Organic Semiconductors
Contenido
Los materiales orgánicos son compuestos que basan su estructura en el átomo de carbono. Este al igual que el silicio y el germanio forman parte del grupo IV de la tabla periódica y como es una característica de los elementos de este grupo, poseen una valencia de cuatro, lo que les permite enlazarse a cuatro átomos más. El carbono, así como los demás elementos del grupo IV presentan una distribución electrónica que termina en sp.
Origin of semiconduction
The structure of organic materials can be defined as molecular solids, since the structural unit, or unit cell, that is repeated in the solid network is a molecule. However, unlike inorganic solids, the nature of the intermolecular interactions that govern solid-state ordering is non-covalent in nature, see Figure 1.
The weakness of non-covalent interactions makes it difficult to adopt an ordered structure that favors an optimal overlap of the orbitals belonging to the nearby molecules with which it is intended to interact. Consequently, except in exceptional cases, the structuring of energy bands is not favored and the energy levels in the solid will be defined by levels located on each molecule. However, it is possible to establish a parallel to that of inorganic semiconductors, this between the valence and conduction bands and the molecular orbitals. Thus, the HOMO orbital (Highest Occupied Molecular Orbital) would be related to the upper limit of the valence band, which will be separated by an energy spacing of the LUMO orbital (Lowest Unoccupied Molecular Orbital) that corresponds to the lower limit of the conduction band, as can be seen demonstrated in Figure 2.
For many organic compounds this energy spacing between the HOMO and LUMO orbitals is within the range of inorganic semiconductors, 3 eV. For this reason, some of these organic molecules, which also have certain structural characteristics, could function as semiconductor materials. However, due to the absence of bands, the movement of electrons is usually described by charge transfers between levels located in the constituent molecules of the solid material, which seriously hinders the conduction of an electric current compared to inorganic materials.
In organic materials, these localized states or sites are: the states of individual molecules in molecular crystals, the states of individual polymer chains, or the states of segments of these chains where conjugation is interrupted by structural or chemical defects. Having defined the localized states or sites and keeping in mind that these sites act as potential wells, the charge transfer between sites occurs through quantum jumps or hopping where the charge carriers through tunneling assisted by phonons (vibrations of the structure of the material) pass from one site to another, under certain special conditions.
Despite the difficulty of charge transport in organic semiconductors, the current density that they can conduct is powerful enough to make them suitable for the manufacture of electronic devices, making them a technologically interesting alternative for the electronics industry.
Classification
Organic materials, by their nature, can form an infinite number of compounds of different sizes, shapes and structures, and consequently there are numerous classification parameters. Regarding organic compounds that have conductive characteristics, they are usually classified into two groups according to their molecular weight, these are: low molecular weight molecules and high molecular weight molecules. The first refer to conjugated molecules, with a size of less than 20 monomers, which are known as oligomers, and the second cover conjugated molecules of more than 20 monomers, which are known as polymers. Both types of organic compounds differ from each other in their size and physical properties, although they share similar electrical properties.[3].
In the field of organic electronics, these molecules are characterized by being made up of a not very large number of atoms, with a conjugated bond structure and by forming molecular type crystals. These crystals differ from atomic type crystals such as those formed by silicon, germanium or carbon (diamond), in that the former are formed by the union of individual molecules that are intramolecularly linked by strong covalent bonds, but that are intermolecularly joined by weak forces such as van der Waals, while atomic type crystals are entirely a single molecule that is strongly linked by covalent bonds. This difference causes atomic type crystals to generate very well-defined band structures because this type of crystals are very strong and the structure of their energy states remains unchanged, while in molecular crystals, being united by such weak attractions, and because they have a very low number of atoms per molecule, their energy coupling is low and their energy structure is altered very easily, causing this type of materials to have very little importance in the energy band structure in the way they conduct. electrical currents. It should be noted that despite the ease with which the energetic order is interrupted, the crystals of small molecules maintain considerable order. Semiconductors manufactured with small molecules stand out for presenting better levels of conduction than polymeric semiconductors, but being very difficult to prepare in solutions prevents them from being deposited with low-cost manufacturing techniques such as conventional printing or spin coating, and for this reason, they cannot form thin films of large area.
Semiconductor polymers
Polymers are characterized by being very long chains, composed of monomers, where each monomer is linked by covalent bonds. This leads to the formation of energy bands within the polymer chains, as in inorganic semiconductors, as there is a large amount of strongly linked monomeric units. However, these chains are of finite length, and a polymeric compound is made up of millions of these chains, which are very weakly coupled, which is why in polymers, as in conjugated organic molecular crystals, the energy band structure does not present much relevance in the total current of the semiconductor. Semiconducting polymers, unlike molecular crystals, are more easily soluble, which allows them to be deposited by conventional printing or spin coating, and thus be deposited in thin films of large area at very low cost. Additionally, semiconducting polymers have the characteristics of polymeric materials, such as flexibility and durability. Regarding conductivity, since polymers have amorphous structures, they have a lower conductivity than molecular crystals.[3].