Ab-Initio Investigation on Dye Conformer Structures and the Interplay between Conformation and Multilayer Aggregation on TiO2 toward Solar Cell Application

Abstract

Self-assembled monolayers (SAMs) have many applications such as the solar cells and water-spitting systems, which mimic the photosynthetic process in the plants and serve as an effective alternative to provide clean energy from the sun. Dye molecules that are self-assembled on semiconductor substrates (usually TiO2) play essential roles in the photon-to-electron conversion process. However, many structural aspects and structure-properties of dyes in SAMs remain elusive, and the conformer structures of dye molecules and their effects on the properties of SAMs are frequently neglected. In fact, dye molecules can adopt multiple conformer structures on the TiO2 substrate due to the existence of flexible alkyl chains and non-rigid backbones, which might affect the structures and properties of the dye-sensitized systems. In this research, we investigate the structures and effects of different conformers via the first principles calculations, employing an organic azo dye 4-[2-[4-(diethyl-amino)phenyl]diazenyl]-benzoic acid, which combines good transparency, aesthetic quality and electricity-generation properties. We find that various conformers of the organic dye could reside stably on the TiO2 substrate, and the conformer effects are especially dominant when the dye aggregation model is considered. The relationship between the azo dye conformation and the azo dye aggregation is explained. Our calculations demonstrate that including the conformer effects leads to diverse structures and properties of the azo dye/TiO2 systems, which could be important for the devices containing the dye/TiO2 systems such as the dye-sensitized solar cells, smart windows and dye-sensitized water splitting systems.