Low-Temperature Solution-Processed SnO₂ Electron Transport Layer for Stable, Lead-Free Cesium-Based Perovskite Solar Cells

The electron transport layer (ETL) plays a vital role in extracting and transporting photogenerated charge carriers in a halide-based perovskite solar cells (PSCs). Tin oxide (SnO2) is a promising alternative to titanium oxide (TiO2) as an ETL due to its better energy level alignment with the perovskite absorber layers, high mobility, stability, and ability to be deposited at lower temperature. However, despite these advantages and extensive research on lead-free perovskite thin film solar cells, the use of SnO2 as an ETL in all-inorganic lead-free PSCs has not been thoroughly investigated. This work presents the effect of SnO2 as an ETL interfaced with the Cs3Bi2I9 and CsBi3I10 lead-free perovskite thin films on their structural, morphological, and photoemission spectroscopic properties. Moreover, all inorganic perovskite solar cells have been fabricated for the first time to analyze the performance in the n-i-p planar architecture with device stacks comprising ITO/SnO2/(Cs3Bi2I9 or CsBi3I10)/Spiro-OMeTAD/Au. The CsBi3I10 device exhibited a photo conversion efficiency (PCE) of ~1% with a fill factor of 59% while the Cs3Bi2I9 device showed a PCE of 0.15%. These findings mark significant progress in the eco-friendly development of lead-free Cs3Bi2I9 and CsBi3I10 perovskite-based solar cells.