Optical, Spectroscopic and Band-Gap Characterisation of Natural Dyes Extracted from Delonix regia Flower, Curcuma longa (Turmeric) and Their 1:1 Mixture as Photosensitisers for Dye-Sensitised Solar Cells

Natural dyes are inexpensive, abundant and environmentally benign photosensitisers for third-generation dye-sensitised solar cells (DSSCs), but their light-harvesting behaviour is governed by pigment chemistry, extraction solvent and the mode of anchoring on the semiconductor surface. In this study, natural dyes were extracted from Delonix regia (royal poinciana) flower and Curcuma longa (turmeric), together with a 1:1 (v/v) mixture of the two extracts, using a 50:50 distilled water-methanol solvent, and their optical properties were characterised in solution and after adsorption onto a nanoporous titanium dioxide (TiO₂) film. Ultraviolet-visible-near-infrared (UV-Vis-NIR) spectra recorded from 230 to 1100 nm showed that all three extracts absorbed strongly in the ultraviolet (333 and 375 nm) and visible regions; the D. regia extract additionally exhibited a broad near-infrared band at 764 nm in solution which extended to 1028 nm after adsorption on TiO₂. From the measured absorbance, the absorption coefficient, an absorbance-based extinction parameter and the light-harvesting efficiency (LHE) were derived; the D. regia extract sustained an LHE of approximately 99% over 305–541 nm. Optical band gaps obtained from Tauc analysis, assuming a direct allowed transition, were 2.02 eV for D. regia, 1.98 eV for turmeric and 2.35 eV for the mixture. The results indicate that D. regia is the most promising single sensitiser of the three and that the 1:1 mixture is optically superior to the single turmeric extract, although co-pigmentation redistributes rather than simply superimposes the component bands. The dataset provides the optical baseline required for rational sensitiser selection and co-sensitisation.