Enhanced fill factor for normal n-i-p planar heterojunction and mesoscopic perovskite solar cells using ruthenium-doped TiO₂ electron transporting layer

Perovskite solar cells (PSCs) fabricated with a normal mesoscopic negative-intrinsic-positive (n-i-p) device structure have shown a great promise for further developments ever since their enhanced performance with high power conversion efficiency (PCE, 25.2%) and excellent reproducibility. The electron transporting layer (ETL) employed in such PSCs has been a critical component for improving their performance. The present work focuses on the synthesis of high-quality Ru-doped compact TiO₂ (c-TiO₂) ETLs (Ru:c-TiO₂) by a simple spin-coating technique. Further, the role of Ru⁴⁺ cation doping in c-TiO₂ is discussed in detail. A systematic study revealed that the Ru-doping not only significantly influences the open-circuit voltage (V_oc), current density (J_sc), and fill factor (FF) but also suppresses the charge recombination in the perovskite devices. The PSCs prepared using Ru-c-TiO₂ ETLs with optimum Ru-doping content exhibited PCEs of 19.48% for planar and 20.87% for mesoscopic device architecture with enhanced photovoltage. Additionally, the fabricated PSC devices based on 1.5% Ru:c-TiO₂ ETLs exhibited air stability over 200 days, which is much higher than that of a control device.