Physics and applications of conjugated polymers semiconductors
description
Transcript of 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
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Outline Overview
Triplet exciton formation
Field-effect transistor
Multi-color LED
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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
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10
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LED Device Operation
Conduction
Valence
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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
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S
T
sT
G γG
Ground State
Free electron-hole pair
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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Free electron-hole pair
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
Free electron-hole pair
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
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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
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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
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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