Solution-Processed Ultrathin TiO₂ Compact Layer Hybridized with Mesoporous TiO₂ for High-Performance Perovskite Solar Cells

The electron transport layer (ETL) is a key component of perovskite solar cells (PSCs) and must provide efficient electron extraction and collection while minimizing the charge recombination at interfaces in order to ensure high performance. Conventional bilayered TiO₂ ETLs fabricated by depositing compact TiO₂ (c-TiO₂) and mesoporous TiO₂ (mp-TiO₂) in sequence exhibit resistive losses due to the contact resistance at the c-TiO₂/mp-TiO₂ interface and the series resistance arising from the intrinsically low conductivity of TiO₂. Herein, to minimize such resistive losses, we developed a novel ETL consisting of an ultrathin c-TiO₂ layer hybridized with mp-TiO₂, which is fabricated by performing one-step spin-coating of a mp-TiO₂ solution containing a small amount of titanium diisopropoxide bis­(acetylacetonate) (TAA). By using electron microscopies and elemental mapping analysis, we establish that the optimal concentration of TAA produces an ultrathin blocking layer with a thickness of ∼3 nm and ensures that the mp-TiO₂ layer has a suitable porosity for efficient perovskite infiltration. We compare PSCs based on mesoscopic ETLs with and without compact layers to determine the role of the hole-blocking layer in their performances. The hybrid ETLs exhibit enhanced electron extraction and reduced charge recombination, resulting in better photovoltaic performances and reduced hysteresis of PSCs compared to those with conventional bilayered ETLs