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?