Small thiophene-based molecules with favorable properties as HTMs for perovskite solar cells or as active materials in organic solar cells

Abstract

Computational investigations are carried out to design new molecules based on thiophene cores substituted in several positions using different end-capped electron-donating groups formed from triphenylamine or OMe-triphenylamine that can be used as HTMs in Perovskite Solar Cells (PSCs), or as an active layer in Organic Solar Cells. The influence of π-extension on the charge transfer process is also probed by inserting two azomethine groups between acceptor and donor (D-π-A-π-D) moieties. The designed molecules (T10,12,13,15) were compared with the two synthesized molecules (T11,14) that have been applied as HTMs in a PSC. Density functional theory (DFT) and time-dependent DFT (TD-DFT) were chosen to calculate different chemical parameters of these molecules. Different chemical parameters were calculated to make the link between geometrical parameters and the efficiency, solubility, and stability of the under-probed compounds. The results show that all molecules can be used adequately with the perovskite matrix because of their sufficient deep HOMOs levels. The optoelectronic properties of the D-A-D molecules show no competition with the light-harvesting of the perovskite layer, while D-π-A-π-D compounds show a slight overlap with the absorption band of perovskites. All molecules indicate a faster hole-transport rate and an easy electron-hole pairs dissociation into free charge carriers, which facilitates the hole transport and enhances open-circuit voltage. Adding a second thiophene unit in the central core reinforces the structure's flexibility and improves Frontier Molecular Orbital's electronic density distribution, while inserting azomethine groups is advantageous for the molecule's solubility. Their short gaps, higher radiative lifetimes, and visible light absorbance make T12,15 good candidates for their usage in organic solar cell applications. A spontaneous solvation process within dichloromethane for all the investigated molecules was proved based on the high negative free energies.