Computational design of phenoxazine and phenothiazine dye sensitizers with improved charge separation for application in DSSCs: a DFT study

This study utilizes DFT and TD-DFT calculations at the B3LYP/6-31G(d,p) level to design and evaluate eight metal-free organic dye sensitizers for DSSCs, comprising phenoxazine (POD1–POD4) and phenothiazine (PZD1–PZD4) donor groups, thiophene-based π-spacers, and 2-cyanoacrylic acid as the acceptor. The phenoxazine dyes (POD series) exhibit superior molecular planarity, enhancing intramolecular charge transfer and reducing band gaps, with POD4 showing the smallest value (1.63 eV). These dyes also possess higher dipole moments and reactivity, promoting more effective charge separation. Among them, POD3 achieves the highest open-circuit voltage and regeneration driving force, while PZD4 demonstrates exceptional light-harvesting efficiency. Furthermore, the POD dyes facilitate faster electron transfer, resulting in higher short-circuit current density, and both POD4 and PZD2 exhibit prolonged excited-state lifetimes, which help suppress charge recombination.