Room-TemperatureSynthesis of Ultrasmall Sulfur-FreeSnO₂ Quantum Dots as an Electron Transport Layer in PerovskiteSolar Cells

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Original 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.

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