Solution-Processed\nUltrathin TiO₂ Compact\nLayer Hybridized with Mesoporous TiO₂ for High-Performance\nPerovskite Solar Cells

Abstract

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