Microfluidic Reactors Provide Preparative and Mechanistic Insights into the Synthesis of Formamidinium Lead Halide Perovskite Nanocrystals

Abstract

Formamidinium lead bromide and iodide (FAPbX 3 , X = Br, I) in the form of colloidal nanocrystals (NCs) exhibit outstanding photoluminescence properties in the green and infrared regions of the electromagnetic spectrum, characterized by narrow emission line widths (below 90 meV) and high quantum yields (above 90%). The controlled formation of Br-I mixed halide NCs is a facile strategy for tuning band-gap energies, in particular between 700 and 800 nm, not accessible with CsPbX3 NCs. Herein, we report a mechanistic and highthroughput parametric screening study of the synthesis of such NCs using droplet-based microfluidic platforms, equipped with in situ optical characterization. We establish the growth conditions that fully suppress the formation of nanoplatelet impurities in the final colloid and demonstrate that the formation mechanism of FAPbBr 3 NCs proceeds via the formation of nanoplatelets as transient species, whereas FAPbI 3 forms directly as cubic-shaped NCs. In contrast to CsPb(Br/I) 3 NCs, the stability of FAPb(Br/ I) 3 NCs increases with iodine content. Such NCs form by first nucleating pure FAPbI 3 NCs, followed by incorporation of bromide ions.