Unveiling the Shape Evolution and Halide-Ion-Segregation in Blue-Emitting Formamidinium Lead Halide Perovskite Nanocrystals Using an Automated Microfluidic Platform

Abstract

Hybrid organic–inorganic perovskites and in particular formamidinium lead halide (FAPbX3, X = Cl, Br, I) perovskite nanocrystals (NCs) have shown great promise for their implementation in optoelectronic devices. Specifically, the Br and I counterparts have shown unprecedented photoluminescence properties, including precise wavelength tuning (530–790 nm), narrow emission linewidths (<100 meV) and high photoluminescence quantum yields (70–90%). However, the controlled formation of blue emitting FAPb(Cl1–xBrx)3 NCs lags behind their green and red counterparts and the mechanism of their formation remains unclear. Herein, we report the formation of FAPb(Cl1–xBrx)3 NCs with stable emission between 440 and 520 nm in a fully automated droplet-based microfluidic reactor and subsequent reaction upscaling in conventional laboratory glassware. The thorough parametric screening allows for the elucidation of parametric zones (FA-to-Pb and Br-to-Cl molar ratios, temperature, and excess oleic acid) for the formation of nanoplatelets and/or NCs. In contrast to CsPb(Cl1–xBrx)3 NCs, based on online parametric screening and offline structural characterization, we demonstrate that the controlled synthesis of Cl-rich perovskites (above 60 at% Cl) with stable emission remains a challenge due to fast segregation of halide ions.