Documento PDF - UniCA Eprints - Università degli studi di Cagliari.

A.3 methods 81tional Theory (DFT) level. The estimate of the electroniccoupling for cofacial dimers has been obtained using theso-called "energy splitting in dimer" method [95]. In thismethod the transfer integral for holes can be computedevaluating the energy difference between the orbitals resultingfrom the overlap of the highest occupied molecularorbitals of the two interacting molecules: J αβ = (ɛ HOMO −ɛ HOMO−1 )/2, where ɛ HOMO and ɛ HOMO−1 are the energiesof the two highest occupied molecular orbitals of the dimer.We used the gradient-corrected PBE density-functional [128]together with a plane-wave basis set and ultrasoft pseudopotentialsas implemented in the CPMD [129] programpackage; to account for dispersion interactions we used theempirical dispersion correction proposed by Grimme [108],that adds a Van der Waals-type term scaling as R −6 into thetotal energy of the system.As for the ab initio methods used in chapter 4, DFT+Ucalculations have been performed by using the Quantum-ESPRESSO package[107]. Total energies have been calculatedby using ultrasoft pseudopotentials[130], by expandingKohn-Sham eigenfunctions on a plane-wave basis set.The cutoff has been set at 35 Ry on the plane waves and at280 Ry on the electronic density. The electronic propertiesof the ZnO/P3HT system, of the ZnO/ZnPc system, and ofthe double-interface ZnO/ZnPc/P3HT system have beeninvestigated by analyzing the electronic eigenvalues calculatedat the Γ point. The exchange-correlation functionalhas been obtained by adding an ab initio non-local vander Waals correlation contribution[131, 132] to the semilocalgradient-corrected PBE functional[128]. An Hubbard Ucorrection [133, 134] has been applied to the Zn 3d and O2p atomic shells, thus allowing for an optimal position ofZnO band edges with respect to the molecule and polymerHOMO-LUMO levels. A P3HT oligomer, formed by fourmonomers has been used to simulate the properties of thepolymer. Finally, optical absorption spectra ranging fromthe near-IR to the near-UV regions have been calculatedby using a recent approach to the solution of the Bethe-Salpeter equation within the framework of time-dependentdensity matrix perturbation theory (TDDFPT) [135, 136].In chapter 5, the interaction between the single THF moleculeand the ZnO surface at DFT level have been perfomed by