OPTO-ELECTRICAL CHARACTERIZATION OF BIO-DYE EXTRACT FROM Adansonia digitata (BAOBAB) AND Azadirachta indica (NEEM TREE) LEAVES FOR DYE SENSITIZED SOLAR CELL APPLICATION

Abstract

Solar power harvesting research has been primarily focused on developing solar cells that are both affordable and environmentally friendly. Among the extensively researched technologies are Perovskite, Quantum Dot (Q.D), and Dye Sensitized Solar Cells (DSSCs). DSSCs utilizing ruthenium-complex dyes have demonstrated high conversion efficiencies. However, due to the scarcity, high cost, and toxicity of ruthenium, there is a growing interest in exploring natural dyes as alternatives. Natural dyes are more cost-effective and less harmful than ruthenium, although they do exhibit lower Incident Photon-to-Current Conversion Efficiency (IPCE), power conversion efficiency, and stability compared to ruthenium-based dyes. To optimize the performance of natural dye-based DSSCs, several approaches were employed. These include increasing charge excitations from Highest Occupied Molecular Orbital (HOMO) to Lowest Unoccupied Molecular Orbital (LUMO), enhancing charge injection into the conduction band of the semiconductor, and reducing recombination rates. In this study, zinc oxide (ZnO) was synthesized through the sol-gel technique using zinc chloride as a precursor. Thin films of ZnO were then deposited on FTO slides using the spin coating method, forming the anode of the DSSCs. Sensitizers for these solar cells were prepared by blending leaf extracts from Baobab and Neem trees. Analysis of ZnO thin film optical micrographs using ImageJ revealed that surface roughness and particle distribution improved with higher calcination temperatures. The sheet resistance of the ZnO thin film decreased as the calcination temperature increased, as determined by four-point probe analysis. UV-Vis spectroscopy of the thin film indicated absorption at 338 nm, attributed to Surface Plasmon Resonance in ZnO. The zinc oxide-based DSSCs were then sensitized with various combinations of Baobab and Neem leaf extracts, ranging from 50% to 100% concentration. The output parameters of these sensitized cells were compared to those of cells sensitized with pure Baobab or pure Neem leaf extracts. By employing UV-Vis spectroscopy in the range of 300 nm to 700 nm and utilizing Tauc's approximation approach to determine the optical bandgap of the dyes, it was discovered that the band gap energy fell within the range of 1.65 eV to 1.757 eV. Notably, blending the dyes in a 33% Baobab to 67% Neem ratio resulted in a significant improvement in the JSC (short-circuit current density) of Baobab DSSC, increasing from 0.133 mA.cm-2 to 1.23 mA.cm-2. Likewise, the open-circuit voltage (VOC) of Neem DSSC increased from 0.402 V to 0.433 V. This blending approach also led to a substantial enhancement in the power conversion efficiency (ƞ) of the Baobab DSSC, improving from 0.15% to 1.11%. These findings demonstrate that blending the two dyes resulted in a broader absorption bandwidth for Baobab dye, thereby improving the light absorption capabilities and consequently the power conversion efficiency of the dye-sensitized solar cell.