⚛️Molecular Electronics Unit 11 – Organic Electronics: Semiconductors

Key Concepts and Definitions

• Organic semiconductors materials composed of carbon-based compounds with semiconducting properties
• Conjugated systems alternating single and double bonds in organic molecules, allowing for delocalized electrons and charge transport
• HOMO (Highest Occupied Molecular Orbital) and LUMO (Lowest Unoccupied Molecular Orbital) energy levels analogous to valence and conduction bands in inorganic semiconductors
  • HOMO-LUMO gap determines the bandgap and optical properties of the material
• Charge carriers in organic semiconductors can be either electrons (n-type) or holes (p-type)
• Excitons bound electron-hole pairs created by the absorption of light in organic semiconductors
  • Exciton binding energy typically higher than thermal energy at room temperature, requiring dissociation for charge generation
• Charge mobility measure of how quickly charge carriers can move through a material under an applied electric field, expressed in units of cm²/Vs
• Doping process of intentionally introducing impurities to enhance the electrical conductivity of a semiconductor

Organic Semiconductor Materials

• Small molecules and polymers two main classes of organic semiconductors
  • Small molecules (pentacene, rubrene) have well-defined molecular structures and can be deposited through thermal evaporation
  • Polymers (P3HT, PEDOT:PSS) consist of long chains of repeating units and are typically solution-processed
• Acenes family of small molecules with linearly fused benzene rings (tetracene, pentacene)
• Thiophenes heterocyclic compounds containing a sulfur atom, commonly used as building blocks for organic semiconductors (P3HT, PBTTT)
• Fullerenes carbon-based molecules with a spherical or ellipsoidal shape (C60, C70), often used as electron acceptors in organic solar cells
• Donor-acceptor copolymers polymers with alternating electron-rich (donor) and electron-poor (acceptor) units, designed to tune the bandgap and improve charge transport
• Stability and degradation organic semiconductors can be sensitive to oxygen, moisture, and light, requiring encapsulation for long-term stability
• Solubility and processability important considerations for solution-based fabrication methods (spin-coating, inkjet printing)

Electronic Structure and Band Theory

• Molecular orbitals formed by the overlap of atomic orbitals in organic molecules
  • Bonding orbitals lower in energy and contribute to molecular stability
  • Antibonding orbitals higher in energy and contribute to molecular instability
• π-conjugation system of alternating single and double bonds that allows for electron delocalization and charge transport in organic semiconductors
• Bandgap energy difference between the HOMO and LUMO levels, determining the optical and electrical properties of the material
  • Materials with smaller bandgaps absorb and emit light at longer wavelengths
• Density of states (DOS) distribution of available electronic states as a function of energy
  • Gaussian distribution of states in organic semiconductors due to disorder and localization
• Fermi level energy level at which the probability of finding an electron is 0.5, located near the middle of the bandgap in intrinsic semiconductors
• Polaron quasiparticle consisting of a charge carrier (electron or hole) and its associated lattice distortion in organic semiconductors
• Disorder and localization organic semiconductors often have significant energetic and positional disorder, leading to localized electronic states and hopping transport

Charge Transport Mechanisms

• Hopping transport dominant charge transport mechanism in organic semiconductors, involving the thermally activated tunneling of charge carriers between localized states
  • Hopping rate depends on the distance and energy difference between sites, as well as the temperature and applied electric field
• Multiple trapping and release (MTR) model describes charge transport in the presence of shallow trap states, where carriers are repeatedly trapped and released by thermal activation
• Poole-Frenkel effect field-dependent increase in the charge carrier mobility due to the lowering of the potential barrier for hopping in the presence of an electric field
• Grain boundaries and interfaces can act as barriers for charge transport, leading to reduced mobility and increased recombination
• Charge injection and extraction processes of introducing and removing charge carriers from the organic semiconductor at the contacts
  • Energy level alignment between the contacts and the semiconductor is crucial for efficient charge injection and extraction
• Mobility anisotropy charge carrier mobility can be different in different crystallographic directions due to the anisotropic nature of molecular packing
• Charge-transfer states intermediate electronic states formed at the interface between two organic semiconductors with different electron affinities, playing a role in charge separation and recombination

Device Physics and Architectures

• Organic light-emitting diodes (OLEDs) devices that emit light when an electric current is passed through an organic semiconductor layer
  • Consist of an emissive layer sandwiched between a cathode and an anode, with additional charge transport and injection layers
• Organic photovoltaics (OPVs) devices that convert light into electrical energy using organic semiconductors as the active layer
  • Typically have a donor-acceptor heterojunction architecture to facilitate exciton dissociation and charge separation
• Organic field-effect transistors (OFETs) devices that modulate the current flow between a source and a drain electrode by applying a voltage to a gate electrode
  • Can be used as switches or amplifiers in electronic circuits
• Charge injection and transport layers materials with suitable energy levels and conductivity to facilitate charge injection and transport in organic electronic devices
• Exciton blocking layers materials with a wide bandgap that prevent excitons from reaching the electrodes and quenching, improving device efficiency
• Tandem and multi-junction architectures stacking multiple organic semiconductor layers with complementary absorption spectra to enhance light harvesting and power conversion efficiency
• Device stability and degradation mechanisms (chemical, thermal, electrical) that can lead to the deterioration of device performance over time

Fabrication Techniques

• Vacuum thermal evaporation deposition of small molecule organic semiconductors by heating them in a vacuum chamber and condensing them onto a substrate
  • Allows for precise control over layer thickness and composition, but requires high vacuum and is limited to thermally stable materials
• Solution processing deposition of organic semiconductors from a liquid phase, such as spin-coating, inkjet printing, or blade coating
  • Enables large-area and low-cost fabrication, but may result in less control over film morphology and purity compared to vacuum deposition
• Organic vapor phase deposition (OVPD) technique that combines the advantages of vacuum deposition and solution processing, allowing for the deposition of multiple layers with high purity and control
• Patterning methods (photolithography, shadow masking, direct printing) for defining the active areas and electrodes of organic electronic devices
• Encapsulation and packaging techniques to protect organic electronic devices from environmental factors (oxygen, moisture) and ensure long-term stability
• Roll-to-roll processing continuous, high-throughput fabrication method suitable for large-area and flexible organic electronic devices
• Substrate choice (glass, plastic, metal foil) depending on the application requirements, such as transparency, flexibility, or thermal stability

Applications and Real-World Examples

• Displays (smartphones, televisions) one of the most successful applications of organic semiconductors, with OLEDs offering high contrast, wide viewing angles, and thin form factors
• Solid-state lighting OLED-based lighting panels and fixtures for energy-efficient and flexible lighting solutions
• Photovoltaics organic solar cells as a potential low-cost and environmentally friendly alternative to silicon-based photovoltaics
  • Suitable for building-integrated and portable applications
• Wearable electronics organic electronic devices can be integrated into clothing and accessories for health monitoring, energy harvesting, and fashion
• Medical devices organic electronic sensors and stimulators for biomedical applications, such as drug delivery and prosthetics
• Radio frequency identification (RFID) tags low-cost and disposable organic electronic circuits for item tracking and supply chain management
• Smart packaging organic electronic sensors and indicators for monitoring the quality and safety of packaged goods
• Neuromorphic computing organic electronic devices that mimic the structure and function of biological neural networks for energy-efficient and adaptive computing

Challenges and Future Directions

• Improving charge carrier mobility and conductivity through molecular design, processing, and doping strategies
• Enhancing device efficiency and stability by optimizing device architectures, materials, and encapsulation techniques
• Developing new materials and device concepts for specific applications, such as transparent, stretchable, or self-healing electronics
• Scaling up fabrication processes for large-area and high-volume production of organic electronic devices
• Addressing environmental and sustainability concerns related to the production, use, and disposal of organic electronic materials and devices
• Integrating organic electronics with other technologies, such as sensors, energy storage, and communication systems, for smart and multifunctional devices
• Exploring the use of organic semiconductors in emerging applications, such as artificial intelligence, quantum computing, and space technology
• Fostering interdisciplinary collaborations between chemistry, physics, materials science, and electrical engineering to advance the field of organic electronics