Impact of Alkyl Chain Length on the Formation of Regular- and Reverse-Graded Quasi-2D Perovskite Thin Films.

Crystallization of low-dimensional perovskites is a complex process that leads to multidimensional films comprising two-dimensional (2D), quasi-2D, and three-dimensional (3D) phases. Most quasi-2D perovskite films possess a regular gradient with 2D phases located at the bottom of the film and 3D phases at the top. Recently, multiple studies have reported reverse-graded perovskite films, where the location of the 2D and 3D structures is inverted. The underlying reasons for such a peculiar phase distribution are unclear. While crystallization of regular-graded quasi-2D perovskites has been described as starting with 3D phases from the liquid-air interface, the film formation of reverse-graded films has not been investigated yet. Here, we examine the impact of the alkyl chain length on the formation of regular- and reverse-graded perovskites using alkylammonium ions. We find that long alkyl chains reverse the phase distribution gradient. By combining photoluminescence spectroscopy with optical absorption measurements, we demonstrate that crystallization starts at the liquid-N interface, though as 3D phases for short-chain alkylammonium ions and as quasi-2D phases for long chains. We link this behavior to enhanced van der Waals interactions between long-chain alkylammonium ions in polar solvents and their tendency to accumulate at the liquid-N interface, creating a concentration gradient along the film thickness.