Understanding the Structural Dynamics of 2D/3D Perovskite Interfaces

The use of 2D perovskite capping layers to passivate the surface defects of 3D perovskite active layers has become ubiquitous in high performance lead halide perovskite solar cells. However, these 2D/3D interfaces can be highly dynamic, with the structure evolving to form various mixed dimensional phases when exposed to thermal stress or illumination. Changes in the photoluminescence spectrum of formamidinium lead iodide (FAPbI₃) films capped with alkylammonium-based 2D perovskites as they age at 100 °C or under simulated 1 sun illumination indicate that the 2D perovskite transforms to progressively larger inorganic layer thicknesses (denoted by layer number <i>n</i>), eventually approaching a steady-state condition where only the 3D perovskite (<i>n</i> = ∞) is detectable. We find that this transformation slows by a factor of ∼2 when the length of the alkyl chain in the organic monoammonium ligand is increased from butylammonium to dodecylammonium. Furthermore, replacing dodecylammonium with its diammonium ligand counterpart, 1,12-dodecanediammonium, slows the structural transformation by 10-fold. These results point to the use of diammonium ligands as a possible pathway to form stable 2D/3D interfaces.