Thin Films of Dendritic Anatase Titania Nanowires Enable Effective Hole-Blocking and Efficient Light-Harvesting for High-Performance Mesoscopic Perovskite Solar Cells

To achieve high-performance perovskite solar cells, especially with mesoscopic cell structure, the design of the electron transport layer (ETL) is of paramount importance. Highly branched anatase TiO2 nanowires (ATNWs) with varied orientation are grown via a facile one-step hydrothermal process on a transparent conducting oxide substrate. These films show good coverage with optimization obtained by controlling the hydrothermal reaction time. A homogeneous methylammonium lead iodide (CH3NH3PbI3) perovskite thin film is deposited onto these ATNW films forming a bilayer architecture comprising of a CH3NH3PbI3 sensitized ATNW bottom layer and a CH3NH3PbI3 capping layer. The formation, grain size, and uniformity of the perovskite crystals strongly depend on the degree of surface coverage and the thickness of the ATNW film. Solar cells constructed using the optimized ATNW thin films (220 nm in thickness) yield power conversion efficiencies up to 14.2% with a short-circuit photocurrent density of 20.32 mA cm(-2), an open-circuit photovoltage of 993 mV, and a fill factor of 0.70. The dendritic ETL and additional perovskite capping layer efficiently capture light and thus exhibit a superior light harvesting efficiency. The ATNW film is an effective hole-blocking layer and efficient electron transport medium for excellent charge separation and collection within the cells.