Conducting PolymersConducting Polymers

10-21-08

Questions: erica.forzani@asu.edu

Conducting Polymers - AdvantagesCombination of properties

Metals Plastics

High conductivity LightnessHigh conductivity LightnessEase of processing (spin coating)Low costTailored synthesis

Conductivity of typical materials

-Semiconducting polymers comprise a wide range of conductivity

- The higher conductivities values are obtained when polymers are “charged“

Polymers

HH HH HH HH

CH2 CH2n

- Traditional plastic : Polyethylene“Always insulator”

CC

HH

CC

HH

CC

H

H

H

CC

H

H

H

C CH

H

H

H HH HH HH HHHH

Ethylene

- Conjugated polymer : Trans-polyacetylene CH CHn“insulator conducting”

HOW ?

C HCH CC

CC

CC

CC

CCH

H

H

H

H

H

H

H

H

HHOW ?

H H H H HAcetylene

+n

3 I2n

+ 2 I3-

2 +

Clue: alternate single-double bonds

Polyacetylene (PA)

PA i th i l t j t d l

Doped Polyacetylene (doped-PA)

•After exposure to iodide:•PA is the simplest conjugated polymer

• Conductivity at room temperature:- c-PA: 10-7 Sm-1

After exposure to iodide:

Conductivities are as high as 105 Sm-1

- t-PA: 10-3 Sm-1

compared to cooper 108 Sm-1

PA suffer a drastic increase of conductivity (108-1013)

moderate insulatorThe plastic becomes conductive

possibility of plastic wiresPA

+ 3 I2 + 2 I3-

2 +

PA doped-PA

+n

3 I2n

+ 2 I3

+n

3 I2n

+ 2 I3-

2 +

Nobel Prize in Chemistry 2000

“For the Discovery and Development of Conductive Polymers”

Hideki ShirakawaUniversity of TsukubaUniversity of Tsukuba

Alan HeegerUniversity of California

Alan MacDiarmidUniversity ofy

at Santa Barbaray

Pennsylvania

Other Conjugated Polymers Conducting Polymers

n Sn

N

Hn

Trans-polyacetylene (t-PA) Polythiophene (PT) Polypyrrole (PPY)

nn

Poly(p-phenylene) (PPP)

n

Poly(p-phenylenevinylene) (PPV)PEDOT (very conductive

NH NH N N

PEDOT (very conductive and stable)

ny 1-yPolyaniline (PAN)

π electrons from double bonds (sp2)

How the conjugated organic polymer is converted into a conducting polymer ?

It can not move pieces It can move pieces

A full orbital can not conduct electrons,

so to get a conjugated material to conduct, we must add or remove charges (doping process)

Two options:- Remove electrons from the HOMO (create holes)

Chemically (oxidizing/reducing agents) or electrochemically (applied potential)

Remove electrons from the HOMO (create holes)- Add electrons to LUMO

Doping process

p-doping Partial oxidation of polymer chain

Per functional unit: [P] + y A- [(Py+) A - ] + y e

-[P]x- + xy A- -[(Py+) Ay- ]-x + xy e

Per functional unit: -[P]- + y A- -[(Py+) Ay- ]- + y e

n-doping Partial reduction of polymer chain

Per functional unit: -[P]- + y e + y M+ -[(Py-) My+ ]-

-[P]x - + xy e + xy M+ -[(Py-) My+ ]x-

Doping process: involve ionic transportDoping process: involve ionic transport

can be described in terms of diffusion/reaction eqs. (e.g. Barttlet,Gardner)

Differences of the doping process withd i i l t l i d tdoping process in elemental semiconductors

• Significant doping levels (until 10 mole %)

• There is a charge transfer between the incorporated dopant atom and the polymer chainincorporated dopant atom and the polymer chain = the lattice is partially oxidized or reduced

What are the consequences of doping ?

Ionic transportCharge transport:

Polaron (+.)Bipolaron (++)Soliton (0,+, -)

A-

A-, A-p-doping

n-doping

+

A-

Let’s draw on the board a generic case !

R1 R1 R1A

Intra and inter chain charge transport R2 R2 R2

R1 R1 R1

Intra and inter chain charge transport R2 R2 R2R2 = neutral groupR2 = anionic group (A-)

Molecular versus Band Models

M. Lögdlund et al.E

1

2 3

4

56

7

8nE (eV)

Electronaffinity

n

HOMO

LUMOaffinity

Bandgap

Ionizationpotential

p-doping, example 1: p-doping, example 1: Generation of Polaron and Bipolaron defects in Conjugated PolymersCB

VBRadical-cation / Polaron (spin)

+

VB

+

Polaron

Dication / Bipolaron (spin less)

++

Bipolaron

Produced as far as is possible to produce quinoid resonance structures

CB

p-doping, example 2: Generation of Polaron and Bipolaron defects in Conjugated Polymers

Polypyrrole (PPy)

VBVB

Polaron0.5 eV

Bipolaron

H il d d l

Band structure of a conjugated polymer as a function of

p-doping levelSli htl d d

E

Undoped polymer Heavily-doped polymerCB

Slightly-dopedpolymer

LUMO (L)

CB CB CB

LL L

POL2POL2BIP2

HOMO (H)

VB VB VB

HH H

POL1 BIP1

VB

Neutral PolymerVB full, CB empty

VB VB

VBPolarongenerationvia fisrt

Bipolaron generationvia second Further oxidation

tVB full, CB empty via fisrt1e oxidation

via second1e oxidation generates

bipolaron bands

J. Cornil et al., Adv. Mater. 8, 447 (1996)

Conductivity dependence as a function of polymer length

C d tConductance

Length of polymer

Why ?What is the eq. associated to this trend ? And its meaning ?

CBSoliton structures in polyacetylene – Band structure

VB

. Neutral SolitonSpin 1/2

+Positive Soliton

Spin 0

..Negative Soliton

Spin 0

So far…• Conducting polymer features:

- conjugated structure (alternate doble-single bonds) free electrons from double bond of conjugated polymer (π electrons)- dopping process need it- mixed electronic and ionic conduction- conductivity dependent on charge transport carriers: polaronconductivity dependent on charge transport carriers: polaron, bipolaron, soliton.

Dig into more basic studiesDig into more basic studies…• Conducting polymer techniques used for characterization:

- EC (electroactivity)- Quartz Crystal Microbalance (QCM) and Probe Beam Deflection (PBD) (ionic movement towards and from the polymer)- UV-Vis-Near IR spectroscopy (absorption)- Fourier Transform Infrared Spectroscopy (FTIS) (chemical structure)- … many other surface characterization techniques based on X-Rays or optical detection

Applications- Energy storage (batteries) Easy to producee gy sto age (batte es)

-Electrochromic windows (intelligent windows)

Easy to produceUltra high capacitance

- Actuators (Artificial muscles)

Sensors (field effect transistors)

- Electrostatic coatings

- Sensors (field effect transistors)

Baytron P (Bayer) / Orgacon (AGFA)

S

nn

O O

SO3-

PEDOT PSS

- Electromagnetic shielding

Electrochromic windows (intelligent windows)

SS

SS

SS

ts) Bipolaron

ty (a

rb. u

nit

Neutral

Polaron

Inte

nsit Polaron

Energy (eV)600 800 1000 1200 1400 1600 1800 2000

Energy (nm)

J. Guay et al.Chem. Mater. 4, 1097 (1992)

Electrochromic windows, sensorsSee “electrochomic display”

& “polyaniline video” from youtube.com

Actuators (Artificial muscles) ionic movement swelling-shrinking

PPy

A-

A

PPy

Reduction:Anion release

Oxidation:Anion intake

A-

A solid state artificial muscle based on polypyrrole(PPy) andpoly(epichlorohydrin-co-ethylene oxide) [P(ECH-co-EO)]/LiClO4 (solid polymeric electrolyte) is developed

See “artificial muscles” from youtube.com

TF Otero, et.al

Conducting Polymer FET Sensors

1 cm

1 mmBSi3N4 window

Recognizingelement on the

junctiongold pads

5µm5µm

1µmpolymerGap smaller than 60 nm

Glucose NanosensorGlucose Oxidase

Conducting polymer

20-60

Oxidant by-product the increases p-doping level

Time response: ~ 200 ms20-60 nm

AGold nanoelectrode

Glucose, O2

H O

Sensitivity: 1nA/mM

Detection Limit: 1 µM

H2O2

PANIox(more conducting) Detection Limit: 1 µM

A>0

( g)

E. Forzani, H. Zhang, L. Nagahara, I. Amlani,R. Tsui, and N.J. Tao Nano Letters 4 ,9 2004

Time course of drain current at Eg = 35 mV vs SCE (Vsd = -20 mV) recorded on a PANI-PAA/PANI-bisulfite/GOx-PDAB nanojunction (20-60 nm) in 20 uL Mc Ilvaine buffer,

0.5 M Na2SO4 pH 5 upon 1 µL successive additions of 40 mM glucose.