Room-Temperature Synthesis of Ultrasmall Sulfur-Free SnO ₂ Quantum Dots as an Electron Transport Layer in Perovskite Solar Cells

SnO 2 quantum dot (QD) electron transporting layers (ETLs) have garnered significant attention as an alternative to conventional SnO 2 ETLs in perovskite solar cells (PSCs). Despite the advancements in replacing conventional SnO 2 with SnO 2 QDs, current synthesis strategies are complex and rely on hazardous facilitators such as thiourea, raising concerns regarding environmental impact alongside long-term device stability. To address these issues, we report a room-temperature, ink-based approach for synthesizing ultrasmall SnO 2 QDs (∼1 nm) under ambient conditions, employing novel, environmentally benign, and sulfur-free urea-based ligands. We explore the influence of ligand methylation on the properties and performance of the SnO 2 QD inks and thin films, and analyze the chemical, morphological, crystallographic, electronic, and optoelectronic characteristics of SnO 2 QD thin films to optimize precursor formulation for depositing phase-pure SnO 2 QDs. Our optimized urea-based SnO 2 QDs deliver device PCEs up to 20.01%, outperforming conventional thiourea-derived SnO 2 QDs at 18.71%. Furthermore, the urea-based devices retained about 90% of their initial efficiency after 90 days in a drybox and over 93% under 72 h of continuous ambient illumination, compared to 83% and 90% retention, respectively, for the thiourea-based reference devices. This novel approach may offer a pathway to stable, highly efficient, and flexible photovoltaic cells via low-temperature processing.