Role of Molecular Thermodynamical Processes at Functionalized Polymer/Metaloxide Interfaces for Photovoltaics

We present a combined theoretical and experimental investigation of the role of pyridine-based functionalizers (IMs) at the hybrid titania (TiO2)/poly(3-hexylthiophene) (P3HT) interface. We first used density functional theory to study the electronic structure and adsorption energy of several IMs, isolated and self-assembled, on a TiO2 anatase (101) surface. Small details in the molecular structure are found to induce strong differences in the morphology of the corresponding self-assembled monolayer. We then used model potential molecular dynamics simulations to study the translational and rotational diffusion of a P3HT oligomer on the naked and functionalized TiO2 surfaces. We correlate the photovoltaic performance measured on TiO2/IM/P3HT with the degree of order of the interface. In particular, we find that the thiol group of 4-mercaptopyridine is able to stabilize the corresponding self-assembled interlayer. This, in turn, increases the polymer mobility on the inorganic surface, yielding a larger polymer/substrate interface area.