Semiconductors and Polymers

Open Table of Contents

Polycrystalline Materials
- Growth Methods:
Conduction in Dielectrics
- Types of Conduction in Dielectrics
- Breakdown Mechanisms
Conduction in Polymers
Organic Light-Emitting Diodes (OLEDs)

Polycrystalline Materials

Growth Methods:

Single Crystal Czochralski Growth: melting of polysilicon doping - introduction of seed crytstal, beginning of crystal, crystal pulling, formed crystal with a residue of melted silicon (rotate wiring while pulling). If you cool down a boule too fast, then there will be defects (minimum thickness is 3rd of mm). Race - who can make bigger and bigger single crystals of silicon (used in high-end applications, such as solar cells in satellites). In other applications, no one has figured out a way to reduce cost of silicon single crystals.

Conduction in Dielectrics

Dielectric materials are characterized by their ability to store electrical energy in an electric field while exhibiting minimal electrical conduction. However, under certain conditions, dielectrics can exhibit conduction mechanisms.

Types of Conduction in Dielectrics

1. Electronic Conduction

Schottky Emission: Thermionic emission of electrons over potential barriers at metal-dielectric interfaces
Poole-Frenkel Effect: Field-assisted thermal excitation of trapped electrons to the conduction band
Fowler-Nordheim Tunneling: Quantum mechanical tunneling through triangular barriers at high electric fields

2. Ionic Conduction

Bulk Ionic Conduction: Movement of ions through crystal lattice defects or interstitial sites
Interface Ionic Conduction: Enhanced ionic mobility at grain boundaries and interfaces
Temperature Dependence: Follows Arrhenius behavior with activation energy

3. Space Charge Limited Conduction (SCLC)

Occurs when injected charge carriers exceed the intrinsic carrier density
Current-voltage relationship: $J \propto V^2$ (for trap-free case)
Important in thin film devices and organic electronics

Breakdown Mechanisms

Intrinsic Breakdown: Fundamental limit due to impact ionization
Thermal Breakdown: Due to Joule heating and thermal runaway
Electromechanical Breakdown: Mechanical failure due to electrostatic forces

Conduction in Polymers

Polymers can exhibit a wide range of electrical properties, from highly insulating to highly conducting, depending on their molecular structure and processing.

Classification of Polymers by Conductivity

1. Insulating Polymers

Examples: Polyethylene (PE), Polystyrene (PS), PMMA
Conductivity: σ < 10⁻¹² S/cm
Applications: Cable insulation, packaging, structural materials

2. Semiconducting Polymers

Examples: Polyacetylene, Polythiophene, Poly(p-phenylene vinylene) (PPV)
Conductivity: 10⁻¹⁰ to 10² S/cm (depending on doping)
Mechanism: π-conjugated backbone enables charge transport

3. Conducting Polymers

Examples: Doped polyacetylene, PEDOT:PSS, Polyaniline
Conductivity: Up to 10⁵ S/cm
Applications: Antistatic coatings, electromagnetic shielding, flexible electronics

Charge Transport Mechanisms in Polymers

1. Band Transport

Conditions: High crystallinity, low temperature, high purity
Characteristics: High mobility, temperature-independent at low T
Mobility: Can exceed 1 cm²/V·s in high-quality organic single crystals

2. Hopping Transport

Variable Range Hopping (VRH): Mott’s T^(-1/4) temperature dependence
Nearest Neighbor Hopping: Thermally activated with Arrhenius behavior
Polaron Transport: Formation of localized charge-lattice distortions

3. Multiple Trapping and Release (MTR)

Mechanism: Charge carriers trap and detrap during transport
Characteristics: Dispersive transport, frequency-dependent mobility
Applications: Understanding organic field-effect transistors (OFETs)

Doping in Conjugated Polymers

p-Type Doping (Electron Acceptors)

Dopants: I₂, FeCl₃, TCNQ, F4TCNQ
Effect: Creates holes (positive polarons/bipolarons)
Conductivity Enhancement: Several orders of magnitude increase

n-Type Doping (Electron Donors)

Dopants: Alkali metals (Na, K), reducing agents
Effect: Creates electrons (negative polarons/bipolarons)
Challenges: Air sensitivity, limited stability

Organic Light-Emitting Diodes (OLEDs)

OLEDs represent a revolutionary display and lighting technology based on the electroluminescence of organic compounds.

OLED Structure and Operation

Basic Device Structure

Cathode (Al, Ca/Al, LiF/Al)
    ↓
Electron Transport Layer (ETL)
    ↓
Emissive Layer (EML)
    ↓
Hole Transport Layer (HTL)
    ↓
Hole Injection Layer (HIL)
    ↓
Anode (ITO, PEDOT:PSS)
    ↓
Substrate (Glass, Plastic)

Operating Principle

Charge Injection: Electrons from cathode, holes from anode
Charge Transport: Through respective transport layers
Exciton Formation: Electron-hole recombination in emissive layer
Light Emission: Radiative decay of excitons

Types of OLED Emitters

1. Fluorescent Emitters (First Generation)

Mechanism: Singlet exciton emission
Efficiency: ~25% internal quantum efficiency (IQE)
Examples: Alq₃, NPD, TPD
Advantages: Simple synthesis, good color purity

2. Phosphorescent Emitters (Second Generation)

Mechanism: Triplet exciton harvesting via heavy atom effect
Efficiency: Up to 100% IQE
Examples: Ir(ppy)₃, PtOEP, Ir(MDQ)₂(acac)
Challenges: Triplet-triplet annihilation, roll-off at high brightness

3. TADF Emitters (Third Generation)

Mechanism: Thermally Activated Delayed Fluorescence
Efficiency: Up to 100% IQE without heavy metals
Examples: 4CzIPN, DACT-II, TXO-TPA
Advantages: Pure organic, cost-effective, tunable emission

OLED Performance Parameters

Efficiency Metrics

External Quantum Efficiency (EQE): Photons out per electron in
Current Efficiency: Luminance per current density (cd/A)
Power Efficiency: Luminance per electrical power (lm/W)
Color Rendering Index (CRI): Quality of color reproduction

Operational Stability

T₅₀ Lifetime: Time for brightness to decay to 50% of initial value
Degradation Mechanisms:
- Chemical degradation of organic materials
- Electrode migration
- Moisture and oxygen ingress
Encapsulation: Critical for device lifetime