Size-Engineered ZnO and CdS Nanomaterials for Photovoltaics: Optical Properties and Charge Transport

This study examines the size-dependent structural, optical, and electrical properties of ZnO nanoparticles and CdS quantum dots as candidate materials for photovoltaic architectures. Chemical precipitation and aqueous synthesis are described alongside X-ray diffraction, transmission electron microscopy, ultraviolet–visible absorption, photoluminescence, temperature-dependent conductivity, and dark current–voltage characterization. The reported ZnO band-gap values increase from 3.37 eV (bulk) to 4.10 eV (2 nm), consistent with a quantum-confinement interpretation. ZnO nanocrystals show an approximately 8.5 nm characteristic size; the reported transport activation energies decrease from 0.80 eV (bulk) to 0.62 eV (5 nm). CdS absorption features shift from approximately 540 nm (12 nm particles) to 380 nm (3 nm particles), indicating tunable spectral response. The integrated discussion links size effects, structural quality, defect-related emission, and junction transport to possible roles in sensitizers and electron-transport layers. These materials-level observations do not establish an improvement in solar-cell efficiency because calibrated illuminated current–voltage measurements were not performed. Future device studies should test charge extraction, stability, and photovoltaic performance under standardized illumination.