Metal halide perovskites have opened exciting opportunities in the nanocrystal-based optoelectronic applications including solar cells,¹ lasers² and light emitting devices.³ In recent years, all-inorganic perovskite nanocrystals (NCs), CsPbX₃ (X = Cl, Br, I), have gained significant interest due to their high quantum yields of up to 90% and facile compositional tuning of their emission and absorption energies spanning the whole visible spectral region.⁴ However, our understanding of the reaction parameters governing their physicochemical and photophysical characteristics is limited due to the rapid kinetics involved in ionic-metathesis-based synthesis.

To date, microfluidic platforms incorporating integrated optical systems allow for controlled synthesis of NCs together with real-time characterization of material properties in a facile and rapid manner.⁵ Most efforts have focused on the on-line or in-line implementation of analytical techniques, including fluorescence, absorption, x-ray, backscattering and correlation spectroscopies, which mostly focus on unveiling information relating to particle size, shape and size distribution.⁵ Recently, we demonstrated the microfluidic synthesis of CsPbX₃ together with an in-situ rigorous and rapid mapping of reaction parameters, including molar ratios of Cs, Pb and halide precursors, reaction temperatures and reaction times. The combination of online photoluminescence (PL) and absorption measurements and the fast mixing of reagents within such a platform allowed for the investigation of early-stage kinetics and provided optimized and transferrable synthesis parameters to the conventional flask-based reaction.⁶ In contrast, and in the context of particle photophysics, further investigations are required to develop alternate detection strategies that are efficient in extracting real-time information regarding fluorescence quantum efficiencies and radiative and non-radiative deactivation pathways.

In this direction, we report a novel microfluidic approach, which combines for the first time on-line PL measurements and Time-Correlated Single Photon Counting (TCSPC) measurements for a systematic investigation of the effect of different reaction parameters including temperature, I/Br, Pb/Cs ratios and entropically riched organic ligands on the average fluorescence lifetime of the synthesized CsPbX₃ NCs. TCSPC measurements in flow allowing for in-situ decoding of fluorescence lifetimes in the range of 5 to 42 ns without the need of purification steps. Moreover, concurrent PL and TCSPC measurements are recorded for each individual droplet to allow correlations of PL intensity fluctuations with fluorescence lifetime changes.

[1] I. J. Kramer et al. Chemical Reviews, 2014, 114, 863-882.

[2] S. Yakunin et al. Nature Communications, 2015, 6, 8056.

[3] C. Dang et al. Mrs Bulletin, 2013, 38, 737-742.

[4] L. Protesescu et al. Nano Letters, 2015, 15, 3692-3696.

[5] T. W. Phillips et al. Lab on a Chip, 2014, 14, 3172-3180.

[6] I. Lignos et al. Nano letters, 2016, 16, 1869-1877.