Density functional theory study of the dipole across the P3HT : PCBM complex: the role of polarization and charge transfer

Using density functional theory (DFT) calculations we study the electronic structure and the dipole moment of the complex formed by an oligomer of poly(3-hexylthiopene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). We scanned the total energy of the P3HT : PCBM dimer as a function of the relative angular orientation between the two molecules in the search for stable angular configurations. After finding four stable geometries, we applied DFT calculations to characterize the electronic properties of those configurations. Using extended charge decomposition analysis, we calculated the contributions of the occupied and unoccupied orbitals from the individual species to form the electronic states of the complex. This method allowed us to determine the charge effectively transferred from the polymer to the PCBM upon the formation of the complex. Our results indicate that this amount of transferred charge is small but extremely sensitive to the detailed arrangement of the molecules in the dimer. However, the direction of the dipole moment of the complex is strongly influenced by the orientation of the permanent dipole of the PCBM relative to the chain of the oligomer. In the light of the dipole-assisted exciton dissociation models, we identified only one configuration that has a dipole moment with the right orientation to help the dissociation of a photo-excited electron–hole pair in the polymer. This configuration is just the molecular arrangement with the lowest total energy. Moreover, we study the contributions of charge transfer and electronic polarization to the composition of this dipole as a function of the intermolecular separation. We finally discuss how the results found using our simple dimer model can be related to electronic properties obtained using more-realistic models of the P3HT/PCBM interface. (paper)