Organic semiconductors offer several crucial advantages over their inorganic counterparts in electronic and spintronic applications. Besides offering structural advantages such as variety and flexibility, organic semiconductors can be manufactured with cheaper processes and at lower temperature. These promising potentials call for the development of a complete theoretical framework, without any need for experimental input, for description of charge and spin transport in these materials. A possible strategy can be to employ a multiscale method where electronic structure is calculated with abinitio methods and the information so obtained is used to construct a material specific model Hamiltonian. This Hamiltonian can then be solved with statistical techniques to extract transportrelated quantities, like mobility, spindiffusion length etc. Since, in a real device, the organic semiconductor will be attached to conducting electrodes, the interface between the two systems will play a crucial role in the device functionality. Keeping these in mind, in this thesis, we attempted to calculate several important properties and parameters of organic crystals related to electron and spin transport, both for the bulk material and at the interface.
A modification of the popular Density Functional Theory (DFT) method known as
constrained DFT (cDFT) has been used to calculate the charge transfer energies between
a graphene sheet and a benzene molecule absorbed on it. We have computed these energy
values for several modifications of the systemconfiguration and have rationalized the results in terms of classical electrostatics. Next, we have developed a method, within the framework of calculations employing localized basis orbitals, to determine the accurate forces when the energy of the system depends on a subspace population. Such method, in conjunction with cDFT, has been used to evaluate the reorganization energy of a pentacene molecule adsorbed on a flake of graphene. We have also developed the excitonic DFT method for calculating the optical gap of materials with cDFT, by confining certain number of electrons within a subspace of the KohnSham eigenfunctions. We have shown that this method predicts the optical gaps of organic molecules with appreciable accuracy. We have also tried to extend this method to periodic solids.
As a step toward describing spinrelated phenomena, we have extracted the spinorbit coupling matrix elements, which can be responsible for spinrelaxation in organic crys
tals, with respect to a set of maximally localized Wannier functions. We have applied this
on several materials and showed that the spinorbit split band structures calculated from the Wannier functions match those obtained directly with first principles calculations. Since, in organic crystals, lattice vibrations play a major role at finite temperature, we have extended the aforementioned work to include the effects of phonons. To this end, we have calculated, with respect to the Wannier functions, the spinphonon coupling, namely the effect of various phonon modes on modification of spinorbit coupling. We have performed such calculation on a crystal of durene and showed that there is no apparent correlation between the electronphonon and the spinphonon coupling terms.
