MSE150-Electrical Properties -One Lecture

8/2/2019 MSE150-Electrical Properties -One Lecture

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Copyright of Prof. Qibing Pei

Materials Science & Engineering

1

MSE 150: Introduction to Polymer

Electrical Properties of Polymers

Professor Qibing Pei

Department of Materials Science & EngineeringUniversity of California, Los Angeles

[email protected]

Extension: 310-825-4217



Materials Science

& Engineering

2

Band Diagrams of Materials





3

Electrical Properties of Common Materials

• Room Temperature s values (W-m)-1

Polystyrene <10-14

Polyethylene 10-15

- 10-17

Silver 6.8 x 107

Copper 6.0 x 107

Iron 1.0 x 107

METALS:

Silicon 4 x 10-4

Germanium 2 x 100

GaAs 10-6

SEMICONDUCTORS:

Soda-lime glass 10-10

Concrete 10-9

Aluminum oxide <10-13

CERAMICS:

POLYMERS:

conductors

semiconductors insulators


Materials Science

& Engineering

4

A dielectric material is

electrically insulating and exhibitsor may be made to exhibit an

electric dipole structure.

Application: capacitors that

store charges.O

HH

..

:

A parallel-plate capacitor (a)whena

vacuumis present and (b)whena dielectric

material is present.

Q AC

V l

e : permittivity of the dielectrice0 : permittivity of vacuum

=8.85x10-12 F/m

e r : relative permittivity or

dielectric constant

0 r

Dielectric Properties





5

Dielectric Constant and Dielectric Strength


Materials Science

& Engineering

6

Dipole moment:

Density of charge (dielectric displacement):

p qd

Q D E

A

(a) Imposed forces (torque) acting ona dipole by an electric field.

(b ) Final dipole alignment with thefield.

An electric dipole generated bytwo electric charges (ofmagnitude q) separated by thedistance d; the associatedpolarization vector p

Dipole and Charge Density





7

p = pe + pi + p o

Totalpolarization:

Origin of Polarization


Materials Science

& Engineering

8

Dipole orientations for one

polarity of an alternating

electric field and for thereversed polarity.

Variation of dielectric

constant with frequency of

an AC field.

Relaxation of Polarization





9Ferroelectric and Piezoelectric Polymers

Ferroelectric: The polar crystallites are aligned (poled) by high electricfield. Relaxation is very sluggish due to large crystallite size

P

b-phase

Polyvinylidene difluoride (PVDF)


Materials Science

& Engineering

10

Electromechanical transduction in dielectric elastomers

Pressure generated by field: p = e e o E 2 = e e o (V / z)2

Small longitudinal strain: s z = - p / Y = - e e o E 2 / Y

Small biaxial transverse strain: s x = s y = - 0.5 e e o E 2 / Y

Energy density (/volume): e v = 0.5 p (- s z ) = 0.5 (e e o )2 E 4 /Y

Polymer

filmVoltage off

Compliant electrodes(on top and bottom

surfaces)V

Voltage on

- - - - - - - - - - -+ + + + + + + + + + +

Dielectric Elastomers





11Dielectric Elastomer Actuators and Robots

Electromechanical transduction in dielectric elastomers

Artificial Muscles


Materials Science

& Engineering

12

Polyphenylene

Poly(phenylene vinylene)

S

S

SPolythiophene

Polyacetylene

N

N

NPolypyrrole

H

H

H

Conjugated Polymers





13

C

C

C

C

C

C

H

H

H

H

H

H

p

p*

2p 4p 8p np

LUMO

HOMOEg

p-electron delocalization in conjugated polymers

p-Conjugation


Materials Science

& Engineering

14

Electronic and optical properties of conjugated polymers

LUMO

HOMOEg

+

p-doping

(Electro)chemical doping:conductive polymers

-

+

Optical absorption,photoluminescence,

and photovoltaics

-

+

electroninjection

holeinjection

Charge injection andelectroluminescence

-

n-doping

Charges and Doping in Conjugated Polymers





15Conductivity of doped conjugated polymers

Doped polyacetylene, 1x105

S/cm

Doped oriented polyphenylenevinylene, 1x104

Stretch-oriented dopedpolypyrrole: 1x103

Doped polythiopehen: 100-1000

Electrical conductivity, s = ne m

Conjugated polymers are doped tointroduce large number of charge

carriers, one charge per 2-4monomer units

n = 1021 ~ 10 22 /cm3

m = 10 -3 ~ 102 cm2 /Vs


Materials Science

& Engineering

16Undoped Conjugated Polymers as Semiconductor

1. Polymer Field Effect Transistor

2. Polymer Light Emitting Diodes

3. Polymers Solar Cells





17Field Effect Transistor (FET)

Source, Drain, Gate electrodes: Au, Al, PEDOT, doped silicon, etc.

Dielecrtric layer: SiO2, SiNx, polyvinyl alcohol, etc., 100-400 nm thick

Semiconductor: 30-100 nm thickness solution cast polymers

Source-drain channel length, L: 10~100 m m

Channel widths, W : 100 m m.


Materials Science

& Engineering

18FET modulates source-drain current

Channel length L

Charge mobility m

Drain current I d

Source/drain voltage VD

Source/gate voltage VG

Channel width W

Insulator capacitance COX

Threshold voltage VT

Drain current in linear region:

Drain current in saturated region:





19Ordering polymer chains for high carrier mobility

Regioregular Poly(3-hexylthiophene)

Liquid crystalline

polyfluorene copolymer


Materials Science

& Engineering

Flexible active-matrix displaycombines Philips' ultra-thin

polymer TFT backplane with E-Ink's electronic ink frontplane.

20Polymer FETs for thin film transistor array





21Polymer Light Emitting Diodes

electrons

holes

●

☼

● ●

○○○

Al/Ca

Light Emitting Polymer

Conducting Polymer

Indium-tin oxide (ITO)


Materials Science

& Engineering

22Charge Injections in a polymer LED

2.8 eV 2.9 eV

4.7 eV

ITO

Ca

MEH-PPV

4.9 eV

MEH-PPV

ITO Ca

e-

h+

Reverse bias

ITO

Ca

Al

Au

e-

h+

Forward bias

Energy levels

relative to vacuum

O

On

MEH-PPV

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0

10

20

30

40

50

60

70

-6 -4 -2 0 2 4 6

Current [mA]

Light Intensity

C u r r e n t [ m A ]

Bias[V]

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0

10

20

30

40

50

60

70

-6 -4 -2 0 2 4 6

Current [mA]

Light Intensity

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0

10

20

30

40

50

60

70

0

10

20

30

40

50

60

70

-6 -4 -2 0 2 4 6-6 -4 -2 0 2 4 6

Current [mA]Current [mA]

Light IntensityLight Intensity

C u r r e n t [ m A ]

Bias[V]





23Light Emitting Polymers

0.0

0.2

0.4

0.6

0.8

1.0

1.2

400 500 600 700 800

L i g h t I n t e n s i t y ( a . u . )

Wavelength (nm)

OO


Materials Science

& Engineering

24Colors





25OLED Displays

LG 55” OLED TV

Samsung Galaxy S2


Materials Science

& Engineering

26OLED Lighting





27Polymer Solar Cells

Al/Ca

Polymer

ConductingPolymer

ITO

Power conversion efficiency

SC OC I V FF

PCE IncidentSolarPower


Materials Science

& Engineering

28Photocurrent and Photovoltage





29Bulk Heterojunction

Bulk heterojunction in p- and n- semiconductor blend


Materials Science

& Engineering

30Materials in polymer solar cells





31Summary of Polymer Semiconductors

1. Processable semiconductor

2. Low mobility

3. FET

4. LEDs and displays

5. Solar cells

6. Large-area, thin-film, low-cost fabrication

7. Further materials R&D may overcome theremaining hurdles to flexible electronics


Materials Science

& Engineering

32Electrical Properties: Summary

1. Polymers are mostly insulators

2. Dielectric polarization

3. Dielectric constant (relative permeability)

4. Dielectric elastomers

5. Conductive polymers comprised ofconductive fillers

6. Ionically conductive polymers

7. Dope conjugated polymers

8. Change of properties of conjugatedpolymers as a function of doping

MSE150-Electrical Properties -One Lecture

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