The term organic semiconductor is used for denoting the class of materials that are based on carbon, which takes part in semiconducting characteristics. And therefore, many of the attractive optoelectronic features exhibited in organic semiconductors emerge from the properties of carbon atoms. Organic semiconductor materials fall into two broad categories, i.e., low-molecular weight organic materials and polymers (Ishii et al. 2009). The individual energy properties are different as can be seen below.
The bonding nature manifested in organic semiconductors is fundamentally dissimilar from those seen in their inorganic counterparts. The solid molecular crystals of organic semiconductors are bonded with weak van-der-Waals forces meaning that they do have considerably weaker intermolecular bonding in comparisons to other semiconductors such as GaAs or Si, which are covalently bonded (Hill et al. 2011). The consequences or results of these weak intermolecular bonding can be seen in their thermodynamic and mechanical characteristics like lower melting points, reduced hardness, and much more inadequate properties to delocalize electronic wave functions within the neighboring molecules. The latter property has direct implications for charge carrier support and that of optical features.
However, the situation that exists in polymers is somehow different because the morphology of the chains in polymers can cause improvement in the mechanical characteristics of this category of organic semiconductors (Niedzialek, & Friend, 2012). For instance, polymers have multiple carbon-carbon covalent bonds which tend to increase their material strengths. Similarly, they have excellent thermodynamic and mechanical features which explain their increased application in the aircraft building.
Again, In order to support the charge carrier support, charge separation in organic semiconductors occurs only through the hot-state charge for the delocalized molecules and not driven by the energy gradients of intermolecular hopping. Overall, low-molecular weight organic semiconductors have the best energy for optical applications while polymers have the desired material energy for mechanical applications.
References
Ishii, H., Sugiyama, K., Ito, E., & Seki, K. (2009). Energy level alignment and interfacial electronic structures at organic/metal and organic/organic interfaces. Advanced Materials, 11(8), 605-625.
Hill, I. G., Rajagopal, A., Kahn, A., & Hu, Y. (2011). Molecular level alignment at organic semiconductor-metal interfaces. Applied Physics Letters, 73(5), 662-664.
Bakulin, A. A., Rao, A., Pavelyev, V. G., van Loosdrecht, P. H., Pshenichnikov, M. S., Niedzialek, D., & Friend, R. H. (2012). The role of driving energy and delocalized states for charge separation in organic semiconductors. Science, 335(6074), 1340-1344.
