Computational Insight on the Working Principles of Zinc Porphyrin Dye-Sensitized Solar Cells

National Science Foundation of China [21073168, 21290193]; National Basic Research Program of China [2011CB808501]A step-by-step theoretical protocol based on the density functional theory (DFT) and time-dependent DFT (TD-DFT) at both the molecular and periodic levels have been performed to study a zinc porphyrin complex (named YDoc) sensitized TiO2 solar cell including dye excitations, electron injection, the regeneration of photooxidized dyes and the effect of electrolyte additives. Our study reveals the possibility of a favorable electron transfer from the excited dye to the semiconductor conduction band (CB) and suggests three possible pathways of the electron injection from the dye to the nanoparticle (TiO2)(38). One is the direct one-step injection by photoexcitation, and the other two are from the different parts of the excited dye to the nanoparticle. The influence of the electrolyte composition on the geometric and electronic features of the dye/TiO2 system has also been studied. It is found that, with the additive of the lithium ion, the energy gap between the LUMO of dye and the TiO2 CB edge increases, which subsequently increases the driving force for the ultrafast excited-state electron injection, contrary to the effect of 4-tert-butylpyridine additive. The computational results of the oxidized dye interacting with I- and I-2(-) reveal that there are a few possible mechanisms for the regeneration of oxidized dye. The effective mechanisms of the regeneration are suggested