Green synthesis of ZnO nanoparticles for DSSC photoanode : a joint experimental and density functional theory study

Abstract

Green synthesis, a biological method for nanoparticle preparation, has been suggested as a possible eco-friendly alternative to chemical and physical methods. In this study, we report on first principles calculations and the green synthesis of zinc oxide (ZnO) nanoparticles (NPs) from Erythrina abyssinica stem bark extract calcined under different temperatures (300-700 ℃) for application as a photoanode in dye sensitized solar cells (DSSCs). Synthesized ZnO NPs were subjected to characterization using X-Ray diffraction, Scanning Electron Microscopy, Energy Dispersive X-Ray spectroscopy, Ultraviolet–Visible spectroscopy and photoluminescence analysis. The analysis revealed that highly crystalline hexagonal ZnO NPs were formed at 700 ℃, with the nanospheres agglomeration into non-uniform distinct NPs with a band gap energy of 3.12 eV. The DSSC exhibited a short circuit current density (Jsc) of 56 µA cm-2, open circuit voltage (Voc) of 161 mV, a fill factor of 0.265, and a power conversion efficiency of 0.0024% using 100 mWcm-2 illumination. Density Functional Theory (DFT) calculations were performed on the structural, electronic, and dielectric properties of ZnO at the atomic level. The Projected Density of States (PDOS) analysis revealed that Zn-4s and O-2p orbitals contributed significantly to the conduction band minimum (CBM) and valence band maximum (VBM), respectively, and a direct band gap at Gamma in the electronic band structure. Dielectric function analysis revealed anisotropy in the refractive index and dielectric function, with noticeable transparency in the visible spectrum and strong absorption in the ultraviolet, making them potential candidates in a set of photoelectrochemical applications.