Star-Shaped Molecules with a Triazine Core: (TD)DFT Investigation of Charge Transfer and Photovoltaic Properties of Organic Solar Cells

We investigate theoretically the design and application of nine triazine (Tr)-core star-shaped molecules built with thiophene (Th), phenyl (Ph), and carbazole (Cz) moieties in dye-sensitized solar cells (DSSCs) and bulk heterojunction (BHJ) organic solar cells. Density functional theory and time-dependent density functional theory are employed to rationalize the electronic structure, charge transfer properties, and photovoltaic performance of these systems. Tr-Th-Cz and Tr-Ph-p-Th-Cz display power conversion efficiency (PCE\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\text{PCE}$$\end{document}) values close to 30% due to a favorable alignment of the energy levels. Computed open-circuit voltage values show that lack of pi-bridges, as in the Tr-Th and Tr-Cz, favor higher voltage generation. Systems containing thiophene (Th) and carbazole (Cz) present the most favorable values for efficient electron injection and dye regeneration. Furthermore, the incorporation of these Th and Cz donor substituents into the triazine core combined with pi-bridges significantly impacts other optoelectronic properties of the organic solar cells. The computed high singlet-triplet gap values indicate limited potential of these molecules as thermally activated delayed fluorescence (TADF) emitters for applications in organic light-emitting diodes (OLEDs).