Dye@Semiconductor

The ultimate goal in the computer simulation of DSC from first principles is to calculate the full operational device mechanism for realistic models under realistic conditions. This ideally corresponds to simulate the dye adsorbed onto extended semiconductor surfaces (typically TiO₂), including, for liquid phase DSC, an explicit description of the solution containing the electrolyte.
The first computational investigation from first principles we reported on an inorganic dye was performed in 2004 where we investigated the electronic structure, adsorption geometry and optical absorption spectra of [Fe(CN)₆]^4- on TiO₂. This system represented an interesting case since experimental evidences pointed towards a direct electron injection mechanism, in which an electron was directly transferred from the dye ground state to unoccupied TiO₂ states which were spatially localized in the proximity of the dye coordination site. We computed the absorption spectra and analyze the electronic structure of the system.
The same strategy was then used on a TDDFT investigation of the adsorption geometry and excited states of the anionic di-protonated N3 dye, as a model of the N719 dye. Since some of the sensitizer’s protons can be transferred to the TiO₂ surface, different representative configurations were analyzed suggesting that protonation of the sensitizer and/or of the surface can have an important influence on the electronic dye/semiconductor coupling (i_ph) and on the position of the TiO₂ conduction band (V_OC) in DSCs.

Recently, we have investigated the absorption spectrum and the alignment of ground and excited state energies for the prototypical N719 Ru(II) sensitizer adsorbed on an extended TiO₂ model.

An ultrafast, almost instantaneous, electron injection component was predicted on the basis of the strong coupling and of the matching of the visible absorption spectrum and density of TiO₂ unoccupied states. On the basis of our calculations it seems therefore that no sizable lower bound to an “injection time” exists, rather the timings of electron injection are mainly ruled by electron dephasing in the semiconductor.
Due to the increasing interest on the employment of ZnO as semiconductor in DSCs, the adsorption configuration, electronic structure and excited states of organic dyes on ZnO realistic models have been analyzed to rationalize the similarities and differences with the dye@TiO₂ interfaces.