What are Organic Semiconductors?

Organic semiconductors are materials, ranging from small molecules to polymers, that can transport charge. Unlike in conductors, where electrons move freely across the material, organic semiconductors rely on a structure primarily composed of carbon and hydrogen atoms. These atoms are arranged to enable overlap of π-orbitals, allowing electron delocalization within each molecule. This delocalization facilitates electron movement, or conductivity, along the molecule. Additionally, the π-orbitals can overlap with those of neighboring molecules, creating intermolecular electronic coupling. As a result, delocalized electrons can "hop" between molecules in the organic semiconductor, enabling charge transport. These unique properties make organic semiconductors useful in applications like organic light-emitting diodes (OLEDs) and organic photovoltaics.

Main classes of Organic Semiconductors

• Small Molecule Semiconductors
• Fullerenes
• Non-Fullerene Acceptors
• Polycyclic aromatic hydrocarbons (PAHs)
• Polymeric Semiconductors
• Classic conjugated polymers
• Donor-Acceptor (D-A) copolymers
• Polymeric blends
• 2D Organic Semiconductors
• 2D Covalent Organic Frameworks
• Graphene Oxide / Reduced Graphene Oxide
• Self-Assembled Monolayers

Advantages of Organic Semiconductors

Organic semiconductors offer several advantages, especially for applications in flexible and lightweight electronics. Here are some key benefits:

Low-cost Preparation

Light Weight

Mechanical Flexibility

Easy Processing

Tunable Function Through Molecular Design

Rich Availability

Wide Range of Optical Properties

Thin Film Formation

Low Energy Consumption

Chemistry of Organic Semiconductors

Organic semiconducting materials are versatile and can have many components. One of the most crucial components to organic semiconductors is conjugated units. This is where there is an area of sp²-hybridised carbons. This kind of bonding means only three of the four valence electrons within carbon atoms hybridise. The fourth electron occupies the perpendicular p_z orbital.

Interactions between these p_z orbitals form weaker π (bonding) and π* (anti-bonding) orbitals, which flatten the molecule. This planarity facilitates the delocalization of π and π* states across multiple carbon atoms and is known as conjugation. The more conjugation a material possesses the closer the HOMO and LUMO levels are as the difference between the bonding and antibonding molecular orbitals reduces.

Every sp² hybridised carbon that is able to engage in π bonding introduces another variable into the molecular orbitals. The number of p-orbitals increases as the conjugated system grows, with each p-orbital containing both a positive and a negative lobe. This arrangement allows for variations in π-orbital interactions since each lobe can align with either a like-charged or an opposite-charged neighboring lobe. This variability contributes to the richness of the electronic states within the material, offering multiple pathways for electron delocalization across the π-system. As a result, the increased possibilities for orbital overlap enhance the overall stability and flexibility of the molecular orbitals, providing greater control over the material's electronic properties and its suitability for various organic electronic applications.

The π orbitals of neighboring molecules can also overlap, allowing electrons to spread across multiple layers of semiconducting molecules. This "π–π stacking" is important for helping with electron movement and also lowers the energy needed for light absorption. By combining the inter- and intramolecular electron delocalization with other methods to adjust the HOMO and LUMO energy level gap these organic molecules offer huge advantages. Such features means organic semiconductors have very strong absorption and emission in the visible spectral range making them advantageous for organic electronic applications like photovoltaics and LEDs.

Charge Transport in Organic Semiconductors

Charge transport in organic semiconductors is thought to be heavily dependent on the degree of crystallization of the material. The charge transport theories associated with degree of crystallinity is described in the table below:

The first important feature of organic semiconductors (OSCs) is the relatively weak bonding strength and electronic interactions between neighboring molecules, which result in the density of states (DOS) having a narrow Gaussian distribution Whether small or polymeric, OSCs are held together by weak van der Waals forces. OSCs have lots of structural defects and strong lattice vibrations as a result. This means there is some fluctuation in electronic coupling between molecules as they are more free to move. Electronic coupling is typically electron-phonon coupling. This makes determining electronic processes in organic semiconductors quite complicated.

In OSCs, electron-phonon coupling occurs because electrons can’t move through the organic material without interacting with molecular vibrations. This interaction has several important effects:

• Polaron Formation: When an electron or hole moves through an OSC, it can locally distort the molecular structure, creating what’s known as a polaron—a combination of the charge carrier and the local distortion. This is considered a "self-trapped" charge andmoves more slowly than a free charge due to the energy cost of maintaining the molecular distortion as it moves. This reduces overall charge mobility.

• Impact on Charge Mobility: Strong electron-phonon coupling in OSCs means that the energy needed for charge carriers to hop from one molecule to another is relatively high. The strength of electron-phonon coupling impacts how strongly a charge carrier is localized within this polaron.

• Reorganization energy: energy cost of overcoming the molecular vibrations between each hop. In materials with high electron-phonon coupling, these vibrations can impede movement, making charge transport thermally activated—meaning that it improves with temperature as thermal energy helps overcome this barrier.

• Temperature Dependence: Due to electron-phonon coupling, the temperature of the material significantly impacts charge transport. As temperature increases, molecular vibrations increase, which can assist charge carriers in hopping between molecules, improving mobility. In contrast, lower temperatures can decrease charge mobility as less thermal energy is available to aid hopping.

• Broadening of Energy Levels: Electron-phonon coupling in OSCs also causes energy levels to fluctuate, particularly due to the organic material's structural flexibility. This can lead to a broadening of energy levels or a Gaussian distribution in the density of states (DOS). A narrow Gaussian distribution means there are only a few energy levels close together, making it more challenging for charge carriers to find states with the energy needed for transport. This broadening effect can increase disorder, further slowing charge mobility.

• Hopping and Tunneling in Amorphous Regions: In highly disordered or amorphous OSCs, electron-phonon coupling influences the mechanisms of hopping and tunneling. For hopping transport, electron-phonon interactions mean that electrons must "jump" between energy states on different molecules. In cases where molecules are very close, tunneling may occur, where charges pass through energy barriers instead of hopping over them. This tunneling is also impacted by electron-phonon coupling, especially in terms of how well charge carriers maintain their path without scattering off of vibrations.

Difference between Organic Semiconductors and Inorganic Semiconductors

There are a few key differences between organic semiconductors and their inorganic counterparts due to the difference in chemical structure and composition. Here is a summary of some of the key differences:

• Organic semiconductors have low dielectric constants (e): They don't shield charges from each other as much as inorganic semiconductors. This makes it harder to separate electron-hole pairs and as such they are described as "Frenkel excitons". Good for LEDs but not so good for photovoltaics.

• Organic semiconductors are considered electronically "soft": The delocalized electrons tend to stay close to a small number of atoms rather than spreading across the whole system. This localizations causes changes in the local electronic structure. Along with low dielectric constant this causes distortion in molecular structure in that area. Therefore the charge carrier is a quasiparticle known as a polaron. When polarons move between different parts of the material, like hopping from one polymer chain to another, it requires a lot of energy because the material has to rearrange itself each time. This reorganization energy slows down the movement of charges, reducing the material’s conductivity.

• For organic semiconductors the limiting factor in charge transport is the "hopping" of charge carriers between molecules: Due to the asymmetry of organic semiconductors, charge transport can vary significantly throughout the system (anisotropic). For a given molecule, intrachain transport through the conjugated plane will be much faster than intermolecular transport via π–π stacking. Inter-molecular "hopping" requires significant molecular reorganization and is therefore dependent on temperature and electronic disorder. The same disorder impacts how easily excitons diffuse through the system by limiting the energetic accessibility of nearby sites.

• Organic semiconductors are easily processed: Small molecule organic semiconductors can be easily processed into well-defined high quality films and layered devices without the need for intense heat or ultra-clean conditions. Additionally, the solubility of the molecules can be tuned depending on the desired processing solvent through flexible side chain groups. This tunable solubility means organic semiconductors are solution processed making flexible and highly fracture resistant films.

Applications of Organic Semiconductors

OSCs are attractive across science disciplines including chemistry, physics, materials science, medicine, and biology. Crucially, OSCs are becoming key elements in the preparation of flexible, printable, and scalable electronics such as:

• Light-emitting diodes
• Solar cells
• Transistors
• Thermoelectrics
• Sensors
• Bioelectronics
• Biomimetics

Diverse synthetic/processing technologies can provide versatile platforms to approach various functional materials.

Limitations of Organic Semiconductors

Organic semiconductors (OSCs) offer flexibility, tunability, and potential for low-cost production, but they also have limitations that impact their performance and broader application including:

• Low Charge Carrier Mobility: Charge carrier mobilities in OSCs rarely exceed 10 cm² V⁻¹ s⁻¹, a value which is orders of magnitude lower than in c-Si or graphene, where mobilities in the order of 10³ cm² V⁻¹ s⁻¹ and 10⁶ cm² V⁻¹ s⁻¹.
• Stability: Some organic materials are sensitive to environmental factors like oxygen, moisture, and UV light. They may need to be processed in a glove box to ensure inert conditions and encapsulated to preserve properties. This adds complexity and costs to device production.
• Complex Charge Transport Pathways: As described charges can transport via numerous routes with different levels of efficiency. Issues of reorganization energy, slow hopping and tunnelling and strong electron-phonon coupling.
• High Exciton Binding Energy: The low dielectric constants of organic semiconductor materials means that electron-hole pairs are more tightly bound due to less shielding.

Research is ongoing to improve these limitations of organic semiconducting materials. The modifiable nature of organic molecules has lead to significant advancements in tailoring specific properties required for various applications. In particular, non-fullerene acceptors have seen a huge amount of development and expansion within the field of photovoltaics.

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