Modeling ZnO phases using a periodic approach: from bulk to surface and beyond.

A comprehensive investigation of one of the basic components of ZnO-based dye-sensitized solar cells (DSSC) is presented, carried out using hybrid density functionals combined to a periodic formalism. Both semiconductor bulk and surfaces are discussed thoroughly, with a particular attention to structural and electronic aspects. Next, three possible adsorption modes of formic acid are compared and discussed at the same level of theory. The results confirm that formic acid appears as a suitable choice for an efficient anchoring of large organic molecules, such as the dyes commonly used for DSSC, to semiconductor surfaces since it allows both a stable adsorption and few but significant contributions to the density of states for all adsorption modes considered. More in general, our results suggest that hybrid functionals and, in particular the parameter free PBE0 (PBE denotes Perdew-Burke-Ernzerhof), can be considered as a reliable tool for modeling complex molecule-semiconductors interfaces such as the one of interest in DSSC, thus providing a powerful computational protocol for the in silico design of new systems for photovoltaic applications.