Solution-Processed Ultrathin TiO Compact Layer Hybridized with Mesoporous TiO 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 TiO ETLs fabricated by depositing compact TiO (c-TiO) and mesoporous TiO (mp-TiO) in sequence exhibit resistive losses due to the contact resistance at the c-TiO/mp-TiO interface and the series resistance arising from the intrinsically low conductivity of TiO. Herein, to minimize such resistive losses, we developed a novel ETL consisting of an ultrathin c-TiO layer hybridized with mp-TiO, which is fabricated by performing one-step spin-coating of a mp-TiO 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-TiO 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.