Effect of Lead:Halide Precursor Ratio on the Photoluminescence and Carrier Dynamics of Violet- and Blue-Emitting Lead Halide Perovskite Nanocrystals

Abstract

Highly luminescent cesium lead halide (CsPbX3, X = Cl, Br, I) perovskite nanocrystals (NCs) are promising materials for a number of optoelectronic applications like LEDs and other display technologies. However, low photoluminescence (PL) quantum yield (QY) of the large-bandgap violet- and blue-emitting CsPbCl3 and CsPb(Cl/Br)3 NCs is an obstacle to the development of blue- and white-emitting LEDs. In this work, we show that these NCs with high PLQY can be obtained directly by employing an appropriate halide precursor and by optimizing the Pb:X precursor ratio. Specifically, employing N-chloro- and N-bromophthalimides as halide precursors and varying the Pb:X precursor ratio, we have obtained stable and highly luminescent (PLQY 80–99%) perovskite NCs emitting in the blue-violet region extending to green by direct synthesis. Time-resolved PL and ultrafast pump–probe studies of these systems reveal the effect of Pb:X precursor ratio on the carrier recombination processes. Rapid carrier trapping is found to be the dominant process that impairs the PLQY of the NCs obtained by using a stoichiometric (1:3) Pb:X precursor ratio. This trapping of carriers is effectively alleviated by using an excess amount of the halide precursor during the preparation of the NCs. The results brighten the potential utility of these high-quality perovskite NCs emitting in the blue-violet region in optical applications.