2-Hexylthiophene-substituted Alizarin-based (D–π–A) Organic Dyes for Dye-sensitized Solar Cell Applications: Density Functional Theory and UV–Vis Studies

This work reports density functional theory and time-dependent density functional theory calculations of the optimized geometries, electronic structures and optical properties of molecular dyes D1, D2, D3, D4, D5, and D6 formulated through substitution of 2-hexylthiophene to alizarin using the hybrid functional B3LYP and 6-31G (d,p) basis sets. The dyes are considered as potential pigments for dye-sensitized solar cells. For all dyes, HOMO/LUMO (Highest Occupied Molecular Orbital/Lowest Unoccupied Molecular Orbital) analysis results in positive outcomes upon electron injection to semiconductors and subsequent dye regeneration by the electrolyte. It is found that charge transfer is from the thiophene and unsubstituted ring of alizarin to the substituted ring of alizarin containing C=O and OH groups. The C=O groups are observed to be very important in strengthening the dyes as they are revealed to be the anchoring group bonding to the TiO2 semiconductor. Comparatively, dye D6 is observed to possess high absorption ability and electron injection power through a study of the light-harvesting efficiency and injection driving force (Δ Ginject). The estimated values of open-circuit voltage ( Voc) for the computed dyes are also presented. Decisively, all the considered dyes prove to be useful as potential photosensitizers in solar cells using a TiO2 semiconductor and [Formula: see text] coupling electrolyte.