Computational molecular insights of natural carotenoid photosensitizers for dye-sensitized solar cells: A DFT and TD-DFT study

Natural carotenoid dyes have emerged as promising sustainable alternatives to conventional metal- based sensitizers in dye-sensitized solar cells (DSSCs) because of their low toxicity, natural availability, and strong visible-light absorption characteristics. In this work, a comprehensive density functional theory (DFT) and time-dependent DFT (TD-DFT) investigation was performed to evaluate the photovoltaic potential of four natural carotenoid dyes: bixin (BXN), norbixin (NBXN), lutein (LTN), and zeaxanthin (ZXTN). The ground-state geometry optimization and subsequent electronic structures calculation were performed employing B3LYP/6–311 G(d,p) level, while TD-DFT at the CAM-B3LYP/6–311 G(d,p) level. The calculated HOMO–LUMO gaps range from 2.223 to 2.300 eV, indicating comparable electronic excitation characteristics. Electronic structures analyses show predominantly delocalized π-electron distributions, with electron-rich oxygen-containing end groups as principal anchoring sites toward TiO 2 . Positive values of ∆ 𝑁 for the studied dyes indicate charge transfer from the dyes toward the TiO 2 . The lowest-energy excited states are dominated by H→L configurations. TD-DFT calculations predict intense visible-light absorption, with 𝜆 m a x ranges 465–482 and 495–515.70 nm in gas and acetonitrile respectively. All dyes exhibit favorable energetics for electron injection into TiO 2 . However, BXN and NBXN show more feasible energy alignments and dye-regeneration energetics, while their carboxyl/ester functionalities provide a more advantageous anchoring motif for TiO 2 than the hydroxyl groups of LTN and ZXTN. Overall, BXN and NBXN provide comparatively balanced electronic, optical, photovoltaic, and anchoring characteristics, whereas LTN and ZXTN of face limitations associated with regeneration and semiconductor anchoring. These findings provide molecular-level guidance for the development of sustainable carotenoid-based sensitizers for DSSCs.