Physics and applications of
conjugated polymers
semiconductors
孟心飛 交通大學物理所
1
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
2
Conjugated polymer:
organic semiconductor with
direct bandgap of 2-3 eV
3
Outline

Overview

Triplet exciton formation

Field-effect transistor

Multi-color LED
4
Technologies of conjugated
polymers




1970-80, metallic conductivity reached by
molecular doping
1990, first polymer LED was made
1998-99, polymer flat-panel-display was
demonstrated, other opto-electronic
devices are underway
Solution processing, large area, lightweight, high-brightness, flexible
5
Science of conjugated
polymers
1D semiconductor
 Electron-electron and electronphonon enhanced in 1D
 Quasi-particle: solitons, polarons ..
 Complicated recombination
 Spin-triplet exciton formation
 Transport in disordered
materials

6
7
PPV semiconductor band structure
E(k)
y
x
C : 1s2 2s2 2p2
 2s,2px,2py
sp2 hybridization -bond
 2pz -bond
One -electron for each carbon atom
8
9
10
LED Device Operation
Conduction
Valence
11
Triplet exciton formation
in polymer LED
12
_
+
Electron-hole
pair
Coulomb capture
+
_
Exciton (large
binding energy)
Radiative decay
photon
13
Total spin of exciton (electronhole bound state)
Electron spin = 1/2 , Hole spin = 1/2
Exciton spin =
0 (Singlet)
1 (Triplet)
14
Spin-independent recombination
γ= 3
Free electron-hole pair
G
3G
Triplet
Singlet
Radiative:
light
S
T
Nonradiative:
heat
Ground State
15
Not so simple
T.-M. Hong and H.-F. Meng, Phy. Rev. B, 63, 075206 (2001)
Bottleneck
Radiative
Decay
Non-radiative
Decay
16
Detection of singlet and triplet excitons
No quantitative relation available!
Free electron-hole pair
G
γG
S
T
Visible light
emission
s
Induced absorption
at near IR (1.3-1.6 eV)
T
Ground State
17
How do we measure γ ?
Compare EL and PL rate equations
EL : electric excitation
PL : optical excitation
18
1. EL Rate equation
 EL
EL
Ns  G
Free electron-hole pair
G
γG
S
s
T
T
Ground State
 EL
1
s
N T  G 
N sEL
1
T
N TEL
G: generation rate for
singlet exciton
τs: singlet exciton lifetime
τt: triplet exciton lifetime
19
2. PL Rate equation
PL
Free electron-hole pair
S
pump
 isc
T
T
 PL
NT 
1
 isc
N
PL
S

1
T
N
PL
T
 isc :intersystem
crossing lifetime.
Ground State
20
Steady-state
G
G 
1
 isc
1
s
N
1
T
N
PL
S
EL
s
N
0
EL
T

1
T
0
 NsEL
= NtPL
S N

 isc N
EL
T
PL
T
N TPL  0
21
MEH-PPV LED
Al
ITO
Al 100nm
Ca 10nm
MEH-PPV (100nm or 50nm)
PEDOT 40nm
ITO 80nm
Glass
22
Experiment
setup
Pump
Laser
Attenuator
be
p ro
nm r
850 lase
Beam
expender
Singlet
detector
PL
Triplet
detector
lens
lens
Preamplifier
s
len
Lock-in
sample holder
EL
Function
generator
23
Optical table
24
Infrared semiconductor probe lasers
25
Cooling system (under construction)26
10
9
8
5
R/R x 10
(Triplet exciton induced-absorption)
EL-induced absorption (EA)
spectrum due to the triplet
exciton
7
6
5
4
3
2
1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 2.1
Probe photon energy (eV)
27
Triplet and singlet exciton
density
4 4.5
3.5
25
3
5
6 6.5
5.5
100nm EA vs EL
50nm EA vs EL
100nm PA vs PL
50nm PA vs PL
5
5.5
15
4.5
2
4
2 2.5 3
3.5
2.5
20
7
6
30
1.5
10
1
linear
1 1.5
Triplet (mV)
35
5
0
-20
0
20
40
60
80
100 120 140 160
Singlet (mV)
28
Luminescence intensity (a.u.)
Time-resolved PL, s=0.64 ns
1
0.1
0.01
1E-3
0.64 ns (592nm)
1E-4
0
2
4
6
8
10
12
Time (ns)
29
Phys. Rev. Lett., 90, 036601
(2003)
E  (V  Vbi ) / d


d : thickness of
MEH-PPV.
Vbi : built-in
voltage
Triplet/Singlet ratio 
20
18
16
14
12
50nm
100nm
10
8
6
4
2
0
5
5
5
5
5
5
5
5
1x10 2x10 3x10 4x10 5x10 6x10 7x10 8x10
E (V/cm)
30
Two possible explanations
Free carrier continuum
S2
0.3ev
S1
1/4
3/4
1. Field dissociation
T2
Phonon bottleneck
0.1ev
1ev
T1
2. Quenching by
polarons
Ground state
31
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

32
Parallel transport and
field effect transistors
33
Light emitting polymers
have very low carrier
mobility
34
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
Perpendicular transport
(high mobility)
Parallel transport
(low mobility)
j
j
Glass substrate
Glass substrate
35
Theoretical basis:
High intrachain mobility can be achieved even
with many conjugation defects
Yi-Shiou Chen and Hsin-Fei Meng,
Phys. Rev. B, 66, 035191 (2002)
36
Parallel hole transport
d
d = 2 micron
h = 100 nm
Au
h polymer
Au
glass
37
38
Thermal coater
39
Mask aligner for photo-lithography
40
Spinner
41
1μm gold source/drain channel on
glass or SiO2/ITO
42
Interdigited
1 μm channel
43
ITO/PPV/Au sandwich
device

9 V2
 3
8
L


Hole-only device
T=307K
2
SCLC
9 Vmodel
 3
J=
8
L
p=510-11m2/Vs
=510-7cm2/Vs
R1=CH3, R2=C10H21
   0 r
PRB55,R656(1997)
44
Space charge limited
current


Steady state: J=nqE
Poisson’s eq.:
 dE
q dx
n
J dE

E
 dx
1
 2 J  2 12
E (0)  0  E ( X )  
 x
  
1
 8J  2 32
V ( L)  V ,V  
 L
 9 

9 V2
J   3
8
L
……Mott-Gurney law
45
fixed T, variable SD distance
d
10000
d=2.5micron
d=4.5micron

2
current density J(A/m )
8000

6000
4000
Ohmic:
J=n0 q p E
There is little
dependence
between p and d.
2000
0
bias(V)
46
J-E plot

4
10

2
current density J(A/m )
d=2.5micron
d=4.5micron
3
10
2
10
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
6
7
10
10
field E(V/m)
p=3.810-3 cm2/Vs
47
sandwich device:
ITO/MEH-PPV/Ca/Al

bias>3V: SCLC
J= 9
V2
-2
10
2
current density(A/m )
-3
10
Ohmic
-4
10
0.01
0.1
1
bias voltage(V)
10
L3
=3
L =1200Å
p =1.44×10-5cm2V-1s-1
 bias<3V: Ohmic
J=n0 q p E
n0 =7.84×1021m-3
r
SCLC
8
 0 r  p
48
Compare with other sandwich
devices

4
10
p=3.810-3 cm2/Vs
3
10
Chen:
p =1.44×10-5 cm2/Vs
 Friend:
p =2×10-7 cm2/Vs
 Hegger:
p =2.24×10-7 cm2/Vs

2
10
2
current density J(A/m )
Our horizontal device:
1
10
0
10
d=2.5micron
d=4.5micron
Chen (Sandwich)
Heeger (MEH-PPV)
Friend (PPV)
-1
10
-2
10
-3
10
-4
10
0
7
1x10
7
2x10
7
3x10
field E(V/m)
7
4x10
7
5x10
7
6x10
49
fixed T, variable d

4
2.5x10

4
2
current density J(A/m )
2.0x10
4
1.5x10

4
1.0x10
d=0.9micron
d=2.5micron
d=4.5micron
d=9.6micron
d=14.7micron
3
5.0x10
T=297K
There is little
dependence
between p and d
No domain down
to 1 micron
0.0
0
7
1x10
7
2x10
7
3x10
field E(V/m)
7
4x10
7
5x10
7
6x10
50
Temperature dependence
fixed d, variable temperature
d=0.9micron
T : from 297K to 235K
J=n0 q p E
4
2.5x10
297K
282K
267K
256K
235K
4
2
current density J(A/m )
2.0x10
4
1.5x10
4
1.0x10
3
5.0x10
0.0
0
7
1x10
7
2x10
3x10
7
4x10
7
5x10
7
7
6x10
field E(V/m)
51
fixed d, variable temperature
-6
10
2
p= 0exp(-/kBT)
=0.233eV
Horizontal ~ Sandwich/2
hole mobility(m /Vs)
d=0.9micron
-7
10
-8
10
3.2
3.4
3.6
3.8
4.0
4.2
4.4
-1
1000/T(K )
52
Field effect transistor and
its applications
53
Bottom gate transistor
structure
d
d = 2 micron
h = 100 nm
Au
h polymer
Au
SiO2
ITO
glass
54
P-type transistor
with hole accumulation
channel
VGS<0
source
drain
insulator
gate
glass
55
Application: active matrix
flat-panel-display
56
Passive matrix display
Scan line
1.One row each scan
2.Fast degradation
3.Voltage drop in lines
4. Uniformity problem
Data line
57
One active matrix
Pixel
Driving
TFT
Data line
Scan line
Switchin
g TFT
58
Design by Cambridge and Seiko-Epson
59
Our design : Pixel and FET
share same semiconductor

Side view
Metal
PPV
S
ITO
D
I
G
S
D
I
G
60
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
61
MEH-PPV FET characteristics
Vsd < 0
FL023
-8
1.0x10
-9
5.0x10
0.0
-9
-5.0x10
Ids(A)
-8
-1.0x10
-8
-1.5x10
-8
Vgs=0V
-5V
-10V
-15V
-20V
-2.0x10
-8
-2.5x10
-8
-3.0x10
-8
-3.5x10
-30
-25
-20
-15
Vds
-10
-5
0
62
FET characteristics
Vsd > 0
FL016
-6
1.2x10
1 A
Vg=-30
-25
-20
-15
-10
-5
-6
1.0x10
-7
8.0x10
ISD
-7
6.0x10
-7
4.0x10
-7
2.0x10
0.0
0
10
20
VSD
30
40
63
Frequency response
 Setup：
G
Function
generator
S
SiO2
D
MEH-PPV
R1
oscilloscope
64
1KHz frequency
response: not bad
Gate
voltage
R1
Voltage
65
Conclusion
• Same-polymer pixel+FET is possible
• Simplified active-matrix display design
• Processing on flexible substrate is
possible
66
Voltage-tunable full-color
PLED
67
Motivation
 Full
color display without
pixel patterning
 Signaling
 Lighting
68
Working principle
 Hole
mobility is much larger
than electron mobility
 Electron mobility increases
rapidly with field
 Recombination zone pushed by
field
69
Suitable structure
with electron blockade
e
e
Ca: 2.9
MgAg: 3.7
AL: 4.2
ITO
4.8~5.0
h
AU
5.2
h
70
Red
(610~640nm, 2.03~1.94eV)

1. MEH-PPV: 605nm, 2.8—5.0eV
~1.0%(PRB, 53, 15815(1996))
71
Green
(505~555nm, 2.46~2.23eV)

1. A proprietary material of Dow
Chem.
536nm, ??--??eV, ~0.9%
(SM, 111, 159(2000))
72
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))
73
Electron blocking

PVK: 1.2—6.1eV(APL, 65, 1272(1994)),
tetrahydrofuran(THF), chloroform
(APL, 74, 3613(1999)),
trichloroethane
(JAP, 79, 934(1996))
74
4V
9V
11V
13V
75
9v
5v
17v
13v
20v
76
PEDOT/PVK/PFO/PF/MEH
4V 592
6V 588
8V 584
10V 580
12V 576
14V 572
16V 568
18V 556
1.0
Y Axis Title
0.8
0.6
0.4
0.2
0.0
300
400
500
600
700
800
X Axis Title
77
PEDOT/PVK/PFO/G/MEH
6V 584
8V 576
10V 572
12V 572
14V 568
16V 560
18V 556
20V 552
1.0
Y Axis Title
0.8
0.6
0.4
0.2
0.0
300
400
500
600
700
800
wavelength
78