Tracking the Fluorescence Lifetimes of Cesium Lead Halide Perovskite Nanocrystals During Their Synthesis Using a Fully Automated Optofluidic Platform

Semiconductive metal halide perovskites have opened exciting opportunities in a range of optoelectronic applications including solar cells, photodetectors, lasers, and light emitting devices. Recently, all-inorganic cesium lead halide (CsPbX3; X = Cl, Br, I) nanocrystals have become attractive light sources due to their high photoluminescence quantum yields, narrow emission linewidths, and emission color tunable over the entire visible region. Their radiative rates are higher (i.e., luminescence lifetimes shorter) than those of more conventional quantum dots, making perovskite NCs brighter emitters, highly attractive as both classical and quantum light sources. Multiple factors-primarily synthesis parameters and postsynthetic experience-govern the observed radiative lifetime and other optical characteristics. High-throughput experimentation in microfluidic platforms equipped with in-line optical characterization had proven to be highly instrumental for rapid and accurate assessment of optical properties, mainly in a steady-state mode. Thus far, in-line measurement of the radiative lifetimes and hence the proper use of high-throughput experimentation for tailored engineering of radiative rates have been elusive. Herein, we showcase a fully automated optofluidic platform that integrates time-correlated single photon counting measurements in droplet-based flow for the rigorous extraction of fluorescence lifetimes of CsPbX3 nanocrystals. The sensitivity of the experimental setup allows for measurements at a single-droplet level. Such concurrent time-resolved photoluminescence allows mapping the parametric space in a time-efficient manner (similar to 1000 lifetime measurements in 5 h of operation) and with high reagent economy (200 nL reaction volume per measurement). We elucidated the effects of composition and ratios of judiciously chosen reagents, as well as temperature on the fluorescence lifetimes (5-42 ns). Specifically, the average lifetime as well as the emission spectra of all halide compositions tested was strongly dependent on Pb-to-Cs variations. Accordingly a correlation between the steady-state luminescence amplitude and fluorescence lifetimes was established, thus providing a simple method to differentiate between the photoluminescence quantum yields, concentration effects, and effects due to the nonradiative recombination at the surface traps. Such a microfluidic tool will aid in analyzing the physicochemical and photophysical properties of diverse perovskite nanocrystals and other luminescent materials produced in the liquid-state synthesis.