The role of lead precursors in driving competitive crystallization reactions during the formation of 2D perovskites

Two-dimensional (2D) lead halide perovskites are an exciting class of materials currently being extensively explored for both photovoltaics and optoelectronic applications. The ionic nature of these materials makes them ideal candidates for solution processing into both thin films and nanostructured crystals. However, a complete mechanistic description of 2D perovskite crystallization in solution is still missing due to the intricacy of process parameters and intermediates. Here, we investigate the role of different solid lead precursors (PbO2, PbI2, PbCO3) on the crystallization of pure-phase, n=1, Ruddleson-Popper 2D perovskite BA2PbI4, during a two-step drop-cast-based synthesis. While BA2PbI4 is formed in all cases, the nucleation and resulting morphology are strongly dependent on the choice of precursor, where the three lead precursors differ from each other in terms of their Pb-ion oxidation state, crystal structure, and material class. We use in-situ optical live imaging during synthesis to reveal clear differences in crystallization kinetics of the same 2D perovskite as a function of the lead precursor. We discern three competing mechanisms in the Pb-precursor for the formation of BA2PbI4: dissolution/complexation, BAI intercalation, and solid-state conversion. The differences in the oxidation state and solubility of the starting lead precursor in halide-rich solution play a key role in defining the crystallization pathway(s). This work demonstrates the importance of lead precursors in defining the nucleation and growth of perovskites thereby advancing the existing solution-processing techniques. Understanding how 2D perovskite crystals form in solution is key towards full control over their growth and optoelectronic properties, which will enable new types of physical phenomena and devices.