Deciphering the Difference Between Molecular Cation and 2D Perovskite Surface Passivation in Perovskite Solar Cells

The deposition of large ammonium cations onto perovskite surfaces to passivate defects and reduce contact recombination has enabled exceptional efficiency/stability in perovskite solar cells. These ammonium cations can either assemble as a thin molecular layer at the perovskite surface or induce the formation of a low-dimensional - usually two-dimensional (2D) - perovskite capping layer on top of the three-dimensional (3D) perovskite. The formation of these two different structures is often overlooked by researchers although they impact differently on device operation. In this presentation we carefully distinguish between the two passivation types and discuss the impact of these two techniques on world record device performance. Using first-hand experimental data combined with a meta-analysis derived from hundreds of research articles, we provide a set of conditions required to form 2D perovskite atop of 3D. We then exam the electronic properties of the two structures and demonstrate that 2D perovskite forms a blocking layer for electron transport within a solar cell stack. We discuss the mechanisms by which 2D perovskite passivation and molecular passivation can improve photovoltaic efficiency and stability, and we summarise the knowledge gaps needed to be addressed to better understand and optimize ammonium cation-based passivation strategies.