Lawrence Berkeley National Laboratory

Abstract Using in situ photoluminescence measurements during the spin‐coating and annealing steps, this study probes the formation of 2D layers on 3D triple cation perovskite films comparing phenylethylammonium and 2‐thiophenemethylammonium iodide bulky cations. This study elucidates the formation mechanisms of the surface layers for both cases and reveals two regimes during 2D layer formation: a kinetic‐driven and a thermodynamic‐driven process. These driving forces result in different compositions of the 2D/3D interface for each treatment, namely, different ratios of pure 2D ( n  = 1) and quasi‐2D ( n  > 1) structures. This study shows that a higher ratio of quasi‐2D phases is more beneficial for device performance, as pure‐2D layers may hamper current extraction. Due to a more evenly distributed formation energy profile among 2D and quasi‐2D phases, highly concentrated 2‐thiophenemethylammonium iodide appears to be more suited for effective surface passivation than its phenylethylammonium analog.