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What are molecular crystals and why are they so important in organic semiconductors?
Molecular crystals are solids formed by stacking covalently bonded molecules, most of which are composed of carbon and hydrogen atoms, and sometimes also contain foreign atoms such as nitrogen, sulfur and oxygen. In organic semiconductors, the structure of the material is closely linked to its function and forms an important pillar of today's technology. These molecular crystals can exist as crystalline or amorphous films and are usually non-conductive, but can become semiconductors under the right conditions (such as charge injection from electrodes, doping, or photoexcitation).
Molecular crystals typically have energy band gaps between 2.5 and 4 electron volts, which means that in the absence of an applied electric field, they behave essentially like insulators.
Generally speaking, the electrical properties of organic semiconductors are determined by their molecular structure. Specifically, the charge carrier mobility of these molecular crystals mainly depends on the injected charge carriers, while their conductivity properties are usually brought about by photoexcitation or voltage applied by electrodes. Therefore, a deep understanding of the properties of molecular crystals is crucial to improving the performance of organic semiconductors.
Historical BackgroundThe history of molecular crystal research can be traced back to the 19th century. In 1862, Henry Litherby discovered a material that was partially conductive, which was thought to be polyaniline. In the 1950s, researchers discovered that polycyclic aromatic compounds form semiconducting charge transfer complexes with halogens, further confirming the potential of organic compounds to conduct electricity.
In 1977, Hideo Shirakawa et al. reported the high conductivity of oxidized and iodine-doped polyacetylene, for which they were awarded the 2000 Nobel Prize in Chemistry.
With the increasing application of organic semiconductors, various targets such as organic light-emitting diodes (OLEDs), solar cells (OSCs) and organic field-effect transistors (OFETs) have been developed one after another. The operating principles of these devices are Closely related to the electronic properties of organic materials.
Applications of organic semiconductors
Molecular crystal materials play an important role in electronics, especially in optoelectronic devices. Nowadays, rigid skeleton organic semiconductors are not only used in organic light-emitting diodes, but also in organic solar cells and organic field-effect transistors. Compared with inorganic materials, organic semiconductors offer many advantages, such as easy fabrication, good mechanical flexibility and low cost.
OLED devices utilize the recombination of electrons and holes to emit light, a process that is particularly dependent on the work function of the electrodes.
Combined with appropriate materials and technologies, organic semiconductors can not only drive existing electronic products, but also have the potential to create new functions and applications, such as environmentally friendly biosensors. These advances have triggered exploration and thinking among all sectors on the eco-friendliness and multifunctionality of future electronic products.
Material characteristics and classification
Organic semiconductor materials include amorphous films, molecular doped polymers, molecular crystals, etc. Each material has different manufacturing methods and properties, which further affect its performance in different applications. For example, organic materials with a crystalline structure generally have high charge carrier mobility and are very suitable for use as active elements in organic field-effect transistors.
Self-assembled aromatic peptides are considered a promising class of bio-inspired nanoscale semiconductors with tunable conductivity and structural properties.
The diversity of these materials not only expands their uses, but also promotes significant impact on the development of new organic devices.
Charge transport mechanism
Compared with previous organic crystals, most modern organic semiconductors have a certain degree of disorder, which makes the charge transfer process and structural design more complicated. Due to the weak van der Waals forces between molecules, charge carriers cannot move along a stable conduction band and are instead transferred through discontinuous jumps, the efficiency of which is often affected by the material structure and the external environment.
As a result, the mobility of charge carriers in modern organic semiconductors is often significantly lower than in equivalent crystalline systems.
Understanding these mechanisms is critical for optimizing material design and improving device performance.
Mechanical properties
The mechanical properties of organic semiconductors are also an important consideration in the design. Its elastic modulus and yield point determine how the material behaves under physical stress. Conducting stretch tests can help us understand these characteristics so that we can design electronic devices that are more resistant and last longer.
Variable viscoelasticity makes organic semiconductors very promising for use in wearable devices because they can absorb energy during applied stress, thereby enhancing durability.
With a deeper understanding of material properties, future industrial applications will become more diverse and innovative.
The role of molecular crystals in organic semiconductors sparks people's imagination of future technology. How will such technology once again change the way we live and interact?
