Colloidal SnO2-Assisted CdS Electron Transport Layer Enables Efficient Electron Extraction for Planar Perovskite Solar Cells

The cadmium sulfide (CdS) is a promising electron transport layer (ETL) material for perovskite solar cells (PSCs) due to its low photocatalytic activity toward perovskite materials under UV light. The critical problem responsible for the moderate performance of CdS-based PSCs is the parasitic light absorption of CdS, which drives researchers to deposit ultrathin ETLs. However, the ultrathin ETL often involves the undesirable shunt current leakage because of the direct contact between conducting substrate and perovskite layer. Herein, a fully low-temperature solution-processed colloidal SnO2-assisted CdS (S-CdS) ETL for planar CH3NH3PbI3 PSCs is constructed. The detailed characterizations of morphological, optical, and energy levels confirm that the assistance of colloidal SnO2 provides the ameliorated continuity, reduces surface roughness and superior wettability of ETLs for high-quality perovskite formation as well as the favorable cascade band structure for efficient charge transfer. The study of charge transfer mechanisms reveals that the S-CdS ETL effectively inhibits the shunt leakage, promotes the electron extraction and suppresses the charge recombination at the ETL/perovskite interface. Consequently, the S-CdS ETL-based PSCs deliver an appreciable efficiency of 16.26%, doubling that of conventional CdS-based devices. To the best of our knowledge, this value is the champion efficiency reported for CdS-based CH3NH3PbI3 PSCs.