L9: Conductive Polymers and Organic Semiconductors

Lecture 9: Conductive Polymers and Organic Semiconductors

1. Introduction to Organic Semiconductors

• Definition: Organic semiconductors are materials primarily made of carbon and hydrogen, with some heteroatoms (e.g., sulphur, oxygen, nitrogen). They exhibit semiconducting properties, such as light emission in the visible range and electrical conductivity.

• Applications: Used in organic light-emitting diodes (OLEDs), organic solar cells (OSCs), and organic field-effect transistors (OFETs).

2. Conjugated Polymers

• Structure: Conjugated polymers have a backbone of alternating single and double bonds, allowing for delocalized ππ-electrons.

• Examples:

  • Polyacetylene: The first conductive polymer, discovered by Heeger, MacDiarmid, and Shirakawa (Nobel Prize in Chemistry 2000).

  • Polypyrrole, Polythiophene, PEDOT: Common conductive polymers with sp²-conjugated backbones.

3. Organic vs. Inorganic Semiconductors

• Inorganic Semiconductors:

  • Examples: Silicon (Si), Germanium (Ge), Gallium Arsenide (GaAs).

  • Band gaps: Typically low (e.g., 1.1 eV for Si).

  • Conductivity: Intrinsic, due to thermal excitation of electrons.

• Organic Semiconductors:

  • Band gaps: Larger (2–3 eV), making thermal excitation less effective.

  • Conductivity: Extrinsic, due to doping, charge injection, or photogeneration.

  • Dielectric constant: Lower (εr≈3.5), leading to stronger Coulomb interactions.

4. Doping in Conductive Polymers

• Doping: Introduces charge carriers (e.g., via oxidation or reduction) to increase conductivity.

• Example: Polyacetylene can be doped to increase conductivity from 10−8 to 102 S/cm

5. Molecular Orbitals and Conjugation

• Conjugation: Requires a continuous array of p-orbitals for π-bonding overlap.

• Molecular Orbitals:

  • σσ-orbitals: Electrons centred around the bond axis.

  • ππ-orbitals: Electrons found above and below the bond axis.

• HOMO-LUMO Gap: Determines the energy required for electron excitation. Smaller gaps in larger conjugated systems (e.g., pentacene).

6. Applications of Conductive Polymers

• Organic Photovoltaics (Solar Cells): Use of polymers like P3HT (Poly(3-hexylthiophene)) for light absorption and charge carrier generation.

• Organic Light-Emitting Diodes (OLEDs): Polymers like MEH-PPV and PEDOT:PSS are used for light emission.

• Organic Field-Effect Transistors (OFETs): Polymers like P3HT are used for their high charge carrier mobility.

7. Crystallization of Conductive Polymers

• Crystallinity: Affects the electrical properties of polymers. Higher crystallinity improves charge carrier mobility.

• Effect of Molecular Weight (Mw):

  • Low Mw: Forms smaller crystalline lamellae (<30 nm).

  • High Mw: Forms larger crystalline lamellae (>30 nm), leading to higher carrier mobility.

• Effect of Additives:

  • 2-methylpentane (2-MP): Increases supersaturation, promoting nucleation.

  • Ultrasound: Provides energy for fibre nucleation.

8. Effect of Epitaxy

• Epitaxial Crystallization: Oriented growth of polymer crystals on a substrate (e.g., P3HT on 1,2,4,5-tetrachlorobenzene), leading to improved electrical properties.

9. Polythiophene (P3HT)

• Structure: Regioregular P3HT has a head-to-tail (HT-HT) configuration, leading to better crystallinity and charge carrier mobility.

• Applications: Used in OFETs and OSCs due to its light absorption, charge carrier mobility, and environmental stability.

• Crystallization: Forms nanofibers with lamellar stacking, where the fibre length correlates with the number of chains in π-stacking.

10. HOMO-LUMO Levels

• HOMO (Highest Occupied Molecular Orbital): Energy level of the highest occupied electron state.

• LUMO (Lowest Unoccupied Molecular Orbital): Energy level of the lowest unoccupied electron state.

• Example: In P3HT/PCBM solar cells, the HOMO-LUMO levels determine the efficiency of charge separation and light absorption.

11. Summary of Key Points

• Conductive Polymers: Have conjugated backbones with alternating single and double bonds, enabling delocalized ππ-electrons.

• Applications: Used in OLEDs, OSCs, and OFETs due to their semiconducting properties.

• Crystallinity: Higher crystallinity improves electrical properties, and molecular weight, additives, and epitaxy influence crystallization.

• Doping: Increases conductivity by introducing charge carriers.

• HOMO-LUMO Gap: Determines the optical and electronic properties of organic semiconductors.

Learning Outcomes

By the end of this lecture, you should be able to:

1. Understand the structure and properties of organic semiconductors, including conjugated polymers like polyacetylene, P3HT, and PEDOT.

2. Compare organic and inorganic semiconductors in terms of band gaps, conductivity, and dielectric constants.

3. Explain the role of doping in increasing the conductivity of polymers.

4. Describe the molecular orbitals involved in conjugation and how they affect the electronic properties of polymers.

5. Understand the applications of conductive polymers in OLEDs, OSCs, and OFETs.

6. Explain how crystallinity and molecular weight affect the electrical properties of polymers like P3HT.

7. Discuss the role of additives and epitaxy in promoting crystallization and improving polymer performance.

8. Understand the importance of HOMO-LUMO levels in determining the efficiency of organic solar cells.