Molecular cation and low-dimensional 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 and 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) perovskite capping layer on top of the three-dimensional perovskite. The formation of these two different structures is often overlooked by researchers, although they impact differently on device operation. In this Review, we seek to distinguish between these two passivation layers. We consider the conditions needed for the formation of low-dimensional perovskite and the electronic properties of the two structures. We discuss the mechanisms by which each method improves photovoltaic efficiency and stability. Finally, we summarize the knowledge gaps that need to be addressed to better understand and optimize ammonium cation-based passivation strategies.