Room-Temperature Synthesis of Ultrasmall Sulfur-Free SnO₂ Quantum Dots as an Electron Transport Layer in Perovskite Solar Cells

Abstract

SnO2 quantum dot (QD) electron transporting layers (ETLs) have garnered significant attention as an alternative to conventional SnO2 ETLs in perovskite solar cells (PSCs). Despite the advancements in replacing conventional SnO2with SnO2 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 SnO2 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 SnO2 QD inks and thin films, and analyze the chemical, morphological, crystallographic, electronic, and optoelectronic characteristics of SnO2 QD thin films to optimize precursor formulation for depositing phase-pure SnO2 QDs. Our optimized urea-based SnO2 QDs deliver device PCEs up to 20.01%, outperforming conventional thiourea-derived SnO2 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.