TiO 2 /WO 3 Bilayer as Electron Transport Layer for Efficient Planar Perovskite Solar Cell with Efficiency Exceeding 20%

Abstract It is crucial to retard the carrier recombination and minimize the energy loss at the transparent electrode/electron transport layer (ETL)/perovskite absorber interfaces to improve the performance of the perovskite solar cells (PSCs). Here, a bilayered TiO 2 /WO 3 film is designed as ETL by combining atomic layer deposition (ALD) technology and spin‐coating process. The ALD‐TiO 2 underlayer fills the fluorine‐doped tin oxide (FTO) valleys and makes the surface smoother, which effectively avoids the shunt pathways between perovskite layer and FTO substrate and thereby suppresses electron–hole recombination at the interface. Moreover, the presence of hydrophilic TiO 2 underlayer is helpful in forming a uniform and compact WO 3 layer which is beneficial for extracting electron from perovskite to ETL. Meanwhile, the lower valance band minimum level of TiO 2 relative to WO 3 can efficiently enhance the hole‐blocking ability. By employing the optimized TiO 2 (7 nm)/WO 3 bilayer as ETL, the resulting cell exhibits an obviously enhanced power conversion efficiency of up to 20.14%, which is much better than the single WO 3 or TiO 2 ETL based device. This work is expected to provide a viable path to design ultrathin and compact ETL for efficient PSCs.