Co-Adsorbent Effect on the Sensitization of TiO2 and ZnO Surfaces: A Theoretical Study

Co-adsorbents are often employed in DSSC to avoid electron/ hole recombination at the hybrid organic/inorganic interface. Here we investigate, by means of density functional theory simulations, the changes induced in the structural and electronic properties of a hybrid heterostructure by the presence of a co-adsorbent. In particular, we focus on a prototype interface obtained by co-grafting a dye molecule (the hemi-squaraine dye) and imidazole at the oxide surfaces mostly employed in DSSC (anatase-TiO2(101) and ZnO(1100)). Our results show that beside avoiding dye aggregation, the co- adsorbent plays an active role in stabilizing the binding between the chromophore and oxide substrate and in tuning the optoelectronic properties of the system. We found that imidazole shifts downward the dye energy levels (HOMO and LUMO) while raising the oxide valence and conduction bands, strongly influencing the energy alignment of the interface. As such, a co-adsorbent can either modify the type of heterostructure (straddling, staggered, or broken gap) or change the open circuit voltage (Voc), with potentially both beneficial or detrimental effect of the final device performance.