孟心飛 交通大學物理所 polymers semiconductor devices. coworkers research group: 洪勝富...
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孟心飛 交通大學物理所
Polymers Semiconductor Devices
Coworkers
• Research group: 洪勝富 教授 清華大學電機系
• Ph.D. students 廖華賢 , 曾信榮 , 趙宇強 , 劉建成 , 楊家銘
• Facilities and material: 許千樹 教授 交通大學應化系
是否物理一個物理逃兵的感言
• 知識之累積– 物理
• 知識用於生活– 物理 ??
• 今天不談物理 , 卻談一個物理工作者遇到的一些重要問題
PPV
nn
OR
MeO
Polyfluorene
Conjugated Polymers
Organic Electronics
• Microelectronics -- Si• Macro-electronics -- C• Large area, flexible, light weight• High performance device by Solution process• Human interface
– Display, Lighting, Biosensor, Robot skin…
• Life and Life• Solar energy
Brief History
• First small-molecule OLED, Kodak 1987• First polymer LED, Cambridge 1990• Production of OLED since 2000• Production of organic transistor scheduled
2008• No commercial success as of 2007• Polymer solar cell becomes red hot since
2005
Recent research topics(2003-2007)
Triplet exciton PRL (2003), PRB (2003, 2005, 2006), APL(2007)
Gas effect PRB (2007, 2007)
Multi-layer PLED APL (2004, 2005, 2006), JAP(2007)
Metal-base transistors APL (2005,2006)
Solar cell Biochemical and distance sensor
What are special about carbon:1. Midway in periodic table to form covalent bonding with many atoms2. Small atomic size enables strong and stable bonding to itself, huge amount of organic molecules
Organic vs Inorganic
• “Old fashioned” semiconductor device– Inorganic– Epitaxy in vacuum– Lithography– Top-down, brute force
• “New” semiconductor device– Organic by synthesis– Self-assembled in solution– New device concept and physics– Bottom-up, Nature’s choice
Thin and lightweight
Flexible OLED
Illumination (Organic EL,IMES)
A lot of applications
Some organic materials
Small molecule
Alq3: electron transportand emissive layer
TPD: hole transport layer
PBD: elelctron transport layer
Conjugated polymers
alternation of single and double bonds
PF
sp3
sp2
2s+px+py+pz: sp3 hybridization
2s+px+py : sp2 hybridization
pz: no hybridization
120o
C bonded to four atoms, sp3
C bonded to three atoms, sp2
C bonded to three atoms, sp2
π* orbital
π orbital
π-electron system: πand π* form energy bands
Polyacetylene
sp2
σ bond
Energy bands of conjugated system
π-conjugated system
Electron
Hole
Fermi level EF
Conduction (LOMO,π*)
Valence (HOMO, π)
electron
hole
Band gap Eg
Wave number k
Energy E
PPV semiconductor band structure
One -electron for each carbon atom
E(k) xy
Model polymer: PPV
ITOPEDOT
Polymer
Ca/Al
MEH-PPV
nn
OR
MeO
Polymer Light-emitting diode (PLED)
Exciton formation
Electron-holepair
Coulomb interaction
Excitonstate
Large binding energyLarge binding energy
0.4eV 0.4eV 4641K 4641K
Very stableunder
room temperature
Singlet and triplet statesSpin-independent
1:3
Singlet exciton Triplet exciton
,
Radiative Non-radiativeDipole selection rule
Higher level
Lower level
Exchange energy
Multi-layer PLED for balanced recombination
Anode5 eV
Hole transport layer
Emission layer
ElectronBlockinglayer Hole
Blockinglayer
ElectronTransportlayer
Cathode 3 eV
Cannot be made by multiple spin coating due to dissolution
Liquid buffer layer method
substrate
Layer 1
substrate
Layer 1
Layer 2
A
B
substrate
Layer 1
substrate
Layer 1
Layer 2
C
(b)
Buffer layer(Liquid)
Buffer layer(Liquid)
Buffer liquid:1. High viscosity2. Low boiling point3. Non-solvent
Alcohol with multiple H-bond
Appl. Phys. Lett. 88, 163501 (2006)
Blue Polymer, closed structure
300 400 500 600 700 800
0.0
0.2
0.4
0.6
0.8
1.0
wavelength(nm)
A.U
.
BP105 700A TFB/BP105 700A
LiF
2.3
TFB
5.3
3.04
BP105
5.8PEDOT
5.2
Ca2.9
High hole mobility
Blue PLED with 9 cd/A (Photon/Electron 5.5 %)
0 2 4 6 8 10-1
0
1
2
3
4
5
6
7
8
9
10
V(V)
Y(c
d/A
)
BP105 700A TFB/BP105 700A
2.62%
5.54%
TFB
0 2 4 6 8 10
-2000
0
2000
4000
6000
8000
10000
12000
14000
16000
18000
20000
22000
24000
26000
28000
V(V)
L(c
d/m
2)
BP105 700A TFB/BP105 700AHigh Brightness
28,000 cd/m2
White PLED with polymer blendBP105:PDY132=100:4
4V(0.31,0.53)
6V(0.28,0.48)
8V(0.26,0.46)
BP105 (0.14,0.17)
300 400 500 600 700 800
0.0
0.2
0.4
0.6
0.8
1.0
EL
sp
ect
ra
nm
TFB/BP105:PDY @4V TFB/BP105:PDY @6V TFB/BP105:PDY @8V
0 2 4 6 8 10
-10123456789
10111213141516
Yie
ld(C
d/A
)
Voltage
TFB/BP105 tolu. 1a TFB/BP105 tolu. 2a (dark) TFB/BP105(tolu):PDY132 1a TFB/BP105(xy):PDY132 2a
BP105:PDY132 16 cd/A, 70,000 cd/m2
0 2 4 6 8 10
-50000
50001000015000200002500030000350004000045000500005500060000650007000075000
L(C
d/m
2)
Voltage
TFB/BP105 tolu. 1a TFB/BP105 tolu. 2a (dark) TFB/BP105(tolu):PDY132 1a TFB/BP105(xy):PDY132 2a
-10 -5 0 5 100.01
0.1
1
10
100
1000
J(m
A/c
m2)
Voltage
TFB/BP105 tolu. 1a TFB/BP105 tolu. 2a (dark) TFB/BP105(tolu):PDY132 1a TFB/BP105(xy):PDY132 2a
0 2 4 6 8 10
-0.20.00.20.40.60.81.01.21.41.61.82.02.22.42.62.83.03.2
Y
V
400kPFO(800A) #1a 400kPFO:190kTFB=100:1(800A) #3b 190kTFB(300A)/400kPFO(700A) #6a 190kTFB(300A)/400kPFO:190kTFB=100:1(700A) #7a 190kTFB(300A)/400kPFO(600A)/B(200A) #9a 190kTFB(300A)/400kPFO:190kTFB=100:1(600A)/B(200A) #10a 190kTFB(300A)/400kPFO:190kTFB=100:1(600A)/B(200A) #11a
3.99%
High Mw PFO, Open chemical structure3 cd/A (4 %) in deep blue
Tri-layer structure
High Brightness of Tri-layer PFO LED
0 2 4 6 8 10-2000
0
2000
4000
6000
8000
10000
12000
14000
L
V
400kPFO(800A) #1a 400kPFO:190kTFB=100:1(800A) #3b 190kTFB(300A)/400kPFO(700A) #6a 190kTFB(300A)/400kPFO:190kTFB=100:1(700A) #7a 190kTFB(300A)/400kPFO(600A)/B(200A) #9a 190kTFB(300A)/400kPFO:190kTFB=100:1(600A)/B(200A) #10a 190kTFB(300A)/400kPFO:190kTFB=100:1(600A)/B(200A) #11a
300 400 500 600 700 800
0.0
0.2
0.4
0.6
0.8
1.0
AU
WL
400kPFO:190kTFB=100:1(800A) #3b 190kTFB(300A)/400kPFO:190kTFB=100:1(700A) #7a 190kTFB(300A)/400kPFO:190kTFB=100:1(600A)/B(200A) #10a 190kTFB(300A)/400kPFO:190kTFB=100:1(600A)/B(200A) #11a
Organic Metal-base Junction Transistor
•Sandwich device structure•Vertical charge transport•Low voltage, high current, high speed•FET driving unstable for active matrix
Substrate
Gate Dielectric
Source Drain
Organic Layer
Driving voltage ~ 1/L (40 V)Switching time ~ vd/L ~ 1/L2 (ms)
Vertical structure •Short channel length•Low voltage•High speed•On-top of OLED
Base (metal)
Emitter
Collector
Vertical metal-base junction transistor
L
L
Bipolar junction transistor
Base transport factor α=IC/IE
Common-emitter current gainβ= α/(1-α)=IC/IB
Need large IC, large β, low VCE
First all-organic hot-carrier transistor
Current gain (Ic/Ib) = 30, Current density low 0.1mA/cm2
OLED current density 20-100 mA/cm2
Hot-carrier transistor:• Operation not limited by mobility• Large current, low voltage, high-speed• Attempted for Si at 1960’s but failed • Better than FET for organics
Appl. Phys. Lett. 87, 253508 (2005)
ITO(~1500A)
P3HT(1300A) Al(rate~20A/s total100A)
Emitter Au(rate~10A/sec total300A)
700A
PEDOT(200A)
LiF(7A)
Hot carrier transistor with high current density
High vertical mobilityEfficient hole injection
-10 -8 -6 -4 -2 0 2 4
-1.0x10-3
-8.0x10-4
-6.0x10-4
-4.0x10-4
-2.0x10-4
0.0
IE 0A 0.2E-3 0.4E-3 0.6E-3 0.8E-3 1E-3
IC(A)
VC(V)
0.931E-3
Common Base Current gain=13.7
IC max = 46 mA/cm^2
P3HT/Al/LiF/P3HT/Au
-10 -8 -6 -4 -2 0-1.5x10-3
-1.0x10-3
-5.0x10-4
0.0
IB 0A 1E-5 2E-5 3E-5 4E-5 5E-5
IC(A
)
VC
P3HT/Al/LiF/P3HT+PVK(5:1)/Au
Common Emitter Current gain=25
IC max = 126 mA/cm^2
EBC
Vout1
~
Vout2
10V
100Ω
ΔV=0~5 V
IC ~ 10-4 AVEC ~ 10 VREFFECT ~ 100 KΩ >> 100Ω
Response time of hot carrier transistor
Radio Frequency Tag works at13.6 MHz
Organic FET works at 1 kHz
0 100 200 300 400 500-20
0
20
40
60
80
100
HOT #167 VEC
=10V Vmax
EB=5V
10 kHz with resistance 100
VR , I
C
stander reversed
Am
p.
Time
Responding up to 1 MHzfastest organic transistor
0 100 200 300 400 500-150
-100
-50
0
50
100
150 V
R , I
C
stander reverseHOT #167 V
EC=10V Vmax
EB=5V
100kHz with resistance 100
Am
p.
Time
0 100 200 300 400 500-150
-100
-50
0
50
100
150HOT #167 V
EC=10V Vmax
EB=5V
1 MHz with resistance 100 V
R , I
C
stander reversed
Am
p.
Time
Space-charge-limited transistor
• Solid state vacuum tube
• Classical device versus Quantum device
• High current gain
• High on-off ratio
• Easier fabrication
Vacuum tube : space-charged limited current controlled by grid potentialVacuum-tube
Semiconductor-Metal gird -Semiconductor StructureA solid-state organic “vacuum tube”
-2.4x10-7
-1.8x10-7
-1.2x10-7
-6.0x10-8
0.0
VB = 0.50V
VB = 0.02V
VB = -0.22V
VB = -0.46V
VB = -0.70V
I C (
A)
(a)
-3 -2 -1 0-1.0x10-9
-5.0x10-10
0.0
5.0x10-10
1.0x10-9
I B (
A)
VC (V)
(b)
Current gain is 506 when VC is -3V.
5 μm
Polymer “Vacuum tube” with low voltageRandom holes on Al using PS spheres as shadow masks
2000 A sphere
5000 A sphere
Appl. Phys. Lett. 88, 223510 (2006)
Scale 5 × 5 um
1000 A
2000 A
-3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0
-5.0x10-6
-4.0x10-6
-3.0x10-6
-2.0x10-6
-1.0x10-6
0.0
Vb = -0.8V Vb = -0.48V Vb = -0.16V Vb = 0.16V Vb = 0.48V Vb = 0.8V
Ic
Vc
-8 -7 -6 -5 -4 -3 -2 -1 0
-1.0x10-5
-8.0x10-6
-6.0x10-6
-4.0x10-6
-2.0x10-6
0.0
Vb = -0.8V Vb = -0.48V Vb = -0.16V Vb = 0.16V Vb = 0.48V Vb = 0.8V
IcVc
Novel way for high PS sphere density on hydrophobic polymer surfaceCurrent density 2 mA/cm2
Al grid (5 μm x5 μm)
evaporation control of PS sphereCurrent density 1 mA/cm2
Polymer field-effect transistor on Glass : Gate metal effect
•High mobility only on Si substrate with thermal oxide•Glass substrate devices needed for real applications
Polymer field-effect transistor
Au (source) Au (drain)
SiO2(3800A or 2000A)
glass
Gate
P3HT(p-type)
Regioregular and regiorandom
Sirringhaus et al Science 401, 685 (1999)
Sample Gate Oxide roughness (nm)
Mobility (cm2/V-s) On/off ratio
(a) ITO 3.3 Below 10-4 (not stable) Below 100
(b) Al 9.5 6.3 x10-4 970
(c) Cr 2.6 8.77 x 10-4 750
(d) Cr (after O2
plasma)
1.2 3.3 x 10-3 10000
(e) Cr (after O2
plasma)
1.2 5 x10-2 320
(f) Cr (after O2
plasma)
0.7 3 x10-1 1100
Our work: Gate metal and polymer mobility on glass
Glass
Gate SiO2
Source DrainP3HTITO Al
Cr Cr
APCVD SiO2 surface
(a)
-40 -30 -20 -10 0
-140.0u-130.0u-120.0u-110.0u-100.0u-90.0u-80.0u-70.0u-60.0u-50.0u-40.0u-30.0u-20.0u-10.0u
0.010.0u
Dra
in c
urr
en
t (u
A)
Drain-source Voltage (V)
10V0V
-10V
-20V
Gate-source voltage=-30V
Dip-coated P3HT surface morphology on Cr gate
Dip-coated Spin-coated
Mobility = 0.3 cm2/Vs, as high as Si substrate, on-off 30,000
Appl. Phys. Lett. 89, 243503 (2006)
Summary on polymer devices• High performance blue PLED can be made by
multilayer spin-coating (9 cd/A) (5.5 %)• High efficiency blue PLED for simple PFO (3
cd/A) (4%)• Metal-base vertical transistor delivers high
current at high speed without high mobility.• Old alternative concepts (vacuum tube, hot
carrier) can be applied to organic semiconductors successfully.
• Physics is essential
Other topics
• Solar cell– Device modal– Limit of low Voc and Fill Factor– Trinary blend for high Jsc
• Biochemical sensor
• Distance sensor
Outlooks
• Bad and good time for conjugated polymers
• Bad time– Difficult to make commercial success with
polymers
• Best time– Harvest for 20 years of research– Display in not the only thing– Light in the horizon
• Physics?