Spin-Organics
Organic semiconductors present few key advantages over its inorganic counterparts. Mechanical flexibility, chemical tunability to realize different electronic functionalities, ease in structural modifications are some of its advantages. Organic molecules with low molecular weight elements have a lower spin-orbit coupling and weaker hyperfine interactions. This leads to a larger spin relaxation time which makes them feasible for applications in spin-electronics. Further due to weaker spin-orbit interactions, the selection rules for optical excitations are relaxed leading to an increased optical efficiency, something that has been shown with gadolinium as electrodes in OLED’s. Due to their larger spin relaxation time, multiple operations can be performed before the polarized electrons reaches its equilibrium state (i.e. lose information). The other term often used is the spin relaxation length which depends on the conductivity/mobility of the charge carriers in the organic material. This implies that electrons travel a greater distance through conducting materials than insulators before the spin information gets annihilated. Thus efforts are currently on way to realize organic semiconductors with high mobility. The two most eligible candidates currently in study are rubrene and pentacene.
Molecular Orbitals
Organic semiconductors consist of alternate double bonds that cause bond delocalization. One of the most commonly used theoretical model to determine the wave functional form of these delocalized electrons is the Linear Combination of Atomic orbitals (LCMO). Consider a benzene ring having 6 Carbon atoms. The electrons in the pz orbital are delocalized in the ring. An appropriate solution would be to consider a linear combination of the six pz orbitals centered on each carbon atom. By invoking the Schrödinger wave condition, one can determine the molecular levels of the pz orbitals. The other orbitals s, px and py are localized and provide a backbone structure to the aromatic compound.
Transport Mechanism
Band like structures as in the case of inorganic semiconductors is not the appropriate model for studying the conduction mechanism in organic semiconductors. Organic molecules are bounded by a weak Vander Wall’s force which causes the material to have disordered structures. Transport mechanism is predicted to occur through hopping/tunneling process through the localized molecular states. Highly ordered systems however are more close to having conduction and valence bands for transport. Thus at low temperatures where the mean free path of the electron exceeds the intermolecular distance, overlap of the HOMO levels and LUMO levels occur to form the valence and conduction band respectively. Thus efforts are being made to grow highly ordered single crystal organic semiconductors to achieve higher mobility by formation of HOMO and LUMO bands that extend throughout the crystal.
Structural Complexity and Interfaces
The complexity in studying (theoretical and computational) the organic structures arises due to higher degree of freedom (six) that exist in modeling than their inorganic counterparts (three). In addition to the 3 degrees used in identifying the location of the molecule the remaining 3 is used in determining the orientation of each molecule in the structure. With similar arguments interface modeling are difficult too. Moreover, since the organic semiconductors do not have free charge carriers, interfaces are dominated by dipolar interactions.
Carrier injection across the metal-organic interface is determined by the energy barrier height and the density of states at the Fermi level of the metal contact. Contact resistance can be a result of a mismatch of the HOMO/LUMO with respect to the work function of the metal. The resulting Schottky barrier gives rise to non linear behavior. The interface resistance depends exponentially on the barrier height and linearly on the DOS of the metal contact at Fermi level.