Novel Bilayer SnO₂ Electron Transport Layers with Atomic Layer Deposition for High-Performance α-FAPbI₃ Perovskite Solar Cells

Perovskite solar cells (PSCs) based on the SnO₂ electron transport layer (ETL) have achieved remarkable photovoltaic efficiency. However, the commercial SnO₂ ETLs show various shortcomings. The SnO₂ precursor is prone to agglomeration, resulting in poor morphology with numerous interface defects. Additionally, the open circuit voltage (V_oc) would be constrained by the energy level mismatch between the SnO₂ and the perovskite. And, few studies designed SnO₂-based ETLs to promote crystal growth of PbI₂, a crucial prerequisite for obtaining high-quality perovskite films via the two-step method. Herein, we proposed a novel bilayer SnO₂ structure that combined the atomic layer deposition (ALD) and sol-gel solution to well address the aforementioned issues. Due to the unique conformal effect of ALD-SnO₂, it can effectively modulate the roughness of FTO substrate, enhance the quality of ETL, and induce the growth of PbI₂ crystal phase to develop the crystallinity of perovskite layer. Furthermore, a created built-in field of the bilayer SnO₂ can help to overcome the electron accumulation at the ETL/perovskite interface, leading to a higher V_oc and fill factor. Consequently, the efficiency of PSCs with ionic liquid solvent increases from 22.09% to 23.86%, maintaining 85% initial efficiency in a 20% humidity N2 environment for 1300 h.