Ce-Doped SnO₂ Electron Transport Layer for Minimizing Open Circuit Voltage Loss in Lead Perovskite Solar Cells

In the planar heterostructure of perovskite-based solar cells (PSCs), tin oxide (SnO₂) is a material that is often used as the electron transport layer (ETL). SnO₂ ETL exhibits favorable optical and electrical properties in the PSC structures. Nevertheless, the open circuit voltage (V_OC) depletion occurs in PSCs due to the defects arising from the high oxygen vacancy on the SnO₂ surface and the deeper conduction band (CB) energy level of SnO₂. In this research, a cerium (Ce) dopant was introduced in SnO₂ (Ce-SnO₂) to suppress the V_OC loss of the PSCs. The CB minimum of SnO₂ was shifted closer to that of the perovskite after the Ce doping. Besides, the Ce doping effectively passivated the surface defects on SnO₂ as well as improved the electron transport velocity by the Ce-SnO₂. These results enabled the power conversion efficiency (PCE) to increase from 21.1% (SnO₂) to 23.0% (Ce-SnO₂) of the PSCs (0.09 cm² active area) with around 100 mV of improved V_OC and reduced hysteresis. Also, the Ce-SnO₂ ETL-based large area (1.0 cm²) PSCs delivered the highest PCE of 22.9%. Furthermore, a V_OC of 1.19 V with a PCE of 23.3% was demonstrated by Ce-SnO₂ ETL-based PSCs (0.09 cm² active area) that were treated with 2-phenethylamine hydroiodide on the perovskite top surface. Notably, the unencapsulated Ce-SnO₂ ETL-based PSC was able to maintain above 90% of its initial PCE for around 2000 h which was stored under room temperature condition (23-25 °C) with a relative humidity of 40-50%.