 =7.84×1021m-3

3

2

08

9

L

Vpr

r

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Compare with other sandwich devices

0 1x107 2x107 3x107 4x107 5x107 6x107

10-4

10-3

10-2

10-1

100

101

102

103

104

curr

en

t de

nsi

ty J

(A/m

2 )

field E(V/m)

d=2.5micron d=4.5micron Chen (Sandwich) Heeger (MEH-PPV) Friend (PPV)

Our horizontal device:

p=3.810-3 cm2/Vs Chen:

p =1.44×10-5 cm2/Vs Friend:

p =2×10-7 cm2/Vs Hegger:

p =2.24×10-7 cm2/Vs

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fixed T, variable d

T=297K There is little

dependence between p and d.

No domain down to 1 micron

0 1x107 2x107 3x107 4x107 5x107 6x107

0.0

5.0x103

1.0x104

1.5x104

2.0x104

2.5x104

curr

en

t de

nsi

ty J

(A/m

2 )

field E(V/m)

d=0.9micron d=2.5micron d=4.5micron d=9.6micron d=14.7micron

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Temperature dependence

fixed d, variable temperatured=0.9micronT : from 297K to 235KJ=n0 q p E

0 1x107 2x107 3x107 4x107 5x107 6x107

0.0

5.0x103

1.0x104

1.5x104

2.0x104

2.5x104

curr

en

t de

nsi

ty J

(A/m

2 )

field E(V/m)

297K 282K 267K 256K 235K

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fixed d, variable temperature

p= 0exp(-/kBT)=0.233eVHorizontal ~ Sandwich/2

3.2 3.4 3.6 3.8 4.0 4.2 4.410-8

10-7

10-6

d=0.9micron

ho

le m

ob

ility

(m2 /V

s)

1000/T(K-1)

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Field effect transistor and its applications

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Bottom gate transistor structure

ITO

SiO2

polymer

glass

Au Au

d

hd = 2 micronh = 100 nm

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P-type transistorwith hole accumulation

gate

insulator

glass

source drain

channel

VGS<0

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Application: active matrix flat-panel-display

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Passive matrix display

Scan line

Data line

1.One row each scan2.Fast degradation3.Voltage drop in lines4. Uniformity problem

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One active matrix

Scan line

Data line

Pixel

Switching TFT

Driving TFT

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Design by Cambridge and Seiko-Epson

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Our design : Pixel and FET share same semiconductor

Side view

S SD D

G GITO

PPV

Metal

I I

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Transistor target

LED turn-on current density j = 10 mA/cm2

Pixel area A = 0.1x0.1 mm2

Driving current = A j = 1 A = 1000 nA

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MEH-PPV FET characteristicsVsd < 0

FL023

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FET characteristicsVsd > 0

FL016

1 A

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Frequency response SetupSetup：：

FunctionFunctiongeneratorgenerator

FunctionFunctiongeneratorgenerator

G

SiO2

S D

oscilloscopeoscilloscopeoscilloscopeoscilloscope

MEH-PPVMEH-PPV

R1

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1KHz frequency response: not bad

Gate Gate voltagevoltage

RR1 1

VoltageVoltage

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Conclusion

• Same-polymer pixel+FET is possible• Simplified active-matrix display design• Processing on flexible substrate is

possible

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Voltage-tunable full-color PLED

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Motivation

Full color display without pixel patterning

Signaling Lighting

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Working principle

Hole mobility is much larger than electron mobility

Electron mobility increases rapidly with field

Recombination zone pushed by field

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Suitable structurewith electron blockade

e

h

e

h

ITO4.8~5.0

AU5.2

Ca: 2.9MgAg: 3.7

AL: 4.2

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Red(610~640nm, 2.03~1.94eV) 1. MEH-PPV: 605nm, 2.8—5.0eV

~1.0%(PRB, 53, 15815(1996))

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Green(505~555nm, 2.46~2.23eV) 1. A proprietary material of Dow Chem.

536nm, ??--??eV, ~0.9% (SM, 111, 159(2000))

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Blue(460~480nm, 2.70~2.58eV)

2.PFO: 440nm, (SM, 125, 55(2002))

2.12—5.8eV(APL, 73, 2453(1998))

2.95—5.9eV, ~1.2% (JCP, 116,1700(2002))

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Electron blocking PVK: 1.2—6.1eV(APL, 65, 1272(1994)),

tetrahydrofuran(THF), chloroform (APL, 74, 3613(1999)),

trichloroethane(JAP, 79, 934(1996))

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4V

9V

11V

13V

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5v 9v 13v

17v 20v

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PEDOT/PVK/PFO/PF/MEH

300 400 500 600 700 800

0.0

0.2

0.4

0.6

0.8

1.0

Y A

xis

Title

X Axis Title

4V 592 6V 588 8V 584 10V 580 12V 576 14V 572 16V 568 18V 556

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PEDOT/PVK/PFO/G/MEH

300 400 500 600 700 800

0.0

0.2

0.4

0.6

0.8

1.0

Y A

xis

Title

wavelength

6V 584 8V 576 10V 572 12V 572 14V 568 16V 560 18V 556 20V 552

physics and applications of conjugated polymers semiconductors

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