Mechanism of Conduction in Conducting Polymers

Introduction

Conducting polymers are organic polymers that conduct electricity due to the presence of conjugated π-electron systems along their backbone. Unlike conventional insulators (like polyethylene), these materials exhibit electrical conductivity ranging from semiconducting to metallic levels when properly doped.

Key examples: Polyacetylene, Polyaniline, Polypyrrole, Polythiophene, and PEDOT.

1. Conjugated Structure – The Foundation

Conduction begins with alternating single and double bonds (conjugation) in the polymer chain:

    –C=C–C=C–C=C–   (Polyacetylene backbone)
        

• π-electrons are delocalized over the chain.
• Overlap of p-orbitals forms π and π* bands (like valence and conduction bands in semiconductors).
• In undoped state: Large band gap (∼1.5–3 eV) → insulating or semiconducting.

2. Doping – Enabling Charge Carriers

Doping introduces charge carriers (electrons or holes) by redox processes or protonation.

Types of Doping:

Type	Mechanism	Example	Charge Carrier
p-type (Oxidative)	Removal of electrons → cation formation	I2 doping of polyacetylene	Positive polarons/bipolarons
n-type (Reductive)	Addition of electrons → anion formation	Na/naphthalene doping	Negative polarons/bipolarons
Protonation (Acid doping)	H+ adds to nitrogen → charged site	HCl doping of polyaniline	Positive charges on chain

3. Charge Carriers: Polarons and Bipolarons

• Polaron: A radical cation/anion + local lattice distortion (quinoid segment).
• Bipolaron: Two like charges on same chain → dication/dianion (more stable at high doping).
• These quasi-particles move along the chain or hop between chains.

4. Conduction Mechanisms

a) Intrachain Transport

• Charge carriers move along the conjugated backbone.
• High mobility if chain is planar and defect-free.
• Limited by chain length and conjugation breaks.

b) Interchain Hopping

• Charge jumps between adjacent polymer chains.
• Governed by variable range hopping (VRH) model (Mott’s law):

σ = σ0 exp[ – (T0/T)1/(n+1) ]
            

• n = 3 → 3D hopping
• n = 1 → 1D systems

c) Metallic Conduction (High Doping)

• At high doping (>10%), bipolaron bands merge with valence/conduction bands.
• Forms partially filled bands → metallic state.
• Conductivity up to 10⁵ S/cm (e.g., stretched polyacetylene).

5. Factors Affecting Conductivity

Factor	Effect
Doping level	Increases → conductivity rises (up to a maximum)
Chain orientation	Aligned chains (e.g., stretch-aligned) → higher σ
Morphology	Crystalline domains → better interchain transport
Temperature	Metallic: σ ∝ T–1; Semiconducting: σ ∝ exp(–Ea/kT)
Counterions	Larger ions may disrupt packing → lower mobility

Summary

Conduction in polymers relies on:

1. Conjugated π-system for electron delocalization.
2. Doping to generate mobile charge carriers (polarons → bipolarons).
3. Intra- and interchain transport via band motion or hopping.
4. At high doping → metallic islands and percolation paths.

This unique blend of organic flexibility and tunable electronic properties makes conducting polymers ideal for flexible electronics, sensors, OLEDs, and solar cells.

Polarons and Bipolarons in Conducting Polymers

• Polaron: A polaron is a quasi-particle formed when a charge (electron or hole) in a conducting polymer becomes coupled with a local distortion of the polymer chain. This occurs when the polymer is doped, creating a single charge with an associated local lattice distortion. Polarons have a charge of ±1 and a spin of ½, allowing them to move along the polymer backbone and contribute to conduction.
• Bipolaron: A bipolaron arises when two like charges (often +2 from double oxidation) are stabilized together with a larger lattice distortion. Bipolarons are spinless (spin 0) and carry a charge of ±2. They are typically dominant in heavily doped polymers and can move along the chain, further enhancing conductivity.

Role in Conduction

• When an electric field is applied, polarons and bipolarons move along the polymer backbone, carrying charge and enabling electrical conduction.
• In heavily doped polymers, bipolarons may outnumber polarons, increasing conductivity as they are more mobile in some systems.

Summary Table

Species	Charge	Spin	Structure	Role in Conduction
Polaron	±1	½	Charge + local distortion	Primary carrier (lightly doped polymer)
Bipolaron	±2	0	Two charges + larger distortion	Dominant in heavy doping