Conventional synthesis routes for functional materials in energy applications often involve significant economic and environmental costs, posing a challenge to the sustainable energy transition. Addressing this, we report a novel, green synthesis of Cerium Oxide (CeO₂) and Tin-doped CeO₂ (Sn-CeO₂) nanoparticles using aqueous extracts of pineapple leaf waste. This agricultural byproduct serves as a source of biodegradable reducing and capping agents, offering a sustainable alternative to standard chemical precursors. The nanoparticles were characterized and evaluated for their potential as an Electron Extraction Layer (EEL) in Perovskite Solar Cells (PSCs). Characterization confirmed the formation of crystalline, cubic-phase nanoparticles. Notably, Sn-doping induced a favourable widening of the optical bandgap from 2.36 eV to 2.76 eV and conferred enhanced thermal stability compared to pure CeO₂. These findings confirm a waste-to-resource paradigm for fabricating critical photovoltaic components, showing a model for sustainable innovation that bridges material science and green chemistry.