The Synergetic Effect Between Extracted Dyes from Stigma and Petals of Saffron for Sensitizing TiO2 Nanoparticles for Harvesting the Light

Introduction: Dye-sensitized solar cells (DSSCs) have emerged as promising alternative to conventional solar cells, largely due to their low cost and relatively simple fabrication process. The core of a DSSC is the photo-sensitizer dye, which absorbs sunlight and injects electrons into a semiconductor, typically nanostructured titanium dioxide (TiO₂). While ruthenium-based dyes have achieved high efficiencies, their high cost and toxicity have spurred research into more affordable and environmentally friendly alternatives, particularly natural dyes. Saffron, Crocus Sativus L., is a valuable spice and has been studied for its health-promoting properties. The stigmas and petals of saffron contain various natural dyes, such as crocetin and pelargonidin, which are responsible for its vibrant color. This study investigates, for the first time, the potential of natural dyes extracted from the stigma and petals of Birjand saffron as co-sensitizers in DSSCs. We aim to explore the photoelectrochemical properties of these dyes and determine whether a synergistic effect exists when used together.
 
Materials and Methods: Fresh saffron flowers were sourced from a farm in Noferest, Birjand, Iran. Dyes were extracted from the stigma, petals, and a 1:1 mixture of both, using absolute ethanol. These extracts were then used to sensitize a commercial anatase TiO₂​ paste, which was applied to an FTO conductive glass substrate using the doctor-blading technique and then sintered at 400 °C. A layer of larger TiO2​ particles (300 nm) was applied as a reflector. UV-Vis diffuse reflectance spectroscopy (DRS) was employed to analyze the absorption characteristics of the dyes that were adsorbed onto the TiO₂​ nanoparticles. The photoelectrochemical properties of the prepared DSSCs were measured under simulated sunlight (100 mW/cm2) using I-V (current-voltage) measurements and electrochemical impedance spectroscopy (EIS) via an AUTOLAB model. The resulting data facilitated the calculation of key parameters for the calculation of key parameters such as short-circuit current density (JSC), open-circuit voltage (VOC), fill factor (FF), and overall energy conversion efficiency (η).
 
Results and Discussion: UV-Vis DRS analysis revealed that the dye from saffron stigma produced an absorption peak around 450 nm, while the dye from the petals showed a peak around 625 nm. When a mixture of the dyes was adsorbed, the resulting film displayed two distinct absorption peaks, demonstrating that both dyes successfully co-adsorbed and collectively broadened the absorption range of the TiO₂ film from 375 nm to 750 nm.
The photoelectrochemical performance of the DSSCs was evaluated. The cell sensitized by the mixed dye extract exhibited a significantly higher energy conversion efficiency (η) compared to the cells sensitized by the individual stigma and petal extracts. Notably, the efficiency of the co-sensitized cell was 46% higher than the linear superposition of the individual cell efficiencies. This non-linear effect confirms a synergistic interaction between the dyes, where their combined performance exceeds the sum of their individual contributions. This synergistic effect is likely due to more efficient energy transfer and enhanced light harvesting across the broader absorption spectrum.
Electrochemical impedance spectroscopy (EIS) data further supported this finding. The EIS Nyquist plots showed a deviated semicircle for each cell, with the characteristic frequency for electron lifetime being 14.3 Hz, 20.3 Hz, and 4.9 Hz for the stigma, petal, and mixed-dye cells, respectively. These frequencies correspond to electron lifetimes of approximately 70 ms, 50 ms, and 200 ms. The significantly longer electron lifetime in the co-sensitized cell indicates that the co-adsorption of the saffron stigma and petal dyes effectively reduces the recombination rate of electrons, contributing to the enhanced performance.
 
Conclusion: This study successfully demonstrated the use of natural dyes extracted from both the stigma and petals of saffron as photo-sensitizers for DSSCs. We found that co-adsorption of these dyes on nanostructured TiO₂ particles results in a broad and effective light absorption spectrum. The energy conversion efficiency of the co-sensitized cell was found to be 46% higher than the linear sum of the individual dye-sensitized cells, revealing a significant synergistic effect. Furthermore, impedance spectroscopy confirmed that this synergistic effect is coupled with a reduced electron recombination rate and an increased electron lifetime. These findings suggest that saffron extracts, particularly when used in combination, hold great promise as a viable, low-cost, and non-toxic alternative to conventional synthetic dyes for solar cell applications.