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 (SnO 2 ) is a promising alternative to titanium oxide (TiO 2 ) 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 SnO 2 as an ETL in all-inorganic lead-free PSCs has not been thoroughly investigated. This work presents the effect of SnO 2 as an ETL interfaced with the Cs 3 Bi 2 I 9 and CsBi 3 I 10 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/SnO 2 /(Cs 3 Bi 2 I 9 or CsBi 3 I 10 )/Spiro-OMeTAD/Au. The CsBi 3 I 10 device exhibited a photo conversion efficiency (PCE) of ∼1% with a fill factor of 59% while the Cs 3 Bi 2 I 9 device showed a PCE of 0.15%. These findings mark significant progress in the eco-friendly development of lead-free Cs 3 Bi 2 I 9 and CsBi 3 I 10 perovskite-based solar cells.