Fluorinated Cation-Based 2D Perovskites for Efficient and Stable 3D/2D Heterojunction Perovskite Solar Cells

Three-dimensional (3D) perovskite solar cells (PSCs) containing additives capable of forming two-dimensional (2D) structures in neat films have attracted attention due to their ability to enhance power conversion efficiency (PCE) in combination with improved operational stability. Herein, a newly designed fluorinated ammonium salt, 2-(perfluorophenyl)ethanaminium bromide:chloride_50:50 (FEABr:Cl_50:50), is introduced into CsMAFAPbI₃-based PSCs with a standard n-i-p architecture. FEABr:Cl_50:50 was used as an additive in the tin(IV) oxide (SnO₂) electron transporting layer (ETL) as well as a surface treatment for the perovskite film. Used in this dual way, the additive was found to passivate charge-trapping defects within the SnO₂ ETL and regulate the crystal growth of the perovskite layer. When FEABr:Cl_50:50 was deposited onto the surface of the 3D perovskite film, it formed a thin hydrophobic 2D capping layer. Adopting this dual strategy led to the perovskite film having larger grain sizes, improved quality, and overall better device performance. As a result, the best-performing device exhibited a PCE of over 23% with negligible hysteresis in an n-i-p device architecture with an area of 0.2 cm². Furthermore, unencapsulated devices with the hydrophobic 2D capping layer showed improved stability compared to the control device when measured under continuous light irradiation at a maximum power point (MPP) at 80 ± 5 °C in a humid (≈50%) environment.